Settling of SiC particles in an AlSi7Mg melt

Abstract

The effect of SiC particle size and volume fraction on the settling behaviour was studied by isothermal holding of particulate metal matrix composite melts with initially homogeneous distributions of particles. The melts were held in cylindrical steel crucibles at 700 °C from which one crucible was quenched every minute. Longitudinal sections of the solidified samples were polished, and a sharp transition between particle-free and particle-enriched areas could be observed. The distance from the bottom of the sample to the particle-free area was measured, and used for calculating the settling rate. The effect of particle sizes ranging from 9 to 23 μm and volume fractions from 10 to 20 vol.% was studied. The observed settling rate varied from 3 to 26 mm min −1, depending on the combination of particle size and volume fraction. For particle size 23 μm and 10 vol.% reinforcement the settling rate was 26 mm min −1; increasing the reinforcement to 20 vol.% reduced the settling rate to 3 mm min −1. On reducing the particle size to 9 μm, a settling rate of 3 mm min −1 was observed for 10 vol.% reinforcement. Measured settling rates were approximately 5 times higher than what is predicted from Stokes' law. The measured settling rates indicate that selection of the proper combination of size and amount of SiC reinforcement is of major importance for obtaining a homogeneous distribution of the SiC particles in castings with medium and slow solidification rates. In cases where the objejctive is to obtain local reinforcement at the bottom of the casting, large particle size and/or reduced volume fraction should be used.