Dynamic Simulation And Analysis Of Short Circuiting Transfer In GMAW

Short circuiting transfer is that the droplet at the end of wire is contact with the molten pool,and the droplet make the transition to molten pool directly under the effect of electromagnetic pinch force and strong overheating.As one of the important transition forms of CO₂ welding,it has advantages of high efficiency,energy saving and low hydrogen,and is commonly utilized in thin sheet and all-position welding.However,there are two key problems exist in CO₂ welding,such as serious welding spatter and bad formation.In the welding of new materials,a lot of experiments are needed to determine the appropriate welding parameters.The cost of those experiments are high,long cycle,and consume a lot of manpower and financial resources.Aiming at those problem,establishing the accurate short circuiting transfer model and studying the dynamic mechanism of transfer process can effectively improve the stability of the welding process and optimize welding parameters.The predictions of previous models for short circuiting transfer process focus on analysis and control the signals of current and voltage,but ignore the dynamic behavior of the droplet.The analysis of contact droplet sizes is also limited to the equilibrium state under single transition period,and the results are often just mean value.So this is appropriate to analyze jet transition with high frequency.For the short circuiting transfer process,due to the different transition period,the droplet size distribution is in accordance with the statistical law,so only focus on the equilibrium state behavior of the droplet under single transition period is defective.Firstly,the welding test platform is built.Overall experimental system comprises welding system,high speed camera system and electric signal recording system.At the same time,the experimental results of short circuit transition are analyzed with the signal synchronization system.The image processing technology using Canny operator,which has a strong ability to remove the noise and can produce a clear edge,is discussed and used to measure the equivalent radius of the droplet.Based on this algorithm,the equivalent radius of the contact droplet and the residual droplet are all statistically distributed,but the equivalent radius distribution of residual droplet is relatively small.At the optimum welding conditions,the average values of them reaches the minimum.In according to the characteristics of arcing phase and short circuiting phase in welding process,in consideration of the variation of arc length,liquid bridge volume and resistance,a metal transfer dynamic model and a welding circuit model have been developed respectively.Based on the two models and some assumptions,an improved“mass-spring”model for describing the whole process of short circuiting transfer have been put forward.Based on the dynamic model,the short circuiting frequency,welding current and voltage,the equivalent radius of the contact droplet and the residual droplet?including mean value and statistical distribution?,and dynamic variation of droplet equivalent radius are calculated and analyzed under the welding condition of different wire feed rate.The results demonstrate that the predicted results are in broad agreement with the experimental.In addition,the variation of displacement of the droplet mass center,oscillation velocity of droplet,wire extension and liquid bridge shape are discussed.The results show that the displacement,velocity and wire extension are minimum at the optimum arc point.The smaller the radius of curvature of the initial liquid bridge is more conducive to the stability of the short circuiting transfer process.