Effect of alloying additions on threshold pressure for infiltration and porosity of aluminium-based melt infiltrated silicon carbide powder compacts

An instrumented heated pressure vessel has been used to determine the effect of alloying additions on the pressurised infiltration of SiC powder preforms by a series of Al-based melts in an air environment. The melts contained (wt%) 3.4Mg, 5.0Ti, 2.5Cr, 6.0Cu, 3.8Si, 2.1Sr, 1.4Pb, or 0.8Ca as binary alloying additions to pure aluminium. Addition of up to 3.4 wt% Mg to pure Al reduced the threshold pressure slightly while additions of 5.0 %Ti, 6.0 %Cu, 2.5 %Cr or 3.8 %Si did not affect threshold pressure significantly. Additions to pure Al of 2.1 % Sr or 0.8 %Ca increased the threshold pressure dramatically while a considerable reduction was observed by the addition of 1.4 %Pb. Porosity was significantly reduced by addition of Mg, did not change by addition of Cr, Cu or Si and was considerably increased by Sr, Pb or Ca. SEM, EDX and X-Ray powder diffraction analysis showed formation of clustered Al2CaSi2 phase in the matrix at its interface with the SiC particles, when Ca was added.