Semi-solid Microstructure And Properties Of7075Aluminum Alloy With Forced Convection Mixing Process And Simulation

Based on the principle of mechanical stirring, the Forced convection rheoforming machine (FCR) was designed and developed by ourselves for the preparation and forming of semi-solid slurry. The rheo-die castings with perfect quality were prepared. The effect of processing parameters on the microstructure and morphology of7075semi-solid slurry were investigated. The changes of various physical fields in the preparation of semi-solid slurry were analyzed by using numerical simulation. And the mechanism of forced convection effect on the primary particle nucleation and growth was discussed. Based on the critical diameter method, the hot cracking susceptibility in different process was researched by the methods of experiment and computer simulation for hot cracking problem of7075aluminum ingot. The main contents and results are as follows:The desirable semisolid slurry with average primary solid phase diameter of70μm and shape factor of0.8can be obtained at pouring temperature of650-640℃, barrel temperature of610-590℃and rotational speed of200-400r/min. The semi-solid slurry has few defects. Because it can prevent the turbulence and splash in filling process and reduce the shrinkage defects. After T6heat treatment, the property of7075rheo-die casting biscuit was superior to the traditional process with the tensile strength of538MPa and elongation of3.32%. Some plastic regions could be found and some dimples had been observed on the fracture surfaces of rheo-die casting samples. Compared to the HPDC process, The R-HPDC process with the FCR device decreased the total cross-sectional area of pore defects by about27times in the cross section of shock absorber part.The mixing forced convection of the melt in FCR device changed the unilateral heat transfer and slow mass transfer state in traditional solidification. The rapid mixing of the material and the heat in the melt was beneficial to a uniform distribution of various physical fields quickly, all the melt was reached a uniform super-cooled state. The evenly distributed effective nucleation particles can nucleate largely. The grains adhered on the wall interface was washed and free to the melt, which increased the nucleating ratio. The uniform temperature, the small temperature difference and the grain spin is conducive to growing into spherical. Reducing the barrel temperature and the pouring temperature can increase the undercooling degree and nucleation rate. Increasing the rotational speed appropriately was beneficial to decrease the non-uniform time of the melt and the temperature difference of the maximum temperature and minimum temperature in the melt, which is good for grain growing in all directions.The edge and the core of the slender rod had the fastest cooling speed. It had long and thin dendrite arm and easy to cause the hot cracking. The hot cracking level of traditional7075aluminum alloy casting in different parameters was256mm2. Increasing the mold temperature and the pouring temperature of FCR process can improve the grain morphology of semisolid microstructure and increase the crack propagation resistance, which is beneficial to decrease the hot cracking tendency significantly. The hot cracking level of semisolid rods was100mm2with the FCR parameters of the pouring temperature650℃, the barrel temperature580℃,the rotational speed300r/min, the mixing time30s and the mode temperature230℃.