Al-Mn Intermetallics in High Pressure Die Cast AZ91 and Direct Chill Cast AZ80

Abstract

Manganese-bearing intermetallic compounds (IMCs) are important for ensuring adequate corrosion performance of magnesium-aluminium alloys and can be deleterious to mechanical performance if they are large and/or form clusters. Here, we explore the formation of Al-Mn IMCs in Mg-9Al-0.7Zn-0.2Mn produced by two industrial casting processes, high-pressure die casting (HPDC) and direct chill (DC) casting. As Al8Mn5 starts forming above the α-Mg liquidus temperature in this alloy, we consider its formation during melt handling as well as during casting and heat treatment. In HPDC, we focus on sludge formation in the holding pot, partial solidification of IMCs in the shot chamber, and Al-Mn IMC solidification in the die cavity. In DC casting, we focus on interactions between Al-Mn IMCs and oxide films in the launder system, Al-Mn IMC solidification in the billet, and the partial transformation of Al8Mn5 into Al11Mn4 during solution heat treatment. The results show that minimising pre-solidification in the shot sleeve of HPDC and controlling pouring and filtration in DC casting are important for ensuring small Al-Mn intermetallic particles in these casting processes.

Eutectic Solidification in Mg-9al-0.7zn: From Divorced to Coupled Growth

“…The 0.2 wt% Mn addition reacts with Al and impurity Fe to form Al-Mn-Fe intermetallics [38][39][40], starting at temperatures above the α-Mg liquidus and continuing throughout the solidification sequence including during the main eutectic reaction involving Mg 17 Al 12 [41][42][43][44]. Nave [24] found that a 0.3 wt% Mn addition had no significant effect on the eutectic morphology in AZ91, and a recent 3D imaging study on Al 8 Mn 5 in high pressure die cast AZ91 [45] found some Al-Mn particles in contact with Mg 17 Al 12 but the majority were surrounded by α-Mg.…”