Metal-related castability effects in aluminium foundry alloys

Abstract

Many foundries have intermittently experienced a major outbreak of casting rejects in which the alloy, although clearly within specification, nevertheless appears to be the primary cause of the problem. Minor changes in metal composition or metal quality can be sufficient to facilitate the formation of casting defects, within normally castable products, even when normal process conditions prevail. In the Al-Si based foundry alloys, the formation of porosity and shrinkage defects is sensitive to both metal composition and metal quality. However, the casting design and the actual processing conditions used can alter the sensitivity of the response to variations occurring within the metal. A number of metal-dependent factors of particular relevance to the castability of aluminium foundry alloys are discussed in this review. These include hydrogen level, oxidation tendency, the presence of inclusions, total volumetric shrinkage, alloy freezing range, dendritic growth conditions (dendrite arm spacing, coherency and grain size), formation of intermetallic phases, interdendritic feedability and the mode of eutectic solidification. The review also considers the sources of impurities found in aluminium alloys.