Fatigue behavior of additively manufactured 17-4 PH stainless steel: Synergistic effects of surface roughness and heat treatment

Abstract

In this study, the synergistic effects of heat treatment and surface roughness on the microstructure and mechanical properties of laser powder bed fusion (L-PBF) 17-4 precipitation hardening (PH) stainless steel (SS) were investigated under monotonic tensile and fatigue loading. Five different heat treatment procedures, with and without primary solution treating cycle, i.e. Condition A (CA), were applied on L-PBF specimens in both as-built and machined surface conditions. Monotonic tensile tests were conducted at a strain rate of 0.001 s−1 and, uniaxial fully-reversed (R = −1) strain-controlled fatigue tests were carried out at various strain amplitudes ranging from 0.001 to 0.004 mm/mm. The heat treatment procedures were found to have a significant effect on the microstructure and mechanical properties of L-PBF 17-4 PH SS with the CA thermal cycle improving the fatigue strength considerably. This is due to the more homogenized microstructure obtained after performing the CA thermal cycle. In addition, eliminating the effect of surface roughness by machining and polishing processes enhanced the fatigue strength of the material, somewhat comparable to the one of the wrought counterpart. Fractography analysis revealed surface micro-notches and internal pores as the primary crack initiating factors in the as-built and machined specimens, respectively.