Image processing and control of weld pool in switch-back welding without backing plate

Abstract

To achieve high quality in multilayer welding, it is important to obtain stable weld results for the first layer.^1–5 For this reason, welding is usually conducted by using a backing of the same kind as the base material or a ceramic backing plate. In using a backing metal, the weight of the base material increases. Also it sometimes induces notches to occur at the weld joint between the base material and the backing. This can cause cracking. On the other hand, in using the backing plate, its removal is required after welding, and this makes the method unsuitable for automating the process. Therefore, in the authors' previous report, a proposal was made of the switch-back welding method in which welding is executed without using a backing plate, and also an investigation was made of the welding conditions for the root gap.⁶ In this welding method, it is necessary to adjust the wire feeding rate, the torch motion and the power property in accordance with the gap so that the back bead and the deposit can be controlled. Even if the gap is measured before welding, it may change owing to welding distortion. Hence, when welding is conducted by fixing the welding conditions, the change in the gap causes the back bead to become unstable. For this reason, in this study, a method is discussed to detect the gap on-line by processing images of the molten pool observed by a CCD camera. Then, using the results, the back bead was feed-forward-controlled in real time. Moreover, to examine the controlling performance, welding was conducted by varying the gap between 4.6 mm and 2.5 mm without tack welding. Very good weld results were obtained.