Directed energy deposition additive manufacturing of CoCrFeMnNi high-entropy alloy towards densification, grain structure control and improved tensile properties

Abstract

Additive manufacturing (AM) of high-entropy alloys (HEAs), such as CoCrFeMnNi, is of high interest owing to their potential applications for extreme conditions (e.g., cryogenic environment). However, a major drawback of additively manufactured (AMed) metallic components is their poor and anisotropic mechanical properties, which primarily originate from the presence of metallurgical defects and coarse columnar grains. This study herein aims to (i) manipulate directed energy deposition (DED) AM parameters towards densification and (ii) suppress the formation of coarse columnar grains in DED AMed CoCrFeMnNi HEA. First, the effects of laser powers and laser scan speeds on the formation of various metallurgical defects in the DED AMed CoCrFeMnNi samples were investigated. Contrary to the laser powder bed fusion (L-PBF) AMed CoCrFeMnNi HEAs which were reported to form lack-of-fusion defects in the regime of low laser powers and high laser scan speeds, the lack-of-fusion defects in DED AMed CoCrFeMnNi HEAs tend to form in the regime of high laser powers and low laser scan speeds. This abnormal observation was rationalized by recourse to the resultant large layer thickness and hence insufficient melting. Cracking was observed in the regime of high laser scan speeds, and these cracks were classified into solidification cracks based on the observation of the intruding cellular/dendritic features on the cracked surface. High angle grain boundaries were found to be more sensitive to solidification cracking and the inherent mechanisms were discussed. Through such a high throughput library of samples, a printability map and near-full density were achieved for DED AMed CoCrFeMnNi HEAs. Second, for the grain structure control, contrary to the previous works that tried to manipulate the solidification conditions or introduce the secondary heterogenous particles, the current paper employed ultrasonic-assisted DED AM processing of CoCrFeMnNi HEA. Despite the measured lowered cooling rate, the coarse columnar grains were replaced by finer equiaxed grains in the presence of ultrasound, i.e., the average grain size decreases from 140 to 44 μm. The formation of fine equiaxed grains were rationalized by the combined effects of the ultrasonic-facilitated dendrite fragmentation as heterogenous nucleation sites and the enhanced liquid supercooling in the columnar front. As a direct result, the tensile yield strength of the ultrasonic-assisted DED AMed CoCrFeMnNi HEA increases by ∼ 17%, with no obvious tensile ductility drop. The strength increase was well explained by the grain refinement effect based on the classic Hall-Petch relationship. These findings provide opportunities to additively manufacture high-performance and complex-geometry HEAs components for critical applications.