Effect of low-temperature annealing on the mechanical anisotropy and microstructure of Selective Laser Melting (SLM) AlSi7Mg alloy

This study investigates the microstructural evolution and mechanical anisotropy of Selective Laser Melting (SLM) AlSi7Mg alloy before and after low-temperature annealing. The effect of heat treatment on the anisotropic behavior of the material was examined through tensile tests, SEM, quasi in-situ Electron Backscatter Diffraction (EBSD) and Transmission Electron Microscopy (TEM). The results show that the SLM AlSi7Mg alloy exhibits pronounced anisotropy: in the XY orientation, the yield strength (YS), ultimate tensile strength (UTS), and elongation (Ef) were 310 ± 4 MPa, 500 ± 4 MPa, and 10.3 ± 0.3%; in the Z orientation they were 280 ± 7 MPa, 476 ± 19 MPa, and 4.9 ± 0.9%. After low-temperature annealing (170 °C/4 h), strength increased in both orientations and anisotropy decreased: XY-HT reached 339 ± 11 MPa (YS), 503 ± 4 MPa (UTS), and 9.0 ± 1.5%; Z-HT reached 310 ± 4 MPa, 516 ± 14 MPa, and 6.4 ± 0.7%. Correspondingly, the elongation gap narrowed from 5.4% to 2.6% (∼52% reduction), and the UTS gap shrank from 24 MPa to 13 MPa (∼46%). The anisotropic tensile elongation was mainly caused by differences in crack propagation behavior, with cracks propagating along melt pool boundaries in the Z samples and along cellular boundaries in the XY samples. The Z samples exhibited a higher work-hardening rate due to the activation of additional slip systems, facilitating dislocation motion. Low-temperature annealing promoted the formation of deformation twins of Si, coarsening of Si precipitates, and the reduction of stacking faults, which enhanced dislocation mobility and improved overall mechanical properties. This work highlights the potential of low-temperature annealing in refining microstructure and optimizing mechanical properties for SLM AlSi7Mg alloy, particularly in applications requiring balanced strength and ductility.