In-situ Preparation And Characterization Of The Particle Reinforced Aluminum Matrix Composite Based On Zircon Tailing Sand

In this thesis study,a novel aluminum matrix composite(AMC)reinforced by hybrid particles was developed and studied based on zircon tailing sand.The main composition of the abandoned zircon tailing sand was SiO2 with a small fraction of aluminum silicate.The Al2O3/SiO2(p)/Al composite,with uniformly distributed particles and excellent mechanical properties,was fabricated by mixing the particles and pure Al using stir casting technique.The special duplex phase within SiO2 particles surrounded by the Al2O3 transition layers,micron Al2O3 particles,the Si phase at the grain boundary of α-Al phase and a few aluminum silicate particles are revealed in the matrix of the manufactured composite.The existence of the special duplex phase indicates that the in-situ SiO2-Al reaction could take place at 700 ℃,whilst the Al2O3 transition layer will generate around the SiO2 particle in the matrix.In order to know the effect of stir casting parameters on the uniformity of the particles dispersed in the composite,the microstructures of the composites fabricated by these different parameters were observed.In addition,the uniformities of the particles dispersed in the composites were quantified.At first,the effect of the addition amount of the particles and the size of the particles on their dispersion in AMCs were investigated.The result suggests that particles with smaller size are more difficult to be dispersed.Meanwhile,the dispersion level of the particles with the same size is decreased with the increase of particle addition amount.The effect of key process parameters,such as particles pretreatment(calcination),stirring temperature,stirring rate,stirring duration and particles feeding rate on the dispersion level of the particles in AMCs was investigated on the basis of the microstructural observation.The optimal process parameters are: pretreatment(calcinating)of the particles at 700℃ × 1h,stirring temperature of 700℃,stirring rate of 500 rpm,stirring duration for 10 min and the particle feeding rate of 10 g/min.The flow field of molten Al was predicted in terms of the computational fluid dynamic(CFD)simulation.The simulation result indicates that the higher the stirring rate,the higher of the velocity would the flow obtained.In addition,the mechanism of the difference of particle feeding rates on their dispersing process has been explained with the help of CFD simulation.In order to know the effect of SiO2 addition amounts and the reaction time of SiO2-Al on the microstructure of the composites,the solidification character of the Al and molten composites were investigated based on thermal analysis method.With the increase of the addition amount of SiO2 particles in the melt,the solidification temperature was decreased and the duration of eutectic stage was extended.The cooling rate,nucleation temperature(TN)of α-Al,solidification time duration and fraction solid curve of the melt were also varied by changing the addition amount of SiO2 in molten Al.Meanwhile,the cooling curves of the molten composite at different durations of the reaction indicated the solidification point,duration of the Al-Si eutectic stage and the latent heat released from solidification course were increased with the extending of SiO2-Al reaction time.However,the time duration of Al-Si eutectic stage did not show a significant change with the extending of reaction time.These results indicated that the consumption of SiO2 particles was related to the time duration of the in-situ reaction.The in-situ SiO2-Al reaction might approach to the extreme state after it was sustained for a certain moment.For the sake of disclosing the consuming rule of SiO2 particles in molten Al,the method of immersing the quartz rod(with the same crystal form as the SiO2 particles in zircon tailing sand)into the molten Al was carried out.By observing the microstructure of reacted quartz rod,the thickness of Al2O3 transition layers at the SiO2/Al interface were measured.Based on the experimental data,the apparent coefficient of reaction-diffusion(K)was determined with the help of Ginstling modeling.The result showed that the larger the diameter of the SiO2 rod,the higher the K value,i.e.,the sooner of the SiO2 would be consumed.Meanwhile,the consumption rule of the SiO2 particles(with the diameter of 185 μm)in molten Al(700 ℃)was predicted.The Brinell hardness and tensile tests were carried out on the composites with different particle addition amounts and particle diameters.The result indicated that the hardness of the composite persistently increased with the increase of the particle addition amount.The yield strength(YS),elasticity modulus,ultimate tensile strength and percentage elongation of the composites fabricated by using the 185 μm particles were better.These mechanical properties of composites prepared with different stirring duration were also investigated.It indicated that the hardness,elasticity modulus and yield strength were higher while using the longer stirring duration,but the percentage elongation was decreased.The fracture analysis showed the agglomeration of the particles would remarkably affect the strength and percentage elongation and the hardening exponent.