Process Monitoring
OCT for Efficient High Quality Laser Welding High-speed, high-resolution online seam tracking, monitoring and quality control Nataliya Deyneka Dupriez and Christian Truckenbrodt
High requirements for duration and quality of laser welding are the main topics in the line production nowadays, especially in the automotive industry. With acquisition rates up to 100 kHz and microseconds duration capture times, optical coherence tomography (OCT) is a powerful adaptive tool, with a few microns resolution, for real-time direct non-contact investigation of material processing. The measurement system used to acquire the workpiece topography enables high accuracy automatic seam tracking, process monitoring and quality assurance (QA), and these functions are executed simultaneously. Starting from 1973, when Ford Motor Co. implemented a laser welding system for an assembly line, the use of laser technology in automotive industry dramatically increased. The driving force for this are end product requirements such as miniaturization, weight and cost reduction, achieved by higher productivity, increased process speed and shorter cycle times. Moreover, part dimensional accuracy and reduced width of the flange results in reduced vehicle weight with higher structural stiffness and enables to laser-join the small components. However, in order to achieve high production rates and respectively high profits, a stable laser manufacturing process must be run, remaining in the narrow process window of allowable process parameters. For this purpose, both exact real-time positioning of the processing beam and online quality with the simultaneous control of process parameters control, are essential. It demands an online non-destructive non-contact testing system. The weld faults need to be automatically detected,
autocorrected before the weld fails, solving weld irregularities faster, thus reducing scrap rates, test costs and rework. To achieve this objective, Lessmüller Lasertechnik developed several online tracking and quality control systems for laser welding. Starting from process monitoring for industrial serial production the WELDEYE system conquered the market across the globe offering online seam tracking, visualization, monitoring and quality assurance of welding or brazing processes [1]. The system performs synchronous live image ana lysis and process parameters. It can be simply adapted to imaging or QA device (like, for example, camera, photodiode control tool or OCT) and can be easi ly integrated in many laser processing heads, communication performed via standard fieldbuses. In contrast to indirect measurements of surface reflections or 2D camera imaging, process emission light, energy or temperature, that need to be specified during previous teach-in process, the innovative Lessmüller Lasertechnik OCT system for laser welding stands for 3D imaging. OCT enables omni directional high-resolution online seam tracking, monitoring, quality control and permits to perform a step forward in direct exact and reliable product line traceability to ensure weld quality standards.
OCT operation principal OCT is an interferometric imaging system capable of producing high-resolution 3D images of the reflective surface topography or semi-transparent undersurface cross section. A spectrometer-based OCT system consists of a high-speed x-y scanner unit
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Fig. 1 Schematic layout of the OCT sensor setup.
(OCT scanner) and an interferometer (OCT sensor) with a light source and spectrometer. The light beam from a broadband source is incident to a beam splitter, where it is split into a reference arm and a sample arm. The split light beams are reflected at the reference reflector and at the sample and are recombined at the beam splitter and evaluated by a spectrometer. In case of an optical path length difference between the ref-
Company Lessmüller Lasertechnik GmbH Munich, Germany
Providing tailor-made solutions for online closed loop controlled seam tracking, visualization, monitoring and quality assurance Lessmüller Lasertechnik is the leading manufacturer of online quality control systems for laser welding or brazing processes. Based on the proven WELDEYE, with over 400 systems installed worldwide, the brand new Lessmüller OCT is an innovative technology for laser welding or brazing which permits to make a step forward in faster, more accurate, and thus more cost-effective series production.
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erence arm and the sample arm within a coherence length of the source, optical interference occurs. Optical interference spectra are processed into an OCT image. The higher the difference in optical path length between the two arms, the greater is the resulting modulation frequency over the interference spectrum [2]. Thus, by the spectral analysis of the interferograms, the system measures with µm-accuracy the distance to every surface point, yielding a reflection in the imaging area. Three-dimensional profiles are obtained by simultaneous lateral scanning of the sample. A schematic layout of the OCT sensor setup is shown in Fig. 1.
Application of OCT for laser welding processes Since OCT can provide surface profile and/or depth information, it opens up a number of capabilities for high resolution real-time non-destructive inspection during the welding process. OCT systems with an associated OCT scanner, data acquisition and digital signal processing unit, can be easily integrated into established laser heads by connecting to the existing camera port, while laser, process fiber and head configurations remain unchanged. Fig. 2 shows the typical configuration setup including welding optics, integrated OCT system, data processing hardware, and communication passes. The split beam (the sample arm) from the OCT sensor is fiber-guided to the OCT scanner that is connected through a) Post-process
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Fig. 3 Schematic representation of the OCT measurements (doted blue lines) performed on the fillet weld in 0° incidence configuration (a); OCT images with respective cross-section photographs of the joint (pre-process; b, c) and the weld bead (post-process; d, e).
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OCT sensor OCTcontrol unit
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b) Fig. 2 The welding setup with integrated OCT system and fieldbus communication. Photograph of the OCT system setup; example for system integration to Lessmüller LSO welding optics (a). Schematic layout of the setup including photograph of the workpiece surface with the OCT measurement figure (doted blue lines) during welding process (b).
the camera port to the laser head. Then it passes the deflecting mirror coaxially to the laser head, pursuing the process beam to directly measure the processed surface area and its close approximation with high acquisition rate of up to 100 kHz. The reference arm is folded to keep the fiber length identical to the sample arm and to allow the setup to be as compact as possible. Communication is established by standard fieldbuses. As it was shown earlier by Schmitt et al. [3], OCT systems can also be successfully set up with processing laser head supplied by telecentric F-theta lens. The aberrations caused by telecentric F-theta lens can be compensated through a dispersive element in the reference path on the one hand. On the other hand, preliminary calibration can be performed and the calibration data stored in the system software can be used for the online correction of these aberrations. Up to now, OCT was successfully used for welding quality control particularly for online high speed profiling of keyhole depth [4]. By implementing the principles and algorithms of the
WELDEYE software platform, optimally adapted to the OCT system, fast and exact estimation of the welding location, the maximum keyhole depth which gives an evidence about the power of the processing laser, and the detection of the quality of the resulted weld are possible. The probe beam rapidly sweeps across the collocation line of the joining pieces (see photograph in Fig. 2b) for accurate profiling of the seam joint. Then it penetrates the keyhole along the welding direction (see photograph in Fig. 2b) yielding the maximum laser penetration depth which is therefore minor influenced by the welding speed. After that, it tests the finished surface directly after the process (see photograph in Fig. 2b). All these measurements are performed and processed online, simultaneously, during every single weld, in one cycle that is repeated every few milliseconds. Quality criteria can be specified by the user in terms of target vs. actual divergence thresholds of the welding position or focusing, keyhole depth limits, and seam profile tolerances together with the allowable process
© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Process Monitoring
parameters ranges. As a result of automated QA analysis performed online by the WELDEYE software, a prompt pass / fail signal is generated permitting either to correct in real time beam position, focus, and process parameters, or reject immediately any parts that do fail as soon as the weld is complete.
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Advantages over laser triangulation and other QA systems Non-contact seam tracking and process control of laser welding can be fulfilled by several optical techniques. One widely used technique is laser triangulation. In contrast to two-dimensional camera images, OCT provides the user with the tree-dimensional quantitative information of the workpiece surface topography. While the laser triangulation technique is based on the displacement analysis of the projected lines, OCT performs singular point measurements, the lateral distribution in the scan region where the number of which are user defined. OCT can be applied for nearly any joint shapes (Fig. 3 – 5). It is marked by high axial measuring accuracy regardless of focusing. The lateral resolution of the instrument is determined by the diameter of the spot that is generated by the focus length of both the OCT optics and the laser head optics. The axial resolution depends on the optical distances of the optical system through which the measuring beam passes. The decoupling of lateral and axial resolution allows the measurement of structures with high or small aspect ratios like joints with sharp edges, holes and pits, etc.
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Fig. 4 OCT images with respective photographs of the different joint configurations: flange joint of zinc coated steel sheets in 0° incidence configuration (a); fillet weld joint of zinc coated steel sheets in 45° incidence configuration (b); radial butt joint of stainless steel in 0° incidence configuration (d); powertrain V-joint (0.5 mm wide) in 0° incidence configuration (e). Arrows on Figure (b) and (e) mark secondary reflections. Schematic demonstration of the origin of secondary reflections obtained by testing fillet weld joint in 45° incidence configuration (c) and of powertrain V-Joint in 0° incidence configuration (f).
It has also a long and variable working distance to look through the welding optics while maintaining high resolution. Thus, the axial mechanical scanning of the workpiece is unnecessary while using an OCT that permits to maintain moderate mechanical complexity of the instrument. This, together
with the high lateral resolution of OCT measurements, allows seam tracking and weld inspection with the precision and reliability not obtainable with any other conventional technique. The inherently three-dimensional nature of the OCT data is in many respects superior to the traditional techniques.
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Application examples
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ing set-ups and configurations of the external line projector and of the cross jet can be omitted. Compact and lightweight designed, OCT itself has small interfering contour. All these enable highly productive and flexible production line layouts, making welding in series production faster, more accurate, and thus cost-effective.
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Fig. 5 OCT images with respective photographs of the welding faults (marked by arrows) on the fillet welds: a pore of the size of 0.2 mm on the welded seam of zinc coated steel sheets in 45° incidence configuration (a); welding apart the joint on the bottom sheet of aluminium in 0° incidence configuration (b); welding apart the joint on the top sheet of zinc coated steel in 0° incidence configuration (c). OCT imaging allows to make an immediate automated “fail” decision.
In contrast to laser triangulation, under the coaxial measurement configuration the interfering factors, like clams and fastening fixtures, are not affecting the tracking and QA results allowing OCT to perform in limited space. Frequency analysis of the spectral data makes OCT immune to the white-hot process glow and speckles during welding. Thus, OCT is not influenced from outside. Another important aspect of spectral domain OCT is that it enables also to achieve high scanning rates that the fast simultaneous confirmation of the processing position and of the weld quality is obtained. This considerably reduces the cycle time. Additionally, OCT is easy to integrate through the camera port, it is simple in operation, it is unsusceptible to contaminations and robust beyond question. Time-consum-
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To demonstrate the performance of the OCT system, the imaging of a variety of joints, welds and faults was performed offline. The sets of point locations were imaged and analyzed. Each point is approximately 0.05 mm in diameter. The scans presented in Figs. 3 – 5 contain in total 100 measurement points that were made along the line: across the seam joint for seam tracking feedback, or across the weld bead for quality control (see schematic representation in Fig. 3a). The sampling frequency was 100 kHz, axial resolution was 12 µm. The measurement shown in Fig. 3b represents the reflection of the OCT measuring beam at the fillet joint perpendicular to the workpiece surface, where the rapid change of the surface height (a gap between two lines on the OCT image) corresponds to the joint location. The joint profile is visually comparable with those visible on the cross-section photograph in Fig. 3c. Fig. 3d shows the weld bead topography obtained with OCT from a premium quality weld. The surface profile is in accordance with the cross-section image in Fig. 3e. Using the system, the different joint configurations (Fig. 4) were imaged. Fig. 4 demonstrates that due to the large working distance and high precision of the OCT tool, the detection of the joints with extraordinary high aspect ratio (flange joint in Fig. 4a) or small aspect ratio (butt joint and powertrain gear V-joint (Fig. 4d, e), can be achieved. Profiles of the small joint gaps (0.3 – 0.5 mm) in Fig. 4d, e, can be easily resolved with highly sensitive OCT. The OCT images in Fig. 4a, b, e show clear reflections from the tilted surfaces, indicating the capability of the instrument to perform the measurements under variable angle of incidence. One example is the fillet joint tested in the
configuration of 45° angle of incidence. The results presented in Fig. 4b shows a well defined joint proving the angular flexibility of the tool. Under certain measurement configurations, the secondary reflections (marked in Fig. 4b and c by arrows) can appear. Fig. 4f illustrates the manner, how the incident OCT beam (blue arrows) reflects from the surface of the bottom sheet underneath the upper sheet and then reflects from the underside of the upper sheet the same way back to OCT sensor. Similary, the reflections from the tilted edge of the V-joint to the vertical edge can be driven (Fig. 4f). Reflected by this way, beams have obviously longer optical way compared to the reflections from the top workpiece surfaces (blue dots). Thus, secondary reflections from the edges, marked on Figs. 4e, f with the red dots, appear in OCT image below the joint profile and do not affect the automated seam tracking. Moreover, the analysis of those reflections can be useful to evaluate the gap between upper and lower sheets. OCT was also applied to measure selected unsuccessful welds (Fig. 5). Both high lateral and axial resolution of this three-dimensional imaging module, allow to resolve the pores. Fig. 5 demonstrates, as an example, the 0.1 – 0.2 mm large pores (arrow mark in Fig. 5a) which are obviously visible for OCT even in 0° of incidence configuration. In Fig. 5 b, c, the surface profiles of the welded seams are not gradually increasing like in Fig. 3d, indicating weld location faults.
Summary To increase product value and to perform full precise online monitoring of laser welding, a powerful diagnostic tool is presented. OCT is a unique and valuable technique with a few microns resolution for non-destructive real-time diagnostic during laser welding processes performed coaxially to processing beam through the existing laser optics. OCT is useful for real-time process seam tracking, process monitoring and quality assurance of laser welding and brazing, simultaneously done by a single instrument. It is possible to determine the seam joint location, focal position, weld penetration depth as well
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Process Monitoring
as the profile of the finished weld bead. Online quality control allows to detect and eventually correct the manufacturing faults during early processing stages. It is demonstrated that OCT has the potential to determine the joints and faults, which are not accessible for common online process monitoring tools. These features are of great advantage not only because of the reduced overall process time but also because they enable to achieve an increased process stability and product quality with reduced rejected parts number. OCT may enhance the whole manufacturing process by the capability of real-time direct and independent measurements unaffected by environmental factors and processed laser light. OCT opens up new possibilities for modern online laser guiding with simultaneous process monitoring and provides a simple trend-setting QA solution for high performance, high-throughoutput laser welding at low per-part cost.
DOI: 10.1002/latj.201600020
[1] Ch. Truckenbrodt: Laser Technik Journal 10 (2013) 46-50. [2] M. Brezinski: Optical coherence tomography – Principles and Applications, Elsevier (2006) 130-134.
[3] R. Schmitt, G. Mallmann: Photonik international 11 (2013) 57-59. [4] J. J. Blecher et al.: Science and Technology of Welding and Joining 19 (2014) 560-564.
Authors Nataliya Deyneka Dupriez
studied physics before completing her doctoral thesis in solid state physics at the University of Ulm. After years of academic research in the field of material science, she joined Lessmüller Lasertechnik GmbH in 2015.
Christian Truckenbrodt
is a COO of Lessmüller Lasertechnik GmbH. Since his study and PhD, he has been working on development of the systems for tracking and monitoring of industrial laser processes. Starting the world wide implemented WELDEYE system based on the camera with external illumination for the 2D process imaging, he put the company focus on next generation process monitoring, namely 3D imaging with OCT.
Lessmüller Lasertechnik GmbH, Gollierstraße 12, D-80339 Munich, Germany, Tel.: +49(0)893609048104, Fax: +49(0)89360904829, e-mail:
[email protected]
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