Effect Of Injection Speed On Microstructure And Mechanical Properties Of ADC12 Die Castings

Die casting is an advanced near net shape manufacturing process. Aluminum alloy die casting part has the characteristics of lightweight and high strength. As environmental problem becomes increasingly prominent, automobile lightweight has become an important tendency of the automotive industry. Due to liquid filling with very high speed and complex solidification conditions, there exist inevitable pore defects and special microstructure in conventional die castings, which have important effect on mechanical properties of castings. Thus, selection of proper parameters to get required microstructure, so as to improve the performance of the die castings, is one of the most important factors.In this paper, sample shape and dimensions of die-cast tensile specimens and layout of the cavity were determined, and the corresponding die structure and process parameters were designed. The mold filling with different fast shot speeds was analyzed using Flow-3D software. The results showed that the higher fast shot speed increased the filling flow velocity of liquid metal, and significantly accelerated the collision between liquid metal and the mold. Also, the temperature of the liquid metal becomes higher at the end of mold filling. The die casting experiment was then carried out based on the selected process parameters.The microstructures(skin, core and special microstructures) with different fast shot speeds were investigated. Besides, the pore characteristics were analyzed using 3D X-ray tomography technology. The results show that there are three kinds of skin microstructure : normal microstructure of primary α-Al and eutectic phase followed by layer with rich α-Al(Skin2), layer with rich α-Al(Skin1), normal microstructure of primary α-Al and eutectic phase(No Skin). The core microstructure mainly consists of externally solidified crystals(ESCs) and special microstructure of cold flake. As fast shot speed increased, the type of the skin microstructure changed from Skin2 to Skin1 and the thickness of both skin decreased. The ESCs gradually become smaller and more round and the areal fractions of cold flake significantly reduced because of the intense shear on the liquid metal at the higher melt velocity. Also, transmission of the intensification pressure become better and the volume of pores with complex shape and low sphericity decreased. The fitting degree of the three parameter lognormal distribution of the pore size gradually increased.The tensile properties and micro-hardness were studied, and the effect of microstructure on the properties was discussed. The results showed that the increase of fast shot speed improved the tensile strength and elongation of castings. The casting tensile properties and elongation were mainly affected by fractographic defects. The smaller area fraction of the c defects, the higher the tensile strength and elongation. Yield strength of the die castings was mainly affected by externally solidified crystals. The larger area fraction of ESCs, the lower the yield strength. The Weibull analysis of the tensile strength showed that the increase of fast shot speed helped to improve the stability of the mechanical properties.