Microstructure and material soundness in liquid and rheocast AZ91: effect of section thickness

Abstract

Rheocast and liquid cast AZ91 magnesium alloy were compared to explore the feasibility of Rheocasting commercial Mg alloys using the RheoMetal process. A close-to industrial scale squeeze casting machine was used, with a mold having rectangular shaped cavities of section thicknesses 7, 13 and 20 mm. The pore content and its distribution as well as the microstructure of the castings were analyzed. The average size of the globular primary α-phase particles in the rheocast samples was around 50 μm. The shape factor of the primary particles in the rheocast samples had a bell-shaped distribution with a peak at 0,4-0,6 while the conventionally cast material had more stochastic distributions. The porosity in the rheocast samples was coarser than in the liquid cast samples, even though both types had similar levels of overall porosity. There was a layer of segregated porosity at around 30% of the section thickness from the sample surface; severe in the liquid cast samples and slight or non-existent in the rheocast samples.