State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing,Beijing 100083
Cite this article:
CUI Li, GUO Mingxing, PENG Xiangyang, ZHANG Yan, ZHANG Jishan, ZHUANG Linzhong. INFLUENCE OF PRE-DEFORMATION ON THE PRECIP- ITATION BEHAVIORS OF Al-Mg-Si-Cu ALLOY FOR AUTOMOTIVE APPLICATION. Acta Metall Sin, 2015, 51(3): 289-297.
To reduce the weight of car body, Al-Mg-Si-Cu alloys have been widely used to produce outer body panels of automobiles due to their favorable high strength-to-weight ratio, corrosion resistance, weldability and good formability. Al-Mg-Si-Cu alloys belong to age-hardenable aluminium alloys, whose strength derives mainly from the matrix precipitation during aging treatments. However, their bake hardening response still need to be further improved to enhance their dent resistance. A novel thermo-mechanical treatment consisting of conventional pre-aging, pre-deformation and re-aging was developed to enhance the tensile properties and bake hardening increment of Al-Mg-Si-Cu alloys. In this work, the effect of pre-deformation on the precipitation behaviors of Al-Mg-Si-Cu alloy was studied by DSC, mechanical property measurement and TEM. The results show that, the GP zone dissolution rate decreases with increasing pre-deformation during the slow heating up process for the pre-aged alloy, the corresponding activation energies of 0, 5% and 15% pre-deformed alloy calculated by Avrami-Johnson-Mehl method are 137.1, 189.5 and 141.3 kJ/mol, respectively. If the pre-deformed alloys are directly bake hardened at 185 ℃ for 20 min, precipitation and bake hardening increment can be greatly improved by pre-deformation (the highest bake hardening increment is 160 MPa), but the bake hardening increment rate gradually decreases if the pre-deformation is above 10%. In addition, the GP zone dissolution rates of pre-deformed alloys after bake hardening treatment are much lower when the heat treatment temperatures are below one certain value, but if the treatment temperatures above it, the corresponding GP zone dissolution rates are higher than that of alloy without pre-deformation, finally, the activation energy changes from high value to low value even can be observed in the ln[(dY/dT)φ/f(Y)]-1/T curve. For the precipitation in the alloys, with increasing pre-deformation, its activation energy gradually decreases, corresponding gradually increase of precipitation rate.
Fund: Supported by National High Technology Research and Development Program of China (No.2013AA032403), National Natural Science Foundation of China (No.51301016), Fundamental Research Funds for the Central Universities (No.FRF-TP-14-097A2) and Beijing Higher Education Young Elite Teacher Project in 2013 (No.YETP0409 )
Fig.1 DSC curves of Al-0.6Mg-0.9Si-0.2Cu-0.07Mn alloys after T4P treatment with different pre-deformations
Fig.2 Determination of GP zone dissolution activation energy for Al-0.6Mg-0.9Si-0.2Cu-0.07Mn alloys after T4P treatment with 0, 5% and 15% pre-deformations and GP zone dissolution kinetics of the theoretical prediction
(a) Y-T curves (b) dY/dT-T curves
(c) ln[(dY/dT)φ/f(Y)]-1/T curves (d) theoretical prediction of Y-t curves at 185 ℃
Fig.3 Stress-strain curves of the Al-0.6Mg-0.9Si-0.2Cu-0.07Mn alloys after T4P treatment with 0, 5%, 10% and 15% pre-deformations bake hardened (BH) at 185 ℃ for 20 min
Fig.4 DSC curves for Al-0.6Mg-0.9Si-0.2Cu-0.07Mn alloys with 0, 5% and 15% pre-deformation after aging at 185 ℃ for 20 min
Fig.5 GP zones dissolution peaks and the determination of GP zone dissolution activation energies
Fig.6 β″ precipitation peaks and the determination of β″ precipitation activation energies (a) β″ precipitation peaks (b)Y-T curves (c) dY/dT-T curves (d) ln[(dY/dT)φ/f(Y)]-1/T curves
Peak of GP and
Q (kJmol-1)
k0 min-1
Kinetics expression
GP zone dissolution for the (T4P+BH)
128.0
2.9×1013
Y=1-exp[-2.9×1013exp(-15401/T)t]
GP zone dissolution for the (T4P+5%+BH)
133.8
1.4×1014
Y=1-exp[-(1.4×1014exp(-16103/T)t)1.5]
GP zone dissolution for the (T4P+15%+BH)
97.0
2.2×1010
Y=1-exp[-(2.2×1010exp(-11664/T)t)1.8]
precipitation for the (T4P+BH)
176.9
5.2×1017
Y=1-exp[-(5.2×1017exp(-21285/T)t)2]
precipitation for the (T4P+5%+BH)
164.9
3.5×1016
Y=1-exp[-(3.5×1016exp(-19842/T)t)2]
precipitation for the (T4P+15%+BH)
127.1
5.3×1012
Y=1-exp[-(5.3×1012exp(-15321/T)t)2]
Fig.7 Precipitation kinetics of the alloys bake hardened at 185 ℃ (a) GPzone dissolution kinetics (b) β″ precipitation kinetics
Fig.8 TEM images of the bake hardened alloys heated up from 20 ℃ to 250 ℃ with 10 ℃/min under 0 (a), 5% (b) and 15% (c) pre-deformations
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