Vision Sensing And Control Of The Backside Keyhole Dynamic Behaviors In Controlled-pulse Keyholing Plasma Arc Welding Process

The controlled-pulse keyhole plasma arc welding (PAW) is a novel improvement on the traditional pulse PAW. It has potential to produce high quality joints under wider range of process parameters. However, the developed system used the efflux plasma voltage to reflect the keyhole state indirectly, such a voltage signal can only describe whether the keyhole is fully-penetrated or not, but can not give more comprehensive information of the keyhole dynamic behaviors. In this study, vision sensing technology is employed to capture the keyhole image from backside of the test pieces, and the keyhole image sequence directly shows the keyhole evolution process at different instants on the welding current pulse waveform. The research results lay foundation for deeply understanding the thermal behaviors of both the keyhole and weld pool during the keyhole PAW process, and hence have important theoretical significance and will promote the practical applications of the controlled-pulse keyhole PAW process.The vision sensing system is constructed based on a common industrial CCD camera. It can measure not only the length and width of the backside keyhole exit, but also the deviation distance from backside keyhole exit center point to the welding torch axis. Different levels of welding current, welding speed and plasma gas flow rate have been selected in series of constant-parameter keyhole PAW processes. From these tests, it is observed that keyhole parameters, including the keyhole size and keyhole deviation distance, vary as the plasma arc penetration ability changes, but the keyhole deviation distance experiences much larger variation than the keyhole size does when a welding parameter has the same varying amplitude. Changing of the heat input alters the thermal state in the keyhole and weld pool and will induce the variation of the melting condition around the keyhole, so that the weld width and the front keyhole wall inclination will be both changed. The inclined degree of the front keyhole wall directly determines how far is the backside keyhole exit deviated away from the torch axis (deviation distance). Thus, the keyhole deviation distance is a very useful parameter to reflect the keyhole front wall melting state and its thermal condition. As the thermal state changes in the fully-penetrated keyhole process, the backside keyhole deviation distance varies with much larger scope and faster speed than the keyhole size parameters do. The deviation distance hence is a better variable to describe the keyhole dynamic characteristics.The quantity of the backside keyhole deviation distance, i.e. the inclined degree of the keyhole channel, is one of the critical factors to affect the weld defects formation. If the keyhole deviation distance is too large, porosity will easily occur with a bad front weld surface. If the deviation value reduces to nearly zero, the high level heat energy in the keyhole will speed up the melting process of the solid metal around the keyhole, and the weld pool will over-grow so that it is easy to collapse or even burn-through. Hence, the backside keyhole deviation distance can be used to predict the formation of four kinds of weld defects:low quality front weld surface, porosity, weld pool collapse and burn-through.In each pulse cycle of the controlled-pulse keyhole PAW process, backside keyhole deviation distance reaches its peak value when the fully-penetrated keyhole firstly forms; then, it decreases fast during the keyhole open period. It indicates that the keyhole front wall is inclined most severely at the instant of the fully-penetrated keyhole just forms or the closed keyhole period ends. Increasing of the keyhole open period decreases the keyhole front wall inclined degree and hence decreases the backside keyhole deviation distance. Adjusting of the current falling rate at the trailing edge of the welding current pulse will control the duration of the keyhole open period, hence control the level of the backside keyhole deviation distance.Taking the backside keyhole deviation distance as the controlled variable and current falling rate at trailing edge of the welding current pulse as the controlling variable, a modified controlled-pulse keyhole PAW system has been developed based on the predictive control algorithm. The welding tests have been carried out on stainless steel plates of thickness up to9.5mm. It demonstrates that the control process is stable, keyhole opens and closes smoothly in every pulse, and high quality welds have been obtained.