Processing and Microstructure–Property Relations of Al-Mg-Si-Fe Crossover Alloys

Abstract

This study investigates the processing and microstructure–property relations in new crossover aluminum alloys derived from 6xxx and 8xxx foil stock alloys. The new alloys combine the age-hardening capability of Al-Mg-Si alloys with the microstructure-controlling effect on processing of primary intermetallic phases used in foil stock. A highly Fe-rich intermetallic phase content was deployed to conceptually mimic high scrap content. Fast and slow solidification rates were applied to represent thin strip and direct chill casting, respectively. The effects of adding Fe and Mn to alloy 6016 were examined, while the Si consumed in primary phases was partly adjusted to maintain age-hardening potential. It was shown that upon thermomechanical processing, primary intermetallic phases in the new alloys are finely fragmented and well dispersed, resulting in strong grain refinement and a uniform texture. Attractive combinations of strength and ductility were revealed, also in material processed under direct chill casting conditions. The new alloys’ high elongation values of up to 30%, and their age-hardening response, were similar to those seen in commercial alloy 6016, while their strain hardening capacity was significantly greater. This can be attributed mainly to the formation of geometrically necessary dislocations near primary Fe-rich intermetallic phases. The study discusses microstructure refinement on the basis of particle stimulated nucleation. It uses a simple model to describe the individual contributions to yield strength, including the effect of primary phases. It also models the effect of these particles on increased strain hardening and ductility.

Keywords: Aluminum alloys, crossover alloys, Fe-rich intermetallic phases, grain refining, particle stimulated nucleation