Effect of heat treatments on fatigue behavior and fracture of Inconel 625 additively manufactured by Laser Powder Bed Fusion

This investigation examined the impact of Heat Treatments (HT) on the fatigue and fracture characteristics of additively manufactured Inconel 625. Following the fabrication of net-shaped specimens by the Laser Powder Bed Fusion (L-PBF) technique, the material underwent four distinct heat treatments: Stress Relief (SR), Direct Aging (DA), Solubilization and Aging (S+A), and Annealing (AN), all conducted in a high vacuum furnace. The effects of HT on microstructure, porosity, and residual stress state were comprehensively examined utilizing metallographic methods, X-ray diffraction, and computed tomography. The specimens, which included a batch retained in as-built (AB) condition for comparative analysis, underwent fatigue testing for up to 2E7 cycles. In all cases, HTs led to improved fatigue properties; however, AN produced the most significant enhancements, exhibiting the highest fatigue strength and a flatter slope of the S-N curve in the finite life zone. Post-fatigue residual stress measurements indicated that the heat-treated conditions with better fatigue performance were generally associated with a more favourable final compressive stress state; however, these measurements should be interpreted mainly as qualitative support, since cyclic loading and crack propagation may have modified the initial residual stress field. The examination of fracture surfaces by SEM and quantitative assessments of surface topography yielded additional insights into the relationships between HT-induced microstructures and fatigue failure processes.