| Plasma arc welding (PAW) process can fully penetrate steel plates with middle thickness by one single pass without groove. As the one side welding technology, PAW has great potential applications in joining steel plates with middle thickness. One side welding of PAW is based on forming of a keyhole throughout the work piece thickness. Keyhole status plays an important role in stabilizing PAW process and ensuring weld quality. However, keyhole is sensitive to the variation of welding parameters, and its stability is weak. These problems restrict the wide application of PAW in manufacturing. Therefore, it is of important theoretical significance and practical meaning to study the plasma arc heat and pressure distribution on the dynamic keyhole wall, to calculate temperature profile and fluid flow in weld pool, and to simulate keyhole evolution. So that, the mechanism of PAW process is revealed, and the knowledge base of PAW process is enriched.Plasma arc welding is a complex physical process involving heat transfer from plasma arc to work piece, fluid flow in weld pool, keyhole dynamic evolution, and coupling action among them. According to the characteristics of PAW process, a three dimensional instantaneous model of PAW is established with considering the coupling relationship among heat transfer, fluid flow and keyhole dynamics. With re-development, the ANSYS Fluent software is used to establish the model. The physical boundary during welding process is defined by δ function on the basis of tracking the keyhole surface by VOF (Volume of fluid) method. The plasma arc heat and pressure on the keyhole wall are specially dealt with as an energy equation source term and a momentum equation source term, respectively.The plasma arc heat is delivered to the work piece through the keyhole wall. According to the feature of plasma heat during keyhole establishing process, a keyhole dependent surface heat source model is developed. And the distribution of heat flux on the keyhole wall is studied, which is varying with the keyhole shape and size.The arc pressure on the work piece is produced by the fast flowing plasma impacting on the work piece. An approximate analytic formula of plasma arc pressure is derived based on the accelerated plasma impacting theory. The analytic formula quantitatively describes the relationship between plasma arc pressure and welding parameters (welding current, plasma gas flow rate, nozzle size, arc voltage, etc.). And the keyhole dependent distribution model of plasma arc pressure is developed with considering the characteristic of plasma arc pressure acting on the dynamic keyhole wall. By applying this model, the dynamic distribution of plasma arc pressure on the keyhole wall is researched.The plasma arc welding model, plasma heat model and arc pressure distribution model are applied in numerically analyzing plasma arc welding process. The dynamic evolution of temperature profile on work piece, fluid flow in weld pool and keyhole size and shape is simulated during PAW process. The distribution of streamlines in weld pool is obtained and researched. The backward inclined keyhole channel is also numerically simulated, which is only observed in experiment before. And the distribution of heat flux and plasma arc pressure on the keyhole wall is calculated. Finally, the effect of both welding current and plasma gas flow rate on establishment time of open keyhole is quantitatively studied in this work.The plasma arc welding experiments are conducted. The establishment time of open keyhole, the displacement between the keyhole center at back side and torch axis, keyhole length and keyhole width are detected during the PAW process. After welding, the welded samples are sectioned, polished and etched to get the macro graphs, and then the weld zone and fusion line are measured. The calculated results basically agree with the experimental data. |
PDF Full Text Request