Study On The Technologies Of Variable Polarity Plasma Arc (VPPA) And Activating Flux TIG (A-TIG) For Magnesium Alloy Welding

As the lightest structure material, Magnesium alloys have many advantages, such as low density, high specific strength, damping capacity, better mechanical properties, recyclable and so on, which are attracting more attention from the domestic and foreign. With the wide use of magnesium alloy structure in manufacture, it is requested to investigate and apply the high-quality welding technique of magnesium alloy.The variable polarity plasma arc (VPPA) welding is used successfully to weld the AZ31B magnesium alloy. We found that the welding parameter interval of variable polarity plasma arc for magnesium alloy welding was narrow and influenced a lot by variable parameters. The microstructure of the weld is investigated and the results show that favorable joint can be obtained, whose heat-affected zone is not evident, and the grains of fusion zone (FZ) are tiny. Besides, the distribution of element Al and Zn are even in the joint in spite of the vaporizing of Mg. The element segregation and Mg17Al12 phase can not be found in FZ. It proves that variable polarity plasma arc welding is a perfect welding technique of magnesium alloy.The activating flux TIG (A-TIG) welding of AZ31B magnesium alloy was also studied in this thesis. The experimental results show that the weld penetration can be increased up to 300 percent compared with the traditional TIG welding under the same welding parameters. The tensile strength of the weld with flux is about 90 percent of base metal. Furthermore, with technologies of high-speed camera and infrared temperature measurement, the arc profiles and the temperatures were recorded and analyzed. The captured images of the arc clearly demonstrate that the flux greatly influenced the welding arc. The temperatures of the weld pool increased significantly when the flux was used, which due to the increasing of the arc temperature. Based on these results, the mechanism for A-TIG welding magnesium alloy is discussed. The cation of flux affects the arc by capturing the free electrons and this destroys the electrical balance of the arc and makes a great deal of the free electrons lose. Thus, more Mg atoms should be ionized, leading to the increase of the degree of ionization. This results in the increase in ionization degree and arc temperature, ultimately the increase of the weld penetration. The oxide (TiO2) changes the the Marangoni convection direction in the welding pool and which accords with the theory reversal of the Marangoni convection.