Abstract
In this study, the suitable alloys for semi-solid forming are searched considering the advantage of carrying out fabrication of near-net shaped products at a lower temperature than die-casting. Firstly Mg–20%Al alloy as selected as a model alloy for preliminary evaluation of fluidity. Furthermore, in order to improve mechanical properties, Mg–20%Al alloy was chosen as a base alloy, and alloys containing Zn and Ca with reduced Al content were also investigated. These investigated alloys show that solid fraction becomes about 57 to 65% just above the melting temperatures of eutectic regions, and solid particles become spherical due to the introduced compressive strain. Besides, eutectic regions, which are rapidly cooled from the semi-solid temperatures during press forming exhibit fine microstructure. The fluidity of Mg–20%Al alloy and alloys containing Zn is excellent, while the alloys containing Ca are extremely bad. It is possible to harden all of the investigated alloys by aging. However, heat-treated high Al containing alloys show low tensile strength and elongation because a network of the eutectic compound, Mg17Al12 is retained along the grain boundaries even after solution heat treatment, and this becomes the source of crack initiation and propagation. On the other hand, in the alloys having reduced Al content and containing Zn or Ca, the tensile properties increase due to the changes of the morphologies or kinds of eutectic compounds.
