THE REDUCED PRESSURE TEST AS A MEASURING TOOL IN THE EVALUATION OF POROSITY HYDROGEN CONTENT IN AL-7 WT PCT SI-10 VOL PCT SIC(P) METAL-MATRIX COMPOSITE

Porosity is one of the important factors critical to the production of optimum aluminum alloy castings. Hydrogen is mainly responsible for t he ''gas porosity'' in such castings, which is also affected by other factors including melt cleanliness. The importance, therefore, of obta ining a reliable estimate of the melt hydrogen level prior to casting has led to the development of several techniques, among which the redu ced pressure test (RPT), basically a comparative, qualitative test, ap pears to be the one popularly used in foundries due to its simplicity and easy adaptation to the foundry floor. Attempts have been made to q uantify the test by correlating the densities of reduced pressure samp les with the hydrogen contents of their melts. In the present study, t he RPT was tested as a means of determining the hydrogen content in Al -7 wt, pct Si-10 vol pct SiC composite melts as part of an on-going st udy being carried out in our laboratories on such composites. The resu lts reveal that rather than indicating the hydrogen content of the mel t, the RPT is a better indicator of the porosity content of the cast s ample and can be employed as a melt quality measuring tool, provided t he sample density is correctly related to said porosity. Qualitative a nalysis is substantiated throughout by pore size and distribution data obtained from image analysis. It is also found that compared to the u nreinforced A356 matrix alloy, the composite material has a beneficial effect on the formation of porosity due to the tendency of the SiC re inforcement particles to restrict the growth of the pores. This, coupl ed with the microporosity associated with the presence of the SiC part icles, results in the skewed pore size distribution curves typically o bserved for the composite samples.