Studies on the Effect of Substrate Preheating, Interlayer Dwell, and Heat Treatment on Microstructure, Residual Stress, and Mechanical Properties of IN625 Superalloy built by Direct Metal Deposition

Abstract

In this investigation, IN625 alloy samples were processed by directed energy deposition (DED) under various metal deposition strategies such as substrate preheating, inter-layer dwell, and with combined substrate preheating, inter-layer dwell as well as post-heat treatment. The processed sample's microstructural characteristics, residual stress, microhardness, and tensile properties are assessed in comparison to the manufacturing strategies. Rapid heat dissipation caused finer microstructure near the substrate. There is a growth of columnar grain structure epitaxially in the build direction. The progressive microstructure change seen in the build direction across the cross-section was due to the gradual rise of heat accumulation between subsequent layers. The inter-dendritic zones contained Laves phases. Laves phases have a high Nb, Mo as well as Si content, according to the EDS spectrum. The FESEM microstructural morphology of the deposited samples after their post-heat treatment has shown a new microstructure with the combination of equiaxed (recrystallized) and columnar dendritic structure with the reconstruction of columnar dendritic solidification microstructure into equiaxed grains. Heat treatment caused the Laves phases to dissolve in the matrix of IN625 alloy, which led to the precipitation of nanometric γ″ phases. The deposition strategies with substrate preheating significantly decreased the residual stress with moderately improved mechanical properties. The combination of substrate preheating, inter-layer dwell, and post-heat treatment has shown an outstanding reduction of residual stress along with a remarkable improvement in tensile strength with the retainment of an equivalent ductility compared with the other strategies.