# Nano-texturing orientation effect on the piercing behavior of five stacked amorphous electrical steel sheets

SHIRATORI Tomomi<fnref n="1" />,ᵃ*, ITO Kentaro<fnref n="2" />,ᵇ, FURUHATA Suguri<fnref n="2" />,ᶜ and
AIZAWA Tatsuhiko<fnref n="3" />,ᵈ

**Keywords:** Punching, Nano-Texturing, Amorphous Electrical Steel Sheets

**Abstract.** Higher efficiency motor becomes an expecting keyword in the sustainable society. The iron core in these motors is fabricated by piercing the amorphous electrical steel sheet with sufficient dimensional accuracy and without defects. However, this material has 2.2 GPa in the tensile strength, no ductility, and over 900 HV in hardness. Furthermore, tool life was limited to be short. In recent years, some new tooling technologies for piercing amorphous electrical steel sheets have been developed to reduce the induced damages into punched sheets. Especially, when using nano-textured tool, the process affected width was more reduced than using the non-textured tools. In the present study, nano-texturing orientation effect on the piercing behavior is investigated and its mechanism is discussed.

## Introduction

The iron core of a motor has been processed by punching an electrical steel sheet. Non-oriented electrical steel sheets have been adapted to efficiently motor. An amorphous electrical steel sheet has been expected to the high efficiency motors. An amorphous electrical steel sheet has a 2.2 GPa in the tensile strength, no ductility, and over 900 HV in hardness. Thus, when punching amorphous electrical steel sheets, tool life was limited to be short. There are some piercing technologies has been developed in recent years. In the field of amorphous electrical steel sheets piercing, Chemical Vapor Deposition (CVD) diamond coated and adding nano-texturing new tool was developed [1] and the effect of reducing the process affected width was confirmed. A tool adding nano-texturing on SKD11 material has been developed [2]. It has been reported that changing the angle at which nano-texturing is applied reduces piercing load and process affected width. The effects of nanometer and micrometer textures on sheared surface formation were investigated [3]. This study showed that even if the direction of microgroove formation was changed by 90 degrees, the process affected width in surface of pierced hole in amorphous electrical steel sheets did not change much, and nano-texture became main factor of reduced the process affected width [3]. In the field of non-oriented electrical steel sheet piercing, reduction of adhesion was observed with CVD diamond and nano-textured punch [4]. These results indicate that nano-texture has the potential to improve the sliding properties during piercing process with the workpiece, contributing to increased tool durability. In addition to these studies, ion sharpening tools are also being developed [5]. It has been confirmed that the homogeneity of nanometer-sized tool tips has the effect of lowering the fracture surface ratio.

It has been found that nanotextures and tools with nanometer precision can suppress the process affected width during sheared surface formation and piercing process of amorphous electrical steel sheets. However, the optimization and mechanism of machining conditions such as

<figure><img src="image_2325.png" /></figure>

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<footnote n="1">¹Faculty of engineering, University of Toyama, Toyama-city, Toyama 930-8555, Japan</footnote>

<footnote n="2">²Graduate School of Engineering, University of Toyama 930-8555, Japan</footnote>

<footnote n="3">³Surface Engineering Design Laboratory, SIT; Tokyo 144-0045, Japan</footnote>

<footnote>ᵃshira@eng.u-toyama.Fuac.jp, ᵇm23c1411 @ems.u-toyama.ac.jp,</footnote>

<footnote>ᶜs2279404@ems.u-toyama.ac.jp, ᵈtaizawa@sic.shibaura-it.ac.jp</footnote>
tool type selection and optimal angle for nanotextured tools have not been fully elucidated. Therefore, in this study, we aimed to investigate the influence of the direction in which nanotexture is applied. An amorphous electrical steel sheet is pierced under different conditions with different tool conditions and nano-texture application angles, and the mechanism of nano-texture is studied by investigating the process affected width in pierced holes and piercing load.

## Experimental Methods

The five stacked an electrical amorphous steel sheet with a thickness of 25 µm was used as workpiece. The material was composed with Fe-3Si. This stacked material was employed as a work for piercing experiments as shown in Fig. 1a. A small-scaled screw servo-stamping system with the loading capacity of 30 kN (DT-J311 system, Microfabrication Laboratory, LLC.; Tokyo, Japan), was used for the present piercing experiment in dry without lubrication as shown in Fig. 1b. The punching speed was set at 5.0 mm/s. The piercing load was measured by the load cell (LMBT-A-2KN, Kyowa Electric Co., Ltd.; Tokyo, Japan) which was embedded into the lower die set. The die stroke was measured by the laser displacement meter (LK-G30, KEYENCE Co., Ltd.; Tokyo, Japan). The whole measured data were transferred to PC through the interface (NR-600, KEYENCE Co., Ltd.; Tokyo, Japan).

<figure><img src="image_2335.png" /><figcaption>Fig. 1 A small-scaled screw servo-stamping system. (a) A schematic view on the piercing procedure of amorphous electrical steel sheet stack, and (b) outlook of press machine.</figcaption></figure>

Five punch-die pairs were prepared for tooling as listed in Table 1. The punch diameter used in the experiment was unified at 5.000 mm and basically clearance between punch and die was set at 0.010 mm. No. (a) and (b) punch was adopted CVD diamond. The nano-texturing angle from punch bottom was set at 90 degrees and 30 degrees. No. (c) was set at normal ground WC+Co punch. No. (d) and (e) punch was adopted WC+Co material with 90 degrees and 30 degrees nano-texturing from punch bottom. The condition of die material and finishing method was unified with WC+Co and ion-sharpening.

The piercing process was set to 10 shots under each condition. The punch after shearing was observed using a scanning electron microscope (SEM). From this SEM observation results, we measured the process affected width at the edge of the pierced hole. In addition, the piercing loads were compared.
tool type selection and optimal angle for nanotextured tools have not been fully elucidated. Therefore, in this study, we aimed to investigate the influence of the direction in which nanotexture is applied. An amorphous electrical steel sheet is pierced under different conditions with different tool conditions and nano-texture application angles, and the mechanism of nano-texture is studied by investigating the process affected width in pierced holes and piercing load.

## Experimental Methods

The five stacked an electrical amorphous steel sheet with a thickness of 25 µm was used as workpiece. The material was composed with Fe-3Si. This stacked material was employed as a work for piercing experiments as shown in Fig. 1a. A small-scaled screw servo-stamping system with the loading capacity of 30 kN (DT-J311 system, Microfabrication Laboratory, LLC.; Tokyo, Japan), was used for the present piercing experiment in dry without lubrication as shown in Fig. 1b. The punching speed was set at 5.0 mm/s. The piercing load was measured by the load cell (LMBT-A-2KN, Kyowa Electric Co., Ltd.; Tokyo, Japan) which was embedded into the lower die set. The die stroke was measured by the laser displacement meter (LK-G30, KEYENCE Co., Ltd.; Tokyo, Japan). The whole measured data were transferred to PC through the interface (NR-600, KEYENCE Co., Ltd.; Tokyo, Japan).

<figure><img src="image_2326.png" /><figcaption>Fig. 1 A small-scaled screw servo-stamping system. (a) A schematic view on the piercing procedure of amorphous electrical steel sheet stack, and (b) outlook of press machine.</figcaption></figure>

Five punch-die pairs were prepared for tooling as listed in Table 1. The punch diameter used in the experiment was unified at 5.000 mm and basically clearance between punch and die was set at 0.010 mm. No. (a) and (b) punch was adopted CVD diamond. The nano-texturing angle from punch bottom was set at 90 degrees and 30 degrees. No. (c) was set at normal ground WC+Co punch. No. (d) and (e) punch was adopted WC+Co material with 90 degrees and 30 degrees nano-texturing from punch bottom. The condition of die material and finishing method was unified with WC+Co and ion-sharpening.

The piercing process was set to 10 shots under each condition. The punch after shearing was observed using a scanning electron microscope (SEM). From this SEM observation results, we measured the process affected width at the edge of the pierced hole. In addition, the piercing loads were compared.
<table><caption>Table 1 Punch and die conditions in experiment.</caption><thead><tr><th rowspan="2">No.</th><th colspan="3">Punch</th><th colspan="3">Die</th><th rowspan="2">Clearance<br>[mm]</th></tr><tr><th>Material</th><th>Finishing</th><th>Outer dia.<br>[mm]</th><th>Material</th><th>Finishing</th><th>Inner dia.<br>[mm]</th></tr></thead><tbody><tr><td>(a)</td><td>Diamond</td><td>Nano-texturing 90 deg.</td><td>4.982</td><td>WC+Co</td><td>ion-sharpening</td><td>5.003</td><td>0.0105</td></tr><tr><td>(b)</td><td>Diamond</td><td>Nano-texturing 30 deg.</td><td>4.985</td><td>WC+Co</td><td>ion-sharpening</td><td>5.007</td><td>0.0110</td></tr><tr><td>(c)</td><td>WC+Co</td><td>Normal ground</td><td>4.998</td><td>WC+Co</td><td>ion-sharpening</td><td>5.019</td><td>0.0105</td></tr><tr><td>(d)</td><td>WC+Co</td><td>Nano-texturing 90 deg.<br>with ion-sharpening</td><td>4.999</td><td>WC+Co</td><td>ion-sharpening</td><td>5.021</td><td>0.0110</td></tr><tr><td>(e)</td><td>WC+Co</td><td>Nano-texturing 30 deg. with<br>ion-sharpening</td><td>4.998</td><td>WC+Co</td><td>ion-sharpening</td><td>5.021</td><td>0.0115</td></tr></tbody></table>

## Experimental Results

Punch edge SEM observation results at after 10 shot punching is shown in Fig. 2. From Fig. 2(a) and (b), Surface of Diamond punches were covered with adhesion. Inhomogeneous edge is shown in Fig.2(c) Normal ground WC+Co punch. It's hard to see in Fig. 2(d) and (e), but the adhesion covers part of the nano-texturing with ion-sharpening punch.

<figure><img src="image_2336.png" /></figure>

<figure><img src="image_2337.png" /><figcaption>Fig. 2 Punch edge SEM observation results at after 10 shot punching.</figcaption></figure>

Fig. 3 shows the SEM observation results of surface of a hole made by piercing five stacked sheets and results of process affected width. From this figure, from No. 2 to No. 4 sheet, there are cracks on the surface of the holes. Also, for sheet No. 1, the process affected width changes depending on the tool conditions. The process affected width from No. 1 to No. 5 sheet changes continuously under all tool conditions. Fig. 4 shows a graph of the process affected width measured from Fig.3. In the sheet No.1, (c) normal ground punch condition recorded the maximum value of 170.3 µm. Next, (b) Diamond nano-texturing 90 deg. recorded 72.2 µm. The other three conditions showed small values of around 11 µm. From No. 2 sheet results, blue color (c) result decreased from No.1 sheet data and red color (b) data remained the same from No.1 sheet result. On the other hand, the other three conditions showed a value of 100 µm or more, and these results were about
<table><caption>Table 1 Punch and die conditions in experiment.</caption><thead><tr><th rowspan="2">No.</th><th colspan="3">Punch</th><th colspan="3">Die</th><th rowspan="2">Clearance<br>[mm]</th></tr><tr><th>Material</th><th>Finishing</th><th>Outer dia.<br>[mm]</th><th>Material</th><th>Finishing</th><th>Inner dia.<br>[mm]</th></tr></thead><tbody><tr><td>(a)</td><td>Diamond</td><td>Nano-texturing 90 deg.</td><td>4.982</td><td>WC+Co</td><td>ion-sharpening</td><td>5.003</td><td>0.0105</td></tr><tr><td>(b)</td><td>Diamond</td><td>Nano-texturing 30 deg.</td><td>4.985</td><td>WC+Co</td><td>ion-sharpening</td><td>5.007</td><td>0.0110</td></tr><tr><td>(c)</td><td>WC+Co</td><td>Normal ground</td><td>4.998</td><td>WC+Co</td><td>ion-sharpening</td><td>5.019</td><td>0.0105</td></tr><tr><td>(d)</td><td>WC+Co</td><td>Nano-texturing 90 deg.<br>with ion-sharpening</td><td>4.999</td><td>WC+Co</td><td>ion-sharpening</td><td>5.021</td><td>0.0110</td></tr><tr><td>(e)</td><td>WC+Co</td><td>Nano-texturing 30 deg. with<br>ion-sharpening</td><td>4.998</td><td>WC+Co</td><td>ion-sharpening</td><td>5.021</td><td>0.0115</td></tr></tbody></table>

## Experimental Results

Punch edge SEM observation results at after 10 shot punching is shown in Fig. 2. From Fig. 2(a) and (b), Surface of Diamond punches were covered with adhesion. Inhomogeneous edge is shown in Fig.2(c) Normal ground WC+Co punch. It's hard to see in Fig. 2(d) and (e), but the adhesion covers part of the nano-texturing with ion-sharpening punch.

<figure><img src="image_2327.png" /></figure>

<figure><img src="image_2328.png" /><figcaption>Fig. 2 Punch edge SEM observation results at after 10 shot punching.</figcaption></figure>

Fig. 3 shows the SEM observation results of surface of a hole made by piercing five stacked sheets and results of process affected width. From this figure, from No. 2 to No. 4 sheet, there are cracks on the surface of the holes. Also, for sheet No. 1, the process affected width changes depending on the tool conditions. The process affected width from No. 1 to No. 5 sheet changes continuously under all tool conditions. Fig. 4 shows a graph of the process affected width measured from Fig.3. In the sheet No.1, (c) normal ground punch condition recorded the maximum value of 170.3 µm. Next, (b) Diamond nano-texturing 90 deg. recorded 72.2 µm. The other three conditions showed small values of around 11 µm. From No. 2 sheet results, blue color (c) result decreased from No.1 sheet data and red color (b) data remained the same from No.1 sheet result. On the other hand, the other three conditions showed a value of 100 µm or more, and these results were about
<figure><img src="image_2338.png" /><figcaption>Fig. 3 SEM observation results of surface of a hole made by piercing five stacked sheets and results of process affected width.</figcaption></figure>
<figure><img src="image_2329.png" /><figcaption>Fig. 3 SEM observation results of surface of a hole made by piercing five stacked sheets and results of process affected width.</figcaption></figure>
<figure><img src="image_2339.png" /><figcaption>Fig. 4 Process affected width in stacked piercing hole edge.</figcaption></figure>

10 times larger than the No. 1 sheet data. From No.3 to No.5 sheet results, the value of color blue (c) decreases linearly. The other four conditions results were within the range of 80 ± 20 µm. In piercing of an stacked five amorphous electrical steel sheet, it was found that the process affected width in pierced hole changes with tool conditions.

Fig. 5 shows the load – stroke curve in piercing of five stacked sheet. Five stacked sheet material thickness is 125 µm. However, the minimum breaking stroke was 0.26 mm for green color (d) and orange color (e). It has a stroke of 0.26 mm for a plate thickness of 0.125 mm, and fracture occurs at a stroke that is over twice value of the five stacked sheet thickness. The red color (a) and blue color (b) using a diamond punch require a longer stroke to break than the normal ground condition for black color (c), and the maximum load is also increased.

<figure><img src="image_2340.png" /><figcaption>Fig. 5 Load – stroke curve in piercing of five stacked sheet.</figcaption></figure>

Discussion

From the Fig. 2, when using diamond punch with nano-textured, there are radius shape of several micrometers. This large radius shape is considered to be the cause of the increase in the process affected width in No. 1 sheet as shown in Fig. 4 (b). In addition, this large radius shape is thought to be the cause of the increase in the fracture stroke and maximum load as shown in Fig. 5 (b). On the other hand, when using (a) diamond nano-texturing 90 deg., from Fig. 2(a), the process affected width of No. 1 sheet is suppressed to 12.8 µm, while the piercing load is maximized as shown in Fig. 5(a). This means that when piercing sheet No.1, it is possible to efficiently apply shear stress to the workpiece material by setting the nano-texturing at 90 degrees, regardless of the large radius shape. In addition, it is thought that by setting the direction in which the nano-texturing is applied
<figure><img src="image_2330.png" /><figcaption>Fig. 4 Process affected width in stacked piercing hole edge.</figcaption></figure>

10 times larger than the No. 1 sheet data. From No.3 to No.5 sheet results, the value of color blue (c) decreases linearly. The other four conditions results were within the range of 80 ± 20 µm. In piercing of an stacked five amorphous electrical steel sheet, it was found that the process affected width in pierced hole changes with tool conditions.

Fig. 5 shows the load – stroke curve in piercing of five stacked sheet. Five stacked sheet material thickness is 125 µm. However, the minimum breaking stroke was 0.26 mm for green color (d) and orange color (e). It has a stroke of 0.26 mm for a plate thickness of 0.125 mm, and fracture occurs at a stroke that is over twice value of the five stacked sheet thickness. The red color (a) and blue color (b) using a diamond punch require a longer stroke to break than the normal ground condition for black color (c), and the maximum load is also increased.

<figure><img src="image_2331.png" /><figcaption>Fig. 5 Load – stroke curve in piercing of five stacked sheet.</figcaption></figure>

Discussion

From the Fig. 2, when using diamond punch with nano-textured, there are radius shape of several micrometers. This large radius shape is considered to be the cause of the increase in the process affected width in No. 1 sheet as shown in Fig. 4 (b). In addition, this large radius shape is thought to be the cause of the increase in the fracture stroke and maximum load as shown in Fig. 5 (b). On the other hand, when using (a) diamond nano-texturing 90 deg., from Fig. 2(a), the process affected width of No. 1 sheet is suppressed to 12.8 µm, while the piercing load is maximized as shown in Fig. 5(a). This means that when piercing sheet No.1, it is possible to efficiently apply shear stress to the workpiece material by setting the nano-texturing at 90 degrees, regardless of the large radius shape. In addition, it is thought that by setting the direction in which the nano-texturing is applied
at 90 degrees, the fracture stroke will be reduced compared to 30 degrees even if with a large radius shape.

When using normal ground punch, inhomogeneous tool edge shape as shown in Fig. 2(c) became maximum process affected width in No.1 sheet as shown in Fig. 4(c). This suggests that the non-uniform edge condition acts as a resistance to the start of piercing of the sheet, resulting in a deformed state that bends the workpiece and causes cracks [6].

When using nano-texturing with ion-sharpening punch, as can be seen from Fig. 4 (d) and (e), by using a punch that combines ion-sharpening and nano-texture, the process affected width of No. 1 sheet and No. 2 sheet are smaller than that of normal ground punch condition. As can be seen from Fig. 5 (d) and (e), the breaking stroke is minimized. There are no significant differences in processing characteristics due to differences in the direction of nanotexture application. This means that it can be considered that the effect of ion-sharpening exceeded the effect of adding nano-texture.

From the above considerations, it has been shown that in piercing stacked amorphous electrical steel sheets, homogeneous and sharp edges due to ion-sharpening is effective for suppress the process affected width and reducing the fracture stroke. There was a tendency for the fracture stroke changing with direction of the nano-texturing. It is thought that advanced piercing of amorphous electrical steel sheets can be achieved by optimizing the size of the edge radius, the homogeneity of the tool edge, the direction of nano-texturing, etc.

## Conclusions

The following conclusions were obtained by shearing five laminated amorphous electrical steel sheets.

1) The homogeneity of the tool edge is a factor that reduces the process affected width of hole at the first sheet from the punch side.

2) An increase in the corner radius of the tool causes an increase in piercing load and breaking strokes.

3) By applying nano-texture at 90 degrees from the bottom of the punch, the breaking stroke tends to be reduced.

4) For the advanced piercing of stacked amorphous electrical steel sheets can be achieved by optimizing the size of the edge radius, the homogeneity of the tool edge, the direction of nano-texturing.

## Acknowledgement

This study was partly funded by Grants-in-Aid for Scientific Research No. 21K03789 in Japan.

## References

[1] T. Aizawa, T. Shiratori, Y. Kira, T. Yoshino, Y. Suzuki, Femtosecond laser trimming with simultaneous nanostructuring to fine piercing punch to electrical amorphous steel sheets, Micromachines, 12-568(2021) 1-14. https://doi.org/10.3390/mi12050568

[2] T. Shiratori, Y. Komori, Y. Suzuki, K. Abe, T. Aizawa, Fine piercing of amorphous electrical steel sheet stack using micro-/nano-textured punch, Proc. ICTP2023, 4(2023)89-99. https://doi.org/10.1007/978-3-031-42093-1_9

[3] Y. Komori, Y. Suzuki, K. Abe, T. Aizawa, T. Shiratori, Fine piercing of amorphous electrical steel sheet stack by micro-/nano-textured punch, Materials, 15(5)-1682(2022)1-13. https://doi.org/10.3390/ma15051682
at 90 degrees, the fracture stroke will be reduced compared to 30 degrees even if with a large radius shape.

When using normal ground punch, inhomogeneous tool edge shape as shown in Fig. 2(c) became maximum process affected width in No.1 sheet as shown in Fig. 4(c). This suggests that the non-uniform edge condition acts as a resistance to the start of piercing of the sheet, resulting in a deformed state that bends the workpiece and causes cracks [6].

When using nano-texturing with ion-sharpening punch, as can be seen from Fig. 4 (d) and (e), by using a punch that combines ion-sharpening and nano-texture, the process affected width of No. 1 sheet and No. 2 sheet are smaller than that of normal ground punch condition. As can be seen from Fig. 5 (d) and (e), the breaking stroke is minimized. There are no significant differences in processing characteristics due to differences in the direction of nanotexture application. This means that it can be considered that the effect of ion-sharpening exceeded the effect of adding nano-texture.

From the above considerations, it has been shown that in piercing stacked amorphous electrical steel sheets, homogeneous and sharp edges due to ion-sharpening is effective for suppress the process affected width and reducing the fracture stroke. There was a tendency for the fracture stroke changing with direction of the nano-texturing. It is thought that advanced piercing of amorphous electrical steel sheets can be achieved by optimizing the size of the edge radius, the homogeneity of the tool edge, the direction of nano-texturing, etc.

## Conclusions

The following conclusions were obtained by shearing five laminated amorphous electrical steel sheets.

1) The homogeneity of the tool edge is a factor that reduces the process affected width of hole at the first sheet from the punch side.

2) An increase in the corner radius of the tool causes an increase in piercing load and breaking strokes.

3) By applying nano-texture at 90 degrees from the bottom of the punch, the breaking stroke tends to be reduced.

4) For the advanced piercing of stacked amorphous electrical steel sheets can be achieved by optimizing the size of the edge radius, the homogeneity of the tool edge, the direction of nano-texturing.

## Acknowledgement

This study was partly funded by Grants-in-Aid for Scientific Research No. 21K03789 in Japan.

## References

[1] T. Aizawa, T. Shiratori, Y. Kira, T. Yoshino, Y. Suzuki, Femtosecond laser trimming with simultaneous nanostructuring to fine piercing punch to electrical amorphous steel sheets, Micromachines, 12-568(2021) 1-14. https://doi.org/10.3390/mi12050568

[2] T. Shiratori, Y. Komori, Y. Suzuki, K. Abe, T. Aizawa, Fine piercing of amorphous electrical steel sheet stack using micro-/nano-textured punch, Proc. ICTP2023, 4(2023)89-99. https://doi.org/10.1007/978-3-031-42093-1_9

[3] Y. Komori, Y. Suzuki, K. Abe, T. Aizawa, T. Shiratori, Fine piercing of amorphous electrical steel sheet stack by micro-/nano-textured punch, Materials, 15(5)-1682(2022)1-13. https://doi.org/10.3390/ma15051682
[4] T. Aizawa, T. Shiratori, Y. Kira, T. Inohara, Simultaneous nano-texturing onto a CVD-diamond coated piercing punch with femtosecond laser trimming, Applied Sciences, 10-2674(2020)1-12. https://doi.org/10.3390/app10082674

[5] T. Shiratori, T. Yoshino, Y. Suzuki, S. Nakano, M. Yang, Effects of ion processing of cutting edge of tools on tool wear and punching characteristics in micropunching process, J. JSTP, 60-698(2019)58-63. https://doi.org/10.9773/sosei.60.58

[6] Y. Komori, T. Yoshino, Y. Suzuki, T. Aizawa, T. Shiratori, Effects of interfacial resin coating on punching behavior of amorphous electrical steel sheet stack, J. JSTP, 64-746(2023)43-48. https://doi.org/10.9773/sosei.64.43
[4] T. Aizawa, T. Shiratori, Y. Kira, T. Inohara, Simultaneous nano-texturing onto a CVD-diamond coated piercing punch with femtosecond laser trimming, Applied Sciences, 10-2674(2020)1-12. https://doi.org/10.3390/app10082674

[5] T. Shiratori, T. Yoshino, Y. Suzuki, S. Nakano, M. Yang, Effects of ion processing of cutting edge of tools on tool wear and punching characteristics in micropunching process, J. JSTP, 60-698(2019)58-63. https://doi.org/10.9773/sosei.60.58

[6] Y. Komori, T. Yoshino, Y. Suzuki, T. Aizawa, T. Shiratori, Effects of interfacial resin coating on punching behavior of amorphous electrical steel sheet stack, J. JSTP, 64-746(2023)43-48. https://doi.org/10.9773/sosei.64.43
# Improving room temperature formability of twin-roll cast Mg-Al-Zn-Sn alloy by repeated bending

TOZUKA Hotaka¹,ª, WATARI Hisaki¹,ᵇ* and HAGA Toshio²,ᶜ

¹Division of Mechanical Engineering, Faculty of Science and Engineering, Tokyo Denki University, Ishizaka Hatoyama-machi Hiki-gun Saitama 3500394, Japan

²Department of Mechanical Engineering, Osaka Institute of Technology, 5-16-1 Omiya Asahi-ku Osaka 573-0196, Japan

ª19udmtoshioa01@ms.dendai.ac.jp, ᵇwatari@mail.dendai.ac.jp, ᶜtoshio.haga@oit.ac.jp

**Keywords:** Twin-Roll Casting, Solution Heat Treatment, Cold Formability, Mg-Al-Zn-Sn Alloy

**Abstract.** A twin-roll cast Mg-9mass%Al-1 mass%Zn-2mass%Sn alloy was used for the purpose of fabricating for wrought magnesium alloy which can be bent in the room temperature. An appropriate solution annealing time was found in order to reduce beta phase precipitation, which may cause cracking without coarsening of crystal grain size while for improving cold formability of twin-roll cast Mg-9mass%Al-1 mass%Zn-2 mass%Sn alloy. After solution heat treatment of the twin-roll casting of the material, repeated bending by three rolls of the solution heat treated strip was performed in order to examine of bendability of the materials. It has been clarified that repeatedly bending and reverse bending after solution heat treatment were effective for improving formability of the Mg-9mass%Al-1 mass%Zn-2 mass% Sn alloy at room temperature.

**Introduction**

It has been considered that magnesium (Mg) alloys are attractive materials for manufacturing lightweight products due to their light weight and high specific strength. However, applying practical use of Mg alloys for plastic forming at room temperature is difficult due to its crystal structure [1-3]. The low ductility and poor formability are related to the hexagonal close packed crystal structure of magnesium which cannot provide sufficient slip systems for room temperature deformation. The dominant deformation mechanisms in magnesium and its alloys are basal slip, secondary (prismatic and pyramidal) slip, and twinning [4]. The critical resolved shear stress (CRSS) of has been reported for basal slip is less than 1.0 MPa. In addition, the CRSS of prismatic slip is approximately 40 to 50 MPa in pure Mg [5].

Under the situation of Mg alloys, the possible ratio between bending radius (R/t) and sheet thickness shows 5.0 at room temperature. Many studies have been conducted with the aim of improving formability of wrought magnesium alloy sheet at room temperature. It is reported that adding 0.13 mass% of Ca into Mg-3 mass%Al-1 mass% Zn alloy was effective for tilting the pole of the texture with respect to the sheet thickness direction as well as a repeated bending and reverse bending with a curvature of 15 times sheet thickness. The paper reported that a minimum bending radius R/t of 2.5 in cold V-bending was achieved by applying strain to the rolled material and annealing [6].

However, when the Ca is added to magnesium alloys, there is low corrosion resistance problem due to the effect of adding Ca. Generally, it has been recognized that Sn, Ca, reduces the CRSS of prismatic slip when in adding to magnesium alloys. Ebrahim et al. achieve a 5% cold rolling without formation of a basal texture using a solution-treated Mg-9 mass%Al-1 mass%Zn alloy cast material, applying asymmetrically cold rolling [7]. Also, Mg-Sn based alloys have attracted increasing attention due to the considerable strengthening ability of Mg₂Sn precipitates due to the

<figure><img src="image_2341.png" /></figure>
# Improving room temperature formability of twin-roll cast Mg-Al-Zn-Sn alloy by repeated bending

TOZUKA Hotaka¹,ª, WATARI Hisaki¹,ᵇ* and HAGA Toshio²,ᶜ

¹Division of Mechanical Engineering, Faculty of Science and Engineering, Tokyo Denki University, Ishizaka Hatoyama-machi Hiki-gun Saitama 3500394, Japan

²Department of Mechanical Engineering, Osaka Institute of Technology, 5-16-1 Omiya Asahi-ku Osaka 573-0196, Japan

ª19udmtoshioa01@ms.dendai.ac.jp, ᵇwatari@mail.dendai.ac.jp, ᶜtoshio.haga@oit.ac.jp

**Keywords:** Twin-Roll Casting, Solution Heat Treatment, Cold Formability, Mg-Al-Zn-Sn Alloy

**Abstract.** A twin-roll cast Mg-9mass%Al-1 mass%Zn-2mass%Sn alloy was used for the purpose of fabricating for wrought magnesium alloy which can be bent in the room temperature. An appropriate solution annealing time was found in order to reduce beta phase precipitation, which may cause cracking without coarsening of crystal grain size while for improving cold formability of twin-roll cast Mg-9mass%Al-1 mass%Zn-2 mass%Sn alloy. After solution heat treatment of the twin-roll casting of the material, repeated bending by three rolls of the solution heat treated strip was performed in order to examine of bendability of the materials. It has been clarified that repeatedly bending and reverse bending after solution heat treatment were effective for improving formability of the Mg-9mass%Al-1 mass%Zn-2 mass% Sn alloy at room temperature.

**Introduction**

It has been considered that magnesium (Mg) alloys are attractive materials for manufacturing lightweight products due to their light weight and high specific strength. However, applying practical use of Mg alloys for plastic forming at room temperature is difficult due to its crystal structure [1-3]. The low ductility and poor formability are related to the hexagonal close packed crystal structure of magnesium which cannot provide sufficient slip systems for room temperature deformation. The dominant deformation mechanisms in magnesium and its alloys are basal slip, secondary (prismatic and pyramidal) slip, and twinning [4]. The critical resolved shear stress (CRSS) of has been reported for basal slip is less than 1.0 MPa. In addition, the CRSS of prismatic slip is approximately 40 to 50 MPa in pure Mg [5].

Under the situation of Mg alloys, the possible ratio between bending radius (R/t) and sheet thickness shows 5.0 at room temperature. Many studies have been conducted with the aim of improving formability of wrought magnesium alloy sheet at room temperature. It is reported that adding 0.13 mass% of Ca into Mg-3 mass%Al-1 mass% Zn alloy was effective for tilting the pole of the texture with respect to the sheet thickness direction as well as a repeated bending and reverse bending with a curvature of 15 times sheet thickness. The paper reported that a minimum bending radius R/t of 2.5 in cold V-bending was achieved by applying strain to the rolled material and annealing [6].

However, when the Ca is added to magnesium alloys, there is low corrosion resistance problem due to the effect of adding Ca. Generally, it has been recognized that Sn, Ca, reduces the CRSS of prismatic slip when in adding to magnesium alloys. Ebrahim et al. achieve a 5% cold rolling without formation of a basal texture using a solution-treated Mg-9 mass%Al-1 mass%Zn alloy cast material, applying asymmetrically cold rolling [7]. Also, Mg-Sn based alloys have attracted increasing attention due to the considerable strengthening ability of Mg₂Sn precipitates due to the

<figure><img src="image_2332.png" /></figure>
dynamic range of solubility as a function of temperature [8,9]. It has been reported that the formation of thermally stable Mg₂Sn could precipitates also imparts Mg-Sn based alloys with high thermal stability at elevated temperatures [10,11,12].

In this paper, it has been clarified effectiveness of repeatedly bending and reverse bending after solution heat treatment for improving formability of twin-roll cast Mg-Al-Zn-Sn alloys at room temperature.

## Experiment method

*Horizontal twin roll caster.* The material was cast by a horizontal twin roll caster as shown in Fig.1. The material was rapidly cast by using copper rolls in the Figure 1. The roll diameters were 300 mm, and the roll width was 100 mm. The material of upper and lower rolls was pure copper.

A spring was attached to the top of the upper plate at the upper roll, which can give a certain reduction when the strip was go through between the upper and the lower roll. The spring force was varied from 6 kN, 10 kN, 20 kN, in order to examine the effects of roll load to obtain a desired cast strip during twin roll casting.

*Material used in twin roll casting experiment.*

Mg-9mass%Al-1mass%Zn-2mass%Sn alloy (AZT912) was selected to the differential cold speed rolling the experiment after solution treatment. Sn content of the materials was about 2.0% into AZ91 which is one of the popular casting materials in the magnesium alloys. The chemical composition of the Mg-9mass%Al-1mass%Zn-2mass%Sn alloy used in the roll casting experiment is shown in Table 1.

<figure><img src="image_2342.png" /><figcaption>Fig. 1 Schematic illustration of horizontal twin roll caster.</figcaption></figure>

<table><caption>Table 1 Chemical composition of Mg-9mass%Al-1mass%Zn-2mass%Sn alloy.</caption><thead><tr><th colspan="10">Mass %</th></tr><tr><th>Al</th><th>Zn</th><th>Mn</th><th>Si</th><th>Fe</th><th>Cu</th><th>Ni</th><th>Be</th><th>Sn</th><th>Mg</th></tr></thead><tbody><tr><td>8.99</td><td>0.72</td><td>0.21</td><td>0.02</td><td>0.001</td><td>0.001</td><td>0.0006</td><td>0.0011</td><td>1.86</td><td>Bal.</td></tr></tbody></table>

*Heat treatment of twin-roll cast strip.* In the heat treatment process of the roll cast material, the solution treatment temperature was set to 404°C, and the solution treatment time was varied from to 0.5 h, 1 h, 2 h, 3 h, 4 h, 5 h, 6 h, 8 h, and 24 h. After that, a tensile test piece for a tension test was cut from the solution heat-treated strip by a wire electric discharge machine. From the relationship between solution treatment time between the maximum elongation of the tension test, an appropriate heat treatment condition was determined to obtain a desired microstructure while keeping minimizing of the solution treatment time. The selected heat treatment condition of the twin roll cast material was found as shown in Table 2.

<table><caption>Table 2 Solution heat treatment conditions of roll cast material.</caption><tr><td>Temperature [°C]</td><td>406</td></tr><tr><td>Time [h]</td><td>0.5 h, 1 h, 2 h, 3 h, 4 h, 5 h, 6 h, 8 h, 24 h</td></tr><tr><td>Quenching condition</td><td>Hot Water at 60</td></tr></table>
dynamic range of solubility as a function of temperature [8,9]. It has been reported that the formation of thermally stable Mg₂Sn could precipitates also imparts Mg-Sn based alloys with high thermal stability at elevated temperatures [10,11,12].

In this paper, it has been clarified effectiveness of repeatedly bending and reverse bending after solution heat treatment for improving formability of twin-roll cast Mg-Al-Zn-Sn alloys at room temperature.

## Experiment method

*Horizontal twin roll caster.* The material was cast by a horizontal twin roll caster as shown in Fig.1. The material was rapidly cast by using copper rolls in the Figure 1. The roll diameters were 300 mm, and the roll width was 100 mm. The material of upper and lower rolls was pure copper.

A spring was attached to the top of the upper plate at the upper roll, which can give a certain reduction when the strip was go through between the upper and the lower roll. The spring force was varied from 6 kN, 10 kN, 20 kN, in order to examine the effects of roll load to obtain a desired cast strip during twin roll casting.

### Material used in twin roll casting experiment.

Mg-9mass%Al-1mass%Zn-2mass%Sn alloy (AZT912) was selected to the differential cold speed rolling the experiment after solution treatment. Sn content of the materials was about 2.0% into AZ91 which is one of the popular casting materials in the magnesium alloys. The chemical composition of the Mg-9mass%Al-1mass%Zn-2mass%Sn alloy used in the roll casting experiment is shown in Table 1.

<figure><img src="image_2333.png" /><figcaption>Fig. 1 Schematic illustration of horizontal twin roll caster.</figcaption></figure>

<table><caption>Table 1 Chemical composition of Mg-9mass%Al-1mass%Zn-2mass%Sn alloy.</caption><thead><tr><th colspan="10">Mass %</th></tr><tr><th>Al</th><th>Zn</th><th>Mn</th><th>Si</th><th>Fe</th><th>Cu</th><th>Ni</th><th>Be</th><th>Sn</th><th>Mg</th></tr></thead><tbody><tr><td>8.99</td><td>0.72</td><td>0.21</td><td>0.02</td><td>0.001</td><td>0.001</td><td>0.0006</td><td>0.0011</td><td>1.86</td><td>Bal.</td></tr></tbody></table>

*Heat treatment of twin-roll cast strip.* In the heat treatment process of the roll cast material, the solution treatment temperature was set to 404°C, and the solution treatment time was varied from to 0.5 h, 1 h, 2 h, 3 h, 4 h, 5 h, 6 h, 8 h, and 24 h. After that, a tensile test piece for a tension test was cut from the solution heat-treated strip by a wire electric discharge machine. From the relationship between solution treatment time between the maximum elongation of the tension test, an appropriate heat treatment condition was determined to obtain a desired microstructure while keeping minimizing of the solution treatment time. The selected heat treatment condition of the twin roll cast material was found as shown in Table 2.

<table><caption>Table 2 Solution heat treatment conditions of roll cast material.</caption><tr><td>Temperature [°C]</td><td>406</td></tr><tr><td>Time [h]</td><td>0.5 h, 1 h, 2 h, 3 h, 4 h, 5 h, 6 h, 8 h, 24 h</td></tr><tr><td>Quenching condition</td><td>Hot Water at 60</td></tr></table>
Repeatedly rolling and reverse bending process. Two methods of applying pre-strain to the twin-roll cast strips were selected. A differential cold speed rolling experiment to give a pre-strain to the twin-roll cast strips were performed in order to obtain grain refined magnesium alloy in static recrystallization after solution heat treatment as shown in Fig. 2.

Two passes of rolling were performed without lubrication using rolls of 45 mm and 50 mm diameters rolls with a different speed ratio of 1.1. The rolling reduction ratio of the rolls is 4% for the first pass and 6% for the second pass, so that the total rolling reduction ratio become 10%, in order to obtain same curvature of the rolled sheet after the first pass and the second pass. After the second rolling pass, the solution heat treatment of the pre-strained material was carried out under the conditions of 406 °C in 5 hours, and hot water quenching at 60 °C.

Figure 3 shows how to apply pre-strain with bending and reverse bending using three rolls of 30 mm diameters. In this experiment, bending was repeated under two

conditions; one in which the strip curvature was the same as the first and second passes respectively in differential speed rolling in Fig. 2, and the other one in which the strip curvature was fifteen times of the cast strip thickness. After the repeated bending using three rolls, the solution heat treatment of the pre-strained material was carried out under the conditions of 406 °C in 5 hours, and hot water quenching at 60 °C.

V-bending test. A 90-degrees bending test was conducted for solution heat treated cast materials were performed as shown in Fig.4. Table 2 shows the V-bending test conditions.

<table><caption>Table 3 V-bending test conditions.</caption><tr><td>Punch angle [A<sub>p</sub>/deg.]</td><td>90</td></tr><tr><td>Punch radius [R/mm]</td><td>7.2, 6.0, 4.8</td></tr><tr><td>Die angle [A<sub>D</sub>/deg.]</td><td>90</td></tr><tr><td>Width of die [W<sub>D</sub>/mm]</td><td>14</td></tr><tr><td>Test piece thickness [t/mm]</td><td>1.8±0.05</td></tr><tr><td>Test piece width [w/mm]</td><td>20</td></tr><tr><td>Stroke speed [V/mm · min<sup>-1</sup>]</td><td>1.00</td></tr></table>

<figure><img src="image_2343.png" /><figcaption>Fig. 2 Repeated bending by differential speed rolling.</figcaption></figure>

<figure><img src="image_2344.png" /><figcaption>Fig. 3 Repeated bending by using three rolls.</figcaption></figure>

<figure><img src="image_2345.png" /><figcaption>Fig. 4 Punch and die of V-bending test.</figcaption></figure>

## Result and Discussion

Microstructure of cast strip. Figure 5 shows strips fabricated with different spring loads. The plate was 2.5 mm thick and 100 mm wide. Cast condition of the strips is shown in Table 4. From the observation of the microstructure of the strips, we can see macroscopic segregation around
Repeatedly rolling and reverse bending process. Two methods of applying pre-strain to the twin-roll cast strips were selected. A differential cold speed rolling experiment to give a pre-strain to the twin-roll cast strips were performed in order to obtain grain refined magnesium alloy in static recrystallization after solution heat treatment as shown in Fig. 2.

Two passes of rolling were performed without lubrication using rolls of 45 mm and 50 mm diameters rolls with a different speed ratio of 1.1. The rolling reduction ratio of the rolls is 4% for the first pass and 6% for the second pass, so that the total rolling reduction ratio become 10%, in order to obtain same curvature of the rolled sheet after the first pass and the second pass. After the second rolling pass, the solution heat treatment of the pre-strained material was carried out under the conditions of 406 °C in 5 hours, and hot water quenching at 60 °C.

Figure 3 shows how to apply pre-strain with bending and reverse bending using three rolls of 30 mm diameters. In this experiment, bending was repeated under two

conditions; one in which the strip curvature was the same as the first and second passes respectively in differential speed rolling in Fig. 2, and the other one in which the strip curvature was fifteen times of the cast strip thickness. After the repeated bending using three rolls, the solution heat treatment of the pre-strained material was carried out under the conditions of 406 °C in 5 hours, and hot water quenching at 60 °C.

V-bending test. A 90-degrees bending test was conducted for solution heat treated cast materials were performed as shown in Fig.4. Table 2 shows the V-bending test conditions.

<table><caption>Table 3 V-bending test conditions.</caption><tr><td>Punch angle [A<sub>p</sub>/deg.]</td><td>90</td></tr><tr><td>Punch radius [R/mm]</td><td>7.2, 6.0, 4.8</td></tr><tr><td>Die angle [A<sub>D</sub>/deg.]</td><td>90</td></tr><tr><td>Width of die [W<sub>D</sub>/mm]</td><td>14</td></tr><tr><td>Test piece thickness [t/mm]</td><td>1.8±0.05</td></tr><tr><td>Test piece width [w/mm]</td><td>20</td></tr><tr><td>Stroke speed [V/mm · min<sup>-1</sup>]</td><td>1.00</td></tr></table>

<figure><img src="image_2334.png" /><figcaption>Fig. 2 Repeated bending by differential speed rolling.</figcaption></figure>

<figure><img src="image_2335.png" /><figcaption>Fig. 3 Repeated bending by using three rolls.</figcaption></figure>

<figure><img src="image_2336.png" /><figcaption>Fig. 4 Punch and die of V-bending test.</figcaption></figure>

## Result and Discussion

Microstructure of cast strip. Figure 5 shows strips fabricated with different spring loads. The plate was 2.5 mm thick and 100 mm wide. Cast condition of the strips is shown in Table 4. From the observation of the microstructure of the strips, we can see macroscopic segregation around
the center of the thickness of the strip as shown in Fig.6 (a). In Fig.6(a), we can see very fine globular microstructure in the middle of the strip thickness as roll cast materials.

<table><caption>Table 4 Twin-roll casting conditions.</caption><tr><td>Roll gap [mm]</td><td>2.43</td></tr><tr><td>Initial load [kN]</td><td>21.0</td></tr><tr><td>Roll contacting length [mm]</td><td>50</td></tr><tr><td>Temperature of molten Mg at roll contact [°C]</td><td>596</td></tr><tr><td>Roll velocity [m/min]</td><td>36</td></tr></table>

<figure><img src="image_2346.png" /><figcaption>Fig. 5 Twin-roll cast AZT912.</figcaption></figure>

<figure><img src="image_2347.png" /><figcaption>Fig. 6 Microstructure of twin-roll cast AZT912.</figcaption></figure>

(a) Microstructure of as cast strip (b) 406 °C, 1 h heat treatment (c) 406 °C, 8 h heat treatment)

*Microstructure after solution heat treatment.* Microscopic observation of cast strips after heat treatment were indicated as Figs. 6 (b) and 6 (c). An abnormal grain growth was confirmed at the central part in the plate thickness direction when the solution treatment time was eight hours or more in Fig6 (c). In addition, no grain growth was observed except for the central part at any solution treatment time. In the case of 4h solution time, we could not see any beta phase in the material. It can be seen that in the case of one hour solution heat treatment time, we can see very small remained beta phases Mg₁₇Al₁₂. It can be concluded that four hour and more is an acceptable solution heat treatment time after twin roll casting process.

*Effect of differential speed rolling on microstructure.* Figure 7 shows a cold rolled strip with a curvature (15 times of strip thickness) after differential cold speed rolling. After two passes of rolling, annealing was performed along with heating strain correction under the conditions of holding temperature 404 °C, holding time one hours, and hot water quenching at 60 °C. The
the center of the thickness of the strip as shown in Fig.6 (a). In Fig.6(a), we can see very fine globular microstructure in the middle of the strip thickness as roll cast materials.

<table><caption>Table 4 Twin-roll casting conditions.</caption><tr><td>Roll gap [mm]</td><td>2.43</td></tr><tr><td>Initial load [kN]</td><td>21.0</td></tr><tr><td>Roll contacting length [mm]</td><td>50</td></tr><tr><td>Temperature of molten Mg at roll contact [°C]</td><td>596</td></tr><tr><td>Roll velocity [m/min]</td><td>36</td></tr></table>

<figure><img src="image_2337.png" /><figcaption>Fig. 5 Twin-roll cast AZT912.</figcaption></figure>

<figure><img src="image_2338.png" /><figcaption>Fig. 6 Microstructure of twin-roll cast AZT912.</figcaption></figure>

(a) Microstructure of as cast strip (b) 406 °C, 1 h heat treatment (c) 406 °C, 8 h heat treatment)

*Microstructure after solution heat treatment.* Microscopic observation of cast strips after heat treatment were indicated as Figs. 6 (b) and 6 (c). An abnormal grain growth was confirmed at the central part in the plate thickness direction when the solution treatment time was eight hours or more in Fig6 (c). In addition, no grain growth was observed except for the central part at any solution treatment time. In the case of 4h solution time, we could not see any beta phase in the material. It can be seen that in the case of one hour solution heat treatment time, we can see very small remained beta phases $Mg_{17}Al_{12}$. It can be concluded that four hour and more is an acceptable solution heat treatment time after twin roll casting process.

*Effect of differential speed rolling on microstructure.* Figure 7 shows a cold rolled strip with a curvature (15 times of strip thickness) after differential cold speed rolling. After two passes of rolling, annealing was performed along with heating strain correction under the conditions of holding temperature 404 °C, holding time one hours, and hot water quenching at 60 °C. The
obtained mean grain size was about 22 µm in the central part of the strip and about 54 µm in the surface part as shown in Fig. 8. Microscopic observation of the strip after annealing revealed fine microstructure as shown in Fig. 8 especially around the surface area of the strip which was about 100 µm in the case of as cast strip.

<figure><img src="image_2348.png" /><figcaption>Fig. 7 Rolled strip by differential speed rolling.</figcaption></figure>

<figure><img src="image_2349.png" /><figcaption>Fig. 8 Microstructure of annealed strip after differential speed rolling.</figcaption></figure>

<figure><img src="image_2350.png" /><figcaption>Fig. 9 Bent strip by three rolls.</figcaption></figure>

<figure><img src="image_2351.png" /><figcaption>Fig. 10 Microstructure of annealed strip after bending by three rolls.</figcaption></figure>

*Effect of bending and reverse bending on microstructure.* Figure 9 shows a bent strip with a curvature (15 times of strip thickness) after bending by three rolls. After second bending, annealing was performed along with heating strain correction under the conditions of holding temperature 404 °C, holding time one hour, and hot water quenching at 60 °C.

The obtained mean grain size was about 28 µm in the central part of the strip and about 108 µm in the surface part as shown in Fig. 10. The microstructure of the bent strips by three rolls was almost the same as that of the twin-roll as cast material, and grain refinement was not observed. We could not confirm the effect of the difference in the curvature of the bent strips on the microstructure by comparing a curvature of cast strips (R=21t) and a curvature (R=15t).

*Estimation of bendability at room temperature by V-bending.* A V-bending test was conducted to investigate bendability at room temperature. One of the V-bending test results in the case of R=7.2 (R/t=4) is shown in Table 5. Bendability at room temperature was evaluated as the minimum bending radius value in the V-bending test. The minimum bending radius is the one of the important parameters of V-bending. Generally, the smallest bending radius that can be bent without cracking is considered as limiting bending ratio of R/t. It is well known that commercial
obtained mean grain size was about 22 µm in the central part of the strip and about 54 µm in the surface part as shown in Fig. 8. Microscopic observation of the strip after annealing revealed fine microstructure as shown in Fig. 8 especially around the surface area of the strip which was about 100 µm in the case of as cast strip.

<figure><img src="image_2339.png" /><figcaption>Fig. 7 Rolled strip by differential speed rolling.</figcaption></figure>

<figure><img src="image_2340.png" /><figcaption>Fig. 8 Microstructure of annealed strip after differential speed rolling.</figcaption></figure>

<figure><img src="image_2341.png" /><figcaption>Fig. 9 Bent strip by three rolls.</figcaption></figure>

<figure><img src="image_2342.png" /><figcaption>Fig. 10 Microstructure of annealed strip after bending by three rolls.</figcaption></figure>

*Effect of bending and reverse bending on microstructure.* Figure 9 shows a bent strip with a curvature (15 times of strip thickness) after bending by three rolls. After second bending, annealing was performed along with heating strain correction under the conditions of holding temperature 404 °C, holding time one hour, and hot water quenching at 60 °C.

The obtained mean grain size was about 28 µm in the central part of the strip and about 108 µm in the surface part as shown in Fig. 10. The microstructure of the bent strips by three rolls was almost the same as that of the twin-roll as cast material, and grain refinement was not observed. We could not confirm the effect of the difference in the curvature of the bent strips on the microstructure by comparing a curvature of cast strips (R=21t) and a curvature (R=15t).

*Estimation of bendability at room temperature by V-bending.* A V-bending test was conducted to investigate bendability at room temperature. One of the V-bending test results in the case of R=7.2 (R/t=4) is shown in Table 5. Bendability at room temperature was evaluated as the minimum bending radius value in the V-bending test. The minimum bending radius is the one of the important parameters of V-bending. Generally, the smallest bending radius that can be bent without cracking is considered as limiting bending ratio of R/t. It is well known that commercial
wrought magnesium alloys, such as AZ31 and AM 60 has a limiting value of R/t, about 4 to 5, which is a large value compared to zero to 0.5 for aluminum alloy A5052, or zero to 2 for cold rolled steel.

<table><caption>Table 5 V-bending test results depending on processes (R=7.2).</caption><thead><tr><th>Materials</th><th colspan="8">AZT912</th><th>AM31-O</th><th>AM60-O</th></tr><tr><th>Process</th><th colspan="2">As cast</th><th colspan="2">Differential speed rolling</th><th colspan="2">Bending and reverse bending (R=21t)</th><th colspan="2">Bending and reverse bending (R=15t)</th><th colspan="2">Rolling</th><th colspan="2">Rolling</th></tr></thead><tbody><tr><th>Bent direction/ degrees</th><td>0</td><td>90</td><td>0</td><td>90</td><td>0</td><td>90</td><td>0</td><td>90</td><td>0</td><td>90</td><td>0</td><td>90</td></tr><tr><th>Cracks</th><td>AAB</td><td>ABB</td><td>ABB</td><td>ABB</td><td>AAA</td><td>AAB</td><td>AAA</td><td>AAA</td><td>CCC</td><td>AAC</td><td>CCC</td><td>BCC</td></tr><tr><th>Springback/ degrees</th><td>10.9</td><td>10.9</td><td>19.8</td><td>15.6</td><td>15.5</td><td>14.2</td><td>13.6</td><td>13.1</td><td>—</td><td>15.5</td><td>—</td><td>—</td></tr></tbody></table>

It is assumed that the minimum bending radius will be reduced by performing the grain refinement process on the twin roll cast materials. The minimum bending radius of the bent material was examined using a V-bending test as indicated condition in Table 3. In the V-bending test in Table 5, the test pieces were classified into three crack ratings. Namely, A shows a case where there is no cracking, B indicates a case where a crack occurs on the inside or outside of the bending, and C shows a case where the test piece was completely separated in two pieces after V-bending. In the comparison with commercial wrought magnesium alloys, AZ31-O (t=1.2) and AM60-O (t=2.4) were tested for the V-bending test. The test piece was subjected to V-bending in two directions: 0° to the rolling direction, and 90° to the rolling direction, as shown in Table 5. Magnitude of the springback angles of test pieces after bending were also measured. As a result of the V-bending test, it was found that the AZT912 sheet plate manufactured by twin-roll casting was not completely separated, and all test pieces in the three-roll cyclic bending test at a curvature of 15 times the plate thickness did not show cracks.

Regarding the amount of springback, it is thought that the amount of springback decreases as the minimum bending radius decreases, except for the twin-roll as cast material. It was considered that the reason why the amount of springback in the twin-roll cast material was smaller than in other conditions was due to the occurrence of very small cracks that could not difficult to be identified with naked eyes in the as cast twin-roll cast materials. Figure 11 show a result in the case that the bending radius was 4.8 mm (R/t=2.7).

<figure><img src="image_2352.png" /><figcaption>Fig. 11 Surface of bent test pieces and springback (R/t=2.7). ((a) Surface condition of bent strip. (b) Measuring of springback.)</figcaption></figure>

As shown in Fig.11, the AZT912 alloy by twin-roll casting and after bending and reverse-bending, shows the minimum bending radius, 2.7t. Also, the obtained magnitude of the spring back angle is 4.2 degrees. It is suggested that the magnesium alloy obtained by solution heat treatment after twin roll casting is effective to improve the bendability at room temperature, and it could be
wrought magnesium alloys, such as AZ31 and AM 60 has a limiting value of R/t, about 4 to 5, which is a large value compared to zero to 0.5 for aluminum alloy A5052, or zero to 2 for cold rolled steel.

<table><caption>Table 5 V-bending test results depending on processes (R=7.2).</caption><thead><tr><th>Materials</th><th colspan="8">AZT912</th><th>AM31-O</th><th>AM60-O</th></tr><tr><th>Process</th><th colspan="2">As cast</th><th colspan="2">Differential speed rolling</th><th colspan="2">Bending and reverse bending (R=21t)</th><th colspan="2">Bending and reverse bending (R=15t)</th><th colspan="2">Rolling</th><th colspan="2">Rolling</th></tr></thead><tbody><tr><th>Bent direction/ degrees</th><td>0</td><td>90</td><td>0</td><td>90</td><td>0</td><td>90</td><td>0</td><td>90</td><td>0</td><td>90</td><td>0</td><td>90</td></tr><tr><th>Cracks</th><td>AAB</td><td>ABB</td><td>ABB</td><td>ABB</td><td>AAA</td><td>AAB</td><td>AAA</td><td>AAA</td><td>CCC</td><td>AAC</td><td>CCC</td><td>BCC</td></tr><tr><th>Springback/ degrees</th><td>10.9</td><td>10.9</td><td>19.8</td><td>15.6</td><td>15.5</td><td>14.2</td><td>13.6</td><td>13.1</td><td>—</td><td>15.5</td><td>—</td><td>—</td></tr></tbody></table>

It is assumed that the minimum bending radius will be reduced by performing the grain refinement process on the twin roll cast materials. The minimum bending radius of the bent material was examined using a V-bending test as indicated condition in Table 3. In the V-bending test in Table 5, the test pieces were classified into three crack ratings. Namely, A shows a case where there is no cracking, B indicates a case where a crack occurs on the inside or outside of the bending, and C shows a case where the test piece was completely separated in two pieces after V-bending. In the comparison with commercial wrought magnesium alloys, AZ31-O (t=1.2) and AM60-O (t=2.4) were tested for the V-bending test. The test piece was subjected to V-bending in two directions: 0° to the rolling direction, and 90° to the rolling direction, as shown in Table 5. Magnitude of the springback angles of test pieces after bending were also measured. As a result of the V-bending test, it was found that the AZT912 sheet plate manufactured by twin-roll casting was not completely separated, and all test pieces in the three-roll cyclic bending test at a curvature of 15 times the plate thickness did not show cracks.

Regarding the amount of springback, it is thought that the amount of springback decreases as the minimum bending radius decreases, except for the twin-roll as cast material. It was considered that the reason why the amount of springback in the twin-roll cast material was smaller than in other conditions was due to the occurrence of very small cracks that could not difficult to be identified with naked eyes in the as cast twin-roll cast materials. Figure 11 show a result in the case that the bending radius was 4.8 mm (R/t=2.7).

<figure><img src="image_2343.png" /><figcaption>Fig. 11 Surface of bent test pieces and springback (R/t=2.7). ((a)Surface condition of bent strip. (b) Measuring of springback.)</figcaption></figure>

As shown in Fig.11, the AZT912 alloy by twin-roll casting and after bending and reverse-bending, shows the minimum bending radius, 2.7t. Also, the obtained magnitude of the spring back angle is 4.2 degrees. It is suggested that the magnesium alloy obtained by solution heat treatment after twin roll casting is effective to improve the bendability at room temperature, and it could be
possible that the minimum bending radius R was the 2.7 times of the sheet thickness t in this experiment conditions.

## Summary

A twin-roll cast Mg-9mass%Al-1 mass%Zn-2mass%Sn alloy was cast by using a horizontal twin roll caster to fabricate wrought magnesium alloy, which can be bent at room temperature. An appropriate heat treatment condition was clarified. By adopting a V-bending test, it has been suggested that a bending and reverse-bending process was effective for improving formability of twin-roll cast magnesium alloy AZT912 at room temperature, comparing to different speed cold rolling.

## References

[1] P.G. Partridge, The crystallography and deformation modes of hexagonal closed-packed metals, Int. Metall. Rev. 12 (1967) 169-194. https://doi.org/10.1179/mtlr.1967.12.1.169

[2] M.H. Yoo, Slip, twinning and fracture in hexagonal close-packed metals, Metall. Trans. A 12 (1981) 409-418. https://doi.org/10.1007/BF02648537

[3] T.M. Pollock, Weight loss with magnesium alloys, Science 328 (2010) 986-987. https://doi.org/10.1126/science.1182848

[4] J. Koike, T. Kobayashi, T. Mukai, H. Watanabe, M. Suzuki, K. Maruyama, K. Higashi, The activity of non-basal slip systems and dynamic recovery at room temperature in fine-grained AZ31B magnesium alloys, Acta Mater. 51 (2003) 2055-2065. https://doi.org/10.1016/S1359-6454(03)00005-3

[5] W.B. Hutchinson, M.R. Barnett, Effective values of critical resolved shear stress for slip in polycrystalline magnesium and other hcp metals, Scr. Mater. 63 (2010) 737-740. https://doi.org/10.1016/j.scriptamat.2010.05.047

[6] E. Tolouie, R. Jamaati, Asymmetric cold rolling, A technique for achieving non-basal texture in AZ91 alloy, Mater. Lett., 249 (2019) 143-146. https://doi.org/10.1016/j.matlet.2019.04.087

[7] R.L. Dietrich, Surface orientation and rolling of magnesium sheet, JOM 1 (1949) 621-626. https://doi.org/10.1007/BF03398906

[8] T.T. Sasaki, J.D. Ju, K. Hono, K.S. Shin, Heat-treatable Mg-Sn-Zn wrought alloy, Scripta Mater., 65 (2009) 80-83. https://doi.org/10.1016/j.scriptamat.2009.03.014

[9] T.T. Sasaki, K. Oh-ishi, T. Ohkubo, K. Hono, Enhanced age hardening response by the addition of Zn in Mg-Sn alloys, Scripta. Mater., 55 (2006) 251-254. https://doi.org/10.1016/j.scriptamat.2006.04.005

[10] W.L. Cheng, H.S. Kim, B.S. You, B.H. Koo, S.S. Park, Strength and ductility of novel Mg-8Sn-1Al-1Zn alloys extruded at different speeds, Mater. Lett. 65 (2011) 1525-1527. https://doi.org/10.1016/j.matlet.2011.03.010

[11] H. Liu, Y. Chen, Y. Tang, Y. Tang, S. Wei, G. Niu, Tensile and indentation creep behavior of Mg-5% Sn and Mg-5% Sn-2% Di alloys, Mater. Sci. Eng., A, 464 (2007) 124-128. https://doi.org/10.1016/j.msea.2007.02.061

[12] S.H. Park, S.H. Kim, H.S. Kim, J. Yoon, B.S. You, High-speed indirect extrusion of Mg-Sn-Al-Zn alloy and its influence on microstructure and mechanical properties, J. Alloys Compd., 667 (2016) 170-177. https://doi.org/10.1016/j.jallcom.2016.01.163
possible that the minimum bending radius R was the 2.7 times of the sheet thickness t in this experiment conditions.

## Summary

A twin-roll cast Mg-9mass%Al-1 mass%Zn-2mass%Sn alloy was cast by using a horizontal twin roll caster to fabricate wrought magnesium alloy, which can be bent at room temperature. An appropriate heat treatment condition was clarified. By adopting a V-bending test, it has been suggested that a bending and reverse-bending process was effective for improving formability of twin-roll cast magnesium alloy AZT912 at room temperature, comparing to different speed cold rolling.

## References

[1] P.G. Partridge, The crystallography and deformation modes of hexagonal closed-packed metals, Int. Metall. Rev. 12 (1967) 169-194. https://doi.org/10.1179/mtlr.1967.12.1.169

[2] M.H. Yoo, Slip, twinning and fracture in hexagonal close-packed metals, Metall. Trans. A 12 (1981) 409-418. https://doi.org/10.1007/BF02648537

[3] T.M. Pollock, Weight loss with magnesium alloys, Science 328 (2010) 986-987. https://doi.org/10.1126/science.1182848

[4] J. Koike, T. Kobayashi, T. Mukai, H. Watanabe, M. Suzuki, K. Maruyama, K. Higashi, The activity of non-basal slip systems and dynamic recovery at room temperature in fine-grained AZ31B magnesium alloys, Acta Mater. 51 (2003) 2055-2065. https://doi.org/10.1016/S1359-6454(03)00005-3

[5] W.B. Hutchinson, M.R. Barnett, Effective values of critical resolved shear stress for slip in polycrystalline magnesium and other hcp metals, Scr. Mater. 63 (2010) 737-740. https://doi.org/10.1016/j.scriptamat.2010.05.047

[6] E. Tolouie, R. Jamaati, Asymmetric cold rolling, A technique for achieving non-basal texture in AZ91 alloy, Mater. Lett., 249 (2019) 143-146. https://doi.org/10.1016/j.matlet.2019.04.087

[7] R.L. Dietrich, Surface orientation and rolling of magnesium sheet, JOM 1 (1949) 621-626. https://doi.org/10.1007/BF03398906

[8] T.T. Sasaki, J.D. Ju, K. Hono, K.S. Shin, Heat-treatable Mg-Sn-Zn wrought alloy, Scripta Mater., 65 (2009) 80-83. https://doi.org/10.1016/j.scriptamat.2009.03.014

[9] T.T. Sasaki, K. Oh-ishi, T. Ohkubo, K. Hono, Enhanced age hardening response by the addition of Zn in Mg-Sn alloys, Scripta. Mater., 55 (2006) 251-254. https://doi.org/10.1016/j.scriptamat.2006.04.005

[10] W.L. Cheng, H.S. Kim, B.S. You, B.H. Koo, S.S. Park, Strength and ductility of novel Mg-8Sn-1Al-1Zn alloys extruded at different speeds, Mater. Lett. 65 (2011) 1525-1527. https://doi.org/10.1016/j.matlet.2011.03.010

[11] H. Liu, Y. Chen, Y. Tang, Y. Tang, S. Wei, G. Niu, Tensile and indentation creep behavior of Mg-5% Sn and Mg-5% Sn-2% Di alloys, Mater. Sci. Eng., A, 464 (2007) 124-128. https://doi.org/10.1016/j.msea.2007.02.061

[12] S.H. Park, S.H. Kim, H.S. Kim, J. Yoon, B.S. You, High-speed indirect extrusion of Mg-Sn-Al-Zn alloy and its influence on microstructure and mechanical properties, J. Alloys Compd., 667 (2016) 170-177. https://doi.org/10.1016/j.jallcom.2016.01.163
# The influence of signal type and distance to the sound source on sound transmission through small 3D printed polymer panels

HRITUC Adelina<sup>a*</sup>, MIHALACHE Andrei Marius<sup>b</sup>, DODUN Oana<sup>c</sup>, NAGÎŢ Gheorghe<sup>d</sup> and SLĂTINEANU Laurenţiu<sup>e</sup>

"Gheorghe Asachi" Technical University of Iași, Department of Machine Manufacturing Technology, Blvd. D. Mangeron, 39 A, 700050 Iași România

<sup>a</sup>adelina.hrituc@student.tuiasi.ro, <sup>b</sup>andrei-marius.mihalache@academic.tuiasi.ro,

<sup>c</sup>oanad@tcm.tuiasi.ro, <sup>d</sup>nagit@tcm.tuiasi.ro, <sup>e</sup>slati@tcm.tuiasi.ro

**Keywords:** Polymer Panel, 3D Printing, Sound Intensity, Distance, Sound Type, Empirical Model

**Abstract.** The sound insulation of some spaces is possible using panels made of polymer materials manufactured by 3D printing. It is expected that the intensity of the sound signal after passing through a small wall made of polymeric material will be affected by the type of sound signal and the distance at which the sound is received. In the case of previous experimental research, the influence exerted by some parameters of the 3D printing process on the sound insulation capacity of walls made of polymeric materials was largely considered. In the case of the present paper, experimental research was carried out aiming to identify empirical mathematical models capable of highlighting the influence of the sound type and distance on the intensity of the sound signal after passing through the panel. The experimental tests were carried out on equipment designed to investigate different factors' influence on the sound signal's impact when passing through small panels. An empirical mathematical model was identified through the mathematical processing of the experimental results. The empirical mathematical model highlights the influence exerted by the shape of the electrical signal used to generate the sounds and, respectively, the distance between the sound source and the sensor of a sound level meter on the energy absorbed by the small panel from a polymeric material.

**Introduction**

Sounds are mechanical vibrations that the human ear can perceive. Human beings can perceive such vibrations if they have a frequency between 20 Hz and 20 kHz and if sounds also have a certain intensity. It should be noted that mechanical vibrations with frequencies in the mentioned range are called *sounds* when they are judged to have a pleasant effect and, respectively, *noises* when the previous condition is not met.

On the other hand, there are practical situations when the intensity of sounds is desired to be as low as possible. For this purpose, it is possible to use enclosures that do not allow the penetration of sound waves from the outside or the exit of such waves to the outside. For this purpose, the walls of the enclosure must have a high capacity to absorb sound waves from the indoor or outdoor environment. Such walls are made in the form of panels made by different manufacturing processes and from different materials, characterized in principle by high sound insulation properties.

Some of the materials used to make the panels are polymers. The development of additive manufacturing processes in recent decades has led to the manufacture of sound insulation panels through these processes. In principle, additive manufacturing involves gradually generating the part to be obtained by depositing successive layers. One of the additive manufacturing processes frequently used today is 3D printing. In this case, the polymeric material in a molten state and advanced through a nozzle is deposited layer by layer as a result of some controlled relative

<figure><img src="image_2353.png" /></figure>
# The influence of signal type and distance to the sound source on sound transmission through small 3D printed polymer panels

HRITUC Adelina<sup>a*</sup>, MIHALACHE Andrei Marius<sup>b</sup>, DODUN Oana<sup>c</sup>, NAGÎŢ Gheorghe<sup>d</sup> and SLĂTINEANU Laurenţiu<sup>e</sup>

“Gheorghe Asachi” Technical University of Iași, Department of Machine Manufacturing Technology, Blvd. D. Mangeron, 39 A, 700050 Iași România

<sup>a</sup>adelina.hrituc@student.tuiasi.ro, <sup>b</sup>andrei-marius.mihalache@academic.tuiasi.ro,

<sup>c</sup>oanad@tcm.tuiasi.ro, <sup>d</sup>nagit@tcm.tuiasi.ro, <sup>e</sup>slati@tcm.tuiasi.ro

**Keywords:** Polymer Panel, 3D Printing, Sound Intensity, Distance, Sound Type, Empirical Model

**Abstract.** The sound insulation of some spaces is possible using panels made of polymer materials manufactured by 3D printing. It is expected that the intensity of the sound signal after passing through a small wall made of polymeric material will be affected by the type of sound signal and the distance at which the sound is received. In the case of previous experimental research, the influence exerted by some parameters of the 3D printing process on the sound insulation capacity of walls made of polymeric materials was largely considered. In the case of the present paper, experimental research was carried out aiming to identify empirical mathematical models capable of highlighting the influence of the sound type and distance on the intensity of the sound signal after passing through the panel. The experimental tests were carried out on equipment designed to investigate different factors' influence on the sound signal's impact when passing through small panels. An empirical mathematical model was identified through the mathematical processing of the experimental results. The empirical mathematical model highlights the influence exerted by the shape of the electrical signal used to generate the sounds and, respectively, the distance between the sound source and the sensor of a sound level meter on the energy absorbed by the small panel from a polymeric material.

**Introduction**

Sounds are mechanical vibrations that the human ear can perceive. Human beings can perceive such vibrations if they have a frequency between 20 Hz and 20 kHz and if sounds also have a certain intensity. It should be noted that mechanical vibrations with frequencies in the mentioned range are called *sounds* when they are judged to have a pleasant effect and, respectively, *noises* when the previous condition is not met.

On the other hand, there are practical situations when the intensity of sounds is desired to be as low as possible. For this purpose, it is possible to use enclosures that do not allow the penetration of sound waves from the outside or the exit of such waves to the outside. For this purpose, the walls of the enclosure must have a high capacity to absorb sound waves from the indoor or outdoor environment. Such walls are made in the form of panels made by different manufacturing processes and from different materials, characterized in principle by high sound insulation properties.

Some of the materials used to make the panels are polymers. The development of additive manufacturing processes in recent decades has led to the manufacture of sound insulation panels through these processes. In principle, additive manufacturing involves gradually generating the part to be obtained by depositing successive layers. One of the additive manufacturing processes frequently used today is 3D printing. In this case, the polymeric material in a molten state and advanced through a nozzle is deposited layer by layer as a result of some controlled relative

<figure><img src="image_2344.png" /></figure>
displacements between the nozzle and the table of the equipment on which the gradual generation of the desired part occurs.

By varying the parameters of the 3D printing process, some physical-mechanical properties of the materials of the printed parts can be decisively influenced. This fact led to the idea of researching how, through an appropriate selection of the values of the input factors in the printing process, the sound insulation properties of polymer panels can be improved.

On the other hand, researching the sound insulation capacity of polymer panels involves using appropriate equipment designed, for example, to evaluate the energy of sound waves after they pass through the panels and the influence of different factors on the sound insulation capacity of different materials [1-8].

Thus, Perrot et al. analyzed the acoustic properties of some samples from foams, focusing mainly on the extent to which the internal structure of such materials can affect the sound properties of the cellular foams [1]. They found that the level of absorption of sound waves by materials in the form of foams is directly affected by cell size.

Zahiruddin and Yarlagadda manufactured by 3D printing a polyethylene terephthalate glycol plate with a porosity of 60 % and tested the material's ability to absorb sounds using the impedance tube method [2]. One of the research conclusions was that the plate material has good soundproofing properties for high-frequency sounds.

King and Teo considered the soundproofing possibilities of a room in modern homes. They appreciated that it is possible to improve the ability of materials to absorb sound waves by using materials with optimized structures [3]. Using samples of materials with such structures manufactured by 3D printing, they showed that the produced materials have a high capacity to absorb sounds with frequencies lower than 4000 Hz.

The development of polymer matrix composite materials has led to an expansion of concerns regarding the properties of such materials to absorb sound energy. In a study published in 2021, Sujon et al. concluded that there are great prospects for using polymer matrix composites for sound insulation [4].

In the last decades, the problem of using some polymers as insulating materials from a sound point of view has been a continuous concern of researchers interested in identifying new solutions that allow a better absorption of sound waves [5-8].

In previous research, some of the authors of this paper presented the results of research that focused on the influence of factors such as panel thickness, printing speed, infill percentage, infill pattern, layer thickness, sound frequency, and sound volume on the insulation capacity of small panels made of plastic materials by 3D printing [9,10]. During these researches, questions were formulated regarding the extent to which the shape of the sound waves or the distance between the wall and the sensor of the pressure sound level measuring device can exert, themselves, a certain influence on the results of some measurements of the sound insulation capacity of the panels manufactured by 3D printing.

Experimental research was developed to highlight the influence exerted by the two factors and possibly the intensity of the influence exerted by the two factors on the evaluation of the sound-insulating capacity of the polymer panels. In the present work, equipment intended for researching the sound insulation capacity of small panels made of polymeric materials was used to highlight the theoretical conditions involved in the generation, propagation, and absorption of sound energy. Later, the experimental research had to provide information on the intensity of the influence exerted by the shape of the sound signal and, respectively, by the distance between the polymer panel and the sensor of the sound pressure level evaluation device on the measurement results.

## Experimental conditions

There are various solutions for generating sounds by converting electrical, hydraulic, etc., energy into mechanical vibrations whose intensity can be increased in various ways. Currently, the most
displacements between the nozzle and the table of the equipment on which the gradual generation of the desired part occurs.

By varying the parameters of the 3D printing process, some physical-mechanical properties of the materials of the printed parts can be decisively influenced. This fact led to the idea of researching how, through an appropriate selection of the values of the input factors in the printing process, the sound insulation properties of polymer panels can be improved.

On the other hand, researching the sound insulation capacity of polymer panels involves using appropriate equipment designed, for example, to evaluate the energy of sound waves after they pass through the panels and the influence of different factors on the sound insulation capacity of different materials [1-8].

Thus, Perrot et al. analyzed the acoustic properties of some samples from foams, focusing mainly on the extent to which the internal structure of such materials can affect the sound properties of the cellular foams [1]. They found that the level of absorption of sound waves by materials in the form of foams is directly affected by cell size.

Zahiruddin and Yarlagadda manufactured by 3D printing a polyethylene terephthalate glycol plate with a porosity of 60 % and tested the material's ability to absorb sounds using the impedance tube method [2]. One of the research conclusions was that the plate material has good soundproofing properties for high-frequency sounds.

King and Teo considered the soundproofing possibilities of a room in modern homes. They appreciated that it is possible to improve the ability of materials to absorb sound waves by using materials with optimized structures [3]. Using samples of materials with such structures manufactured by 3D printing, they showed that the produced materials have a high capacity to absorb sounds with frequencies lower than 4000 Hz.

The development of polymer matrix composite materials has led to an expansion of concerns regarding the properties of such materials to absorb sound energy. In a study published in 2021, Sujon et al. concluded that there are great prospects for using polymer matrix composites for sound insulation [4].

In the last decades, the problem of using some polymers as insulating materials from a sound point of view has been a continuous concern of researchers interested in identifying new solutions that allow a better absorption of sound waves [5-8].

In previous research, some of the authors of this paper presented the results of research that focused on the influence of factors such as panel thickness, printing speed, infill percentage, infill pattern, layer thickness, sound frequency, and sound volume on the insulation capacity of small panels made of plastic materials by 3D printing [9,10]. During these researches, questions were formulated regarding the extent to which the shape of the sound waves or the distance between the wall and the sensor of the pressure sound level measuring device can exert, themselves, a certain influence on the results of some measurements of the sound insulation capacity of the panels manufactured by 3D printing.

Experimental research was developed to highlight the influence exerted by the two factors and possibly the intensity of the influence exerted by the two factors on the evaluation of the sound-insulating capacity of the polymer panels. In the present work, equipment intended for researching the sound insulation capacity of small panels made of polymeric materials was used to highlight the theoretical conditions involved in the generation, propagation, and absorption of sound energy. Later, the experimental research had to provide information on the intensity of the influence exerted by the shape of the sound signal and, respectively, by the distance between the polymer panel and the sensor of the sound pressure level evaluation device on the measurement results.

## Experimental conditions

There are various solutions for generating sounds by converting electrical, hydraulic, etc., energy into mechanical vibrations whose intensity can be increased in various ways. Currently, the most
widespread way of generating sounds is based on the use of speakers. In this case, a membrane attached to a coil moves due to the interaction between the electromagnetic field of the coil and the field of a permanent magnet. The variation in the magnitude of the electromagnetic field generated by the coil occurs as a result of the controlled change in the intensity of the current flowing through the coil. The displacement of the membrane leads to the occurrence of air compressions and decompressions, which contribute to the propagation of sounds through the air (Fig. 1). A sensor placed in the direction of sound propagation will receive the pressure variations, allowing both an assessment of the intensity of the sound waves (of their energy) and a possible graphic illustration of the time variation of the sound intensity.

<figure><img src="image_2354.png" /><figcaption>Fig. 1. Generation, propagation, and absorption of sound waves</figcaption></figure>

When sound insulation is required, obstacles of materials characterized by a high capacity to absorb sound wave energy are placed along the direction of sound wave propagation. This sound insulation capacity of a certain space is influenced by different properties of the material of the obstacle but also by some characteristics of the sound waves. Such a characteristic can be the shape of the electrical signal used to generate the sounds, the form transmitted by the propagation of the sounds in the air, and the material of the obstacle.

On the other hand, as one moves away from the sound-generating source or the obstacle intended to reduce the energy of the sound waves, it is expected that the intensity of the sounds will decrease due to the absorption of the energy of the sounds by the air layers traveled. The sound level sensor could pick this up.

The so-called *sound impedance tube* is used to investigate the sound insulation capacity of some materials incorporated in small disc-shaped samples. In the case of these tubes, sound waves with known characteristics produced by a sound generator must pass through the disc-shaped test sample. Later, certain characteristics of the sounds passed through the panel are evaluated or recorded using appropriate sensors and devices [11,12].

In its more commonly used variant, the sound impedance tube considers the sounds reflected by the walls and not only the sound waves coming from the sound generator. Also, only relatively small test samples can be inserted into the sound impedance tube. For these reasons, equipment was designed and used to allow the testing of small panels (100x100 mm²) manufactured from polylactic acid (PLA) by 3D printing. The test samples were manufactured on an Ultimaker 3 (Netherlands) printer. The thickness of the test sample was 2 mm. The main parameters of the 3D printing process were the following: printing speed $v=20$ mm/s, infill $i=78\%$, gyroid infill pattern, layer thickness $l=0.1$ mm).
widespread way of generating sounds is based on the use of speakers. In this case, a membrane attached to a coil moves due to the interaction between the electromagnetic field of the coil and the field of a permanent magnet. The variation in the magnitude of the electromagnetic field generated by the coil occurs as a result of the controlled change in the intensity of the current flowing through the coil. The displacement of the membrane leads to the occurrence of air compressions and decompressions, which contribute to the propagation of sounds through the air (Fig. 1). A sensor placed in the direction of sound propagation will receive the pressure variations, allowing both an assessment of the intensity of the sound waves (of their energy) and a possible graphic illustration of the time variation of the sound intensity.

<figure><img src="image_2345.png" /><figcaption>Fig. 1. Generation, propagation, and absorption of sound waves</figcaption></figure>

When sound insulation is required, obstacles of materials characterized by a high capacity to absorb sound wave energy are placed along the direction of sound wave propagation. This sound insulation capacity of a certain space is influenced by different properties of the material of the obstacle but also by some characteristics of the sound waves. Such a characteristic can be the shape of the electrical signal used to generate the sounds, the form transmitted by the propagation of the sounds in the air, and the material of the obstacle.

On the other hand, as one moves away from the sound-generating source or the obstacle intended to reduce the energy of the sound waves, it is expected that the intensity of the sounds will decrease due to the absorption of the energy of the sounds by the air layers traveled. The sound level sensor could pick this up.

The so-called *sound impedance tube* is used to investigate the sound insulation capacity of some materials incorporated in small disc-shaped samples. In the case of these tubes, sound waves with known characteristics produced by a sound generator must pass through the disc-shaped test sample. Later, certain characteristics of the sounds passed through the panel are evaluated or recorded using appropriate sensors and devices [11,12].

In its more commonly used variant, the sound impedance tube considers the sounds reflected by the walls and not only the sound waves coming from the sound generator. Also, only relatively small test samples can be inserted into the sound impedance tube. For these reasons, equipment was designed and used to allow the testing of small panels (100x100 mm²) manufactured from polylactic acid (PLA) by 3D printing. The test samples were manufactured on an Ultimaker 3 (Netherlands) printer. The thickness of the test sample was 2 mm. The main parameters of the 3D printing process were the following: printing speed $v=20$ mm/s, infill $i=78\%$, gyroid infill pattern, layer thickness $l=0.1$ mm).
A schematic representation of the equipment can be seen in Fig. 2. The equipment uses a mini audio speaker, which can be operated by existing software on a smartphone. The sounds with known characteristics generated by the mini audio speaker pass through the polymer panel, whose sound insulation properties must be determined. Later, the sounds whose energy has been reduced due to passing through the panel made of polymeric material reach the sensor of a sound level meter. The signal generated by the sound level meter can be sent to a computer, whose software allows the illustration of the evolution of some sound characteristics over time on a monitor screen. The audio minibox, the polymer material panel, and the sound level meter sensor are placed in compartments with wooden walls lined inside with a material that exhibits good sound wave absorption properties. A HY1361-type equipment (China) was used to determine the sound pressure level. The audio mini speaker type Andowl M10 allowed the generation of sounds with different signal shapes. The mini audio speaker received the signal from a smartphone through the Frequency Generator application (produced by LuxDeLux).

<figure><img src="image_2355.png" /><figcaption>Fig. 2. Schematic representation of the equipment used to perform the experimental tests</figcaption></figure>

In the context of some previous research [9,10], for different polymeric materials, empirical mathematical models were determined to highlight the difference in acoustic pressure corresponding to the energy absorbed by the panel made of polymeric material as a function of the thickness *t* of the panel, type *m* of panel material (*m*=1 for polylactic acid, *m*=2 for high-impact polystyrene, *m*=3 for polyethylene terephthalate glycol), printing speed *v*, infill *i*, infill pattern *i*<sub>p</sub>, layer thickness *l*, sound frequency *f* and sound volume *s*. Such an empirical mathematical model, established by considering the results obtained for 18 samples, is the following [10]:

$$ \Delta p_{ac} = 169.745 - 263.917t + 43.719t^2 + 5.097m - 1.639m^2 - 0.141v + 0.00153v^2 + 0.180i - 0.00177i^2 + 1.311i_p - 0.571i_p^2 - 27.377l + 222.774l^2 + 0.00853f - 3.253 \cdot 10^{-7}f^2 + 0.685s - 0.00480s^2. \quad (1) $$

During the experimental tests aimed at determining the mathematical model corresponding to Eq. (1), the idea appeared to carry out some tests that would allow highlighting the influence of the shape of the electric signal used to generate sound waves and respectively of the distance between the wall and the sensor of sound meter level on the difference Δ*p*<sub>ac</sub> of sound pressure level.

For this purpose, it was used to carry out some experimental tests in which the four types of sound signals generated using the existing software on smartphones and, respectively, two distances *d* between the panel made of polymeric material and the sensor of the sound meter level
A schematic representation of the equipment can be seen in Fig. 2. The equipment uses a mini audio speaker, which can be operated by existing software on a smartphone. The sounds with known characteristics generated by the mini audio speaker pass through the polymer panel, whose sound insulation properties must be determined. Later, the sounds whose energy has been reduced due to passing through the panel made of polymeric material reach the sensor of a sound level meter. The signal generated by the sound level meter can be sent to a computer, whose software allows the illustration of the evolution of some sound characteristics over time on a monitor screen. The audio minibox, the polymer material panel, and the sound level meter sensor are placed in compartments with wooden walls lined inside with a material that exhibits good sound wave absorption properties. A HY1361-type equipment (China) was used to determine the sound pressure level. The audio mini speaker type Andowl M10 allowed the generation of sounds with different signal shapes. The mini audio speaker received the signal from a smartphone through the Frequency Generator application (produced by LuxDeLux).

<figure><img src="image_2346.png" /><figcaption>Fig. 2. Schematic representation of the equipment used to perform the experimental tests</figcaption></figure>

In the context of some previous research [9,10], for different polymeric materials, empirical mathematical models were determined to highlight the difference in acoustic pressure corresponding to the energy absorbed by the panel made of polymeric material as a function of the thickness *t* of the panel, type *m* of panel material (*m*=1 for polylactic acid, *m*=2 for high-impact polystyrene, *m*=3 for polyethylene terephthalate glycol), printing speed *v*, infill *i*, infill pattern *i*<sub>p</sub>, layer thickness *l*, sound frequency *f* and sound volume *s*. Such an empirical mathematical model, established by considering the results obtained for 18 samples, is the following [10]:

$$ \Delta p_{ac} = 169.745 - 263.917t + 43.719t^2 + 5.097m - 1.639m^2 - 0.141v + 0.00153v^2 + 0.180i - 0.00177i^2 + 1.311i_p - 0.571i_p^2 - 27.377l + 222.774l^2 + 0.00853f - 3.253 \cdot 10^{-7}f^2 + 0.685s - 0.00480s^2. \quad (1) $$

During the experimental tests aimed at determining the mathematical model corresponding to Eq. (1), the idea appeared to carry out some tests that would allow highlighting the influence of the shape of the electric signal used to generate sound waves and respectively of the distance between the wall and the sensor of sound meter level on the difference $\Delta p_{ac}$ of sound pressure level.

For this purpose, it was used to carry out some experimental tests in which the four types of sound signals generated using the existing software on smartphones and, respectively, two distances *d* between the panel made of polymeric material and the sensor of the sound meter level
are used. Experimental tests were conducted by the requirements of a factorial experiment with two independent variables (signal shape *s* and distance *d*). Sinusoidal (*s*=1), rectangular (*s*=2), triangular *s*=3), and sawtooth (*s*=4) signals were used. The distances between the panel and the sensor were *d*<sub>min</sub>=40 mm and *d*<sub>max</sub>=135 mm, respectively. These last values were established starting from the hypothesis that they will allow highlighting the extent to which the distance *d* could contribute to the reduction of the intensity of the sounds received by the sensor due to the sounds traveling a greater distance.

<table><caption>Table 1. Performance conditions and experimental results</caption><thead><tr><th rowspan="3">Experiment no.</th><th colspan="3">Input factors</th><th colspan="3">Output parameters</th></tr><tr><th colspan="2">Signal shape, <i>s</i></th><th rowspan="2">Distance <i>d</i> [mm]</th><th rowspan="2">Acoustic pressure level without panel, <i>I</i><sub>0</sub> [dB]</th><th rowspan="2">Acoustic pressure level when using polymer panel, <i>I</i><sub>p</sub>, [dB]</th><th rowspan="2">Difference in acoustic pressure level, Δ<i>p</i><sub>ac</sub> [dB]</th></tr><tr><th>Shape</th><th>Coded value</th></tr></thead><tbody><tr><td>Column no. 1</td><td>2</td><td>3</td><td>4</td><td>5</td><td>6</td><td>7</td></tr><tr><td>1</td><td>Sinusoidal</td><td>1</td><td>40</td><td>117.6</td><td>97.1</td><td>20.5</td></tr><tr><td>2</td><td>Sinusoidal</td><td>1</td><td>135</td><td>102.4</td><td>103.8</td><td>-1.4</td></tr><tr><td>3</td><td>Rectangular</td><td>2</td><td>40</td><td>117.8</td><td>98.3</td><td>19.5</td></tr><tr><td>4</td><td>Rectangular</td><td>2</td><td>135</td><td>102.7</td><td>103.4</td><td>-0.7</td></tr><tr><td>5</td><td>Triangle</td><td>3</td><td>40</td><td>117.9</td><td>97.1</td><td>20.8</td></tr><tr><td>6</td><td>Triangle</td><td>3</td><td>135</td><td>102.1</td><td>102.7</td><td>-0.6</td></tr><tr><td>7</td><td>Sawtooth</td><td>4</td><td>40</td><td>117.5</td><td>99.6</td><td>17.9</td></tr><tr><td>8</td><td>Sawtooth</td><td>4</td><td>135</td><td>105.5</td><td>108.8</td><td>-3.3</td></tr></tbody></table>

The conditions and the results of the experimental tests are mentioned in Table 1. All results were obtained using a flat panel test sample manufactured from polylactic acid by 3D printing. The fused deposition modeling 3D printing technology was used. In column no. 5 of Table 1, the values of the acoustic pressure level indicated by the measuring device have been entered when the polymer material panel is not placed between the mini audio speaker and the sensor of the measuring device. The values of the acoustic pressure when the polymeric material panel was inserted are those in column no. 6. The differences between the two values of the acoustic pressure level for the same shape of the electrical signal and the same distance between the mini audio speaker and the sensor were entered in column no. 7. These differences are basically due to the sound energy absorbed by the panel material.

## Processing of experimental results

The experimental results were mathematically processed using software based on the least squares method [13]. According to this software, the adequacy of the empirical mathematical model to the experimental results is achieved using the Gauss criterion [13,14]. The value of Gauss's criterion is determined as a ratio in which the sum of the squares of the differences between the values determined by using the proposed empirical mathematical model and the values of the ordinates of the points corresponding to the experimental results for the same values of the abscissas, is entered in the numerator. The difference between the number of experimental tests and the number of constants in the proposed empirical mathematical model is used as the ratio's denominator. In
are used. Experimental tests were conducted by the requirements of a factorial experiment with two independent variables (signal shape *s* and distance *d*). Sinusoidal (*s*=1), rectangular (*s*=2), triangular *s*=3), and sawtooth (*s*=4) signals were used. The distances between the panel and the sensor were *d*<sub>min</sub>=40 mm and *d*<sub>max</sub>=135 mm, respectively. These last values were established starting from the hypothesis that they will allow highlighting the extent to which the distance *d* could contribute to the reduction of the intensity of the sounds received by the sensor due to the sounds traveling a greater distance.

<table><caption>Table 1. Performance conditions and experimental results</caption><thead><tr><th rowspan="3">Experiment no.</th><th colspan="3">Input factors</th><th colspan="3">Output parameters</th></tr><tr><th colspan="2">Signal shape, <i>s</i></th><th rowspan="2">Distance <i>d</i> [mm]</th><th rowspan="2">Acoustic pressure level without panel, <i>I</i><sub>0</sub> [dB]</th><th rowspan="2">Acoustic pressure level when using polymer panel, <i>I</i><sub>p</sub>, [dB]</th><th rowspan="2">Difference in acoustic pressure level, Δ<i>p</i><sub>ac</sub> [dB]</th></tr><tr><th>Shape</th><th>Coded value</th></tr></thead><tbody><tr><td>Column no. 1</td><td>2</td><td>3</td><td>4</td><td>5</td><td>6</td><td>7</td></tr><tr><td>1</td><td>Sinusoidal</td><td>1</td><td>40</td><td>117.6</td><td>97.1</td><td>20.5</td></tr><tr><td>2</td><td>Sinusoidal</td><td>1</td><td>135</td><td>102.4</td><td>103.8</td><td>-1.4</td></tr><tr><td>3</td><td>Rectangular</td><td>2</td><td>40</td><td>117.8</td><td>98.3</td><td>19.5</td></tr><tr><td>4</td><td>Rectangular</td><td>2</td><td>135</td><td>102.7</td><td>103.4</td><td>-0.7</td></tr><tr><td>5</td><td>Triangle</td><td>3</td><td>40</td><td>117.9</td><td>97.1</td><td>20.8</td></tr><tr><td>6</td><td>Triangle</td><td>3</td><td>135</td><td>102.1</td><td>102.7</td><td>-0.6</td></tr><tr><td>7</td><td>Sawtooth</td><td>4</td><td>40</td><td>117.5</td><td>99.6</td><td>17.9</td></tr><tr><td>8</td><td>Sawtooth</td><td>4</td><td>135</td><td>105.5</td><td>108.8</td><td>-3.3</td></tr></tbody></table>

The conditions and the results of the experimental tests are mentioned in Table 1. All results were obtained using a flat panel test sample manufactured from polylactic acid by 3D printing. The fused deposition modeling 3D printing technology was used. In column no. 5 of Table 1, the values of the acoustic pressure level indicated by the measuring device have been entered when the polymer material panel is not placed between the mini audio speaker and the sensor of the measuring device. The values of the acoustic pressure when the polymeric material panel was inserted are those in column no. 6. The differences between the two values of the acoustic pressure level for the same shape of the electrical signal and the same distance between the mini audio speaker and the sensor were entered in column no. 7. These differences are basically due to the sound energy absorbed by the panel material.

## Processing of experimental results

The experimental results were mathematically processed using software based on the least squares method [13]. According to this software, the adequacy of the empirical mathematical model to the experimental results is achieved using the Gauss criterion [13,14]. The value of Gauss's criterion is determined as a ratio in which the sum of the squares of the differences between the values determined by using the proposed empirical mathematical model and the values of the ordinates of the points corresponding to the experimental results for the same values of the abscissas, is entered in the numerator. The difference between the number of experimental tests and the number of constants in the proposed empirical mathematical model is used as the ratio's denominator. In
essence, the lower the value of Gauss's criterion, the more it is appreciated that the proposed empirical mathematical model is more appropriate to the experimental results used.

By using the mentioned software, it was found an empirical mathematical model of the second-degree polynomial type (for which the Gauss sum has the value $S_G=3.510862$):

$$ \Delta p_{ac} = 27.948 - 1.0920s_s + 0.937s_s^2 - 0.201d, \qquad (2) $$

where $s_s$ corresponds to the coding mode of the signal shape (Table 1).

Using the empirical mathematical model defined by Eq. (2), the graphical representations in Figs. 3 and 4 were developed.

<figure><img src="image_2356.png" /><figcaption>Fig. 3. The influence of the distance $d$ between the mini audio speaker and the sensor of sound level meter for the four shapes of the sound signal, on the difference in acoustic pressure level $\Delta p_{ac}$</figcaption></figure>

The analysis of the mathematical model constituted by Eq. (2) and the graphic representations in Figs. 3 and 4 allowed the formulation of some observations regarding the influence exerted by the signal shape $s$ and by the distance $d$ between the sensor of the sound level meter and the mini audio speaker on the difference $\Delta p_{ac}$ corresponding to the energy absorbed by the small dimensions polymer panel.

It is thus found that as the distance $d$ between the mini audio speaker and the sensor of the sound meter level increases, there is a decrease in the energy absorbed by the measuring instrument. This fact can be justified by the decrease of the energy that reaches the sensor once the by sound waves of greater distance.

It can also be observed that from the point of view of the influence of the shape of the signal used to generate the sounds, the lowest value of the difference $\Delta p_{ac}$ corresponds to the signal of sinusoidal shape, higher values being registered, in order, for the sinusoidal signal, for the triangle signal and, respectively, for sawtooth signal.
essence, the lower the value of Gauss's criterion, the more it is appreciated that the proposed empirical mathematical model is more appropriate to the experimental results used.

By using the mentioned software, it was found an empirical mathematical model of the second-degree polynomial type (for which the Gauss sum has the value $S_G=3.510862$):

$$ \Delta p_{ac} = 27.948 - 1.0920s_s + 0.937s_s^2 - 0.201d, \quad (2) $$

where $s_s$ corresponds to the coding mode of the signal shape (Table 1).

Using the empirical mathematical model defined by Eq. (2), the graphical representations in Figs. 3 and 4 were developed.

<figure><img src="image_2347.png" /><figcaption>Fig. 3. The influence of the distance $d$ between the mini audio speaker and the sensor of sound level meter for the four shapes of the sound signal, on the difference in acoustic pressure level $\Delta p_{ac}$</figcaption></figure>

The analysis of the mathematical model constituted by Eq. (2) and the graphic representations in Figs. 3 and 4 allowed the formulation of some observations regarding the influence exerted by the signal shape $s$ and by the distance $d$ between the sensor of the sound level meter and the mini audio speaker on the difference $\Delta p_{ac}$ corresponding to the energy absorbed by the small dimensions polymer panel.

It is thus found that as the distance $d$ between the mini audio speaker and the sensor of the sound meter level increases, there is a decrease in the energy absorbed by the measuring instrument. This fact can be justified by the decrease of the energy that reaches the sensor once the by sound waves of greater distance.

It can also be observed that from the point of view of the influence of the shape of the signal used to generate the sounds, the lowest value of the difference $\Delta p_{ac}$ corresponds to the signal of sinusoidal shape, higher values being registered, in order, for the sinusoidal signal, for the triangle signal and, respectively, for sawtooth signal.
<figure><img src="image_2357.png" /><figcaption>Fig. 4. The variation of difference in acoustic pressure level, $\Delta p_{ac}$ depending on the shape of the signal and the distance $d$</figcaption></figure>

## Conclusions

The development of 3D printing technologies has allowed the manufacturing of panels made of polymeric materials that can be used for sound insulation. Several groups of factors are capable of influencing these panels' ability to absorb sound wave energy. The research whose results were included in this paper sought to highlight the influence exerted by the shape of the electrical signal used to generate sound waves and, respectively, the distance between the source of sound vibrations and the sensor of a sound level meter on the difference in acoustic pressure level. The experimental research was carried out using equipment that allows the modification of the input factors considered. The evaluation of the sound insulation capacity of small panels made of polylactic acid was considered. The experimental results were mathematically processed using software based on the least squares method. An empirical mathematical model of the second-degree polynomial type was determined. The model highlights the influence exerted by the two input factors on the difference in acoustic pressure level. As expected, increasing the distance between the source of sound vibrations and the sensor of the sound meter level led to a decrease in the sound energy reaching the sound meter level. In the future, an extension of the experimental research related to the study of the sound insulation capacity is also considered in the case of other polymeric materials. Another possible direction of research could aim to reveal the influence exerted by the internal structure of the polymeric material. This structure can have different aspects by changing the values of the parameters of the 3D printing process.

## References

[1] C. Perrot, M.T. Hoang, F. Chevillotte, An overview of microstructural approaches for modelling and improving sound proofing properties of cellular foams: Developments and prospects, SAE Technical Paper (2018) 01-1564. https://doi.org/10.4271/2018-01-1564

[2] S.K. Zahiruddin, J. Yarlagadda, Experimental investigation of sound absorption properties of 3D printed PETG absorbers, Eur. Chem. Bull. 12(S2) (2023) 5400 – 5407. https://doi.org/10.31838/ecb/2023.12.s3.607

[3] Y.-J. King, K.-K. Teo, Application of 3D printed structured materials as the sound absorption panels, IOP Conf. Series: Earth and Environmental Science 463 (2020) 012032. https://doi.org/10.1088/1755-1315/463/1/012032

[4] M.A.S. Sujon, A. Islam, V.K. Nadimpalli, Damping and sound absorption properties of polymer matrix composites: A review. Polym. Test. 104 (2021) 107388. https://doi.org/10.1016/j.polymertesting.2021.1073

[5] E. Knapen, R. Lanoye, G. Vermeir, W. Lauriks, D. Van Gemert, Acoustic properties of sound absorbing, polymer-modified porous cement mortars. 6th International Conference on Materials
<figure><img src="image_2348.png" /><figcaption>Fig. 4. The variation of difference in acoustic pressure level, $\Delta p_{ac}$ depending on the shape of the signal and the distance $d$</figcaption></figure>

## Conclusions

The development of 3D printing technologies has allowed the manufacturing of panels made of polymeric materials that can be used for sound insulation. Several groups of factors are capable of influencing these panels' ability to absorb sound wave energy. The research whose results were included in this paper sought to highlight the influence exerted by the shape of the electrical signal used to generate sound waves and, respectively, the distance between the source of sound vibrations and the sensor of a sound level meter on the difference in acoustic pressure level. The experimental research was carried out using equipment that allows the modification of the input factors considered. The evaluation of the sound insulation capacity of small panels made of polylactic acid was considered. The experimental results were mathematically processed using software based on the least squares method. An empirical mathematical model of the second-degree polynomial type was determined. The model highlights the influence exerted by the two input factors on the difference in acoustic pressure level. As expected, increasing the distance between the source of sound vibrations and the sensor of the sound meter level led to a decrease in the sound energy reaching the sound meter level. In the future, an extension of the experimental research related to the study of the sound insulation capacity is also considered in the case of other polymeric materials. Another possible direction of research could aim to reveal the influence exerted by the internal structure of the polymeric material. This structure can have different aspects by changing the values of the parameters of the 3D printing process.

## References

[1] C. Perrot, M.T. Hoang, F. Chevillotte, An overview of microstructural approaches for modelling and improving sound proofing properties of cellular foams: Developments and prospects, SAE Technical Paper (2018) 01-1564. https://doi.org/10.4271/2018-01-1564

[2] S.K. Zahiruddin, J. Yarlagadda, Experimental investigation of sound absorption properties of 3D printed PETG absorbers, Eur. Chem. Bull. 12(S2) (2023) 5400 – 5407. https://doi.org/10.31838/ecb/2023.12.s3.607

[3] Y.-J. King, K.-K. Teo, Application of 3D printed structured materials as the sound absorption panels, IOP Conf. Series: Earth and Environmental Science 463 (2020) 012032. https://doi.org/10.1088/1755-1315/463/1/012032

[4] M.A.S. Sujon, A. Islam, V.K. Nadimpalli, Damping and sound absorption properties of polymer matrix composites: A review. Polym. Test. 104 (2021) 107388. https://doi.org/10.1016/j.polymertesting.2021.1073

[5] E. Knapen, R. Lanoye, G. Vermeir, W. Lauriks, D. Van Gemert, Acoustic properties of sound absorbing, polymer-modified porous cement mortars. 6th International Conference on Materials
Science and Restoration, MSR-VI, Aedificatio Publishers (2003) 347-358.
https://www.academia.edu/32064222/Acoustic_properties_of_sound_absorbing_polymer_modifi
ed_porous_cement_mortars

[6] H. Zhou, B. Li, G. Huang, Sound absorption characteristics of polymer microparticles, J. Appl. Polym. Sci. 101 (2006) 2675-2679. https://onlinelibrary.wiley.com/doi/epdf/10.1002/app.23911?saml_referrer

[7] M. Bratu, I. Ropotă, O. Vasile, O. Dumitrescu, M. Muntean. Research on the absorbing properties of some newtypes of composite materials, Rev. Romana Mater. 41(2) (2011) 147 – 154.

[8] X. Li, Z. Cao, L. Xu, B. Liu, Sound absorption of the absorber composed of a shunt loudspeaker and porous materials in tandem, *Polymers* 15 (2023) 3051. https://doi.org/10.3390/polym15143051

[9] A. Hrițuc, O. Dodun, A. Mihalache, G. Nagîț, Equipment requirements for the investigation of sound insulation properties of 3D printed polymeric materials, Acta Tech. Napoc., Ser. Appl. Math. 65 (2022) 1186-1192. https://atna-mam.utcluj.ro/index.php/Acta/article/view/2046

[10] A. Hrițuc, A.M. Mihalache, O. Dodun, G. Nagîț, I. Beşliu-Băncescu, B. Rădulescu, L. Slătineanu, Propagation of sounds through small panels made of polymer materials by 3D Printing, Polymers 16 (2024), 5. https://doi.org/10.3390/polym16010005

[11] ISO 10534-1, Acoustics. Determination of sound absorption coefficient and impedance in impedance tubes. Part 1: Method using standing wave ratio

[12] ISO 10534-2, Determination of sound absorption coefficient and acoustic impedance with interferometer. Part 2: Transfer function method

[13] G. Crețu, Fundamentals of experimental research. Laboratory Handbook, “Gheorghe Asachi” Technical University of Iași, Iași, Romania, 1992 (in Romanian)

[14] A.G. Worthing, J. Geffner, Processing the experimental data, Technical Publishing House, Bucharest, Romania, 1959 (in Romanian)
Science and Restoration, MSR-VI, Aedificatio Publishers (2003) 347-358.
https://www.academia.edu/32064222/Acoustic_properties_of_sound_absorbing_polymer_modifi
ed_porous_cement_mortars

[6] H. Zhou, B. Li, G. Huang, Sound absorption characteristics of polymer microparticles, J. Appl. Polym. Sci. 101 (2006) 2675-2679. https://onlinelibrary.wiley.com/doi/epdf/10.1002/app.23911?saml_referrer

[7] M. Bratu, I. Ropotă, O. Vasile, O. Dumitrescu, M. Muntean. Research on the absorbing properties of some newtypes of composite materials, Rev. Romana Mater. 41(2) (2011) 147 – 154.

[8] X. Li, Z. Cao, L. Xu, B. Liu, Sound absorption of the absorber composed of a shunt loudspeaker and porous materials in tandem, *Polymers* 15 (2023) 3051. https://doi.org/10.3390/polym15143051

[9] A. Hrițuc, O. Dodun, A. Mihalache, G. Nagîț, Equipment requirements for the investigation of sound insulation properties of 3D printed polymeric materials, Acta Tech. Napoc., Ser. Appl. Math. 65 (2022) 1186-1192. https://atna-mam.utcluj.ro/index.php/Acta/article/view/2046

[10] A. Hrițuc, A.M. Mihalache, O. Dodun, G. Nagîț, I. Beşliu-Băncescu, B. Rădulescu, L. Slătineanu, Propagation of sounds through small panels made of polymer materials by 3D Printing, Polymers 16 (2024), 5. https://doi.org/10.3390/polym16010005

[11] ISO 10534-1, Acoustics. Determination of sound absorption coefficient and impedance in impedance tubes. Part 1: Method using standing wave ratio

[12] ISO 10534-2, Determination of sound absorption coefficient and acoustic impedance with interferometer. Part 2: Transfer function method

[13] G. Crețu, Fundamentals of experimental research. Laboratory Handbook, “Gheorghe Asachi” Technical University of Iași, Iași, Romania, 1992 (in Romanian)

[14] A.G. Worthing, J. Geffner, Processing the experimental data, Technical Publishing House, Bucharest, Romania, 1959 (in Romanian)
# Perforation resistance of some materials in 3D printed parts

NICOLAU Mihaela¹,ª, HRITUC Adelina¹,ᵇ*, MIHALACHE Marius-Andrei¹,ᶜ, NAGÎT Gheorghe¹,ᵈ, DUȘA Petru¹,ᵉ, CRĂCIUN Elisaveta¹,ᵉ, MUNTEANU Adriana¹,⁹, DODUN Oana¹,ʰ, SLATINEANU Laurențiu¹,ᵢ

¹Gheorghe Asachi” Technical University of Iași, Department of Machine Manufacturing Technology, Blvd. D. Mangeron, 39 A, 700050 Iași, Romania

ªmihaela.nicolau@student.tuiasi.ro, ᵇadelina.hrituc@student.tuiasi.ro,

ᶜandrei.mihalache@tuiasi.ro, ᵈnagit@tcm.tuiasi.ro, ᵉpdusa@tcm.tuiasi.ro,

felisaveta.craciun@student.tuiasi.ro, gadriana.munteanu@academic.tuiasi.ro,

ʰoanad@tcm.tuiasi.ro, ᵉslati@tcm.tuiasi.ro

**Keywords:** Perforation Resistance, Testing Equipment, 3D Printing, Finite Element Modeling, Experimental Test Empirical Mathematical Model

**Abstract.** Sometimes, for various reasons, some parts manufactured by 3D printing need to have high perforation resistance. In such situations, it is necessary to know the perforation resistance values of the materials incorporated in the parts. In principle, conical tip penetrators are allowed to fall under their weight on the test sample material to be investigated, which allows the conical tip to penetrate and advance into the sample material. A finite element modeling allowed obtaining some information about the behavior of the material during the perforation action. Perforation resistance testing equipment has been designed to use conical tips and different heights from which the penetrator is released. Thus, it became possible to highlight the influence of some input factors in the testing process on the diameter of the hole generated by the penetrator in the thermoplastic polyurethane-type material of the test sample. As input factors, the weight of the penetrator, the drop height of the penetrator, and the thickness of the test sample were considered. The mathematical processing of the experimental results allowed the establishment of some empirical mathematical models, which highlight the influence exerted by the three input factors on the value of the considered output parameter. It was confirmed that as the penetrator weight and drop height increase, the diameter of the hole generated in the sample material increases. In contrast, the test sample thickness exerts the opposite effect.

**Introduction**

In 2009, ASTM defined additive manufacturing (AM) as “a process of joining materials to make objects from 3D model data, usually layer upon layer, as opposed to subtractive manufacturing methodologies” [1,2]. Since its inception, additive manufacturing has been widely used in many applications in different industries and research fields due to the many benefits it brings in manufacturing, design, and distribution, such as freedom of design, flexibility, customization, waste minimization, and sustainability of production in the artistic, medical, marketing, and educational fields [3-6].

Fused deposition modeling (FDM) is an additive manufacturing process in which a thin filament of plastic (thermoplastic polymer) is melted and extruded (dispensed through a nozzle) in a thickness typically of 0.25 mm [5]. Due to the properties of the materials used in the process, FDM can generate rigid and flexible parts. Materials used in this process are PLA (polylactic acid), PC (polycarbonate), PET (polyethylene terephthalate), TPU (thermoplastic polyurethane) (flexible), ABS (acrylonitrile butadiene styrene), polyphenylsulfone, and nylon 6 [7].

Typical applications include the production of parts and functional prototypes with outstanding thermal and chemical resistance and excellent strength-to-weight ratios, such as jigs, fixtures,

<figure><img src="image_2358.png" /></figure>
# Perforation resistance of some materials in 3D printed parts

NICOLAU Mihaela¹,ª, HRITUC Adelina¹,ᵇ*, MIHALACHE Marius-Andrei¹,ᶜ, NAGÎT Gheorghe¹,ᵈ, DUȘA Petru¹,ᵉ, CRĂCIUN Elisaveta¹,ᵉ, MUNTEANU Adriana¹,⁹, DODUN Oana¹,ʰ, SLATINEANU Laurențiu¹,ᵢ

¹Gheorghe Asachi” Technical University of Iași, Department of Machine Manufacturing Technology, Blvd. D. Mangeron, 39 A, 700050 Iași, Romania

ªmihaela.nicolau@student.tuiasi.ro, ᵇadelina.hrituc@student.tuiasi.ro,

ᶜandrei.mihalache@tuiasi.ro, ᵈnagit@tcm.tuiasi.ro, ᵉpdusa@tcm.tuiasi.ro,

felisaveta.craciun@student.tuiasi.ro, gadriana.munteanu@academic.tuiasi.ro,

ʰoanad@tcm.tuiasi.ro, ᵉslati@tcm.tuiasi.ro

**Keywords:** Perforation Resistance, Testing Equipment, 3D Printing, Finite Element Modeling, Experimental Test Empirical Mathematical Model

**Abstract.** Sometimes, for various reasons, some parts manufactured by 3D printing need to have high perforation resistance. In such situations, it is necessary to know the perforation resistance values of the materials incorporated in the parts. In principle, conical tip penetrators are allowed to fall under their weight on the test sample material to be investigated, which allows the conical tip to penetrate and advance into the sample material. A finite element modeling allowed obtaining some information about the behavior of the material during the perforation action. Perforation resistance testing equipment has been designed to use conical tips and different heights from which the penetrator is released. Thus, it became possible to highlight the influence of some input factors in the testing process on the diameter of the hole generated by the penetrator in the thermoplastic polyurethane-type material of the test sample. As input factors, the weight of the penetrator, the drop height of the penetrator, and the thickness of the test sample were considered. The mathematical processing of the experimental results allowed the establishment of some empirical mathematical models, which highlight the influence exerted by the three input factors on the value of the considered output parameter. It was confirmed that as the penetrator weight and drop height increase, the diameter of the hole generated in the sample material increases. In contrast, the test sample thickness exerts the opposite effect.

**Introduction**

In 2009, ASTM defined additive manufacturing (AM) as “a process of joining materials to make objects from 3D model data, usually layer upon layer, as opposed to subtractive manufacturing methodologies” [1,2]. Since its inception, additive manufacturing has been widely used in many applications in different industries and research fields due to the many benefits it brings in manufacturing, design, and distribution, such as freedom of design, flexibility, customization, waste minimization, and sustainability of production in the artistic, medical, marketing, and educational fields [3-6].

Fused deposition modeling (FDM) is an additive manufacturing process in which a thin filament of plastic (thermoplastic polymer) is melted and extruded (dispensed through a nozzle) in a thickness typically of 0.25 mm [5]. Due to the properties of the materials used in the process, FDM can generate rigid and flexible parts. Materials used in this process are PLA (polylactic acid), PC (polycarbonate), PET (polyethylene terephthalate), TPU (thermoplastic polyurethane) (flexible), ABS (acrylonitrile butadiene styrene), polyphenylsulfone, and nylon 6 [7].

Typical applications include the production of parts and functional prototypes with outstanding thermal and chemical resistance and excellent strength-to-weight ratios, such as jigs, fixtures,

<figure><img src="image_2349.png" /></figure>
turbines, rotors, bumpers, and more [8]. According to Melnikova *et al.* (2014), ABS is too brittle for textile industry applications, but a soft version of PLA can be used to manufacture textile-based structures. In this case, the hard PLA is polylactic acid with some useful properties (elongation at break ~ 4 %, tensile modulus 1968 MPa), while a soft PLA is PLA including a softener (with elongation at break up to ~ 200 %) [9].

In the textile area, AM is used to manufacture textiles, flexible structures, and flexible materials, resulting in structural materials frequently used, for example, in the medical field [10].

Any manufactured part is usually tested to confirm that the intended design was achieved. Conventional tests such as tensile, flexure, Poisson's ratio, axial fatigue, and compression tests [11] can be supplemented by any non-destructive tests, such as hardness tests, that do not directly measure a mechanical characteristic [12]. Due to the possibility of obtaining an uneven-density product through the FDM process, an indentation test applies an uneven stress distribution on the sample to predict the mechanical behavior of materials [13].

In the indentation test, the stress experienced by the material is due to both elastic and plastic deformation. In the beginning, elastic deformation is predominant until a certain depth is reached, after which plastic deformation localizes around the indenter edge and then expands along with indentation depth. The test output is usually the indentation depth or the size of the imprint, which correlates the material hardness to indenter geometry and indentation parameters [14]. Low-load indentation techniques have been developed for testing metallic and ceramic thin films or coatings, requiring several tests and statistical data treatment [14]. According to Riccardi *et al.* (2004), a flat-end punch with a diameter of 1 mm or smaller and axial length equal to 1.5 mm was used on a massive frame, dimensioned to have a total elastic deformation of about 1 µm so that elastic modulus and yield stress can be directly determined from pressure-penetration curves by applying simple analytical relationships.

According to Alisafaei *et al.* [13], many studies have been done using a sharp Berkovich tip and show the numerous variables that can affect the results. Lambiase *et al.* (2023) [12] show that the tests using a sharp tip are heavily influenced by the density of the material being tested, which in turn is defined by the thickness of the sample. In [15], the authors investigate the perforation resistance of glass fiber-reinforced polyetherketoneketone (PEKK) composites. The study focuses on evaluating the composite material's ability to withstand perforation and provides insights into its mechanical properties. The findings contribute to the understanding of the performance of glass fiber-reinforced PEKK composites, offering valuable information for applications requiring high resistance to perforation. The impact resistance and compression behavior of carbon fiber-reinforced polyetheretherketone (CF/PEEK) laminates are experimentally investigated after hot-press fusion repair, considering various stacking sequences [16]. The results emphasize the performance of repaired laminates under impact and compression and offer relevant information for the development and maintenance of composite structures. According to Popa *et al.* [17], the AM materials can be numerically modeled and calibrated with the experimental ones.

For textile additive manufacturing, the adhesion of the printed material onto the chosen apparel is a serious issue [18], and it is being managed through adhesion tests, known as mechanical peel tests [19-21]. Other tests include tear strength, bending length measurements, abrasion testing, wetting and cyclic bending, and the effect of washing [18].

The International Standards Organization (ISO) proposed a standard (ISO 17296-3:2014 Additive Manufacturing – General Principles—Part 3: Main characteristics and corresponding test methods [19]) to address the quality characteristics of parts produced by AM [22].

According to Maidin *et al.* [23], a stab test was performed on an AM material (PC-ABS) of various thicknesses, from 4.0 to 10.0 mm, using an impact energy of 24 joules to determine a minimum thickness that resulted in a maximum penetration of 7.0 mm by using Instron CEAST
turbines, rotors, bumpers, and more [8]. According to Melnikova *et al.* (2014), ABS is too brittle for textile industry applications, but a soft version of PLA can be used to manufacture textile-based structures. In this case, the hard PLA is polylactic acid with some useful properties (elongation at break ~ 4 %, tensile modulus 1968 MPa), while a soft PLA is PLA including a softener (with elongation at break up to ~ 200 %) [9].

In the textile area, AM is used to manufacture textiles, flexible structures, and flexible materials, resulting in structural materials frequently used, for example, in the medical field [10].

Any manufactured part is usually tested to confirm that the intended design was achieved. Conventional tests such as tensile, flexure, Poisson's ratio, axial fatigue, and compression tests [11] can be supplemented by any non-destructive tests, such as hardness tests, that do not directly measure a mechanical characteristic [12]. Due to the possibility of obtaining an uneven-density product through the FDM process, an indentation test applies an uneven stress distribution on the sample to predict the mechanical behavior of materials [13].

In the indentation test, the stress experienced by the material is due to both elastic and plastic deformation. In the beginning, elastic deformation is predominant until a certain depth is reached, after which plastic deformation localizes around the indenter edge and then expands along with indentation depth. The test output is usually the indentation depth or the size of the imprint, which correlates the material hardness to indenter geometry and indentation parameters [14]. Low-load indentation techniques have been developed for testing metallic and ceramic thin films or coatings, requiring several tests and statistical data treatment [14]. According to Riccardi *et al.* (2004), a flat-end punch with a diameter of 1 mm or smaller and axial length equal to 1.5 mm was used on a massive frame, dimensioned to have a total elastic deformation of about 1 µm so that elastic modulus and yield stress can be directly determined from pressure-penetration curves by applying simple analytical relationships.

According to Alisafaei *et al.* [13], many studies have been done using a sharp Berkovich tip and show the numerous variables that can affect the results. Lambiase *et al.* (2023) [12] show that the tests using a sharp tip are heavily influenced by the density of the material being tested, which in turn is defined by the thickness of the sample. In [15], the authors investigate the perforation resistance of glass fiber-reinforced polyetherketoneketone (PEKK) composites. The study focuses on evaluating the composite material's ability to withstand perforation and provides insights into its mechanical properties. The findings contribute to the understanding of the performance of glass fiber-reinforced PEKK composites, offering valuable information for applications requiring high resistance to perforation. The impact resistance and compression behavior of carbon fiber-reinforced polyetheretherketone (CF/PEEK) laminates are experimentally investigated after hot-press fusion repair, considering various stacking sequences [16]. The results emphasize the performance of repaired laminates under impact and compression and offer relevant information for the development and maintenance of composite structures. According to Popa *et al.* [17], the AM materials can be numerically modeled and calibrated with the experimental ones.

For textile additive manufacturing, the adhesion of the printed material onto the chosen apparel is a serious issue [18], and it is being managed through adhesion tests, known as mechanical peel tests [19-21]. Other tests include tear strength, bending length measurements, abrasion testing, wetting and cyclic bending, and the effect of washing [18].

The International Standards Organization (ISO) proposed a standard (ISO 17296-3:2014 Additive Manufacturing – General Principles—Part 3: Main characteristics and corresponding test methods [19]) to address the quality characteristics of parts produced by AM [22].

According to Maidin *et al.* [23], a stab test was performed on an AM material (PC-ABS) of various thicknesses, from 4.0 to 10.0 mm, using an impact energy of 24 joules to determine a minimum thickness that resulted in a maximum penetration of 7.0 mm by using Instron CEAST
9340 Drop Impact Tower. The results demonstrated that the thickness must be at least 8.0 mm for the desired results.

The research, the results of which are presented in this paper, aimed to initiate some investigations regarding the resistance to perforation of test samples made of a polymer. The analysis of the conditions in which the perforation resistance can be experimentally tested showed that it is possible to use a dynamic test based on the impact produced by the fall from a certain height of a penetrator with the material of the test sample. Adequate equipment was designed to allow the fulfillment of the specific conditions of the proposed material. Later, through the mathematical processing of the experimental results, empirical mathematical models were identified to highlight the influence of the input factors considered on the diameter size of the hole generated by the penetrator in the sample material.

## Premises for testing the perforation resistance of parts manufactured by 3D printing

For testing the resistance of the material of parts manufactured by 3D printing, it could use test schemes involving a dynamic effect (fast penetration of a penetrator into the material of the part) and quasi-static ones, where there is a slow penetration or in successive stages of the penetrator in the material of the part.

In the case of the present investigation, a dynamic test scheme was adopted based on the fall under the action of the weight of a cone-shaped penetrator from a determined height. Such a way of testing perforation resistance requires the existence of appropriate equipment. As a parameter

to evaluate the perforation resistance, the diameter of the hole generated in the test sample material by the penetrator or the penetration depth of the penetrator in the part material could be considered (Fig. 1). It is expected that for perforation-resistant materials, the diameter of the hole generated by the penetrator or the penetration depth will be smaller.

The main factors that affect the value of the output parameter specific to the perforation test under the mentioned conditions are the shape and weight of the penetrator, the height from which the penetrator falls, and some physical and mechanical properties of the material of the tested part. It is known that such physical-mechanical properties of the material of a part manufactured by 3D printing depend on the material's nature and the material's internal structure. The last factor taken into account (the physical structure of the part material) can be decisively influenced by the values of the factors that characterize the 3D printing conditions (thickness of the deposited layer, the percentage infill, the infill pattern, the type of printing network, the printing temperature, the cooling conditions, etc.).

<figure><img src="image_2359.png" /><figcaption>Fig. 1. Scheme of testing a material from the point of view of resistance to perforation</figcaption></figure>

## Experimental conditions

The experimental research aimed to identify some possibilities of highlighting the influence exerted by several input factors in the perforation process on an output parameter of this process. The diameter of the hole made by the penetrator in the test samples was chosen as such output parameter. The experimental tests were carried out using the equipment schematically represented in Fig. 2. This equipment was designed to be usable on a drilling machine. A cone-shaped penetrator (Fig. 3) was allowed to fall freely from a certain height. The release of the
9340 Drop Impact Tower. The results demonstrated that the thickness must be at least 8.0 mm for the desired results.

The research, the results of which are presented in this paper, aimed to initiate some investigations regarding the resistance to perforation of test samples made of a polymer. The analysis of the conditions in which the perforation resistance can be experimentally tested showed that it is possible to use a dynamic test based on the impact produced by the fall from a certain height of a penetrator with the material of the test sample. Adequate equipment was designed to allow the fulfillment of the specific conditions of the proposed material. Later, through the mathematical processing of the experimental results, empirical mathematical models were identified to highlight the influence of the input factors considered on the diameter size of the hole generated by the penetrator in the sample material.

## Premises for testing the perforation resistance of parts manufactured by 3D printing

For testing the resistance of the material of parts manufactured by 3D printing, it could use test schemes involving a dynamic effect (fast penetration of a penetrator into the material of the part) and quasi-static ones, where there is a slow penetration or in successive stages of the penetrator in the material of the part.

In the case of the present investigation, a dynamic test scheme was adopted based on the fall under the action of the weight of a cone-shaped penetrator from a determined height. Such a way of testing perforation resistance requires the existence of appropriate equipment. As a parameter

to evaluate the perforation resistance, the diameter of the hole generated in the test sample material by the penetrator or the penetration depth of the penetrator in the part material could be considered (Fig. 1). It is expected that for perforation-resistant materials, the diameter of the hole generated by the penetrator or the penetration depth will be smaller.

The main factors that affect the value of the output parameter specific to the perforation test under the mentioned conditions are the shape and weight of the penetrator, the height from which the penetrator falls, and some physical and mechanical properties of the material of the tested part. It is known that such physical-mechanical properties of the material of a part manufactured by 3D printing depend on the material's nature and the material's internal structure. The last factor taken into account (the physical structure of the part material) can be decisively influenced by the values of the factors that characterize the 3D printing conditions (thickness of the deposited layer, the percentage infill, the infill pattern, the type of printing network, the printing temperature, the cooling conditions, etc.).

<figure><img src="image_2350.png" /><figcaption>Fig. 1. Scheme of testing a material from the point of view of resistance to perforation</figcaption></figure>

## Experimental conditions

The experimental research aimed to identify some possibilities of highlighting the influence exerted by several input factors in the perforation process on an output parameter of this process. The diameter of the hole made by the penetrator in the test samples was chosen as such output parameter. The experimental tests were carried out using the equipment schematically represented in Fig. 2. This equipment was designed to be usable on a drilling machine. A cone-shaped penetrator (Fig. 3) was allowed to fall freely from a certain height. The release of the
<figure><img src="image_2360.png" /><figcaption>Fig. 2. Schematic representation of the equipment used to test the perforation resistance of the material of some test samples manufactured by 3D printing</figcaption></figure>

penetrator was carried out by means of a sub-assembly with an electromagnet mounted on the flange of the bushing in which the main shaft of the drilling machine rotates.

The material from which the samples were made was thermoplastic polyurethane (test samples with a diameter of 45 mm and with different thicknesses). The samples were manufactured by 3D printing on an Ultimaker 3 printer (Netherlands). The 3D geometries design included the corresponding samples' 0.4 mm and 0.8 mm thickness values. Due to the particular nature of the flexible filament produced by FormFutura, the 3D printer produced samples of 0.56 mm and 0.94 mm in thickness when using .stl files. Values are higher because of the complexity of the geometry, which contains an array of square holes of 1.5 mm, which forces the extruder to its maximum. The main printing conditions were the following: printing speed v=20mm/s, layer thickness l = 0.1mm, infill density i= 100%, zig-zag infill pattern.

The 3D model was imported as a .stl file in the Ultimate Cura version 5.5.0 software. It was decided upon a fine printing profile that uses 0.1 mm of layer height. The wall line count was set to three with the same value as the layer height for the outer wall wipe distance. Wall ordering was set from outside to inside for better dimensional accuracy. The seam alignment on the Z axis was user-specified so it could allow a back-right position. The top and bottom layers were left with default settings, but the infill density was set to 100% so it would not affect the integrity of samples by using different values and patterns. Because of the number of travels and retractions that have to be made and the special flexible nature of the filament, the infill speed was set to 20 mm/s.

The printing process, the Fused Deposition Modelling (FDM), produced unexpectedly higher dimensional values than the ones from CAD design. Results and observations about this phenomenon are going to be detailed in a future scientific paper. The 3D Python Flex 90A filament
<figure><img src="image_2351.png" /><figcaption>Fig. 2. Schematic representation of the equipment used to test the perforation resistance of the material of some test samples manufactured by 3D printing</figcaption></figure>

penetrator was carried out by means of a sub-assembly with an electromagnet mounted on the flange of the bushing in which the main shaft of the drilling machine rotates.

The material from which the samples were made was thermoplastic polyurethane (test samples with a diameter of 45 mm and with different thicknesses). The samples were manufactured by 3D printing on an Ultimaker 3 printer (Netherlands). The 3D geometries design included the corresponding samples' 0.4 mm and 0.8 mm thickness values. Due to the particular nature of the flexible filament produced by FormFutura, the 3D printer produced samples of 0.56 mm and 0.94 mm in thickness when using .stl files. Values are higher because of the complexity of the geometry, which contains an array of square holes of 1.5 mm, which forces the extruder to its maximum. The main printing conditions were the following: printing speed v=20mm/s, layer thickness l = 0.1mm, infill density i= 100%, zig-zag infill pattern.

The 3D model was imported as a .stl file in the Ultimate Cura version 5.5.0 software. It was decided upon a fine printing profile that uses 0.1 mm of layer height. The wall line count was set to three with the same value as the layer height for the outer wall wipe distance. Wall ordering was set from outside to inside for better dimensional accuracy. The seam alignment on the Z axis was user-specified so it could allow a back-right position. The top and bottom layers were left with default settings, but the infill density was set to 100% so it would not affect the integrity of samples by using different values and patterns. Because of the number of travels and retractions that have to be made and the special flexible nature of the filament, the infill speed was set to 20 mm/s.

The printing process, the Fused Deposition Modelling (FDM), produced unexpectedly higher dimensional values than the ones from CAD design. Results and observations about this phenomenon are going to be detailed in a future scientific paper. The 3D Python Flex 90A filament
<figure><img src="image_2361.png" /><figcaption>Fig. 3. The
penetrator used
for testing the
perforation
resistance of the
material of some
specimens
manufactured by
3D printing</figcaption></figure>

produced by FormFutura has a shore hardness of 90A, can elongate up to 500%, has a tensile strength of 35 MPa, and a tear strength of 90 N/mm². Prints were carried out with respect to the manufacturer's recommendations. Each set of samples included two additional test samples for validation purposes.

Each test sample was rigidly fixed in a support module located, in turn, in a sub-assembly mounted on the table of the drilling machine (Fig. 2). When designing equipment, a solution was also taken into account to allow the evaluation of the impact force. For this purpose, a sub-system with a spring was included in the sub-assembly located on the drilling machine table. The compression of the spring under the action of the impact force can be highlighted using a dial gauge. When it is not intended to evaluate the magnitude of the impact force, spring compression no longer occurs due to the use of a locking screw. The possibility of changing the weight of the penetrator (Fig. 3) has been provided by mounting one or more nuts on the central rod.

Releasing the penetrator causes it to move toward the test sample and make a hole. When the impact force is higher, due to the increase in the height *h* from which the penetrator falls or the increase in the mass of the penetrator, the hole made in the test sample has a larger diameter. The diameter of the hole was determined using an optical microscope type ISM-PM600SA, with magnification 10x-200x, short focus distance, resolution 1600x1200, and software corresponding to this microscope (ISM-PRO). An image of a hole obtained with the help of a digital microscope is shown in Fig. 4. Due to a certain fragility of the sample material, some holes did not have a circular shape but were closer to that of an irregular polygon with partially curved sides. In such situations, the diameter of the irregular polygon's circumscribed circle was considered.

**Finite element modeling of the phenomena that develop during the puncture resistance testing process**

It was intended to model, using the finite element method (FEM), the response of a polymeric circular sample to a solicitation imposed by a perforating element. Experimental tests revealed that

different deformations were recorded depending on the parameters used for 3D printing of samples, which, in visual terms, show irregulated shaped holes. By measuring hole dimensions, time-of-travel of the penetrator, and other parameters, it can establish relationships between solicitation and the response it produces. However, perforation generates stress and strain, which can be very difficult to assess by traditional methods. By simulating the same environment, it aims to achieve the same type of responses as the ones produced during experimental

tests. That would allow us to evaluate stress and strain distribution numerically.

Ansys (researcher license) was the preferred software choice for FEM analysis. It used TPU type of material retrieved from TotalMateria database. Explicit dynamics allow large deformation

<figure><img src="image_2362.png" /><figcaption>Fig. 4. Optical microscope image of a hole made by punching the 3D printed polymer sample</figcaption></figure>
<figure><img src="image_2352.png" /><figcaption>Fig. 3. The
penetrator used
for testing the
perforation
resistance of the
material of some
specimens
manufactured by
3D printing</figcaption></figure>

produced by FormFutura has a shore hardness of 90A, can elongate up to 500%, has a tensile strength of 35 MPa, and a tear strength of 90 N/mm². Prints were carried out with respect to the manufacturer's recommendations. Each set of samples included two additional test samples for validation purposes.

Each test sample was rigidly fixed in a support module located, in turn, in a sub-assembly mounted on the table of the drilling machine (Fig. 2). When designing equipment, a solution was also taken into account to allow the evaluation of the impact force. For this purpose, a sub-system with a spring was included in the sub-assembly located on the drilling machine table. The compression of the spring under the action of the impact force can be highlighted using a dial gauge. When it is not intended to evaluate the magnitude of the impact force, spring compression no longer occurs due to the use of a locking screw. The possibility of changing the weight of the penetrator (Fig. 3) has been provided by mounting one or more nuts on the central rod.

Releasing the penetrator causes it to move toward the test sample and make a hole. When the impact force is higher, due to the increase in the height *h* from which the penetrator falls or the increase in the mass of the penetrator, the hole made in the test sample has a larger diameter. The diameter of the hole was determined using an optical microscope type ISM-PM600SA, with magnification 10x-200x, short focus distance, resolution 1600x1200, and software corresponding to this microscope (ISM-PRO). An image of a hole obtained with the help of a digital microscope is shown in Fig. 4. Due to a certain fragility of the sample material, some holes did not have a circular shape but were closer to that of an irregular polygon with partially curved sides. In such situations, the diameter of the irregular polygon's circumscribed circle was considered.

**Finite element modeling of the phenomena that develop during the puncture resistance testing process**

It was intended to model, using the finite element method (FEM), the response of a polymeric circular sample to a solicitation imposed by a perforating element. Experimental tests revealed that

different deformations were recorded depending on the parameters used for 3D printing of samples, which, in visual terms, show irregular shaped holes. By measuring hole dimensions, time-of-travel of the penetrator, and other parameters, it can establish relationships between solicitation and the response it produces. However, perforation generates stress and strain, which can be very difficult to assess by traditional methods. By simulating the same environment, it aims to achieve the same type of responses as the ones produced during experimental

tests. That would allow us to evaluate stress and strain distribution numerically.

Ansys (researcher license) was the preferred software choice for FEM analysis. It used TPU type of material retrieved from TotalMateria database. Explicit dynamics allow large deformation

<figure><img src="image_2353.png" /><figcaption>Fig. 4. Optical microscope image of a hole made by punching the 3D printed polymer sample</figcaption></figure>
and were the choice for the simulation module. The 3D geometry was modeled inside SolidEdge (Teacher edition) concerning the dimensions of parts used in experimental tests. The penetrator and the ring on which the circular sample sits were assigned structural steel material, whereas the test sample with a thickness of 0.56 mm received the TPU. The setup includes default body interactions and a single bonded contact between the upper facet of the support ring and the polymeric sample's lower facet, which was assigned with an asymmetric behavior.

Mesh-wise, element order was set to linear. Because perforation appears near the center of the sample, it was needed a finer mesh in that area for FEM to capture appropriate results. The solution was a new user-defined coordinate system assigned to a node that was the penetrator's tip. Two body sizing methods based on the sphere of influence for the penetrator and the circular sample were used. Element size was set to 1 mm and 4 mm for the sphere's radius. It used a hex dominant method, which involves hexagons as the preferred type of finite element for the 3D sample. This produced 4013 nodes and 5048 elements, which are tetrahedral elements with 4 nodes (Tet4), hexahedron elements with 8 nodes (Hex8), wedge (prism) elements with 6 nodes (Wed6) and pyramidal elements with 5 nodes (Pyr5).

<figure><img src="image_2363.png" /><figcaption>Fig. 5. Graphical representations of FEM results: a) distribution of equivalent elastic stress; distribution of equivalent (von-Mises) stress.</figcaption></figure>

Conditions include a displacement value of -4 mm on Z axis for the penetrator and a fixed support assigned to the lower facet of the support ring. Analysis settings considered 0.001 s end time with explicit time integration for one load step. Erosion controls allow the solver to continue when the geometric strain limit of 100% is being achieved. Results are visual consistent with the ones retrieved from experimental tests. Equivalent elastic strain (see Fig. 5.a) peaks at 0.55338 mm/mm, whereas the equivalent stress evaluated according to von Mises criteria registers a maximum value of 162.97 MPa (see Fig. 5.b). Comparisons between the results of FEM and experimental tests are not included in the present paper as they are subjected to those mentioned above in future scientific papers. The aim of using FEM in the present paper was to highlight the distribution of stress and strain after the tensile strength of the material is overpassed and the perforation occurs within the area of interest. Deformation-wise, one may observe that the thickness of the material is slightly overpassed when the perforation occurs at high speed, thus not giving the material time to elongate as it should, according to the producer's technical data sheet. The presented results were obtained with significant use of computing power. The authors acknowledge that refinement of the results may be needed and recommend that they should be used with care.

## Experimental results

The input factors in the perforation process were the weight *w* of the penetrator, the distance *h* between the tip of the penetrator and the test sample, and the thickness *t* of the test sample, respectively. It was appreciated to carry out the experimental tests in accordance with the
and were the choice for the simulation module. The 3D geometry was modeled inside SolidEdge (Teacher edition) concerning the dimensions of parts used in experimental tests. The penetrator and the ring on which the circular sample sits were assigned structural steel material, whereas the test sample with a thickness of 0.56 mm received the TPU. The setup includes default body interactions and a single bonded contact between the upper facet of the support ring and the polymeric sample's lower facet, which was assigned with an asymmetric behavior.

Mesh-wise, element order was set to linear. Because perforation appears near the center of the sample, it was needed a finer mesh in that area for FEM to capture appropriate results. The solution was a new user-defined coordinate system assigned to a node that was the penetrator's tip. Two body sizing methods based on the sphere of influence for the penetrator and the circular sample were used. Element size was set to 1 mm and 4 mm for the sphere's radius. It used a hex dominant method, which involves hexagons as the preferred type of finite element for the 3D sample. This produced 4013 nodes and 5048 elements, which are tetrahedral elements with 4 nodes (Tet4), hexahedron elements with 8 nodes (Hex8), wedge (prism) elements with 6 nodes (Wed6) and pyramidal elements with 5 nodes (Pyr5).

<figure><img src="image_2354.png" /><figcaption>Fig. 5. Graphical representations of FEM results: a) distribution of equivalent elastic stress; distribution of equivalent (von-Mises) stress.</figcaption></figure>

Conditions include a displacement value of -4 mm on Z axis for the penetrator and a fixed support assigned to the lower facet of the support ring. Analysis settings considered 0.001 s end time with explicit time integration for one load step. Erosion controls allow the solver to continue when the geometric strain limit of 100% is being achieved. Results are visual consistent with the ones retrieved from experimental tests. Equivalent elastic strain (see Fig. 5.a) peaks at 0.55338 mm/mm, whereas the equivalent stress evaluated according to von Mises criteria registers a maximum value of 162.97 MPa (see Fig. 5.b). Comparisons between the results of FEM and experimental tests are not included in the present paper as they are subjected to those mentioned above in future scientific papers. The aim of using FEM in the present paper was to highlight the distribution of stress and strain after the tensile strength of the material is overpassed and the perforation occurs within the area of interest. Deformation-wise, one may observe that the thickness of the material is slightly overpassed when the perforation occurs at high speed, thus not giving the material time to elongate as it should, according to the producer's technical data sheet. The presented results were obtained with significant use of computing power. The authors acknowledge that refinement of the results may be needed and recommend that they should be used with care.

## Experimental results

The input factors in the perforation process were the weight *w* of the penetrator, the distance *h* between the tip of the penetrator and the test sample, and the thickness *t* of the test sample, respectively. It was appreciated to carry out the experimental tests in accordance with the
requirements of a full factorial experiment, with 3 independent variables at 2 levels of variation [24, 25]. Therefore, two values of the weight *w* of the penetrator were used (*w*<sub>min</sub>=45·10<sup>-2</sup> N, *w*<sub>max</sub>=75·10<sup>-2</sup> N), of the distance *h* between the penetrator and the test sample (*h*<sub>min</sub>=140 mm, *h*<sub>max</sub>=280 mm) and of the thickness *t* of the test sample (*t*<sub>min</sub>=0.56 mm and *t*<sub>max</sub>=0.94 mm).

<table><caption>Table 1. Conditions and results of experimental tests.</caption><thead><tr><th rowspan="2">Exp. No.</th><th colspan="3">Input factors</th><th colspan="3">Diameters of the holes made for three measurements [mm]</th><th rowspan="2">The average diameter of the holes made in the test sample, d [mm]</th></tr><tr><th>Weight, w [g]</th><th>Height, h [mm]</th><th>Thickness, t [mm]</th><th>1</th><th>2</th><th>3</th></tr></thead><tbody><tr><td>1</td><td>45</td><td>140</td><td>0.56</td><td>2.647</td><td>2.596</td><td>2.624</td><td>2.62</td></tr><tr><td>2</td><td>45</td><td>140</td><td>0.94</td><td>0.735</td><td>0.769</td><td>0.791</td><td>0.76</td></tr><tr><td>3</td><td>45</td><td>280</td><td>0.56</td><td>2.675</td><td>2.715</td><td>2.631</td><td>2.67</td></tr><tr><td>4</td><td>45</td><td>280</td><td>0.94</td><td>1.405</td><td>1.277</td><td>1.285</td><td>1.32</td></tr><tr><td>5</td><td>75</td><td>140</td><td>0.56</td><td>5.107</td><td>4.983</td><td>5.05</td><td>5.05</td></tr><tr><td>6</td><td>75</td><td>140</td><td>0.94</td><td>1.979</td><td>1.945</td><td>1.918</td><td>1.95</td></tr><tr><td>7</td><td>75</td><td>280</td><td>0.56</td><td>5.509</td><td>5.435</td><td>5.455</td><td>5.47</td></tr><tr><td>8</td><td>75</td><td>280</td><td>0.94</td><td>3.118</td><td>3.08</td><td>3.101</td><td>3.10</td></tr></tbody></table>

Some of the experimental conditions and the obtained results are listed in Table 1. A single experimental trial was performed for each set of conditions considered.

## Processing of experimental results

The experimental results were mathematically processed using specialized software [25]. The aim was to determine an empirical mathematical model that highlights the influence exerted by the three input factors in the testing process (the weight *w* of the penetrator, the height *h* from which the penetrator is released, and the thickness *t* of the test sample) on the value of the diameter *d* of the hole generated by the penetration of the penetrator into the test sample material.

Said software provides conditions for selecting the most appropriate mathematical model from five predefined models (first-degree polynomial, second-degree polynomial, power-type function, exponential function, hyperbolic function). The selection of the most appropriate model among the five possible models is performed using Gauss's criterion [25, 26]. The lower the value of Gauss's criterion, the more appropriate the mathematical model can be considered in relation to the experimental results used to determine it. By using the specialized software, it was found that the most appropriate mathematical model is a polynomial of the first degree:

<figure><img src="image_2364.png" /><figcaption>Fig. 6. The influence of the size of the weight *w* on the diameter size *d* of the hole in the test sample for different values of the height *h* (*t* = 0.55 mm)</figcaption></figure>

$$d = 2.232 + 0.0683w + 0.00389h - 5.71t, \quad (1)$$

the value of Gauss' criterion being, in this case, $S_G=0.1586501$.

Since power-type functions are commonly used in manufacturing, a mathematical model of this type was also determined:
requirements of a full factorial experiment, with 3 independent variables at 2 levels of variation [24, 25]. Therefore, two values of the weight *w* of the penetrator were used (*w*<sub>min</sub>=45·10<sup>-2</sup> N, *w*<sub>max</sub>=75·10<sup>-2</sup> N), of the distance *h* between the penetrator and the test sample (*h*<sub>min</sub>=140 mm, *h*<sub>max</sub>=280 mm) and of the thickness *t* of the test sample (*t*<sub>min</sub>=0.56 mm and *t*<sub>max</sub>=0.94 mm).

<table><caption>Table 1. Conditions and results of experimental tests.</caption><thead><tr><th rowspan="2">Exp. No.</th><th colspan="3">Input factors</th><th colspan="3">Diameters of the holes made for three measurements [mm]</th><th rowspan="2">The average diameter of the holes made in the test sample, d [mm]</th></tr><tr><th>Weight, w [g]</th><th>Height, h [mm]</th><th>Thickness, t [mm]</th><th>1</th><th>2</th><th>3</th></tr></thead><tbody><tr><td>1</td><td>45</td><td>140</td><td>0.56</td><td>2.647</td><td>2.596</td><td>2.624</td><td>2.62</td></tr><tr><td>2</td><td>45</td><td>140</td><td>0.94</td><td>0.735</td><td>0.769</td><td>0.791</td><td>0.76</td></tr><tr><td>3</td><td>45</td><td>280</td><td>0.56</td><td>2.675</td><td>2.715</td><td>2.631</td><td>2.67</td></tr><tr><td>4</td><td>45</td><td>280</td><td>0.94</td><td>1.405</td><td>1.277</td><td>1.285</td><td>1.32</td></tr><tr><td>5</td><td>75</td><td>140</td><td>0.56</td><td>5.107</td><td>4.983</td><td>5.05</td><td>5.05</td></tr><tr><td>6</td><td>75</td><td>140</td><td>0.94</td><td>1.979</td><td>1.945</td><td>1.918</td><td>1.95</td></tr><tr><td>7</td><td>75</td><td>280</td><td>0.56</td><td>5.509</td><td>5.435</td><td>5.455</td><td>5.47</td></tr><tr><td>8</td><td>75</td><td>280</td><td>0.94</td><td>3.118</td><td>3.08</td><td>3.101</td><td>3.10</td></tr></tbody></table>

Some of the experimental conditions and the obtained results are listed in Table 1. A single experimental trial was performed for each set of conditions considered.

## Processing of experimental results

The experimental results were mathematically processed using specialized software [25]. The aim was to determine an empirical mathematical model that highlights the influence exerted by the three input factors in the testing process (the weight *w* of the penetrator, the height *h* from which the penetrator is released, and the thickness *t* of the test sample) on the value of the diameter *d* of the hole generated by the penetration of the penetrator into the test sample material.

Said software provides conditions for selecting the most appropriate mathematical model from five predefined models (first-degree polynomial, second-degree polynomial, power-type function, exponential function, hyperbolic function). The selection of the most appropriate model among the five possible models is performed using Gauss's criterion [25, 26]. The lower the value of Gauss's criterion, the more appropriate the mathematical model can be considered in relation to the experimental results used to determine it. By using the specialized software, it was found that the most appropriate mathematical model is a polynomial of the first degree:

<figure><img src="image_2355.png" /><figcaption>Fig. 6. The influence of the size of the weight *w* on the diameter size *d* of the hole in the test sample for different values of the height *h* (*t* = 0.55 mm)</figcaption></figure>

$$d = 2.232 + 0.0683w + 0.00389h - 5.71t, \quad (1)$$

the value of Gauss' criterion being, in this case, $S_G=0.1586501$.

Since power-type functions are commonly used in manufacturing, a mathematical model of this type was also determined:
$$d = 0.000309w^{1.551}h^{0.401}t^{-1.670}, \qquad (2)$$

for which the value of Gauss's criterion is $S_G=0.219436$. In the case of the power function type mathematical model, the value of Gauss's criterion is higher than that of the first-degree polynomial. Still, the power type function allows for the acquisition of direct information regarding the intensity of the influence exerted by the input factors on the value output parameter $d$.

The graphical representations in Figs. 6 and 7 were made by using the mathematical model of

the first-degree polynomial type. The analysis of these graphical representations and the mathematical models constituted by equations (1) and (2) shows that, as expected, the diameter $d$ of the hole in the test sample increases when the weight $w$ and height $h$ values increase. This fact can be explained by increasing the kinetic energy when the penetrator hits the test sample material. An increase in the thickness $t$ of the test sample leads to a decrease in the diameter $d$ of the hole due to the absorption by the thicker material of the test sample of a greater amount of the kinetic energy corresponding to the impact between the penetrator and the material of the test sample. It is also found that the strongest influence on the increase in the value of the diameter $d$ is exerted by the weight $w$, which is associated with higher values of the coefficient (in Eq. (1)) and,

respectively, of the exponent (in Eq. (2)). A more suggestive illustration of the intensity of the effects exerted by the three considered input factors is possible by developing a graphic representation of the average responses of the investigated process to the values of the input factors at two levels (Fig. 8).

The average responses were determined as averages of the values of the output parameters corresponding to the placement on the same level of the values of the input factors [24].

Analysis of the graphical representation in Fig. 8 confirms that increasing the values of weight $w$ and height $h$ will increase the value of diameter $d$, while increasing the thickness $t$ of the test sample will decrease the value of diameter $d$. In this case, the intensity of the input factors' influence on the output parameter's value is evidenced by the slope of the line segment corresponding to a certain input factor.

## Conclusions

The resistance to perforation of a material means the resistance that that material opposes to the penetration of a penetrator having a certain shape. The study of specialized literature related to the

<figure><img src="image_2365.png" /><figcaption>Fig. 7. The influence of the height $h$ and the thickness $t$ of the test sample on the diameter size $d$ of the hole in the test sample ($w=75 \cdot 10^{-2} N$)</figcaption></figure>

<figure><img src="image_2366.png" /><figcaption>Fig. 8. Mean response values for the two levels of input factor variation</figcaption></figure>
$$d = 0.000309w^{1.551}h^{0.401}t^{-1.670}, \qquad (2)$$

for which the value of Gauss's criterion is $S_G=0.219436$. In the case of the power function type mathematical model, the value of Gauss's criterion is higher than that of the first-degree polynomial. Still, the power type function allows for the acquisition of direct information regarding the intensity of the influence exerted by the input factors on the value output parameter $d$.

The graphical representations in Figs. 6 and 7 were made by using the mathematical model of

the first-degree polynomial type. The analysis of these graphical representations and the mathematical models constituted by equations (1) and (2) shows that, as expected, the diameter $d$ of the hole in the test sample increases when the weight $w$ and height $h$ values increase. This fact can be explained by increasing the kinetic energy when the penetrator hits the test sample material. An increase in the thickness $t$ of the test sample leads to a decrease in the diameter $d$ of the hole due to the absorption by the thicker material of the test sample of a greater amount of the kinetic energy corresponding to the impact between the penetrator and the material of the test sample. It is also found that the strongest influence on the increase in the value of the diameter $d$ is exerted by the weight $w$, which is associated with higher values of the coefficient (in Eq. (1)) and,

respectively, of the exponent (in Eq. (2)). A more suggestive illustration of the intensity of the effects exerted by the three considered input factors is possible by developing a graphic representation of the average responses of the investigated process to the values of the input factors at two levels (Fig. 8).

The average responses were determined as averages of the values of the output parameters corresponding to the placement on the same level of the values of the input factors [24].

Analysis of the graphical

representation in Fig. 8 confirms that increasing the values of weight $w$ and height $h$ will increase the value of diameter $d$, while increasing the thickness $t$ of the test sample will decrease the value of diameter $d$. In this case, the intensity of the input factors' influence on the output parameter's value is evidenced by the slope of the line segment corresponding to a certain input factor.

## Conclusions

The resistance to perforation of a material means the resistance that that material opposes to the penetration of a penetrator having a certain shape. The study of specialized literature related to the

<figure><img src="image_2356.png" /><figcaption>Fig. 7. The influence of the height $h$ and the thickness $t$ of the test sample on the diameter size $d$ of the hole in the test sample ($w=75 \cdot 10^{-2} N$)</figcaption></figure>

<figure><img src="image_2357.png" /><figcaption>Fig. 8. Mean response values for the two levels of input factor variation</figcaption></figure>
perforation resistance of some materials in the composition of parts manufactured by 3D printing highlighted that, for various reasons, such a problem is a concern of researchers in the field of additive manufacturing. The research, the results of which are presented in this paper, aimed to obtain information regarding the perforation behavior of small-thickness samples made of thermoplastic polyurethane. A finite element modeling was used to identify some aspects of interest in relation to the processes that develop in the perforation zone. A perforation test equipment adapted to a drilling machine was designed to highlight the influence of various factors on the perforation resistance of disc-shaped test samples. As input factors, the size of the penetrator weight, the height from which the penetrator is released, and the thickness of the test sample were considered. The diameter of the hole generated in the test sample by releasing the penetrator from a certain height was measured. Through the mathematical processing of the experimental results, it was found that the factor with the greatest influence on the perforation resistance is the weight of the penetrator. Empirical mathematical models of the first-degree polynomial-type and power-type functions allowed to highlight the intensity of the influence exerted by the input factors on the resistance to perforation. It is intended to continue the research in the future by also taking into account the influence exerted by the 3D printing parameters on some sizes that characterize the resistance to perforation.

## References

[1] T. Sprinkle, The 5 most important standards in additive manufacturing. Information on https://sn.astm.org/features/5-most-important-standards-additive-manufacturing-.html#:~:text=Although%20it%20has%20since%20been,is%20still%20in%20use%20today.

[2] J. Alcisto, A. Enriquez, H. Garcia, S. Hinkson, T. Steelman, E. Silverman, P. Valdovino, H. Gigerenzer, J. Foyos, J. Ogren, J. Dorey, K. Karg, T. McDonald, O.S. Es-Said, Tensile Properties and Microstructures of Laser-Formed Ti-6Al-4V, J. Mater. Eng. Perf. 20 (2011) 203-212. https://doi.org/10.1007/s11665-010-9670-9

[3] M. Mehrpouya, A. Vosooghnia, A. Dehghanghadikolaei, B. Fotovvati, The benefits of additive manufacturing for sustainable design and production, in: Sustainable Manufacturing. Elsevier, 2021, 29-59. https://doi.org/10.1016/B978-0-12-818115-7.00009-2

[4] P. P. Kruth, Material increment manufacturing by rapid prototyping techniques, CIRP Ann. Manuf. Tech., vol. 40, no. 2 (1991) 603-614. https://doi.org/10.1016/S0007-8506(07)61136-6

[5] K. V. Wong, A. Hernandez. A Review of Additive Manufacturing, ISRN Mechanical Engineering (2012) 208760. https://doi.org/10.5402/2012/208760

[6] Information on https://www.materalseducation.org/educators/matedu-modules/docs/Materials_in_FDM.pdf

[7] A.D. Mazurchevici, D. Nedelcu, R. Popa, Additive manufacturing of composite materials by FDM technology: A review, Indian J. of Eng. & Mater. Sc., 27 (2020) 179-192

[8] Information on https://www.stratasys.com/en/stratasysdirect/technologies/3d-printing/fused-deposition-modeling/

[9] R., Melnikova1, A., Ehrmann1, K., Finsterbusch, 3D printing of textile-based structures by Fused Deposition Modelling (FDM) with different polymer materials, *IOP Conf. Ser.: Mater. Sci. Eng.*, 62 (2014) 012018. https://doi.org/10.1088/1757-899X/62/1/012018

[10] D.B. Sitotaw, D. Ahrendt, Y. Kyosev, A.K. Kabish, Additive Manufacturing and Textiles—State-of-the-Art, Applied Sciences, 10, no. 15 (2020) 5033. https://doi.org/10.3390/app10155033

[11] Information on https://advanses.com/mechanical-testing-of-3d-printed-parts-and-materials/
perforation resistance of some materials in the composition of parts manufactured by 3D printing highlighted that, for various reasons, such a problem is a concern of researchers in the field of additive manufacturing. The research, the results of which are presented in this paper, aimed to obtain information regarding the perforation behavior of small-thickness samples made of thermoplastic polyurethane. A finite element modeling was used to identify some aspects of interest in relation to the processes that develop in the perforation zone. A perforation test equipment adapted to a drilling machine was designed to highlight the influence of various factors on the perforation resistance of disc-shaped test samples. As input factors, the size of the penetrator weight, the height from which the penetrator is released, and the thickness of the test sample were considered. The diameter of the hole generated in the test sample by releasing the penetrator from a certain height was measured. Through the mathematical processing of the experimental results, it was found that the factor with the greatest influence on the perforation resistance is the weight of the penetrator. Empirical mathematical models of the first-degree polynomial-type and power-type functions allowed to highlight the intensity of the influence exerted by the input factors on the resistance to perforation. It is intended to continue the research in the future by also taking into account the influence exerted by the 3D printing parameters on some sizes that characterize the resistance to perforation.

## References

[1] T. Sprinkle, The 5 most important standards in additive manufacturing. Information on https://sn.astm.org/features/5-most-important-standards-additive-manufacturing-.html#:~:text=Although%20it%20has%20since%20been, is%20still%20in%20use%20today.

[2] J. Alcisto, A. Enriquez, H. Garcia, S. Hinkson, T. Steelman, E. Silverman, P. Valdovino, H. Gigerenzer, J. Foyos, J. Ogren, J. Dorey, K. Karg, T. McDonald, O.S. Es-Said, Tensile Properties and Microstructures of Laser-Formed Ti-6Al-4V, J. Mater. Eng. Perf. 20 (2011) 203-212. https://doi.org/10.1007/s11665-010-9670-9

[3] M. Mehrpouya, A. Vosooghnia, A. Dehghanghadikolaei, B. Fotovvati, The benefits of additive manufacturing for sustainable design and production, in: Sustainable Manufacturing. Elsevier, 2021, 29-59. https://doi.org/10.1016/B978-0-12-818115-7.00009-2

[4] P. P. Kruth, Material increment manufacturing by rapid prototyping techniques, CIRP Ann. Manuf. Tech., vol. 40, no. 2 (1991) 603-614. https://doi.org/10.1016/S0007-8506(07)61136-6

[5] K. V. Wong, A. Hernandez. A Review of Additive Manufacturing, ISRN Mechanical Engineering (2012) 208760. https://doi.org/10.5402/2012/208760

[6] Information on https://www.materialeducation.org/educators/materialeducation-modules/docs/Materials_in_FDM.pdf

[7] A.D. Mazurchevici, D. Nedelcu, R. Popa, Additive manufacturing of composite materials by FDM technology: A review, Indian J. of Eng. & Mater. Sc., 27 (2020) 179-192

[8] Information on https://www.stratasys.com/en/stratasysdirect/technologies/3d-printing/fused-deposition-modeling/

[9] R., Melnikova1, A., Ehrmann1, K., Finsterbusch, 3D printing of textile-based structures by Fused Deposition Modelling (FDM) with different polymer materials, *IOP Conf. Ser.: Mater. Sci. Eng.*, 62 (2014) 012018. https://doi.org/10.1088/1757-899X/62/1/012018

[10] D.B. Sitotaw, D. Ahrendt, Y. Kyosev, A.K. Kabish, Additive Manufacturing and Textiles—State-of-the-Art, Applied Sciences, 10, no. 15 (2020) 5033. https://doi.org/10.3390/app10155033

[11] Information on https://advances.com/mechanical-testing-of-3d-printed-parts-and-materials/
[12] F. Lambiase, S.I. Scipioni, A. Paoletti, Mechanical characterization of FDM parts through instrumented flat indentation. Int. J. Adv. Manuf. Technol., 125, (2023) 4201-4211. 10.1007/s00170-023-10992-3

[13] F. Alisafaei, C-S. Han, Indentation depth dependent mechanical behavior in polymers. Adv. Condens. Matter. Phys. (2015) 1-20. https://doi.org/10.1155/2015/391579

[14] B. Riccardi, R. Montanari, Indentation of metals by a flat-ended cylindrical punch. Mater Sci Eng: A 381, (1-2) (2004) 281-291. https://doi.org/10.1016/j.msea.2004.04.041

[15] N.A. Nassir, R.S. Birch, W.J. Cantwell, Q.Y. Wang, LQ. Liu, Z.W. Guan, The perforation resistance of glass fiber reinforced PEKK composites. Polymer Testing 72 (2018) 423-431 https://doi.org/10.1016/j.polymertesting.2018.11.007

[16] A. Liu, Y. Zhou, Y. Chen, J., Hu, B. Wang, Experimental investigation of impact resistance and compression behavior of CF/PEEK laminates after hot-press fusion repair with different stacking sequences. Polymer Composites (2023) 1-15. https://doi.org/10.1002/pc.27571

[17] C.F. Popa, T. Krausz, S.V. Galatanu, Numerical and experimental study for FDM printed specimens from PLA under IZOD impact tests, Mater. Today Proc., 78, 2 (2023) 326-330. https://doi.org/10.1016/J.MATPR.2022.11.501

[18] E.M. Keefe, J. A. Thomas, G. A. Buller, C. E. Banks, Textile additive manufacturing: An overview, Cogent Engineering, 9:1 (2022) 1-17. https://doi.org/10.1080/23311916.2022.2048439

[19] Y. Zhang, Y., Liu, X., Ji, C.E., Banks, W., Zhang, Flower-like hydroxyapatite modified carbon paste electrodes applicable for highly sensitive detection of heavy metal ions. J. Mater. Chem., 21 (2011) 7552-7554. https://doi.org/10.1039/c1jm10949a

[20] Information on https://bura.brunel.ac.uk/bitstream/2438/18700/1/FullText.pdf

[21] G.H. Loh, A. Sotayo, E. Pei, Development and testing of material extrusion additive manufactured polymer-textile composites, Fash. Text. 8, 2 (2021). https://doi.org/10.1186/s40691-020-00232-7

[22] Information on https://nvlpubs.nist.gov/nistpubs/ir/2015/NIST.IR.8059.pdf

[23] S. Maidin, S. Ying Chong, T. Kung Heing, Z. Abdullah, R. Alkahari, Stab resistant analysis of body armour design features manufactured via fused deposition modelling process, in: F. Uddin (Ed), Textile Manufacturing Processes. IntechOpen, 2019. https://doi.org/10.5772/intechopen.86439.

[24] R. Ranjit, A primer on the Taguchi Method, 2nd edition, SME, 2010.

[25] G. Crețu, Fundamentals of experimental research. Laboratory handbook. “Gheorghe Asachi” Technical University of Iași, Iași, Romania, 1992 (in Romanian).

[26] A.G. Worthing, J. Geffner, Processing the experimental data. Technical Publishing House, Bucharest, Romania, 1959 (in Romanian).
[12] F. Lambiase, S.I. Scipioni, A. Paoletti, Mechanical characterization of FDM parts through instrumented flat indentation. Int. J. Adv. Manuf. Technol., 125, (2023) 4201-4211. 10.1007/s00170-023-10992-3

[13] F. Alisafaei, C-S. Han, Indentation depth dependent mechanical behavior in polymers. Adv. Condens. Matter. Phys. (2015) 1-20. https://doi.org/10.1155/2015/391579

[14] B. Riccardi, R. Montanari, Indentation of metals by a flat-ended cylindrical punch. Mater Sci Eng: A 381, (1-2) (2004) 281-291. https://doi.org/10.1016/j.msea.2004.04.041

[15] N.A. Nassir, R.S. Birch, W.J. Cantwell, Q.Y. Wang, LQ. Liu, Z.W. Guan, The perforation resistance of glass fiber reinforced PEKK composites. Polymer Testing 72 (2018) 423-431 https://doi.org/10.1016/j.polymertesting.2018.11.007

[16] A. Liu, Y. Zhou, Y. Chen, J., Hu, B. Wang, Experimental investigation of impact resistance and compression behavior of CF/PEEK laminates after hot-press fusion repair with different stacking sequences. Polymer Composites (2023) 1-15. https://doi.org/10.1002/pc.27571

[17] C.F. Popa, T. Krausz, S.V. Galatanu, Numerical and experimental study for FDM printed specimens from PLA under IZOD impact tests, Mater. Today Proc., 78, 2 (2023) 326-330. https://doi.org/10.1016/J.MATPR.2022.11.501

[18] E.M. Keefe, J. A. Thomas, G. A. Buller, C. E. Banks, Textile additive manufacturing: An overview, Cogent Engineering, 9:1 (2022) 1-17. https://doi.org/10.1080/23311916.2022.2048439

[19] Y. Zhang, Y., Liu, X., Ji, C.E., Banks, W., Zhang, Flower-like hydroxyapatite modified carbon paste electrodes applicable for highly sensitive detection of heavy metal ions. J. Mater. Chem., 21 (2011) 7552-7554. https://doi.org/10.1039/c1jm10949a

[20] Information on https://bura.brunel.ac.uk/bitstream/2438/18700/1/FullText.pdf

[21] G.H. Loh, A. Sotayo, E. Pei, Development and testing of material extrusion additive manufactured polymer-textile composites, Fash. Text. 8, 2 (2021). https://doi.org/10.1186/s40691-020-00232-7

[22] Information on https://nvlpubs.nist.gov/nistpubs/ir/2015/NIST.IR.8059.pdf

[23] S. Maidin, S. Ying Chong, T. Kung Heing, Z. Abdullah, R. Alkahari, Stab resistant analysis of body armour design features manufactured via fused deposition modelling process, in: F. Uddin (Ed), Textile Manufacturing Processes. IntechOpen, 2019. https://doi.org/10.5772/intechopen.86439.

[24] R. Ranjit, A primer on the Taguchi Method, 2nd edition, SME, 2010.

[25] G. Crețu, Fundamentals of experimental research. Laboratory handbook. “Gheorghe Asachi” Technical University of Iași, Iași, Romania, 1992 (in Romanian).

[26] A.G. Worthing, J. Geffner, Processing the experimental data. Technical Publishing House, Bucharest, Romania, 1959 (in Romanian).
# Reprocessable vitrimeric composites metallized via cold spray: A preliminary study on the feasibility of novel hybrid structures

PERNA Alessia Serena<fnref n="1" />,ᵃ*, VISCUSI Antonio<fnref n="1" />,ᵇ, MARTONE Alfonso<fnref n="2" />,ᶜ,
PALMIERI Barbara<fnref n="2" />,ᵈ, CILENTO Fabrizia<fnref n="2" />,ᵉ, GIORDANO Michele<fnref n="2" />,ᵉ,
TUCCI Fausto<fnref n="3" />,,g, BORRELLI Domenico<fnref n="4" />,h, CARAVIELLO Antonio<fnref n="4" />,i,
SICIGNANO Nicola<fnref n="4" />,l

**Keywords:** Composite Materials, Vitrimers, Metal Coating, Cold Gas Dynamic Spray

**Abstract.** Due to their excellent mechanical properties and lightweight, fibre-reinforced thermoset composites are appealing materials for high-demand industries like aerospace or automotive. However, the inability to be reprocessed and the difficulty in repairing and recycling the thermoset matrices raise serious environmental issues and greatly increase the cost of materials. In fact, as a result of the irreversible chemical bonds formed during the curing process, it is not possible to reshape the material once it is set into its final form. In this context, the novel vitrimer polymers, characterised by intriguing mechanical and chemical properties as well as the ability to be reprocessed and recycled, have sparked increased attention in the literature [1]. Nevertheless, those composites are still limited by their poor surface properties strongly limiting their functionalities. In this scenario, surface metallisation has proved to be an intriguing opportunity to overcome those issues. Among the metallisation technologies, in recent years Cold Gas Dynamic Spray (CGDS) was widely investigated in the literature, owing to its capacity to produce metallic layers on thermo-sensitive materials as it does not exploit thermal energy to create the coatings. In this work, the possibility of producing hybrid fibre-reinforced vitrimer-based composites coated with metallic particles is analysed.

**Introduction**

The use of carbon fibre-reinforced composites is widespread across industries thanks to their many favourable properties, including lightweight, high strength, resistance to corrosion and fatigue, and low thermal expansion [1,2]. The demand for these composites has been on the rise due to the need for materials that offer a high strength-to-weight ratio in various industrial applications [3,4].

Thermoset matrices are often chosen for these composites due to their impressive combination of characteristics, including a favourable stiffness-to-weight ratio, fatigue resistance, dimensional stability, thermal stability, mechanical strength, resistance to creep, electrical insulation, and chemical resistance [5,6].

<figure><img src="image_2367.png" /></figure>

<footnote n="1">¹Department of Chemical, Materials and Industrial Production Engineering, University of Naples Federico II, Piazzale Tecchio 80, Naples, 80125, Italy</footnote>

<footnote n="2">²National Research Council of Italy (IPCB-CNR), Institute for Polymers, Composites and Biomaterials, Portici, 80055, Italy</footnote>

<footnote n="3">³Department of Industrial Engineering, University of Salerno, Via Giovanni Paolo II 132, Fisciano, Italy</footnote>

<footnote n="4">⁴Sòphia High Tech S.r.l., Via Malatesta 30 A, 80049 - Somma Vesuviana (NA) - Italy</footnote>

<footnote>ᵃalessiaserena.perna@unina.it, ᵇantonio.viscusi@unina.it, ᶜalfonso.martone@cnr.it,</footnote>

<footnote>ᵈbarbara.palmieri@ipcb.cnr.it, ᶜfabrizia.cilento@ipcb.cnr.it, ᵆmichele.giordano@cnr.it,</footnote>

<footnote n="9">⁹ftucci@unisa.it, ᵇdomenico.borrelli@sophiahightech.com,</footnote>

<footnote>ᵢantonio.caraviello@sophiahightech.com, ᵃnicola.sicignano@sophiahightech.com</footnote>
# Reprocessable vitrimeric composites metallized via cold spray: A preliminary study on the feasibility of novel hybrid structures

PERNA Alessia Serena<fnref n="1" />,ᵃ*, VISCUSI Antonio<fnref n="1" />,ᵇ, MARTONE Alfonso<fnref n="2" />,ᶜ,
PALMIERI Barbara<fnref n="2" />,ᵈ, CILENTO Fabrizia<fnref n="2" />,ᵉ, GIORDANO Michele<fnref n="2" />,ᵉ,
TUCCI Fausto<fnref n="3" />,,g, BORRELLI Domenico<fnref n="4" />,h, CARAVIELLO Antonio<fnref n="4" />,i,
SICIGNANO Nicola<fnref n="4" />,l

**Keywords:** Composite Materials, Vitrimers, Metal Coating, Cold Gas Dynamic Spray

**Abstract.** Due to their excellent mechanical properties and lightweight, fibre-reinforced thermoset composites are appealing materials for high-demand industries like aerospace or automotive. However, the inability to be reprocessed and the difficulty in repairing and recycling the thermoset matrices raise serious environmental issues and greatly increase the cost of materials. In fact, as a result of the irreversible chemical bonds formed during the curing process, it is not possible to reshape the material once it is set into its final form. In this context, the novel vitrimer polymers, characterised by intriguing mechanical and chemical properties as well as the ability to be reprocessed and recycled, have sparked increased attention in the literature [1]. Nevertheless, those composites are still limited by their poor surface properties strongly limiting their functionalities. In this scenario, surface metallisation has proved to be an intriguing opportunity to overcome those issues. Among the metallisation technologies, in recent years Cold Gas Dynamic Spray (CGDS) was widely investigated in the literature, owing to its capacity to produce metallic layers on thermo-sensitive materials as it does not exploit thermal energy to create the coatings. In this work, the possibility of producing hybrid fibre-reinforced vitrimer-based composites coated with metallic particles is analysed.

**Introduction**

The use of carbon fibre-reinforced composites is widespread across industries thanks to their many favourable properties, including lightweight, high strength, resistance to corrosion and fatigue, and low thermal expansion [1,2]. The demand for these composites has been on the rise due to the need for materials that offer a high strength-to-weight ratio in various industrial applications [3,4].

Thermoset matrices are often chosen for these composites due to their impressive combination of characteristics, including a favourable stiffness-to-weight ratio, fatigue resistance, dimensional stability, thermal stability, mechanical strength, resistance to creep, electrical insulation, and chemical resistance [5,6].

<figure><img src="image_2358.png" /></figure>

<footnote n="1">¹Department of Chemical, Materials and Industrial Production Engineering, University of Naples Federico II, Piazzale Tecchio 80, Naples, 80125, Italy</footnote>

<footnote n="2">²National Research Council of Italy (IPCB-CNR), Institute for Polymers, Composites and Biomaterials, Portici, 80055, Italy</footnote>

<footnote n="3">³Department of Industrial Engineering, University of Salerno, Via Giovanni Paolo II 132, Fisciano, Italy</footnote>

<footnote n="4">⁴Sòphia High Tech S.r.l., Via Malatesta 30 A, 80049 - Somma Vesuviana (NA) - Italy</footnote>

<footnote>ᵃalessiaserena.perna@unina.it, ᵇantonio.viscusi@unina.it, ᶜalfonso.martone@cnr.it,</footnote>

<footnote>ᵈbarbara.palmieri@ipcb.cnr.it, ᶜfabrizia.cilento@ipcb.cnr.it, ᵆmichele.giordano@cnr.it,</footnote>

<footnote n="9">⁹ftucci@unisa.it, ᵇdomenico.borrelli@sophiahightech.com,</footnote>

<footnote>ᵢantonio.caraviello@sophiahightech.com, ᵃnicola.sicignano@sophiahightech.com</footnote>
Despite being widely used, carbon fibre-reinforced composites can experience mechanical damage in applications involving significant forces, high pressure, temperature, friction, and vibration [7,8]. This damage, which includes creeps, cracks, fractures, and punctures, decreases the lifespan of the product and raises the possibility of unforeseen failures.

Additionally, the insoluble and infusible properties of epoxy resins make it difficult to identify and fix microcracks. The irreversible chemical bonds formed during curing further limit recycling options, as existing techniques often lead to the degradation of the polymer matrix or the retrieval of just the fibres [9,10].

A potential solution to these challenges and the development of more sustainable methods involve the use of vitrimer materials as a viable alternative for recyclable thermoset composites [11]. These materials combine the resilience of thermosets with the ability to be reprocessed at high temperatures. One interesting approach to recycling thermoset composites is by customizing the chemical structure of the thermoset polymer matrix prior to its final production. Extensive efforts have been devoted to creating thermosets that are reprocessable, healable, and recyclable in the last twenty years [10,11]. This has involved incorporating reversible non-covalent interactions and reversible covalent exchanges into polymeric networks. Among these approaches, covalent adaptable networks (CANs) have emerged as a favourable option, as they offer a combination of strong mechanical integrity and thermoplastics-like behaviour at high temperatures [12].

Vitrimers were introduced in 2011 in the work of Leibler et al., as a novel type of polymeric material, utilizing associative covalent adaptable networks (CANs) [13]. These vitrimers were created by introducing a transesterification catalyst to an epoxy/acid polyester network, which allowed for control over the exchange kinetics. Considered the third class of polymer materials, alongside thermoplastics and thermosets, vitrimers exhibit similar thermal and mechanical properties to conventional thermosets at typical operating temperatures. However, when subjected to temperatures above the topology freezing transition temperature (Tv), vitrimers undergo rapid exchange reactions, resulting in a rearrangement of their molecular structure and quick stress relaxation [14,15]. Consequently, epoxy vitrimers can flow like a viscoelastic fluid when subjected to external forces, with their viscosity gradually decreasing as the temperature rises following an Arrhenius-type law.

In this work, experimental research was conducted on a novel epoxy vitrimer formulation optimized for compatibility with the fibres and the metallic coating. The composites produced were then metallized via CGDS employing aluminium powders. The samples produced were then analyzed in terms of surface properties. The influence of the fibres on the coating deposition process was thoroughly investigated.

## Materials and Methods

For this experimental campaign, panels were produced using a combination of carbon fibres and a vitrimeric matrix. The formulation of the vitrimer was carefully studied and optimized to ensure compatibility with the fibres. The resin utilized is a combination of DGEBA (Bisphenol A diglycidyl ether), epoxy resin, THMPA (tetrahydro-methyl phthalic anhydride) curing agent, and of a 2,4,6-tris dimethylaminomethyl-phenol as catalyst, all obtained by Huntsman Corporation. To enhance the vitrimeric behaviour, anhydrous zinc acetate $Zn(Ac)_2$ is added as a transesterification catalyst to modify the standard epoxy system.

The formulation is prepared with equal amounts of epoxy and acyl, with a 10% weight of $Zn(Ac)_2$ relative to the total acyl groups. The manufacturing process involves mixing the epoxy resin and $Zn(Ac)_2$ powder by hand, followed by the addition of cross-linking anhydride and mixing using a planetary centrifugal mixer under vacuum at room temperature.

The accelerator (amine) is then added and further mixing is performed to achieve a homogeneous mixture. The mixture is then cast into Teflon moulds (by hand lay-up) and
Despite being widely used, carbon fibre-reinforced composites can experience mechanical damage in applications involving significant forces, high pressure, temperature, friction, and vibration [7,8]. This damage, which includes creeps, cracks, fractures, and punctures, decreases the lifespan of the product and raises the possibility of unforeseen failures.

Additionally, the insoluble and infusible properties of epoxy resins make it difficult to identify and fix microcracks. The irreversible chemical bonds formed during curing further limit recycling options, as existing techniques often lead to the degradation of the polymer matrix or the retrieval of just the fibres [9,10].

A potential solution to these challenges and the development of more sustainable methods involve the use of vitrimer materials as a viable alternative for recyclable thermoset composites [11]. These materials combine the resilience of thermosets with the ability to be reprocessed at high temperatures. One interesting approach to recycling thermoset composites is by customizing the chemical structure of the thermoset polymer matrix prior to its final production. Extensive efforts have been devoted to creating thermosets that are reprocessable, healable, and recyclable in the last twenty years [10,11]. This has involved incorporating reversible non-covalent interactions and reversible covalent exchanges into polymeric networks. Among these approaches, covalent adaptable networks (CANs) have emerged as a favourable option, as they offer a combination of strong mechanical integrity and thermoplastics-like behaviour at high temperatures [12].

Vitrimers were introduced in 2011 in the work of Leibler et al., as a novel type of polymeric material, utilizing associative covalent adaptable networks (CANs) [13]. These vitrimers were created by introducing a transesterification catalyst to an epoxy/acid polyester network, which allowed for control over the exchange kinetics. Considered the third class of polymer materials, alongside thermoplastics and thermosets, vitrimers exhibit similar thermal and mechanical properties to conventional thermosets at typical operating temperatures. However, when subjected to temperatures above the topology freezing transition temperature (Tv), vitrimers undergo rapid exchange reactions, resulting in a rearrangement of their molecular structure and quick stress relaxation [14,15]. Consequently, epoxy vitrimers can flow like a viscoelastic fluid when subjected to external forces, with their viscosity gradually decreasing as the temperature rises following an Arrhenius-type law.

In this work, experimental research was conducted on a novel epoxy vitrimer formulation optimized for compatibility with the fibres and the metallic coating. The composites produced were then metallized via CGDS employing aluminium powders. The samples produced were then analyzed in terms of surface properties. The influence of the fibres on the coating deposition process was thoroughly investigated.

## Materials and Methods

For this experimental campaign, panels were produced using a combination of carbon fibres and a vitrimeric matrix. The formulation of the vitrimer was carefully studied and optimized to ensure compatibility with the fibres. The resin utilized is a combination of DGEBA (Bisphenol A diglycidyl ether), epoxy resin, THMPA (tetrahydro-methyl phthalic anhydride) curing agent, and of a 2,4,6-tris dimethylaminomethyl-phenol as catalyst, all obtained by Huntsman Corporation. To enhance the vitrimeric behaviour, anhydrous zinc acetate $Zn(Ac)_2$ is added as a transesterification catalyst to modify the standard epoxy system.

The formulation is prepared with equal amounts of epoxy and acyl, with a 10% weight of $Zn(Ac)_2$ relative to the total acyl groups. The manufacturing process involves mixing the epoxy resin and $Zn(Ac)_2$ powder by hand, followed by the addition of cross-linking anhydride and mixing using a planetary centrifugal mixer under vacuum at room temperature.

The accelerator (amine) is then added and further mixing is performed to achieve a homogeneous mixture. The mixture is then cast into Teflon moulds (by hand lay-up) and