Determining keyhole stability for plasma arc welding process control

Stability of the keyhole plays a fundamental role in producing quality welds in keyhole plasma arc welding. Currently, keyhole size is assumed to be a measurement of its stability. To verify this idea, keyhole and weld pool were simultaneously imaged from the opposite side of the welding torch. Experimental results revealed that the width of the keyhole was not correlative with the stability of the keyhole: as long as the keyhole mode was maintained, the width remained constant despite the changes in the welding current and speed. However, it can be used to estimate the lower limit of the pool width for preventing keyhole collapse. Also, the upper limit of the pool width for preventing burn-through is approximately fixed for given applications. Hence, in this study, pool width, its upper limit, and keyhole width were used to determine the margins of the process from collapse and burn-through. To measure the state of the stability of the keyhole process, the stability distance and stability factor were proposed. Based on the imaging system used, the state of the stability can be monitored in real-time.