Effect of Compressive Strain Rate on Microstructure and Mechanical Properties of 7050 Aluminium Alloy

. 7050 aluminium alloy is a superior material used in the areas of aerospace and automobile manufacturing. In this work, homogeneous cast cylinder samples of 7050 aluminium alloy with size of Φ 80mm × 100mm were compressed isothermally at 350 ° C with compressive strain rate of 0.1s − 1 ,1s − 1 , and 10s − 1 , respectively. The samples were then processed into standard tensile specimens, and then tensile testing was done by using the GL8305 universal testing machine. The grain sizes and fracture morphology were analyzed by the SEM observation and cellular automaton (CA) method. The eﬀect of compressive strain rate on microstructure and mechanical properties of 7050 aluminium alloy was investigated. The results show the following. (1) Grain reﬁnement occurred after compression. The grain sizes of the samples decrease with the decrease of compressive strain rate. The grain sizes in the radial edge and the axial center of the cylinder samples are the smallest. (2) The tensile strength and breaking elongation rate were improved when compared to the original alloy. The mechanical properties of samples compressed with compressive strain rate of 0.1 s − 1 are the best. (3) The fracture morphology of the samples shows that the fracture of the samples is ductile fracture. The sizes and depths of dimples increased with the decrease of grain sizes.


Introduction
7050 aluminium alloy is widely used in the fields of aerospace, automobile manufacturing, high-speed rail manufacturing, and shipping industry due to its high strength, good toughness, excellent corrosion resistance, and lightweight [1][2][3][4]. Hot extrusion is one of the main plastic forming process methods for manufacturing 7XXX series aluminium alloy with high performance [5][6][7]. e parameters of hot extrusion, such as extrusion speed, processing temperature, extrusion pressure, and so on, largely affect the properties of aluminium alloy.
In the past decades, research studies have been done to investigate microstructure evolution and mechanical properties of aluminium alloy processed by the hot compression process. e microstructure of aluminium alloys usually experiences dynamic recrystallization (DRX) during the hot extrusion process. Chen et al. [8] investigated the microstructural evolution under compression tests over a wide range of strain rates. Work hardening deceases with the increase of strain rate. Dynamic recrystallization with grain refinement and phase transition occurred during compression. Li et al. [9] found that the fraction of DRX increased with decrease of strain rate, and Huang et al. [10] found that the percentage of DRX increased with the rise of temperature. Zhao et al. [7] found that the recrystallization mechanism during isothermal compression changed from continuous dynamic recrystallization to discontinuous dynamic recrystallization mechanism as the strain rate decreased from 10 −3 s −1 to 5 × 10 −6 s −1 . Cardoso et al. [11] compared the grain sizes and hardness of 7050 aluminium alloy process with equal channel angular pressing (ECAP) at room temperature and 150°C. e sample pressed at 150°C has more refined grains and larger hardness. Yunzhong et al. [12] investigated the grain evolution at different parts of the 7050 aluminium alloy cylinder sample and the result shows the deformation of grain at the central part get larger when the strain increased while the grains becomes smaller and more homogeneous. However, the evolution of grains associated with strain distribution at different parts of cylinder samples during hot compression is much more complicated.
us, the evolution of grains of cylinder aluminium alloys needs to be researched in greater detail. e mechanical properties of alloy are largely related to the microstructure, such as the grain shape, and the type and orientation of texture. Branco et al. [13] researched the effect of strain ratio on cyclic deformation behaviour of 7050 aluminium alloy. e material exhibited a cyclic strain softening behaviour, and the microstructure shows mainly elongated grains aligned with the rolling direction with a certain degree of recrystallization. Wei et al. [14] found that the yield strength in the center of the cylinder sample with a large grain size is higher than that at the edge with the smaller grain size in the thickest section of the cylinder. is is because the effect of texture on the yield strength overweighed the refinement strengthening, where copper-orientated ({112} < 111>) deformation texture with better yield strength dominated the center area and cube-orientated ({112} < 111>) recrystallized texture dominated at the edge. Few investigations have studied the tensile strength and tensile fracture combined with evolution of microstructure of compressed 7050 aluminium alloy. Additional research studies are important to explore the relationship between mechanical behaviours and microstructure of 7050 aluminium alloy resulted from hot compression during the tensile experiment.
In our work, 7050 aluminium alloy cylinder samples with size of φ80 mm × 100 mm were isothermally compressed with three strain rates (0.01 s −1 , 0.1 s −1 , and 1 s −1 ) at 350°C. e total strain value of the compression is 0.6. e cylinder samples are then processed to standard tensile specimens at room temperature. Tensile testing was done by using the GL8305 universal testing machine. e microstructure and the grain evolution were observed via EBSD and simulated with the CA method. e effect of strain rate on alloy microstructure and mechanical properties was investigated. e fracture was characterized and analyzed.

Constitutive Model and Dynamic Recrystallization Model.
e high-temperature rheological behaviour of 7050 aluminium alloy is not only related to the composition of chemical elements but also affected by the deformation temperature and strain rate. e Arrhenius model [15] is used to describe the high-temperature rheological behaviour of aluminium alloy in this work. It is mathematically expressed as follows: where A, b, and n are material constants, which can be calculated via isothermal uniaxial tensile test, _ ε and σ are the strain rate and the stress, Q is the activation energy of deformation (kJ/mol), T is the temperature (K), and R is the gas constant, which is 8.314 J/(mol·K).
Nucleation of dynamic recrystallization is a complicated process which usually occurs locally in the deformed matrix. e decrease of dislocation density causes the decrease of free energy of the sample while formation of new intersurface between new crystal nuclei and the matrix leads to the increase of free energy [16]. e model of dynamic recrystallization can be expressed as follows: where x 1 , x 2 , x 3 , b 1 , b 2 , n 1 , n 2 , m 1 , m 2 , β d , and k d are material constants in the recrystallization model, which can be obtained from isothermal uniaxial tensile test, d 0 is the original grain size of 7050 aluminium alloy (its value is ∼70 μm), Q 1 and Q 2 represent the activation energy of grain growth and the activation energy of recrystallization (kJ/ mol), ε 0.5 is the strain when volume fraction of dynamic recrystallization reaches 50%, and ε p is peak strain of dynamic recrystallization which can be calculated by the following formula: where x 4 , b 3 , and m 3 are material parameters, which can be calculated by second derivative of the function between strain hardening rate and stress. For more details on the aforementioned parameters of 7050 aluminium alloy, the readers are referred to reference [17], and the values are shown in Table 1.

Cellular Automaton (CA) Method. Cellular automaton
method is a mathematical model of dynamic system which is discrete in time and space. It consists of five fundamental concepts: cellular unit, cellular state, cellular neighbour type, cellular space, and cellular transition rules. e cellular unit is defined as 2.5 μm × 2.5 μm square. e cellular state contains the state variable i and the direction variable j. e cellular unit is solid state if the variable i � 1 and the cellular unit is liquid if i � 0. e value of j is any integer between 0-180, and each integer represents a specific orientation. e cellular space is defined as a square cellular grid with cellular unit number of 400 × 400. us, the space size of cellular space is 1 mm × 1 mm according to the cellular unit size of 2.5 μm. e state of the neighbouring cellular units at specific moment t can be expressed as follows: where c t−Δt C i represents the state of the cellular unit at specific moment t, C k is the Kth neighbour of the cellular unit, f is the functional relationship, and Δt is the variation of time. 2 Advances in Materials Science and Engineering

Materials and Experiment
Method. e 7050 aluminium alloy was provided by Southwest Aluminium Group Co. Ltd. e alloy ingots were slowly heated to 470°C at the rate of 0.5°C/s and were annealed for 24 h. en, alloy ingots were then water cooled to room temperature. e heat treatment process helped to preserve the homogenized structure and prevent element segregation near the grain boundary. After homogenization treatment, 7050 aluminium alloy ingots were processed to cylinder samples with size of Φ80 mm × 100 mm. e chemical composition of the alloy was characterized by the PDA-5500S Shimadzu spectrum analyzer. e chemical composition of the sample conforms to 7050 aluminium alloy according to the standard GB/T3190-1996, which is displayed in Table 2.
e single-pass compression test of the cylinder samples was done by using the thermal physics simulator Gleeble 1500. ermocouples were welded on the side of the samples to monitor and collect test temperatures. e compression tests were done by using the thermal physics simulator Gleeble 1500. e samples were heated to 350°C at a rate of 10°C/s and annealed for 60 s to reduce anisotropy before compression. e cylinder samples were compressed at different rates (0.1 s −1 , 1.0 s −1 , and 10.0 s −1 ) until the total tensile strain reached 0.6. e grains in the different parts of the cylinder samples were observed and compared with electron backscattering diffraction (EBSD). Standard tensile specimens were prepared from the cylinder samples according to standard of DB52/T 926-2014. ese tensile specimens were taken from the central axis of the cylinder, and the sampling location is shown in Figure 1(a). e detailed size parameters of the standard tensile specimen are shown in Figure 1(b). Tensile testing was performed to examine the mechanical properties of these compressed samples.
e quasistatic tensile test of specimens was carried out on the GL8305 universal testing machine. e mechanical parameters: tensile strength and fracture derivation rate, were measured according to GB/T228.1-2010 [18]. e fracture was electrochemically polished with an electrolyte solution mixed with 10% perchlorate and 90% ethanol.
e voltage, the current, and the time of electrolytic polishing are 26 V, 1.2 A, and 4 s, respectively. en, the specimen was anodically coated with a solution which consists of 5 mL fluoroborate and 200 mL deionized water. e voltage, the current, and the time of coating are 25 V, 1.5 A, and 50 s. en, the fracture was ultrasonically cleaned after electrochemical polishing and anodical coating. e fracture was observed by using the JSM-6510 scanning electron microscope (SEM).

Finite Element Model.
e thermal compression process and grain evolution of 7050 aluminium alloy cylinder were simulated in the software DEFORM 3D V6. e high-temperature rheological model of the material was assigned according to formula (1), and the dynamic recrystallization model was assigned according to formulas (2)∼(5). e cylinder rod was premeshed in the software Hypermesh14.0. e mesh elements were mainly in a hexahedral shape, and the grids at the notch were locally densified. e total number of the grids was 36278. e punch and the die were set as rigid bodies. e temperature was set at 350°C. e die was fixed, and the punch moved at speeds of 1 mm/s, 10 mm/s, and 100 mm/s, respectively. e punch stopped moving until the nominal strain reached 0.6. e results of the previous step were imported into the postprocessing microstructure simulation module. e evolution of alloy grains during the tensile test was simulated via the cellular automaton (CA) method.
e plastic damage and mechanism of grain evolution were investigated. Figure 2 shows the grain images oriented at the axial center of the cylindrical sample. Figures 2(a) Figures 2(b), (c) and (d) indicate that the grains of the compressed samples were refined after plastic deformation. As shown in Figure 2(b), coarse grains of samples loaded with a strain rate of 0.01 s −1 were crushed. A large number of fine grains appeared both inside the grains and near the grain boundary, which showed equiaxed shape. It reflects that dynamic recrystallization was fully finished in this sample. e microstructure of samples processed with strain rate of 0.1 s −1 at 350°C (Figure 2(c)) reveals that a lot of coarse grains were squeezed. A small number of fine grains appeared near the boundary of coarse grains, suggesting that dynamic recrystallization partially happened. In Figure 2(d), fine grains nucleated near the grain boundaries of most of the grains. It indicates that the dynamic recrystallization was about to happen. Table 3 shows the statistic of mean grain size derived from the EBSD and CA simulation method. e data of Table 3 and the insets in Figure 2 indicate that the mean grain sizes simulated by CA are consistent with the results of the images taken by EBSD. e deviation of mean grain sizes between two ways is smaller than 5%.

e Evolution of Microstructure.
When the compressive rate is small, grain refinement in the alloy driven by the dynamic recrystallization mechanism can fully take place. e recrystallization process is attributed to sufficient degree of freedom for rotation and rearrangement of grain nuclei and subgrains [19]. When the compressive rate increases, there is low degree of freedom Table 1: e value of parameters of the dynamic recrystallization model. Advances in Materials Science and Engineering for rotation and rearrangement of grain nuclei and subgrains. High dislocation density-induced work hardening mechanism dominates the evolution of inner structure while there is not enough time to trigger the dynamic recrystallization of grains. In consequence, grains with small sizes are only formed around the grain boundaries in the samples compressed with high rate, as shown in Figure 2(d). e refinement of alloy grains can be realized by decreasing the compressive rate to improve dynamic recrystallization effect. Figure 3 shows the grain size distribution along the axial and the radial directions of the sample loaded with compressive strain rate of 0.01 s −1 at 350°C. e strains and grain   e image of upper surface shows that the grains at the edge of the surface are the finest. e grain size increased from the edge to the center along the radial direction. e value increases from 2.38 μm to 50.3 μm. e axial change of grains indicates that the grain size increases from the cylinder center to both sides of the cylinder axially. is is because both sides of the sample were in contact with the punch and the die, respectively. e friction between the contact surfaces during plastic deformation obstructs the radial migration of atoms, which leads to the axial reduction of strain. e end face of the cylinder sample is a freely deformed surface with no obstruction of friction. erefore, it migrates outside during the deformation. Furthermore, the local strain at the edge of upper and bottom surfaces of the sample also increased because of the "pull effect" from the end face of the cylinder. e strain distribution is shown by the coloured strain pattern of cylinder sample with strain rate of 0.01 s −1 at 350°C as shown in Figure 4. e sample in Figure 4 presents a bulge shape. e strain is the largest at the edge of the upper and bottom surfaces and at the center of the samples. e strain is the smallest at the center of the upper and bottom surfaces. e grain refinement is closely related to the strain distribution. e larger the strain is, the higher the grain refinement is. Figure 5 shows the curve of strain vs. deformation time of the sample with compressive strain rate of 0.01 s −1 at 350°C. Data of spots P1-P5 marked in Figure 5 were collected. e strain of P1 is 1.78 which is the largest of the alloy after deformation. e largest strain value of P4 after deformation is 1.64. e strains of P2, P3, and P5 show a similar curve as shown in Figure 5. Figure 6 shows the strain distribution along the axial and radial directions. e radial direction takes the center of the circle on the upper surface as the origin point, and the axial direction takes the center of the cylinder as the origin point. e radial curve indicated that the strain of about 80% of spots kept a value of ∼0.5 mm/ mm. e strain along the axial direction decreases from the center to both sides evenly. Figure 7 shows the grain size distribution along axial (Figure 7(a)) and radial (Figure 7(b)) directions with different compressive strain rates. e distribution of grain sizes is in great agreement with distribution of strain. As shown in Figure 7(a), the grain sizes decreased from the  e grains sizes increased from the cylinder center to up and down surfaces axially as shown in Figure 7(b). e grain sizes of the sample with compressive strain rate of 0.01 s −1 were the smallest.

Mechanical Properties.
True stress-strain curve of standard tensile samples was transformed by stress-strain curve obtained by using the universal testing machine GL8305. Figure 8 illustrates that the tensile strength and fracture elongation of sample subjected to the compression deformation both increased in different degrees. e tensile strength and fracture elongation rate of the raw tensile   strengthening is an efficient way to improve mechanical properties of aluminium alloys, which was also reported in other works [20,21]. erefore, the improvement of the tensile strength of the compressed samples is mainly attributed to grain refinement, and this is in great agreement with the refinement degree of samples treated with different compressive strain rates. Figure 9 shows the morphology of tensile fracture of the compression sample with different strain rates observed by SEM. It can be seen from Figure 9 that a large number of dimples were observed at the tensile fracture, indicating that 7050 aluminium alloy has good plasticity after thermal compression [22]. e dimples of the sample with strain rate of 0.01 s −1 were the largest and deepest. Small particles observed in the images were impurity precipitates. e dimples nucleated at the interface between the impurity precipitates and matrix grains and then grew up. Small dimples appeared at the edge of large dimples in the sample with strain rate of 0.1 s −1 . e dimples were relatively shallow, and tearing ridges were formed on the fracture surface. e fracture morphologies of three specimens were in great agreement with the results of the tensile tests.

Conclusion
7050 aluminium alloy cylinder samples with size of V80 mm × 100 mm were isothermally compressed with three strain rates (0.01 s −1 , 0.1 s −1 , and 1 s −1 ) at 350°C. e cylinder samples were processed to standard tensile specimens at room temperature. Tensile testing was done by using the GL8305 universal testing machine. e grain evolution and fracture morphology were analyzed by the CA simulation method and SEM observation. e effect of compressive strain rate on alloy microstructure and mechanical properties was investigated. For 7050 aluminium alloy, the following conclusions can be drawn: (1) Grain refinement occurred during the compression deformation of 7050 aluminium alloy. e sample with strain rate of 0.01 s −1 achieved the smallest grain size. e analysis of sample with strain rate of 0.01 s −1 reveals that the grain size decreased radially from the center of the upper surface. e grain size increased axially from the center of the cylinder to both sides.

Data Availability
No data were used to support this study.

Conflicts of Interest
e authors declare that they have no conflicts of interest.