OCT Improves Process Stability During Laser Metal Deposition

Researchers at the Fraunhofer Institute for Production Technology IPT, in collaboration with five industry partners, have equipped a laser metal deposition (LMD) system with an OCT system. The combination mechanism enabled the scientists to successfully record the welding process in progress, and made it possible to control the quality and process during use, which reduces scrap.

Systems for machine-integrated monitoring are traditionally used in LMD to detect and address abnormalities and deviations without delay to prevent interrupting the process. These systems can check the process directly on the spot and initiate corrections.

The collaborators specifically used a wire-based LMD (LMD-w) system in the work. They believe that the combination allows wire-based material deposition to be used as a fully fledged 3D-printing process in the future.

To date, the complex process development and low process stability of LMD-w have prevented broader industrial use beyond special repair processes and/or the application of wear-resistant coatings.

The research team reported that its system, designed under the “TopCladd: Adaptive Laser Cladding for Precise Metal Coating Based on Inline Topography Characterization” project, is the first time a coaxial wire-based LMD system has been equipped with an OCT system to stabilize and actively control the laser process. LMD-w uses a metal wire as a filler material. The wire is welded onto a workpiece in weld beads with the aid of a laser. The beads produce a layer, and several stacked layers produce a component to complete the additive manufacturing process.

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 deposition process more stable and to produce a higher-quality weld seam, the individual process steps must be recorded. This enables poor-quality weld seams to be repaired and the welding process to be adapted for future production.

Research partners from Germany and Belgium equipped a coaxial LMD-w system with an OCT system to stabilize and actively control the laser process. The use of OCT made it possible to monitor the surface of the weld seam in the phase transition from solid to liquid — and the characteristics of the final weld bead geometry as a result. Courtesy of Fraunhofer IPT.

Using common optics that did not interfere due to their different wavelengths, the researchers coaxially integrated the OCT system into the processing head of the laser. A cone-shaped lens called an axicon and prism-shaped optics ensured that the processing and measurement light remained coaxial.

The optical design allowed the measuring laser to circularly scan the applied weld around the centrally running metal wire, enabling multidirectional measurement independent of the direction in which the welding head moved. In this way, the entire workpiece could be measured without the wire blocking the measuring light.

According to the team, the use of OCT made it possible 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. The data the team obtained indicated that the laser process could be adjusted in the adjacent or overlying weld bead if necessary.

Further, the research showed that the surface structure of the entire melt track could be mapped precisely using the OCT/LMD-w mechanism; the researchers are currently developing a process model for data-based process adaptation and control. The effect this will have on the laser process will open a range of applications, they said.

“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,” said Robin Day, head of the Energetic Beam Processes Department at Fraunhofer IPT.

The additional project partners from Germany and Belgium are Deltatec SA, Dinse GmbH, Laserco SA, Precitec GmbH & Co. KG, and Quada V+F Laserschweißdraht GmbH.

The project was funded by the German Federal Ministry of Education and Research under the funding program “M-ERA.Net: flexible and demand-oriented transnational funding in the field of materials research.”