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Acta Metall Sin  2016, Vol. 52 Issue (2): 191-201    DOI: 10.11900/0412.1961.2015.00334
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Yong LI,Mingxing GUO(),Ning JIANG,Xukai ZHANG,Yan ZHANG,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 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. However, their bake hardening responses still need to be further improved to enhance their dent resistance. In this work, an advanced Al-0.93Mg-0.78Si-0.20Cu-0.30Mn-0.40Fe-3.00Zn (mass fraction, %) alloy has been developed, its precipitation behavior has been investigated systematically through DSC, OM, SEM, TEM and tensile test. Five exothermic peaks were observed in DSC curve of the solution quenched alloy, these peaks were believed to be caused by the formation of GP zones and precipitate phases, the formation and dissolution of these precipitates were analyzed by Avrami-Johnson-Mehl method, a kinetic equation Y=1-exp[-2.03×1019exp(-23573/T)t2] has been established, which can be greatly used to predict the precipitation behavior. After artificial aging at 185 ℃for 90 min, the peak hardness of 133 HV can be obtained, corresponding to the predicted results. Additionally, the tensile properties in peak aging state, i.e. yield strength, ultimate tensile strength and elongation, are 346 MPa, 383 MPa and 13%, respectively, and ductile fracture is the main fracture feature as observed by SEM examination of fracture surface. Although 3.00%Zn is added in the alloy, yet, Mg-Si precipitates are still the main precipitates formed during artificial aging at 185 ℃. Both β′′ and pre-β′′ precipitates can be observed in the peak aging state, and the presence of the latter ones should be resulted from the formation of Zn-containing clusters. In addition, based on the microstructure evolution of the alloy, a schematic diagram of forming precipitates in the Al-Mg-Si-Cu-Zn alloy is put forwarded.

Key words:  Al-Mg-Si-Cu-Zn alloy      GP zone      β" phase      pre-β" phase      precipitation kinetics     
Received:  26 June 2015     
Fund: Supported by National High Technology Research and Development Program of China (No.2013-AA032403), National Natural Science Foundation of China (Nos.51571023 and 51301016), Fundamental Research Funds for the Central Universities (NoFRF-TP-15-051A3), Constructed Project for Key Laboratory of Beijing (NoFRF-SD-B-005B) and Opening Project of State Key Laboratory for Advanced Metals and Materials (No.2014-ZD05)

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Fig.1  OM images of the Al-0.93Mg-0.78Si-0.20Cu-3.00Zn alloy in the different conditions(a) as-casting (b) homogenization (c) cold rolling (d) solution quenching
Fig.2  DSC curve of the Al-0.93Mg-0.78Si-0.20Cu-3.00Zn in the solution quenching state
Fig.3  GP zones formation peaks and dissolution peaks for solution quenched alloys and precipitate formation peaks and their determination of activation energy (T—thermodynamic temperature, Y—mole fraction of excess solute precipitated at time t, f (Y)—implicit function of Y, ϕ—neating rate)
DSC peak Q / (kJmol-1) k0 / min-1 Kinetics expression
GP zone formation 30 2073.2 Y=1-exp[-4.30×106exp(-6374/T)t2]
GP zone dissolution 34 2202.6 Y=1-exp[-4.85×106exp(-8347/T)t2]
Precipitate formation 98 4.5×109 Y=1-exp[-2.03×1019exp(-23573/T)t2]
Table 1  Kinetics parameters of formation and dissolution of GP zones and precipitate formation
Fig.4  Relationship between volume fraction Y of precipitated or dissoluted phase and aging time for Al-0.93Mg-0.78Si-0.20Cu-3.00Zn alloy aging at 185 ℃
Fig.5  Hardness change of the solution quenched Al-0.93Mg-0.78Si-0.20Cu-3.00Zn alloy aging at 185 ℃
Fig.6  Engineering stress-strain curves of the Al-0.93Mg-0.78Si-0.20Cu-3.00Zn alloy in the different conditions
Fig.7  SEM image fracture morphologies of Al-0.93Mg-0.78Si-0.20Cu-3.00Zn alloy in the different conditions and EDS analysis of solution quenched alloy
Fig.8  TEM images of Al-0.93Mg-0.78Si-0.20Cu-3.00Zn alloys for solution quenched (a), 185 ℃, 20 min aged (b) and 185 ℃, 90 min aged (c), and HRTEM images of typical precipitate for 185 ℃, 90 min aged (d~f) and their fast Fourier transform (FFT) images (g, i, k ) and simulation diagrams (h, j, l), respectively
Fig.9  Schematic of the formation of precipitates in the Al-0.93Mg-0.78Si-0.20Cu-3.00Zn alloy
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