Inclusions in Permanent Mold Cast Magnesium ZE41A and AZ91D Alloys

Abstract

This study was part of a pioneering industry-focused AFS research project characterizing and examining the effects of inclusions in magnesium alloys during melting and casting. It seeks an improvement in the mechanical properties of magnesium alloys through a better understanding of the inclusions formed during melting and casting. These inclusions were examined by characterizing the grain size, mechanical properties, and fracture surface inclusion contents of tensile and fracture bar permanent mold castings of ZE41A and AZ91D magnesium alloys. The castings were produced near the start and the end of production runs from four different foundries. For both ZE41A and AZ91D alloy castings, the average yield strength, ultimate tensile strength, and elongation of the castings decreased between the start and end of the production run. For both alloys, an increase in grain size was observed between the start and end of the production run but the reduction in mechanical properties was mainly attributed to particle-type Mg–Al–O inclusions in the AZ91D alloy and film-type Mg–O inclusions in the ZE41A alloy on the fracture surfaces of the tensile samples and fracture bars. The AZ91D castings had very few inclusions, but these inclusions were much larger than those in the ZE41A castings. This research recognized extensive variability of the inclusion levels in the industry and is a precursor to developing industry standards for melt cleanliness in Mg alloys. This will be a major step in enabling improved quality and enhanced use of magnesium alloys in aerospace and automotive industries.