Influence of the wrinkle surface structures on the vapor flow and keyhole stability in 20 kW high power laser welding

•An original multiphase flow model is proposed in this paper, which considers the vapor plume impact, diffusion, and laser ray tracing processes in the extremely narrow keyhole.•A 20kW high-power “sandwich” observation platform is established to observe wrinkle structures (∼100 μm) on the extremely narrow keyhole.•It is found that the wrinkle structures (humps) that exist on the front keyhole wall could seriously interfere with the energy distribution, and break the process stability.•By increasing the laser power, the local drilling effect on the solid-liquid interface and the resultant liquid accumulation could be alleviated, then the humps transport efficiency is improved. The wrinkle structures on the keyhole wall are smoothed, and the welding stability is improved effectively.