Relationship between tensile and impact properties in Al–Si–Cu–Mg cast alloys and their fracture mechanisms

An extensive study related to the investigation of the precipitation behavior of the CuAl2 phase in various 319-type alloys containing strontium (Sr), iron (Fe) and phosphorus (P) alloying elements, and its dissolution during solution heat treatment at 490 °C for times up to 100 h was carried out in the present work. Furthermore, the effect of CuAl2 and other intermetallics on the alloy performance under two different heat treatment conditions (i.e. T5 and T6) was also investigated through an examination of the tensile and impact properties. By comparing the experimental results, conclusions were drawn in terms of the optimum alloying elements, solidification parameters and heat treatment conditions (viz., Sr modification, Fe content, cooling rate and T6 condition). The fracture behaviors of the 319 base alloy and the Sr-modified 319 alloy containing ∼1.2% Fe were also compared through a study of the fracture surfaces of the corresponding alloy samples. The results explicitly reveal that solution heat treatment plays a critical role in the dissolution of the CuAl2 phase. Iron addition leads to an increased precipitation of brittle β-Al5FeSi platelets which act as preferred crack sites and dramatically reduce the impact properties, regardless of the value of dendrite arm spacing. Crack initiation usually occurs through the fragmentation of Si particles and β-Al5FeSi platelets, and the crack propagates through the cleavage of β-Al5FeSi platelets.