Influence of annealing on microstructures and mechanical properties of laser powder bed fusion and wire arc directed energy deposition additively manufactured 316L

The annealing response of 316L stainless steel manufactured with both laser powder bed fusion (PBF-LB) and wire-arc directed energy deposition (DED-Arc) was investigated in the context of microstructural evolution and mechanical properties. Because additive manufacturing (AM) comprises several technologies that differ in heat source, feedstock, and scan strategy, among other build variables, the final products can experience a range of thermal histories and defect (dislocation) populations. These unique thermal histories can subsequently influence as-built properties and annealing response of AM materials, most notably those manufactured with different AM deposition processes. The PBF-LB process (with higher cooling rates) yielded finer austenite microstructures having more lattice strain, higher dislocation densities, and higher yield strength values than the DED-Arc 316L process (with lower cooling rates). Electron backscattered diffraction (EBSD) data of the kernel average misorientation (KAM), boundary density, and geometrically necessary dislocation (GND) density support the differences in lattice strain. As a result, the PBF-LB 316L exhibited more recovery and recrystallization after annealing at elevated temperatures above 873 K based on changes to yield strength, work hardening behavior, and microstructure evolution. The DED-Arc 316L exhibited a δ-ferrite/austenite microstructure with microsegregation that influenced deformation mechanisms active after annealing. Tensile data, deformed microstructures and misorientation histograms from EBSD showed a decreasing amount of deformation twinning when comparing the as-built condition to annealed conditions (up to 1473 K/1h) of PBF-LB 316L. The opposite trend was noted in DED-Arc 316L. The behavior was interpreted to be due to the differences in chemical segregation during solidification and the effects of heat treatment on chemical homogenization and local stacking fault energy.