Acta Metall Sin  2020, Vol. 56 Issue (7): 997-1006    DOI: 10.11900/0412.1961.2019.00386
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Effect of Ageing Routes on Precipitation Behaviors of Al-0.7Mg-0.5Si-0.2Cu-0.5Zn Alloy
ZHU Liang1, GUO Mingxing1,2(), YUAN Bo1, ZHUANG Linzhong1,2, ZHANG Jishan1,2
1. State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing 100083, China
2. Beijing Laboratory of Metallic Materials and Processing for Modern Transportation, University of Science and Technology Beijing, Beijing 100083, China
Abstract

In order to reduce the weight of car body, Al-Mg-Si-Cu alloys have been widely studied and used to produce outer body panels of automobiles due to their favorable high-strength-to-weight ratio, recyclability and good formability. Moreover, the strength of Al-Mg-Si-Cu series alloys can be enhanced by bake hardening treatment. 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 Al alloys in the automotive fields. In this work, the effect of ageing routes on the precipitation behavior of Al-Mg-Si-Cu-Zn alloy with high Mg/Si ratio was systematically studied by DSC, TEM, tensile and hardness tests. The results show that the precipitation activation energies of the β″ phase formed in the non-isothermal ageing processes of the as-quenched and pre-aged alloys are 80.1 kJ/mol and 64.5 kJ/mol, respectively; both the different age hardening rates and microstructure evolution behaviors can be also observed during their isothermal ageing processes, such as, the age hardening rate of as-quenched alloy is much higher, but the peak hardness and strength values of the alloy treated by the two routes were basically the same. However, the elongation and strain hardening rate of the pre-aged alloy in the peak ageing state, and the hardness reduction rate in the over-ageing state are all much higher. In addition, a large number of complex solute clusters can be formed during pre-ageing, which will further grow in high temperature isothermal ageing process, but the sizes of precipitates formed in the under-ageing, peak-ageing and over-ageing states all show a multi-scale characteristics; in comparison, this feature cannot be found in the precipitates formed in the as-quenched alloy aged in the different conditions. The ageing routes cannot change the precipitation sequence of the alloy, but give a significant influence on the nucleation and growth rates of precipitates. As a consequence, a schematic diagram of nucleation and growth process of precipitates in the alloy with ageing routes is proposed.

 ZTFLH: TG146.21
Fund: National Key Research and Development Program of China(2016YFB0300801);National Natural Science Foundation of China(51871029);National Natural Science Foundation of China(51571023);National Natural Science Foundation of China(51301016);Government Guided Program-Intergovernmental Bilateral Innovation Cooperation Project(BZ2019019)
Corresponding Authors:  GUO Mingxing     E-mail:  mingxingguo@skl.ustb.edu.cn
 Fig.1  DSC curves of Al-0.7Mg-0.5Si-0.2Cu-0.5Zn alloys in the as-quenched and pre-aged conditions Fig.2  The determination of active energy for β″ phase precipitation of Al-0.7Mg-0.5Si-0.2Cu-0.5Zn alloys in the as-quenched and pre-aged conditions (Y—volume fraction, T—temperature, t—ageing time, $φ$—heating rate, f(Y)—implicit function of Y)(a) Y-T curves(b) lnln(1/(1-Y))-lnt curves(c) ln[(dY/dT)$φ$/f(Y)]-1/T curves Fig.3  Micro-hardness and peak ageing performance curves of Al-0.7Mg-0.5Si-0.2Cu-0.5Zn alloys during 185 ℃ isothermal ageing (T6—peak ageing, θ—work hardening, σ—strength, σ0.2—yield strength)(a) age hardening curves(b) true stress-true strain curves(c) θ-(σ-σ0.2) Table 1  Tensile mechanical properties of Al-0.7Mg-0.5Si-0.2Cu-0.5Zn alloys in peak ageing state under different ageing processes Fig.4  TEM bright field images (a, b), HRTEM image and fast Fourier transform (FFT) image (c) of precipitates in Al-0.7Mg-0.5Si-0.2Cu-0.5Zn alloys after isothermal ageing at 185 ℃ for 20 min(a) AQ+ageing at 185 ℃ for 20 min (b) PA+ageing at 185 ℃ for 20 min(c) HRTEM image and FFT image (inset) of typical precipitate in Fig.4a Fig.5  TEM bright field images (a, b), HRTEM images and FFT (insets) (c~e), and length distributions of precipitates (f) in Al-0.7Mg-0.5Si-0.2Cu-0.5Zn alloys after isothermal ageing to peak ageing(a) AQ+peak ageing (b) PA+peak ageing (c) HRTEM image of β″ precipitates in Fig.5a (d) HRTEM image of β″ precipitates in Fig.5b (e) HRTEM image of L precipitates in Fig.5b (f) length distribution of precipitates in Figs.5a and b Fig.6  TEM bright field images (a, b), HRTEM images and FFT (insets) (c~f) of precipitates in Al-0.7Mg-0.5Si-0.2Cu-0.5Zn alloys after isothermal ageing to over ageing(a) AQ+over ageing (b) PA+over ageing (c) HRTEM analysis of β′ precipitates in Fig.6a(d) HRTEM analysis of β″ precipitates in Fig.6a (e, f) HRTEM analyses of L and Q′ precipitates in Fig.6b Fig.7  Schematics of precipitation in the Al-0.7Mg-0.5Si-0.2Cu-0.5Zn alloy treated by the ageing routes(a) AQ (b) AQ+short time ageing (c) AQ+peak ageing (d) PA (e) PA+short time ageing (f) PA+peak ageing