Correlating the microstructure and tensile properties of a thixomolded AZ91D magnesium alloy

Abstract

Thixomolding 1 Thixomolding and Thixomolded are registered trademarks of Thixomat Inc., Ann Arbor, MI (USA) 1 , an emerging semisolid technology, was used to process an AZ91D magnesium alloy under experimental conditions designed to yield from 5 to 60% of the primary solid particles. The thixotropic microstructures obtained were characterized in detail and linked to the corresponding tensile properties. An increase in primary solid content was accompanied by its larger microchemical and microstructural inhomogeneity expressed by Al and Zn segregation, sub-micron precipitates of Mg 17Al 12 and eutectic islands. At the same time, the size of α-Mg grains within the eutectic mixture was reduced. For the volume fraction of the primary solid up to about 20%, the tensile strength and elongation remained at the level of 240 MPa and 4.5%, respectively. A further increase of the primary solid caused a reduction in both strength and ductility. The fractographic analysis revealed a correlation between the primary solid content and the morphology of the decohesion surface. It is concluded that for alloys with a solid fraction below approximately 20%, the internal structure of the primary solid and the eutectic mixture control the properties. For a large volume of unmelted fraction, the interface between the primary solid and the eutectic mixture is a key factor which controls the tensile properties of the thixomolded alloy.