| As one of ideal light metals, Magnesium is drawing attention from all works of life. As for parts connecting which is made up of Magnesium, welding is the simplest method during manufacturing. Thus, lots of research has been done on welding of Magnesium, including TIG, MIG, laser welding. But studies on CMT-cold metal transfer-were hardly reported. During CMT welding, filler metal is drew back at the moment when short circuiting occurs to assist droplet transfer. This process has certain advantages, while used in welding Magnesium whose melting and boiling point are very low as well as density. Based on CMT technology, AC CMT or Various Polarity CMT was introduced. AC CMT has good strengths of overlap welding through controlling distribution of energy. Therefor study on CMT welding of Magnesium is necessary for theory and application.In this thesis, droplet transfer and technological test were investigated. By contrast, characteristics of traditional DC MIG, DC PMIG, DC CMT, and AC CMT were studied. Droplet transfer models of the four processes were observed during bead welding with φ1.2mm AZ31B filler metal. Afterward traditional DC MIG, DC CMT and AC CMT were selected to conduct butt welding of 1.5mm thick plates. After welding, tensile strengths, hardness and microscopic structures of joints were tested.During bead welding trails, it was found that steady droplet transfer models can be achieved by all of DC MIG, DC CMT and AC CMT in condition of low current. Critical current of each method was 74A for DC MIG,107A for DC CMT,104A for AC CMT respectively. While above critical current, droplet transfer turned to be unsteady. Along with current increasing, repelled transfer and globular were found in DC MIG, and spatter became severe. Mixture of project transfer and short circuiting transfer was found during DC CMT welding, but spatter was not serious; Mixture of project transfer and short circuiting transfer was also found during EP phase of AC CMT, while EN phase was still short circuiting transfer,but spatter was not serious as well. As to DC PMIG, no steady transfer model was found for 30A to 122A. With pulse frequency of 80Hz, transfer frequency is 6Hz to 12Hz.1.5mm thick AZ31B plates were butt welded by DC MIG, DC CMT and AC CMT and joint performances were studied. With heat input was controlled, good tensile strengths were achieved. Greatest tensile strength by DC CMT was 230MPa, while average of that was 227MPa,89.0% of base metal.Greatest tensile strength by AC CMT was 220MPa, while average of that was 212MPa,83.1% of base metal. Greatest tensile strength by DC MIG was 228MPa, while average of that was 206MPa,80.8% of base metal. However as for elongation, it was only 3.9% by DC MIG, while it was more than 6.4% by both DC CMT and AC CMT. Lateral hardness of joints appeared to be "W" shape, and sections near melting line were very weak. Longitudinal hardness appeared to be "V" shape, and center of joint was hardest.Effects of heat input on microscopic structures were studied. Joint can be divided into three sections:parent metal, heat affected zone (coarse grained region and partial melting region) and weld (columnar zone and equiaxed grained zone). Along with heat increase, microscopic structures in coarse grained would grow and the second phase-Mg17Al12, would be found. And welds character as casting structure. With increasing of heat input, it was found that grains grew to be bigger in coarse grained region, more second phase separated out and secondary and third dendrites appeared to be larger among all of those three processes. However microscopic structures of DC CMT and AC CMT were finer than DCMIG. |
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