Forward mass flow and melt pool tailoring in keyhole laser welding driven by high frequency two-point longitudinal beam oscillation

Abstract Melt pool geometry in laser welding is usually tuned through laser power, scanning speed and beam profile. Here we show that beam wobble can instead tailor its width, depth and shape at constant power and speed, and also impose a directed mass flow inside the pool. Three two point patterns along the scan direction, balanced, front heavy and back heavy, are studied on 316L steel at 1 to 4 kHz and 0 to 1000 µm amplitude, using metallography and in situ synchrotron X ray imaging at 50,400 frames per second. Amplitude alone drives three regimes: a deep, narrow melt pool with a stationary keyhole at low amplitude, an unstable regime at 100 to 300 µm where porosity peaks near 200 µm, and a restabilized onion shaped melt pool above about 400 µm where the keyhole forms only at the extremities. Rear preheating enlarges the rear keyhole, and the resulting pressure asymmetry drives a drag dominated flow that sweeps pores forward for reabsorption. Tracking pores as flow tracers gives forward velocities of up to 240 µm/ms, and the fraction carried forward rises from one half without oscillation to 0.88 at 800 µm. The balanced pattern gives the fastest flow, reaches the onion shape at the lowest amplitude and keeps the deepest penetration up to about 450 µm. The pool contents are swept from rear to front several times before solidification. Dwell distribution, amplitude and frequency thus control both melt pool geometry and internal mass transport at fixed power and speed.