Effect of Cu addition on tensile deformation and fracture behavior of Al-Mg-Si alloys

Al-Mg-Si alloys containing Cu and excess Si were investigated by tensile test, scanning electron, scanning tunnelling and transmission electron microscope observation in order to clarify the difference of their deformation behavior. Strengths and elongations of Cu-bearing alloys were higher than those of Cu-free alloys. Cu-free alloys showed the intergranular fracture, while Cu-bearing alloys showed the transgranular fracture. Frequency of the cracking on a surface of a Cu-free alloy was higher from an early stage of deformation than that of the Cu-bearing alloy. The addition of Cu suppressed the cracking in Al-Mg-Si alloys. Folds and sharp steps of grain boundaries were observed on the surface of Cu-free alloys, while Cu-bearing alloys showed transgranular deformation with wavy slip bands. The width of precipitate free zones (PFZ) in a Cu-bearing alloy was narrower than that in a Cu-free alloy. Dislocations were observed just near grain boundaries in the fractured Cu-free alloy, while there were many dislocations in grains of Cu-bearing alloys. It was considered that the improvement of elongation in Cu-bearing Al-Mg-Si alloys was caused by the suppression of preferential deformation near grain boundaries and the decrement of cracking.