Effect of post heat-treatment on the microstructure and mechanical properties of Hastelloy-X structures manufactured by laser based Directed Energy Deposition

This paper reports a systematic investigation of Hastelloy-X (Hast-X) structures built by Laser Directed
Energy Deposition in as-built and post heat-treated conditions. The optical microscopy shows microstructures with fine dendrites in as-built condition due to higher cooling rate during deposition, while
recrystallized equiaxed grains are observed after post heat-treatment (at 1177 C) due to recrystallization.
X-ray diffraction studies reveal nickel g-matrix with variation in crystallite size and a peak shift after post
heat-treatment, primarily due to change in surface residual stress. Surface topography reveals the
reduction in average roughness with post heat-treatment. Further, the maximum compressive residual
stress of 350 MPa and maximum tensile residual stress of 252 MPa are observed at the surfaces of post
heat-treated and as-built samples, respectively. The average micro-hardness changed from 239 HV1.96N to
208 HV1.96N after post heat-treatment. Single Cycle Ball Indentation studies indicated increase in energy
storage capacity by a factor of 1.55 after post heat-treatment. Automated Ball Indentation studies
quantify yield strength and ultimate strength as 478 MPa and 765 MPa in as-built samples, while
393 MPa and 630 MPa in post heat-treated samples, respectively. Further, crack propagation studies
indicate an improvement in the fatigue life after post heat-treatment, while the specific wear rate
increased by a factor of 1.72 with increased delamination in heat-treated samples. Thus, post heattreatment of Hast-X samples changes the material properties significantly