SETTLING OF SIC PARTICLES IN AN ALSI7MG MELT

The effect of SiC particle size and volume fraction on the settling be haviour was studied by isothermal holding of particulate metal matrix composite melts with initially homogeneous distributions of particles. The melts were herd in cylindrical steel crucibles at 700 degrees C f rom which one crucible was quenched every minute. Longitudinal section s of the solidified samples were polished, and a sharp transition betw een particle-free and particle-enriched areas could be observed. The d istance from the bottom of the sample to the particle-free area was me asured, and used for calculating the settling rate. The effect of part icle sizes ranging from 9 to 23 mu m and volume fractions from 10 to 2 0 vol.% was studied. The observed settling rate varied from 3 to 26 mm min(-1), depending on the combination of particle size and volume fra ction. For particle size 23 mu m and 10 vol.% reinforcement the settli ng rate was 26 mm min(-1); increasing the reinforcement to 20 vol% red uced the settling rate to 3 mm min(-1). On reducing the particle size to 9 mu m, a settling rate of 3 mm min(-1) was observed for 10 vol.% r einforcement. Measured sealing 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 o f SiC reinforcement is of major importance for obtaining a homogeneous distribution of the SIC particles in castings with medium and slow so lidification rates. In cases where the objective is to obtain local re inforcement at the bottom of the casting, large particle size and/or r educed volume fraction should be used.