Research On Swing Arc Narrow Gap MIG Vertical Welding Of Thick-wall Aluminium Alloy

With the development of modern aerospace industry, the welding structure tends to be large, thick walled aluminum alloy has been an important structural material. Taking the welding procedure and the cost of the project into account, vertical welding process is often used in the actual production process. The weld formation is more difficult because of the flowing-down molten pool in the vertical welding process. Narrow gap gas metal arc welding with the swing arc can ensure enough penetration into NGW groove sidewalls and solve the problem of incomplete sidewall fusion, so it is particularly suitable for thick plates welding. The vertical welding process of aluminum alloy has been studied in this thesis on the basis of designed narrow gap welding torch. The influence of various parameters on weld formation is studied. Meanwhile, the causes and control mechanisms of common defects are revealed. After determining the specification interval of best parameters, welded joints without formation defects of 100 mm thick aluminum alloy has been realized.Based on the principle of the swing of the conductive nozzle, a swing arc system for narrow gap MIG welding is designed. Meanwhile, the relationship of various swing parameters and swing trajectory equation are obtained. The designed protective gas nozzle can be inserted into the groove, meanwhile, smoke staining test is done to optimize protective gas nozzle.The droplet transition and arc behavior in the narrow gap of the swing arc welding are studied. The results show that it is drop transition in the swing process, and they are drop transition and short circuit transition when arc staying at the sidewalls. The process in the wall on both sides of the droplet and the mean transition frequency is lower than the average transition frequency in the swing process, and droplet size is slightly greater than the droplet size when in swaying.Through single factor experiment, the influence of various parameters on the weld formation is studied, and the specification interval of best parameters are determined. Through the orthogonal experiment, the significant effects of various parameters on the weld section size is carried out. The results show that the welding speed and wire feeding speed have maximum effects on weld width and weld thickness, and swing angle has the greatest impact on weld concavity.Causes of three common weld defects of flowing-down metal pool, wave shaped weld and convex weld surface are analyzed, and measures are put forward to control these defects. The first measure to control flowing-down molten pool is to raise the welding speed appropriately, the second is to appropriately increase angle of inclination with horizontal plane of the conductive nozzle. The measure to improve the wave shaped weld is to maximize the swing frequency up to 1.4Hz, meanwhile, welding speed is adjusted from 120mm/min to 260mm/min. The measure to avoid the defects of convex weld surface mainly is to increase the swing angle when the weld forming is ensured.The swing arc narrow gap vertical welding process is applied to the 100 mm thick 2219 aluminum alloy. The good formation without obvious defects in the welded joints is obtained through optimizing the parameters and removing oxide film. Microstructure of the center weld is fine equiaxed grain, and there is almost no difference along the thickness direction; the width of the heat affected zone is narrow, it is about 5mm; the weld microstructure near the fusion line is coarse grain, where strength and hardness of welding joint are the lowest.