Melt track formation process during laser powder bed fusion for pure titanium revealed by simultaneous in situ X-ray and thermal imaging

Abstract Optimizing process parameters in laser-based powder bed fusion of metals (PBF-LB/M) is challenging due to the complex interactions among multiple parameters, such as laser power and scanning speed. In particular, support-free induces unstable melting behavior due to the melting region being supported by the powder bed. A continuous melt track formation is crucial for high-quality fabrication, yet the detailed melting behavior remains unclear. This study employed simultaneous in situ X-ray and thermal imaging to investigate the melt track formation process of pure titanium under a constant linear energy density during the PBF-LB/M process. Experimental observations revealed discontinuous melt tracks formed through three mechanisms: (1) powder displacement due to spattering, (2) coalescence of internal pores within the powder bed, and (3) transition of the melting region from a rectangular to a spherical shape due to the phase transition from solid to liquid. Higher laser power and laser scanning speed led to a larger melt pool and increment of the spatter due to the reduction of the thermal diffusion into the powder bed. A gap between the laser irradiation point and the melt track was observed under all laser scanning conditions based on the above three mechanisms. When the gap exceeded approximately 2 mm, the melt pool failed to merge with the melt track, leading to the formation of discontinuities. At higher laser power and scanning speed, discontinuous melt tracks tend to form. These findings enhance the understanding of melt track formation mechanisms and provide process optimization for fabricating support-free structures in PBF-LB/M.