Study On Control Measures Of Weld Pores In TIG Welding Of Die Casting Magnesium Alloy

Due to the high gas content in die casting magnesium alloy, pore issue in fusion welding of die casting magnesium alloy is serious. Therefore, die casting magnesium alloy is usually considered unfavorable to fusion welding in the field of engineering, which limits the development of the technology of welded connection and surface defects repair of magnesium alloy die castings.About the problems above, the plates of die casting magnesium alloy AM60 B were welded by TIG welding by the methods of changing frequency of current(50Hz, 100 Hz, 150 Hz, 200 Hz, 250Hz) and heat treatment before welding(temperature are 150 oC, 200 oC, 250 oC, 300oC)to study the control measures of weld pores. The morphology, distribution features and sizes of pores in the weld were investigated by using scanning electron microscopy(SEM), image processing software MATLAB 8.0, X-ray diffraction(XRD), and particle size analysis software Nano measurer 1.2. The main findings are as follows.① The frequency of weld current has obvious effect on the weld porosity. The mount and size of the holes on the weld surface increase with the increase of AC frequency, and the surface forming becomes worse. On the longitudinal-section, many pores with large sizes are distributed in the central of longitudinal-section, and pores with small sizes can be seen in the roots of the pores with large sizes. On the cross-section, pores are mainly distributed near the fusion line and weld molten zone, and a small amount of pores are distributed at the nearby weld surface. And with the increase of AC frequency, the weld porosity first decreases and then increases.② The influence mechanisms of weld current on weld porosity maybe as follows. Within a certain range, with the increase of ac frequency, on one hand, the pores collide and merge with the nearby ones in the rising process because of the increase of vibration, leading to the increase of the diameter of pores. Therefore, the escape velocity of pores increases, leading to a lower porosity. On the other hand, the AC frequency promotes the regular flow of liquid metal and helps the temperature’s uniformity of the molten pool, effectively reducing the temperature gradient of each part of molten pool and prolonging the time of solidification of molten pool, which is advantageous to the escape of pores, further reducing the porosity. When the AC frequency is higher than a certain value, its influence on the vibration of the molten pool becomes too sharply and it leads the instability of welding arc, so the gas is involved in the molten pool, leading to the increase of porosity.③ Heat treatment before welding has obvious effect on the weld porosity. Compared with the weld without heat treatment before welding, the weld with heat treatment before welding has a better surface forming, the bump becomes lower. However, the bump has an increasing tendency with the increase of heat treatment temperature before welding. On the longitudinal-section, many pores with large sizes are distributed in the central of longitudinal-section, and pores with small sizes can be seen in the roots of the pores with large sizes. On the cross-section, pores are mainly distributed near the fusion line and weld molten zone. Compared with the weld without heat treatment before welding, both the size and mount of pores in the weld with heat treatment before welding decrease. And with the increase of heat treatment temperature before welding, the size and mount of pores have an increasing tendency.④ The influence mechanisms of heat treatment before welding on weld porosity maybe as follows. With the increase of heat treatment temperature, on one hand, it helps gas escape from the surface of base metal, so the weld porosity decreases; on the other hand, the internal gas diffuse uniformity and the gas pressure decreases. Therefore, the diameter of pores decreases when base metal is welded, which is unhelpful for the pores to escape, the porosity increases. When the heat treatment temperature is lower,the former is dominant,on the contrary, the latter is dominant.⑤ Both the frequency of weld current and heat treatment before welding have no obviously influence on weld microstructure in the TIG welding of die casting magnesium alloy AM60 B.