| Magnesium alloys were widely used in aerospace, motor industries,3C Products and other fields, because of the advantages of low density, high strength, fine capacity of anti-impact, excellent conductivity, good electromagnetic shielding capability. Utilizing magnesium alloy can effectively reduce weight to achieve high performance requirements. Although the mechanical performance of wrought magnesium alloy is better than that of casting magnesium alloy, and has less porosity defect than the cast magnesium alloy. But the high cost of wrought magnesium constrains its application in the industrial field. At present, more than 95 percent of magnesium used in industry is casting magnesium. Magnesium alloy is easy to appear defects during the casting and machining process, such as loose, shrinkage, porosity, crack, cold, slag and segregation, leading to nonconforming products. It not only causes the waste of magnesium resources but also affects the production schedule, so the repair welding of defective castings can greatly reduce the production cost of the factory.At present, there are two kinds of methods to solve the problem:one is tungsten inert gas arc welding (TIG), the other is laser welding. However, The TIG contains the disadvantage of shallow penetration, low efficiency, inferior quality and micro defects such as porosity and cracking; Laser welding generally is used for thin walled castings, because of the high cost, melting depth, high welding assembly. Activated tungsten inert gas arc welding (A-TIG), has the advantage of increasing weld penetration and reducing weld defects under the same welding specification compared with TIG welding. Accordingly, the method of A-TIG welding for magnesium alloy is proposed. When the distance from the work piece surface to defects is within 4mm, A-TIG welding can successfully repair the defects without processing a groove. But when the defect from the surface is more than 4mm, it is needed to use multilayer welding with groove.Adopting A-TIG multilayer repair welding can realize high efficiency and low energy consumption of repairing for cast magnesium alloy ZM5 alloy. When repairing 6mm thick plate with a 2mm groove, in order to penetrate joints, TIG welding needs a current 180A, while A-TIG welding with a current 120A, if other parameters remain the same; through observing the microstructure and micro hardness of the welded joints by A-TIG multilayer welding, it is concluded that A-TIG not only to refine the weld zone grain, but also reduce the weld crack defects. From the distribution of electron probe experiment of the welding joint elements, it is found that A-TIG multilayer welding reduce Al element loss and segregation. The distribution of Al elements is more uniform and the composition of the weld is more uniform, thus the mechanical properties of the welded joint is improved.In order to explore the influence of the active agent on welding heat cycle, and the principle of active agent on increasing penetration, thermal cycling curves of TIG and A-TIG welding process were obtained with a thermocouple in the drilled hole on back of the plate. Through the measurement of the internal temperature of molten pool, it was found that the active agent changes the weld pool flow pattern. During the welding process, the arc images are photographed with a high-speed camera, and then transferred to the computer. According to the comparison of the arc shape of the two welding methods, the active agent is found to play a role in contracting the arc, and changing the arc changing mode during the welding process. As a result, the penetration depth increases. |
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