The Effects of Iron-Bearing Intermetallics on the Fitness-for-Service Performance of a Rare-Earth-Modified A356 Alloy for Next Generation Automotive Powertrains

Abstract

Aimed at improving the tensile strength and creep resistance of a rare earth-modified A356 alloy, this study adjusted the Mg and Mn concentration in the alloy, specifically aiming to transform the harmful Al5FeSi and Al9FeSi3Mg5 phase into Al15(Fe,Mn)3Si2. It was found that lowering the Mg concentration from 0.49 to 0.25 wt.% and raising the Mn concentration from 0.10 to 0.41 wt.% resulted in a near complete transformation of the Fe-bearing phases. This transformation led to a greater total volume fraction of Fe-intermetallics (2.9 to 4.1%), without affecting the volume fraction of the desirable, temperature-resistant, AlSiRE phase. Moreover, the chemistry modification led to a shift in the morphology of the AlSiRE phase while reducing its size. Combined with the decreased volume fraction of the harmful Fe precipitates, the chemistry modification improved the yield strength (YS), ultimate tensile strength (UTS) and modulus of elasticity by ~14%, 9%, and 10%, respectively. In addition, the steady-state creep rates of the high Mn alloy were lower at all stresses as compared to the low Mn alloy and the fracture stress was ~15 MPa higher, reaching 100% of the alloy’s original 250 °C YS.