Researchers integrate OCT system into LMD process

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Fraunhofer IPT researchers have coaxially integrated OCT into the processing head of a LMD-w system. (Image: Fraunhofer IPT)

An industry-academia collaboration has integrated optical coherence tomography (OCT) into a coaxial process for wire-based laser metal deposition (LMD-w) – where metal wire is joined onto a workpiece in weld beads with the aid of a laser.

The technology will be used to stabilise and actively control the laser process, reducing scrap. 

It is intended to enable broader industrial use of LMD-w beyond special repair processes or the application of wear-resistant coatings – to which it has so far been limited due to its complex process requirements and low stability. 

LMD-w would thus be usable as a fully-fledged 3D printing process in the future.

The work has been done within the four-year ‘TopCladd’ project (Adaptive Laser Cladding for Precise Metal Coating Based on Inline Topography Characterization), which is being undertaken by the Fraunhofer Institute for Production Technology IPT together with its industrial and research partners.

The quality of LMD-w depends mainly on the surface of the weld seam: the waiver the surface, the lower the component quality. To make the process more stable and produce a high-quality weld seam, the individual process steps can be recorded, enabling poor quality welds to be subsequently repaired and the welding process adapted for future production.

OCT can be used to 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.

The Fraunhofer IPT researchers have therefore coaxially integrated OCT into the processing head of a LMD-w system. The laser and the OCT system use common optics, but do not interfere with each other due to their different wavelengths. 

An axicon and prism-shaped optics ensure that the processing and measurement light remains coaxial. This enables 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. 

The integrated system enables the surface structure of the entire melt track to be mapped precisely. 

Using the process data collected within the project, the Fraunhofer IPT researchers are now developing a process model for data-based process adaptation and control. As a result, the laser process 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,’ said Robin Day, head of the Energetic Beam Processes department at the Fraunhofer IPT. ‘In this way, LMD-w has been upgraded to a full-fledged and sustainable additive manufacturing process.’

The other partners of TopCladd are: Deltatec, Dinse, Laserco, Precitec and Quada V+F Laserschweißdraht.







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