Porosity is one of the important factors critical to the production of optimum aluminum alloy castings. Hydrogen is mainly responsible for the ''gas porosity'' in such castings, which is also affected by other factors including melt cleanliness. The importance, therefore, of obtaining a reliable estimate of the melt hydrogen level prior to casting has led to the development of several techniques, among which the reduced pressure test (RPT), basically a comparative, qualitative test, appears to be the one popularly used in foundries due to its simplicity and easy adaptation to the foundry floor. Attempts have been made to quantify the test by correlating the densities of reduced pressure samples with the hydrogen contents of their melts. In the present study, the 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 study being carried out in our laboratories on such composites. The results reveal that rather than indicating the hydrogen content of the melt, the RPT is a better indicator of the porosity content of the cast sample and can be employed as a melt quality measuring tool, provided the sample density is correctly related to said porosity. Qualitative analysis is substantiated throughout by pore size and distribution data obtained from image analysis. It is also found that compared to the unreinforced A356 matrix alloy, the composite material has a beneficial effect on the formation of porosity due to the tendency of the SiC reinforcement particles to restrict the growth of the pores. This, coupled with the microporosity associated with the presence of the SiC particles, results in the skewed pore size distribution curves typically observed for the composite samples.
