Preparation of AZ91D slurries for semi-solid forming using Al-8(Mn, Fe)(5) precipitates

Abstract

During the solidification of the AZ91D-alloys, the Al-8(Mn, Fe)5 phase is generally precipitated in the melt in advance of the precipitation of the primary alpha-Mg. The basic principle for manufacturing AZ91D-alloy slurries for semi solid forming is to use the Al-8(Mn, Fe)5 precipitates as the heterogeneous nucleation sites for primary a-Mg phases. Microscopic analysis for the location of the Al-8(Mn, Fe)5 precipitate explains that the Al-8(Mn, Fe)5 precipitate is the effective heterogeneous nucleation site for the primary alpha-Mg phase. It was also observed that increase of the Mn content in the melt and the cooling rate below to the solid/liquid two-phase region resulted in smaller and more globular primary alpha-Mg due to the increase of heterogeneous nucleation sites. It was found that the average alpha-Mg diameter grew as a function of t(0.278), where t is the holding time at the solid/liquid two-phase region. This would be attributed to the Ostwald type ripening and coalescence between primary alpha-Mg phases. The cooling rate below to the solid/liquid two-phase region, Mn content in AZ91D alloy, and the holding time and temperature affected on the quality of slurry.