Microstructural control in metal laser powder bed fusion additive manufacturing using laser beam shaping strategy

Additive manufacturing (AM) promises to revolutionize manufacturing by producing complex parts with tailored mechanical properties through local microstructure control. The main challenge is to control or prevent columnar (elongated) growth morphology which is prevalent in AM parts. Here, we elucidate mechanisms of microstructure control that promote favorable equiaxed grains (aspect ratio close to 1) using a laser beam shaping strategy. This requires an accurate thermal profile that is only captured using advanced predictive simulation that couples full laser ray tracing, ultra-fast hydrodynamic melt flow and the cellular automata method for grain growth. We investigate columnar to equiaxed microstructure transition during single-track laser powder bed fusion processing of 316 L stainless steel using Gaussian (circular) and elliptical (transverse and longitudinal) laser beam shapes. We demonstrate that the propensity to produce equiaxed grains through nucleation events correlates with large beam width as delivered by an elliptical transverse laser beam. In addition, we reveal different microstructure evolution mechanisms during transient states such as at start and end of a scan track when the laser is respectively turned on and off. Columnar growth is hard to prevent at the start of a track and the growth morphology in the absence of heat input is dictated by the melt pool width and depth achieved and the degree of thermal undercooling. We expect this fundamental understanding of the physics of local beam shaping for microstructural control would have implications on future complex beam shape designs as well as beam modulation.