Mechanisms determining bendability in Al-Mg-Si-Fe crossover alloys

Abstract

Al-Mg-Si-Fe crossover alloys combine the microstructure-controlling effect of iron-rich intermetallic primary phases with the age-hardening properties of Al-Mg-Si alloys. This study investigates the mechanisms which determine bendability in these alloys. The commercial alloy 6016 was heavily doped with Fe and Si to conceptionally mimic some of the effects of increased end-of-life scrap input. The Si content was adjusted to compensate for the Si trapped in iron-rich phases. Various processing routes, from casting to cold-rolled sheet metal, were also considered. The study concluded that the bendability of Al-Mg-Si-Fe crossover alloys does not correlate strictly with the addition of Fe, but is effected mainly by the composition of the aluminum matrix and the processing conditions. The investigation provides detailed insights into the effects of the type and density of intermetallic particles, the overall strength of the alloy, its strain hardening and the development of porosity due to particle cracking and the dissolution of phases leading to Kirkendall pores. It shows that bending is limited by damage and further void formation near the surface. It links the fracture of iron-rich primary phases which degrade bending performance itself to aluminum matrix strength and strain hardening. It also reveals that Kirkendall pores have a strong negative effect on bending because (similarly to fractured primary phase particles) these pores promote the formation of narrow shear bands which cause early bending failure. Finally, the study demonstrates that pore formation itself is triggered by slow cooling steps during manufacturing, but that the pores can be healed by adjusting the solution treatment time to restore bending capacity in all the alloys and conditions investigated.