This study aims, as a part of studies for improving a plastic workability and for enhancing mechanical properties of magnesium alloys, to control microstructure of common Mg-Al system alloys by adding misch-metal and through a semi-solid process for manufacturing general-purpose casting/processing magnesium alloys with enhanced plastic workability, which are necessary for manufacturing high-quality magnesium alloy parts and components through combined casting/processing processes such as casting/forging, and thixo-forming. The addition of misch-metal, which was conducted in this study to control the microstructure, is popularly utilized to improve the high-temperature formability and mechanical properties of magnesium alloys by adding rare-earth (RE) metallic elements to them in a much cheaper way. Moreover, the semi-solid metal forming process is introduced to increase the solid solubility of the alloying elements added for alloy manufacturing, and to improve the plastic formability of magnesium alloys by fine and granular grain with a fine spherical shape during alloy solidification, other than microstructure with coarse dendritic phase. Accordingly, this study describes the detailed requirements for the manufacturing of semi-solid magnesium alloys to which misch-metal is added, and the effects of the added element affecting the characteristics of the plastic works, such as compression and extrusion of the manufactured alloys.
In Section 4.1 of this paper, the manufacturing of semi-solid magnesium alloys by adding misch-metal, the application of the semi-solid alloy process for controlling the microstructure of magnesium alloys, were investigated. The semi-solid metal manufacturing methods can be classified into the method involving the construction of fine and granular grains with spherical shapes by destroying the coarse dendritic microstructure using mechanical or electromagnetic stirring methods, and the cooling plate method, which can easily manufacture semi-solid metals by simply flowing a molten metal on to a cooling plate. In this study, the cooling plate method was employed. The semi-solid metal manufacturing using the cooling plate method was investigated based on previous studies on the effects of the pouring temperature and added elements on the microstructure and mechanical properties of the semi-solid alloys, and on the mechanical properties of the manufactured alloys at room and high temperatures.
In Section 4.2, the high-temperature compression test was conducted to indirectly estimate the hot formability of the semi-solid AM80-xMM alloy manufactured in an appropriate casting condition for cooling-plate casting. The compressive-deformation characteristics of each alloy were evaluated by the test that was conducted with conditions various compressive-deformation temperatures and strain rate. The high-temperature compression behavior was measured through the compression test to evaluate the formability of the compression process and to investigate the high-temperature deformation behaviors by calculating the values of the flow stress and strain rate sensitivity (m) during the high-temperature compressive deformation of each alloy.
In Section 4.3, the extrusion process, among the various compression processes, was applied to evaluate the compressive formability of the alloys through the hot-extrusion of each of the semi-solid alloys. The extruded materials that were obtained from the hot-extrusion were used to evaluate the mechanical properties of the extruded bars through tensile tests at room and high temperatures.