A comparison study of high purity nickel fabricated by laser powder bed fusion and subjected to dynamic plastic deformation

Abstract

High density additively manufactured (AM) pure Ni samples were fabricated by laser powder bed fusion with 46 sets of processing parameters. The microstructure, thermal stability, and tensile properties of the sample with the optimized set of parameters were evaluated. To compare the inherent size effects related to dislocation cells and grain boundaries on thermal stability and strengthening, ultrafine-grained Ni samples with an identical chemical composition were also prepared by dynamic plastic deformation (DPD) with strain levels ranging from 0.2 to 2.2. The dislocation cells in AM Ni tended to coarsen between 400 and 600 °C, and partially disentangled and recovered at 700 °C−1200 °C but were reluctant to fully recrystallize even at 1200 °C due to the existence of oxide particles. These oxides were unexpected in high purity Ni. Consequentially, the yield strength of AM Ni continuously decreased with the increase of annealing temperature but remained higher than that of the fully recrystallized samples. The DPD Ni samples exhibited a higher strength level than AM Ni but with a poor uniform elongation. Meanwhile, their full recrystallization occurred at 500 °C for all strain levels, indicating inferior thermal stability compared to AM Ni. Fitting the inherent feature size with strength in the Hall-Petch relationship indicated that dislocation cell boundaries were weaker barriers to glissile dislocations compared to grain boundaries. The Taylor's hardening equation also correlated well with the dislocation cell strengthening in AM Ni. The AM Ni filled in a spot on the strength-ductility map that coarse-grained Ni and DPD Ni were unable to access.