EFFECT OF SOLIDIFICATION RATE AND METAL FEEDABILITY ON POROSITY AND SIC/AL2O3 PARTICLE DISTRIBUTION IN AN AL-SI-MG (359) ALLOY

ln the production of particle-reinforced metal-matrix composite castings, particle sedimentation during the melting process and particle redistribution during solidification can lean to particle segregation in the as-cast structure, the effects of which, in addition to those of porosity, can be highly detrimental to the properties and quality of the casting. Solidification rate and metal feedability are considered mainly responsible for the two problems. The present work reports on the influence of these factors on the particle distribution and porosity in 359 alloy composites reinforced with SiC and Al2O3 particles. The results show that the microporosity observed in 359/SiC(p) composites is a consequence of pore nucleation at the SiC particle sites and hindered liquid metal flow due to particle clustering; the former is responsible for the skewed porosity distribution profiles typically observed in these composites, similar to the type I distributions observed in A356 alloy. In the 359/Al2O3(p) composite, limited feedability and the wider range or larger particle sizes of the alumina particles result in the bell-shaped porosity profile observed, as well as the larger maximum pore size range (100-180 mu m(2) against 0-40 mu m(2) for the SiC(p) composites). The interparticle distance distributions for the SiC(p) composites show that finer dendrite arm spacings (DASs) produce a more uniform distribution of the SiC particles, while higher spacings lead to particle clustering, usually at separations of about 5 mu m, the probability increasing with increase in SiC(p) content. In the 359/Al2O3(p) composite, the distribution profile changes front a normal, random distribution to an exponential type as the DAS is increased. Together with the microstructural observations, the distributions indicate that particle pushing is the dominant phenomenon composites during solidification, whereas in the Al2O3(p) composite, mechanical trapping of the particles takes place at smaller DASs, that changes to particle pushing at larger spacings.