Horizontal growth direction of dendritic solidification during selective electron beam melting of a Co-based alloy

A widely known feature of grain growth during powder-bed fusion selective electron beam melting (SEBM) is columnar growth with grains cross many layers along the building direction. Conditions for disrupting this growth direction dominancy have not been well studied. In this study, two different focus offset values were used while other parameters were kept unchanged to result in the shape of melt pool/track being varied significantly and thus the grain growth direction changed considerably for a significant portion of the track. It has been found that as the beam became more focused, the cross section of the melt track became more U-shaped. The lateral heat transfer dominance in top region then resulted in the growth of dendrites, epitaxial and in columnar form, horizontally. The layer thickness of these horizontal dendrites is up to 80 μm and is significant considering the additive layer thickness being 70 μm.