Study On Double Pulse MIG Welding Of Aluminum Alloy And Magnesium Alloy

In this paper, a direct current (DC) double pulse metal inert gas (MIG) welding process is developed as a variation form of the DC MIG welding and the traditional DC pulse MIG welding, in which, the low frequency modulated pulse is superimposed into the normal pulse. Due to the addition of the low frequency pulse, the DC double pulse MIG weld has more adjustable parameters and a wider range for regulating. Thus, the heat input of DC double pulse MIG welding is more controllable, which is benefit for the improvement of the internal and extrinsic quality of the weld joint. The effects of parameters on the weld formation of DC double pulse MIG welding of thin sheet 6061-T6 aluminum alloy are investigated by orthogonal experiments, and the comparison with the traditional DC MIG welding is also given. Based on the characteristic revealed by the DC double pulse MIG welding of Al alloy, and considered to the difficulties of MIG welding of Mg alloy, the effects of welding parameters on the DC double pulse MIG welding of Mg alloys are investigated in detail. What's more, in order to obtain the high quality weld joint of middle thickness plates (5～10mm) by single welding and forming both the sides, the effects of heat input and groove type on DC double pulse MIG welding of Mg alloy is also studied.The high speed camera, optical microscope, scanning electronic microscope, tensile test machine and microhardness test machine et al. are used to capture the welding arc behavior during welding procedure, the microstructure morphology and mechanical properties of the weld joint. Results show that:(1) The DC double pulse MIG welding process is come from the traditional DC pulse MIG welding process, in which the low frequency pluses are superimposed into the normal high frequency pulses. High quality welded joint of thin sheet 6061-T6 Al alloy can be obtained by using the DC double pulse MIG welding process, while the traditional MIG welding process cannot. The effect principles of primary parameters and the tendency of levels on DC double pulse MIG welding of 6061-T6 Al alloy thin sheet affect on are obtained by orthogonal experiments. The optimized parameters for DC double pulse MIG welding of 6061-T6 Al alloy thin sheet are also given.(2) The DC double pulse MIG welding has many advances when compared with the traditional MIG welding, such as more stable arc, easier transmission of droplet, lower heat input, finer weld formation, lower porosity and reliable mechanical properties. The welded joint by DC double pulse MIG welding has nice weld formation and fine micro structure in the fusion zone. The porosity on the cross section of welded joint is less than 1%, and their distribution is primary in the reinforcement height and fusion line of the joint. The ultimate of tensile strength is about 80% of that of the base metal, and the elongation is about 63% of that of the base metal.(3) the difficulties of traditional MIG welding of Mg alloy is avoided by using the DC double pulse MIG process due to the fact that the heat input is more accurate and controllable. High quality welded joint of Mg alloy by DC double pulse MIG welding is achieved at the first time in international. The ultimate tensile strength and elongation is 232 MPa,11.7%, respectively, which is up to 95%, and 69% of that of base metal.(4) The critical heat input for single welded joint of Mg alloy with 6mm thickness is 450J/mm under the groove condition of:single V with angle 80°,2mm root face, 1mm root opening. When the heat input is too low, the welded joint will present no-penetration or partially penetration, and when it is too high, the grains in the fusion zone will be coarser. In conclusion, the heat input play a key role for DC double pulse MIG welding of Mg alloy. Besides, the groove type is also important for welding of Mg alloy plate with middle thickness (5～10mm). The suitable groove type for DC double pulse MIG welding of Mg alloy (5～10mm) is single V with angle of 30～45℃, root face 1～2mm and root opening is 0.5～1.5mm, and the values should be accordingly increased with the increase of the thickness of the plates.