Quantifying Equiaxed vs Epitaxial Solidification in Laser Melting of CMSX-4 Single Crystal Superalloy

Abstract

The competition between epitaxial vs. equiaxed solidification has been investigated in CMSX-4 single crystal superalloy during laser melting as practiced in additive manufacturing. Single-track laser scans were performed on a powder-free surface of directionally solidified CMSX-4 alloy with several combinations of laser power and scanning velocity. Electron backscattered diffraction (EBSD) mapping facilitated identification of new orientations, i.e., “stray grains” that nucleated within the fusion zone along with their area fraction and spatial distribution. Using high-fidelity computational fluid dynamics simulations, both the temperature and fluid velocity fields within the melt pool were estimated. This information was combined with a nucleation model to determine locations where nucleation has the highest probability to occur in melt pools. In conformance with general experience in metals additive manufacturing, the as-solidified microstructure of the laser-melted tracks is dominated by epitaxial grain growth; nevertheless, stray grains were evident in elongated melt pools. It was found that, though a higher laser scanning velocity and lower power are generally helpful in the reduction of stray grains, the combination of a stable keyhole and minimal fluid velocity further mitigates stray grains in laser single tracks.

Epitaxial Growth and Stray Grain Control toward Single‐Crystal Metallic Materials by Additive Manufacturing: A Review

“…Studies also indicate that other conditions may be important to inhibit the SGs in melt pool. [84,94] In laser melting CMSX-4 SX alloy, Jiang et al [94] showed that aside from higher scanning speed and lower laser power, a combination of a stable conduction-mode melt pool (rather than deep keyhole shaped melt pool) and minimum fluid velocity may also be necessary in order to avoid SG nucleation resulting from dendrite fragmentation. Wang et al [84] indicated that smoothness of fusion boundary is affected by laser irradiation position, which affects the element segregation at melt pool bottom and can influence SGs formation.…”