NUMERICAL SIMULATION OF KEYHOLE SHAPE AND TRANSFORMATION FROM PARTIAL TO OPEN STATES IN PLASMA ARC WELDING

It is of great significance to develop a mathematical model of keyhole shape and dimension in order to widen the process parameter window and improve the process stability in keyhole plasma arc welding (PAW). In this study, a keyhole model was developed according to the force-balance conditions on the keyhole wall. The establishing process of quasi-steady state keyhole was numerically simulated for stainless steel plates of 6 mm thickness, and the keyhole shapes and dimensions were obtained under different welding process parameters. The transformation mechanism of the keyhole from blind (partial) to open (complete) states in PAW process was analyzed based on the calculated action forces on the keyhole wall. The values of action forces at different locations on the keyhole wall were calculated. With increasing of welding current, the keyhole depth rised in a nonlinear way. There existed a critical value of welding current, i.e., if welding current was a little bit higher than this value, the keyhole inside the weld pool would suddenly transform from partial state (blind keyhole) into complete state (open keyhole). The fast centralization of the plasma arc force at the keyhole bottom region resulted in the sudden transformation from a partial keyhole to an open keyhole. The keyhole PAW experiments were conducted to validate the numerical analysis results.