Detection of the contact tube to working distance in wire and arc additive manufacturing

Currently, wire and arc additive manufacturing (WAAM) is mainly done by planning the torch movements layer wise. The height step between the layers is derived from preliminary experiments. Small deviation in the determination of the height step can accumulate over the layers and lead to improper shielding gas conditions or a collision the between torch and the work piece. This makes continuous process monitoring necessary. To overcome these problems, a closed-loop layer height control strategy is beneficial. For the development of a closed-loop height control strategy, it is necessary to have knowledge of the effective height step between the layers during manufacturing. The present study focuses on the development of a sensing method, which allows users to detect the contact tube to working distance (CTWD) in WAAM. The system was developed for short circuit mode of gas-metal arc welding WAAM. The system can also provide information on whether the torch passes over weld beads crossing the weld track or other geometric irregularities existing in the z-direction. Several characteristic values of the process were detected and were matched to the actual CTWD. The accuracy of the sensing method was evaluated, and based on the measured correlation and standard deviation, the electrical resistance during short circuit monitored the CTWD best.