Effect of hot isostatic pressing of laser powder bed fused Inconel 625 with purposely induced defects on the residual porosity and fatigue crack propagation behavior

Abstract

Inconel 625 specimens with different levels of purposely induced defects (up to ~3% of specimen volume) were manufactured by laser powder bed fusion. They were subjected to a stress relief annealing followed by hot isostatic pressing. After each step of post-processing, the residual porosity and the pore size distribution were measured using the computed tomography technique. Tensile and fatigue crack growth testing were then carried out to evaluate the impact of residual defects on the mechanical resistance and damage tolerance of the manufactured specimens. Hot isostatic pressing was effective in reducing the processing-induced defect count and size, but near-the-surface defects were still observed. Large clusters of residual defects were also observed for specimens manufactured with ~3% porosity. The elongation at break, and, to a lesser extent, the tensile strength were impacted by an as-manufactured porosity as small as 0.25%. Fatigue crack growth testing showed that HIP increases the threshold stress intensity factor for long fatigue crack propagation from ~7 to ~9 MPa*m1/2 and that irrespective of the level of as-manufactured porosity. An as-manufactured porosity smaller that 1% did not impact the fatigue crack growth rate of hot isostatic pressed specimens significantly, whereas occasional crack retardation was observed for specimens with an as-manufactured porosity of ~3%.