Effect of mischmetal concentration and superheating on the microstructure and tensile properties A319.1: Role of Fe content

The aim of this research is to study the effect of certain metallurgical parameters such as the addition of a mixture of rare earths (mischmetal) and superheating on the microstructure and tensile properties of the Al–Si–Cu alloy A319.2 containing 0.4%, 0.8% and 1.2% iron used in the automotive industry. The mischmetal (MM) precipitates in the form of platelets like the β-Al 5 FeSi leading to a marked refinement in eutectic Si particles at 5% MM. In addition, α- Al 15 (Fe,Mn) 3 Si 2 . segregated particles and mischmetal-bound intermetallics persist in the microstructure even after T6 heat treatment. The tensile properties of 0.8% iron alloys deteriorate when the mischmetal concentration and superheat temperature increase, although the silicon phase undergoes significant changes. Alloys with a high iron concentration (1.2% Fe) showed a slight decrease in the length of the β-Al 5 FeSi phase platelets and of the mischmetal compared to alloys with 0.4% and 0.8% Fe when the concentration of mischmetal increases to 5% and when the superheating temperature reaches 950 °C. This observation explains the increase in the % elongation to fracture of the 1.2% Fe-5% mischmetal alloy cast directly from 750 °C or after superheating at 950 °C, followed by in-furnace cooling to 750 °C. The ultimate tensile strength and yield strength degrade with mischmetal, but improve slightly with superheating; only for the 1.2% Fe-0% mischmetal alloy, its yield strength decreases once superheated to 950 °C.