Effect of accumulative plastic deformation on generation of spheroidal structure, thixoformability and mechanical properties of large-size AM60 magnesium alloy

Accumulative plastic deforming (APD) at multistep temperatures was induced into the recrystallization and partial melting (RAP) route as a novel predeformation method to replace conventional upsetting or extrusion. AM60 magnesium alloy ingots with diameter of 520 mm were produced by semi-continuous casting, and semi-solid slugs with diameter of 320 mm were prepared by partial remelting of as-cast and APD formed alloys. The microstructure evolution during semi-solid remelting was investigated. The tensile mechanical properties of AM60 alloy at room temperatures were determined. The results show that large semi-solid AM60 alloy slugs with fine and homogeneous microstructures can be produced by employing APD in RAP route, and the microstructure consists of spheroidal solid grains surrounded by intergranular liquid matrix. The as-cast AM60 alloy during partial remelting showed inferior microstructures with coarsened solid grains and high proportion of intragranular liquid droplets. During partial remelting, coalescence and Ostwald ripening are dominant in as-cast alloy and APD formed alloy, respectively. The formation and migration mechanism of intragranular liquid droplets were discussed. Although both of APD formed and as-cast AM60 slugs could fill the die during thixoforming process, the tensile mechanical properties of the components thixoformed from APD formed alloys far exceed those from the as-cast alloys.