CHARACTERIZATION AND MICROSTRUCTURAL MODIFICATIONS OF A PRESSURE DIE CAST EUTECTIC ALUMINUM-SILICON ALLOY-GRAPHITE COMPOSITE

The present investigation was carried out to see if intricate shaped c astings can be made from a eutectic Al-Si (BS LM13) alloy dispersed wi th 5.5 mass% graphite particles by pressure die casting technique. A b ushing spring guide (BSG) component used for electrical applications w as selected for the purpose. The study involved the characterization o f a few properties e.g. hardness, density and electrical resistivity o f the pressure die cast (PDC) composite and their comparison with that of the gravity die cast (GDC) one. The influence of heat treatment on the microstructure of the PDC composite has also been studied. Visual examinations revealed that the components were perfect in shape and p ore free indicating that the technique could be a promising route to s ynthesize graphitic aluminium alloys into intricate shapes. Machined s ections of the PDC components indicated reasonably uniform distributio n of graphite particles in various regions of the former. This was als o confirmed by the quantitative analysis of the graphite content recov ered from the dissolved specimens. The variation in hardness, density and electrical resistivity of the composite was quite less agreeing we ll with better uniformity of distribution of graphite particles in the matrix. The matrix microstructure of the PDC composite was considerab ly refined over the one processed by GDC technique, although the morph ology of the microstructural constituents remained unchanged. The high er rate of solidification in this case was found to be responsible for the improvement in the uniformity of graphite distribution in the mat rix and the microstructural refinement. Reduced secondary dendritic ar m spacing (DAS) further confirmed a higher rate of solidification as a result of applying the pressure. Improvement in the graphite/matrix i nterfacial bonding was found to be one of the interesting features of pressure application. This was attributed to the increased solubility of the dissolved gases in the matrix and squeezing out of the entrappe d gases from the latter under the conditions of applied pressure durin g solidification. The graphite particles were found to have fractured in this case probably due to the possible application of a combination of impact, shear and compressive stresses under the influence of the applied injection pressure. Heat treatment of the PDC composite was fo und to have brought about significant and useful modifications in the matrix microstructure at the little loss in properties like hardness, density and electrical resistivity.