Microstructures and tensile properties of Inconel 718 formed by high deposition-rate laser metal deposition

The aim of this study is to characterize microstructures and tensile properties of Inconel 718 (IN718) formed by high deposition-rate laser metal deposition (LMD), and furthermore to verify that the properties of the material are equivalent to those obtained by conventional manufacturing processes, such as casting and forging, and therefore satisfy the specifications for industrial applications. Initially, the powdery additive was characterized in terms of chemical composition, morphology, and porosity. Afterward, blocks for producing tensile specimens were deposited by applying the newly developed high deposition-rate LMD process that has a deposition rate of approximately 2 kg/h. Finally, microstructures and tensile properties of directly deposited and heat-treated material were analyzed, respectively. From the results, precipitation of an irregular shaped phase, which is believed to be Laves phase, and segregation of Nb and Mo were found at interdendritic regions of the directly deposited material. The directly deposited material exhibited relative low tensile strength and 0.2% yield strength but high elongation. Moreover, due to recrystallization that occurred in heat treatment, columnar grains in the directly deposited material transformed to equiaxed grains. By heat treatment, Laves phase was dissolved, and three extra phases, which are believed to be δ phase, strength phases γ′ and γ″, were precipitated. After heat treatment, tensile strength and 0.2% yield strength of the material were significantly enhanced, whereas the plastic elongation decreased by approximately 38%. In comparison to conventional manufacturing technology, the heat-treated IN718 presented superior tensile strength, 0.2% yield strength, and plastic elongation to aerospace material specifications for casted and wrought IN718.