Hump formation mechanism in gas-jet-assisted keyhole laser welding of HR-2 stainless steel

Gas-jet-assisted keyhole laser welding offers the possibility of a breakthrough in the limitations of penetration depth in laser welding, which currently suffers from equipment restrictions. A gas jet of sufficient intensity to assist the keyhole should be used to obtain suppressed plasma, a deepened keyhole, and increased penetration depth. However, an excessively strong gas jet gives rise to humps. The incident angle of the keyhole-assisted gas jet is 60°, with a nozzle ahead of the laser beam. A series of experiments were carried out with different welding velocities and gas parameters by using HR-2 hydrogen-resistant stainless steel and a slab CO₂ continuous-wave laser welding machine. The weld profiles can be categorized into four types, welds of traditional laser welding, welds of enhanced laser welding, undercut welds, and humping welds with increased gas pressure. A high-speed camera was employed in the experiments to monitor the formation of humps under an excessively strong gas jet. The results of analysis show that hump formation can be divided into six stages. Its main driving force is the intense turbulence of gas jet. There are two main reasons for hump formation: premature solidification of the molten pool caused by the large temperature gradient between the front and rear parts of the molten pool, and the emergence of a thin layer liquid bridge with one-directional flow under the enhanced cooling effect of excessively strong gas.