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ANALYSIS: WELD MONITORING


Implementing OCT for industrial weld monitoring


Dr Nataliya Deyneka-Dupriez, of Lessmüller Lasertechnik, describes the benefits that OCT can bring to weld monitoring, and highlights how the technology is starting to find applications in e-mobility


Optical coherence tomography (OCT) is a technology capable of obtaining depth images with micrometre-scale resolution by measuring and processing the interference of light being scattered from a particular point. Since 1992, commercial


OCT systems have been widely used for non-invasive 2D- and 3D-imaging in many in-vivo biomedical applications, and since the early 2000s the technology has also been used in operating theatres to guide laser surgical modules. Today, OCT is adopted in industrial applications, as well as those in medicine, for example in the measurement of coatings, fibre components, MEMS, thin sheets and artworks. This successful implementation of the technology encouraged investigators to also explore the potentials of using OCT to monitor weld depth in real


time during laser processing. As a result, in 2010 Webster et al introduced the infrared spectral-domain low-coherence interferometer, which was aligned coaxially with a processing laser to directly measure and control laser weld depth in-situ. Since then, several companies, including Lessmüller Lasertechnik, have progressed on the way to adapting OCT for industrial laser welding.


Superior monitoring capability By using OCT scanners, not only in-process weld monitoring, but also non-contact seam tracking and non-destructive real-time quality control of welds can be achieved with a precision and reliability unobtainable through conventional process monitoring. Additional benefits of using


OCT over alternative monitoring techniques, for example laser triangulation, include its ability to operate under a coaxial measurement configuration


and perform omnidirectional tracking. The application of the technology is also particularly advantageous when dealing with geometries that are inaccessible to traditional camera-based monitoring techniques, with OCT being able to facilitate rigorous, high-speed welds of metallic automotive workpieces that contain hardly accessible, non- linear seams. With the invention of the dynamic adjustable reference arm, modern OCT systems are also able to achieve large and variable working distances while maintaining high resolution. They can detect joints, welds and defects with high accuracy and with extraordinarily high (or small) aspect ratio. OCT is not only able to detect joint position, but can also detect gap size and the angle of incidence prior to welding, enabling it to compensate for them online. The coaxial alignment of an OCT system empowers users to perform


measurements at a wide work angle range. In the case of steel workpieces, OCT can clearly detect the surface topography under angles of incidence even higher than 50° while looking through welding optics. Other benefits include OCT’s


ability to perform singular point measurements, with the number and location of the points being definable by the user. In addition, interfering factors, such as clamping or other fastening fixtures, do not affect the tracking or quality assurance results of OCT. The technology is also immune to the white- hot process light, speckle and splatter experienced during laser welding. OCT-equipped laser


processing heads offer a number of benefits that affect customer profits through enabling cost-effective production and improved throughput. The technology offers consistent, precise, real-time performance and renders flexible, high-speed, fully automated laser welding possible, with closed-loop control and without the need for time-consuming set-up.


Rising uptake in the automotive industry


The industrial implementation of OCT for laser welding is continuously increasing, with


Microscopic images of numerous transversal cross sections were compared with the depth measured by Lessmüller Lasertechnik’s OCT system. An average discrepancy of 0.04mm or 9 per cent was revealed


22 LASER SYSTEMS EUROPE AUTUMN 2019


OCT acquires 3D surface images to map the weld bead topography with high axial and lateral measuring accuracy. Open pores are detected and evaluated. The size of the pore is crucial for the ‘weld failed’ decision


@LASERSYSTEMSMAG | WWW.LASERSYSTEMSEUROPE.COM @researchinfo | www.researchinformation.info


Lessmüller Lasertechnik


Lessmüller Lasertechnik


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