Influence mechanism and visual monitoring of wire deviation in wire-based electron beam directed energy deposition

Wire-based electron beam directed energy deposition (DED) is acclaimed for its high deposition efficiency, optimal material utilization, and the ambient conditions of vacuum deposition. Nonetheless, the inherent stresses resulting from wire circular stockpiling, coupled with the thermal-induced deformation, readily lead to the deviation of feeding wire from the molten pool, which drastically impacts the forming quality and stability during the deposition process. Therefore, it is imperative to delve into the influence mechanisms of the wire deviation response and develop the corresponding online monitoring method. In this study, the wire deviation simulation model was originally established, and the experiment method was also combined to reveal the effects of wire deviation on the wire melting process, molten pool dynamics, and the as-printed part morphology. Furthermore, a visual sensing system and corresponding image extraction algorithms were also developed, specifically designed to monitor and analyze this behavior. Results indicate with increasing deviation distance, the wire melting pattern shifts from droplet to liquid bridge mode until it fails to melt. When the deviation distance is on a small-scale, it can cause molten pool liquid outflow (liquid transition mode) and a deviation in the deposition path location (droplet transition mode) despite the existence of obvious reflux behavior. In addition, the monitoring system developed in this study can effectively protect the camera lens from being contaminated by the metal vapor and the issue of unclear wire regions caused by the overexposure of the molten pool. The gray-level co-occurrence matrix was adopted to effectively overcome the issue of unclear boundaries at the wire center, and the texture entropy feature's noise ratio only increased from 1.0 to 1.3, demonstrating good noise resistance. Based on the developed algorithm, the wire's deflection distance can be detected with an error below 0.1 mm and a response time under 10 ms. The newly revealed mechanisms and the developed monitoring technologies lay a solid foundation for the subsequent closed-loop control of wire deviation behavior, making a significant enhancement of forming stability and automation level of wire-based DED technology.