Keyhole Behaviors Influence Weld Defects in Plasma Arc Welding Process

The influencing mechanism of keyhole behaviors in relation to weld quality was experimentally analyzed in keyhole plasma arc welding (PAW). To this end, an image-capturing system with two cameras was developed to observe backside keyhole exit behavior and front side weld pool behavior concurrently. From the keyhole exit image, not, only the dimensional parameters were extracted, but also the keyhole exit deviation distance, i.e., the distance of the keyhole exit center point deviated to the torch axis along, the welding direction. Pulsed PAW experiments were conducted both for bead-on-plate ' welding and closed butt joint welding. It was found that if the keyhole exit deviation distance was relatively large, the front weld surface raised substantially, formed high rein-forcement in the front weld surface, and induced porosity in the solidified weld. However, if the keyhole exit deviation distance was relatively small, the weld quality was expected to be sound. This observation result suggests that the keyhole exit deviation distance is a critical keyhole factor to determine the weld quality. The influence mechanism was ' further analyzed. In a keyhole of large deviation distance, the gas flow field was disturbed. More liquid metal was pushed to the front pool surface, and the weld pool was consequently deformed substantially with an inside gas cavity. The demonstrated experimental results provided a comprehensive proof that keyhole PAW weld quality is highly dependent on keyhole behavior, and thus lay a solid foundation to implement the keyhole stability control and process optimization in keyhole PAW.