| In order to fulfill demand of automotive lightweight and make application of automotive structural parts of magnesium alloys possible, new magnesium alloys for complex automotive parts and its corresponding high pressure die casitng(HPDC) technology were studied in the paper.According to the property demand of automotive steering gear housing, a new high strengthening and toughening Mg-Al-Ca-Y alloy with low cost was developed by using multi-alloying method, in which Ca and Y elements were added in the AZ91 alloy. With better mechanical properties at room and elevated temperature than those of commercial AZ91 alloy, the alloy can be used on parts with special mechanical properties demand, especially for key parts with demand of high temperature properties because ultimate tensile strength of alloy can reach 226.8MPa at 200℃. According to the principle of grain refinement strengthening and characteristics of RE-containing Mg alloy, a non-aluminum high strength and heat resistant die casting GWK(Mg-Gd-Y-Zr)magnesium alloy and corresponding cold mould die casting process as well as heat treatment process were developed. The mechanical properties of alloy can be enhanced by short time and low temperature heat treatment. The mechanical properties of die casting GW123K-T4 alloy can reachσb=269.4MPa,σs=251.0MPa,δ5=4.2%. In order to decrease the cost of GWK series alloys, adding Ca and Zn elements and reducing rare earth element Gd content may improve tensile mechanical properties of alloy at room temperature, which have extremely important practice significance to apply the GWK alloy.According to impact test conditions of steering gear housing, the finite element model of the steering gear housing was built and the structural analysis was performed using finite element analysis software and strength theory, which provided a theoretical basis for material substitution of steering gear housing. According to technological demand of steering gear housing, the structure and die casting process characters of die castings were analyzed. The improvement in pouring system of original castings and vacuum exhausting-flooding system was optimized and designed using theoretical calculation and Flow3D computer simulation software. Based on the theory of slow injection, analysis model of slow shot movement of shot plunger in shot sleeve was established. The movement rule of the liquid metal in shot sleeve and best slow shot parameters were investigated using Flow3D software. The optimal slow shot process was uniform acceleration motion with 0.5m/s2 acceleration rate. The fluid analysis model with sleeve filling was built considering influence of slow shot state. The best die casting processing parameters at vacuum condition were obtained using orthogonal simulation experiment as follow: pouring temperature 680℃, mold temperature 200℃and shot speed 3.4m/s(die filling speed 40 m/s).Experimental results of vacumm die casting showed that the optimal processing parameters by simulation were reasonable. Using the optimized processing parameters the AZ91-1.0Y-1.5Ca magnesium alloy castings were found good forming, good production continuity and cutting machining performance.The mechanical properties of magnesium alloy housing were better than those of the original alloy material, and product qualification rate can reach to 94%. The weight of magnesium alloy steering shell housing was decreased 33.5% compared with that of the aluminum steering gear housing, which realized lightweight of product.The vacuum die casting can reduce the gas content and gas porosity size, and make it more uniform distribution, thereby the mechanical properties and product qualification rate were increased. The optimal vacuum processing parameters were found as follow: vacuum starting time 0.8s and cavity vacuum pressure 5KPa. The mechanical properties of vacuum die casting magnesium alloy housing can be enhanced further after heat treatment. The best comprehensive mechanical properties of castings subjected to solid solution and aging treatment were obtained as follow: ultimate tensile strength 280MPa, yield tensile strength 198.8MPa and elongation rate 5%, compared with properties of as-die cast alloy, which increased by 12.5%, 12.9% and 2.0%, respectively.
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