| An experimental investigation concerning the thixoformability, in the continuation of the previous research for control of liquid fraction and microstructure of semisolid AZ91D Mg slurry in the repeated semisolid state, was carried out with grain-refined AZ91D Mg alloy. The grain-refined AZ91D Mg alloy billet was produced by superheating with close control of solidification conditions, and thixoforming was performed using a step die cavity for evaluating thixoformability and using a testing die cavity for thixoforming mechanical testing specimens. Special attention was paid to the variation of hardness with regard to microstructural evolution. All slugs filled the step mold cavity with essentially laminar flow even in the incomplete mold fillings, and the thixoformability was improved by increasing the mold temperature and plunger speed enough to avoid premature freezing. The strength increase of the thixoformed AZ91D Mg alloy was not obtained and the source of this low strength is not yet clearly explained, but it may be due in part to the presence of low strength globular solid phase of about 120 ㎛. Although little work has so far been carried out, the preliminary results showed the improvement of the mechanical properties of thixoformed AZ91D alloy could be achieved by increasing mold temperature and by the heat treatment. |
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