Microstructure and wear response of spray cast Al-Si/Sicp composites

Spray cast composites of Al-Si/SiCp were fabricated by atomizing the alloy melt with a mixture of nitrogen gas and SiCp particles. Microstructure was monitored in the spray cast condition, after hot extrusion, and in the over spray powder. Dry sliding wear response was assessed for the extruded composite, extruded unreinforced Al-Si, and extruded mold cast Al-Si. SiCp particles are embedded in the droplets and result in a homogeneous dispersion of reinforcement. Presolidified droplets containing SiCp and the composite preforms exhibit a uniform distribution of the eutectic silicon phase, and a finer equiaxed grain size than the unreinforced spray cast aluminium alloy. For the unreinforced preform, eutectic silicon crystallizes at the triple points of the aluminium grains with a blocky morphology. For droplets reinforced with SiCp the particles act as nucleation sites for eutectic silicon during solidification and the eutectic silicon retards grain growth during spray casting. The distribution of SiCp particles in the preform is affected by the fraction of liquid phase on the upper surface of the preform and by its solidification rate. Microstructural observations on the spray-cast and hot extruded composites reveal that the average size of the grains and the Si phase become finer, and the aspect ratio of the Si phase decreases, with an increase in v/o SiCp. The morphology of the Si phase changes from rod-like to blocky and globular with an increase in v/o SiCp. Spray-cast and hot extruded composites show improved wear resistance compared to the mold-cast and hot extruded alloy. This is attributed to the presence of a fine grain size and the fine Si phase with a blocky morphology. Increasing the v/o SiCp improves the wear resistance of the composites.