Additive manufacturing and energy-harvesting
performance of honeycomb-structured magnetostrictive
Fe 52 -Co 48 alloys

Hiroki Kurita, Paul Lohmuller, Pascal Laheurte, Kenya Nakajima, Fumio Narita

► To cite this version:

Hiroki Kurita, Paul Lohmuller, Pascal Laheurte, Kenya Nakajima, Fumio Narita. Additive manufacturing and energy-harvesting performance of honeycomb-structured magnetostrictive Fe 52 -Co 48 alloys. Additive Manufacturing, 2022, 54, pp.102741. 10.1016/j.addma.2022.102741. hal-04129592

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# Additive manufacturing and energy-harvesting performance of honeycomb-structured magnetostrictive Fe₅₂–Co₄₈ alloys

Hiroki Kurita<sup>a, *</sup>, Paul Lohmuller<sup>b</sup>, Pascal Laheurte<sup>b, *</sup>, Kenya Nakajima<sup>c</sup>, Fumio Narita<sup>a</sup>

<sup>a</sup> Department of Frontier Sciences for Advanced Environment, Graduate School of Environmental Studies, Tohoku University, Aoba-yama 6-6-02, Sendai 980-8579, Japan

<sup>b</sup> Laboratoire d'Etude des Microstructures et de Mécanique des Matériaux LEM3 UMR CNRS 7239, Université de Lorraine, Arts et Métiers ParisTech Campus de Metz, F-57078 Metz-Cedex 01, France

<sup>c</sup> Department of Materials Processing, Graduate School of Engineering, Tohoku University, Aoba-yama 6-6-02, Sendai 980-8579, Japan

## ARTICLE INFO

### Keywords:

Magnetostrictive

Laser powder bed fusion process (LPBF)

Energy harvesting

Vibration

Impact

## ABSTRACT

Energy harvesting using magnetostrictive materials is currently attracting significant attention. The additive manufacturing process can produce such materials with excellent energy-harvesting performance and various required properties, including mechanical properties—by optimizing the lattice structure of the material. This study optimized the manufacturing parameters of a laser powder bed fusion process to produce a magnetostrictive Fe₅₂–Co₄₈ alloy. We evaluated the energy-harvesting performance of a Fe₅₂–Co₄₈ alloy plate with a honeycomb structure subjected to vibrations and impacts. We compared the results with a fully dense structure and found that the honeycomb structure resulted in a lower resonant frequency. In addition, the honeycomb structure exhibited a power density 4.7 times higher than that of the fully dense structure in a vibration test and 4.9 times higher than that in an impact test. The honeycomb structure is thus an attractive structure for obtaining power efficiently. Furthermore, honeycomb structures and other designs can reduce the weight of a Fe₅₂–Co₄₈ alloy plate and improve its sensitivity for use as a particulate-matter sensor.

## 1. Introduction

The magnetostrictive effect—which involves a change in the shape of a material in an applied magnetic field—was discovered by Joule in 1842 [1]. Subsequently, Villari discovered that changes in the magnetic flux density could be caused by applying stress to a ferromagnetic material, and this phenomenon is well known as the inverse-magnetostrictive or Villari effect.

Among magnetostrictive materials, Tb-Dy-Fe alloys (Terfenol-D) and Fe-Ga alloys (Galfenol) have attracted attention owing to their outstanding magnetostriction of 800–1600 and 400 ppm, respectively. However, Terfenol-D is brittle and expensive, and Galfenol has low workability. Consequently, Yamaura et al. [2] have developed Fe-Co alloys by forging and cold working and have investigated the effects of alloy composition and heat treatment. Fe-Co alloys exhibit high strength and ductility, and hence, they can be processed easily as wires and embedded in resin [3,4]. In this way, non-contact stress sensors using the inverse magnetostrictive phenomenon—i.e., utilizing the stress-induced magnetic flux density—have been developed.

In addition, energy harvesting, which converts mechanical energy into electrical energy, is also attracting attention because changes in the stress-induced magnetic flux density can be converted into voltage using a coil [5–8]. In recent years, Liu et al. [9] have prototyped a laminated cantilever beam consisting of a Galfenol plate and an elastic plate, and they have investigated the effects on the vibration power-generation performance of the laminated configuration, the material and dimensions of the flexible plate, the position and number of bias magnets, and the load resistance. Under optimized conditions, they obtained a maximum power of 13.3 mW and a maximum power density of 3.7 mW·cm⁻³ g⁻¹. Meng et al. [10] analyzed a hyperbolic Galfenol cantilever beam, and they compared the output voltage generated by bending vibrations with those produced by rectangular, trapezoidal, and triangular cantilever beams. They showed that a hyperbolic beam is suitable for harvesting low-frequency vibrational energy. When they connected a load resistance of 50 Ω, they obtained 403.8 μW of power. Also, Liu et al. [11] developed a vibration-energy-harvesting device using a laminated Galfenol cantilever beam, and they obtained a maximum power density of 7.4 mW·g⁻¹ cm⁻³ optimizing it in terms of the bias magnetic field and
stacking number.

On the other hand, Tan et al. [12] mounted a magnetostrictive material under a floor, and they obtained 31.3 mW of electric power from humans walking on the floor. They found that the harvested energy can simultaneously power up to 100 light-emitting diodes. In addition, Yamaura et al. [13] developed a shaft-type-vibration-energy-harvesting device with a shock-sliding structure using a magnetostrictive Fe-Co-V (Permendur) rod, and they obtained output energy of 1592 µJ. Further, Seino et al. [14] produced Fe-Co wire/Al alloy composite materials and evaluated the output power generated by a compressive load. Yang et al. [15] twisted two Fe-Co wires together and embedded them in an Al matrix, and they reported an increase in the output power. Also, Yang et al. [16] fabricated twisted Fe-Co wire/epoxy magnetostrictive composites and used them to explore the effect of the structural design of the composites on their magnetostrictive properties. Wide applications of such magnetostrictive materials are thus expected, and further studies are being carried out [17–20].

As mentioned previously, magnetostrictive materials are generally brittle, which limits the design of complex structures. The development of additive manufacturing (AM) processes—especially laser-based AM—allows the production of complex structures, such as lattices structures [21] and topologically optimized components [22]. Some studies have dealt with the development of AM of magnetostrictive materials such as Fe-Co alloys [23–25], Fe-Co-V alloys [26–28], Fe-Si alloys [29], and Fe-Ga alloys [30]. These previous studies have reported that morphological control of the lattice structure allows the selection of optimized mechanical properties such as rigidity [21], stress concentration [31], and anisotropy [32]. However, only a few studies have focused on the influence of complex structures on the magnetostrictive properties of materials. Scheidler and Dapino [30] have demonstrated that the stiffness and resonant frequency of a composite Galfenol/Al 6061 alloy can be tuned by adjusting the fraction of each constituent material. To our knowledge, however, no study to date has dealt with the influence of architecture on the energy-harvesting performance of magnetostrictive materials.

This study produced and compared the performance of additively manufactured Fe<sub>52</sub>-Co<sub>48</sub> alloys and compared them to commercial materials. Furthermore, we investigated the effect of the honeycomb structure of a plate on the vibration and impact-harvesting performance compared to that of a fully dense plate.

## 2. Experimental procedure

### 2.1. Additive manufacturing of magnetostrictive Fe-Co alloys

Fe<sub>52</sub>-Co<sub>48</sub> alloy cubes [10 × 10 × 9 mm<sup>3</sup>, see Fig. 1(a)] were produced by a laser-powder-bed fusion (LPBF) process (SLM 280 HL, SLM Solutions Group AG) in an argon atmosphere. We used Fe<sub>52</sub>-Co<sub>48</sub> alloy powders (TIZ Advanced Alloy Technology Co. Ltd., China) with a D50 particle size of 39 µm. All specimens were built on a S355 steel platform. Han et al. [33] have investigated that the magnetostriction for the whole spray-cast Fe<sub>(100-x)</sub>-Co<sub>x</sub> binary series and revealed that the

magnetostriction increased with an increase in Co content and reached about 110 ppm for Co compositions between 40 and 60 at%. Fe<sub>52</sub>-Co<sub>48</sub> is one of the reasonable ratios, and the companies (including TIZ Advanced Alloy Technology Co. Ltd.) commercially fabricate Fe-Co alloy powder with this ratio. The processing of new materials with metallic AM technologies often requires the development of process parameters, which generally include four main parameters: the laser power (P), scanning speed (v), hatching distance (h), and layer thickness (t). The volume energy density (E), defined by Eq. (1) below, is frequently used to compare different sets of parameters:

$$E = \frac{P}{vht} \qquad (1)$$

In this investigation, based on previous studies [26,27], we prepared five specimens using different laser outputs and scanning speeds to determine the appropriate energy density. Table 1 shows the experimental conditions we used for manufacturing the specimens. The scanning strategy comprises two border paths and a filling path consisting of back-and-forth scans over a maximal length of 10 mm. For each successive slice, we rotated the scanning paths around the manufacturing direction by an angle of 67° in order to avoid path overlay [Fig. 1(b)].

### 2.2. Characterization of manufactured Fe-Co alloys

Two sections of each specimen were cut along the building direction. Samples were polished using SiC paper with grit size from #600 to #4000, followed by Al<sub>2</sub>O<sub>3</sub> polishing with a final particle size of 0.1 µm, and finally cleaned with ethanol. We observed the porosity of the Fe<sub>52</sub>-Co<sub>48</sub> alloy cubes on two sections along the building direction using optical microscopy (Zeiss Axio Imager, Carl Zeiss Microscopy). We evaluated the microstructure of each Fe<sub>52</sub>-Co<sub>48</sub> alloy cube using scanning electron microscopy (Zeiss Supra 40, Carle Zeiss Microscopy) and X-ray diffraction (XRD) measurements (D8 Brucker, Brucker Corporation) using CoKα radiation. The accelerating voltage and current were 40 kV and 13 mA, respectively. Furthermore, we used energy-dispersive X-ray (EDX) spectroscopy to evaluate the Fe and Co concentrations in the Fe<sub>52</sub>-Co<sub>48</sub> alloy cubes (EDX, Brucker Corporation). We subsequently prepared both fully dense and honeycomb Fe<sub>52</sub>-Co<sub>48</sub> alloy plates with dimensions of 70 × 5 × 1.6 mm<sup>3</sup>. We controlled the wall thickness and cell width of the honeycomb plates to be 250 µm and 2.5 mm, respectively. We used electron backscatter diffraction (EBSD), with the minimal number of detected Kikuchi bands equal to 8, a step size of 0.4 µm, at 20 kV, with a working distance of 21 mm (Oxford instrument) to map the microstructures of the Fe<sub>52</sub>-Co<sub>48</sub> alloys. We evaluated the crystal orientation and grain size of the fully dense Fe<sub>52</sub>-Co<sub>48</sub> alloy plates using Atex software [34].

To investigate the vibration and impact-energy-harvesting performance of the Fe<sub>52</sub>-Co<sub>48</sub> alloy plates, we measured the power density, the output power divided by the volume of the alloy. Fig. 2 shows a schematic illustration of the vibration and impact-energy-harvesting performance tests. The vibration generation system consists of a shaker (ET-

<figure><img src="image_2.png" /><figcaption>Fig. 1. (a) Fe<sub>52</sub>-Co<sub>48</sub> alloy cubes manufactured using different parameters. (b) Schematic representation of the scanning strategy used.</figcaption></figure>
<table><caption>**Table 1**
Manufacturing parameters for LPBF.</caption><thead><tr><th></th><th>Parameter name in this study</th><th>Laser power<br>P (W)</th><th>Scanning speed<br>v (mm/s)</th><th>Hatch space<br>h (mm)</th><th>Thickness<br>t (mm)</th><th>Energy density<br>E (J·mm<sup>-3</sup>)</th></tr></thead><tbody><tr><td rowspan="3">Varying power</td><td>P1V1</td><td>200</td><td>1000</td><td>0.08</td><td>0.03</td><td>104.2</td></tr><tr><td>P2V1</td><td>250</td><td>1000</td><td>0.08</td><td>0.03</td><td>130.2</td></tr><tr><td>P3V1</td><td>300</td><td>1000</td><td>0.08</td><td>0.03</td><td>156.3</td></tr><tr><td rowspan="2">Varying velocity</td><td>P1V2</td><td>200</td><td>775</td><td>0.08</td><td>0.03</td><td>134.4</td></tr><tr><td>P1V3</td><td>200</td><td>660</td><td>0.08</td><td>0.03</td><td>157.8</td></tr></tbody></table>

<figure><img src="image_3.png" /></figure>

<figure><img src="image_4.png" /><figcaption>**Fig. 2.** Schematic illustration of (a) the vibration and (b) the impact-energy-harvesting tests.</figcaption></figure>

132, Labworks Inc., USA), a linear power amplifier (PA-151, Labworks Inc., USA) and a function generator (33250A, Agilent Technologies Inc., USA) to control the waveform and frequency of the output vibration. The acceleration was fixed at 20 m/s². This study employed sinusoidal vibration. 25 mm from the edge of fully dense and honeycomb Fe₅₂–Co₄₈ alloy plates were fixed to the shaker (i.e. the effective length was 45 mm) and then connected with the data logger (NR-HA08, Keyence Corporation, Japan), with a sampling frequency of 50 kHz and a resistance of 1 MΩ to gain the output voltage during the vibration, which was applied between 200 and 600 Hz. For the impact-energy-harvesting test, 25 mm from the edge of fully dense and honeycomb Fe₅₂–Co₄₈ plates were vertically fixed to the mold (i.e. the effective length was 45 mm). 3 specimens were prepared for each structure. Then the fully dense and honeycomb Fe₅₂–Co₄₈ alloy plates and impulse hammer (GK-3100, Ono Sokki Co. Ltd., Japan) were connected with the same data logger (NR-HA08, Keyence Corporation, Japan) for vibration-energy-harvesting tests. Hence, the impact stress produced by the impulse hammer and the output voltage of the specimen can be recorded by a computer. Usually, in order to obtain a large output voltage and power, it is necessary to rotate the magnetic domain of the Fe₅₂–Co₄₈ alloy plate as much as possible in the coil. Therefore, experiments were conducted so that large compressive stresses would be applied to the elongated plate-shaped structures. The coil resistance was 11.42 kΩ, the load resistance was 11.72 kΩ, the coil contained 28,000 turns, and the coil diameter was 0.05 mm. Before the vibration and impact-energy-harvesting tests, we first determined the resonant frequency and the optimal resistance needed to generate the maximum output power from the fully dense and honeycomb-structured Fe₅₂–Co₄₈ alloy plates.

### 3. Results and discussion

Fig. 3 shows the XRD patterns of the Fe₅₂–Co₄₈ alloy cubes obtained using each set of manufacturing parameters. The profile of the alloy

<figure><img src="image_5.png" /><figcaption>**Fig. 3.** XRD patterns of Fe₅₂–Co₄₈ alloy cubes manufactured using each parameter set.</figcaption></figure>

obtained using each set of process parameters contains three strong diffraction peaks, which correspond to the (110), (200), and (211) crystal planes. The profile consists of three strong diffraction peaks, which correspond to (110), (200) and (211) crystal planes of a body-centered cubic (bcc) phase for each process parameter. The constant lattice value was estimated to be 0.2852 nm which is in close agreement with the value of 0.2855 nm corresponding to an arc melted FeCo [35].

Fig. 4 shows the secondary electron image and the EDX maps of the Fe₅₂–Co₄₈ alloy cube manufactured using the P2V1 parameters. The microstructure appears to be homogeneous, without precipitation or
<figure><img src="image_6.png" /><figcaption>Fig. 4. Secondary electron image and EDX maps of the Fe₅₂–Co₄₈ alloy cube manufactured using P2V1.</figcaption></figure>

chemical segregation. Fig. 5 shows the porosity relationship to the energy density of the Fe₅₂–Co₄₈ alloy cubes manufactured using each parameter set. The relative density of each Fe₅₂–Co₄₈ alloy cube exceeded 99.5%, regardless of the manufacturing parameters. The densities of the cubes tended to increase with the volume energy density, except for the case with 300 W of power, where a keyhole regime can be expected. We observed cracks in various Fe₅₂–Co₄₈ alloy cubes [see Fig. 6(a), (c), (d), and (e)]. However, there was no crack in the one fabricated using P2V1 [see Fig. 6(b)]. The colored circles were attributed to residual ethanol remains in porosities. Cracks may come from the accumulation of residual thermomechanical stress during processing. The positions of the cracks in the bottom plane, relative to the building direction, can be partially avoided by using a more robust support structure. Consequently, P2V1 seems to be the best parameter set to use with LPBF for fabricating Fe₅₂–Co₄₈ alloys, and we employed these parameters (which correspond to an energy density of 130.2 J·mm⁻³) to manufacture the fully dense and honeycomb-structured Fe₅₂–Co₄₈ alloy plates.

Fig. 7 shows the EBSD map, the crystal orientation, and the grain size of the Fe₅₂–Co₄₈ alloy cube manufactured using P2V1. The average grain size was 1.5 µm, with a significant fraction of small equiaxial grains, and grains larger than 5 µm exhibit shapes elongated along the manufacturing direction.

Fig. 8 shows the appearance of the Fe₅₂–Co₄₈ alloy plates manufactured using P2V1. We have succeeded in using LPBF to manufacture

plates with both fully dense and honeycomb structures, although the plates were slightly warped during cooling in the air. Fig. 9(a) shows the relationships of the output voltage to the frequency of the Fe₅₂–Co₄₈ alloy plates in the vibration-energy-harvesting tests. The resonant frequencies of the fully dense and honeycomb-structured Fe₅₂–Co₄₈ alloy plates were 487 and 293 Hz, respectively. This result indicates that the structural change of a Fe₅₂–Co₄₈ alloy plate shifts its resonant frequency, with the honeycomb structure having a lower resonant frequency than the fully dense one. Because vibration frequencies in daily life tend to be low, vibration-energy-harvesting devices should have a low resonant frequency. We also investigated the relationship of power density to resistance at these resonant frequencies [see Fig. 9(b)]. The power densities of the fully dense and honeycomb structures were 0.81 and 3.80 µw·cm⁻³, respectively; i.e., the honeycomb structure exhibited a power density 4.7 times larger than that of the fully dense one in the vibration tests. Yang et al. [36] reported that the maximum output voltage of a notched FeCo/Ni-clad plate cantilever was 2.4 times larger than that of a cantilever without a notch. They attributed this result to the stress concentration produced by the notch. Therefore, it seems likely that a high-stress level is also responsible for the remarkable power density obtained from the honeycomb structure. A honeycomb-structured plate thus seems to work effectively in generating electricity from the viewpoint of both the resonant frequency and the power density. However, the honeycomb-structured plates failed after being subjected to vibration cycles for several minutes. It is, therefore, necessary to enhance the honeycomb structure; alternatively, there are probably other optimal structures for harvesting vibration energy.

Fig. 10 shows the power densities of Fe₅₂–Co₄₈ alloy plates in impact-energy-harvesting tests. The power densities of the fully dense and honeycomb structures were 6.99 and 34.0 µm·cm⁻³, respectively; i.e., the honeycomb structure displayed a power density 4.9 times larger than that of the fully dense one in impact tests. However, note that this value is qualitative because the impacts were applied hand in this study. During the impact tests, the honeycomb-structured plates did not fail.

It is well known that magnetostrictive materials also can be used as particulate matter sensors by utilizing the shift in resonant frequency or output voltage [37–39]. The sensitivity of a magnetostrictive particulate-matter sensor is dominated by its weight. Thus, such a sensor must be lightweight to obtain high sensitivity. Hence, it is likely that the honeycomb and other designed structures can be used to provide both high energy-harvesting performance and high sensitivity as a particulate matter sensor.

## 4. Conclusion

This study focused on AM of a Fe₅₂–Co₄₈ alloy. We determined the

<figure><img src="image_7.png" /><figcaption>Fig. 5. Relationship of porosity to the energy density of Fe₅₂–Co₄₈ alloy cubes manufactured using each parameter set.</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>Fig. 6. Optical microscope images of Fe₅₂–Co₄₈ alloy cubes manufactured using (a) P1V1, (b) P2V1, (c) P3V1, (d) P1V2, and (e) P1V3.</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>Fig. 7. (left) EBSD map, (middle) crystal orientation, and (right) grain size of the Fe₅₂–Co₄₈ alloy cube manufactured using P2V1.</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>Fig. 8. Appearance of Fe₅₂–Co₄₈ alloy plates manufactured using P2V1. The (a) fully dense and (b) honeycomb structures. Note that the black line in (a) is the marker to determine where we cramp it.</figcaption></figure>

optimized process parameters for obtaining a homogeneous micro-
structure, low porosity, and no cracks. We evaluated the vibration and
impact energy-harvesting performance for fully dense plates and those
with honeycomb structures. The honeycomb structure shifts the reso-
nant frequency to a lower value. Furthermore, the honeycomb structure
showed power densities 4.7 times larger than fully dense ones in vi-
bration tests and 4.9 times larger in impact tests. However, the increased
rate of power density is not essential. Because there are various types
among the honeycomb structures and all honeycomb structures do not
show the same energy-harvesting performance, this study employed the
honeycomb structure, which the LPBF process could indeed
<figure><img src="image_11.png" /><figcaption>Fig. 9. (a) Relationship of output voltage to frequency and (b) relationship of power density at the resonant frequency to the resistance of the Fe₅₂–Co₄₈ alloy plates manufactured using P2V1, for both the fully dense and honeycomb structures. This study plotted the value every 0.1 Hz and obtained both sets of measurements in the vibration-energy-harvesting tests.</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>Fig. 10. Power densities in impact energy harvesting tests of the Fe₄₉–Co₅₁ alloy plates manufactured using P2V1, for both the fully dense and honeycomb structures.</figcaption></figure>

manufacture.

The honeycomb structure is thus attractive for generating electricity effectively. However, structural optimization is required to avoid failure of the Fe₅₂–Co₄₈ alloy plate and to explore the potential range of vibration and impact-energy-harvesting allowed by the structured material. In addition, the use of a material structure reduces the weight of a Fe₅₂–Co₄₈ alloy plate and probably also increases its sensitivity as a particulate-matter sensor. Numerical method is efficient for searching for optimal structures and are currently under development to support the conclusion.

CRediT authorship contribution statement

**Nakajima Kenya:** Data curation, Investigation, Writing – original draft. **Laheurte Pascal:** Writing – review & editing, Supervision, Project administration, Funding acquisition, Conceptualization. **Narita Fumio:** Writing – review & editing, Validation, Supervision. **Lohmuller Paul:** Data curation, Formal analysis, Investigation, Writing – review & editing. **Kurita Hiroki:** Writing – original draft, Visualization, Supervision, Project administration, Funding acquisition, Formal analysis, Data curation, Conceptualization, Validation.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This research was supported by the Tohoku University–Université de Lorraine Joint Research Fund 2020 from Tohoku University and University of Lorraine, and by Grant-in-Aid for Scientific Research (A) (grant number 19H00733) from the Japan Society for the Promotion of Science.

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