Relation of microstructure, microhardness and underlying thermodynamics in molten pools of laser melting deposition processed TiC/Inconel 625 composites

Laser melting deposition (LMD) was applied to deposit nano-TiC particles reinforced Inconel 625 composite parts. The mechanisms of microstructure evolution and microhardness distinction in the different zones of the individual molten pool which was produced in LMD-processed composites were investigated. The layer-wise microstructural features of the manufactured parts were generally observed with clear outline curves of the molten pool as a result of the layer-by-layer deposition manner of the LMD shaping process. It could be observed that the microstructures in the upper part of the molten pool were mainly cellular structures, whereas which in the bottom and edge region were predominantly columnar dendrites. The increasing ratio of the temperature gradient to the solidification velocity (G/R), which resulted in a gradual change from columnar dendrite growth to cellular grain growth in the solidification regime, accounted for this phenomenon. The different sizes of cellular grains and dendrite spacing were ascribed to the varied cooling rates of diverse regions in the molten pool as well as the heat affecting nearby the overlapping zone. The factors contributing to microhardness variety could be summed up in three aspects, which were sizes of grains, TiC reinforcing particles and solid solution strengthening. (C) 2016 Elsevier B.V. All rights reserved.