Toward the Equiaxed Grain Microstructure in CrMnFeCoNi High-Entropy Alloy Fabricated by Directed-Energy Deposition

Abstract

The columnar grains in additively manufactured alloys increase the tendency to form solidification cracks and cause anisotropy. Studying the effect of process parameters on microstructure development helps to guide the manufacturing of the equiaxed grain microstructure. First, the effect of process conditions on the melt pool dimensions using in situ synchrotron X-ray imaging and thermal profile and solidification condition using finite element simulation and calculation of thermodynamics phase diagrams of CrMnFeCoNi high-entropy alloy fabricated by directed energy deposition is studied. Increasing the laser power reduces the thermal gradient to solidification rate ratio, pushing the solidification closer to the columnar-equiaxed transition. Nevertheless, the simulations still indicate the columnar microstructure for all scan conditions in contrast to the experimental observation that shows single-wall samples built at 200 W consisted of dominantly equiaxed grains, whereas columnar grains are dominant in samples built at 100 W. It is believed that in addition to the effect of thermal gradient and solidification rate, the chemical segregation (Mn and Ni) during solidification may promote dendrite detachment, hence assisting the transition to equiaxed grains. The multitrack deposition results in more solid beneath a new melt pool, increasing the thermal gradient that promotes more columnar grains in comparison to single tracks.