Numerical Study Of Three Dimensional Welding By Double Electrode Micro Plasma Arc Welding

Wire and arc based additive manufacturing that is a mainly category of advanced Direct Digital Manufacturing, produces metallic components directly from the digital model of the part such as a CAD file. Currently, the main challenges for wire and arc based additive manufacturing include low surface finish quality,spatially non-uniform properties of material, and the formation of defects (e.g., porosity). Such challenges originate largely from the limited knowledge of intricate physical processes in additive manufacturing especially the weld pool physics such as melting, molten metal flow, heat conduction, vaporization of alloying elements, and solidification. Direct experimental measurement of weld pool phenomena is highly difficult since the process is localized and transient. Furthermore,current infrared and optical cameras are limited to observe the weld pool surface. As a result,fluid flows in the weld pool,weld pool shape and formation of sub-surface defects are difficult to be visualized by experiment.On the other hand, numerical simulation, based on precise solution of mass,momentum and energy transport equations, can provide important quantitative knowledge of complex transport phenomena taking place in additive manufacturing.First of all, through the analysis of the problems encountered in the wire and arc based additive manufacturing, it is found that manufacturing precision is not high and the poor performance in the wire and arc based additive manufacturing is partly due to the large heat input to the workpiece.Therefore,a double-electrode micro-plasma arc welding (DE-MPAW) has been proposed. The DE-MPAW three-dimensional welding platform is built.The whole platform are composed of three-dimensional motion system,DE-MPAW welding system, signal acquisition system and welding control system. Using a CCD camera to shoot the droplet transition, the thermocouple to measure the heat input to the base metal and the xPC to collect the current voltage during the welding process. A series of basic research on DE-MPAW was carried out. The effect of bypass current on the droplet transfer, the heat input of the base metal and the shape of the weld was analyzed and the optimized process parameters were used for single channel multilayer stacking test.Secondly, the FLOW-3D software, using the finite difference method,was applied under secondary development. Considering the condition of free surface, a three-dimensional transient welding pool model of fixed-point and moving DE-MPAW was established. The flow field and temperature field of the welding pool were simulated and analyzed to deepen the understanding of the fluid flow and heat transfer process in the welding pool. The theoretical basis for obtaining the high quality weld appearance with SUS304 steel is provided. In this model, the droplet was treated as a spherical droplet at a fixed height above the pool, constant temperature and initial velocity. The energy distribution between the main are, the bypass arc and the droplet in the model was analyzed. Heat source and current density used the classic Gaussian distribution model, taking into account the convective heat transfer, radiation and pool heat evaporation in the workpiece, and driving the liquid metal within the pool to produce movement of buoyancy, electromagnetic force, surface tension and plasma flow force and other driving force.Then, the dynamic deposition process of single melting droplet in DE-MPAW three-dimensional welding was analyzed from the simulation point of view. The droplets were driven by gravity, electromagnetic force and arc plasma flow, carrying the mass, momentum and energy of the impact of the substrate periodically, resulting in a liquid pool. The shape of the transient pool and the complex flow in the molten pool mainly affected by the impulse momentum and the surface tension combination are computed.And the effects of total current, bypass current and cooling time on the temperature field,flow field and morphology of the molten pool were analyzed.Finally, the influence of various forces in the molten pool on the flow direction, the maximum flow rate and the molten pool morphology was analyzed from the theoretical point of view. After a comprehensive consideration of many factors, the flow field of DE-MPAW welding pool was studied, and the flow pattern of the liquid metal in the weld pool and the weld structures were analyzed.