Post-fabrication treatments on additively manufactured AISI 316L stainless steel. Part II: Single and combined effects of hot isostatic pressing and shot peening on fatigue behaviour

This study is the second part (Part II) of a two-part investigation aimed at evaluating the microstructural and mechanical properties of laser-powder bed fusion (L-PBF) AISI 316L stainless steel after the individual and combined application of hot isostatic pressing (HIP) and shot peening (SP) post-fabrication treatments. Part I focused on multi-scale microstructural and mechanical characterisation, highlighting a transition of the microstructure from columnar austenite grains, with cellular subgrain structures, to fully recrystallised and equiaxed austenite grains after HIP. A decrease in hardness and yield strength was also observed, together with an increase in tensile ductility. Part II focuses on the effects of individual and combined HIP and SP treatments on rotating bending fatigue properties, which were modelled by a bi-conditional probability-stress-life (P-S-N) model from the literature. Additional analyses (X-ray computed tomography, X-ray diffraction, field emission scanning electron microscopy, and fractographic investigations) clarified the role of SP and the complementary and competitive mechanisms arising when HIP and SP treatments are combined. The combination of HIP and SP treatments yielded the highest fatigue strength – with all tested specimens reaching run-out – thereby interacting to delay or hinder fatigue crack initiation and propagation. As its main novelty, this article deepens the synergistic and competitive effects of combined HIP and SP post-fabrication treatments, providing a solid basis for the structural qualification of fatigue-critical AISI 316L components fabricated by L-PBF.