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Article

Microstructure Evolution and Mechanical Performance of AA6061-7075 Heterogeneous Composite Fabricated via Additive Friction Stir Deposition

by
Qian Qiao
1,
Hongchang Qian
2,
Zhong Li
2,
Dawei Guo
1,3,*,
Chi Tat Kwok
4,
Shufei Jiang
5,
Dawei Zhang
2 and
Lam Mou Tam
1,3,4,*
1
IDQ Science and Technology (Hengqin Guangdong) Co., Ltd., Zhuhai 519031, China
2
National Materials Corrosion and Protection Data Center, Institute for Advanced Materials and Technology, University of Science and Technology Beijing, Beijing 100010, China
3
Institute for the Development and Quality, Macao 999078, China
4
Department of Electromechanical Engineering, University of Macau, Macao 999078, China
5
Aerospace Engineering Equipment (Suzhou) Co., Ltd., Suzhou 215000, China
*
Authors to whom correspondence should be addressed.
Alloys 2025, 4(4), 21; https://doi.org/10.3390/alloys4040021
Submission received: 5 July 2025 / Revised: 22 August 2025 / Accepted: 27 August 2025 / Published: 30 September 2025
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Abstract

An AA6061-7075 composite with a heterogeneous structure was fabricated via the additive friction stir deposition (AFSD) method, and in situ processing data were monitored during the manufacturing process. The results show that the cross-section of the composite subjected to AFSD exhibits a lower degree of plastic deformation behavior compared to the surface and side of the composite, owing to serious heat accumulation during the layer-by-layer stacking process. The denser, heterogeneous structure, consisting of finer (softer) and coarser (harder) grains, which correspond to AA6061 and AA7075, was formed according to transmission electron microscopy (TEM) analysis. Furthermore, the obtained composite subjected to AFSD in this work presents outstanding mechanical properties compared to other as-fabricated AA6061/AA7075 depositions acquired by other additive manufacturing methods along the horizontal building direction, with the ultimate tensile strength (266 MPa) being 89% of that of AA6061-T6 and the elongation 1.1 times that of AA7075-T6. The findings provide useful guidelines for the in situ preparation of Al-based composites and offer ideas for manufacturing high-strength heterostructures for large-scale practical engineering applications.

1. Introduction

Al-based alloys are extensively applied across various fields, including the engineering and aerospace industries, owing to their cost-effectiveness, good machinability, and superior recyclability [1,2]. However, Al alloys also exhibit certain drawbacks, such as insufficient strength and poor corrosion resistance [3]. To address these limitations and fulfill the requirements of practical engineering applications, it is essential to explore alternative solutions.
On the one hand, additive friction stir deposition (AFSD), an innovation additive manufacturing (AM) technique, has been introduced [4]. It is different from the friction stir additive manufacturing (FSAM) method, joining layers sequentially based on friction stir welding (FSW) principles, exhibiting limited flexibility, and resulting in specimens with shapes that are significantly restricted by cumbersome multi-plate assembly procedures [5,6]. Nonetheless, the AFSD process could be regarded as friction surfacing (FS), utilizing a hollow rotating tool enclosed with the tip of the feedstock [7]. The hollow tool restricts the formation of flash, and the pins on the end of the hollow tool penetrate the deposition and stir the interface of the coatings to increase the interlayer bonding strength. Therefore, the surface quality as well as the mechanical properties of deposition could be enhanced using this approach. Hartley and co-workers reported that the Al-Mg-Si deposition acquired by AFSD displayed excellent surface quality and a higher ultimate tensile strength (UTS), with a value of about 250 MPa [8]. Tang and co-workers found that the unit propagation energy (UPE) of the deposited AA6061 specimen was about 132% that of AA6061-T6, meaning that the tear strength of the deposition was clearly improved [9]. On the other hand, forming heterostructures was proven to be an effective technique for improving material properties by creating a significant difference to achieve the desired stress distributions and mechanical performance [10,11]. Heterogeneous structures with superior performance have garnered considerable interest from researchers; however, most current approaches involve incorporating hard particles to induce stress differences, leading to increased dislocations and enhanced work hardening, thereby resulting in improved mechanical properties [12]. For example, Zan et al. fabricated an Al-based heterogeneous composite with the addition of Al2O3, and the corresponding strength was enhanced by 121% compared to pure Al, attributed to the formed lamella heterogeneous structure [13]. Additionally, a method named friction stir vibration processing (FSVP) has been introduced as a modified version of friction stir processing (FSP) to fabricate composites with refined grain structure and enhanced shear punching strength, such as the AZ91-SiC composite [14] and Al5052-SiC specimens [15].
Despite the existing method’s ability to generate high-quality components, it entails substantial investment expenditures and is limited to producing small-sized deposits. Moreover, the current technology exhibits constraints in production efficiency, failing to satisfy the actual industry demand for big components, which is facing long fabrication times and large production costs. Additionally, research on the manufacture of high-performance heterostructure components based on Al-based alloys utilizing the AFSD technique remains scarce, and a comprehensive exploration of the relationship between their microstructure and mechanical properties needs to be conducted. Thus, the objective of this work is to obtain structural heterogeneity of AA6061 and AA7075 alloys in an in situ manner via AFSD. An innovative method for preparing heterogeneous composite structures is proposed, in which the deposition efficiency is superior, reaching up to 9.3 kg/h, and faster than AFSD for seven-series Al alloys [7]. The 6061–7075 composite deposit (in its as-fabricated state) achieved a high strength similar to that of 6061-T6 without the need for heat treatment, therefore improving environmental sustainability and efficiency, reducing processing steps, and facilitating the rapid production of large-scale components by AFSD. The microstructure and mechanical properties, such as the microhardness and tensile properties, of the fabricated composite are investigated in depth. Furthermore, the performance of the composite is also compared with the previous related achievements. The findings of this work may serve as a technical reference for the efficient preparation of large-size components with heterostructures and for recommendations of composite materials used in AFSD and other AM applications.

2. Experimental Methods

As displayed in Figure 1A, the AFSD process was conducted on modified solid-state manufacturing equipment (MSAM-B20X15, Aerospace Engineering Equipment (Suzhou) Co., Ltd., Suzhou, China) with a rotation speed of 600 rpm, a feed speed of 35 mm/min, and a layer thickness of 1 mm. Before depositing, an integrated feedstock was prepared before fabricating the 6061–7075 composite. The AA6061 and AA7075 specimens, both in the T6 state and measuring 7.25 × 14.5 × 400 mm3, were welded using friction stir welding equipment (A50, Aerospace Engineering Equipment (Suzhou) Co., Ltd., Suzhou, China) into an integrated feedstock (Figure 1A).
The chemical compositions of AA6061-T6 and AA7075-T6 feedstocks, and the integrated feedstock, were tested according to the optical emission spectrometric analysis of aluminum and aluminum alloys (Spectro Lam-M10, Kleve, Germany). The integrated feedstock has an average composition level for AA6061-T6 and AA7075 alloys and is detailed in Table 1 [16], providing adequate conditions for the preparation of composite deposited components. A 32 mm diameter AFSD in situ process monitoring kit (iKIT-AFSWD-Gen-BT50, without protrusions, IDQ Science and Technology (Hengqin, Gungdong) Co., Ltd., Zhuhai, China) was used to monitor the processing data, i.e., temperature, upsetting force (Fups), spindle force (Fspi), and spindle torque (Mspi). The detailed structure and monitoring accuracy of the kit have been described in detail in our previous papers [17,18]. The deposition path is shown in Figure 1A: the feedstocks were deposited on the substrate via the in situ process monitoring kit and using a hollow tool at a traverse speed of 65 mm/min along the LD. After traveling for the required distance, the kit was lifted in place and continued to deposit along the same distance in the −LD. The whole process is continuous; by repeating the above steps, a multi-layer deposition with 12 layers was obtained, and the obtained deposition was named AFSDed-composite for further analysis. Photographs of the depositions are displayed in Figure 1B, which shows that two types of depositions were manufactured to analyze the corresponding tensile properties along horizontal (HD) and vertical (VD) directions.
Sampling diagrams of various tests are shown in Figure 1B. Specimens at the middle of the AFSDed-composite were sampled for further observation. Before observation, the specimens were ground and polished using monocrystalline diamond suspensions (EcoMet 30, Buehler, Lake Bluff, IL, USA) and etched with Kroll’s solution. Then, the microstructure and texture evolution of the AFSDed-composite were detected using an optical microscope (OM, GX71, Olympus, Tokyo, Japan), a scanning electron microscope (SEM, S-4300N, Hitachi, Hitachi, Japan), and a field emission scanning electron microscope (FESEM, Zigma 300, Zeiss, Oberkochen, Germany), each equipped with electron back scattered diffraction detectors (EBSDs, Symmetry S2, Oxford, UK). Transmission electron microscopy (TEM, FEI Tecanai G2-F30, Hillsboro, OR, USA) characterization was conducted on a focused ion beam (FIB) lamella of the AFSDed-composite through an FIB Helios 5 UX, eventually thinned to electron transparency. High-angle annular dark field (HAADF), high-resolution transmission electron microscopy (HRTEM) and EDS detection have been explored, and Gatan Micrograph software (Version 3.5) was applied to filter noise and perform fast Fourier transform (FFT) and inverse fast Fourier transform (IFFT) on the obtained images. Meanwhile, referring to ISO standards [19], the AA6061-T6, AA7075-T6, and AFSDed-composite specimens, with a ratio of surface area to volume of 1.5 cm2/mL, were immersed in nitric acid with distilled water (pH = 5~6) to detect the content of the released ions for distinguishing and proving the difference in their composition. Then, the concentration of Cu2+, Mg2+, and Zn2+ ions in the testing solution was detected by inductively coupled plasma mass spectrometry (ICP-MS, ICPOES730, Perkin Elmer, Waltham, MA, USA).
For observation of mechanical properties, firstly, the microhardness of the cross-section of the AFSDed-composite was tested with a microhardness tester (Qpix Control2, Qness, Austria). The microhardness distribution at the center of the cross-section of the composite was tested, and the distance between adjacent indentations was 0.5 mm. The applied load was set at 500 g and the dwelling time was 10 s. Secondly, the tensile tests of the specimens along the HD and VD were carried out at a 2 mm/min tensile speed with a universal tensile machine (WDW-20, Shanghai Bairoe Test Instrument Co., Ltd., Shanghai, China) (Figure 1B). Referring to the ASTM E8/E8M standard, specimens measured 3 mm in thickness, 10 mm in width, and 15 mm in gauge length [20]. Meanwhile, a dynamic strain gauge extensometer with a gauge length of 15 mm was affixed to each specimen during testing to detect 3% tensile strain. To guarantee the consistency and precision of the results, several tensile tests were performed to derive the associated strain–stress curves, the yield strength (YS), the ultimate tensile strength (UTS), and the elongation (EL).

3. Results and Discussion

3.1. In Situ Monitoring Data

Figure 2 displays the temperature, Fups, Fspi, and Mspi profiles of the AFSDed-composite monitored using the in situ process monitoring kit. During the AFSD process, after the feedstock reaches the substrate, the increased temperature is detected because of the friction heating, and then the feedstock is yielded and extruded to fill the space between the tool head and substrate. The above process leads to an increase in the values of temperature, force, and torque (time = 0~300 s) (Figure 2). In the steady-state deposition process (time = 300~1800 s), the temperature remains at a stable value (469.1 ± 17.5 °C, Figure 2a). It is noted that the average temperature of the AFSDed-composite is lower than that of the AFSDed 6061 as obtained in the previous work, which is about 485.8 ± 0.7 °C [18]. The decreased temperature is because of the difference in heat capacity between AA6061 and AA7075, with that of the latter (Cp = 0.96 J/gC) being higher than that of AA6061 (Cp = 0.89 J/gc) [21,22], meaning that the amount of heat required to increase the given degree temperature of the AFSDed-composite is higher than the heat required for AA6061-T6. The value of Fups is steady at 26.8 ± 6.7 kN, indicating the stabilization of the feeding process (Figure 2b). However, Fspi clearly increases with the increase in time, and the average Fspi is 11.7 ± 0.9 kN because of the material flow behavior. The accumulated softened material induces shear strain at the interface, resulting in increased torsional strain and Fspi during the layer-by-layer stacking process. In agreement with the findings of Shreyash and his co-workers [23], simulations of the thermophysical effects and calculations of the strain rate gradient during the AFSD process confirmed that the average strain rate escalated from 21 s−1 to 170 s−1 as the layer number increased from 1 to 4, leading to an elevated Fspi. Additionally, the average Mspi is 61.2 ± 5.3 Nm and exhibits a declining trend due to material softening and the presence of a shear thinning effect. As confirmed by Ghadimi et al. [24], Mspi was proportional to the adhesion coefficient at the interface between the deposited layer and hollow tool. The shear thinning effect was triggered by stress and material stacking during the deposition process, leading to a decrease in the viscosity of the material, a concomitant fall in the adhesion coefficient, and subsequently a lowered Mspi (Figure 2c).

3.2. Characterization of the AFSDed-Composite

Figure 3 displays the optical morphology of the AFSDed-composite, revealing the identified layer-by-layer structure. The absence of cracks and porosity in the composite indicates a homogeneous deposited structure, and robust metallurgical bonding between the adjacent layers is strong [25]. The fully mixed structure is observed consistently across the cross-section, the surface, and the side of the AFSDed-composite, attributed to the syngenetic effect of the axis load and the friction during the AFSD process. As the number of layers increases, internal tension intensifies, resulting in a more pronounced blending effect. The adjacent layers of the AFSDed deposition exhibit enhanced bonding due to the secondary friction experienced by the material at the top surface of the preceding layer, resulting in a mechanically interlocking non-planar interface with improved interface metallurgical bonding at the interface [25]. The amalgamation of materials can be defined by the outcomes of ion release. Figure 4 displays the ion content of solutions soaking AA6061-T6, AA7075-T6, and AFSDed-composite, respectively. It is observed that the AFSDed-composite has a unique content. The released amounts of Cu2+, Mg2+, and Zn2+ ions fall within the range of AA6061-T6 and AA7075-T6, indicating that the synthesized component achieves the mixing of the above two materials so that the macroscopic component content is at the average level of the two.

3.3. Texture Evolution

To analyze the grain structure and the texture evolution of the composite, the large-area inverse pole figures (IPFs) and the other EBSD images of AA6061-T6, AA7075-T6, and the various planes of the AFSDed-composite are displayed in Figure 5, Figure 6 and Figure 7. Among them, planes 1, 2, and 3 represent the cross-section, surface, and side of the AFSDed-composite, respectively (Figure 7a). Firstly, based on EDS mapping, it can be seen that the method proposed in this study achieves full mixing of AA6061 (Figure 5c, Al: 97.63 wt%, Mg: 0.85 wt%, Si: 0.47 wt%, Fe: 0.45 wt%, Mn: 0.39 wt%, Zn: 0.21 wt%) and AA7075 (Figure 6c, Al: 90.63 wt%, Mg: 2.54 wt%, Cu: 1.53 wt%, Fe: 0.31 wt%, Mn: 0.14 wt%, Zn: 4.63 wt%, Cr: 0.21 wt%), while the content of Cu (1.18 wt%), Mg (1.52 wt%), and Zn (3.04 wt%) elements corresponding to the cross-section (plane 1) of the AFSDed-composite (Figure 7b) is significantly increased compared to the AA6061 specimen. Secondly, various elements are evenly distributed, meaning that the two series of Al specimens are completely integrated under the thermal cycles and the plastic deformation during the AFSD process. Furthermore, as given in Figure 7b, the Zn element spectrum exhibits a distinct alternating pattern of light and dark, together with a layer-by-layer stacking morphology. Zn is a distinctive element of AA7075, indicating that the composite microstructure obtained in this work results from the alternate layering of AA6061 and AA7075. The comprehensive examination and study of the composite mechanism of this AFSDed-composite structure is indicated in Section 3.4.
Upon exploring the grain structure, it is seen that the fine and equiaxed grains are distributed regardless of location. In comparison to AA6061 (17.9 ± 1.3 μm, Figure 5) and AA7075 (21.5 ± 1.2 μm, Figure 6), the grains of the composite are clearly refined after the AFSD process, measuring between 10.6 and 12.5 μm (Figure 7c). The AFSDed-composite shows the refined microstructure, which is attributed to the continuous recrystallization (CDRX) behavior under the heat generation during the AFSD process [26]. The CDRX induces the formation of dynamically recrystallized grains in the parent grains, resulting in the finer grains [27]. Additionally, the possibly produced particles, i.e., MgZn2, Mg2Si, and AlFeSi, as confirmed by other researchers [28,29], offer numerous nucleation sites and inhibit grain growth during the recrystallization process, thereby effectively curbing the growth of grains and then decreasing the grain size of the AFSDed-composite. Figure 5a, Figure 6a and Figure 8 show the fractions of low-angle grains (LAGBs) and high-angle grains (HAGBs) of AA6061-T6, AA7075-T6, and the AFSDed-composite at various planes. AA6061-T6 (54.8%) and AA7075-T6 (62.6%) contain a much higher proportion of LAGBs compared to the depositions, which vary from 17.3% to 20.8%. The plastic deformation during the AFSD process initially generates LAGBs, and the presence of recrystallization grains facilitates the transition from LAGBs to HAGBs. Meanwhile, the sub-grains could rotate to the sub-grain boundaries through repeated absorption of dislocation growth, evolving into recrystallized grains with HAGBs [30].
The specimens’ {111} pole figure (PF) and orientation distribution function (ODF) results at two angles, 0° and 45°, are also detected to explore the corresponding texture evolution, as shown in Figure 5, Figure 6 and Figure 7. The idealized components for FCC alloys are also displayed in each PF and ODF image to ensure the texture difference in the composite [31]. For AA6061, the shear texture component ( B ¯ ) is the predominant texture, with a maximum intensity of about 3.7. The PF image (Figure 5d) reveals the presence of a percentage of A texture, with a maximum intensity measured at 2.2. For AA7075 (Figure 6), the predominant shear texture components, B ¯ and C, exhibit maximum intensities of 2.8 and 7.1, respectively, as shown by PF and ODF results. Meanwhile, as displayed in Figure 7, the AFSDed-composite shows a different texture orientation. The major texture phases of plane 1 include B, A2*, and RTc, and the average texture intensity is 3.5 and 4.2 from the PF and ODF results (Figure 7). As shown in Figure 7d-2,e-2 and Figure 7d-3,e-3, B and C are the major texture components of planes 2 and 3, and the average texture intensity is 4.8/7.3 and 2.9/6.5. This indicates that the two planes exhibit a higher strain, causing component A to be replaced by components B and C [31]. Meanwhile, it is seen that plane 2 of the AFSDed-composite has the highest texture intensity, meaning that the degree of plastic deformation along each layer during the AFSD process could be the strongest. It has been confirmed that the FSW-based Al alloys indicated higher texture intensity due to a large amount of shear strain, as well as the stronger shear deformation behavior and adequate CRDX [32,33]. The higher texture intensity at the surface of the AFSDed-composite further facilitates the initiation of coordinated deformation slip systems and enhances the ductility of deposition at that plane [34].
The kernel average misorientation (KAM) of highly magnified images of various planes is shown in Figure 8 and is intended to demonstrate the varying degrees of plastic deformation among these planes [35]. It shows that plane 2 displays a higher KAM value (5.0) than that of planes 1 (4.4) and 3 (4.7). The surface of the AFSDed-composite displays a greater dislocation density and comprises many deformed microstructures. Moreover, the volume fraction of the deformed grains and other types of grains are further detected through the grain orientation spread (GOS) figures of various planes (Figure 8(3)). As organized in Figure 8d, the percentage of recrystallized grains (GOS values less than 2°) is 56.5% in plane 1, which is higher than that in planes 2 (47.7%) and 3 (56.2%) [36,37]. The percentage of the deformed grains (GOS values more than 5°) is about 8.2%, 13.2%, and 8.9% at planes 1, 2, and 3, respectively. The dislocation density in deformed grains is high, and these dislocations are arranged in dislocation structures, leading to local misorientation of several degrees inside the grains. The elevated proportion of deformed grains in plane 2 means that there is a reduced degree of deformation development in planes 1 and 3 [38]. This is because, except for the extrusion and forging behaviors, the accumulation of friction heating during the layer-by-layer stacking process might play a main role in promoting grain recrystallization [39]. However, at the surface of the AFSDed-composite (plane 2), the plastic deformation is the primary behavior under the higher rotation speed of the tool to induce the higher strain rate at that plane, which agrees with the KAM results at that plane (Figure 8(a-2)).

3.4. Heterogeneous Structure Induced in the AFSDed-Composite

Figure 9a presents the IPF images of the AFSDed-composite at a magnification of 2000×. In conjunction with the corresponding EDS mapping results (Figure 9b), it demonstrates that Zn elements have the most pronounced alternating light and dark morphology, thereby confirming that the AFSDed-composite attains a uniform mixture of 6061 (light zone) and 7075 (dark zone) materials. To facilitate analysis, the zones are designated as the 6061 and 7075 zones, respectively. Firstly, a clear difference in grain size between the two zones is observed (Figure 9c). The grain size in the 7075 zone measures 16.1 μm, which is approximately 5.2 times larger than the finer grains of the 6061 zone, measuring 3.1 μm. Referring to the literature [40], the typical microstructure can be defined as heterogenous. The junction position of the 7075 and 6061 zones, as illustrated in Figure 9a, is determined using the FIB method, and the distinction between the two regions is identified through HAADF investigation to verify the existence of this heterostructure. The clear grain boundary can be detected in Figure 10A, and the upper-left corner of Figure 10A might be the 7075 zone. This is because the EDS mapping results (Figure 10A) indicate that Cu and Zn, unique to AA7075 alloy, are enriched in this area. At the same time, when conducting an EDS line scanning test along the marked red line (from X to Y) in Figure 10B, it can be found that the Cu (3.13 wt%) and Zn (0.52 wt%) contents in the 7075 zone are elevated, as indicated by the average elemental composition results presented in Table 2. In the 6061 zone, the proportion of Mg contained is significantly increased (3.54 wt%, Table 2). In addition, the division of the two regions, the 6061 and 7075 zones, can also be proven by the type of precipitation phase. The columnar precipitate phase at the grain boundary is MgZn2, and the line scanning result indicates that the ratio of Mg and Zn is approximately 1:2 (Figure 10B). Consistent with the finding of Abdollahzadeh et al. [41], the MgZn2 particles were evenly distributed among the FSWed Al-Mg alloys because of the identical crystal lattice between Zr and Mg elements, causing the formation of the MgZn2 particles before Al-Mg hard IMCs.
The irregular precipitation in the 6061 zone is enriched in Al, Fe, and Si elements, with line scanning results showing a content ratio of Fe and Si of approximately 1:1. That is, the precipitated phase could be AlFeSi. Figure 11 presents the higher-magnification HAADF images, HRTEM images of the induced precipitates, and the IFFT patterns. The particles in the 7075 zone have a monoclinic structure with lattice interplanar spacing of about 0.219 nm. After comparison with the JCPD card of MgZn2 (00-034-0457), the interplanar spacing of MgZn2 along (2 0 1) is 0.218 nm [42]. Additionally, the particles in the 6061 zone, with a size of about 200 nm × 300 nm, have a monoclinic structure with lattice planar spacing of about 0.198 nm. After comparison with the JCPD card of AlFeSi (071-0238), the interplanar spacing of AlFeSi (4 2 3) is 0.19 nm. As a result, the two particles are MgZn2 and AlFeSi, which are the unique strengthening phases of the AA7075 and AA6061 alloys, further confirming that the deposition prepared in this work is composed of a heterostructure in which the microstructures of two series of aluminum alloys overlap with each other.
Analysis of LAGBs and HAGBs reveals that LAGBs predominantly occur in the 7075 zone (30.4%) in contrast to the 6061 zone (17.1%, Figure 9d). This is consistent with the larger grain size of AA7075 than that of AA6061 as given in Figure 5b and Figure 6b. The higher fraction of HAGBs in the 6061 zone is because the sub-grains rotate to the sub-grain boundaries through repeated absorption of dislocation growth, evolving into recrystallized grains with HAGBs [30]. The GND distribution of the AFSDed-composite is displayed in Figure 9e, showing that the GND density is higher and densely arranged in the 6061 zone (finer-grain zone), while relatively lower GND densities occur in the 7075 zone (coarser-grain zone). This indicates that the 6061 zone has a greater ability to resist plastic deformation evolution. When this distribution is extended to the large-sized components fabricated in this study, it could be inferred that the AFSDed-composite is composed of overlapping 6061 and 7075 zones distributions, showing that the component also has heterogeneity in the stress distribution, further affecting the corresponding mechanical properties. The presence of finer grains with higher dislocation density (GND density) significantly enhances their strength and strength–ductility synergy. As shown by Shin et al. [43], the compatible existence of finer and coarser grains in Ti alloys produced stronger stress and strain gradients, resulting in a better strength and ductility.

3.5. Mechanical Properties

Figure 12a shows the microhardness distribution image of the AFSDed-composite. The bottom layer of the deposition exhibits a lower microhardness (61.5 HV0.5), owing to the growth of precipitates under the continuous thermal cycles. The heat generated during AFSD causes the growth of precipitates and the formation of new precipitates. The β″ and β′ phases in the bottom layer might transform into β′ and β phases and then gradually grow and coarsen due to the longer duration of heat exposure. The coarsening particles (β″), observed by transmission electron microscopy (TEM), in the lower layer of the deposited AA6061 alloy were attributed to the reduction in mechanical properties [44]. Hence, the growth of particles under greater thermal cycles is the reason for the decrease in microhardness of the bottom layer.
Additionally, this shows that, compared to the AA6061 substrate (85.8 ± 1.3 HV0.5, Figure 12c), the microhardness of the AFSDed-composite exhibits a similar average microhardness value of 80.1 ± 4.8 HV0.5, which is clearly higher than the microhardness of the fabricated Al6061 deposition, according to related findings [45]. The enhanced microhardness of the AFSDed-composite could be explained by various reasons. Firstly, the induced equiaxed and fine grains of the AFSDed-composite (Figure 7) may help to increase the microhardness by the grain refinement strengthening effect [46]. Secondly, the higher monitored temperature (469.1 °C, Figure 2a) during the AFSD process causes many particles (β″ and β′ phases) to dissolve into the matrix, where they are transformed into solute atoms [47,48]. The dissolved atoms, i.e., Mg, Zn, and Mg, inside the Al-based matrix act as solute atoms, inducing lattice distortions that hinder lattice integrity and dislocation mobility. Hence, the dislocations encounter greater resistance when attempting to traverse the solid solution, leading to an elevated microhardness of the specimen [49]. So, solid solution strengthening is one factor that helps to increase microhardness. Thirdly, the various induced precipitates, including MgZn2 and AlFeSi (Figure 11), act as the strengthening phase and cause a stronger pinning effect at the grain boundaries and then increase the corresponding microhardness [50,51,52]. It has been confirmed that the Al-Mg composite displayed enhanced hardness, attributed to the hard nature of reinforcing particles and the IMC formation, with hard and brittle characteristics [53].
Figure 12b displays the stress–strain curves of AA6061-T6, AA7075-T6 [52], and the AFSDed-composites, and the results are exhibited in Figure 12c. The YS and UTS values of the AFSDed-composite-HD/AFSDed-composite-VD are 146.9 MPa/155.8 MPa and 262.1 MPa/266.3 MPa, respectively, while the EL value of the AFSDed-composite-HD/AFSDed-composite-VD is 18.7%/14.3%. This means that the tensile specimens along the HD and tensile specimens along the VD show similar tensile properties, and the obtained composite shows stronger interlayer bonding strength corresponding to higher longitudinal tensile properties. Compared to the AFSDed-6061 depositions obtained in our previous work [18], the tensile properties of the composite are increased by about 1.5 times. The enhanced plastic deformation during the AFSD process and the precipitation strengthening effect are factors that increase the corresponding mechanical properties. Meanwhile, the induced heterogeneous structure in the AFSDed-composite contributes to the enhancement of tensile strength; as the softer 6061 zone and harder 7075 zone deform inhomogeneously, back stresses form in the 6061 zone and, correspondingly, forward stresses appear in the 7075 zone, producing hetero-deformation which in turn causes strengthening that increases the YS and UTS values of the AFSDed-composite [54]. The heterogeneous structure also increases the possibility of dislocations multiplying and accumulating, increasing the gradient strengthening effect and effectively increasing the tensile properties [55]. Furthermore, the clearly enhanced tensile strength also could be explained by the existence of strengthening particles (MgZn2 and AlFeSi) (Figure 11). Consistent with the research of Chen et al., the precipitation strengthening effect is the main strengthening mechanism for increasing the YS and UTS of the AFSDed 6061 alloys [50].
The fracture images of AA6061-T6 and AA7075-T6 and the AFSDed-composite specimens are shown in Figure 12d–g. Lots of dimples, with a small area of cleavage planes, are observed in AA6061-T6 (Figure 12d), meaning that the ductile and brittle fracture modes are induced. Yet, only small, randomly and densely distributed dimples are detected on the fracture surface of AA7075-T6 (Figure 12e), which further illustrates its higher tensile properties (Figure 12c). For the AFSDed-composite (Figure 12f–g), in addition to the presence of a small number of dimples, the ductile tearing also appears at the fracture interface, which directly increases the possibility of fracture. Additionally, the induced particles would be observed surrounding the dimples. The existence and coalescence of the formed particles lead to the rupture of the particle/matrix interface. It has been confirmed that the presence of the particles accelerated the crack propagation and the formation of ductile tearing [56].

3.6. Analysis of Increased Performance

The results of this work are compared to the tensile properties of AA6061-T6/AA7075-T6, the deposited 6061/7075 through the AFSD method, and other AM methods, as shown in Figure 13. It is found that, when based on other AM methods [57,58,59,60,61], this method could ensure, with relative certainty, that the deposition has a higher tensile strength (120~230 MPa), but it directly sacrifices the ductility of specimens with a lower elongation (about 15%). When using the AFSD method, values of tensile strength and elongation might maintain a relative equilibrium state [18,62,63]. However, its tensile strength is nearly doubled compared to the AA6061-T6 alloy, meaning that it could not meet the practical demand in the engineering field for lower tensile strength. Although that problem might be solved by the post-heat treatment process to a degree, it could mean higher time costs and expenses. And the suitable treating parameters should be adjusted for the various types of depositions, which further increases the actual operational difficulty. In this work, the fabricated composite presents a UTS and ductility close to those of AA6061-T6, and when comparing the properties of AA7075-T6 and 7075 specimens obtained by means of various AM methods [64,65,66,67,68], it is suggested that the composite manufactured in this work has overall greater tensile strength than that of the valued AFSD method, as well as other AM methods. The AA6061 and AA7075 are mixed adequately during the AFSD process to form a uniform composite with a refined grain microstructure and the existence of strengthening particles, leading to better mechanical performance.
From another perspective, in addition to significant advantages in mechanical performance, the deposition components obtained in this study also have high practical application significance in terms of size. Compared with previous heterostructures of Al-based materials (Figure 13c), although there are various sample preparation methods, such as LPBF [69], WAAM [70], and SLM [71], their corresponding molding sizes are smaller, ranging from 7359 to 14,400 mm3. However, the volume of AFSDed-composite fabricated in this work reached 25,920 mm3, which is closer to the demands of engineering applications. Therefore, the findings of this study not only provide technical guidance for manufacturing high-quality components through AFSD but also provide ideas for manufacturing high-strength heterostructures for large-scale practical engineering applications.
Combined with the above results, the benefits of this study are summarized in Figure 14. Firstly, a novel method aiming to obtain a composited Al-based specimen has been proposed. Secondly, the microscopic heterogeneous microstructure with an alternating distribution of 6061 and 7075 zones is induced within the fabricated AFSDed-composite. Thirdly, when compared with the 6061-T6/7075-T6 or the as-fabricated 6061/7075 specimens acquired by various AM methods, the AFSDed-composite has clearly enhanced mechanical properties. Lastly, the fabricated large-size AFSDed-composite exhibits excellent mechanical performance.

4. Conclusions

This study proposes a unique method for fabricating an AA6061-7075 heterogeneous composite with enhanced performance by additive friction stir deposition (AFSD), utilizing an in situ process monitoring kit to quantify temperature, force, and torque. Microstructure and texture results indicate that the AFSDed-composite shows the refined grain structure attributed to dynamic recrystallization. Cross-sections of the composite display a reduced level of plastic deformation relative to its surface and sides, as confirmed by a lower texture intensity. However, the surface of the AFSDed composite exhibits the highest magnitude of shear strain and the most pronounced shear deformation characteristics.
The heterogeneous structure is characterized using the OM, SEM, EBSD, and TEM methods, revealing that the AFSDed-composite is obtained through mixing the two Al alloys, resulting in a uniform and dense microstructure with an alternating distribution of 6061 and 7075 materials. Texture analysis indicates that the AFSDed-composite has a significant degree of plastic deformation resulting from the development of a heterogeneous structure, including the finer (6061 zone) and coarser grains (7075 zone), throughout the thermal cycles during the AFSD process. Analysis of the precipitates (MgZn2 and AlFeSi) based on HRTEM verifies the formation of the heterostructure.
The AFSDed-composite has superior mechanical properties compared to the AA6061 and AA7075 depositions acquired by other AM methods. It displays a microhardness of 80 HV0.5, a yield strength of 156 MPa, and an ultimate tensile strength of 266 MPa, attributed to the grain refinement effect, precipitated strengthening phases, and a heterogeneous structure with a higher stress gradient. The findings of this work not only provide useful guidelines for the preparation of Al-based composites with superior performance but also allow for new ideas for manufacturing high-strength heterostructures for large-scale practical engineering.

Author Contributions

Methodology, Q.Q.; Validation, H.Q., Z.L., C.T.K., S.J. and D.Z.; Writing—original draft, Q.Q.; Writing—review & editing, Q.Q.; Supervision, D.G.; Funding acquisition, L.M.T. i have this version of the contributions. All authors have read and agreed to the published version of the manuscript.

Funding

The authors are grateful for the financial support of the Science and Technology Development Fund (FDCT) of Macau SAR (0110/2023/AMJ), National Key Research and Development Program of China (2023YFE0205300), and the Joint Fund of Basic and Applied Basic Research Fund of Guangdong Province (No. 2021B1515130009).

Informed Consent Statement

Not applicable.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Conflicts of Interest

Author Qian Qiao and Dawei Guo were employed by the company IDQ Science and Technology (Hengqin Guangdong) Co., Ltd., author Shufei Jiang was employed by the company Aerospace Engineering Equipment (Suzhou) Co., Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Figure 1. Diagrams and photographs of the fabricating process, sampling methods, and obtained depositions, where (A) depicts the AFSD process and the composite feedstock and (B) illustrates the depositions and the sampling for further tests.
Figure 1. Diagrams and photographs of the fabricating process, sampling methods, and obtained depositions, where (A) depicts the AFSD process and the composite feedstock and (B) illustrates the depositions and the sampling for further tests.
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Figure 2. Profiles of the in situ monitored processing data. Among them, (a–c) present the temperature (black line), Fups (black line), Fspi (red line), and Mspi (black line) of the AFSDed-composite monitored with the in situ process monitoring kit.
Figure 2. Profiles of the in situ monitored processing data. Among them, (a–c) present the temperature (black line), Fups (black line), Fspi (red line), and Mspi (black line) of the AFSDed-composite monitored with the in situ process monitoring kit.
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Figure 3. Three-dimensional macrostructure of the AFSDed-composite specimen.
Figure 3. Three-dimensional macrostructure of the AFSDed-composite specimen.
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Figure 4. Concentration of ions released from AA6061-T6, AA7075-T6, and AFSDed-composite.
Figure 4. Concentration of ions released from AA6061-T6, AA7075-T6, and AFSDed-composite.
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Figure 5. EBSD and EDS mapping of the AA6061-T6. Among these, (a) is a large area of the highly magnified (1000×) IPF image, (b) exhibits the grain size distribution map, (c) is the EDS mapping of the highly magnified (1000×) IPF image, and (d,e) indicate the PF and ODF images.
Figure 5. EBSD and EDS mapping of the AA6061-T6. Among these, (a) is a large area of the highly magnified (1000×) IPF image, (b) exhibits the grain size distribution map, (c) is the EDS mapping of the highly magnified (1000×) IPF image, and (d,e) indicate the PF and ODF images.
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Figure 6. EBSD and EDS mapping of the AA7075-T6. Here, (a) shows a large area of the highly magnified (1000×) IPF image, (b) is the grain size distribution map, (c) is the EDS mapping from the highly magnified (1000×) IPF image, and (d,e) present the PF and ODF.
Figure 6. EBSD and EDS mapping of the AA7075-T6. Here, (a) shows a large area of the highly magnified (1000×) IPF image, (b) is the grain size distribution map, (c) is the EDS mapping from the highly magnified (1000×) IPF image, and (d,e) present the PF and ODF.
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Figure 7. EBSD and EDS mapping of the AFSDed-composite at various planes. Here, (a) shows the large 3D area EBSD results of the AFSDed-composite at various planes, including (1), (2), and (3). (b) shows the EDS mapping of plane (1), while (c–e) show the grain size distribution map, PF, and ODF results at the three planes.
Figure 7. EBSD and EDS mapping of the AFSDed-composite at various planes. Here, (a) shows the large 3D area EBSD results of the AFSDed-composite at various planes, including (1), (2), and (3). (b) shows the EDS mapping of plane (1), while (c–e) show the grain size distribution map, PF, and ODF results at the three planes.
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Figure 8. IPF, KAM, and GOS results of the AFSDed-composite at various planes. Here, (a) is the IPF images at 300× magnification, (b) is the KAM images, and (c) shows the GOS figures of the AFSDed-composite at planes 1, 2, and 3. (d) indicates the percentage of various grain types based on GOS results, and black, red, and blue bars represent planes 1, 2, and 3, respectively.
Figure 8. IPF, KAM, and GOS results of the AFSDed-composite at various planes. Here, (a) is the IPF images at 300× magnification, (b) is the KAM images, and (c) shows the GOS figures of the AFSDed-composite at planes 1, 2, and 3. (d) indicates the percentage of various grain types based on GOS results, and black, red, and blue bars represent planes 1, 2, and 3, respectively.
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Figure 9. IPF, EDS mapping, grain distribution results, and GND image of the fabricated AFSDed-composite. Here, (a) shows IPF images at 2000× magnification, (b) is the corresponding EDS mapping and element content, (c) shows the grain size distribution, (d) exhibits the grain boundaries and misorientation distribution, and (e) shows the GND image of the AFSDed-composite.
Figure 9. IPF, EDS mapping, grain distribution results, and GND image of the fabricated AFSDed-composite. Here, (a) shows IPF images at 2000× magnification, (b) is the corresponding EDS mapping and element content, (c) shows the grain size distribution, (d) exhibits the grain boundaries and misorientation distribution, and (e) shows the GND image of the AFSDed-composite.
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Figure 10. TEM results of the heterogeneous structure within the obtained composite. Here, (A) shows the HAADF images of the induced heterogeneous structure, and (B) shows the EDS line scanning results along the XY direction as marked in (A).
Figure 10. TEM results of the heterogeneous structure within the obtained composite. Here, (A) shows the HAADF images of the induced heterogeneous structure, and (B) shows the EDS line scanning results along the XY direction as marked in (A).
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Figure 11. HAADF images, HRTEM images, and IFFT patterns of the induced particles in (A) 7075 zone and (B) 6061 zone.
Figure 11. HAADF images, HRTEM images, and IFFT patterns of the induced particles in (A) 7075 zone and (B) 6061 zone.
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Figure 12. Mechanical property results of the AFSDed-composite. Here, (a) shows the microhardness distribution of the AFSDed-composite, and (b) shows the stress–strain curves of AA6061-T6, AA7075-T6, and the AFSDed-composite specimens along HD and VD. The black, red, blue, and green lines correspond to the results of AA6061-T6, AA7075-T6, the AFSDed-composite-HD, and AFSDed-composite-VD. (c) Details the mechanical properties of the specimens. (d–g) present the fracture images of AA6061-T6, AA7075-T6, and the AFSDed-composite specimens (along HD and VD).
Figure 12. Mechanical property results of the AFSDed-composite. Here, (a) shows the microhardness distribution of the AFSDed-composite, and (b) shows the stress–strain curves of AA6061-T6, AA7075-T6, and the AFSDed-composite specimens along HD and VD. The black, red, blue, and green lines correspond to the results of AA6061-T6, AA7075-T6, the AFSDed-composite-HD, and AFSDed-composite-VD. (c) Details the mechanical properties of the specimens. (d–g) present the fracture images of AA6061-T6, AA7075-T6, and the AFSDed-composite specimens (along HD and VD).
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Figure 13. Comparison of the tensile strength of (a) AA6061 and (b) AA7075 produced by AFSD and other AM methods. (c) The volume of the heterogeneous component obtained by various AM methods.
Figure 13. Comparison of the tensile strength of (a) AA6061 and (b) AA7075 produced by AFSD and other AM methods. (c) The volume of the heterogeneous component obtained by various AM methods.
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Figure 14. Summary of the benefits of this work.
Figure 14. Summary of the benefits of this work.
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Table 1. Chemical compositions of the AA6061-T6, AA7075-T6, and composite feedstock shown in Figure 1A (wt%).
Table 1. Chemical compositions of the AA6061-T6, AA7075-T6, and composite feedstock shown in Figure 1A (wt%).
AlMgCuSiFeMnZnCrTi
AA6061-T6 *97.170.960.230.590.300.520.190.030.01
AA7075-T689.582.531.620.100.280.135.480.220.06
Composite feedstock89.642.521.610.130.320.165.330.230.06
* The raw feedstocks are non-standard specimens, so there are slight deviations in elemental composition compared to the corresponding standards.
Table 2. Average elemental composition of the different magnified planes in Figure 10B.
Table 2. Average elemental composition of the different magnified planes in Figure 10B.
wt%AlMgCuSiZn
7075 zone93.752.083.130.520.52
6061 zone93.863.541.371.070.17
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MDPI and ACS Style

Qiao, Q.; Qian, H.; Li, Z.; Guo, D.; Kwok, C.T.; Jiang, S.; Zhang, D.; Tam, L.M. Microstructure Evolution and Mechanical Performance of AA6061-7075 Heterogeneous Composite Fabricated via Additive Friction Stir Deposition. Alloys 2025, 4, 21. https://doi.org/10.3390/alloys4040021

AMA Style

Qiao Q, Qian H, Li Z, Guo D, Kwok CT, Jiang S, Zhang D, Tam LM. Microstructure Evolution and Mechanical Performance of AA6061-7075 Heterogeneous Composite Fabricated via Additive Friction Stir Deposition. Alloys. 2025; 4(4):21. https://doi.org/10.3390/alloys4040021

Chicago/Turabian Style

Qiao, Qian, Hongchang Qian, Zhong Li, Dawei Guo, Chi Tat Kwok, Shufei Jiang, Dawei Zhang, and Lam Mou Tam. 2025. "Microstructure Evolution and Mechanical Performance of AA6061-7075 Heterogeneous Composite Fabricated via Additive Friction Stir Deposition" Alloys 4, no. 4: 21. https://doi.org/10.3390/alloys4040021

APA Style

Qiao, Q., Qian, H., Li, Z., Guo, D., Kwok, C. T., Jiang, S., Zhang, D., & Tam, L. M. (2025). Microstructure Evolution and Mechanical Performance of AA6061-7075 Heterogeneous Composite Fabricated via Additive Friction Stir Deposition. Alloys, 4(4), 21. https://doi.org/10.3390/alloys4040021

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