Review LSE 2021

Manufacturing processes from the user’s point of view

Research and industry are already setting the course toward producing the most important components in series. At the virtual event, Marc Hummel, a research associate at the Chair of Laser Technology LLT at RWTH Aachen University, reported on the exciting interplay between two research sites: “Together with the experts at the German Electron Synchrotron (DESY) in Hamburg, we are researching how synchrotron radiation can be used to visualize and, thus, analyze the melt pool dynamics and the development of vapor capillaries during laser-based welding.”

A fundamental understanding of laser beam welding is a prerequisite for successfully implementing this joining technique, especially for thermally sensitive components such as battery cells. Marc Hummel offered insights into welding in the truest sense of the word with his remarks on investigations at the German Electron Synchrotron in Hamburg. Visualizing the vapor capillary in the material provides new findings into process development and understanding. Correlations between laser parameters and welding results can, thus, not only be determined after the process in a destructive test, but pores, for example, can be detected while they are still forming. This helps users, especially when they determine process parameters and design products.

And yet lasers are not only used in joining technology. Prof. Achim Kampker, founder and head of the Chair of Production Engineering of E-Mobility Components PEM at RWTH Aachen University, described the important role of this particular operating tool in production. His work focuses on integrating production and process engineering, and he examines manufacturing processes from the user's perspective.

In view of current forecasts, according to which around 60 percent of all vehicles are expected to be equipped with e-drives by 2030 (with 120 million new vehicle registrations), the industry now needs to optimize processes and significantly reduce costs, Kampker said. “Cost drivers are primarily the key components – battery and drive train – because they account for more than 50 percent of total costs,” he stated. “But it’s also about the fuel cell, where we need to get down to a third of the current costs, which are still very high.” He sees new process technologies as an approach: For example, in cell production, the high energy required and, thus, also the costs could be reduced by 30 to 35 percent with the help of lasers. To this end, for example, PEM is working with Fraunhofer ILT on a roll-to-roll system to research how the previous drying of coated metal foils by infrared can be replaced by laser processes. At the LSE, Kampker explained how a powerful VCSEL laser (power: 120 W/cm²) or a focused diode laser could be used to dry coated electrodes over large areas in an energy-efficient manner in the future. However, there is still no system ready for mass production.

Laser saves energy, costs and space

Not only do saving energy and controlling the process well and quickly speak in favor of changing the process: During the symposium the mobile video system provided good virtual view into the current status of the joint research work. It took the online participants into the Battery Lab of Fraunhofer ILT. Here, the researchers have access to a wide range of equipment for laser-based battery production on an area of almost 140 square meters. Furthermore, the research assistant of the neighboring PEM, Simon Voss, presented a completely encapsulated roll-to-roll system from Coatema. The institute chair is now also investigating whether and how laser-based cutting can replace the current mechanical die cutting for future large-scale production. The researchers at PEM are also tackling laser-based microstructuring of the coated electrodes together with Fraunhofer ILT.

The enormous potential of laser processing of functional layers was addressed by Dr. Christian Vedder, head of the Thin Film Processing group at Fraunhofer ILT. The spectrum he presented ranged from cleaning, paint stripping, crystallization, application of sensor technology, integration of electronic conductors in CFRP components, corrosion protection all the way to selective gold plating.

While many of these processes are attractive to companies working in electromobility, one of the most energy-efficient technologies is certainly “thermal post-treatment,” or laser-based drying. In this context, there are some amazing details about the processes that Fraunhofer ILT and the neighboring chair are already setting up: The laser unit dries the wet-coated films within a few seconds, with the laser radiation significantly heating only the layer, not the entire system, a disadvantage of conventional oven processes. Thanks to this efficiency, energy can be reduced by up to 50 percent, depending on the layer system. In addition, laser technology can be used to massively reduce the space required for a drying system. A scanning diode laser or a vertical cavity surface emitting laser (VCSEL) is a suitable beam source.

The latter works with semiconductor laser diode bars in which the laser beam is emitted vertically from the top surface. Fraunhofer ILT is currently integrating this laser into the drying process, along with laser structuring processes to increase the electrode surface area, into a roll-to-roll system of the latest design, which ILT scientist Samuel Fink showed during the lab tour. Also in the lab, ILT scientist Linda Hoff gave a live demonstration of how lasers can sinter future solid-state battery materials at much higher temperatures, up to 1000 °C, without significantly affecting their crystallinity.

How lasers can be used flexibly in electric vehicles was highlighted by Dr. Jan-Philipp Weberpals, who is responsible for the overall strategic planning of laser-beam technology at Audi AG in Neckarsulm. For him, the laser is a resource that can take over several tasks. As an example, Weberpals cited the laser-beam remote welding (BrightLine Weld with an infrared truncated disk laser) of battery cells. For galvanized steel (HV module housings), the automaker uses the process to weld fillet welds at the lap joint with different thicknesses. Full penetration welding was out of the question here due to the design and assembly requirements. The situation is different with laser-based remote welding of I-seams at the lap joint on pure aluminum, which Audi uses instead of full penetration welding to protect the cell. The focus here is on contacting cells to each other.

Lateral oscillation shape ensures increased heat input

What also plays an important role are key factors such as path fidelity (up to a maximum frequency of 250 Hz) and the oscillation shape. Since the circular oscillation shape produces V-shaped seam cross-sections with insufficient bond width, Audi opted for lateral oscillation for aluminum welding: Here, heat input is increased to generate a U-shaped seam cross-section and sufficient bond width.

When welding galvanized steel, Audi decided to use beam shaping and adapted path planning, where the path correction prevented full penetration welding. In addition, two major developments characterize steel remote welding: First, as the power distribution was adapted, the bond width increased and edge notches decreased. Second, the increased lateral angle lowered the feed rate and improved lateral connection.

Detecting defects in real time

Stabilizing the process through targeted beam oscillation or adjusting the power density distribution alone is not sufficient, however: Another very important step is real-time monitoring. For this, Audi relies on the Laser Welding Monitor 4.0 from Precitec GmbH & Co. KG, headquartered in Gaggenau-Bad Rotenfels, in which a sensor unit with three photodiodes detects temperature, plasma and back reflection. The monitor makes it possible to record the joint quality in a non-destructive, real-time manner by recording process emissions with its trio of photodiodes in three wavelength ranges. This real-time data enables 100 percent quality assurance with reliable defect detection. Audi opted for the light-sectioning method because it offers significantly higher resolution than the more complex optical coherence tomography (OCT).

To evaluate the significance of this data, Audi investigated how laser power and focus position influence tensile force and contact resistance. The tensile force changed linearly along with the laser power, while the contact resistance entered a saturation range after a certain time – both with varying laser power and focus position. Audi also uses the monitor data for detailed AI analysis. These steps point the way towards a Smart Factory with a real-time control loop, in which the completely continuous process chain is operated and regulated in an automated manner. 

Test facility for laser-based battery carrier production

Benteler AG in Paderborn has set up a flagship project to show how flexible manufacturing can work: It contains a battery carrier tested as a demonstrator component on a multi-stage, interlinked pilot plant under the typical conditions of automotive production. A component weighing around 100 kg (0.8 to 8 mm material thickness) made of aluminum and steel has been produced in an eight-hour shift in batches of 220 to 230. Lasers do the welding and cutting, the bending process is mechanical.

Copper also plays an important role for TRUMPF: It uses a green laser beam source to directly bond copper to ceramics (DCB: direct copper bonding). Within 100 ms, a green laser (TruDisk 1020) bonds the double copper layer (2x 0.3 mm) to a ceramic carrier. A video showed how the DCB process works safely and spatter-free with a very low welding depth (0.05 mm). 

Hairpin: Reliable ablation of the insulation layer 

There has also been a positive development in hairpin welding, which is now a fully automated, highly productive and high-quality laser-based process without spatter. A nanosecond pulsed laser is used to selectively ablate the insulation layer (laser de-coating), thus reducing material usage; unlike mechanical processes, this process does not damage the copper layer. In the next step, the hairpins are welded. A benchmark for quality is pore purity, which TRUMPF achieves with a BrightLine Weld laser that works with a two-channel laser-light fiber as opposed to a single-spot beam source. Thanks to the additional ring beam, the keyhole opening is enlarged, allowing the metal vapor to escape more easily. The result, according to Möller, is a virtually pore-free surface. The hairpins are then measured using camera-based technology (OCT) and the optimum welding point is recorded. 

Conclusion: Electromobility not only awakens curiosity online, but also a great deal of discussion. The question-and-answer session in the live chat was often livelier than at a face-to-face event. Finally, Dr. Arnold Gillner also piqued the visitors’ curiosity. He showed a blue and white road sign with the provocative inscription “Electromobility: Next Exit” and recommended leaving the highway in September 2021 and February 2022 for the next two electromobility highlights at Fraunhofer ILT: the LKH₂ Laser Colloquium on Hydrogen (September 15 and 16, 2021) and the Laser Symposium on Electromobility LSE (February 8 and 9, 2022).