On aspects of microstructure and mechanical properties of laser-powder bed fused IN718 part: impact of scanning speed and post-heat treatment

Purpose This study aims to fabricate IN718 parts through laser-powder bed fusion (L-PBF) at varied scanning speeds. In the as built condition, studies on location-specific microstructure evolution followed by characterization of mechanical properties such as tensile properties (ultimate tensile strength and % elongation) and microhardness are carried out. The obtained mechanical properties are correlated with part porosity/relative density. The as built specimens are then post-heat treated (homogenization treatment followed by double-stage aging). The microstructure and microhardness of the as built specimens are compared to that of the post-heat treated as well as conventional wrought counterparts. Design/methodology/approach Required numbers of tensile coupons (flat orientation; 0° w.r.t. the build platform) are fabricated through L-PBF at two different scanning velocities (s = 700 mm/s and s = 1,000 mm/s) keeping constant laser power, layer thickness, hatch distance and scan pattern. Tensile properties are determined through uniaxial tensile testing while microstructural features are observed through optical microscopy, scanning electron microscopy and field emission scanning electron microscopy. Hardness measurements are carried out at Vickers micro-indentation hardness tester. The post-heat treatment schedule is designed with homogenization treatment (1186°C, 40 min + air quenching) followed by double-stage aging – 1st stage aging (720°C, 8 h + furnace cool) and 2nd stage aging (625°C, 8 h + air quenching). Findings It is experienced that scan speed exhibits a direct relationship with part porosity and an inverse relationship with part tensile strength. With increase in scan speed, microhardness declines due to increased part porosity. The as built microstructure is decorated with very fine columnar/cellular dendrites. Along the building plane, majority of the grains exhibit directional solidification and hence columnar dendrites are mostly aligned along the building direction. On the other hand, microstructure consists of mostly Equiaxed cells when viewed on a plane perpendicular to the build direction. The fine dendritic features of the as built microstructure disappear after post-heat treatment. The post-heat-treated microstructure is found to be almost similar to the wrought alloy. The post-heat treated specimens exhibit higher microhardness than the as built IN718 due to precipitation strengthening taking place during aging treatments. Originality/value L-PBF is an efficient additive manufacturing route appropriate to fabricate small-to-medium sized parts with extreme degree of design complicacy. IN718 is a widely used aerospace superalloy known as “difficult-to-machine.” When compared to conventional machining, additive fabrication of IN718 is beneficial and it improves buy-to-fly ratio. However, the L-PBF process is greatly influenced by a huge parametric set which is different to control. Moreover, the process experiences heat accumulation effect and exhibits directional dependency on microstructure as well as mechanical properties. The post-heat treatment is thus attempted to homogenize the as built microstructure and to ensure isotropic properties. Hence, precise control of L-PBF parameters and determining an optimal heat treatment schedule are indeed required. The present research provides an insight to aforesaid issues. The outcome of the work is helpful for academic practitioners and it has strong industrial relevance.