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Defects in Aluminum Alloy Castings

Most of the major defects in Al alloy castings are the result of entrainment processes. The entrainment of the surface of the liquid creates bifilm defects, and the entrainment of air creates bubbles and bubble trails. Occasionally the entrainment of foreign inclusions, such as sand inclusions, can also be a problem. Bifilms form the initiators of gas porosity, shrinkage porosity, hot tears, and cracks. Since bifilms can be controlled by improved melting and casting techniques, all these defects are controllable. In addition, bifilms control the mechanical properties of castings, particularly tensile elongation, toughness, and fatigue. The other important effects caused by bifilms such as invasive corrosion behavior including pitting, stress corrosion cracking, and possibly hydrogen embrittlement, are beyond the scope of this review.