Effect of heat treatments on microstructure evolution and grain morphology of alloy 625 with 0.4 wt% boron modification fabricated by laser wire deposition

Abstract

Alloy 625 modified with 0.4 wt% B was deposited on stainless steel 304 substrate by laser wire deposition. The microstructure was characterized and grain morphology was analyzed in the as-deposited and two post heat-treated conditions. In the as-deposited condition, continuous eutectics were observed to segregate in the inter-dendritic regions. The eutectics mainly consist of Laves phase and small amount of NbC precipitates. It was also found that the solidification features in the layer boundary were coarser than the layer core due to the recalescence mechanism. Small equiaxed grains nucleated in the layer boundary and eliminated the epitaxial growth which is normally formed during the additive manufacturing process. In the two-step aging condition, the eutectics were almost fully dissolved in the layer boundary. This has been associated with the layer boundary's larger elemental diffusion path when compared to the layer core. Partial recrystallization occurred in the layer core and led to nucleation of equiaxed grains. The grain size in both the layer boundary and the layer core coarsened during heat treatment. In the annealed and aged condition, the eutectics observed in the as-deposited condition were fully re-melted, resulting in the formation of large sized M5B3 borides during re-solidification. The fraction of borides in the layer boundary was lower, with smaller sized precipitates than those in the layer core. Similarly, to the two-step aged condition, partial recrystallization and grain growth developed during annealing and aging treatment.