Role of defects on the high cycle fatigue behavior of selective laser melted Al-Mg-Sc-Zr alloy

Al-Mg-Sc-Zr alloys manufactured using selective laser melting (SLM) possess outstanding static mechanical properties and thus have great potential application in the aerospace industry. However, their fatigue performance has not been clearly investigated and understood. In this work, the high cycle fatigue properties of an Al-Mg-Sc-Zr alloy manufactured via SLM with optimised process parameters (relative density over 99%) was studied with focus on the defects, whose size, shape and distribution were determined through X-ray tomography. Microstructure analysis results show that the Al-Mg-Sc-Zr alloy produced using SLM presents a periodic, inhomogeneous structure consisting of equiaxed grains at the boundary and columnar grains at the centre of one molten pool, which causes a significant difference in deformational mismatch under the fabrication conditions. Furthermore, X ray-CT results show that the defects, mainly porosity, are randomly distributed with the critical flaw size to initiate fatigue crack, thereby reducing fatigue life. When the stress ratio (R) is 0.1, the annealed sample presents good fatigue strength (sigma(f)) reaching 100.5 MPa, which is close to that of high-strength wrought 7075Al. Finally, a fatigue crack propagation mode connected with defect microstructure characteristics was established for prediction of fatigue properties.