Study On The Humping Formation And Inhibition With The Double Wire GMAW Process

Gas metal arc welding(GMAW) is widely used in modern manufacturing industry for its low equipment cost, simple process and ease of automation, etc. The double wire GMAW process can be used to suppress the humping defects appeared in the high speed GMAW process, so as to realize the cost-efficient arc welding. In recent years, researchers have studied the mechanism of the humping formation in high speed GMAW process and weld pool convection in the double wire GMAW process, but there are many problems unresolved:(1) systematic description of the difference of flow pattern in the high-speed GMAW and normal speed GMAW weld pools;(2) the influence of the surface tension and the backward fluid flow on the formation of the humping;(3) a three-dimensional numerical model of double wire GMAW process should be the established to study flow pattern in the weld pool, and the mechanism of humping inhibition.In this paper, the three-dimensional numerical models of normal speed GMAW, high speed GMAW, double wire GMAW processes are established to study the temperature and flow fields, the models take into account the gravity, electromagnetic force, buoyancy, arc pressure, surface tension, and dynamic Impingement of the droplets, radiation, convection and evaporation are considered in the energy boundary conditions. The high speed photography system is also built to observe pool geometry In the welding experiment.The comparison of the simulation results in normal speed and high speed GMAW processes shows that both inward and outward flow patterns exists in normal speed GMAW weld pool, the flow of the liquid metal forms a counterclockwise circulation in the high speed GMAW weld pool.Simulation and experimental results show that two conditions are responsible for the formation of humping in high speed GMAW process: the first factor is the accumulation of liquid metal in the tail of the weld pool leads to the formation of "Swelling", which marks the beginning of the humping formation; The second factor is the elongation, contraction, premature solidification of the liquid channel, which can slow down the transfer of the liquid metal and energy from the front portion to the tail of the pool, it marks the end of the humping formation. The backward fluid flow can promote generation and growth of the “Swelling”, and prevent the backfill of liquid metal. When surface tension is high, the liquid metal in the “Swelling” can not overflow the sides of the weld pool and flatten out onto the surface of the baseplate. Meanwhile, the imbalance of normal surface tension force can lead to contraction of the liquid channel. Any factors that reduce the momentum of the backward fluid flow and the surface tension of the liquid metal, can suppress the formation of the humping.Both the "push- pull" and outward flow patterns exist in the double wire GMAW weld pool. This "push- pull" way inhibits a large number of liquid metal flow to the rail of the weld pool, and reduces the momentum of the backward fluid flow, so the “Swelling” can not form. The outward flow of liquid metal makes the weld width larger, the normal surface tension force acted on the liquid channel is balanced, the shrinkage and premature solidification of the liquid channel will not appear, In this way, the formation of humping is suppressed.