Mechanical properties of laser powder bed fusion processed Inconel alloy IN718 in different heat treatment conditions through small scale specimen testing

In this study, IN-718 samples were printed using laser powder bed fusion (LPBF) process to investigate the effect of printing parameters (in terms of Volume Energy Density, (VED)), printing orientation, heat treatment and test temperature on the microstructure and mechanical properties. Tensile properties were evaluated using small-scale tensile specimens extracted from samples printed with 15 different processing parameters (by varying scan speed and scan power) in two print orientations (XY and XZ plane), in three microstructural conditions (as-printed (AP), printed + solution treated and aged (STA), printed + hot isostatically pressed (HIP) + STA), at two different temperatures (room temperature and 650 °C). The mechanical properties obtained from testing of small-scale tensile specimens were compared with results obtained from testing of specimens following ASTM E8 standard, to understand effect of specimen size on mechanical properties. Based on microstructural analysis, it was observed that below a VED value of 50 J/mm³, the conduction region prevailed, with the presence of irregular lack of fusion porosities. In VED range of 50–100 J/mm³, the transition prevailed with highly dense samples (over 99 % of theoretical density) with minimal porosity. Above a VED of 100 J/mm³, the keyhole regime prevailed with the presence of spherical keyhole porosities. STA heat treatment improved YS and UTS to ∼1200 and ∼1400 MPa, respectively, while reducing ductility to ∼15 %. HIP + STA had similar effect on tensile properties as STA heat treatment. Based on tensile strength data, it was observed that HIP is not essential for parts printed with optimum parameters. Sample size in the present study was found to have an insignificant effect on tensile properties. In conclusion, the study provides an insight into the role played by printing parameters, printing orientation, and post-processing on defects, microstructures and thereby on the mechanical properties.