Numerical Analysis Of Dynamic Heat Transfer And Fluid Flow In PAW Weld Pool And Keyhole

In keyhole plasma arc welding, the formation and sustainment of the keyhole involve complicated physical mechanisms. The dynamic evolution of keyhole (formation, sustainment, closure) plays an important role in determining weld penetration and weld quality. The interaction between keyhole and weld pool occurs when the keyhole appears inside the weld pool. The change of the keyhole shape and dimensions has direct effect on the fluid flow and heat transfer in the weld pool, and the latter will also influence the keyhole shape and dimensions. Up to present, the study on the coupled behaviors of weld pool and keyhole is still far from satisfactory. Thus, it is of great theoretical and practical significance to conduct numerical analysis of the interaction of weld pool and keyhole as well as the fluid flow and heat transfer phenomenon. It will lay solid foundation for optimizing the welding parameters and improving the stability of PAW process.The PAW experiments are conducted on stainless steel plates. Image vision system is used to capture the backside image of the keyhole under different welding conditions. The weld dimensions are obtained based on the metallograph of weld cross sections. The effects of the welding process parameters on the keyhole formation are discussed.Based on the in-depth analysis of the fluid flow and heat transfer in PAW, a three dimensional transient model of fluid flow and heat transfer in weld pool and keyhole is established. It considers the interaction between keyhole and weld pool. Double elliptical distribution of plasma arc pressure, electromagnetic force, buoyancy force, gravity, and surface tension are included in the model. The volume of fluids method (VOF) is used to track the keyhole boundary. The latent heat and momentum sink due to the solidification and melting are dealt with by enthalpy porosity technique. The FVM (finite volume method) is used to discrete the control equations and PISO (Pressure-Implicit with Splitting of Operators) algorithm is used to solve the discretization equations. A further development of Fluent software is made to perform the numerical simulation.In view of the characteristics of PAW process, a combined volumetric heat source model is established. Some of its distribution parameters are adjusted dynamically with the variation of the depth of keyhole is established. To consider bugle-like weld configuration resulted from the strong digging action of the plasma arc in keyhole PAW, the upper part of heat source is double-ellipsoid source, and the lower part is the conic body source. During the computational process, some of distribution parameters of the volumetric heat source will adjust according to the depth of keyhole, so that it describes the thermal effect variation with thekeyhole depth along the thickness of the test plate.Numerical simulation of PAW process is conducted on stainless steel plates of thickness 6mm and 8mm. The physical phenomenon, such as the weld pool development, the keyhole formation, the evolution of fluid flow and thermal field, the full-penetration of the tese plates, and the emergency of an open keyhole, are quantitatively analyzed. The numerical results reveal the regularity of fluid flow in weld pool and the pressure on keyhole wall before and afer an open keyhole is formed.The calculated keyhole shape at backside and the transverse cross-section of PAW welds are compared with the experimental measurements. Both agree with each other generally.It indicates that the developed model can be useful for different welding conditions.