Analysis Of Keyhole Shape In Plasma Arc Welding Based On Force-balance Condition

In keyhole plasma arc welding (PAW), the forming and sustaining of the keyhole involves complicated physical mechanism. The shape and dimensions of the keyhole directly determine the heat interaction along the thickness direction of the workpiece, the penetration depth and the weld bead quality. It has theoretical and practical significance to deeply analyze and study the keyhole behaviors and its effect on the forming of the weld bead.First of all, the experiments of Keyhole PAW on the 6 mm-thickness and 8 mm-thickness stainless steel workpieces are conducted. And the efflux plasma voltage, and the shape and sizes of PAW welds are measured under different welding conditions in order to get the experimental data to verify the models of the keyhole and the weld pool.Based on in-depth analysis of the forces acting on the weld pool, the keyhole model is established according to the force-balance condition on the keyhole wall. The shape and dimensions of quasi-steady state keyhole are calculated. And the results of the simulation agree well with the experimental measured keyholing cionditions which demonstrate the rationality and applicability of keyhole model.Primary discussion on the transformation mechanism of the keyhole from "blind (partial)" to "open (complete)" in PAW process is conducted based on the each action force on the keyhole wall. The magnitude of each force is calculated. The results indicate that the forming of the open keyhole is caused by the drastic change of the arc force with increasing the welding current, and the sudden transformation of the depth of the keyhole is owing to the concentration of the arc force in partial areas. To integrate the keyhole model with the model of combined volumetric heat source, the shape and dimensions of the keyhole are used to calibrate some parameters of the combined volumetric heat source. The temperature fields of the 6 mm-thickness and 8 mm-thickness stainless steel plates are calculated respectively. The calculated fusion line at the transverse cross-section of PAW welds are compared with the experimental measurements. It is found that the predicted results are in agreement with the experimental data.