Prasad, B K and Dan, Tapan Kumar and Rohatgi, P K
(1993)
Characterization and microstructural modifications of a pressure die cast eutectic aluminum-silicon alloy-graphite composite.
Materials Transactions of JIM, 34 (5).
pp. 474-480.
ISSN 1345-9678
Abstract
The present investigation was carried out to see if intricate shaped castings can be made from a eutectic Al-Si (BS LM13) alloy dispersed with 5.5 mass% graphite particles by pressure die casting technique. A bushing spring guide (BSG) component used for electrical applications was selected for the purpose. The study involved the characterization of a few properties e.g. hardness, density and electrical resistivity of 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 pore free indicating that the technique could be a promising route to synthesize graphitic aluminium alloys into intricate shapes. Machined sections of the PDC components indicated reasonably uniform distribution of graphite particles in various regions of the former. This was also confirmed by the quantitative analysis of the graphite content recovered from the dissolved specimens. The variation in hardness, density and electrical resistivity of the composite was quite less agreeing well with better uniformity of distribution of graphite particles in the matrix. The matrix microstructure of the PDC composite was considerably refined over the one processed by GDC technique, although the morphology of the microstructural constituents remained unchanged. The higher rate of solidification in this case was found to be responsible for the improvement in the uniformity of graphite distribution in the matrix and the microstructural refinement. Reduced secondary dendritic arm spacing (DAS) further confirmed a higher rate of solidification as a result of applying the pressure. Improvement in the graphite/matrix interfacial 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 entrapped gases from the latter under the conditions of applied pressure during 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 found to have brought about significant and useful modifications in the matrix microstructure at the little loss in properties like hardness, density and electrical resistivity.
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