Novel control model of Contact-Tip-to-Work Distance (CTWD) for sound monitoring of arc-based DED processes based on spectral analysis

Additive manufacturing (AM) is a rising technology bringing new opportunities for design and cost of manufacturing, compared to standard processes like casting and machining. Among the AM techniques, Direct Energy Deposition (DED) processes are dedicated to manufacture functional metallic parts. For all DED technologies, the Contact-Tip-to-Work Distance (CTWD)-namely the work distance between the deposition head and the workpiece-is a critical process parameter, strongly affecting deposition conditions and bead geometry. To date, the work distance is mainly considered through geometrical control. However, geometrical control methods are hardly implementable in real-time and may have a limited application range due to their sensor setup. Thus, there is a need to provide for simple CTWD control methods to keep a constant work distance throughout manufacturing. Current researches in the laboratory focus on a multi-sensor monitoring method dedicated to DED processes. In this research, original monitoring strategies using spectral analysis are investigated for arc-based DED processes. This paper aims to be a proof of concept illustrated on the Cold Metal Transfer mode (WAAM-CMT). Based on a study of the acoustic signature of the process, a specific frequency correlated to the CTWD is identified in the WAAM-CMT sound spectrum. This frequency is used to develop a novel CTWD control model, enabling to indirectly assess the work distance with a 0.5-mm precision. The proposed CTWD control method is simple, low-cost, and applicable to a large application range. At the end of the paper, perspectives for integrating the proposed model within layer-wise and real-time monitoring loops are discussed.