Study On Semi-solid Forming And Slurry Fabrication Of AZ91D Magnesium Alloy By Self-inoculation Method

As effective method of grain refinement, the inoculation was widely used. There is one of methods for preparation of semi-solid slurry by chemical grain refinement. To obtain high-quality semi-solid slurry, one of the conditions must be generated in the initial enough free grain. Small initial free grain will evolve in the small solid particles. So the initial free grain should be small and adequate.In this thesis,the semi-solid slurry of magnesium alloy AZ91D was prepared by a novel Self-Inoculation Method(SIM) which is with high efficiency and low energy consumption. Beginning with the nucleus and growth grain, two different routes to obtain semi-solid slurry were discussed, such as dendrite fracture, controlled nucleation and restrained growth method. A series of experiments on AZ91D magnesium alloy were carried out to investigate the effects of processing parameters on the semi-solid microstructure.The primary phase evolution and formation mechanism of non-dendritic structures were investigated.The partial remelting microstructure of AZ91D alloy produced by various parameters, and then rheo-diecasting and thixo-diecasting were studied.The results indicate that:1.Under SIM process,the microstructure of self-inoculants should be fine even with non-dendritic structure, this is the important parameter to obtain high quality semi-solid microstructure and the addition of the self-inoculants should be 5% to 8%(mass fraction).2.The suitable condition can increase effective nucleation rate and obtain high quality semi-solid slurry, which requires the parameters match of the melt treatment temperature (650-680℃)and angle of the slope channel.If the two parameters, addition of the self-inoculants and angle of the slope channel, are in the fitting range, the grain growth morphology of primary a-Mg from coarse dendritic to Small block grain, and even to be spherical with the decrease of the melt treatment temperature.3.Semi-solid slurry with much fine and uniform primary phase particles is gained by self-inoculation method, the particle average size is approximate 36μm after the water quenched. The primaryα-Mg phase particles are with non-entrapped liquid, and the average equivalent diameter is 60-70μm after the semi-solid slurry held at the semi-solid range of temperature for a short time(5-10min).When the gate velocity in 2 m/s and lOmin holding at 585℃for 10min, the rheo-diecast can improve the microstructure and obtain good performance of the semi-solid castings.The UTS and hardness of thixo-diecasted AZ91D alloy are 185MPa and HV30=67.88. 4. The billet prepared by SIM was with good thixotropy and the partial remelting microstructure was finer and uniformer. the large size of billet(Φ75mm) was held at 585℃for 110min, the primary solid phase particles distribute uniformly in the microstructure with an average diameter of 80-90μm and solid fraction of 58%. Meanwhile, the UTS and hardness of thixo-diecasted AZ91D alloy are 192MPa and HV30=68.74 respectively.