Monitoring and control of penetration in GTAW and pipe welding

Gas tungsten arc welding (GTAW) is the most widely used arc welding process for joining of metals in critical applications such as welding of pipes that are subject to high temperature and pressure. For such applications, assurance of weld joint penetration is the most critical requirement. Unfortunately, industry lacks a simple yet effective approach to monitor and control weld joint penetration. Existing automated orbital welding systems use welding parameters preprogrammed per welding position and heavily rely on accurate joint preparation and specific groove shape, both of which are costly and time consuming, to produce the required weld joint penetration. Highly skilled pipe welders often have to be employed to manually weld pipes to ensure the weld joint penetration. In this paper, a novel method is proposed to determine the weld joint penetration from arc voltage measurements. This method is based on an observation of the authors on the dynamic development of the weld pool surface during the penetrating process in GTAW; the weld pool surface tends to first expand toward the electrode and then be pushed away from the electrode after complete penetration is established. The development of the arc voltage that reflects the change in the arc length and weld pool surface can thus be analyzed to determine the weld joint penetration. A control algorithm has been accordingly designed and implemented to monitor and control weld joint penetration in pipe welding using a square butt joint at the 5G fixed position. Smooth and consistent weld beads were made with guaranteed/feedback-controlled complete joint penetration for both automated orbital welding and manual welding by an electrical engineer as a novice welder.