Effects of laser shock peening on the ultra-high cycle fatigue performance of additively manufactured Ti6Al4V alloy

Although additive manufacturing (AM) allows to fabricate metallic parts with a high static strength comparable to their forged counterparts, the fatigue strength of an AM-ed part is generally inferior, restricting AM from many critical applications. In this study, laser shock peening (LSP) was applied to modify the surface properties of selective laser melting fabricated (SLM-ed) Ti6Al4V titanium alloy. This study performs systematic tests and analyses on the fabricated alloy specimens to characterize their microstructures and mechanical properties including residual stress, tensile strength, ultra-high cycle fatigue (UHCF) strength. The results reveal that LSP can refine microstructure, suppress residual stresses, and delay crack propagation in the affected area. However, the inherent defects in an SLM-ed part, such as unmelted powders, lack of fusion and clusters of alpha phase, dominate the fatigue failure of the specimens especially in the UHCF regime, resulting in their poor fatigue performance. Meanwhile, The LSP processed specimens showed a lower S-N curve than that of specimens without LSP processing especially in the UHCF regime, which not only results from the inherent defects, but also the increased surface roughness and non-uniform residual stresses.