Dr Nataliya Deyneka-Dupriez 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.
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. (Image: Lessmüller Lasertechnik)
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.
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. (Image: Lessmüller Lasertechnik)
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 the offered time-saving and material economy benefits being particularly attractive for automobile manufacturers.
Due to laser welding being a key process in the production of batteries and motors for electric vehicles, the number of possible applications of OCT monitoring is increasing further with the ongoing global transition towards emission-free transport, and the resulting emergence of the e-mobility market.
Among these are the welding of power storage and power train components, such as copper hairpins in the stator of an electric motor. It has been shown that OCT technology can be successfully used for fast and exact localisation of hairpins, and fast quantitative quality assessment of the weld bead during processing. The benefit of OCT over other inspection techniques here is that it offers not only three-dimensional visualisation of the hairpins, but also direct real-time height measurements. The exact height of each pin is crucial for the adjustment of the focus and power density of the processing beam, which enables the copper to be welded with minimised heat input and without spatter. The height difference of the hairpin couple before and after welding gives an insight into the volume of molten metal, which, together with the other measured surface profile parameters of the bond, are determining factors for the hairpins’ weld quality, which in turn is essential to ensure proper electrical efficiency and mechanical strength.
Another application in the e-mobilty sector is the welding of busbars in various material combinations and geometries. The weld has to be profound enough to guarantee a sufficient bonding cross section for a minimum resistance and maximum strength, while not penetrating through the busbar into the highly sensitive battery cell. Lessmüller Lasertechnik has tested this with alumina material (2.0mm Al onto 1.2mm Al) at a penetration depth of around 2.6mm. OCT technology was used for the online measurement of the weld depth during processing, and achieved results that matched those later obtained with microscopic images taken of the weld cross section.
OCT is starting to be used in e-mobility for determining the quality of welds made between copper hairpins used in electric motors. (Image: Lessmüller Lasertechnik)
OCT is also a potential technology for enabling the growing use of electrical propulsion systems in aircraft, as, like automotive, new laser processes with the highest precision and reproducibility will be required in their manufacture.
Together with the industrialisation of OCT for laser welding, the development of industrial solutions for sequential online pre-, in- and post-process control with OCT are ongoing. An essential future development will be the use of OCT keyhole depth data for closed-loop laser power control to maintain constant weld depth.
All these potential merits of OCT and the 3D nature of its data, make it in many respects superior to traditional process monitoring technologies. It empowers highly productive and flexible production-line layouts with increased throughput and yield. OCT makes welding in series production faster, more accurate, and thus more cost-effective than currently achievable with today’s conventional sensor
technology.
Dr Nataliya Deyneka-Dupriez is the technical editor of Lessmüller Lasertechnik