Controlling grain nucleation and morphology by laser beam shaping in metal additive manufacturing

Gaussian laser intensity profiles are standard in laser-based metal additive manufacturing, although recent work in single-layer melt tracks showed that beam shaping could offer a feasible route towards microstructural control. Since thermal cycling and grain orientation templating in multilayer builds can alter microstructures, we compare three-dimensional 316 L stainless steel cubes built using Gaussian and elliptical laser intensity profiles. Microstructural characterization confirms that elliptical beams result in a modified and improved microstructure compared to Gaussian beams. This assessment favoring the elliptical beam is based on: (1) the observed refinement of the columnar and equiaxed grains; (2) more importantly, the volume fraction occupied by equiaxed grains increases dramatically such that the average grain area is reduced by nearly 50%; (3) reduced texture in cubes built using an elliptical beam. The random orientation of small equiaxed grains in samples built using an elliptical beam also suggests a higher nucleation frequency. High-fidelity finite element simulations that deliver accurate thermal profiles by incorporating laser ray tracing and fluid dynamics were performed. Using a time-dependent solidification map based on local thermal gradients (G) and growth rates (R), our simulation results confirm the experimentally observed trend that an elliptical beam results in a favorable thermal profile.