Simultaneous Sensing And Control Of Both Weld Pool And Keyhole In Pulse Plasma Arc Welding

Keyhole plasma arc welding (PAW) has narrow ranges of appropriate welding parameters to form a stable keyhole. To solve this problem, the controlled-pulse keyhole plasma arc welding has been developed, which widens the application filed. In order to make optimal use of controlled-pulse keyhole strategy, it is necessary to capture images of both the keyhole and weld pool simultaneously. However, the existing system can only obtain the keyhole image rather than providing more details about the keyhole and weld pool dynamic behaviors. In this study, vision sensing technology is used to detect both the keyhole and weld pool from backside of work pieces and demonstrate both of their dynamic characteristics under different welding conditions. The research results have important theoretical significance and practical application values for developing novel PAW system, optimizing plasma arc welding parameters, and widening the process parameter window.The vision sensing system is developed through the combination of visual sensor and infrared camera. By using external triggering, two-sensor system can observe both the keyhole and its surrounding thermal field at the same time. Through image processing and coordination system transformation, the thermal field with keyhole can be combined in the same image. The thermal field is calibrated by thermal couples. The relationship between temperature and gray level is constructed and the boundaries of weld pool and keyhole are extracted.Moreover, spectral analysis of plasma arc and investigation about camera principle are used to select a proper infrared filter and to develop a new vision sensing system. The new vision system only uses one single CCD camera and can detect both keyhole and weld pool images during a whole controlled pulse keyholing period without illumination. During the peak current period, the dimension and position of keyhole and weld pool can be obtained. While the keyhole is closed at base current period, the dynamic variation of weld pool can also be captured.In order to explain the relationship between plasma arc shape and welding process, different welding parameters have been selected in constant-parameter keyhole PAW tests. According to the keyhole and weld pool behaviors, the plasma arc welding process is divided into following 6 stages:non-penetrated keyhole (blind keyhole) stage, unstable open keyhole stage, open keyhole expansion stage, weld pool growth stage, completely quasi-steady state stage, keyhole closeness and weld pool contract stage when plasma arc is extinguished. It is observed that the keyhole locates in the front of weld pool and the liquid metal in front of weld pool is thin. Keyhole position decides the weld pool position. Thermal conditions of the keyhole and weld pool are independent. There is a time lag between the moment when keyhole and weld pool reach the quasi-steady state. The keyhole position and the maximum position of weld pool locate behind the welding torch axis. The initial keyhole position determines the location with the maximum position of weld pool width under the action of constant heat input.For a whole period of controlled-pulse PAW, the CCD camera can capture the weld pool or keyhole images regardless of the keyhole status. Based on infrared radiation theory, an image processing method is designed to detect the weld pool boundary in open keyhole status and blind-keyhole status. In each pulse cycle, the positions of the keyhole and weld pool are dynamic, and both influence each other. In each pulse, the position where keyhole forms is also the maximum width position of weld pool, and the position relationship between keyhole and weld pool determines the weld quality. It provides an online method to judge whether backside weld is continuous or not.Taking the keyhole size and weld pool position as the controlled variables and peak welding current and its duration as the controlling variables, a keyhole PAW system based on fuzzy logic control has been developed. To verify the effectiveness of the control system, a series of welding tests (bead-on-plate welding with keyhole control and keyhole-weld pool control, varied-thickness plate with keyhole-weld pool control and varied-welding speed with keyhole-weld pool control) were conducted. It has proved that the developed system with weld pool position control can improve backside weld quality in welding of stainless steel plates. When plate thickness and welding speed changed, welding current and its duration time can be adjusted rapidly to keep the stability of the welding process. It demonstrates that the control system runs in a stable way, the backside weld bead is continuous, the weld width and penetration depth are constant, and the weld bead quality is satisfied.