Abstract: A welding head is designed for high welding quality and long continuous welding time of thin-walled pipe. In the industrial field, it is used to realize the all-position welding, because of the fixed position of the pipes. Firstly, the working principle and structure of closed welding head are introduced. Then, the design of heat sink close to gear ring and annular shunt around the two key technologies—cooling and gas supply are discussed. Numerical simulation method is used to analyze the water cooling effect and the distribution of argon, thereby the optimized designs of cooling and gas supply systems are obtained. In the research of gas supply for the welding head, through numerous changes of the distribution of shunt holes and unevenness evaluation of shunt design, excellent argon flow field is obtained. Ultimately, the prototype of the developed all-position closed pipe welder is tested. The results show that when the inlet argon flow is about 5 L/min，the arc burning is stable and the protection effect is optimal, which is close to the simulated result 5.4 L/min. The welding machine features long continuous welding time, stable and reliable welding quality and low fault rate of welding head.

Key words: Circumferential shunt, Closed welding head, Numerical simulation, Water-cooled, Laser beam-variable polarity plasma arc hybrid welding Heat source model Temperature field numerical simulation