Research On Droplet Transfer And Weld Formation Of Swing Arc Narrow Gap Mag Welding For Various Positions In Space

In the manufacture of larger metal thick-wall structure, welding in non-flat position is inevitable due to the difficulty of changing welding position. The larger welding load also demands a higher welding efficient. Narrow gap MAG welding is a high efficient welding technology, currently it is mainly used in the flat welding of thick plate. In this paper the swing arc method was applied in the narrow gap MAG welding to develop swing arc narrow gap MAG welding process suitable for various positions in space. Based on the development of swing arc narrow welding torch, the droplet transfer process and weld formation of swing arc narrow gap welding in various positions were researched, and the characteristics and evolution of molten pool in flat welding, vertical down welding, overhead welding and vertical up welding were analyzed. According to weld bead characteristics in different welding positions, the statistical models of narrow gap all position GMA weld bead geometry were developed, and then the optimal welding parameters were obtained by numerical optimization. Finally the swing narrow gap all-position welding of thick-wall pipeline was achieved. The good formation and defect free all-position welding joint was obtained.According to requirement of narrow gap MAG welding, the contact tip swing mode was employed in this paper in considering merits and drawbacks of other siwng modes. Based on the contact tip swing mode, the welding torch and matching nozzle for swing arc narrow gap MAG welding was designed. The welding tests show that the torch can be used in narrow gap welding with long duration and large current. It lays the foundation for the subsequent research of droplet transfer and weld pool behavior.The metal transfer in swing arc narrow gap MAG welding was researched by using the high-speed camera and the current acquisition system. It was discovered the droplet transfer frequency close to sidewall is higher than that at the middle of the groove. The arc swing in the narrow gap goove causes the regularly change of arc length and welding current due to the change of distance between the wire and the sidewall, as a result the droplet transfer in swing arc narrow gap changes regularly. The influentces of wire feed rate, welding voltage, swing angle, dwell time and welding position on droplet transfer were analyzed.The droplet transfer in narrow gap flat welding, vertical down welding, overhead welding and vertical up welding were studied. Droplet transfer frequency and diameter change as the welding position changes. When welding position changes from flat position to overhead position, droplet transfer frequency decreases and droplet diameter increases. But it doesn’t change as the welding position changes that the droplet transfer frequency close to sidewall is higher than that at middle of narrow gap groove. The welding position has great effect on the droplet transfer close to sidewall, while it has limited effect on the droplet transfer at middle of narrow gap groove.In order to have a deep understanding of heat and mass transfer process in swing arc narrow gap welding molten pool for various welding positions, the transient three-dimensional numerical model combined with fearture of swing arc narrow gap welding was establisned. This model takes account into not only the influences of arc pressure, electromagnetic force and surface tension, but also the effect of droplet, swing trajectory, narrow gap groove and welding position. The comparsion between cross-section of acture weld bead and simulation result indicates that the transient thermol fluid model is reliable and accurate.The temperature field fearture of swing arc narrow gap welding, the molten pool characteristics and evolution in flat weling, vertical donw welding, overhead welding and vertical up welding were explored based on the above numerical model. In the initial stage the molten pool is concave shape in all position, while the final molten pool shape is concave or convex in diffirent positons. The molten pool in the flat and vertical down welding is concave shape, while in the vertical up and overhead welding the molten pool is convex shape. Molten metal spreads a certain height along the melt zone of sidewall, and the spreading height determined the final height of the molten pool close to the sidewalls. When the amount of the filled metal in the rear of molten pool is too little that the height of the middle of molten pool is lower than the spreading height, the molten shape is concave, on the contrary it is convex. The final molten pool shape is determined by the flow direction of liquid metal, the amount of filled metal, the cooling rate of the fusion metal.Base on the study of the above metioned, the weld formation of swing arc all-position welding was optimized. According to the formation and molten pool behavior of all-position welding, the all-position welding is divided into two ranges, namly 0-180° and 180-360°. The cross section of weld bead is concave shape and with shallow depth in the welding position of 0-180°, while it is convex and with deep depth in welding position of 180-360°. The statistical models of narrow gap all position GMA weld bead geometry were developed using response surface methodology (RSM) based on central composite design (CCD). The optimal welding parameters at typical welding position were obtained by numercal optimization method, and the good formation and defect free weld joint in flat welding, vertical down welding, overhead welding and vertical up welding were achived.Finally the swing arc narrow gap all-position welding was applied in welding thick pipeline to test the practicability of the welding process. The swing arc narrow gap all-position welding of thick pipeline were achieved, and the swing arc narrow gap weld joint with good formation and excellent mechanical properties of thick pipeline were obtained.