Study on Semi-Solid Magnesium Alloys Slurry Preparation and Continuous Roll-Casting Process

Abstract

Magnesium and its alloys, as the lightest functional and structural materials, have great increasingly application in the automobile, electronic and aeronautical industries due to their low density, high specific stiffness and strength, good heat conductivity, high electromagnetic interference shielding and damping capabilities, etc. However, at present the major problem to the application of magnesium alloys is still lack of suitable forming process. As a new type of metal forming method, Semi-solid metal (SSM) processing technology has attracted close attention all over the world, since M.C Flemings in MIT firstly introduced this concept in 1970s (Spencer et al., 1972), (Flemings, 1991). Compared with the conventional processes, SSM process has many advantages: higher viscosity than fully liquid, leading to laminar flow and even filling of a die; low solidification shrinkage, leading to more precise dimension product; lower casting temperature and less latent heat resulting in less thermal shock to the die. Compared with solid forming processed, SSM process needs lower loads and can fill more complicated shapes and thinner sections (Xie, 2002). The mechanical properties of SSM product are close to the forged product. So, it is significant to combine the semi-solid process techniques with continuous roll-casting techniques to produce the high quality magnesium alloy strip with non-dendritic structure. However, the key of SSM process is to prepare non-dendritic structure semi-solid magnesium slurry with rosette or globular microstructure or billets with non-dendritic structure. Usually, the nondendritic structure can be obtained by continuous stirring or controlling nucleation and growth processes during the early stages of solidification. In recent years, many new method of the semi-solid slurry preparation have been developed. These include UBE’s new rheocasting (NRC) process (Hall et al., 2000) and the Cooling Slope method (Haga & Suzuki, 2000; Haga et al, 2004; He et al, 2009); the SLC (subliquids casting) process (Jorstad et al, 2002); the “Continuous Rheoconversion Process” (CRP) (WPI, 2002), the SIMA (Strain-Induced Melt Activation) Process, the SEED (Swirled Enthalpy Equilibration Device) Process; the SCR (Shear-Cooling Roll) process and etc. In essence, all of the above-mentioned processes utilize the same fundamental concept: nucleation and dispersion of the nuclei to achieve the semi-solid structure as the alloy melt is cooled below the liquidus temperature.