Study On The Preparation And Squeeze Casting Process Of In Situ Al₂O₃ Particles Reinforced Aluminum Matrix Composites

Based on the Al-SiO2 system, in situ Al2O3p/Al composites were synthesized and then formed by squeeze casting. The reaction mechanism of Al-SiO2 system and the microstructure of the obtained samples at different temperatures were investigated by powder metallurgy method. SiO2 powder was added into semi-soild aluminum melt by mechanical stirring and in situ Al2O3 particles reinforced aluminum matrix composites were prepared by direct melt reaction, then the composites were formed by squeeze casting. The microstructure and the phase content of the composites, and the morphology, size as well as the distribution of the particles were respectively analyzed by XRD, SEM, EDS and other analytical methods. The effect of Mg content on the in situ reaction products in the Al-SiO2-Mg system was discussed. The room temperature tensile properties, the fracture behavior and hardness of the composites were analyzed and the effect of different sizes of Al2O3 particles on the composite was studied. And the optimization of the structure and properties of the composites by squeeze casting was discussed.In the Al-SiO2 system, Al and SiO2 started to react by powder metallurgy technology at 620℃, but the reaction rate was slow and it was controlled by atomic diffusion process. The reaction of Al-SiO2 was much faster at 850℃, and crystal transformation and cracking could be observed on the Al2O3 particles during the process of heat preservation, so the particle size was smaller and the particles were distributed uniformly at 850℃. The size and existing state of Al2O3 particles were different at different temperatures. α-Al2O3 was generated at 850℃, while the obtained particles of Al2O3 were probably amorphous at 620℃.Composites were prepared by direct melt reaction with the addition of 5wt.%SiO2 powder. The reaction product was composed of a large amount of MgAl2O4 and a little Al2O3 when the Mg content in the aluminum melt was 1wt.%. However, when the Mg content had decreased from 1wt.% to 0.3wt.%, the reinforced particles were mainly Al2O3. At the same time, there was small amount of MgAl2O4. The morphology and size of the original SiO2 powder had a significant impact on themorphology and size of the obtained Al2O3, and the reinforced particles almostly retained the morphology of SiO2 particles. When the size of SiO2 is 30μm, the size of Al2O3 particle is large, and since the volume reduction during the reaction process from SiO2 to Al2O3, many cracks were observed inside Al2O3 particles. These cracks provided diffusion channels for the atoms involved in the reaction. When the size of SiO2 powder was 3μm, the generated Al2O3 particles were relatively small at the size of 3μm, and many cracks were also observed.Squeeze casting improved the surface quality of casting and enhanced the effect of shrinkage supplement. The defects such as holes in the casting were eliminated and the microstructure of casting was more compact during the solidification under high pressure. At the same time, high pressure also broke the dendrite, which reduced the amount of coarse dendrites, so that the dendrites could be refined and spherized. The spheroidization and refinement were better with higher pressure. And high pressure could promote the uniform distribution of Al2O3 particles in the matrix and eliminate the coexistence of particles and holes during squeeze casting process. The hardness of Al2O3p/Al composites reinforced with 30μm and 3μm Al2O3 were respectively65.52 HBW and 58.46 HBW, both higher than those of the matrix alloy. After squeeze casting with a pressure of 60 MPa, the tensile strength of Al2O3p/Al composites with the Al2O3 particle size of 30μm and 3μm were 162 MPa and 189 MPa, while the elongation were 2.5% and 3.1%, both higher than those formed by gravity die casting.And the strengthening mechanism mainly included the second phase strengthening and grain refinement strengthening.