Impact of machine parameters and wall thickness on microstructural characteristics and microhardness of laser powder bed fusion Inconel 718 parts following heat-treatment

Abstract

This study investigates a series of geometric feature build plates manufactured by multiple laser powder bed fusion (L-PBF) machine configurations, examining seven different wall thicknesses ranging from 0.1 to 2.0 mm. These build plates and thin wall specimens completed a full heat treatment cycle: stress relief (SR), HIP, solution, and aging per standards. The fully heat-treated (FHT) Inconel 718 wall specimens were sectioned from sixteen different geometric feature build plates built across fifteen different L-PBF machines. They were characterized by optical microscopy and EBSD image mapping. The microstructural evolution from the As-built sample, SR, HIP to FHT wall specimens from a single machine configuration was also obtained along with cooling rate simulations for 0.1 mm, 0.2 mm, 0.5 mm, and 0.8 mm wall thicknesses. The difference in cooling rates between the thick and thin walls was simulated to provide an understanding of microstructural evolution and evaluate the computer simulation as a tool to predict microstructural features. For FHT wall specimens, results indicate mostly equiaxed grain structures containing annealing twins, ranging in size from ∼21 μm to 93 μm parallel and perpendicular to the build direction. For most of the samples, the grain size was shown to increase with the increasing wall thickness due to slower solidification rates. The double aging composing the fully heat-treated thin walls also fully age-hardened the grain structures with gamma double-prime precipitates. This precipitation produced a median Vickers (HV) hardness for nominal section thicknesses >0.6 mm of ∼ HV 472; consistent with commercially heat-treated and optimized Inconel 718 products. Feature sizes >0.6 mm were reproducible for all L-PBF machine configurations.