The quality index (Q) was first proposed by French foundrymen to explain the changes in strength and elongation of tensile bars during an aging heat treatment. This index was later given a theoretical basis by Caceres and co-workers, making it possible to answer the question: “Does this casting have the highest quality possible; or can further improvements be made?” The quality of A356-T6 castings made in an AA test mold are then considered. A considerable improvement in quality has occurred during the last 50 years. Improved degassing is the primary reason. The effect of Fe in A356-T6 B108 test bars is also presented. These results show that in rapidly cooled sections, a well degassed and low iron metal produces a quality close to the maximum theoretically possible. Recent tests of low-pressure suspension components show current best practices produce the same highest quality in commercial castings. Contrary to the claims of Campbell and co-workers, when only considering tensile properties, the presence of oxide films is of lessor importance compared to good degassing and rapid freezing rates. Examples are presented. When fatigue strength is important, however, the situation is more complicated. Factors controlling fatigue life are discussed in some detail.
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