Elucidating the spatially asymmetric fluctuation behavior of the keyhole during laser melting

The process stability and formation quality are directly affected by the keyhole fluctuation behavior during laser welding and laser additive manufacturing. In this work, a fully coupled diffuse-interface lattice-Boltzmann model (DI-LBM) is developed to characterize the keyhole fluctuation behavior. The model achieves the implicit capturing of gas-liquid diffuse-interface, and develops a progressive ray tracing algorithm that considers the diffuse-interface. The results reveal that the rear keyhole wall exhibits a fluctuation amplitude exceeding 3 times that of the front keyhole wall, while its fluctuation frequency reaches 20 kHz. The rear keyhole wall absorbs energy primarily through laser beam reflection, with direct irradiation playing a minor role. This results in an average wall temperature below 3000 K, significantly lower than the evaporation point. Consequently, the insufficient recoil pressure cannot withstand the impact of molten pool, causing severe wall fluctuations with peak forward velocities reaching 7.5 m/s. The modeling results suggest that the core mechanism for improving keyhole stability lies in enhancing the supporting effect of recoil pressure by optimizing the uniformity and adequacy of laser beam energy on the rear keyhole wall. This work has both theoretical value and engineering application prospects.