Transition mechanism of melt depth in vacuum during laser powder bed fusion using in-situ X-ray and thermal imaging

This research investigates the transition mechanism of melt depth in Ti powder bed during laser powder bed fusion process using in-situ X-ray and thermal imaging. A fiber laser beam of 300 W was irradiated on a powder bed at a scan speed of 15 mm/s for 3.5 s in a vacuum chamber. The X-ray images demonstrated that the keyhole depth ( L d ) increased immediately after laser irradiation, gradually decreased, and became constant. Additionally, the keyhole width ( L w ) increased immediately after laser irradiation, subsequently decreased and increased, and finally became constant. Furthermore, the thermal images that measured the temperature on the powder bed revealed that the high-temperature width ( L h ) gradually increased and became constant. A model of the driving force was developed and examined by analyzing the volume and scattering speed of the molten droplets. The model indicated the recoil pressure generated by the vaporization of powder metal was the driving force for the scattering of the molten droplet. The transition mechanism of keyhole depth was explained as: the increase in L d at the beginning of the irradiation was due to the increase in the recoil pressure ( P T ). This was due to the decrease in L w and a large quantity of vaporization. Subsequently, the decrease in L d was due to the decrease in P T . This was observed due to the increase in L w and decrease in the quantity of vaporization. Finally, the transition to the constant L d was caused by the stabilization of L w and L h followed by the stabilization of P T .