Laser Powder Bed Fused Al-Mg-Zr-Mn alloy: Microstructure, Mechanical and Fatigue performance

In this study, an Al-Mg-Zr-Mn alloy (Al5X1) was designed for laser powder bed fusion (LPBF) to address the demand for a cost-effective high-strength AM aluminum alloy. Mechanical properties were optimized through a single-step aging treatment (6 hours @400°C followed by gas quench). The material exhibited a hardness of 0.98 ± 0.03 GPa (92.57 ± 2.42 HV) in the as-built condition and 1.39 ± 0.03 GPa (131.30 ± 2.84 HV) in the aged condition, as determined by depth-sensing nanoindentation testing. In the aged condition, the tensile properties include yield strength (YS) of 390 ± 2 MPa, ultimate tensile strength (UTS) of 428 ± 2 MPa, and elongation to failure (EL) of 14.26 ± 0.94%. Solid solution strengthening by Mg and Mn, along with precipitation strengthening through Zr (Al₃Zr precipitates), contributed to the material's enhanced strength and grain refinement. Microstructure analysis via scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) revealed the presence of precipitates and a bimodal grain distribution with ultrafine (~1 µm) and fine grains (~5 µm). The fatigue behavior of this material was investigated for the first time, confirming a fatigue strength of ~140 MPa (at 5 million cycles) in the aged condition. SEM-based fractography analysis identified process-induced defects, especially those near the surface, as the primary cause of failure. Fracture exhibited a ductile nature, characterized by the formation of high-density dimples in the final fracture region, with the presence of tiny precipitates based on Al₃Zr. Based on these findings, the next steps for improving fatigue properties may involve further optimizing the process to reduce defects or introducing post-processing techniques such as hot isostatic pressing to mitigate defect volume.

Keywords: Laser powder bed fusion; Al-Mg-Zr alloy; Al5X1; Precipitation hardening.