Controlling the columnar-to-equiaxed transition during Directed Energy Deposition of Inconel 625

In this work, the grain orientation and mechanical properties of Inconel 625 are tailored by varying the process parameters during directed energy deposition. Under the same deposition speed, increasing the current is effective in promoting the columnar-to-equiaxed transition due to modifications on the thermal cycle. The solidification conditions (temperature gradient and cooling rate) were characterized during the process. A comparison with an existing solidification map for Inconel 625 indicates that the temperature gradients in the melt pool of the sample fabricated with larger current decrease sufficiently to permit the nucleation and growth of equiaxed grains. Uniaxial tensile testing showed that the sample with equiaxed grain microstructure exhibits a higher yield strength (increase by 36 %) when compared to the sample with columnar grains. Contributions of various strengthening mechanisms to the yield strength are quantified in terms of grain boundary strengthening, dislocation strengthening, and solid-solution strengthening. It is found that the higher yield strength of samples that possess equiaxed grains can be attributed to the enhanced dislocation strengthening arising from the large average Taylor factor.