Metal bridging characteristics and button-hole suppression mechanisms in pulse waveform-integrated oscillation laser beam welding (OLBW) of Ti6Al4V sheets under an air gap condition: A hydrodynamic perspective

In the aerospace manufacturing industry, conventional laser welding often fails due to assembly errors, resulting in discontinuous long-straight welds in titanium cabin skin structures. This study introduced a novel oscillation laser beam welding (OLBW) approach, combined with a low-frequency, medium-duty cycle pulse waveform. We applied this method to fabricate tailor-welded blanks (TWBs) of Ti6Al4V alloy under reserved air gap conditions. Experiments involved various combinations of beam trajectory and pulse waveform parameters. Keyhole and weld pool dynamics were numerically simulated by developing a validated multi-phase thermo-fluid coupling model. Key findings include: At an oscillation frequency of 100 Hz, the process enhances metal bridge formation between workpieces, with the keyhole maintaining a normal shape and an opening area of 0.196 mm²—smaller than the focused beam spot—enabling continuous weld beads. At 200 Hz, however, the average metal bridge area drops by 23.2 % indicating a weakened gap bridging capacity, and the keyhole opening expands to over 0.8 mm² after several cycles, creating a button-hole geometry. This is primarily due to insufficient hydrostatic pressure response on the keyhole's posterior wall under the rapid anterior wall's movement, along with a surface tension coefficient increasing from 1.1 N/m to 1.2 N/m. Introducing a 50 Hz square pulse waveform addresses this by promoting periodic keyhole collapse and weld pool cooling, keeping the keyhole in a semi- or full-penetration state and therefore suppressing the button-hole effect. Additionally, increasing beam oscillation frequency or incorporating pulse reduced α′-martensite grain size, primarily driven by an average cooling rate over 2.6 × 10⁵ K/s or cyclic remelting in overlap regions.