For a better world: OCT unlocks real-time quality control during oscillation welding
Nataliya Deyneka-Dupriez and Alexander Denkl, of Lessmüller Lasertechnik, describe how OCT can be used to perform online process monitoring when using an oscillating laser beam to weld e-mobility components
The concentration of CO2 in the atmosphere is contributing to the warming of our planet. This year, however, the biggest carbon emission drop ever recorded (4 to 8 per cent) was observed during the widespread lockdowns brought about by Covid-191. The biggest impact was the reduction in global average road transport, which fell to 50 per cent of 2019 levels by the end of March. At that time in Paris, atmospheric CO2 levels decreased up to 72 per cent compared to normal2.
Immediate environmental responses were observed, as global travel fell sharply, including cleaner air, cleaner water and flourishing vegetation. This deadly pandemic has therefore highlighted the importance of reducing global CO2 emissions. But, reducing such emissions does not necessarily have to happen via restrictions of surface transport and reduced car demand. The world could also move towards greener sources of energy.
According to long-term scenarios for low-carbon societies, the future generation and consumption of conventional fossil fuel energy, which causes environmental pollution, will decline. Use of renewable energy sources and clean energy consumption is therefore expected to increase, which will reduce the emission of greenhouse gases3. The more renewable energy is generated and used, the more important cost-effective and sustainable high-performance energy storage systems become. Among them, batteries are considered as one of the most suitable solutions for energy storage4, and for their production, highly efficient laser processing is required.
Oscillation (or wobbling) of the laser processing beam during welding or post-process treatment is known to improve weld quality and stability. Laser surface modification with an oscillating beam during post-process treatment results in superior surface topography, in terms of morphology and roughness. Welding with oscillating laser beams enables adjustment of the bonding geometry. One- or two-dimensional laser beam oscillation, superimposed with feed movement, is applied for gap bridging during fillet welds. The oscillation parameters are adapted to the gap size, in order to generate sufficient molten material and melt pool width, or to influence melt pool dynamics, such as for better degassing5.
In addition, using laser beam oscillation, process emissions such as spatter and melt ejections can be reduced. A single- or low-order-mode near-infrared fibre laser with small spot size, and thus high power density, can be used to weld materials such as aluminium and copper, which are used in battery production. The combination of high-power density and beam oscillation can overcome the challenges of the high reflectivity and high heat conductivity of these materials, while creating a stable keyhole that avoids pore formation6.
In the case of joining dissimilar materials such as aluminium and copper, in addition to their different thermophysical properties, their weldability is limited by the formation of coarse brittle intermetallic phases and the formation of hot and cold cracks7. By using optimal oscillation parameters, the metallurgy formation of these intermetallic phases in the weld can be specifically controlled and weld defects can be minimised. This achieves strong welds with the required electrical conductivity and heat transfer. These benefits have opened up new applications for manufacturing not only electric vehicle batteries, but also electrical power storage products: busbar welding, critical electrical contacts of individual cells, metal bipolar plates, or for the sealing of the entire battery enclosure.
In order to improve laser processing productivity, optical coherence tomography (OCT), as a direct and accurate height] sensing technology, can be widely used in laser welding.
OCT enables fast, precise and omnidirectional online detection of varying seam joint location, focal position, weld penetration depth, as well as weld faults8. This enables automated process adjustment before the weld fails, reducing the positioning time, scrap rate, test costs and rework required. These advantages of OCT are certainly attractive for its use for process monitoring and control during welding with an oscillating focal spot. To realise this approach, the first technological attempts have already been made. Lessmüller Lasertechnik has developed such an OCT system for laser beam oscillation applications.
Performing pre-, post- and in-process measurements using OCT
The OCT and laser processing beam are coaxially directed to the workpiece surface by the welding optics. Because the OCT beam deflects together with the processing laser, it undergoes the same oscillation movements. This means OCT, with an oscillating processing beam, can be used for offline measurements. For seam tracking, for example (see figure 1a), the OCT system is used to ‘teach-in’ a path to the welding control system. The laser head then welds with an oscillating laser beam along the ‘taught-in’ path. After processing is complete, the OCT system is used to obtain the surface profile of the cooled seam (see figure 1b), enabling surface roughness or defects at the seam surface to be assessed.
Even though offline OCT measurements with a deactivated processing laser are fast and flexible, while offering high precision, this procedure obviously results in longer cycle times. Another option is performing online OCT measurements during oscillation welding. In this case, the OCT beam oscillates in the same manner as the processing beam, but the OCT scanner deflects the OCT measuring beam in front of the welding spot (point measurement relative to the TCP position, see figure 2) to detect in real time the exact joint location. Similarly, this can be done behind the processing point for quality monitoring purposes. However, the use of this approach creates some constraints. The joint must have an exact contour with no rounding and oscillations must have a large amplitude.
Figure 1: 3D view of joint surface topography as measured by OCT (a) before (pre-process) and (b) after (post-process) laser processing.
By decoupling the OCT measuring beam and laser processing beam, online pre-, post- and in-process measurements can be realised in situ during beam oscillation, offering the user two functions in one tool: high-resolution real-time seam tracking and precise quality monitoring. A continuous joint trajectory and the geometry of the weld bead have to be measured with the OCT beam simultaneously with the oscillating movement of the processing beam in any welding conditions. Since the OCT scanning figure does not correspond to the movement pattern of the processing laser, the OCT system must compensate for these movements. This compensation is only possible with systems that work independently of each other, and which enable high-speed real-time data communication between each other. Lessmüller Lasertechnik has developed scanner hardware and software solutions for industrial implementation of this concept (see figure 3).
Figure 2: (a) Schematic presentation of the OCT measuring beam position (cyan) relative to the oscillating processing laser beam, whose exemplarily sinusoidal movement is superimposed with a feed movement; (b) Short extract (20ms) from a pre-process OCT measurement on a lap joint.
An online compensation strategy can be applied when manufacturing eco-friendly products, for example when welding the powertrain of an electric vehicle. One application which highly benefits from the decoupling of the OCT scan and the movement of the processing beam is hairpin welding. When the OCT scans independently of the processing beam, the synchronous precise acquisition of hairpin positions in pre-process and of weld bead profiles in post-process can be achieved (see figure 4). Copper hairpins of electric drive stators have to be welded very fast and with welds of superior quality to ensure good electrical contact. If the distance between hairpin couples is within the lateral measurement range of OCT (that varies depending on the optical configuration of the welding head), simultaneous evaluation of hairpin geometry and quality assessment with OCT during welding with an oscillating laser beam is possible.
Figure 3: Lessmüller Lasertechnik’s OCT scanner with a built-in connector serving for fast (10μs) data transfer, necessary for the compensation of oscillating movements of the processing beam.
While the oscillating laser beam is welding one hairpin couple, its motion pattern, frequency and amplitude are compensated by the OCT measuring beam. This allows the OCT system to scan the next hairpin couple simultaneously along four lines (see figure 4). After evaluating the four OCT scan lines, the offset of the real measured centre from the theoretical centre is calculated. With four scan lines, the gap size and position between the pins, the pin width and the misalignment of the pins with respect to each other (lateral offset, see figure 4), are automatically measured. The exact height of each pin is crucial for adjusting the focus and power of the processing beam. The total measuring and evaluation time of this is approximately 55ms. The correction data for the centre position can then be sent to the welding optics control system.
Figure 4: Application of online oscillation compensation in the welding of hairpins.
The quality of the weld bead of each hairpin couple can be also assessed online using OCT. The OCT measuring beam compensates the oscillation of the processing beam and scans the surface of the already welded hairpin couple. Several scans are recorded, each taking approximately 10ms. Measurements across or side by side are possible. The height and surface profile characteristics of the weld bead help draw conclusions about weld quality, namely electrical resistance and required mechanical strength.
For applications in the cell production of batteries, large intermetallic cross-sections are required with low penetration depths. The welding parameters have to be adjusted to match the weld depth, while keeping the bonding width constant, thereby preventing active material damage in the battery cell. Therefore, for the welding of battery or bipolar cells – where 100 per cent quality control is essential – the weld depth has to be permanently, precisely controlled. The welding head needs an integrated high-dynamic quality control system, such as OCT, enabling the compensation of processing laser oscillations. It requires real-time co-ordinate exchange between the welding optics and OCT scanner to carry out synchronised movement compensations.
Compared to conventional laser welding with a static focal spot, welding with an oscillating laser beam results in better weld quality. However, to ensure the broad industrial application of this technique, an easy-to-handle integrated system for precise seam tracking and quality monitoring is required. OCT can be applied to a welding process with an oscillating laser beam to influence the process by improving control of the weld position, depth and geometry. This is done by compensating the oscillating laser movements. Hence, laser welding of power storage and electric engine components using an oscillating beam can become more robust using a highly dynamic OCT system.
Dr Nataliya Deyneka-Dupriez is technical editor and Alexander Denkl is head of application engineering at Lessmüller Lasertechnik
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