Revealing thermal behavior, cracking behavior, phase and microstructure formation of a ternary equiatomic alloy additively manufactured using directed energy deposition

Abstract

Additive manufacturing (AM) of multi-principal element alloys (MPEAs) has recently attracted considerable attention. However, few studies focus on the thermal behavior, cracking behavior, and microstructure tunability of AM-processed MPEAs, which can significantly affect the final performance of AM MPEA parts. In this study, a ternary equiatomic MPEA CrCoNi, with a single-phase face-centered-cubic (FCC) structure, was fabricated by the AM process via directed energy deposition (DED) at different laser scan speeds (10, 30, and 50 mm/s), and special focus was given to the thermal behavior, cracking behavior and microstructure formation. The increase in the laser scan speed from 10 to 50 mm/s causes a sharp increase in temperature gradients and cooling rates by five-fold and seventeen-fold, reaching up to 1148 K/mm and 57,778 K/s, respectively, as in-situ measured by a high-speed and high-resolution thermal pyrometer. Furthermore, the increased laser scan speed induces the severe cracking, which propagates along high angle grain boundaries and is classified as solidification cracking based on the observed protruding dendrites from the cracked plane. Although the Scheil-Gulliver solidification predicts a very narrow critical temperature range of 16 K which is indicative of a low solidification cracking susceptibility, the high temperature gradient and the resulting high thermal stress, as evidenced from the high density of dislocations and stacking faults, are believed to trigger the severe solidification cracking of the CrCoNi MPEA deposited at a high laser scan speed of 50 mm/s. With increasing the laser scan speed, the grain structure changes from elongated grains, which are roughly oriented along the build direction, to a more heterogenous grain structure with elongated grains converging towards the centerline and equiaxed grains arranged between the columns of elongated grains. Furthermore, with increasing the laser scan speed, the cellular structures are refined down to ∼ 2 µm due to the increased cooling rates. These findings not only contribute to better understanding the thermal behavior, cracking behavior, and microstructure formation of the AM-processed MPEAs, but also pave a road for further enhancing the mechanical properties of AM parts via tuning the thermal behavior and microstructures.