Towards prediction of microstructure during laser based additive manufacturing process of Co-Cr-Mo powder beds

Abstract

Processing parameters during laser based additive manufacturing affect the fluid flow, heat transfer and solidification characteristics in the melt pool, leading to microstructural variants of texture, grain size, and morphology. A finite volume based Computational Fluid Dynamics (CFD) model coupled with solidification physics has been developed to predict melting, flow, solidification, and resulting microstructural characteristics during such processes. The variation of texture, grain size and columnar/equiaxed morphology of the grains with laser power and speed has been verified against experiments. The difference in cooling curves and evolution of temperature gradients and cooling rates during solidification leading to a difference in each microstructural variant has been identified. Lower laser power and higher scan rates lead to “unconstrained” solidification with small variation of solidification times across the melt pool depth leading to finer grain structure with lower grain boundary (GB) misorientations. On the other hand, higher power and lower scan rates lead to “constrained” solidification, with huge variations in solidification times, coarser grains and highly misoriented grain boundaries. The desirable microstructure has been found to be predominantly fine grained based on mechanical strength measured using nanoindentation tests. Hunt's criterion-based processing maps for understanding the morphological variants has been prepared for systematic design of microstructure during the process.

Keywords

• Co-Cr-Mo
• Selective laser melting
• Computation fluid dynamics
• Columnar to Equiaxed transition
• Unconstrained and constrained solidification