Numerical Analysis Of Transient Weld Pool And Keyhole Geometry In Controlled Pulse Keyhole PAW

The controlled pulse keyhole plasma arc welding (PAW) is a novel welding process, which uses a special pulse current waveform including two slow-dropping stages with different slopes of K1and K2at the pulse trailing-edge of welding current from the peak level to the base level. The thermophysical behaviors of both weld pool and keyhole can be actively controlled by adjusting the values of the K1and K2, and the keyhole undergoes periodic variation of the "establishing-expanding contracting-closing" process. The "controlled pulse keyhole" strategy can ensure the open keyhole establishment and full penetration under the lowest heat input, so that it has a wide application prospects. However, this novel process involves more process parameters, and its physical mechanisms need to be studied. Therefore, a deep study of the thermal processes and transient variations of weld pool and keyhole geometry in controlled pulse keyhole PAW is very important for the optimization of welding procedure and understanding of the thermophysical mechanisms.The controlled pulse keyhole PAW experiments are conducted on stainless steel plates. The efflux plasma voltage depicting the open keyhole size is measured under different welding conditions. And the backside images of the keyhole are captured by a vision system. The metallograph of weld cross sections is prepared after welding, and the fusion zone dimensions are obtained. The experimental results provide guidelines for developing weld pool and keyhole models in controlled pulse PAW, and can be used to verify the reliability of simulation results.By the method of "macroscopic thermal effect", a combined volumetric heat source model "double-ellipsoid+cylinder" is established to simulate the transient variation of the temperature field and weld pool in a pulse cycle during the controlled pulse keyhole PAW. Under the condition of controlled pulse keyhole PAW, the weld pool undergoes the periodic variation of "partial penetration-full penetration" with the specially designed welding current waveform. Compared with the square wave pulse PAW process, the evolution process of weld pool with pulse current is smoother in controlled pulse PAW, which demonstrates the advantages of controlled pulse keyhole PAW process.Based on the characteristics of force-action and surface-deformation of the weld pool in controlled pulse keyhole PAW, the keyhole model is established with considering the dynamic balance conditions related to keyhole opening and closing. The transient variation process of the keyhole undergoing "establishing-expanding-contracting-closing" is numerically simulated in a pulse cycle under different process conditions. According to the periodic variation of weld pool in a pulse cycle, the keyhole is numerically simulated for two cases (partial or full penetration). Based on the detected efflux plasma voltage and keyhole images at backside, the keyhole dimension and sustaining period are calculated, which are consistent with the measured ones. The calculated weld cross-section is in agreement with the measured ones.The calculated weld pool surface deformation and the relevant keyhole shape purely based on the "force-action" conditions illustrate a larger humping at the rear part of weld pool. In fact, the coupled "thermal-mechanical" actions exerted on the keyhole wall from the arc plasma determine the keyhole shape and size. The calculated keyhole shape based on the force balance condition is modified by considering the thermal balance condition. The question of bigger humping at the rear of weld pool is solved to some extent. The effect of thermal balance on keyhole formation is preliminarily researched.