The influence of Cu content and Mg/Si ratio on the strength and formability in Al-Mg-Si-Cu alloys

Abstract

The effects of Mg/Si ratio and Cu content on the strength and formability of four 6000 series alloys with Mg-rich and high Cu content were investigated by means of three-point bending testing, tensile testing and hardness measurement, scanning electron microscope and transmission electron microscope. It was addressed that alloys composition affects strength and formability by microstructure including intermetallic particles, grain size, texture and precipitates. The high Mg and Cu contents lead to lower yield strength in T4P temper but higher strength after paint baking due to the formation of large numbers of Cu-containing precipitates. The Mg-rich alloys show better formability which is attributed to the lower fraction of coarse α-AlFe(MnCu)Si intermetallic particles (controlled by the Mg/Si ratio) in these alloys. Additionally, high intensity Cube texture in Si-poor alloys, a feature that is also harmful to hemming performance, was found to compete with particle-stimulated nucleation (PSN). This observation indicates that the influence of coarse intermetallic particles on formability is greater than texture in Mg-rich alloys.