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Optical Coherence Tomography Improves LMD Process Stability

Process stability and control are of great importance in laser metal deposition: Ideally, abnormalities and deviations in the process must be immediately addressed without delay to prevent interrupting the process. This is where systems for machine-integrated monitoring are needed – they can check such processes directly on the spot and initiate corrections. Together with its industrial and research partners, the Fraunhofer Institute for Production Technology IPT has, therefore, integrated the imaging inspection method optical coherence tomography (OCT) into a coaxial process for wire-based laser metal deposition, or LMD-w. The new OCT measurement methodology can not only record the welding process in progress, but also control the quality and process during use, reducing scrap. LMD-w can, thus, be used as a fully-fledged 3D printing process in the future.

LMD-w is an additive manufacturing process in which a metal wire as a filler material is welded onto a workpiece in weld beads with the aid of a laser. Several of these weld beads next to each other produce a layer, and several layers on top of each other produce a component. Since LMD-w only applies material where it is needed, it is a resource-saving process. The complex process development and low process stability have so far prevented broader industrial use beyond special repair processes or the application of wear-resistant coatings.

In the research project ‘TopCladd – Adaptive Laser Cladding for Precise Metal Coating Based on Inline Topography Characterization’ the Aachen research partners have, for the first time, equipped a coaxial LMD-w system with an OCT system to stabilize and actively control the laser process. OCT, which has its origins in ophthalmology, is a measurement process based on short-coherent interferometry used make non-contact, high-resolution images of tomographic cross-sections. In combination with a high measuring frequency, OCT can be used to check and optimize the surface quality of the material deposition directly during the process.

Laser Metal Deposition Plus OCT – The Path To Process Stability

The quality of laser metal deposition depends mainly on the surface of the weld seam: the wavier the surface, the lower the component quality. To make the laser metal deposition process more stable and to produce a high-quality weld seam, the individual process steps must be recorded. Poor quality welds can, thus, be subsequently repaired and the welding process adapted for future production. OCT can check the surface of the weld seam in the phase transition from solid to liquid and, hence, the characteristics of the final weld bead geometry. Based on the data obtained, the laser process can be adjusted in the adjacent or overlying weld bead if necessary.

To exploit the advantages of OCT for the laser welding process, Fraunhofer IPT researchers coaxially integrated the OCT system into the processing head of the laser. The laser for machining and the OCT system thereby uses common optics, but do not interfere due to their different wavelengths. A so-called axicon, which is a cone-shaped lens, and some prism-shaped optics ensure that the processing and measurement light remains coaxial. This optical design allows the measuring laser to circularly scan the applied weld around the centrally running metal wire, making multidirectional measurement possible, independent of the direction that the welding head moves. In this way, the entire workpiece can be measured without the wire blocking the measuring light.

With OCT to Active Process Control

When OCT is integrated into the laser metal deposition process, the surface structure of the entire melt track can be mapped precisely. Using the process data collected in the project, the Aachen researchers are developing a process model for data-based process adaptation and control. As a result, the laser processes will become more robust, opening a wide range of new fields of application. “With OCT, we will be able to apply not only one or two layers on top of each other during laser metal deposition in the future, but any number of layers. In this way, LMD-w has been upgraded to a full-fledged and sustainable additive manufacturing process”, says Robin Day, head of the ‘Energetic Beam Processes’ department at the Fraunhofer IPT.

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