Al
2O
3/Al composites manufactured by in-situ reaction between molten Al and SiO
2 were investigated. Near net shape composites were obtained after the reaction. The effects of molten Al temperature and dipping time on the reaction rate, density, and structure of the composites were investigated. The maximum reaction rate of approximately 1.1 mm/ks was obtained in molten Al at 1138 K and 1473 K. When a SiO
2 rod is dipped in molten Al at 1273 K and above, the density is 3.5 Mg/m
3 and the hardness is 300-330 HV. Intermediate layers can be observed between the layer where the reaction occurred and the SiO
2, which incurred no reaction. The hardness of the intermediate layer is about 660HV10, and the structure of this layer is composed mainly of fine Al
2O
3. This composite material is composed of continuous Al
2O
3. Al, and a small amount of Si. The volume fraction of Al
2O
3 is approximately 70%, while the remainder is Al-Si alloy. In order to stabilize the layer where the reaction occurs, the temperature of the molten Al should be preferably above 1273 K.
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