Research On Molten Pool Behaviour And Control Technology Of Rotating Arc Narrow Gap Horizontal Welding

A novel rotating arc system for narrow gap horizontal welding is developed for solving the poor weld formation caused by the improper control of the molten pool during the horizontal welding. The droplet transfer characteristics and molten pool behavior of rotating arc horizontal welding are explored. The characteristics of temperature distribution of rotating arc welding process are studied. The reasons of defects formation in the horizontal weld are analyzed. The optimum welding parameters of rotating arc narrow gap horizontal welding are determined. Finally, the rotating arc narrow gap horizontal welding process is applied to the heavy plate high strength steel. The good formation and defect free horizontal joint is obtained.The metal transfer and molten pool behaviour in rotating arc narrow gap horizontal welding is successfully observed by the high-speed photography system. The theory foundation of horizontal weld formation is established. It is discovered that the metal transfer is different at the different location of the groove not only in the direction of the weld width, but also in the downwards motion and the upwards motion of the wire in a rotating period. It is because the difference of the arc length induced by the molten pool surface shape in the rotating arc horizontal welding. The rotating arc process can improve the metal transfer. And the results indicated that the centrifugal force induced by the arc rotating has great effect on the globular transfer mode which has lower transfer frequency and larger droplet size. Regarding the spray transfer mode, it has limited effect.The oscillation of the molten pool in the rotating arc narrow gap horizontal welding is observed. The arc force and droplet impact can affect the molten pool periodically. It induces the fusion metal flowing in the direction of weld width. This oscillation occurs during all the molten pool formation process. It can improve the fusion metal at the lower side to flow to the upper side. It is crucial for the horizontal weld formation.The molten pool formation process is explored. The final molten pool shape is determined by the melt area of the base materials, the amount of the filler metal and the cooling velocity of the fusion metal. The molten pool is filled from the lower side to the upper side, and the tips of the molten pool surface on the both sidewall are still during the molten pool forming process. So the satisfied weld formation can be obtained only in the proper welding parameter range.The welding trajectory is changed due to the arc rotating. So the characteristics of temperature distribution of rotating arc welding process are studied firstly for deepening the understanding of the rotating arc welding process. A novel mathematical model of heat source for rotating arc welding process is developed. A transient three-dimensional (3D) model is established and computed by a finite element analysis computer program MARC as well as its subroutine. Good agreement is shown between the computed results and experimental results of thermal circle in rotating arc welding process. The results indicate that the temperature changes in the style of stepladder. Moreover in the high temperature zone, there are multiple peak temperature points. These characteristics of temperature distribution are induced by the arc rotating which leads the repetitive heating or fusing during the welding process. In contrast with non-rotating arc (NRAW) process, the width of temperature distribution enlarges, and the peak temperature of weld pool decreases.Base on the study of the above metioned, the welding process of the rotating arc narrow gap horizontal welding is studied. The causes of the defects of sagging surface, under cut and convex surface are explored. The relationship of the defects and with the welding parameters is established. The technology advantages of the rotating arc process on the narrow gap horizontal welding are represented from the aspects of welding heat input and molten pool behaviour. The optimum welding parameters of rotating arc narrow gap horizontal welding are determined. The characteristics of joint formation in rotating arc horizontal GMAW is studied. The asymmetry of the microstructure in the joint due to the rotation of the arc is discovered. The reason and the mechanism of this phenomenon is analyzed and interpreted.The rotating arc narrow gap horizontal welding process is applied to the 35mm thick 12Ni3CrMoV steel. Interlayer defects can be eliminated by removing slag, adjusting groove size and resetting weld torch position. The good formation, defect free and excellent properties horizontal joint is obtained.