Mechanical property enhancement of Inconel 625 through pulse modulations in LDED and further improvement via increased γ’’ precipitation through tailored heat treatment

Abstract

This work dealt with pulsed wave laser additive manufacturing of Inconel 625 (IN625) using a laser directed energy deposition (LDED) route, envisioning refined microstructure evolution with superior mechanical characteristics and greater chemical homogeneity. A comparative analysis between microstructural and mechanical characteristic evolution under continuous wave (CW) and pulsed wave (PW) mode with varying duty cycle and frequency was analysed. In-process infrared pyrometer captured the temperature dynamics of the molten pool revealing greater insights into the periodic thermal fluctuations under four different PW modes. The study showed microstructural refinement with visible restriction in columnar grain growth and a transition towards higher equiaxed grain evolution under PW mode at lower duty cycle and frequency. Moreover, the extent of elemental segregation was also limited through PW laser irradiations leading towards greater chemical homogeneity. PW modes resulted in the evolution of a weak mixed texture comprising of copper, brass and R/S components. PW laser irradiation under low duty cycle and frequency thus led to the evolution of IN625 parts with near isotropic microstructural properties. Greater high angle grain boundary evolution and significant presence of low angle grain boundaries enhanced the mechanical characteristics of sample fabricated under PW mode especially under 50 Hz frequency compared to CW mode as evident from microhardness and tensile analysis. Further improvement in mechanical properties was obtained through direct double aging heat treatment (DA1 and DA2) attributed to greater γ’’ and δ phase precipitation proximity to grain boundaries, the presence of which was confirmed through scanning electron microscopic and transmission electron microscopic analysis. An improvement of 10–12 % in tensile stress was registered for DA1 and DA2 samples compared to corresponding non-heat-treated ones.