Preparation of AZ91D Slurries for Semi-Solid Forming Using Al_8 (Mn, Fe)_5 Precipitates

摘要

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 α-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 α-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 α-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 α-Mg due to the increase of heterogeneous nucleation sites. It was found that the average α-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 α-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.