A study of the mechanical properties of a Mg-Al-Zn alloy (AZ91) produced via Thixomolding

In this work, the mechanical properties and failure characteristics of a Mg-9.0Al-0.8Zn alloy (AZ91) fabricated via the Thixomolding process (TC) were investigated and compared with those of alloys formed via other casting processes, including cold-chamber die-casting (CD) and hot-chamber die-casting (HD). The microstructure of the three cast samples mainly consisted of the alpha-Mg phase and eutectic Mg17Al12. Fine, uniformly organized spherical alpha-Mg grains formed in the TC sample, and this morphology can be related to the TC process. A relatively small microstructure, comparable to the TC sample, was also obtained with CD because of the high solidification rate. However, large grains and cast defects were observed in the HD sample. Shrinkage porosity cast defects were dominant with HD, and they were caused by the low casting pressure. The ultimate tensile strength and 0.2% proof stress of the TC sample were 279 MPa and 192 MPa, respectively, which was similar to 10% higher than for the CD sample. The cast defects and large grains for the HD sample caused a significant reduction in its tensile properties, although its 0.2% proof stress was relatively similar to the CD sample. High fatigue strength and high crack growth resistance were realized with TC. Failure analysis was also carried out to probe the excellent mechanical properties of the TC sample.