How ready is laser material processing for serial e-mobility production?

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Christian Geiger and Tony Weiss, of the TUM Institute for Machine Tools and Industrial Management, evaluate laser technology readiness for EV mass-production

Co-authored by Christian Stadter and Professor Michael Zaeh, from TUM Institute for Machine Tools and Industrial Management

To meet CO2 emission reduction requirements, a transformation of the mobility sector from fossil-fuel-powered vehicles to electrified vehicles is underway. To increase the acceptance of e-mobility in society, the drawbacks compared to fossil-fuel-powered vehicles, like higher prices + smaller driving ranges, must be overcome.

Research is focused on mass reduction by lightweight designs and on the development of energy storage devices with high energy densities, such as lithium-ion batteries (LIBs) or all solid-state batteries (ASSBs) to advance zero-emission mobility solutions. With the advent of technologies in e-mobility, new materials, miniaturisation and individualisation of components can all be realised in new manufacturing processes for series production.

Laser material processing can contribute to reducing production costs and enabling designs due to its high level of flexibility, productivity and wear-free operation.

At the Institute for Machine Tools and Industrial Management at the Technical University of Munich (TUM), relevant laser technologies currently being researched were evaluated and their technology readiness levels (TRL) methodically determined. The TRL is intended to enable companies to determine at what point laser materials processing is ready for use in an industrial environment.

TRL evaluation

The identification of threats and opportunities, as well as technology needs, is a necessary step for companies to maintain their competitiveness1. The systematic analysis and the structured management of production technologies support manufacturing companies during this process and ensure market competitiveness, especially in emerging fields of application such as battery production2. In 2011, Reinhart et al. introduced a method based on the concept of the TRL3 to evaluate the maturity of technologies. Within the study that is presented in the following, experts with years of experience in the field of laser manufacturing technologies evaluated the maturity of processes, means of production and sensors used for laser materials processing to create a maturity profile that determines the required expenditure for R&D along a maturity scale.

This scale includes basic research activities (TRL 1), feasibility studies (TRL 2), technology development (TRL 3) and demonstration (TRL 4), integration in production resources (TRL 5) and in environments (TRL 6), as well as the application in serial production (TRL 7)4. In this work the method was used to determine the TRL of currently researched technologies in laser materials processing. The evaluation of the TRL is summarised for this article. A more detailed discussion can be found in the proceedings of the Lasers in Manufacturing Conference 20215.

Laser beam welding for the production of power electronics

For the design and manufacturing of power electronics, laser beam welding (LBW) opens up possibilities due to its high precision and flexibility. To enable the joining of copper with a high process efficiency, new laser beam sources emitting radiation in the visible spectrum (blue and green) have been developed in recent years to make use of the higher absorptivity of copper for these wavelengths compared to infrared radiation6. Green laser radiation for example, at a wavelength of 515nm, can be used to join copper to a ceramic substrate, as shown by a metallographic cross section in figure 1a.

Figure 1a: metallographic cross-section of a joint made with green continuous-wave laser beam radiation.

The evaluation of the current technological maturity level for LBW in the production of power electronics showed that further experimental investigations are necessary, with consideration to the characteristics of the workpiece, such as the properties of the substrate and the metalised semiconductor. Additionally, further adaptations are necessary to join complex geometries. Combined with the development of suitable methods for automatic inline quality assurance, the high productivity of LBW can be fully exploited to reduce manufacturing costs and enable innovative products in the future.

Digital laser beam welding

Digital laser beam welding (DLBW) involves the use of relevant quality characteristics derived from real-time process variables – also known as closed loop processing – to fulfill process requirements regarding control and resultant product quality. Process monitoring is becoming increasingly important for this.

To a large extent, optical methods are largely suited to process monitoring for DLBW7. Low coherence interferometry, for example, allows dimensional quantities of process variables to be recorded inline. This enables the recording of process variables independent from process emissions, which enables precise and temporally highly-resolved geometric measurements.

As a result, the surface topography8 and the capillary depth9 can be assessed despite process emissions prevailing in the process zone at high intensities. The complex signal structure of the measured inline data requires a holistic investigation of the necessary data processing pipeline – as outlined in figure 1b (based on VDMA (2020)10) – to use the measured weld depth profile for the prediction of the surface quality11.

Figure 1b: Process model for the concept of digital laser beam welding.

The evaluation of DLBW’s TRL showed an advanced level of development for already-demonstrated approaches and systems, indicating that the concept of DLBW can be transferred to series applications for e-mobility in the near future.

Further research has to address the optimisation and qualification of the technology for series production facilities. Based on a technology demonstrator, prototypes have to be integrated into a real production environment to evaluate the full spectrum of possible process and environmental boundary conditions.

Laser-based surface pre-treatment for joining metals and reinforced plastics

As shown in figure 2a, laser materials processing can be used for functionalising metallic surfaces to improve the bonding properties to polymers, such as strength12 or tightness13. The quantitative evaluation of the technological readiness is oriented towards a subtractive modification using pulsed and continuous-wave (CW) laser radiation as the dominant area of research. Thorough and structured research into theoretical fundamentals of the interaction between the laser radiation and material has been carried out, as indicated in the evaluation of the TRL.

Figure 2a: Metal-plastic hybrids with applied laser pre-treatment.

Nevertheless, the technology has not yet been widely established in industrial use. In addition to financial aspects, it is not sufficiently qualified towards the particular industry-specific, technical requirements – the productivity of the processing, the complexity of the defined geometry or the quality of the joint. For this reason, a combination of process acceleration and high-performance systems will be necessary to process at higher area rates.

To ensure the surface quality under industrial manufacturing conditions, an efficient quality assurance for the pre-treatment and the joining process is necessary. Integrable measuring systems, whose inline measurements enable process control, are an elegant approach in this case.

Additive manufacturing using laser metal deposition (LMD) with coaxial wire feeding

In science and industry, additive manufacturing of metals to apply protective coatings, to build up complex, lightweight structures and to repair worn or damaged components, is currently the subject of intensive R&D. For e-mobility, the ability to produce lightweight structural components is of particular interest.

Commonly, powder or wire is continuously and locally fed to a substrate into a laser-induced molten pool. Areas of application are still emerging through specialised additive manufacturing processes, such as wire-based coaxial LMD, as shown in figure 2b.

Figure 2b: High-speed camera image of LMD using coaxial wire feeding.

One advantage of using wire as feedstock is a 100 per cent usage of the material. Additionally, the effort required to protect the operator and the environment is significantly reduced compared to a powder-based LMD process. Recent developments in laser optics that enable coaxial wire feeding in the centre of an annular laser beam profile14-16 represent a relevant step towards an industrial relevance of wire-based LMD.

One of the main challenges of the process is that the implementation of a new system, as well as changes in influencing parameters, for example, through the use of a different material, require extensive parameter studies to achieve a stable process. For this reason, suitable closed-loop control approaches are currently an important part of R&D in LMD processes17. In this context, the qualification of sensor technologies for the real-time monitoring of process variables is crucial.

Considering the TRL evaluation, theoretical fundamentals of the beam-matter interaction were investigated, along with feasibility studies of the process. Further investigations are needed to increase the process reliability, as well as the automation of LMD and to integrate prototypes into a series of applications to reach a standardised industrial use.

Battery cell and module production

Within the context of battery production, laser materials processing can be used as a versatile tool to improve production steps or modify components. In lithium-ion cell-based battery storages, up to several thousand electrical contacts have to be manufactured18.

LBW – with its advantageous properties such as a high degree of automation, a low cycle time and a precise local energy input – is a promising technology19. However, LBW of highly electrically and thermally conductive materials, such as aluminium and copper, is challenging20. In several publications, the high reflectivity of copper using near-infrared laser radiation and the advantages of laser beams with wavelengths in the visible spectrum were discussed21, 22. A battery pack demonstrator welded using green laser radiation is shown in figure 3a. From the evaluated TRL it can be concluded that LBW for the joining of LIBs and cell connectors is about to achieve the technological maturity to enter serial or mass production. However, process windows for novel beam sources, challenging material combinations and an inline quality assurance need to be investigated more deeply.

Figure 3a: A battery pack demonstrator welded using green laser radiation.

Additionally, laser processes are promising with respect to the cell-internal contacting of LIBs19. In this production step, an arrester tab is welded to the uncoated parts of the metallic current collector foils of the electrodes, as shown in figure 3b. The weld seam ensures the mechanical and electrical connection. Commonly, ultrasonic welding is used for joining the electrodes. In comparison, laser welding enables novel product designs by a higher degree of geometric flexibility and thus improved seam properties23. LBW, with its high welding speed, contributes to an industrially-scalable process.

Figure 3b: Laser-contacted cell-cab of a lithium-ion battery.

In previous studies, laser beam sources emitting in the infrared and visible wavelength spectrum, as well as different welding strategies, for example, CW and pulsed welding, were investigated24, 25. The evaluation of the TRL showed that further investigations on suitable process parameters for a varied number of foils and the analysis of the thermal cell load during joining have to be carried out. Verifying the technology on large-format LIBs and the transfer to an industrial production facility are also necessary to increase the overall TRLs.

Furthermore, laser materials processing can be used to improve the electrochemical properties of LIBs. Increasing the proportionate share of active materials to passive materials, such as the casing, enhances the energy density of a LIB. Therefore, thick- and highly-compressed electrode coatings in the battery can be used. However, a decreased fast charging ability caused by diffusion limitations in the electrodes is a major drawback.

Introducing microscopic holes in the electrode coatings, as presented in figure 4a, can address this disadvantage. It was shown that short-pulsed laser beams are a suitable tool for the creation of such structures with micrometre precision26. Besides the enhancements of the charging and discharging performance of batteries with structured electrodes27, an increase in their lifetime also could be shown in empirical studies28. Furthermore, with laser structured electrodes a facilitated wetting with electrolyte was achieved29, 30.

Figure 4a: Scanning electron microscopy image of a laser-structured graphite anode

The TRL evaluation showed that the impact of electrode structuring on varying substrate materials and the thermophysical ablation phenomena are not fully understood. Additionally, for enabling industrial production, a strong increase in the process speed of laser structuring is required and industrial quality standards have to be met.

Moreover, laser materials processing advances new battery types such as the ASSBs. ASSBs with favourable performance characteristics, for example the high energy density, are considered to be a promising battery technology31. Market availability of ASSBs is expected in the next few years32.

However, no production routines have been established so far. The electrodes and the solid electrolyte layers must be cut to shape, as shown exemplarily in figure 4b, to assemble an ASSB. The solid electrolytes can be polymeric, ceramic or glass-ceramic materials33 and the highly reactive lithium metal is used as an anode material34, which requires the integration of the production equipment into an inert gas atmosphere.

Figure 4b: Laser-cut edge of a ceramic solid electrolyte processed with a picosecond pulsed laser system.

The evaluation of the TRL showed that parameter studies must be performed and quality measures have to be quantified. With production demonstrators, in-depth investigations under near series production conditions can be conducted. After that, the integration into pilot and industrial-scale equipment considering the periphery and quality monitoring systems should be initiated.

Outlook

Laser materials processing in the field of e-mobility can contribute to a significant improvement in many applications. As shown in this article, this includes the enabling of fast charging through the cell-internal contacting of current collector foils, the introduction of microscopic structures and the additive manufacturing of structural components. Additionally, laser-based manufacturing processes also reduce costs, due to their high processing speed and wear-free operation.

For the future, four major trends can be identified: new fields of application, sustainable production, cheaper beam sources and digitisation. As it was presented, laser materials processing is indispensable for e-mobility and it can be assumed that in the long term it will become an essential part of the production of hydrogen fuel cells since processes for cutting, surface modification and joining are also needed in this field – even more so for new materials.

In addition to stable and reliable manufacturing techniques to avoid scrap, other processes, such as rework or disassembly, will also be required for a sustainable production. Laser processes allow for rework due to their high flexibility. With the high precision of the laser beam, components can be disassembled without harming the surroundings, and therefore it can enable a process for products that cannot be disassembled otherwise. As a result, laser-based applications promote the circular economy. For safety-critical products, such as battery storages or hydrogen fuel cells, a 100 per cent quality assurance is essential. To be economically competitive, this can only be done by inline process monitoring and suitable evaluation algorithms. Digitisation can also extend the availability of laser sources through predictive maintenance and increasing throughput via data-based optimisation

Christian Geiger and Tony Weiss are research associates at the Department of Laser Technologies at the Institute for Machine Tools and Industrial Management, part of Technical University of Munich’s Department of Mechanical Engineering

References

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