• Ehlers, Henrik

Abstract

To increase the quality and thus the usability in safety-relevant areas such as medicine or aerospace technology for additively manufactured parts, methods for process monitoring are increasingly being used. For metallic components produced with Laser Powder Bed Fusion, a variety of methods are used for process monitoring. Eddy Current Testing, which is suitable for detecting defects below the surface, has hardly been used in this area to date. In combination with layer-by-layer testing during the manufacturing process, this is a promising approach for process monitoring. In this work, a system for online Eddy Current Testing for process monitoring of Laser Powder Bed Fusion is developed and its feasibility is demonstrated. In the context of the development, the essential steps to make such a system possible are shown. GMR sensor arrays are used in combination with single-wire excitation to produce eddy current probes with high sensitivity and high spatial resolution, which can also be parallelized to adapt the system to different testing widths. The probes presented here use 128 single GMR elements with a pitch of 125 μm resulting in a total testing width of 16 mm. In comparative studies, it is shown that MR-based eddy current probes provide a significantly improved Signal-to-Noise Ratio when testing 400 μm blind holes. For the large number of GMR elements, a compact and economical system is not available, so that the development of the test hardware and software is shown in detail before demonstrating online Eddy Current Testing. Using offline Eddy Current Testing with the developed system in comparison to a conventional setup with lock-in amplifier, shows that the system is suitable for Eddy Current Testing of Laser Powder Bed Fusion parts. Compared to testing with a single element, the system developed here can reduce the testing time by a factor of 225. Another advantage of the system is the low material cost of €10,000, when the entire width of the build platform is inspected. This makes online Eddy Current Testing a competitive tool for online process monitoring of Laser Powder Bed Fusion. The feasibility of online Eddy Current Testing for process monitoring of Laser Powder Bed Fusion is demonstrated by manufacturing a step-shaped specimen made of Haynes 282. For this purpose, the Eddy Current Testing system is integrated into a machine for Laser-based Powder Bed Fusion of Metal by SLM Solutions, and each layer is tested throughout the entire manufacturing process and evaluated afterwards. It is shown that not only the geometry of the specimen of the topmost layer can be detected, but also of further layers in up to 400 μm depth.

Topics

• impedance spectroscopy
• surface
• selective laser melting
• defect
• wire