Grain growth during selective laser melting of a Co–Cr–Mo alloy

Modes of solidification during selective laser melting (SLM) of metallic alloys, including Co–Cr–Mo alloy, are still not fully understood. This understanding is important in SLM to achieve acceptable properties and part reliability. Using a typical SLM condition and Co–Cr–Mo alloys, microstructures of tracks were characterized in this study. As is commonly observed, solidification starts from epitaxial growth in the boundary of melt track. Cells were found to grow immediately from the melt boundary, without forming a planar zone. This is explained by the growth velocity being sufficiently high that planar growth condition is not favorable. Epitaxial growth has been found to have two possible crystallographic orientations of <100>: either a continuation of the same <100> orientation as in previous track or a change of 90° to another <100> orientation. The selection is in response to scan direction-dependent heat flux direction. The crystal growth direction in relation to heat flux direction also explains that a grain (a group of cells) after epitaxial growth could either stop or continue to the track surface. No equiaxed grains were observed, and this can be explained by the continuation of cellular growth in the whole track.