Effect of columnar-to-equiaxed microstructural transition on the fatigue performance of a laser powder bed fused high-strength Al alloy

Aluminum alloys that are additively manufactured using the laser powder bed fusion (LPBF) suffer from relatively poor high cycle fatigue (HCF) resistance. In an effort to alleviate this, a high-strength Al alloy, Al-Mn-Mg-Sc-Zr, with columnar, equiaxed, and bi-modal microstructures was produced by varying the scanning velocity and the substrate temperature during the LPBF process. The tensile strength of LPBF Al-Mn-Mg-Sc-Zr alloy is 475 ± 5 – 516 ± 6 MPa with favorable elongation of approximately 11 %, higher than that of most of the other Al alloys, including conventional high-strength rolled/ECAP Al alloys and AM Al-Mg-Sc-Zr alloys. Specimens with bimodal microstructure and specimens with fully equiaxed microstructure both show a fatigue strength of 230 MPa (at 10⁷ loading cycles), which is the highest among those reported for the LPBF Al alloys. The deformation synergy in the bimodal microstructure also improves the fatigue resistance in the strain-controlled low cycle fatigue (LCF) regime. The equiaxed microstructure restricts the to-and-fro dislocation motion during cyclic loading, which, in turn, minimizes the strain localization. At the later stages of strain accumulation, microcracks form at the grain boundaries, limiting the further improvement of the alloy's fatigue strength. This study demonstrates microstructural tailoring through AM enables improvement of the fatigue resistance of aluminum alloys.