Effect of Mg on microstructures and properties of Al-Mg-Si-Cu aluminium alloys for automotive body sheets

The effects of variation of Mg content on microstructures, the tensile properties and the formability of Al-Mg-Si-Cu alloys for automotive body sheets were investigated by means of scan electron microscopy, optical metallographic analysis, tensile and Ericsson tests. The results show that for Al-Mg-Si-Cu aluminium alloys with excessive Si, with an increment of Mg content, the strength enhances, the specific elongation and Erisson values of alloys decrease, and the number of Mg2Si constituent increases and that of Al(MnFe)Si type constituents reduces. Al-Mg-Si-Cu aluminium alloys with excessive Si for automotive body sheets can present obviously the paint bake hardenability during the paint bake cycle (i.e. artificial aging at 170 degrees C for 30 min immediately after the solution treatment and quenching). Suitable Mg content should be controlled in the range of 0.8% and 1.2% (mass fraction).