Research On All-digital Variable Polarity MIG Welding System And Welding Process Optimization Of Magnesium Alloy

Magnesium alloy has a broad application prospect in the fields of electronics, automobile, machinery, chemical industry and others because of its advantages of high specific stiffness and strength, excellent electromagnetic shielding and radiation resistance performance and easy to recycle, however, its characteristics of large thermal expansion coefficient, high thermal conductivity, small temperature difference between melting point and boiling point have limited its further application, therefore, research of advanced welding technology on magnesium alloy will help to promote the wide application of magnesium alloy. Variable polarity pulse MIG welding technology could accurately adjust the total heat input of welding process through the means of welding current waveform control, so it suitable for the welding application of heat sensitivity materials, such as magnesium alloy, so it is very important and meaningful to carry out the research of variable polarity MIG welding technology of magnesium alloy.In view of the welding characteristics of magnesium alloy and problems existed in the conventional magnesium alloy welding, this thesis proposed the method of variable polarity pulsed MIG welding in magnesium alloy welding, and developed a set of welding system according to the welding technology. This welding system is composed of three parts, which are power source, digital wire feeding system and interactive system, the microcontroller STM32F405RGT6 based on ARM Cortex TM-M4 architecture was used as the key control unit to achieve accurate control of welding current waveform with its logic sequential control functions, so as to realize the adjustment of heat input in welding process of magnesium alloy. Main circuit of the power source adopts the secondary inverter topology, the primary invert circuit realize constant current control, and the secondary invert circuit realize the current polarity shift. Proposed the main forms of droplet transition and the reasonable range of welding process parameters of VP MIG welding of magnesium alloy by analyzing the characteristics of VP MIG welding technology and comparing with traditional DC pulsed MIG welding technology.Testing the performance of the VP MIG welding system with the experimental analysis platform which consist of high-speed camera system, electrical signal synchronous detection and analysis system and welding robot, and then have the welding experiment of magnesium alloy AZ31 B with the VP MIG welding system. The testing result shows that the VP MIG welding system has an excellent output characteristic, precise control ability and stable operation, and the welding experiment result indicate that VP MIG welding method is suitable for magnesium alloy, the welding process and droplet transfer is very stable, in the case of matching the welding parameter, projected transfer mode and better welding seam could be achieved without welding defect, such as welding spatter, slag inclusion, hot cracking and so on.