Phase fraction mapping in the as-cast microstructure of extrudable 6xxx aluminum alloys

The mapping of Mg2Si and beta-AlFeSi phase fractions in the as-cast microstructure of Al-Mg-Si-Fe-Mn (6xxx series) alloys has been performed over the useful composition range (0-1.2 mass%) of the principal alloying elements Mg and Si. The calculations were based on the Scheil-Gulliver assumption of infinite diffusion in the liquid and limited diffusion in the solid state. The computed phase fractions were validated with experimental measurements of phase fractions. The mapping procedure allows the control of intermetallic phases in the as-cast microstructure, the minimization of the beta-AlFeSi phase in particular, which is a significant prerequisite in obtaining enhanced extrudability, combined with high strength in this alloy series. Construction of maps for different levels of Mn has shown that addition of Mn could allow for higher alloying with Mg and Si, in order to obtain higher amounts of Mg2Si, without at the same time increasing the beta-AlFeSi phase in the as-cast microstructure.