Influences of Si and Mg contents on microstructures of Al-xSi-yMg functionally gradient composites reinforced with in situ primary Si and Mg2Si particles by centrifugal casting

Hypereutectic Al-xSi-yMg functionally gradient composites reinforced with primary Si and Mg2Si particles were fabricated by centrifugal casting. The influence of Si and Mg contents on microstructures of the Al-xSi-yMg functionally gradient composites was investigated. Calculations of the volume fractions and the sizes of primary Si and Mg2Si particles in the cross section of each tube along the radial direction from the inner-to-outer surface revealed that this type of gradient composite tube can be fabricated by centrifugal casting when the contents of Si and Mg are more than or equal to 19 and 4%, respectively. The tubes consist of an inner layer, the middle layer, and the outer layer measured in the radial direction on the cross section. The inner layer segregates blocky primary Si and Mg2Si particles, the middle layer contains no primary Si and Mg2Si particles, and the outer layer contains few primary Si and Mg2Si particles. We compared resulting mean volume fractions and average sizes of produced primary Si and Mg2Si particles with initial Si and Mg contents to determine their relationship in this process. The morphology of the Mg2Si phase is another key factor in the formation of the composites. It was found that the blocky primary Mg2Si particles have greater centroclinal velocity than that of primary Si particles due to the lower density of the Mg2Si particles. The primary Si particles are pushed by blocky primary Mg2Si particles. The two kinds of particles move toward the inner wall of the tube together during the solidification. A model of particles movement has been established according to the experimental results.