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Acta Metall Sin  2017, Vol. 53 Issue (8): 907-917    DOI: 10.11900/0412.1961.2016.00480
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Optimization and Controlling on the Microstructure, Texture and Properties of an Advanced Al-Mg-Si-Cu-Zn Alloy Sheet
Yi CHEN, Mingxing GUO(), Long YI, Bo YUAN, Gaojie LI, Linzhong ZHUANG, Jishan ZHANG
State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing 100083, China
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To reduce the weight of car body, Al-Mg-Si-Cu series alloys have been widely used to produce outer body panels of automobiles due to their favorable high-strength-to-weight ratio, corrosion resistance and good formability. Moreover, the strength of Al-Mg-Si-Cu series alloys can be enhanced by artificial ageing treatments. However, their formability and final strengths still need to be further improved compared to steels, which are the major obstacles to wide-scale application of aluminum in the automotive fields. In this work, both the effect of different thermomechanical processes on formability, microstructure and texture of Al-Mg-Si-Cu-Zn alloy, and the influence of ageing treatment on its precipitation behavior were studied through mechanical property tests, OM, SEM, TEM and EBSD measurements. The results reveal that both the strengths and strain-hardening component n value of the T4P treated alloys are not affected by the two thermomechanical processes, but the r?, Δr and elongations in the different directions are significantly affected. The microstructure and texture evolution of the alloy in the two thermomechanical processes are different from each other. Both the microstructure of a little coarser and bi-modal grain size distribution, and the texture characteristics of much more components but with quite lower intensities can be seen in the solution treated alloy sheet which possesses a better formability after the T4P treatment. The hardness increment of 65 HV can be achieved in the quenched alloy after artificial ageing treatment of 185 ℃, 20 min. And then the peak-ageing state can be obtained after ageing 5 h, the hardness, yield strength, ultimate tensile strength and elongation, are as follows, 132 HV, 318 MPa, 364 MPa and 13%, respectively, and ductile fracture is the main fracture feature as observed by SEM examination of fracture surface. Mg-Si precipitates, such as β", β' and Q' phases, are still the main precipitates formed after artificial aging at 185 ℃, and β" phases mainly grow along its b axis and finally transform into β' and Q' phases, which is the main reason for the observed better ageing stability during long time artificial ageing treatment.

Key words:  Al-Mg-Si-Cu-Zn alloy      thermomechanical processing      texture      formability      precipitation     
Received:  27 October 2016     
ZTFLH:  TG146.2  
Fund: Supported by National Key Research and Development Program of China (No.2016YFB0300801), National Natural Science Foundation of China (Nos.51301016 and 51571023), National High Technical Research and Development Program of China (No.2013AA032403) and Beijing Natural Science Foundation (No.2172038)

Cite this article: 

Yi CHEN, Mingxing GUO, Long YI, Bo YUAN, Gaojie LI, Linzhong ZHUANG, Jishan ZHANG. Optimization and Controlling on the Microstructure, Texture and Properties of an Advanced Al-Mg-Si-Cu-Zn Alloy Sheet. Acta Metall Sin, 2017, 53(8): 907-917.

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Fig.1  Engineering stress-strain curves of T4P treated Al-Mg-Si-Cu-Zn alloy in different directions (a) processing I (b) processing II
Processing Direction / (o) r r? Δr n n? A / % Rp / MPa Rm / MPa
I 0 0.638 0.585 0.020 0.306 0.308 30.48 134.4 276.5
45 0.575 0.307 28.54 133.9 273.1
90 0.551 0.311 26.64 132.7 270.0
II 0 0.726 0.654 -0.031 0.304 0.309 30.28 133.5 277.7
45 0.670 0.311 27.42 131.5 272.9
90 0.551 0.312 30.15 130.4 270.3
Table 1  Mechanical properties of T4P treated Al-Mg-Si-Cu-Zn alloy samples in different directions
Fig.2  OM images of microstructure evolutions in Al-Mg-Si-Cu-Zn alloy prepared by processing I (a~c) and processing II (d~f) (a, d) intermediate annealing (b, e) cold rolling (c, f) solid solution
Fig.3  SEM images (a, b) and EDS analysis of particle A in Fig.3b (c) of the intermediate annealed Al-Mg-Si-Cu-Zn alloy prepared by processing I (a) and processing II (b, c)
Fig.4  ODF maps of cold rolled sheets with processing I (a) and processing II (b)
Fig.5  EBSD analysis maps of grain orientation (a, c) and grain size distributions (b, d) of solution treated alloy sheets with processing I (a, b) and processing II (c, d)
Fig.6  ODF maps of solution treated alloy sheets with processing I (a) and processing II (b)
Component Processing I Processing II
Intensity Volume fraction Intensity Volume fraction
CubeND{001}<310> 5.62 16.80% - -
Cube{001}<100> 1.94 6.60% 4.23 5.10%
Goss{011}<100> 1.80 2.63% 1.04 1.15%
P{011}<122> 1.06 4.71% 0.75 2.94%
Q{013}<231> 1.46 13.40% 1.15 12.20%
R{124}<211> 1.14 9.22% 1.12 11.50%
H{100}<011> 1.56 3.72% 1.11 2.91%
Brass{011}<211> - - 1.03 4.03%
S{123}<634> - - 1.54 9.85%
Table 2  Intensities and volume fractions of textures components in the solution treated alloy sheets
Fig.7  Hardness change of the solution treated alloy sheet with processing II after ageing at 185 ℃ (a) and the engineering stress-strain curve in the peak ageing condition (185 ℃, 5 h) (b)
Fig.8  SEM images (a, b) and EDS analysis of particle A in Fig.8b (c) of processing II treated alloy sheet in the peak ageing condition (T6: 185 ℃, 5 h)
Fig.9  Low (a, b) and high (c, d) magnified HRTEM images of microstructure in solution treated alloy sheet with processing II after ageing at 185 ℃ for 5 h (a, c) and 30 h (b, d) (Insets show the corresponding SAED patterns)
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