| Cold metal transfer (CMT) welding is a new kind of welding technology. It is suitable for high speed welding of thin sheets or ultra-thin sheets. Based on other researchers’work, in the view of understanding CMT welding physics, expanding welding craft window and improving welding efficiency, the effects of current waveform parameters of CMT welding on welding stability, especially on high speed CMT welding stability, and weld bead formation is studied in this thesis.Backlight shadow method is used to abtain the droplet transfer images, arc images and weld pool images. Also the welding electrical parameters is collected synchronously. A series of experiments about CMT welding current waveform parameters, especially about boost current Ib and boost duration tb, is carried out. Results show that boost duration tb, can significantly influence the droplet weight (droplet size), transfer frequency, arc power distribution and weld bead formation, however boost current Ib has little effects. Statistical method is used to analyze the effects of above-mentioned two parameters on the probability distribution of each welding stage. It is found that when tb, is set as 1.6-3.6ms, stable welding process can be obtained. Welding process is unstable when tb, is beyond this range. Based on droplet transfer images and corresponding welding current/voltage, abnormal CMT welding processes and their causes are explored. When tb, is set as a low value, the temperature of molter metal is low. Its viscosity is large, thus molten metal adheres to filler wire and leads to difficulity to break the liquid bridge. So short circuit stage will be too long. If tb, is set as a high value, transien short circuit occurs and it makes too short short circuit stage.Using orthogonal test, current waveform parameters of high speed CMT welding in different wire feed rate are optimized. Comprehensive evaluation method is used to analyze the impact of different waveform parameters. Highest welding speed is established under different welding conditions. Results show that peak wire feed speed has the largest influence on weld bead appearance and Ib has the smallest effects. Welding parameters are optimized, the highest welding speed can reach to 2.1m·min-1. Probability distribution of each welding stage and U-I diagram are developed to evaluate high speed welding stability. "Wire adhesion", as a unique phenomenon in high speed CMT welding, is studied combined with corresponding droplet transfer images. It is found that too fast peak wire feed speed, low temperature of weld pool and droplet drag phenomenon are the reason of "Wire adhension".Increase weld pool temperature and boost wire molten speed can weak or eliminate the "wire adhesion" phenomenon in high speed CMT welding, so heat source assisted hybrid CMT welding is developed. Preliminary test of CMT+STT welding and CMT+GMAW-S welding are conducted to explore the probability of hybrid welding. The weld bead formation, droplet transfer process and welding stability of CMT welding are studied under above-mentioned two hybrid methods. With welding process kept stable and sound weld bead formation, higher weld speed is accomplished, the highest welding speed can reach to 2.4m·min-1. Welding speed and efficiency are significantly improved. |
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