Enhanced As-Built Microstructure and Residual Stress Characteristics of IN718 Processed via Laser-Based Powder Bed Fusion with Laser Beam Shaping

Beam shaping technology holds great potential for improving productivity, minimizing spatter formation, and controlling melt pool morphology in laser-based powder bed fusion of metals (PBF-LB/M) by modifying the laser intensity profile from the conventional Gaussian distribution. This adjustment affects key microstructural features such as grain size, shape, and orientation, which influence mechanical properties like tensile strength and anisotropy. Simulations show that changing the laser intensity profile modifies the thermal gradient in the melt pool, thereby altering grain growth. Since both the thermal gradient and the resulting microstructure are linked to residual stress in the final part, beam shaping could be a useful tool for managing as-built residual stress. However, the connection between beam shaping and residual stress has only been briefly studied, leaving a significant opportunity for further research. In this study, we investigate two rotational symmetric laser beam profiles, Gaussian and Ring, and one axially asymmetric, simulation-optimized profile designed to achieve a uniform melt pool temperature, referred to as the chair-shaped beam. All beam profiles have a diameter of 250 µm, measured using the second-moment method. The selected process parameters result in build rates between 11.2 and 14.4 mm³/s, with porosity levels kept below 1%. Extensive microstructural and residual stress analyses were carried out. Microstructural characterization of the as-built volumetric parts was performed using optical microscopy and electron backscatter diffraction (EBSD). To qualitatively assess residual stress and deformation, double-sided cantilever specimens were fabricated. Quantitative residual stress measurements were obtained using the hole-drilling method on samples processed with selected parameter sets. Furthermore, melt pool temperature behavior was monitored in situ using a custom-developed on-axis Multi-Spectral Imaging system. The results reveal variations in grain orientation, size, and morphology, along with a significant difference, up to 15 %, in the measured von Mises residual stress among samples produced with different laser beam profiles.