Stability of molten pool and microstructure evolution of Ti-6Al-4 V during laser powder bed fusion with a flat-top beam

The energy distribution pattern of a laser beam has a significant influence on the molten pool stability and the defect formation mechanism in the additive manufacturing (AM) process. An in-depth understanding of this influence is necessary to improve the molten pool stability and reduce defects. In this paper, a high-fidelity model was proposed to reveal the fluid dynamics within the melt pool under different beam patterns during the laser powder bed fusion (PBF-LB) process of Ti-6Al-4 V. The transient temperature and flow field of the molten pool were computed with different process parameters for comparison between Gaussian and flat-top beams. Samples of Ti-6Al-4 V were also fabricated to validate the simulation. The results show that a Gaussian beam with high heat input produced a steep keyhole wall, which makes laser reflection unpredictable. Variation of absorbed energy leads to an unstable recoil pressure acting on the keyhole tip, increasing the probability of the keyhole collapsing and gas pore formation. The temperature field within the flat-top beam is more uniform compared to the Gaussian pattern. The evenly distributed recoil pressure pushes the melt from the center to the sides, forming a keyhole wall with a small inclination which reduces the reflection number and improves the stability of the flow field. The adoption of flat-top beam substantially expands the process window. High density samples could be obtained when the volume energy density (VED) meets the minimum requirement. Hatch spacing (HS) has an influence on the constituent phase of the sample. A large proportion of β phase formed with a small HS.