The Numerical Simulation To Die Casting Processes Of Magnesium Alloy And Discussion About The Formation Of Defect Bands

Magnesium alloy die casting process is development with the automobile industry. Atpresent, the majority of magnesium alloy products are made by die casting process, at thesame time, there are large amounts of magnesium storage in ocean and lithosphere. Usingmagnesium alloy products instead of steel is good for energy conservation and emissions’reduction. In order to expand the scope of application of magnesium alloy die castingproducts and enhance its market competitiveness, the casting process must be improved toobtain more excellent performance for magnesium alloy die castings.In this paper, use the casting simulation software ProCAST to simulate the magnesiumalloy die casting technologies. Set up pressure curve and the initial filling pressure is0.2MPa,the supercharging pressure is50MPa. When the pouring temperature is640℃,the moldtemperature is180℃and the injection speed is1.6m/s, the filling will be smooth, butcastings are prone to cold defects. When the pouring temperature is690℃, the moldtemperature is220℃,and the injection speed is1.6m/s, the filling will be smooth. But thesolidification time of the castings become longer, and big body contraction would be easy toproduce porosity or shrinkage in castings. When the pouring temperature is670℃, the moldtemperature is200℃and the injection speed is1.0m/s or4.5m/s, in the process of fillingwill appear turbulence, splash, air entrainment, the filling effect is bad. After a lot ofsimulation, put forward reasonable die casting process parameters: the injection pressure is50MPa, the pouring temperature is670℃, the mold temperature is200℃, the injection speedis in the scope of1.2~4.3m/s, the filling effect is good, the coagulation time is shorter, andthe casting defects such as shrinkage and porosity in castings is less.Through the XRD and SEM-DES analysis, AM50die casting magnesium alloys arecomposition of α-Mg matrix phase and α-Mg+β-Mg17Al12eutectic phase in grain boundary. Inthe defect bands exist small shrinkage, but in those locations the grains are tiny, averagegrains’ size are only10μm, meshed brittle precipitated phase distribute at the grain boundary.In the center of castings exists shrinkage, and the grains are relatively coarse and uneven distribution, many grains’ size are up to40μm, brittle precipitated phase is mushy zone orscattered islands in the grain boundary. In the surface of castings, grains are relatively coarseand many grains’ size are up to40μm, brittle precipitated phase is dot or granular scattered inthe grain boundary.This paper has discussed the formation mechanism of defect bands: coarse dendritesexist in die casting, of which a large part is due to crystals solidified from the liquid metal inshot sleeve, and turn into ESCs (Externally Solidified Crystals). The ESCs flowing with themolten metal into the mold cavity, in the solidification process of castings, ESCs particlesmigrate to the center of castings slowly, as the core of heterogeneous nuclei, growing intocoarse dendrite, and growing direction is oppose to the normal dendrite, then at the two sidesof the front dendrite formed mushy zone, where result in shear deformation under the pressure.At the later period of solidification, both sides of dendrite contact each other, local dendritenetwork may be fragmented because of collisions, then formed defect bands.Test60samples’ tensile properties, yield strength is in the range of110~140MPa,tensile strength is in the range of180~250MPa, and elongation is in the range of6~18%.According to stretching curve, tensile strength and elongation’ changing trend is consistent,the larger tensile strength corresponds to large elongation. According to the fracturemorphology, AM50die casting magnesium alloys are ductile-brittle mixed fracture.Thecasting defects, such as stomatal, shrinkage porosity and crack, will be reduced mechanicalproperties of castings. When the content of brittle precipitated phase in defect bands is higherand distributed in the grain boundary with mesh structure, it will be also reduce the alloy’smechanical properties.