Research On Arc Behaviors Of Narrow-Gap MAG

Narrow gap MAG (NG-MAG) welding is a high-efficient welding method aiming at thick plate welding. There are many concrete forms of this method, among which the high speed rotating arc method is widely used because of its stable welding process and the twin-wire method is of its high efficiency. As for the methdod of high-speed rotating arc welding, its high rotation frequency brings with its special arc characteristics, droplet transfer characteristics and wire melting characteristics. On the contraty, the method of twin-wire welding owns poor arc stability because of the interaction between the two wires. Therefore, the arc performance and its influence law on the weld formation is chosen as the research object of this paper. This study enriches theories of the NG-MAG welding arc and lays foundations for the choice of welding parameters.Based on the binary coning motion mode, the welding torch for high-speed rotating NG-GMAW is designed with a flexible conductor connecting with welding source. This equipment can be used in continuous welding because of its stability and reliability, and solve problems of tips wearing and poor sidewall fusion. At the same time, according to the hydrodynamic calculation and gas dyeing test, the insertive nozzle for shielding gas is designed, which is able to offer effective protection for deep welding grooves.During experiments, the transitional characteristics of droplets and the arc shape of high-speed rotating arc in narrow gap are observed with the help of high-speed camera. The welding voltage is also collected. Results show that fluctuation of welding voltage will appear with the variation of arc length because of the sidewall, and this will lead to variations of droplet transitional form and melting rate of welding wires. Comparison of droplet sizes and transitional forms in different rotating frequency proves that rotating arc can promote the droplet transition and reveals that centrifugal force plays an important role in the droplet transition.According to the calculation results about droplet transitional time and speed based on high-speed images in welding grooves at different positions, the reason why there is little welding spatter in rotating arc is that the transition time is short when the arc is close to sidewalls. When the welding arc is at the center part of the groove, the transition time is long while the length of weld pool is large and the droplet can fall into it. In the course of metal transfer, the acceleration effect of plasma flow force is small.Radius of the rotating arc in narrow gap increases with the rotating frequency increasing. Through analyzing forces of welding arc and metal liquid column at the terminal of welding wires and comparing arc shapes with and without liquid column, we know that the variation of arc rotating radius is the result of electromagnetic force between the metal liquid column and the welding arc. The welding arc will deflect with the help of electromagnetic force, the bigger the swinging angle is, the bigger the arc rotating radius is. Therefore, the swinging angle will increase with the rotating frequency increasing. In contrast, the effect of welding current and CO2 content on the swinging angle is small.Researches show that the high-speed rotating arc welding is only suited for DCEP welding. The influence law of welding parameters of rotating arc on weld formation is also investigated in this paper. Increasing rotating frequency and decreasing CO2 content in shielding gas can increase the bending of weld surfaces and the penetration of sidewalls. NG-rotating arc can enlarge the range of welding parameters but have little effect on melting efficiency of welding wires, so the improvement of weld formation is not from change of whole welding heat input but from its reasonable distribution. Therefore, penetration increasing of groove sidewall is mainly the interaction of both increasing rotating radius and increasing welding current when rotating arc is close to groove sidewall. With the increasing of heat input on sidewall, the wetting ability between sidewall and weld pool enhances and then lead to increase the surface bending of welds. At the same time, rotating frequency increasing will lead to increase the swing range of the liquid volume at the end of the welding wire, then it is liable to form short circuit with sidewall. Frequent short circuit transfer will deteriate weld formation, thus the premise for the choice of welding parameters is to ensure that there is no short circuit between the liquid volume and the sidewall.Twin-wire NG-MAG welding is capable of improving efficiency and is suitable for large wire feeding speed. However, the arc-breaking phenomenon is serious because of the electromagnetic force between two welding arc. Researching experiments shows the influence law of welding parameters on arc breaking, based on this the occurrence process of arc breaking and change law of welding voltage signal are analyzed. According to these analyses, the narrow gap welding model and the physical model of arc force are builded, and then influence factors of welding parameters on the deviation amount of welding arc. Through experimental comparisons of the weld formation and the dip angle between two wires, it is proved that the deviation amount of welding arc is the interaction between the electromagnetic force and the vibration of weld pool. The reason why the welding arc is stable when twin-wire distance is small is that the vibration of weld pool is small in the effect of welding arc force and the flow of weld pool. Meanwhile, the cathode spot cannot move freely. Furthermore, we put forword measures to ensure the stability of the welding arc.