Multiscale characterization of microstructures and mechanical properties of Inconel 718 fabricated by selective laser melting

A multiscale investigation of the microstructures and mechanical properties of Inconel 718 fabricated by selective laser melting (SLM) was performed on the as-SLM and heat-treated (HT) samples at various build locations. The microstructures were characterized by optical microscopy, high-resolution scanning electron microscopy, and electron backscatter diffraction over a wide range of length-scales. The as-SLM samples exhibited unique multi-scale microstructure features, including a few hundred-micrometer melt pools and columnar cube-textured grains, micrometer-sized dendritic and cellular sub-grain structures from chemical segregation and precipitates. Upon heat treatment, recrystallization occurred in the sample, replacing the unique SLM microstructures with homogenized grains, annealing twins, and nano-scale precipitates. Dry sliding wear tests, Vickers hardness tests, and force modulation microscopy in the atomic force microscopy were conducted to characterize the mechanical properties of the SLM parts. It was found that the wear resistance of the as-SLM sample was sensitive to the grain-level structures, exhibiting a relatively large spatial variation (∼11%) at different build locations. In contrast, the Vickers hardness barely changed (<1%) with locations. Heat treatment reduced the wear rate from 7.2 × 10⁻⁴ to 5.83 × 10⁻⁴ mm³/Nm and enhanced the hardness from 320 to 458 HV, resulting from the precipitation of nanoscale strengthening phases resolved from force modulation microscopy. Finally, the correlations of the mechanical properties with the hierarchical SLM microstructures were discussed, implying the importance of multi-scale conditions on tuning the SLM part performance.