Effect of clustering on particle pushing and solidification behaviour in TiB2 reinforced aluminium PMMCs

The behaviour of titanium diboride particles in molten aluminium was investigated by conducting casting experiments at different cooling rates and particle addition levels, starting with a master alloy containing in situ formed TiB2 particles. The particle pushing/engulfment phenomena and particle clustering effects were studied for two matrix alloy systems, commercial purity (CP) aluminium and an Al-4% Mg (A514) alloy. The critical velocity, which is the solidification velocity below which the particles are pushed and above which they are engulfed, was calculated. The results were validated against the predictions of the models available in the literature. The assumptions, limitations and comparative success of the different models were discussed. The critical velocity observed in CP-Al ranged between 4 and 8 mu tm/s at low particulate concentration (< 1 vol%) and decreased to less than 2 mu m/s for the high concentration of particles. In the case of A514, the critical velocity was lower at the low particulate concentration (between 2 and 3 mu m/s). The hypothesis that particle clustering could result in erroneous critical velocity predictions was explored and it was concluded that particle clustering must be included for accurate prediction of particle pushing in these alloys. (c) 2004 Elsevier Ltd. All rights reserved.