Effect of Two-Step Solution Treatments on the Microstructure and Mechanical Properties of B357 Aluminum Alloy

Abstract

This study investigates the effect of two-step solution treatments and aging times on the microstructure and mechanical properties of B357 Al–Si alloy, part of the most widely used cast aluminum alloy. Three sets of artificial aging heat treatments were conducted on the tensile samples prepared from B357 alloy produced by low-pressure die casting. Firstly, the conventional artificial aging heat treatment (T6) was carried out by solutionizing the tensile samples at 543 °C for 8.5 h, water quenching at 60 °C, and then artificially aging for 8.5 h at 160 °C. In the two-step solution treatment, the samples were solutionized at 400, 440, 480, and 520 °C for 4 h and then solutionized at 543 °C for 8.5 h and water quenched to 60 °C and then artificially aged for 8.5 h at 160 °C. Thirdly, the samples were solutionized at 480 °C for 4 h, solutionized at 543 °C for 8.5 h, water quenched to 60 °C, and artificially aged for 3–192 h. Despite different solutioning and aging processes, no significant differences were observed in the microstructures of the samples. Si particle coarsening was observed with increasing solution temperature (400–520 °C) and aging times (3–192 h). Si-containing dispersoids and dispersoid-free zones (DFZ) were observed in the primary-α matrix. While DFZ width increased with temperature and aging, dispersoid zones in primary-α dendrites significantly decreased. Differential Scanning Calorimetry (DSC) analysis shows that two-step solution treatment in B357 alloy increases β′ precipitates for Mg2Si precipitation strengthening. The two-step solutionized and aged sample showed the best combination of strength and ductility among all aged samples. B357 alloy exhibited the highest yield and tensile strength (309.7, 366.1 MPa) with 6% elongation for two-step solutionizing and 48 h aging. All aged B357 alloys showed ductile fracture as the primary fracture mode. However, brittle fractured Si particles were observed on the fracture surfaces.