Knowledge and Use of Heat-Treating Procedures Employed to Analyze the Cause of a Pair of Aluminum Piston Fractures

Abstract

Aluminum alloy pistons, one each from two of the same model internal combustion engines, experienced fracture. The alloy would have been heat treated—solution treated and aged—to obtain an intended as-manufactured hardness. Hardness tests suggested that both pistons had been subjected to elevated temperature and, hence, were overaged. To prove the point, material from an undamaged piston was purposely overaged. The overaged hardness was consistent with the hardness of the materials from the fractured pistons. Fragments from a damaged piston were then solution treated and optimum aged, the result being restoration of material hardness. The heat-treatment experiments determined that the reduced hardness of the fractured pistons was incident related and not manufacturing related. Physical evidence also indicated that the pistons had seized in the engines. The result was fracture of the pistons because of overaging by heating when the pistons seized. The result of the heating was to reduce the hardness and strength of the piston material.