Research On The System Of Controlled Pulse Key-holing Plasma Arc Welding

Keyhole plasma arc welding (PAW), as one of the high energy density welding processes, can fully penetrate the workpieces of middle thickness with single pass, so that it has great potential of application in industry. However, since key-holing condition during PAW process is sensitive to the variation of welding process parameters, the ranges of appropriate welding parameters are narrow, and the tolerance is less, which limits its wide applications in manufacturing. In this study, the traditional keyhole PAW was novelly improved to make it be adaptive to any variation of welding conditions. The ranges of appropriate welding parameters were expanded with higher process stability and weld quality. It is of significant value for promoting wider applications of keyhole PAW and increasing welding productivity in modern manufacturing industry.Based on the 'controlled pulse key-holing' strategy, a keyhole PAW experiment system was developed with the functions such as signal acquisition, data processing, process control etc. The interface circuits and the system software were designed to integrate the main control computer and the PAW machine. Efflux plasma voltage signal acquisition device was constructed, and large amount of sensing experiments were conducted. The quantitative correlation between the efflux plasma voltage, backside weld width and welding current were established through data analysis. It was found that efflux plasma voltage can reflect and characterize the keyhole status and size reliably. In addition, a new method for sensing keyhole- NTWV was proposed with preliminary experimental verification.The welding current waveform for controlled pulse key-holing strategy was developed and applied in the keyhole PAW system. Two slow-decreasing slopes were added at the dropping point from peak current to base current to further reduce welding heat input and improve the controllability. Such current waveform assured more stable and smoother transition of the keyhole from 'open' to 'close' status. The sudden change of keyhole signals like an inverted 'V' shape under square-waveform current was avoided. The test results showed that when the welding current waveform for controlled pulse key-holing strategy was applied, the corresponding keyhole signal changed with a smoother mode, like an inverted 'U' shape. It proved that the transition of keyhole from 'open' to 'close' status is more stable, which results in higher weld quality.Keyhole PAW process was analyzed according to the different time intervals of keyhole establishing and sustaining, and the keyhole signal describing the averaged keyhole size during a pulse cycle. Two control methods were determined, i.e., taking the keyhole establishing time and the averaged efflux plasma voltage as the controlled variables, respectively, while taking the peak current and its slow-dropping slopes as the controlling variables. Based on the classical control theory, system identification and model construction were conducted, and PI controller parameters were acquired.Through integrating the keyhole sensing device and the PI controller, a system of controlled pulse key-holing plasma arc welding was developed. To verify the effectiveness of the control system, various kinds of welding experiments (bead-on-plate welding with constant setpoint and varied setpoint, varied-thickness plate with constant setpoint) were conducted. It has proved that the developed system works stably and the control algorithms are reliable. When the setpoint of controlled variable varies, the controlling variable can be adjusted rapidly to maintain the dynamic stability of the welding process. When the workpiece thickness changes continuously, the system adjusts the peak current and its decreasing slopes according to the deviation of the feedback signal from the setpoint. Therefore, the controlled variable fluctuates around the setpoint within the permitted range, and keyhole size and backside weld width can be guaranteed to be uniform although the plate thickness changes. Through comparing the welding results under two control methods, it was found that the controller taking the averaged keyhole size as the controlled variable is better.