Effects of direct aging treatment on microstructure, mechanical properties and residual stress of selective laser melted AlSi10Mg alloy

Abstract

• The as-fabricated microstructure benefits short-circuit diffusion of Si. • Direct aging treatment promotes unique precipitation behaviors. • Direct aging removes roughly 40% of the residual stress. • Mechanisms of residual stress removal are explored. • Over-aging treatment produces better mechanical properties. Direct aging treatment is an important post-processing procedure, yet little research has been done on how it balances the mechanical properties and the stress removal for selective laser melted (SLMed) AlSi10Mg alloys. Here, we proposed a typical direct aging treatment on SLMed AlSi10Mg alloys, and studied the effects on their microstructure, properties and residual stress evolution. The results indicate that the as-built microstructure is mainly composed of fine cellular α-Al and reticulated Si phases, and some pre-existing precipitates and dislocations are found in these cells. The direct aging treatment promotes the precipitation of the nano-scaled Si phase and preserves the network-like Si structure. Therefore, the strength of the peak-aged alloy increases while the ductility decreases. As the aging temperature increases from 160 to 200°C, aging hardening behavior was accelerated significantly. Aging at 160 °C for 4-9 h removes 32.0%-43.0% of the residual stress, which is attributed to the decomposition of the supersaturated α-Al matrix, the precipitation of the nano-Si phase and the exposure of low-angle grain boundaries (LAGBs). Considering the overal alloy performance obtained, over-aging at 160°C for 4 h is the optimized heat treatment regime. Under this condition, the yield strength (YS), ultimate tensile strength (UTS) and elongation (EL) of the alloy in the transverse and longitudinal direction are 309.5 MPa, 464.4 MPa and 8.3% and 286.4 MPa, 464.9 MPa and 5.1%, respectively.