Effect of copper content on bendability of Al–Mg–Si alloy sheets

Al–Mg–Si alloys are used for automotive closure panels as a weight saving option. Improvement of the Al–Mg–Si alloy sheet bendability is required for better hemming performance. In this study, the effects of the copper content on the bendability of the Al–Mg–Si alloy sheets were investigated. The Al–Mg–Si alloys with less than 0.01 mass%Cu, 0.4 mass%Cu and 0.8 mass%Cu were prepared, and the time of solution heat treatment was changed to obtain different dispersion conditions of the second phase particles and to obtain different shear band formation conditions by bending. For the samples with less than 0.01 mass%Cu and 0.4 mass%Cu, no cracks were observed during the bending. For the sample with 0.8 mass%Cu, the maximum depth of the crack by bending increased with the time of solution heat treatment up to 75 s, and then decreased over 75 s. The second phase particles decreased with increasing the solution heat treatment time, while the number of shear bands by bending increased with increasing the solution heat treatment time and the copper content. The cause of the occurrence and the propagation of cracks by bending are considered to be the combined effect of the shear band formation in the grains and the micro-voids formed around the second phase particles. The improvement of the bendability requires a decrease in the size and number of the second phase particles and/or the restraint of the shear band formation during the bending.