Effects of Intermetallics on the Fracture Behavior of Al-Si-Cu-Mg Base Alloys

Fracture behavior, as well as impact toughness, is influenced by alloy composition, solidification rate, and the heat treatment conditions applied. This study was undertaken to investigate the effects of intermetallics on the fracture behavior of non-modified and Sr-modified Al-Si-Cu-Mg base alloys. Castings were prepared from both experimental and industrial 319 alloy melts containing 0-0.6wt% Mg. Test bars were cast in two different molds, namely a star-like permanent mold and an L-shaped permanent mold, which provided cooling rates corresponding to secondary dendrite arm spacing (SDAS) values of 24 and 50 mu m (0.00095 and 0.00197 in.), respectively. The bars were tempered at 180C (356F) (T6 treatment) and 220C (428F) (T7 treatment) for 2-48 hours. It was observed that the addition of Mg resulted in the precipitation of the beta-Mg2Si, Q-Al5Mg8Cu2Si6, pi-Al8Mg3FeSi6, and the blocklike theta-Al2Cu phases. The fractured surfaces of non-modified alloys exhibited long Si particles with cracks in the interior, while those of the Sr-modified alloys displayed a dimpled structure throughout the matrix. Increasing the Mg content up to 0.6% resulted in the appearance of fractured particles of Q-Al5Mg8Cu2Si6 and pi-Al8Mg3FeSi6 phases. Decreasing the cooling rate or increasing the aging temperature did not alter the fracture mechanism with respect to the alloy composition.