| The high-speed gas metal arc welding (GMAW) has great potential in application because of its low cost, easiness to use and high productivity. However, high welding speed usually brings some unusual problems, such as undercutting, humping, which seriously restrict its application in practical manufacture. In order to prevent the humping defects, many efforts have been done to investigate the mechanism of humping formation in recent years, and great progresses have been made. But many problems still exist, for example: (1) Humping formation mechanism is still analyzed qualitatively, lack of quantitative explanation. (2) Many mathematical models can not reflect the essential process of high-speed welding quantificationally because they are based on the oversimplification of the weld pool shape. (3) The study of three-dimensional geometry of weld pool and dynamic temperature fileds for humping bead is rare. Thus, based on characters of high-speed GMAW process, mathematical models are needed to be further developed to describe the humping formation mechanism quantificationally. This is of great theoretical and practical significances for optimizing welding parameters and popularizing high-speed GMAW.The mathematical models are developed to investigate the dynamic physical behaviors during high-speed GMAW. Considering the serious surface deformation and long shape of weld pool, body-fitted coordinate is used to deal with complex physical boundaries, and the governing equations are discretized by this method. The additional source term method is employed to solve energy boundary conditions. To reduce error evoked by data transfer and improve calculation efficiency, parabola interpolation method is adopted to inherit former data in non-uniform dynamic grids. Modular structure is adopted to improve the readability and maintainability of programs. A new surface deformation equation is deduced by considering influence of backward flowing liquid which is one of the major characters during high-speed GMAW. After simply calculating the maximum liquid velocity, an appropriate distribution of liquid speed is proposed based on experimental observation to simplify simulation process. The results show that this backward liquid obviously changes the force acting on weld pool, which causes metal accumulated at rear of weld pool and the thinner liquid layer occurred in the middle of weld pool. Moreover, this metal accumulation and thinner liquid layer are the key factors contributing to humping bead.Based on another major character of high-speed GMAW, different modes for droplet heat content distribution are compared and studied. Finally, an appropriate mode is proposed as follows, droplet heat content is distributed in bottom layer of gouging region, and is averagely distributed in the whole layer at rear of weld pool. The results show that this mode can reflect the forming and evolving of weld pool in high-speed GMAW.By using the developed models, the whole process of humping bead formation(including forming serious deformed weld pool, weld pool and humping growth, thinner layer early solidification, accumulated metal solidification) is numerical simulated. It is found that lack of droplet heat and arc heat in the middle of pool is the major reason for early solidification for thinner layer. And early solidification also means that a humping will be formed and a new humping will be gestated. It is a periodic process. The temperatures fileds on top surface and in longitudinal section of workpiece, transient growth of 3-D shape of weld bead are obtained under different welding parameters to quantificationally analyses humping bead formation process and the influence of welding parameters.Experiments are carried out, and the predicted results are in good agreement with measured data. It means that those models developed above can reflect major characters of high-speed GMAW. Based on numerical analysis, the methods of restraining humping bead are proposed, and the experimental results show they work well.
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