Pressure casting is a method that liquid or semi-solid alloy melt is pressed into the mold cavity at high speed under the action of high pressure, and the melt is solidified under pressure, so as to obtain the casting with accurate size and clear outline. Magnesium alloy is especially suitable for pressure casting because of its low melting point (pure magnesium is about 650 ℃), low latent heat, fast solidification speed, low viscosity and good fluidity.
Magnesium alloys produced by die casting, such as AZ91D and AM60, have good room temperature performance and corrosion resistance, and have been used in the production of some non important parts, such as valve cover and instrument panel. However, due to air entrapment and other reasons, the conventional die-casting parts can not be heat treated, and the properties can not be further improved by aging strengthening. Vacuum die casting, oxygen filling die casting and semi-solid die casting developed in recent decades can solve this problem. In the process of vacuum die casting, the gas in the mold cavity is removed first, so as to reduce or even eliminate the air holes and dissolved gas in the die casting, and improve the mechanical properties and surface quality of the casting.
Oxygen filling die casting is to fill the cavity with oxygen or other active gas to replace the air in the mold before the melt filling. During the filling process, the active gas reacts with the liquid metal to form dispersed metal oxides, so as to eliminate the gas and pores in the die casting. Semi solid die casting can be divided into rheologic die casting and thixotropic die casting. Rheo diecasting is to transfer the prepared semi-solid slurry directly to the pressure chamber for die casting, while thixo diecasting is to reheat the pre fabricated semi-solid ingot with fine structure to the semi-solid section for die casting. Whether it is rheo diecasting or thixo diecasting, the filling of liquid metal in semi-solid region is stable, and turbulent filling mode is avoided. Therefore, the porosity and shrinkage of die casting can be significantly reduced, and the mechanical properties of the alloy can be improved. For example, for AM60 and AZ31 alloy, the tensile strength and elongation of rheo diecasting alloy after solution treatment are increased by 24% and 19%, 141% and 46% respectively. After heat treatment, the tensile strength of thixodie cast za74 alloy can be increased by 52%.