INFLUENCE OF METALLURGICAL PARAMETERS ON THE IMPACT TOUGHNESS OF NEAR EUTECTIC AL-SI ALLOYS

摘要

The effects of Sr and Mn additions, cooling rate, and a range of aging temperatures and times on the impact toughness of a near-eutectic Al-11 % Si-2.5 % Cu-0.3 % Mg-0.45 % Fe alloy were investigated, using unnotched samples in order to emphasize the role of metallurgical parameters on both crack initiation and crack propagation processes. Surfaces of tested samples were also examined to determine the fracture behaviour. Results show that the eutectic silicon and the high volume fraction of Cu- and Fe-containing intermetallic phases present provide a wide number of crack initiation sites, thereby reducing the impact energy values of these alloys. While these alloys display a similar trend under both high and low cooling rate conditions, the best impact values are observed in alloys solidified at the high cooling rate. Increasing the aging time up to 44 h at the peak-aging temperature of 180 degrees C (356 degrees F) produces virtually no discernible change in the impact values of the alloys investigated. However, aging at 240 degrees C (464 degrees F) for 44 h significantly increases the impact values as a result of softening, regardless of alloy composition.