Effect of Pre-Deformation on the Microstructure and Precipitation Behavior of Spray-Formed 7xxx Series Aluminum Alloys

Abstract

This study investigates the effect of pre-deformation on the microstructure and precipitation behavior of spray-formed 7xxx series aluminum alloys. Pre-deformation introduces a high density of dislocations, increasing the proportion of low-angle grain boundaries from 40% to 66%. After solution treatment at 580 °C, grain size significantly increases, ranging from 35 µm to 315 µm, with a higher proportion of larger grains observed in pre-deformed samples. Subsequent aging treatment refines the microstructure, resulting in grain sizes between 30 µm and 270 µm, and leads to a more uniform precipitate distribution.

1. Introduction

The 7xxx series aluminum alloys are derived from the Al-Zn-Mg-Cu alloy system, which is known for its high strength and excellent mechanical properties. These alloys are currently considered to be the highest-strength aluminum alloys [1]. Due to their excellent combination of high strength and low density, they are widely used in critical components such as aircraft wings, fuselage panels, and fairings [2]. According to the statistical data from the International Al Institute and the National Bureau of Statistics, the 7xxx Al alloy accounts for 70–80% in aircraft structural components, making it an indispensable lightweight structural material in aircraft [3]. With the development of the aviation industry, the requirements for structural components [4] have become more and more strict, and the research and development of 7xxx series aluminum alloys with even higher strength have become a development trend [5,6].

The 7xxx series aluminum alloys are typical heat-treatable strengthened aluminum alloys [7], and their common precipitation sequence is as follows: supersaturated solid solution (SSS) → GP zones (Guinier-Preston zones) → metastable η′ precipitate → equilibrium η phase [8,9,10,11,12,13]. The high strength of 7xxx series aluminum alloys is mainly due to the large amount of dispersed and fine metastable η′ precipitates [14] in the matrix formed during aging process [15,16,17]. Increasing the volume fraction of η′ precipitates can enhance the strength of 7xxx series aluminum alloys [18,19,20,21]. Qin et al. [22] studied the influence of alloying elements on the mechanical properties of Al-Zn-Mg alloys. They found that appropriate Zn and Mg contents led to more η′ precipitates in the matrix, which improved the alloy strength. Han et al. [23] investigated the influence of pre-deformation on the structure and properties of the Al-Zn-Mg-Cu alloy. Their findings indicate that pre-deformation-induced dislocations can serve as additional heterogeneous nucleation sites for precipitates, leading to a more uniform distribution of precipitates. Feng et al. [24] found that pre-deformation significantly enhances the aging precipitation of 7050 aluminum alloy, a high-strength Al-Zn-Mg-Cu alloy, which results in improved strength of the material. The 7050 alloy is widely used in aerospace applications for its excellent strength-to-weight ratio and resistance to stress corrosion. Similarly, the 7075 aluminum alloy, another Al-Zn-Mg-Cu alloy, is also known for its high strength and is often used in structural components that require superior mechanical properties. The 7050 aluminum alloy performs better in terms of corrosion resistance and machinability, while the 7075 aluminum alloy has higher strength but poorer corrosion resistance and machinability. Both alloys are strengthened primarily by the formation of η′ precipitates during aging. Lin et al. [25] investigated the mechanical strength of aged Al-Zn-Mg-Cu alloys subjected to varying levels of pre-deformation. The T6 aging process comprises a series of heat treatment steps: solution heat treatment at temperatures ranging from approximately 480 to 500 °C, followed by rapid quenching and, subsequently, aging at a lower temperature, typically between 120 and 160 °C. This process facilitates the formation of fine η′ precipitates within the alloy matrix, which significantly enhances the material’s strength by effectively impeding dislocation movement. Yu et al. [26] performed cryogenic T6 aging treatment on 7075 aluminum alloy. They found that with the prolonged aging time, the density of η′ precipitates initially increased and then decreased, and the alloy strength followed a similar trend. Therefore, the η′ precipitate [27] in the 7xxx series aluminum alloys dominates the strength of 7xxx series aluminum alloys [28].

In this paper, the spray-formed 7xxx series aluminum alloy was studied. Spray form-ing is an advanced rapid solidification technique that involves atomizing molten metal into fine droplets and depositing them onto a substrate, which significantly refines grain size and reduces segregation, thereby yielding a homogeneous microstructure and excel-lent mechanical properties [29]. Building on the advantages of this technology, this study further explores the method of introducing high-density dislocations through rolling pre-deformation during solid solution treatment (SSS) [30]. The influence of pre-deformation on the recrystallization structure [31] of the alloy, as well as its impact on the subsequent precipitation behavior of η′ during aging [32] were investigated. This heat treatment condition allows for the partial retention of dislocations introduced by pre-deformation, which plays a key role in promoting the nucleation of η′ precipitates during the subsequent aging process [33,34]. This study provides insights into increasing the volume fraction of η′ phases to enhance the strength of the 7xxx series aluminum alloy [35]. Although the effects of pre-deformation on aluminum alloys have been extensively studied, the specific role of pre-strain in spray-formed 7xxx series alloys remains poorly understood. In particular, the interaction between pre-deformation, precipitate evolution, and grain boundary characteristics has not been systematically investigated. This study aims to address this gap by examining the microstructure and precipitation behavior of spray-formed 7xxx alloys pre-deformation.

2. Materials and Methods

2.1. Materials and Processing

A total of 12 samples were prepared in this study, of which 6 samples were pre-deformed, and 6 samples remained undeformed as a control group. The composition of 7xxx series aluminum alloys was determined by Advant/p-381 X-ray fluorescence (XRF) spectrometry, with a measurement uncertainty of ±0.1 wt.% and a detection limit of 0.01 wt.% for the main elements. The results are listed in

Table 1. Chemical composition of the spray-formed 7xxx series aluminum alloy.

Elements	Weight Percent (wt.%)	Atomic Percent (at.%)
Al	90.5%	94.89%
Zn	6.14%	2.66%
Cu	1.70%	0.76%
Mg	1.36%	1.58%
Zr	0.172%	0.05%
Si	400 ppm	0.04%
Fe	300 ppm	0.02%

. The samples were provided by CITIC Dicastal Co., Ltd. (Qinhuangdao, Hebei Province, China), Qinhuangdao XinNeng Energy Equipment Co., Ltd. (Qinhuangdao, Hebei Province, China). The desired alloy composition is an Al-Zn-Mg-Cu-based alloy, which typically contains Zn as the primary alloying element, along with Mg and Cu, to achieve high-strength and excellent mechanical properties. The samples were cut into 10 mm × 10 mm × 10.4 mm specimens along the ND × RD plane using a Hua fang HF320Z electrical discharge CNC wire-cutting machine (Hangzhou Huafang CNC Machine Tool Co., Ltd., Hangzhou, China). The plane is defined by the normal direction (ND) and the rolling direction (RD). A detailed cutting schematic is shown in Figure 1. The samples were divided into two groups: one group went without pre-deformation treatment; while the other group underwent pre-deformation by cold rolling using a Crown CH350 rolling mill (Zhengzhou Shengyuan Machinery Co., Ltd., Zhengzhou, China). The initial thickness of the processed sample is 10.4 mm, while that after rolling is 9.8 mm, which means the thickness reduction is about 5%. Cold rolling was chosen as the pre-deformation method to introduce dislocations and strain into the material, which can enhance precipitation kinetics during subsequent aging treatments. Both groups were then placed in a furnace and held at 580 °C for 3 h, followed by being air-cooled. This stage, known as solution heat treatment, aims to dissolve the alloying elements (Zn, Mg, and Cu) into a solid solution and homogenize the microstructure. Although the solidus of 7xxx series aluminum alloys is usually between 460 and 490 °C, we chose 580 °C for heat treatment because higher temperatures can more effectively dissolve the original precipitates, allowing the size and distribution of the precipitates to be controlled during the subsequent aging process, thereby improving the mechanical properties. Subsequently, both groups were held at 120 °C for 24 h, followed by air-cooling. This stage, referred to as artificial aging, is designed to promote the formation of fine precipitates (such as η′ and η phases) within the matrix, which significantly enhance the alloy’s strength and hardness. The combination of solution treatment and artificial aging is commonly referred to as the T6 temper, which is used to enhance the mechanical properties in 7xxx series aluminum alloys.

2.2. Microstructure Characterization

The samples at different processing stages were cut into dimensions of 10 mm × 5 mm × 3 mm using an NH7720A-G wire-cutting machine (Beijing Ninghua Technology Co., Ltd., Beijing, China). The specimens were polished using a YMP-2B polishing machine (Changzhou Hengjiang Instrument Equipment Co., Ltd., Beijing, China). After polishing, the microstructure was directly observed under an IE500M metallographic microscope (Ningbo Sunny Optical Instruments Co., Ltd., Ningbo, China). The polished samples were also electrolytically polished in a 10% perchloric acid + 90% methanol solution, with a voltage of 18V and a duration of 10 s. Then, the samples were measured using a Zeiss Gemini300 field emission scanning electron microscope (SEM, Hitachi, Tokyo, Japan) equipped with an Electron Backscatter Diffraction (EBSD) detector. EBSD measurements were performed at 20 kV, with a working distance of 12 mm, a heading angle of 70°, and a scan step of 1 μm. The EBSD data were analyzed using OIM Analysis 8 software (version, OIM Analysis™ v8).

Specimens with a thickness of 400 μm were cut using an NH7720A-G wire-cutting machine (Beijing Ninghua Technology Co., Ltd., Beijing, China) and then ground to a thickness of 30 μm. Subsequently, a double-jet polishing process was conducted in a 30% nitric acid +70% methanol solution, using a TenuPol-5 double-jet polishing instrument (Stirling, Denmark) at −23 °C, which is cooled by liquid nitrogen. After double-jet polishing, the thin region around the micro-holes in the samples were observed by using a FEI Talos F200X field emission transmission electron microscope (TEM, JEOL, Tokyo, Japan), and energy-dispersive X-ray spectroscopy (EDS, JEOL, Tokyo, Japan) was employed to analyze the elemental distribution. TEM operating voltage is 200 KV, TEM point resolution is 0.25 nm, information resolution is 0.12 nm, and energy spectrum energy resolution is 136 eV.

3. Result

3.1. Microstructure After Rolling Pre-Deformation

Figure 2 shows the metallographic microstructures of the spray-formed aluminum alloy without and with pre-deformation. In the images, the solid solution is represented by light-grey areas, while the black regions along the grain boundaries indicate the eutectic structures formed during solidification. The dark-grey points within the grains represent secondary phases and etching pits. By comparing Figure 2a,b, it can be observed that the pre-deformation with a reduction of 5% did not cause significant changes in the microstructure of the alloy.

Figure 3 presents the EBSD images of the alloy without and with pre-deformation, where Figure 3a,b are inverse pole figure (IPF) maps indicating different crystal orientations by various colors. Figure 3a,b show inverse pole figure (IPF) maps, where different crystal orientations are indicated by various colors. In the alloy without pre-deformation, the grain colors are relatively uniform, while the alloy with pre-deformation exhibits more variation in color within the grains, marked by the red square, indicating small orientation variations. Figure 3c,d show the local kernel average misorientation (KAM) maps corresponding to Figure 3a,b. Notably, the alloy with pre-deformation exhibits localized bright areas in both small and large grains, indicating residual stresses due to grain growth during solidification. The bright areas, mainly located near grain boundaries, suggest that pre-deformation caused dislocation slip, with dislocations accumulating at grain boundaries. Figure 3e,f represent the grain boundary (GB) maps corresponding to Figure 3a,b, where the red and green lines represent small-angle grain boundaries, while the blue lines represent large-angle grain boundaries. From Figure 3e,f, it can be observed that the proportion of small-angle grain boundaries in the range of 1–5° is increased from 40% to 66% by pre-deformation, while the proportion of high-angle grain boundaries in the range of 15–180° is decreased from 56% to 31%.

3.2. The Microstructure of the Alloy After High-Temperature Heat Treatment

Figure 4 illustrates the metallographic microstructures of the alloy without and with pre-deformation after holding at 580 °C. In Figure 4a, the alloy without pre-deformation exhibits localized formation of non-equilibrium solidification eutectic structures (marked by the red rectangle), indicating that 580 °C exceeds the eutectic temperature of the alloy, which caused partial melting. In Figure 4b, the microstructure of the alloy with pre-deformation after holding at 580 °C appears relatively uniform, without apparent eutectic structures. However, the grain size is significantly larger compared to Figure 4a, suggesting that pre-deformation promotes rapid growth of the hypereutectic grains and suppresses the non-equilibrium solidification eutectic structures.

Figure 5 depicts the EBSD images after holding at 580 °C. Both specimens exhibit small, recrystallized grains, and the recrystallized grains are noticeably larger with pre-deformation (Figure 5b). Figure 5c,d show the KAM maps of the specimen. Comparing with Figure 3d, it can be observed that the bright areas in Figure 5d are significantly reduced. Furthermore, by comparing Figure 3f and Figure 5f, it is evident that the distribution of small-angle boundaries is significantly reduced after being held at 580 °C. This is attributed to the rearrangement and disappearance of dislocations during the heating process, leading to a decrease in dislocation density. The dislocations introduced by pre-deformation are likely retained because the 580 °C for 3 h heat treatment is not sufficient to completely eliminate them. These retained dislocations are visible in Figure 5c,d. Additionally, this process induces grain boundary migration, resulting in a significant increase in grain size after heating.

3.3. Microstructure of the Alloy After Aging Treatment

Figure 6 shows the metallographic images of the alloy after aging treatment at 120 °C. Comparing Figure 4a with Figure 6a, it can be observed that the eutectic structures in the alloy without pre-deformation are significantly decreased. Similarly, a comparison between Figure 4b and Figure 6b reveals that there is no significant grain growth observed in the alloy after the aging treatment. This suggests that the aging process at this temperature primarily affects the eutectic structures without substantially altering the grain size.

The EBSD analysis after the aging treatment is shown in Figure 7. The grain size distribution is similar to that after holding at 580 °C, indicating that grain growth is not significant. This may be due to the relatively low aging temperature (120 °C), which is insufficient to induce significant grain growth. The KAM maps in Figure 7c,d show local bright areas within the grains. In the image after aging treatment (Figure 7d), stronger bright areas are observed, especially in small grains, indicating more stress concentration in these small grains. Figure 7e,f show the distribution of low-angle and high-angle grain boundaries. After aging treatment, the proportion of low-angle grain boundaries 1–5° increases, possibly due to the accumulation of dislocations promoting the formation of low-angle grain boundaries. The fewer high-angle grain boundaries 15–180° suggest that the grains did not grow significantly during the aging process.

To intuitively compare the evolution of grain sizes, a statistical analysis of grain sizes in different states was conducted, and the results are shown in Figure 8. The values for the alloy without pre-deformation are marked in red, while those for the alloy with pre-deformation are marked in blue. Figure 8a shows the grain size distribution of the alloy before heat treatment. The grain size distribution of the alloy does not change much by pre-deformation, which is due to the small thickness reduction during pre-deformation. Figure 2 shows the corresponding SEM image of the alloy before heat treatment, where the grain structure is still relatively coarse and uniform across both groups. Figure 8b shows the grain size distribution of the alloy after holding at 580 °C. Compared to Figure 8a, the grain size is significantly increased and distributed in the range between 35 µm to 315 µm. After pre-deformation, the proportion of grains in larger size is higher, as seen in the SEM image (Figure 4), which indicates more pronounced coarsening of the grain structure due to the elevated temperature and time at 580 °C. Figure 7c shows the grain size distribution of the alloy after aging treatment. The grain size of the alloy without pre-deformation is distributed in the range between 30 µm to 150 µm, while that of the alloy with pre-deformation is distributed in the range between 30 µm to 270 µm; both are relatively smaller than that after holding at 580 °C. The corresponding SEM image in Figure 6 illustrates the finer grain structure after aging, with more uniform precipitate distribution and smaller grain sizes than those observed after the solution heat treatment.

The microstructures of the aged samples, both without and with pre-deformed, were observed using TEM, as illustrated in Figure 9. The black regions in the images represent the precipitates (marked by red circles). Comparing the bright-field (BF) images in Figure 9a,b, it is evident that the number of precipitates is increased by pre-deformation, and their distribution becomes more uniform. The high-resolution transmission electron microscopy (HRTEM) images are shown in Figure 9c,d, which reveals that the alloy without pre-deformation has fewer precipitates. In order to compare the dislocation distributions in Figure 9c,d more clearly, inverse fast Fourier transform (IFFT) was performed, and the results are shown in Figure 10, which reveals that the alloy without pre-deformation has more dislocations. Comparing Figure 5c,d with Figure 9c,d, it can be inferred that the abundance of dislocations after pre-deformation promotes the nucleation of precipitates, leading to a significant dislocation consumption during aging. Therefore, in Figure 9d, dislocations are almost absent, and larger-sized precipitates are present. To determine the crystal structure of the precipitates, fast Fourier transform (FFT) analysis was performed, as shown in Figure 9e. It indicates two sets of diffraction patterns, with one set corresponding to the matrix and the other set corresponding to η′ precipitate. The orientation relationship between η′ and the matrix is determined as follows. To further verify that the black regions marked in Figure 10 are η′ precipitates, an elemental distribution analysis of η′ precipitate was conducted, which is shown in Figure 11, where the regions enriched in Zn and Mg are η′ precipitates. In summary, pre-deformation induces a large number of dislocations, which are consumed during the aging process to promote the nucleation of η′ precipitates. The presence of numerous η′ precipitates can effectively hinder the movement of dislocations during plastic deformation, thereby enhancing the strength of the alloy.

4. Discussion

In this study, we investigated the effects of pre-deformation and aging on the micro-structure of 7xxx series aluminum alloys. The TEM analysis revealed that pre-deformation prior to aging resulted in a more refined and homogeneous distribution of precipitates compared to undeformed samples. This refinement can be attributed to the increased dis-location density introduced by pre-deformation, which provides more η′ precipitates during aging [36]. The formation of these fine, uniformly distributed precipitates is critical for enhancing the mechanical properties of the alloy as they act as effective barriers to dislocation motion [37].

The improved homogeneity of precipitates in pre-deformed samples suggests that pre-deformation could be a viable strategy for enhancing the mechanical properties of 7xxx series aluminum alloys in industrial applications, such as aerospace components. However, it is important to note that this study focused on a single aging temperature. Future work should explore the effects of varying aging temperatures and times to optimize the microstructure and properties of these alloys.

The formation of η′ precipitates during aging is significantly influenced by the dislocations introduced by pre-deformation. These dislocations act as preferential nucleation sites, leading to a more uniform distribution of fine precipitates. According to the Zener pinning mechanism, these precipitates effectively inhibit grain boundary migration, contributing to grain refinement and improved mechanical properties. The refined grain structure (average size reduced to 15 μm) further enhances the yield strength via the Hall-Petch relationship [38]. While the current study did not focus on the precise measurement of precipitate size, future work will employ advanced techniques such as HRTEM or APT to quantify these parameters and provide deeper insights into the strengthening mechanisms.

In conclusion, this study provides important insights into the effects of pre-deformation and heat treatment on the microstructure and mechanical properties of spray-formed 7xxx series aluminum alloys. The findings demonstrate that pre-deformation enhances dislocation density and promotes a more uniform distribution of precipitates, ultimately improving the alloy’s strength. These results contribute to a deeper understanding of how processing conditions influence the performance of 7xxx alloys, which is critical for advancing the design of high-strength, lightweight materials for aerospace and other high-performance applications. Future research should focus on correlating these microstructural changes with mechanical properties, such as tensile strength and fatigue resistance, and exploring alternative pre-treatment methods to further optimize alloy performance.

5. Conclusions

This study focuses on the effects of pre-deformation on the microstructure of the spray-formed 7xxx series aluminum alloys. The following conclusions were drawn:

• A large number of dislocations are formed by pre-deformation, which forms small-angle boundaries in grains. The proportion of small-angle grain boundaries is increased from 40% to 66% by pre-deformation.
• Upon heating at 580 °C, the solidification-formed second phases are dissolved. The alloy without pre-deformation shows eutectic structures, while the eutectic structures are suppressed by pre-deformation. In addition, pre-deformation promotes rapid grain growth during heating. In addition, some of the dislocations introduced by pre-deformation are retained after being held at 580 °C.
• During the aging process, the alloy with pre-deformation that has a lot of dislocations will facilitate the nucleation of η′ precipitates. As a result, dislocations are extensively consumed, and more η′ precipitates are formed in the alloy.
• The metallographic analysis shows clear effects of pre-deformation on the micro-structure. The group with pre-deformation exhibits more small-angle grain boundaries and a higher density of stresses, as shown by the EBSD KAM maps. This indicates that pre-deformation significantly alters the grain boundary character and precipitate distribution, influencing the alloy’s mechanical properties.