Fatigue characteristics of a newly developed laser powder bed fused scandium-free Al-Mg-Zr-Mn alloy

This study investigates the fully reversed force-controlled fatigue response of a newly developed laser powder bed fused (LPBF) Al-Mg-Zr-Mn alloy (EOS Al5X1) in the post-aged condition. The fatigue behavior revealed a defect-driven response with a fatigue strength of approximately 140 MPa at 5 million cycles. Comprehensive microstructural analyses, including grain size, texture, and precipitate characterization, were performed using advanced microscopy techniques. Additionally, X-ray computed micro-tomography (XCT) was employed to assess defect size and distribution, yielding a relative density of 99.93 %. Fracture surfaces of all fatigue-failed specimens were examined using optical and scanning electron microscopy to determine the primary failure mechanisms, with a focus on distinguishing between defect-driven and microstructural causes. The results indicated that nearly all specimens, tested across seven stress levels, exhibited crack initiation from process-induced volumetric defects, such as pores and lack of fusion. At lower stress levels (up to 195 MPa), single crack initiation sites driven by defects were identified at either surface or subsurface locations. In contrast, at higher stress levels (234 to 351 MPa), multiple crack initiation sites were observed, also at the surface or subsurface.