Influence of cooling rate on microstructure, dislocation density, and associated hardness of laser direct energy deposited Inconel 718

This work deals with the effect of cooling rate on microstructure, dislocation density, and microhardness of laser direct energy deposited Inconel 718. Thermocycles were captured during direct energy deposition process using an infrared pyrometer. Cooling rate was estimated from the thermocycles at various laser powers and scanning speeds. In addition, a numerical model was developed to calculate cooling rate at different laser process parameters, and the same was verified with the experimental results. Microstructure and phases of the direct energy deposited Inconel 718 were observed using a scanning electron microscope and X-ray diffractometer, respectively. Top layer of the cladding was found to consist of fine equiaxed grains, whereas columnar dendrites were observed at the interface region of cladding layer and substrate. This is attributed to the variation in cooling rates between the top layer of the cladding and the interface region. γ, γ′, γ″ and Laves phases were identified to be the primary phases in the cladding layer. Moreover, niobium content was found to be high and varying with the cooling rate in the direct energy deposited Inconel 718. Dislocation density at varying scanning speed, i.e., cooling rate was estimated using the Williamson-Hall method. An increase in the dislocation density and concomitant improvement in the hardness was found with an increase in the cooling rate.