Local flashing events at the keyhole front in laser welding

• By high speed imaging waves were observed at the keyhole front in laser welding.
• The waves appear continuous, but they are composed of many flashes.
• The flashes indicate that laser-induced boiling is pulsating, not continuous.
• This can be a key to understand the fundamental keyhole pressure mechanisms.

For sufficiently high power density, high power lasers induce a vertical boiling front that enables keyhole welding. Waves streaming down the keyhole front were recently observed by ultra-high speed imaging. Although the wave flow appears continuous, deeper analysis has revealed that it is composed of flashing events. The evaluation of eleven events for five parameter cases confirms a strong modulation of the recorded grayscale of bright peaks, achieving up to 50% higher brightness. The flash can even end 50% darker than initially, probably in the shadow of the laser beam. The flashing events took place every 5-15 µs, lasted for typically 10–70 µs and moved at a speed of 10–15 m/s. The flashing events are of fundamental significance to understand the keyhole front. They are associated with temperature peaks and with temporary local boiling action, accompanied by ablation pressure that accelerates the melt.