# Microstructural and Mechanical Properties of AFSDed Materials

Ismail Zabeeullah Kolimi, Julie Marteau, Salima Bouvier

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Ismail Zabeeullah Kolimi, Julie Marteau, Salima Bouvier. Microstructural and Mechanical Properties of AFSDed Materials. 2025. hal-05320194

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# Microstructural and Mechanical Properties of AFSDed Materials

Authors : Ismail Zabeeullah KOLIMI¹*, Julie MARTEAU¹, Salima BOUVIER¹

¹ Université de technologie de Compiègne, Laboratoire Roberval (Mechanics Energy and Electricity), Centre de recherches de Royallieu, CS 60319 – 60203 Compiègne Cedex, France

Microstructural and Mechanical Properties of AFSDed Materials © 2025 by Ismail Zabeeullah KOLIMI, Julie MARTEAU & Salima BOUVIER is licensed under Creative Commons Attribution 4.0 International

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## Introduction

In recent years, a number of solid-state metal additive manufacturing (AM) technologies have emerged, offering a low-energy pathway to produce near-net-shape metallic components with reduced heat input, lower thermal gradients, and minimized residual stresses. Unlike conventional fusion-based AM processes, such as powder bed fusion (PBF) or directed energy deposition (DED), which rely on selective melting and rapid solidification, solid-state techniques operate below the melting temperature of the material. This fundamental distinction eliminates many metallurgical challenges associated with solidification, including hot cracking, high residual stresses, and porosity.

Among the various solid-state processes developed in the past decade, such as cold spray additive manufacturing (CSAM), ultrasonic additive manufacturing (UAM), friction stir additive manufacturing (FSAM), and additive friction stir deposition (AFSD), the latter has received increasing attention for its ability to combine severe plastic deformation and controlled thermomechanical bonding in a single step. The AFSD process integrates the friction stir principle with a continuous material feeding system, enabling the deposition of bulk, fully dense metallic structures with excellent bonding integrity and fine recrystallized microstructures in the as-deposited state.

This report provides a comprehensive overview of key research findings across several alloy systems, including aluminium, magnesium, stainless steels, duplex stainless steels, Inconel 625, and copper alloys, highlighting the relationships between processing parameters, microstructure, and resulting mechanical properties.

E-mail address: kolimiis@utc.fr (I.Z.Kolimi)

<footnote>* Corresponding author at Roberval Laboratory, Université de Technologie Compiègne, CS 60319 – 60203 Compiègne, France</footnote>
# 1. Aluminium Alloys

Aluminium alloys are well-known for their lightweight, corrosion-resistant, and electrically conductive properties, making them a popular choice in industries that require materials with low density and high strength, such as aerospace, automotive, and electronics. These alloys are generally divided into two groups: non-age hardenable alloys (such as 1, 3 and 5 series) and age-hardenable alloys (such as 2, 6 and 7 series). Despite their many advantages, aluminium alloys present challenges in terms of thermal sensitivity and weldability, especially with conventional fusion-based processes, as the steep cooling rates associated with the process, these alloys tend to micro-crack during processing, resulting in parts with poor structural integrity [1]. These limitations, however, are mitigated when aluminium alloys are processed using the AFSD technique, which offers enhanced control over thermal cycles and material flow.

<figure><img src="image_4.png" /><figcaption>Figure 1: Optical micrographs of FSPAM made multi-layer deposition of Al 6061 alloy: (a) cross-section, (b) third deposition layer, and (c) first deposition layer [2].</figcaption></figure>

Several studies have highlighted the potential of AFSD for improving the mechanical properties and microstructure of aluminium alloys. For instance, Chaudhary et al. [2] explored the deposition of Al6061 alloy using Friction Stir Powder Additive Manufacturing (FSPAM), a novel
variation of AFSD. The study demonstrated that the deposition process, when performed at temperatures of up to 417 °C, resulted in the formation of fine sub-grains and a homogeneous distribution of the alloying elements, particularly Mg₂Si and Al precipitates, as shown in Figure 1. These microstructural improvements contributed to enhanced mechanical properties, with the AFSDed Al6061 exhibiting tensile strength and hardness comparable to the commercially available AA6061-T4 alloy, as reflected in Figure 2.

<figure><img src="image_5.png" /><figcaption>Figure 2: Stress-strain curve of multi-layer deposition of Al 6061 alloy and AA6061-T4 Al alloy [2].</figcaption></figure>

Li et al. [3] studied the use of AFSD for cladding aluminum 6061 Al alloy-T651 onto annealed 316 stainless steel, focusing on the influence of deposition strategies on the strength and adhesion between the two materials. They observed that overlapping deposition provided adequate material flow and eliminated the microstructural defects, like holes in the boundary region observed in the AFSD Al-clad steel sample without overlapping, as shown in Figure 3. They found that employing an overlapping deposition strategy significantly improved the bonding strength, achieving a shear strength of 109.0 MPa, which was superior to single-pass deposition. This approach holds promise for large-scale AFSD applications in cladding, joining, and coating, expanding the versatility of AFSD for aluminium alloys in various structural applications.
<figure><img src="image_6.png" /><figcaption>Figure 3: Optical micrograph of the cross-section of (a) the Single-pass deposition (S), (b) Overlap deposition of 6 mm (O-6), and (c) Overlap deposition of 8 mm (O-8) samples; (d–k) High magnification scanning electron microscope (SEM) under backscattered electron (BSE) mode located in (a–c) [3].</figcaption></figure>

In another study, Gong et al. [4] examined the effect of preheating temperatures on the bonding strength in composite plates made from 6061-T6 aluminium alloy and TA1 titanium alloys. The results revealed that higher preheating temperatures (up to 200 °C) enhanced the interfacial diffusion and shear strength of the bond. The study underscores the importance of process optimization for achieving strong, reliable bonds in aluminium-based composites.
<figure><img src="image_7.png" /><figcaption>Figure 4: Schematic illustration of the different stages of the repair and cross-sectional dimensions of the blind hole, which is repaired using AFSD [5].</figcaption></figure>

The repair of high-strength aluminium alloys, such as the 7075 aluminium alloy, is another area where AFSD has shown significant promise. Wang et al. [5] demonstrated that AFSD could successfully repair the 7075 alloy, overcoming common defects observed in conventional welding processes, such as solidification cracking. The illustration for repair is shown in Figure 4. Moreover, post-repair heat treatment (PRHT) further enhanced the tensile strength of the repaired specimens, although the elongation was slightly reduced due to precipitate aggregation at the interface, as reflected in Figure 5.

<figure><img src="image_8.png" /><figcaption>Figure 5: Engineering stress-strain curve with fractured tensile [5].</figcaption></figure>
Griffith et al. [6] explored the relationship between process parameters and microstructure in AFSD of both copper and aluminium-magnesium-silicon alloys. They observed that aluminium-based alloys processed with AFSD exhibited significant geometric and dynamic recrystallization as Al has high stacking fault energy (SFE), leading to a homogenous microstructure and well-developed sub-grain structures. In contrast, the copper material (Cu 110) showed more heterogeneous grain structures, with variations in grain size and lower shear texture due to Discontinuous Dynamic Recrystallization (DDRX) as reflected in Figure 6.

<figure><img src="image_9.png" /><figcaption>Figure 6: AFSD Deposition along with feedstock and deposited microstructure and recrystallization mechanisms [6].</figcaption></figure>

Microstructural refinement during AFSD is a critical factor influencing the mechanical properties of aluminium alloys. For example, Zeng et al.[7] observed a significant reduction in grain size from 163.6 µm to 3.3 µm in aluminium alloys (Al 6061) processed using AFSD. The as-deposited Al6061 parts underwent T6 heat treatment. The process consisted of two steps. First, during the solution treatment, the samples were heated to 530 °C at a rate of 10 °C/min, held at that temperature for 45 minutes, and then quenched in room-temperature water. This was followed by artificial ageing, where the samples were heated to 175 °C at the same rate, held for 8 hours, and finally quenched in room-temperature water. Heat treatment after deposition led to further grain growth, but the overall grain size remained much smaller compared to the base material, as shown in Figure 7. Quantitatively, the grain sizes for the Feedstock, AD, and ADHT samples were 163.5 ± 96.2 µm, 8.5 ± 3.1 µm, and 12.5 ± 5.2 µm, respectively. This grain refinement enhanced the
mechanical properties, although hardness decreased after heat treatment due to the coarsening of precipitates.

<figure><img src="image_10.png" /><figcaption>Figure 7: EBSD inverse pole figure (IPF) orientation maps of the (a) feedstock, (b) as-deposited (AD) sample and (c) as-Deposited Heat Treated (ADHT sample (c)). The EBSD image for the feedstock is at a 10-time lower magnification [7].</figcaption></figure>

Similarly, Philips et al. [8] demonstrated that the AFSD process led to a 96% reduction in grain size in aluminium alloy 5053, resulting in improved tensile strength. However, challenges such as carbon contamination during deposition were noted, which hindered interlayer bonding and caused premature failure. This highlights the importance of controlling the deposition environment to avoid defects that may undermine the mechanical properties of the final product.

The mechanical performance of AFSDed aluminium alloys can also be influenced by the type of feedstock used. Beck et al.[9] compared the effects of using solid rods and chips as feed materials for AFSD of AA5083 aluminium alloy. The results showed that chips led to a greater reduction in grain size and an increase in tensile strength. However, the fatigue performance was found to be similar for both feed materials, suggesting that AFSD can produce components with comparable fatigue resistance regardless of the feedstock used.

In a study by Rivera et al. [10] precipitation-hardened AA2219 aluminium alloy was successfully deposited using AFSD. The results indicated that the top layers of the deposited structure exhibited the highest tensile and yield strength, which were superior to the base material, as shown in Figure 8. Additionally, the mechanical properties of the deposited AA2219 material showed improvement under both quasi-static and high-strain-rate conditions, demonstrating the versatility of AFSD for producing high-strength aluminium alloys with superior mechanical performance.
<figure><img src="image_11.png" /><figcaption>Figure 8: As-deposited tensile results comparing deposition layer dependency in (A) quasi-static and (B) high rate. For both strain rates, the plots identify that the top layer is strongest, followed by the middle and bottom layers [10].</figcaption></figure>

Finally, Zhu et al. [11] and Li et al. [12] examined the residual stress distribution in AFSDed
aluminium alloys, particularly AA6061 and Al–Li alloys. They found that the top layers of the
deposited structures exhibited higher residual stresses, which were attributed to the thermal cycles
and ageing effects during the deposition process. These residual stresses can affect the long-term
performance of AFSDed components, making it essential to optimize process parameters and post-
deposition heat treatments to minimize their impact.

## 2. Magnesium Alloys

Magnesium alloys are renowned for their excellent damping properties, high specific
strength, and low density, making them ideal for lightweight applications in aerospace, automotive,
and biomedical industries. However, magnesium alloys also present challenges related to their poor
formability and weldability due to their high reactivity with oxygen and moisture at elevated
temperatures, leading to oxidation and potential degradation. The AFSD process offers a promising
solution for overcoming these challenges by enabling the deposition of magnesium alloys with
fine, homogeneous microstructures. Furthermore, AFSD can be conducted under a protective
environment, such as an inert gas atmosphere, to mitigate oxidation and ensure high-quality
deposition.

Extensive studies on AZ31 magnesium alloys have demonstrated the potential of AFSD to
enhance the microstructural and mechanical properties of these materials. Robinson et al. [13]
observed that the AFSD process significantly intensified texture in the X–Z and Y–Z planes
compared to the feedstock, while the X–Y plane retained a texture intensity similar to the feedstock
but lacked fiber texture, as shown in Figure 9. Here, X denotes the transverse direction, Y
corresponds to the tool traverse (longitudinal) direction, and Z represents the build direction during AFSD processing. Furthermore, quasi-static tensile tests revealed that AFSD AZ31B samples exhibited mechanical properties and microstructures comparable to forged counterparts, with notable improvements over rolled plates, O-temper, and additive friction stir layer welding (AFSLW) samples, as reflected in Figure 10.

<figure><img src="image_12.png" /><figcaption>Figure 9: Representative texture of feedstock in the (a) X-Y, (b) X-Z, and (c) Y-Z planes. Representative texture of AFSD bulk deposition region in the (a) X-Y, (b) X-Z, and (c) Y-Z planes [13].</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>Figure 10: Stress-strain plot comparing the quasi-static tensile test results of the AFSD Mg AZ31B-F, extruded feedstock, rolled plate, O Temper, and AFSLW [13].</figcaption></figure>
Joshi et al. [14] produced AZ31B magnesium alloy samples using AFSD under different processing parameters (400 rpm at 6.3 mm/min and 400 rpm at 4.2 mm/min), achieving a density of ≥ 99.4%. The AFSD process induced a slight increase in grain size and the evolution of a (0001) texture at the surface, as shown in Figure 11. Compared to the feedstock, AFSD samples exhibited a reduction in Mg₁₇Al₁₂ fraction, decreased 0.2% proof stress by approximately 30 MPa, and lower tensile strength by 10–30 MPa, alongside a 4–10% reduction in elongation. As the traverse speed decreased, a marginal increase in average grain size (from 15 ± 4 µm to 18 ± 3 µm) was observed, accompanied by a corresponding decline in mechanical properties.

<figure><img src="image_14.png" /></figure>

<figure><img src="image_15.png" /><figcaption>Figure 11: The EBSD results containing IPF and PF, and the distributions of grain size from (a) feedstock, (b) 82 J/mm2, and (c) 116 J/mm2 samples. The 82 J/mm2 sample was associated with a process parameter of 400 rpm and 4.2 mm/s. The 116 J/mm2 sample was associated with a process parameter of 400 rpm and 6.3 mm/s [14].</figcaption></figure>
Luo et al. [15] demonstrated the successful preparation of multilayer deposits of high-performance Mg-8Gd-3Y-0.5Zr alloys via force-controlled AFSD, referred to as Friction Extrusion Additive Manufacturing (FEAM). Their results highlighted a non-uniform grain distribution across the deposit width, as shown in Figure 12. The advancing side (AS) exhibited a coarse-grained zone with an average grain size of 12.8 ± 1.5 µm, leading to reduced fine-grain strengthening and a lower UTS of 269.9 MPa. Conversely, the middle and retreating side (RS) zones displayed significant grain refinement, resulting in higher UTS values of 284.4 MPa and 295.4 MPa, respectively.

<figure><img src="image_16.png" /><figcaption>Figure 12: Optical Microscope pictures of (a-c) middle regions, (d-f) coarse grain area formed on AS and (g) starting materials, (h) layer-by-layer grain size statistics, and (i) XRD results [15].</figcaption></figure>
Further work by Williams et al. [16] investigated the influence of process parameters on WE43 magnesium alloy deposits. A process parameter study involving several four-layer builds identified an optimized window for producing 68-layer WE43 structures with minimal surface defects and high interlayer bonding. The AFSD WE43 samples exhibited fine and uniform microstructures and texture variations compared to the forged material. While AFSD WE43 showed a reduction in hardness and tensile properties relative to forgings, the fatigue performance under high-cycle conditions remained comparable. Low-cycle fatigue performance, however, showed a decline, reflecting microstructural sensitivity to cyclic loading.

McClelland et al. [17] compared AFSD WE43 samples to various conditions reported in the literature, including underaged and T6-tempered WE43. The AFSD-produced material exhibited higher yield strength (YS) and ultimate tensile strength (UTS) than the underaged and T6-tempered versions while maintaining similar elongation (EL). These results underscore the ability of AFSD to manufacture load-bearing structural components with performance comparable to conventionally processed magnesium alloys.

## 3. SS316 Stainless Steel

Agrawal et al. [18] explored the deposition of SS316 stainless steel using the AFSD process, focusing on the optimization of processing parameters, microstructural evolution, and recrystallization behaviour. The study revealed that the deposited SS316 exhibited an equiaxed ultrafine-grained microstructure, with an average grain size of approximately 5.0 ± 0.5 µm, which is significantly smaller than that of the feedstock material, as shown in Figure 13.
<figure><img src="image_17.png" /><figcaption>Figure 13: (1) shows the location of the imaging area with respect to build direction; (a1), (a2) are inverse pole figure and inverse pole figure triangle for the base material; (b1), (b2); (c1), (c2); (d1), (d2) are inverse pole figure and inverse pole figure triangles for samples S1 (400RPM), S2 (600 RPM) and S3 (800RPM) respectively [18].</figcaption></figure>

This microstructural refinement was a result of the high-temperature shear deformation incurred during processing, which triggered the recovery mechanism and promoted dynamic recrystallization (DRX) during deposition. The authors observed a unique "necklace-like" microstructure in the deposited SS316, attributed to discontinuous dynamic recrystallization (DDRX), a mechanism where new grain boundaries form during deformation. The results of the study demonstrated that the recrystallization kinetics of SS316 during AFSD were influenced by thermal cycles induced by the friction stir effect. These thermal cycles varied with the process parameters, leading to fluctuations in temperature, strain, and strain rate, which in turn influenced the microstructure and tool wear. The variable recrystallization kinetics were quantitatively modelled using the Johnson-Mehl-Avarami-Kolmogorov (JMAK) model, providing insights into the evolution of the microstructure during the AFSD process.

One of the most notable findings of the study was the synergetic improvement of both strength and ductility in the AFSDed SS316. In the study, the yield strength (YS) of the base material was found to be 406 ± 5 MPa, while the YS of the AFSD samples ranged from 410 to 425 MPa. The total elongation of the AFSD samples shows a significant improvement, ranging from 70% to 79%, compared to the base material's 48%, nearly 1.5 times increase in ductility. The combination of deformation-induced transformations and the occurrence of twinning contributed
to this improvement, along with the formation of a gradient in the twinning and martensitic phases
following deformation. This work-hardening effect allowed the material to maintain high strength
while improving its ductility, making AFSD a promising technique for enhancing the mechanical
performance of SS316 stainless steel.

4. DSS2507 Duplex Stainless Steel

Gor et al. [19] successfully applied the AFSD process to DSS2507 duplex stainless steel, studying the microstructural evolution and mechanical properties of the deposited material. The study found that the AFSD process had a profound impact on the banded microstructure of the feedstock material. The austenite grains in the deposited DSS2507 exhibited a fine, uniform, and equiaxed structure, while the ferrite grains were somewhat larger and elongated, as shown in Figure 14. The microstructure was significantly influenced by the thermo-mechanical conditions created during AFSD, which induced both discontinuous dynamic recrystallization (DDRX) in the austenite phase and continuous dynamic recrystallization (CDRX) in the ferrite phase.
<figure><img src="image_18.png" /><figcaption>Figure 14: EBSD phase maps and individual Kernel Average Misorientation (KAM) maps of austenite and ferrite phase of as-deposited DSS2507 a, b, c) BD-WD orientation from the top, middle, and bottom region, respectively [19].</figcaption></figure>

<figure><img src="image_19.png" /><figcaption>Figure 15: Mechanical properties characterized by a) microhardness variation with distance from the top in the as-deposited material. b) shows the engineering stress vs strain plots of the as-received and as-deposited materials (the</figcaption></figure>
insert shows the schematic of the insert shows the schematic of the tensile specimens with all dimensions in mm)
[19].

The mechanical properties of the AFSD-deposited DSS2507 were found to be superior to
those of other additive manufacturing (AM) DSS2507 parts. The improvements in both strength
and ductility were attributed to the fine, equiaxed microstructure and the absence of excessive σ
phase precipitation in the upper layers, as shown in Figure 15. These results suggest that AFSD
processing, when optimized, can effectively enhance the mechanical performance of duplex
stainless steels, offering an alternative to traditional welding methods that can be detrimental to the
material's microstructure and properties.

5. Inconel 625

Rivera et al. [20] studied the deposition of Inconel 625 using the AFSD process, observing a significant refinement in the grain structure of the deposited material. The grain size of the AFSD-deposited Inconel 625 was reduced to 1 µm, compared to the 30 µm grain size of the feedstock. This grain refinement was accompanied by the formation of banded microstructures, which were attributed to the interface layers between each deposited layer. These interfacial layers exhibited an even finer grain size, around 0.25 µm, as shown in Figure 16. Tensile tests on the as-deposited AFSD material showed higher yield strength and ultimate tensile strength (UTS) compared to both wrought and cast Inconel 625 materials. The mechanical behaviour produced an average yield strength of 730 MPa, an ultimate tensile strength (UTS) of 1072 MPa, and a strain to failure of 0.32. The improvement in mechanical properties is attributed to the refined microstructure and the reduction in the size and distribution of the M23C6 and M6C carbides.

In further work by Avery et al. [21], the AFSD process was shown to cause significant
changes in the carbide distribution within Inconel 625. The severe plastic deformation encountered
in the shear zone during deposition resulted in the breakdown of M23C6 and M6C carbides into
smaller, more uniformly distributed fractions, which contributed to the enhancement of mechanical
properties. Fatigue testing revealed that the AFSD-processed Inconel 625 exhibited higher fatigue
life compared to the feedstock material, laser additively manufactured samples, and cast Inconel
625 samples. The fatigue life improvement was primarily attributed to the grain refinement and
carbide distribution. However, some delamination was observed between interlayers during testing,
which led to failures at very low cycles for two of the samples, though no deposition parameters
were provided in the study.
<figure><img src="image_20.png" /><figcaption>Figure 16: (A) Euler EBSD maps representative location on sample, (B) Euler EBSD maps of 5 locations along build direction, all images are at the same magnification and same location in the corresponding axis, (C) Average grain size and number of grains (location correlates with the EBSD map locations in A and B) [20].</figcaption></figure>

## 6. Alloy Cu 110

Priedeman et al. [22] studied the deposition of Alloy Cu 110 using the AFSD process at a tool rotational speed of 275 rpm and a traverse speed of 127 mm/min. The study observed the formation of some porosity at the interface between the substrate and the first deposited layer, as well as oxidation on the outer surfaces of the deposit. No oxidation was observed in the interior, suggesting that the oxide layer formed during the cooling phase after deposition. The grain size
and hardness of the AFSD-deposited Cu 110 material were found to be lower than those of the feedstock, attributed to dynamic recrystallization during the AFSD process. The hardness of the deposited material was found to range from 63 to 102 HV, lower than that of the feedstock, which was expected due to the refinement of the microstructure.

Griffiths et al. [6] also investigated the deposition of Cu 110 and reported similar findings. However, one parameter combination, involving a high tool rotational speed (600 rpm) and high traverse speed (with specific parameters not provided), resulted in a larger grain size in the deposited material compared to the feedstock, as shown in Figure 6. This study highlights the influence of processing parameters on the final microstructure of the deposited copper alloy.

## 7. Conclusion

In summary, Additive Friction Stir Deposition (AFSD) has emerged as a promising solid-state manufacturing technique capable of processing a wide range of metallic materials, including aluminium, magnesium, stainless steels, Inconel 625, and copper alloys. The process offers distinct advantages over conventional fusion-based methods, such as refined microstructures, improved mechanical properties, and reduced defect formation. Across different material systems, AFSD promotes dynamic recrystallization, enhances grain refinement, and contributes to superior strength-ductility combinations, making it a viable option for structural and high-performance applications.

Key findings indicate that AFSD enables the deposition of aluminium and magnesium alloys with enhanced mechanical properties, overcoming challenges related to thermal sensitivity and oxidation. Stainless steels and duplex stainless steels exhibit microstructural modifications that improve strength and ductility, while Inconel 625 benefits from grain refinement and carbide redistribution, leading to enhanced fatigue performance. Additionally, the process demonstrates its ability to deposit copper alloys with controlled grain structures and mechanical stability.

Despite these benefits, challenges such as residual stress development, interlayer bonding, and process optimization remain critical considerations for further research. Continued advancements in AFSD process parameters, post-deposition treatments, and alloy-specific modifications will be essential to unlocking the full potential of this technique for industrial applications.
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