| 其他摘要 | Tungsten inert gas welding (TIG or GTAW) has been widely used in industries for its high quality welds and lower equipment investment. However, the relative shallow penetration of the TIG welding restricts its ability to weld thicker structures in a single pass, and thus reduces its productivity. In order to increase the TIG welding productivity, the demand for control of the weld pool shape with a deep penetration is steadily increasing, and has been a concern for a long time. A novel modification of the TIG process, namely, active flux TIG (A-TIG) and mixed shielding TIG welding process were developed with deep penetration. However, there is still no common agreement on the understanding of the increased penetration. Research on the mechanism and charanteristics of high efficiency TIG welding process can enrich the theories of welding physics, and is benefit for the development of the new welding process and the application of the process in industry. In this paper, an one-phase continuum mixture model, which is suitable for calculating fluid flow, temperature field and liquid field on the problems of solid to liquid phase transformation for multi-phase systems by solving a set of conservation equations and supplementary equations for TIG welding arc and welding pool, is established by combining magnetofliud flow dynamical equations and porous medium flow equations. By writing user defined functions and using software FLUENT 6.2, the fluid flow and heat transfer in TIG welding pool are investigated and the effect of the active element, oxygen content, on the flow patterns in the welding pool is analyzed. Finally, a mathematical model coupling welding arc and moving TIG welding pool is used to investigate the influence of the active element oxygen content and welding parameters on the fluid flow and heat transfer under argon and helium shielding gas, respectively.
First, the effect of the active element oxygen on the convection in the welding pool and the weld pool shape is investigated by a stationary TIG weld pool model. The results indicates that when the oxygen content is increased, the convection pattern in the welding pool changed from a dominant outward convection, to outward on the pool center together with inward on the pool periphery, and finally to a dominant inward convection. Accordingly, the weld pool evolved from a shallow wide shape, a spoon-like shape to a deep narrow one. The simulated weld pool shape agrees well with the experimental data qualitative. The calculated and the experimental results showed that the change of the Marangoni convection induced by different active element oxygen contents plays an important role on the fluid flow and heat transfer during TIG welding process, and is considered to be one of the principal factors for the increased penetration.
In order to consider the effect of the welding arc on the weld pool shape, numerical simulation to the welding arc is proceed based on the stationary TIG welding pool model. The welding arc model is established and the experimental data in literature verified the validity of the model. Three kinds of arcs under argon, helium and nitrogen are simulated respectively, and the major properties of the three arc plasmas are obtained. The results show that the arc charateristic properties for both helium arc and nitrogen arc are higher than those of the argon arc. The lower electrical conductivity for helium and the higher specific heat for nitrogen make the helium arc and nitrogen arc more constricted than the argon arc. The welding current and electrode gap have important effects on the heat flux, current density and plasma drag force on the anode surface.
The heat flux, current density and gas shear stress on the anode surface from the welding arc model are set as the boundary conditions on the pool surface to establish a coupling model including the welding arc and the moving TIG welding pool, which makes the simulated results approach the real welding process. By using the model, the effects of the welding arc (gas shear stress), active element (Marangoni convection), electromagnetic force and buoyancy on the convection in the welding pool and hence the weld pool shape are investigated with argon and helium shielding gas, respectively. The results showed that the weld D/W ratio sharply increases and then maintains a constant value with the increasing active element oxygen content. The active element directly affects the vortex direction, strength and numver in the liquid pool. By comparing the weld pool shapes between the argon shielding and the helium shielding with the same welding conditions and active element oxygen content, the weld depth and D/W ratio in helium arc are larger than the one in argon arc because of the stronger inward electromagnetic convection under helium arc. Different welding parameters including welding speed, welding current and electrode gap will change the temperature distribution on the pool surface, and therefore, affect the strength of Marangoni convection and the weld pool shape. The reasonable agreement for the quantitative size of the weld pool is obtained between the experiment and the calculation.
The simulation results of the velocity and temperature fields with the different driving forces including buoyancy, electromagnetic force, surface tension and plasma drag force showed that the surface tension on the pool surface is one of the main forces affecting the weld pool shape independent of the shielding gas. Under argon arc, another dominant force is the plasma drag force. However, the electromagnetic force is stronger than the plasma drag froce under helium arc because the helium arc is more constricted than the argon arc and transfer more heat flux and current density to the weld. |
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