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Evolution of Microstructure, Mechanical Properties and Homogeneity of Al-Zn-Mg-Cu-Zr Alloy Plates Prepared by Different Percentage Reductions per Pass  PDF

  • Yan Liangming 1,2 ✉
  • Hu Qiang 1
  • Wang Wenbo 1
  • Hou Xiaohu 1,2
  • Xu Junrui 1,2
  • Liu Huimin 1,2
  • Shen Jian 3
1. School of Materials Science and Engineering, Inner Mongolia University of Technology, Hohhot 010051, China； 2. Inner Mongolia Key Laboratory of Light Metal, Hohhot 010051, China； 3. GRINM Group Co., Ltd, Beijing 100088, China

Updated：2021-08-06

DOI：XX.XXXX/j.issn.1002-185X.2021.07.009

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Abstract

The effects of percentage reduction per pass (PRPP) on the effective strain, microstructure, mechanical properties and through-thickness homogeneity of AA7055 alloy plates were investigated through experiments and numerical simulations. Results show that with consistent total strain, the difference between the effective strain values in the surface and middle layer of AA7055 alloy plates decreases with the increase of PRPP. In the plate rolled with small PRPP, the surface layer of plate contains higher recrystallization fraction, while the middle layer comprises of large recrystallized grains. In contrast, the plate rolled with large PRPP has consistent recrystallized grain size with consistent recrystallization fraction along the thickness direction. Thus, an improved homogeneity of the microstructure and mechanical properties along the plate thickness direction can be obtained through the rolling process with large PRPP.

Science Press

Al-Zn-Mg-Cu aluminum alloy thick plates with high-strength and low density were extensively used in aero-space[

1-3]. Homogeneity of microstructures and mechanical properties is an important consideration during the rolling process of Al-Zn-Mg-Cu alloy plates[4-8]. In order to evenly generate severe plastic deformation along the sheet thickness direction, sheets of 7xxx series alloys are generally produced by asymmetric rolling[8,9]. The homogeneity of 7050 Al alloy sheet improves when thickness reduction per pass decreases during asymmetric rolling[8]. However, there are still some challenges in the asymmetric rolling process for producing thick plates of 7xxx series Al alloys, such as controlling the plate shape. In contrast, it is easy to control the symmetric rolling process which can improve the microstructure and mechanical properties of alloys[10,11]. In the case of traditional symmetric rolling, the effects of total reduction[12,13], tempe-rature[14,15], and deformation mode[16,17] on the microstructures, textures, and mechanical properties of 7xxx series alloy plates were investigated. Increasing the total reduction leads to an enhancement in the proportion of recrystallized grains and a decrease in size of sub-grains, thereby improving the strength[12,18]. Reports also revealed that the rolling process of higher percentage reduction per pass (PRPP) can refine the grains of Mg sheets[19-21]. However, the mechanism of PRPP effects on the mechanical properties and microstructure of aluminum alloy plates prepared by symmetric rolling is unclear and needs to be further studied.

Therefore, 7055 aluminum alloy plates in this research were rolled with small and large PRPP separately. The effective strain distribution was analyzed by finite element (FE) analysis. The influence of PRPP on the microstructures was studied by electron back-scattering diffraction (EBSD) and transmission electron microscopy (TEM). Mechanical proper-ties were investigated by tensile tests. Finally, the effects of PRPP on uniformity along thickness direction of AA7055 plates were investigated.

1 Experiment

The chemical composition of the AA7055 aluminum alloy in this study is listed in Table 1. After homogenization at 470 °C for 24 h, AA7055 ingot was machined into a cuboid specimen with a height of 60 mm. Samples were firstly heated to 430 °C and held for 20 min, and then rolled at 410 °C. The specimens were finally rolled to a height of 12 mm by small PRPP (process-I) and large PRPP (process-II) separately, as listed in Table 2. These rolled plates were solution treated at 470 °C for 70 min and 480 °C for 20 min, and immediately aged at 120 °C for 24 h. Specimens of surface layer and mi-ddle layer for the microstructure observation and the tensile tests were cut at specific position, as shown in Fig.1.

Table 1 Chemical composition of AA7055 Al alloy (wt%)
ZnMgCuZrMnAl
7.80 2.20 2.23 0.10 0.1 Bal.
Table 2 Rolling process parameters (mm)
ProcessRolling schedule
Ⅰ 60→58.44→56.88→55.28→53.68→52.12→49.88→47.49→44.31→41.16→38→34.82→31.67→28.5→26.37→22.2→19→15.6→12
Ⅱ 60→58→55→52→48→43→37→31→24→17→12

Microstructures of the plates were observed by TEM operated at 180 kV and a SSX-550 scanning electron micro-scope (SEM) equipped with an HKL Technology Channel5 EBSD system. TEM specimens were prepared by machining the discs of 3 mm in diameter and thinning them to 50 μm in thickness. The specimens were then treated by twin-jet electro-polishing using a solution of 1vol% HF+2vol% HCl+3vol% HNO3 at -20 °C. EBSD specimens sectioned along the normal direction (ND)-rolling direction (RD) plane were also prepared by electro-polishing with the same solution at -20 °C after mechanical polishing.

Tensile tests were conducted on a MTS810 tester at room temperature. Flaky test specimens were prepared according to the GB/T228.1-2010 standard[

22], as shown in Fig.2.

Fig.2 Standard test specimen for tensile test

FE simulations of the rolling process were performed using the FORGE FE analysis software package. The rollers were considered as rigid bodies. Because the deformation of rollers is far less than that of rolling plate, it will not be discussed in this research. The flow stress model of AA7055 alloy used in the study comes from Ref.[

23]. The related rolling parameters obtained from the earlier reports[9,24-26] are listed in Table 3.

Table 3 Related rolling process parameters
ParameterValueRef.
Temperature/°C 410
Work roll radius/mm 500
Work roll angular velocity/rad·s-1 3.2 [24]
Friction coefficient 0.35 [9], [24]
Coefficient of thermal conductivity between the rolls and plate/W·m-2·K-1 182.8 [26]
Young's modulus/GPa 31.5 [24]
Poisson's ratio 0.3 [25]

2 Results and Discussion

2.1 Microstructure of rolled AA7055 alloy

Fig.3 shows the typical microstructure along the transverse direction (TD)-ND plane (TD-ND section) of the as-rolled AA7055 plate. It is found that grains are elongated along the transverse direction, resulting in the cake-like structure. Also the inhomogeneity in the sizes of the deformed grains along the thickness direction is evident, as shown in Fig.3a and 3b. Orientation imaging microscope (OIM) image of the plate surface layer rolled by process-I shows that the largest grain size along thickness direction is around 60 μm, and the grains with the size of 30~40 μm account for 60%, as shown in Fig.3a. At the initial grain boundary, there are some recrystallized grains with the size of 2~15 μm, and the volume fraction of recrystallized grain is 20%. Fig.3b shows the OIM map of the plate middle layer rolled by process-I. The size of deformed grain significantly varies along the thickness direction, and some grains have a large size of ~100 μm, which is similar to the situation of as-homogenized grains. The volume fraction of recrystallization is 12%, and the recrystallized grain size is smaller. The volume fraction of recrystallized grains with the size more than 8 μm is about 6%. Fig.3c shows the OIM map of the plate surface layer rolled by process-II. The largest size of deformed grain reaches 60 μm, and the grains with the size of 30~40 μm account for 70%. There are many recrystallized grains with sizes of ~10 μm at the original grain boundary. The volume fraction of recrystallized grains is 15% and the size is less than 5 μm. Fig.3d shows the OIM map of middle layer of the plate rolled by process-II. The volume fraction of recrystalli-zation reaches 12% and the maximum size of the deformed grains along the thickness direction is ~60 μm. From Fig.3c and 3d, it is evident that the grain size along the thickness direction of the plate prepared by process-II is more homogenous than that of the plate produced by process-Ⅰ.

Fig.3 OIM maps of surface (a, c) and middle (b, d) layers of AA7055 alloys rolled by process-I (a, b) and process-II (c, d)

2.2 Microstructure of solution-treated plate

Fig.4 shows the grain boundary maps and grain size distribution of rolled AA7055 plates after solution treatment. The plate rolled by process-I exhibits a recrystallization fraction of 51% in surface layer (Fig.4a1) and 3% in middle layer (Fig.4b1). The average grain size is around 34 μm in surface layer (Fig.4a2) and 40 μm in middle layer (Fig.4b2). For the plate rolled by process-II, the recrystallization fraction is about 44% in surface layer (Fig.4c1) and 39% in middle layer (Fig.4d1). The average grain size is about 32 μm (Fig.4c2) in surface layer and 37 μm in middle layer (Fig.4d2).

2.3 Mechanical properties

Tensile strength and elongation of the T6-treated AA7055 alloy plate are shown in Fig.5. It is evident that the mecha-nical properties are influenced by the microstructure shown in Fig.4. Compared with that of the middle layer, the tensile strength at surface layer of plate rolled by process-I increases by 5.45%, whereas as the elongation increases by 9.68%. However, the tensile strength and elongation of the surface layer of the plate rolled by process-II are 1.42% and 4.37% larger than those of the middle layer of the plate rolled by process-II, respectively. The difference of mechanical proper-ties between the surface and middle layers for the plate rolled by process-Ⅱ is less than those of the plate rolled by process-Ⅰ.

Results show that the uniformities of microstructure and mechanical properties along the thickness direction of AA7055 alloy plate are affected by PRPP. From Fig.3~5, it is observed that the mechanical properties and microstructures along the thickness direction in AA7055 plate prepared by both processes are obviously nonuniform. However, the large PRPP can improve the uniformity of AA7055 plate. Microstructure and mechanical properties in plate are relative to strain distribution[

8,27,28]. Fig.6 shows the equivalent strain contours of the rolled AA7055 plates. The equivalent strain of the surface and middle layers of the plate rolled by process-I is 0.768 and 0.511, respectively, as shown in Fig.6a. The equivalent strain difference between the surface and middle layer is 0.257. The large accumulative strain is beneficial to breaking the coarse second phase, which improves the second phase dissolution during the solid solution and precipitation after aging treatment, improving the mechanical properties[5,18]. A large amount of accumulative strain causes the formation of fine grains and sub-grains (Fig.3a) and high deformed storage energy in the surface layer of AA7055 plate rolled by process-I. High deformed storage energy provides a large number of recrystallization sites during the solution treatment, resulting in a large recrystallization fraction (Fig.4a1). However, the lower strain accumulation and deformed coarse grains in the middle layer lead to fewer recrystallization sites and that recrystallized grains locally grow (Fig.4b1) during the solution treatment. The plate rolled by process-II has equivalent strain of 0.717 in surface layer and 0.563 in the middle with the strain difference of 0.154. The strain distribution along thickness direction in plate rolled by process-II is more uniform than that in plate rolled by process-I. A relatively homogenous strain distribution leads to the uniform microstructure and mechanical properties.

Fig.6 Effective strain distribution along longitudinal cross-section of AA7055 alloy plate rolled by process-I (a) and process- II (b)

2.5 Effect of PRPP on recrystallization after solid solution

For as-homogenized AA7055 alloy, the grains are nearly equiaxed, and the distribution of Al3Zr dispersoid is inhomo-geneous. The density of Al3Zr dispersoid gradually decreases from the grain center to the grain boundary and becomes almost none in the area close to the boundary (Fig.7). A simi-lar phenomenon has been reported recently for 7xxx series alloys[

29-32]. After rolling, the Al3Zr particles are concentrated in the grain center, while the coarse second phase particles are distributed along the grain boundaries parallel to the rolling direction. During the solid solution, new grains nucleate near the grain boundary by strain induced boundary migration (SIBM) and/or particle stimulated nucleation (PSN)[33,34] and grow into the initial grain. Growth of the new grains stops when the new grain boundaries are hindered by Al3Zr particle[30].

Fig.7 Al3Zr particles in as-homogenized AA7055 alloy

The plate prepared by process-I has the same total reduction as the plate produced by process-II does. However, the central grain size of the plate rolled by process-I along the normal direction is larger than that of plate rolled by process-II. Thus, the distribution of Al3Zr particles in the plate rolled by process-I is different from that of the plate rolled by process-II. Fig.8 shows the distribution model of Al3Zr particles in different plates. The distance along the normal direction between the precipitation free zones (PFZs) of Al3Zr particles in the plate rolled by process-I is greater than that in plate rolled by process-II. The new grain boundaries in the plate rolled by process-II are hindered by Al3Zr particles when the new grain size is relatively small. Hence, the grain size in plate rolled by process-II is smaller than that of the plate rolled by process-I.

Fig.8 Schematic diagram of Al3Zr particles distribution in grains of different plates

3 Conclusions

1) The percentage reduction per pass (PRPP) significantly affects the strain distribution of AA7055 plate, which is consistent with the microstructure and mechanical properties.

2) The homogeneity of effective strain is significantly im-proved by the rolling process of large PRPP, leading to different recrystallization behavior.

3) The volume fraction of recrystallized grain increases with the increase of effective strain, resulting in the increase of homogeneity of recrystallized grain in volume fraction and in grain size for the plate prepared by large PRPP.

4) The tensile strength, elongation, and homogeneity of AA7055 plate along thickness direction also improve after the process of large PRPP.

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