Effect of heat treatment on microstructural evolution and mechanical properties of eutectic Al-6Mg2Si alloy processed by laser powder bed fusion

Abstract

It is critical to tailor specific schemes of heat treatment for alloys manufactured by laser powder bed fusion (L-PBF) according to their typical microstructural characteristics. In this study, systematic investigation was carried out to understand the influence of direct ageing (DA), annealing treatment (AT), and solution ageing treatment (SAT) on the microstructural evolution and mechanical properties of a L-PBFed Al-6Mg₂Si alloy. The results demonstrated that the hierarchical microstructure of the L-PBFed Al-6Mg₂Si alloy mainly consists of fine α-Al grains, Al/Mg₂Si eutectic cellular structures, solute atoms, and high-density dislocations, providing yield strength (YS) of 363 MPa, ultimate tensile strength (UTS) of 436 MPa, and elongation (El) of 9.3 %. Under DA condition, the Al/Mg₂Si cellular structure was preserved, promoting the precipitation of fine β'′ precipitates. The DA sample exhibited significant improvements (compared to L-PBFed sample) in mechanical properties, with YS, UTS, and El of 420 MPa, 496 MPa, and 9.5 %, respectively. In contrast, the eutectic Al/Mg₂Si cellular structure was completely disappeared and subsequently transformed to coarsened Mg₂Si particles after AT and SAT, resulting in decrease in strength. Direct ageing is a suitable heat treatment process to acquire a good strength-ductility synergy of the L-PBFed Al-Mg₂Si alloy.