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For numerical investigation of the quasi-continuous laser beam welding (Q-CLBW) process, the existing research usually considered a constant welding heat input, which to some degree loses accuracy in representing the welding dynamics. This paper developed a validated CFD model for the simulation of Q-CLBW of Ti6Al4V alloy based on a near-reality heat source model. The coupled transient temperature, velocity, and phase fields were solved under a high-frequency pulsed laser power (HF-PLP) at 5000 Hz as well as a constant laser power (CLP) which has been conventionally employed. Comparative analysis was carried out with respect to keyhole stability, melt flow pattern, and weld pool dimensions resulting from different heat sources and different heat input values with laser peak power of 2800 ~ 3800 W and welding velocity of 0.02 ~ 0.04 m/s. Results indicate that the time-dependent laser input yields less stabilized dynamics featured with middle-depth keyhole collapse, chaotic melt flow, and periodic vapor eruption, and such characteristics are difficult to be observed with CLP. The maximum temperature, pressure, and velocity magnitude of molten metal present remarkable oscillations in relation to welding time. With decreasing heat input in HF-PLP welding cases, the keyhole geometry tends to collapse in the tip segment while the melt flow becomes regular. Furthermore, the employment of the HF-PLP condition also enhances the accuracy in predicting the cross-sectional seam profile in terms of width and depth. Pulsed laser input should be taken into full consideration in the numerical simulation of the Q-CLBW process. [ABSTRACT FROM AUTHOR]

A multi-phase and multi-physics coupling model was proposed and experimentally verified. The simulation of keyhole and weld pool dynamics was conducted during the pulsed laser beam welding (PLBW) of Ti6Al4V titanium alloy. Different pulse frequencies were employed in numerical cases to investigate the influences on thermal transfer and fluid flow behavior and the resultant weld pool dimensions. The calculation results reveal that the welding dynamics go through three stages, that is, keyhole formation/rebuilding, keyhole shrinking, and keyhole collapsing, within a typical laser pulse period. Keyhole collapsing always induces porosities in the lower weld pool, which can be non-contact with a rising-up tendency or captured by the liquid-solid interface unilaterally or multilaterally. Compensation flows are commonly observed near the concaves at keyhole shrinking stage, and then evolve to circulations between porosity and collapsed keyhole. Besides, the keyhole and weld pool show an oscillatory growth on depth values with the oscillation amplitude of keyhole being much higher. A reduced laser pulse frequency can increase the initial weld pool dimensions, obviously, while having little impact on the final welding penetration. The configuration of pulse frequency should be critical to a desirable welding formation in terms of continuity and homogeneity. [ABSTRACT FROM AUTHOR]