Controlling grain nucleation and morphology by laser beam shaping in metal additive manufacturing
Tien T. Roehling, Rongpei Shi, Saad A. Khairallah, John D. Roehling, Gabe Guss, Joseph T. McKeown, Manyalibo J. Matthews
Abstract
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.