Effect of multiple hit and break of oscillating pulse laser-TIG hybrid welding on porosity suppression of ultra-high strength steel

Abstract

Oscillating pulse laser can significantly inhibit welding porosity, but the mechanism is not clear at present. In this paper, a new oscillating pulse laser-TIG hybrid welding (OPL-THW) was first employed to weld D406A ultra-high strength steel (UHSS), and a novel hit and break theory for porosity suppression was proposed. First, the welding experiment revealed that the porosity decreased significantly with the increase of oscillating velocity vo, especially when vo reached 200 mm/s, the porosity was only 0.21 %. Further, by means of “sandwich” high-speed imaging, the interactions between oscillating pulse laser and bubbles were directly observed. It was found that the large bubbles would be hit and broken into multiple small bubbles when they meet with the rotating laser, and the small bubbles were more easily captured by the keyhole and released with metal vapor in the next oscillation. And with the increase of vo, the trajectory overlap rate of laser increased consequently, which greatly improved the encounter probability of bubbles and laser. As a result, the above process would multi-repeatedly act on the broken bubbles. In addition, the oscillating pulse laser can also help to homogenize the temperature and accelerate the flow of the molten pool, which also contributed to the reduction of porosities. Finally, perfect welded joints with “0” porosity were successfully achieved for D406A ultra-high strength steel.