Evolution of Microstructure and Properties of SLM Formed Inconel 738 Alloy during Stress Relief Annealing

Abstract:

Due to the characteristics of rapid melting and solidification in the process of preparing Inconel 738 alloy by selective laser melting (SLM), there is residual stress in alloy, so alloy cannot be directly used. Stress relief annealing is required. The evolution of residual stress, precipitation behavior, microstructure and properties in the alloy was mainly studied after annealing at different temperatures (700~900 ℃) for 24 h. The results show that the microstructure evolution mechanism is recovery，dislocation migration and rearrangement and grain boundary style changes were occured，a bimodal distribution of spherical γ "phase (50~250 nm) and short rod-shaped γ" phase (240~440 nm) are formed，the grain boundary carbides change from point to continuous and the type of carbides changed from MC to M23C6; After annealing at 800 ℃, the residual stress decreased from 380.94 MPa (AS-SLMed) to -66.7 MPa, and the residual stress was eliminated; With the annealing temperature increased, the microhardness and tensile strength first increased and then decreases, and the elongation decreases first and then increases. The alloy has the highest microhardness and tensile strength (581.2 HV, 1330 MPa) which are 1.42 times and 1.41 times of the properties of casting (410 HV, 945 MPa) after annealing at 800 ℃.

Key words:SLM;Inconel 738;residual stress;phase;Mechanical properties