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

Development and evaluation of a closed-loop z-axis control strategy for wire-and-arc-additive manufacturing using the process signal

Wire-and-arc-additive manufacturing (WAAM) is an additive manufacturing technology with a high deposition rate. WAAM usually employs a layer wise build-up strategy. This makes it necessary to know the height of each deposited layer to determine the height the z-axis has to travel after each layer. Current bead geometry models (BGM) lead to variations, which can gradually accumulate over the layers. The present study focuses on the development of a closed-loop control system capable of keeping the contact tube working distance (CTWD) constant during short-circuit gas metal arc welding (GMAW) based WAAM. The algorithm calculates the CTWD based on the resistance during the short circuit. The closed-loop strategy is compared to an open-loop control strategy, which moves along a predefined height step after each layer. Using the proposed control strategy, WAAM becomes a fully automated process without the need for preliminary experiments to determine the height step. Only a short calibration slope is necessary for a complete closed-loop additive build-up. To study the influence of the control strategy on the workpiece the energy input, mechanical strength, microhardness, porosity, and microstructure were analyzed. It is shown that the CTWD of the open-loop deposited component increases slowly. Due to the novel control approach, this is prevented by the closed-loop control, while the mechanical strength and microhardness remain