Effect of 3%Zn Addition on the Non-Isothermal Precipitation Behaviors of Al-Mg-Si-Cu Alloys
YUAN Bo1, GUO Mingxing1,2(), HAN Shaojie1, ZHANG Jishan1,2, ZHUANG Linzhong1,2
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
Cite this article:
YUAN Bo, GUO Mingxing, HAN Shaojie, ZHANG Jishan, ZHUANG Linzhong. Effect of 3%Zn Addition on the Non-Isothermal Precipitation Behaviors of Al-Mg-Si-Cu Alloys. Acta Metall Sin, 2022, 58(3): 345-354.
To reduce the weight of a car body, Al-Mg-Si-Cu alloys have been extensively studied for outer body panels of automobiles owing to their high strength-to-weight ratio, recyclability, and good formability. Moreover, the strength of Al-Mg-Si-Cu series alloys can be enhanced using the bake-hardening treatment. However, compared with steel, the formability and final strengths of the alloys need further improvement, which is a major challenge to the large-scale application of Al alloys in the automotive fields. In this study, the non-isothermal precipitation behavior of Al-0.8Mg-1.2Si-0.5Cu-0.3Mn-0.5Fe(-3.0Zn) (mass fraction, %) alloy was systematically investigated using DSC, TEM, tensile test, and hardness measurements. The results show that adding Zn can simultaneously increase the formation and redissolution of solute clusters in the alloys during low- and high-temperatures non-isothermal heat treatments and promote precipitation. The kinetic equations of precipitation in the two alloys were established based on the activation energy of precipitation obtained through DSC analyses and other material parameters, which can effectively predict the corresponding precipitation rates. Further, adding 3.0%Zn to the alloy can effectively increase the nucleation rate of the precipitates in the alloy during non-isothermal heat treatment, resulting in higher hardness. Additionally, with the increase of ageing temperature, the hardness increased gradually, but a hardness plateau appeared at approximately 100oC, and the peak hardness values appeared at approximately 250oC, followed by a decrease in hardness. TEM microstructural characterization showed that non-isothermal heat treatment could result in the formation of multiscale β'' precipitates in the two alloys in the peak ageing state. In comparison, adding Zn to the alloy increased the number density of the precipitates and significantly changed the lattice parameters of β'' phases formed in the peak aged alloy. Finally, based on the obtained microstructure and mechanical properties, the relationship between the precipitate distribution and microhardness of the two alloys was established.
Fund: National Natural Science Foundation of China(51871029);National Key Research and Development Program of China(2016YFB0300801);Government Guided Program-Intergovernmental Bilateral Innovation Cooperation Project(BZ2019019);Opening Project of State Key Lab of Advanced Metals and Materials(2020-ZD02)
Fig.1 DSC curves of the as-quenched alloys heated with the rate of 10oC/min
Fig.2 GP zones formation peaks and dissolution peaks for solution quenched alloys, precipitate formation peaks, and their determination of activation energy (T—thermodynamic temperature, Y—volume fraction of excess solute precipitated at time t, f(Y)—implicit function of Y, φ—heating rate, n—constant)
DSC peak
Q / (kJ·mol-1)
k0 / min-1
Kinetics expression
1# alloy GP zone formation
42.2
6.3 × 105
1 - exp[-6.3 × 105texp(-5082 / T)]
2# alloy GP zone formation
51.6
1.5 × 107
1 - exp[-1.5 × 107texp(-6209 / T)]
1# alloy GP zone dissolution
80.7
5.0 × 108
1 - exp[-5.0 × 108texp(-9713 / T)]
2# alloy GP zone dissolution
67.0
1.8 × 107
1 - exp[-1.8 × 107texp(-8063 / T)]
1# alloy Precipitate formation
100.1
8.2 × 109
1 - exp[-6.8 × 1019t2exp(-24090 / T)]
2# alloy Precipitate formation
98.8
7.3 × 109
1 - exp[-5.4 × 1019t2exp(-23780 / T)]
Table 1 The kinetics of precipitation in different state in the 1# and 2# alloys
Fig.3 Curves of volume fraction (a) and hardness (b) of precipitates with ageing time for 1# and 2# alloys aged at 185oC
Fig.4 Hardness curves of the 1# and 2# alloys during the non-isothermal ageing process
Fig.5 TEM characterizations of the as-quenched alloys 1# (a-c) and 2# (d-f) heat treated from 20oC to 250oC with a rate of 10oC/min
Fig.6 The stress-strain curves of the two alloys in solution state
Alloy
α
M
β
rc / nm
b / nm
G / GPa
σ0 / MPa
Y / %
σss / MPa
1#
0.55
3.06
0.5
2.5
0.286
26.9
10
5.0
75.9
2#
5.7
84.0
Table 2 The parameters used in the yield strength model
Fig.7 The relationship between yield strength and microhardness in the Al-Mg-Si-Cu(-Zn) alloys
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