Microporosity in cast aluminum alloys can be due to interdendritic shrinkage, hydrogen gas evolution, or a combination of both. A simple means of distinguishing between shrinkage and gas porosity, based on the nearest-neighbor cluster analysis of image analysis data, is explained. The method is applied to castings of Al-7 percent Si (A356) aluminum alloy, in order to evaluate the effect of melt hydrogen content on the relative importance of shrinkage and gas microporosity. It is shown that the two predominant types of porosity are gas porosity and combined gas-shrinkage porosity. Pure shrinkage porosity occurs only at very low gas levels.
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