Effect of heat treatment on the microstructure and mechanical properties of 7075 aluminum alloy cold-spray coatings

Cold Spray (CS) exhibit unique features due to the low temperatures involved. However, the CS coating are harder than the corresponding powders and bulk alloys, which results in a low toughness and then greatly limits the application of CS. To overcome this shortage, preheat treatment of powder and post-heat treatment of CS coating are applied to improve the performance of cold-sprayed 7075 aluminum alloy coatings in this work. With increasing temperatures of post-heat treatment, the tensile strength of the coatings increased from 228 MPa to 309 MPa with elongation of 2.46%. Microstructure analysis of the heat-treated coatings revealed that the improvement in mechanical properties was primarily due to an increase in the secondary phase. Accordingly, 7075 aluminum alloy powder was preheat treated at 200 °C and then used for cold spraying, which resulted in improved mechanical properties in the resultant coatings exhibiting a tensile strength of 302 MPa and an elongation of 3.87%. These findings provide valuable insights into the practical application of cold-spraying technology in the aviation field.


Introduction
7075 aluminum (Al) alloy has been widely used in the aviation industry, particularly as load-bearing members and in constructing aircraft undercarriages, because of its high specific strength and strengthening via heat treatment. However, owing to the hot cracking tendency of this alloy, traditional thermal spraying, welding, and other thermal processing methods are difficult to use on it. Recently, Cold spray (CS) is an advanced spraying technology based on aerodynamics that uses the adhesion mechanism of adiabatic shear instability [1]. During the cold-spray process, collisions induce deformation, dislocation, and slippage of the grain structure within the target materials [2]. Compared with thermal surface processing methods, CS can achieve good consistency in terms of chemical composition and microstructure as well as advantages, including insignificant phase change and pore formation owing to spraying [3]. Therefore, CS technologies have a broad range of potential applications in the aviation field. However, the coatings often have very low toughness due to the strain hardening induced by severe plastic deformation, significantly limiting their application [4]. Usually, heat treatment is performed after CS to improve coating toughness [5]. Previous research indicated that T6 heat treatment could remarkably promote the comprehensive mechanical properties of cold-sprayed Al alloys [6]. Rokni et al confirmed that the hardness, strength, and plasticity of 7075 Al alloy cold-spray coatings improved significantly after heat treatment [7]. Kim [8] and Zhao et al [9] found that the mechanical properties of the alloy improved after heat treatment of 2xxx, 7xxx Al alloy owing to severe plastic deformation (SPD). This is because the SPD promotes the heterogeneous nucleation of precipitates in the alloy. This approach has been reported for 7xxx Al alloys being processed using SPD techniques [10]. Zhu et al analyzed the characterization of microstructural and precipitate evolution in cold-sprayed AA7050 after in situ laser heat treatment [11]. However, the post-heat treatment methods are unsuitable for in situ repair situations because the heat treatment of the coating will inevitably have a thermal effect on the matrix. Furthermore, in the case of heat treatment of coatings, the volume of the matrix is uncontrollable.
Commonly, 7075 Al alloy is a precipitation-hardening alloy [12]. The heat treatment method is generally used to regulate its phase structure and microstructure, thereby improving mechanical properties [13]. The aging or dissolution of the alloy can be divided into three stages. When the heat treatment time is short or the temperature is relatively low, the main generated phase is the GP zone that is coherent with the matrix. This is a zone with a clustering of solute elements rather than a phase; the GP zone plays a significant role in strengthening the alloy [14]. When the treatment duration is longer or the temperature is higher, the primary generated phase is η′ phase, which is semi-coherent with the matrix. The η′ phase is a metastable transitional phase with a spherical morphology of about 10 μm, which usually acts as the primary strengthened phase in 7075 Al alloy [15]. When the treatment duration is even longer or the temperature is higher, the GP zone and η′ phase will transform into the η phase, which is a stable secondary phase that is incoherent with the matrix [16]. Based on the above theory, William et al reported that the deposition efficiency of 7075 Al alloy powder was greatly improved through solid solution heat treatment [17].
Herein, to solve the problem of poor toughness in the CS of 7075 aluminum alloy, the alloy powder is treated with low-temperature heat treatment combined with high-temperature spraying to achieve the optimal microstructure and properties of the coating, resulting in the preparation of high-strength and tough aluminum alloy coatings. Therefore, two different approaches, i.e., pre-heat treatment of powder and post-heat treatment of CS coating, are used to improve the performance of cold-sprayed 7075 aluminum alloy coatings in this work. The results confirm the feasibility of the preheat treatment of powder in practical application. These findings provide valuable insights into the practical application of cold-spraying technology in the aviation field.

Materials
The commercial gas-atomized 7075 Al alloy powders were used as feedstock powders in this study, and the morphology and microstructure of the powders are shown in figure 1. Figures 1(a) and (b) show that some satellite particles are attached to the surface of the large particles. The microstructure of the gas-atomized powder is the typical dendritic cells, as shown in figure 1(c). Moreover, the particle size distribution of powders was investigated using a laser particle size analyzer (Zetasizer Nano ZS, US) with a D 50 of 25.7 μm and a D 90 of 53.7 μm, respectively, and the particle size distribution of the powder is shown in figure 1(d).

Cold spray deposition
The CS coatings were produced using a kinetic metallization system (SST-3000, Canada). The spray nozzle had a stand-off distance of 25 mm and traveled at 30 mm s −1 . Helium was used as the carrier gas at 3.0 MPa and 400°C. The substrate materials used for CS were pure aluminum plates.

Heat treatment
The heat treatment of coating was first conducted in a high vacuum tube furnace (5 × 10 −3 Pa, HTK-1200, Qingdao) with a temperature range of 120°C-200°C for 1 h, and a heat rate of 10°C min −1 . Based on the optimization results of the coating, the 7075 Al alloy powder was placed in a rotary tube furnace (JK-HD-15, China) for heat treatment and then its microstructure was regulated at 200°C for 1 h with a high vacuum (5 × 10 −3 Pa); the heating time from room temperature (25°C) to 200°C was 1.5 h. The rotary tube furnace continued to rotate at a speed of 20 r min −1 , preventing the powders from sintering during heat treatment. The heat-treated powders were used as raw material for CS to prepare the coating.

Mechanical properties
The coatings were first removed from the substrates using wire-electrode cutting and then machined to the subsize test specimen specifications as shown in figure 2. The sample surfaces for tensile testing were polished to avoid the influence of surface defects on the test results. The tensile tests were performed using an INSTRON 5967 tensile tester with a displacement rate of 1 mm min −1 . Each group of samples was tested three times, and the average value was considered for analysis.

Microstructure characterization
The powders and coatings were placed in epoxy resin prior to standard metallographic preparation (grinding with P600, P1200, and P2500 silicon carbide paper and then polishing with colloidal silica suspension). The cross-section and fractured surface microstructure of the powders and coatings, respectively, were investigated using field-emission scanning electron microscopy (FESEM, Inspect F20, FEI, and JSM-7900F, JEOL), with secondary imaging (SE) and backscattered electron (BSE). Further, microstructure investigations were conducted using an FE transmission electron microscope (FETEM) (FEI Talos F200x) with a high-angle annular dark field detector (HAADF) and an energy-dispersive x-ray detector (EDX) (Super-X) at an acceleration voltage of 200 kV. Thin discs with a 3-mm diameter were removed from the deposition and polished, dimpled, and ion milled.   Figure 3 shows the cross-sectional microstructure of the coating after heat treatment at different temperatures. The CS deposit exhibits well-bound powder particles with few defects, which indicates that the powders experienced sufficient particle deformation. The microstructure of the cold-spray coating contains three principal areas: (i) the internal region of the powder with less plastic deformation; (ii) a plate-like region with more plastic deformation; and (iii) the ultrafine grain (UFG) structures with considerable plastic deformation. The deformation of the particles in the deposited body is heterogeneous; the interior of the less deformed particles remains dendritic, whereas the more deformed particles produce an ejection tendency at the junctions between the particle [18]. Subcrystals are also produced at the particle junctions, indicating that the coldspraying process affects the refinement of the particles [19]. The primary morphological feature of the slate-like crystal zone is a streamlined morphology in which the content of the precipitated phase is minimal. In contrast, the UFG structures contain numerous precipitated phases and very small grains; it can be inferred from the observation that the UFG structures are the primary strengthening zone of the coating and coatings with a higher content of the fine crystal structure have superior mechanical properties. Based on the literature on SPD processes [20], developing these UFG structures can also enhance the mechanical properties of the processed materials, e.g., strength, hardness, and ductility. With the increase in heat treatment temperatures, the main changes observed are the partial dissolution of the dendritic structure and the formation of a fine secondary phase along the grain boundaries. Because the dendrite structure inside the particles in the coating dissolves, the solute element redissolves in the matrix, and the internal structure of the particles is relatively more uniform. Fine precipitated phases form in the UFG region and the development of these UFG structures can enhance the mechanical properties, e.g., strength, hardness, and ductility. Figure 4(a) shows the fracture morphology of the as-deposited sample. The morphology indicated that bonding in the as-deposited material relied substantially on mechanical interlocking, whereas the particles that undergo less plastic deformation are seen in the middle of the layers. In the case of the coating not subjected to heat treatment, these particles are the source of fractures, and many voids can be seen in the figure. These voids are formed by the shedding of particles that undergo minor plastic deformation. Numerous weak bonding regions (marked with red arrows) can be observed in figure 4, indicating that the fracture mainly occurred along the particle interfaces [21]. With increasing load, the cracks expand boundary particles and induce particle shedding [22]. Figures 4(b) and (c) show the fractures in the sprayed coating after the powder was heat treated at 140°C and 160°C, respectively. Compared with the fracture observed in the coating that was not subjected to heat treatment, the shedding of the whole particles is decreased in the heat-treated powders. Because of the heat treatment, the fracture layer is flatter, and the fracture mode starts to transition toward the internal fracture of particles. Moreover, with increasing heat treatment temperature, the fracture morphology changes to a certain extent, among which the most significant changes are the gradual reduction in the weak bonding regions (marked with red arrows) and the gradual increase in the dimple regions (marked with blue arrows). This change indicates that the number of crack sources at the boundary of fine particles decreases but increases inside the particles and in the UFG region [21]. This demonstrates that the particle-to-particle bonding is strengthened due to the heat treatment of the powder, and small cracks and pores decrease during the cold-sprayi process [23]. Figure 4(d) shows the fracture morphology in the cold-sprayed layer that was heat treated at 200°C. We found that the fracture morphology changed considerably for samples subjected to heat at 200°C. First, the overall particle shedding situation is improved; the primary fracture mode is the internal fracture of the particles.  (a) Grain morphology of deformed area and undeformed area inside the particle, (b) the internal structure of a particle, (c) the boundary between the region with slabshaped structures and the ultrafine crystal area, (d) the boundary between the three crystal regions; (e) the lath crystal region at a particle-particle boundary, and (f) the fine-grained region at the particle-particle boundary.

Results and discussion
During heat treatment at 200°C, owing to the precipitation of the secondary phase within the particles, the binding strength between the particles increases and dimple regions increase around the particles. The particles no longer break along the dendrite network. The primary tensile region of the coating is the strength of joints between particles and the strength of the particles themselves [24]. Figure 5 shows the BSE images of the 7075 alloy powder after heat treatment at various temperatures of 120°C-200°C. When the heat treatment temperature is less than 160°C, the network structure remains unchanged and no precipitate phase forms. Because the network structure is formed at the grain boundary owing to solute segregation, the mesh size (represented by the red circle) can be regarded as the grain size. With increasing heat treatment temperature, the mesh grows radially in some regions (represented by a yellow circle), Figure 7. (a) HAADF-STEM images of the transition zone between deformed grains and undeformed grains and the corresponding EDS mapping of (a)-(f); (g) is the enlarge image of (a) in the undeformed region, (h)-(i) is the corresponding EDS mapping. and the mesh area in this region has increased considerably. Therefore, we hypothesize that the grain increases as the heat treatment temperature increases. When the heat treatment temperature exceeded 160°C, the internal structure of the powder changed remarkably. The dendritic network began to dissolve, and the produced phase precipitated along the grain boundary (depicted by blue arrows), indicating the phase transformation of solute elements such as Zn, Mg, and Cu. The change in the internal structure of the powder indicates a change in the mechanical properties of the material. Because the dendrite segregation network has a brittle phase, partial dissolution of the dendrite network increases the plasticity, toughness, and strength of the powder. When the heat treatment temperature reaches 200°C, numerous dispersed precipitates (depicted by red arrows) can be seen in the powder, which have a considerable strengthening effect on the 7075 aluminum alloy. Figure 6 show transmission electron microscopy (TEM) images of the junctions between different regions present in the cold-sprayed layer of the 7075 aluminum alloy. Figure 6(a) demonstrates the coexistence of the normal and slate-like regions within the particles [25]. The small microcracks are present at the interface between the two regions due to the significant difference in the extent of the plastic deformation within the two regions during the cold-spraying process. The slate-like crystal region, which undergoes a larger extent of plastic deformation, cannot combine well with the particles. The difference between the particles and the precipitated phase can be observed. Before CS, the internal morphology of the particles was unchanged, and the precipitated phase retained a dendritic structure. In the region with a plate-like structure, owing to the considerable plastic deformation that occurs during the cold-spraying process, the precipitated phase is intermittently distributed via collisions and extrusions as well as renucleated in this region. Figure 6(b) shows the internal morphology of the particles. The microstructure is similar with that of the powder, implying that the areas inside the particles were slightly deformed during the cold-spraying process and that the internal structure of the particles was hardly affected by the cold-spraying process. The black line shown in figure 6(b) shows the dendritic structure of the powder and the absence of precipitated phase production in the matrix [26]. Figure 6(c) shows a TEM image demonstrating the coexistence of the slate-crystal and UFG structure regions; microcracks can also be observed at the junction of these two crystal regions. The same process generates these microcracks as those observed in the particle and slate-crystal regions, owing to the varying degrees of plastic deformation within the regions. The structure between the two regions is also considerably different. A very dense precipitation phase in the ultrafine crystal region can be observed. Figure 6(d) shows a TEM image demonstrating the coexistence of the three crystal regions. No microcracks can be observed, indicating that the best precipitation quality occurs at the union of the three crystal types [27]. Figure 8(e) shows the slate-like structure at the junctions between particles and grains. show the transition zone of particle-to-particle junction region. (d) shows the inner area of the particles. (e) shows the region with a slab-like structure that occurs at the particle-to-particle boundary. (f) shows the fine-grained region at the particle-to-particle boundary.
Based on the TEM images, the grains in this region are compressed into slate-like shapes, with large round precipitates nucleating near grain boundaries. The large size of these precipitates is attributed to early heterogeneous nucleation and accelerated growth due to grain boundary diffusion. Figure 6(f) shows UFG structures at grain-grain junctions. This is owing to the considerable plastic deformation during the coldspraying process. This region exhibits more deformation than the slate-grain region. Thus, the precipitates in this region are almost all extruded, with an ultrafine crystal morphology, and the region contains some spherical and refined precipitates [28]. Figure 7 shows HAADF-STEM images of the transition zone between deformed and undeformed grains and the corresponding EDX mapping. The white precipitates in the images comprise Zn, Mg, and Cu, indicating that the cold-spraying process did not change the phase structure. However, although the grains in the outer layer of the powder underwent SPD, the intergranular precipitates remained in a continuous distribution state, resulting in decreasing plasticity of the sprayed body. This is because the continuous segregation structure weakens the binding strength of grain boundaries and reduces the solute content of matrix. Moreover, the matrix grains inside the particles have not yet undergone deformation and are also surrounded by thick precipitated phases, which are closely related to the grain structure of the original powders. Figure 8 shows TEM images of the coating prepared with powder subjected to heat treatment at 200°C for 1 h. Figures 8(a)-(c) show TEM images of multiple coatings after heat treatment at 200°C. In contrast to the non-heated coating, no visible defects, such as microcracks or pores, were observed at the bonding interface between distinct regions. This indicates that the plasticity of the powder after heat treatment at 200°C is considerably higher than that of the untreated powder and that considerable plastic deformation occurred during the cold-spraying process. Owing to the increasing plasticity of the particles, the bonding quality of the zone is also improving [29]. Finer precipitates were generated near the dendritic structures. This is because the solute elements in the dendritic structure are regenerated as intermetallic compounds after heat treatment. The interface lines between grains are no longer continuous in plate-like structures (marked as red circles). The proportion of larger-size sediments (marked as blue arrows) was reduced, and numerous fine spherical precipitates can be seen to have precipitated into the matrix. This is because the heat treatment deforms the powder to a greater extent during the cold-spray process; thus, the diffusion of grain boundaries is hindered. This is the primary strengthening phase of the 7xxx Al alloys. In the UFG structures ( figure 8(f)), the proportion of the precipitated phase increased substantially and formed a certain number of black spherical precipitates (marked as red arrows), which are further confirmed to be the intermetallic compound η′(MgZn 2 ) [30]. The heat treatment of the powder dissolves the dendritic structure inside the powder and promotes the generation of the precipitated phase, and the main strengthening phase η′ phase precipitates out at 200°C during the heat treatment. The cold-spraying process meets the conditions for generating the η′ phase. It can thus be hypothesized that due to the pre-treatment of the powder, some solute elements were dissolved from the   segregation structure into the matrix. So the solute elements in the matrix are present in the form of a η′ phase. Therefore, the strength of the coating is greatly enhanced [31]. Figure 9 shows TEM images of the UFG structures. There are two types of precipitation phases in this area. One is the square-precipitated phase of 200-500 nm. Crushing and renucleation occur during the cold-spray process. The other is a circular η′ phase with a diameter of less than 50 nm. There are many dislocation loops in the square-precipitated phase, which indicates that when the dislocation passes through the fine-grained region, Since bending a dislocation is a process of dislocation elongation, which requires higher energy and makes it difficult for the dislocation to move, the coating is strengthened, as a dislocation passes through the η′ phase, it is a process of overcoming resistance to do work, dislocation slip is difficult, and the coating is strengthened, that is, second phase reinforcement. The UFG structures play a very important role in structural strengthening [32]. Figure 10 shows HAADF images and EDX mappings of the transition zone of particle-to-particle junction obtained using heat treatment of powders at 200°C, which exhibit significant differences from the untreated coating in the slate-crystal and the UFG structures. The interfaces in the slate-crystal region change from being continuous in the untreated coating to discontinuous in the coating subjected to a heat treatment at 200°C, and numerous needle-like and flaky secondary phases precipitate around the interface. Numerous secondary phases can also be seen in the hyperfine-crystal region. The fine precipitated phases in the TEM images represent η′ phases [33]. Figures 11 and 12 shows the tensile strength and elongation of the coatings before and after heat treatment. For comparison, the coating properties obtained by direct cold spraying of the powder after heat treatment are also shown in the figure (denoted as CS-P). The CS specimens (RT) had the worst tensile strength and elongation of 228 MPa and 1.22%, respectively. Moreover, the tensile strength of the CS samples increased with increasing heat treatment temperature and reached a maximum value of 309 MPa at 200°C.However, the elongation of the CS samples almost stays the same when the heat treatment temperature does not exceed 180°C, while it increased abruptly to 2.46% at 200°C. The tensile strength of the CS-P sample reached 302 MPa compared to that of the coating after heat treatment at 200°C. Similarly, the elongation reached 3.87%, which is much larger than that of the heat-treated coating. According to the microstructure analysis above, the increase in strength and plasticity is mainly due to the dissolution of segregation and the formation of the second phase. The dissolution of segregation enhances the binding force between grain boundaries and makes solute elements dissolve in the matrix. The formation of the second phase also produces the second phase reinforcement.For the post-heat treated coating, the heat treatment promotes the formation of the second phase, and more energy is consumed when the dislocation bypasses these second phases, so the tensile strength is enhanced.For the preheat treated coating, in addition to the above precipitation strengthening mechanism, due to the increased plastic toughness of the powder, the particles are more deformed during the cold spraying process, resulting in finer grains in the UFG structure. Due to the different lattice structure orientation of the adjacent grains, more energy is needed for the dislocation to change direction and move into the adjacent grains. Grain boundaries are also more disordered than within grains, which also prevents dislocation from moving across the continuous slip plane.

Conclusion
Heat treatment was applied to cold-sprayed coatings and enhanced their mechanical properties. This is mainly due to the ability of heat treatment to eliminate residual stresses, refine grain structures, and eliminate dendritic networks. Furthermore, heat treatment can generate new strengthening phases. Heat treatment of the powder before CS can also enhance the mechanical properties of the coating. This is primarily because the heat treatment changes the microstructure of the powder, improves the bonding between particles, eliminates a portion of the dendritic network, and generates a strengthening phase. Under low-temperature heat treatment, tensile strength and elongation of 309 MPa and 2.46%, respectively, of the cold-sprayed coating can be obtained, whereas the preheat treatment of powder yields tensile strength and elongation of 302 MPa and 3.87%, respectively, of the coating. It has been observed that pre-heating the powder results in a coating with higher mechanical properties, providing insights into the cold spray repair sector.

Data availability statement
All data that support the findings of this study are included within the article (and any supplementary files).