CAPS

Economical and Automated Manufacturing Processes of FRP for Use in Lightweight Construction Applications

The use of modern materials such as fiber-reinforced plastics (FRP) enables weight savings due to their excellent stiffness-to-mass ratio, reduces the consumption of fossil raw materials, and lowers exhaust emissions from motor vehicles, for example. The range of electric vehicles is increased, thereby enhancing their appeal.

Manufacture and Further Processing of Pultruded Fiber Composite Components

The transition from conventional materials to new lightweight materials is being hampered by unresolved challenges in manufacturing. The properties and machinability of composite materials differ significantly from those of metals, which means that established metalworking manufacturing processes must be replaced by technologies that are adapted to the challenges of the new materials. For large-scale production, there is a need for economical, automatable manufacturing processes for machining (e.g., cutting) and functionalization, as well as for the production of reliable multi-material components that firmly connect metallic elements to FRP components.

Technology Gap

There is currently no satisfactory separation method for components made of fiber-reinforced composite materials that also contain metal components. In the field of mixed construction methods, the interface surfaces between the different subcomponents play a decisive role in manufacturing and in the resulting load-bearing capacity. Examples of this are interfaces between FRP components and additively supplemented geometries or interfaces between FRP and applied functional coatings (e.g., by thermal spraying). Furthermore, composite components with integrated metal layers can be manufactured in the pultrusion process, which improves crash behavior and enables the subsequent use of classic fasteners (screws, rivets). In all these cases, adapted pretreatment of the interfaces between the materials is necessary in order to achieve resilient components through optimal material/form-fitting connections. In pultrusion, this is achieved, for example, by means of complex powder coating of the metal inserts, which, however, significantly reduces recyclability, among other things.

Laser-based pretreatment in the form of selective fiber exposure or targeted structuring of the FRP or other components represents a promising option for the sustainable and economical production of composite structures.

Use of USP Lasers

As an alternative to mechanical processing or chemical pretreatment, material removal with ultrashort pulse lasers offers the potential to overcome existing challenges. The laser powers available with CAPS sources also open up the possibility of multiplying the still somewhat uneconomical process speeds for structuring and efficient cutting of metal-reinforced pultrusion components.

The project aims to significantly increase the marketability of hybrid materials and associated manufacturing processes, particularly with regard to the use of ultrashort pulsed lasers with high output power.

Development Objectives

• Cutting of FRP components, especially those with integrated metal components
• Surface modification of FRP for the secure bonding of geometries in additive plastic printing
• Foreign material-free interface pretreatment of metal inserts for the pultrusion process
• Optimization and upscaling of interface pretreatment of FRP components for thermal spraying
• Testing of new coating systems (using thermal spraying) for extended functions in lightweight components: coatings for fire protection and abrasion protection

Previous Results of the Investigations

Laser Cutting

In the field of cutting composite materials, comparative investigations were carried out between established laser processes (cw high-power lasers) and UKP lasers. It was shown that multi-material composites made of metal and FRP can only be cut with high-quality UKP lasers. Even with high-power UKP lasers, comparatively long process times and the avoidance of heat-affected zones (HAZ) due to heat accumulation remain a challenge. This is particularly true for small cutting contours and larger material thicknesses of several millimeters.

When cutting FRP with low substrate thicknesses, it was demonstrated that high UKP laser powers can also be achieved through beam splitting in the process. This resulted in cutting speeds that were many times higher than with single beams – without any noticeable deterioration in the quality of the cut edges. For example, when cutting thin CFRP (<1 mm) with UKP lasers, better cut edges were achieved than with cw lasers (very low heat-affected zone, no spalling of the plastic matrix). 

Surface Structuring for Pretreatment

The feasibility of new technological approaches to surface structuring for improving joints in mixed construction methods has been demonstrated very successfully. Laser structuring of the interface surfaces led to a multiplication of the composite strengths. In the case of geometry construction using additive plastic printing on thermoplastic FRP, the bond strength was increased ninefold compared to the reference (without surface pretreatment or only targeted matrix reduction). This opens up new possibilities in the design and construction of lightweight components by combining established fiber composite construction methods with the potential of additive manufacturing.

During the pretreatment of steel sheets for further processing in the pultrusion process, it was demonstrated that laser structuring can replace the previous use of powder coatings. Composite strengths were achieved that exceeded the strengths of the FRP material. In previous shear tensile tests, laser-structured samples achieved strengths of up to 17 MPa. Reference samples with powder coating also achieved a maximum of 17 MPa. The investigations also showed that surface pretreatment with the laser is not the only dominant factor influencing composite strength. Further investigations are necessary to gain a comprehensive understanding of the sub-processes and their optimization. This opens up the possibility of manufacturing more sustainable lightweight components in the future.

Work on new coating systems and the functions they enable is currently still ongoing.

Lack of mechanical stability of hierarchical surface structures reduces application potential in industrial use

Surfaces with defined topographies are applied in various fields such as bioeconomy, medicine or also to increase resource efficiency and in climate technologies. Examples include surface topographies to reduce friction and improve cell and reduce bacterial adhesion. Despite the significant application potential in current and future lead markets, the potential of such surfaces has not yet been fully exploited. In many applications, this is due to the lack of mechanical stability. In particular, this applies to hierarchical surface structures, i.e. topographies consisting of the superposition of structures of different scales.

Although a large number of laboratory experiments now make it possible to reproduce such hierarchical structures artificially on a small scale, these manufacturing processes are both cost- and time-intensive and thus not very attractive from an economic point of view. Consequently, there is currently a lack of realistic application scenarios to realize the great potential of hierarchical structures on an industrial scale.
 

Photonic processes for large-scale fabrication of functional hierarchical structures

The overall objective of the submitted project is the development and use of photonic processes for the large-area fabrication of functional hierarchical structures on organically modified metal surfaces to develop long-term stable surface functionalities. For this purpose, a combination of several scalable roll-to-roll processes (R2R) will be used. Two structuring processes will be sequentially combined to fabricate the hierarchical structures. First, (sub)micrometer structures are written directly onto the metal foil using direct laser interference patterning in the R2R process. In the second process, the metal foils are coated and nanostructures are applied to the (sub-)micrometer structures using a plasma process, also in the R2R process, resulting in hierarchical structures in combination.
 

Opportunities for the USP laser

The main challenge for the proposed project is to achieve economically attractive process speeds for laser-based roll-to-roll patterning. The beam sources developed in CAPS and available in the user facilities offer the best prerequisites for this. High average laser powers in the kW range, which have not been available until now, open up the possibility of reaching process speeds in the range of up to 10 m/min structuring speed.

In the project it is planned to use the USP beam sources available at Fraunhofer IWS in the power range of 300 W combined with the remote processing optics developed at Fraunhofer IWS for the necessary preliminary investigations. The processes and systems developed in this way will then be transferred to the high power sources of the user facility. Only then will it be possible to achieve the targeted process speeds.

Efficiency enhancement of transformer and electrical sheets for energy savings

A globally increasing energy demand as well as the development and use of regenerative energy resources through the imperative reduction of greenhouse gas emissions and the expansion of electromobility are significant drivers for the development of electrical machines with the highest efficiency. Their efficiency is significantly influenced by the core material used in generators, transformers and motors. The property profile of the soft magnetic material used is crucial here. In addition to improvements in material design, laser-based surface processes offer particular potential for application-oriented and application-specific property modification.
 

Conventional process is being further developed

An established laser treatment technology for loss reduction of highly permeable grain-oriented electrical sheets, which are used as core material in transformers, is the so-called laser-induced  magnetic domain refinement (LMDR). In this process, thermal stresses are introduced into the electrical steel at a periodic distance of a few millimeters perpendicular to the rolling direction, which under optimum conditions can lead to a reduction in core losses in the range of approx. 10%. However, subsequent heat treatment of the laser-treated electrical sheets leads to a reduction of the previous optimization due to relaxation of the thermally induced stresses. If annealing or heat resistance is required, such as for toroidal cores, this can be achieved by means of structural defects. Instead of introducing thermally induced stress, trenching grooving by means of a mechanical rolling unit, chemical etching or laser treatment is a promising approach. However, compared to the other methods, laser treatment is characterized by relatively simple integration, a non-contact and thus closure-free characteristic, and high flexibility.

Feasibility studies at Fraunhofer IWS with high-brilliance cw laser beam sources in the kW range have demonstrated the fundamental use of laser technology for structuring grain-oriented electrical sheet. In addition to high process speeds of several meters per minute, a high quality of the trench structure is crucial for industrial implementation. The project focuses on the increase of the efficiency of grain-oriented electrical sheets as well as screen-printed electrical sheets with a new material design by laser treatment using multi-kW-USP lasers.To ensure the insulating effect of the thin insulation layer (2 µm to 5 µm), no material residues of the Fe-Si matrix may remain on the sheet surface during surface structuring. 
 

Use of USP lasers

The application of USP lasers offers the potential to meet the high quality requirements, which will be tested within this project. 

Increasing the service life of components – Laser shock peening instead of shot peening

Materials are increasingly used at the edge of their load limits due to lightweight construction, resource conservation (etc.). Therefore, processes that increase the (fatigue) life of components/parts are particularly interesting. Industrially, shot peening is widely used, which introduces compressive stresses into the surface layer and also strengthens it. However, only slight changes in the process parameters such as too high/too low pressure, working distance and quality/wear of the balls sometimes even lead to a weakening of the component (introduction of additional notches or defects). In order to minimize these negative effects, various quality controls, e.g. irradiation and evaluation of dummy material or Almen tests, must be carried out. If these quality controls are negative, the entire part is classified as a reject.

Alternative method – LSP

Laser Shock Peening (LSP) is a technology in which high-intensity laser pulses vaporize material on the surface to be machined, thereby inducing mechanical shock waves on the surface. At sufficiently high pressures, the associated local plastic deformation is used to solidify and harden the surfaces and build up residual compressive stresses.

Due to the long process times, the method is not yet competitive. In addition, most LSP systems require additional surface pre-treatment (usually coating) and/or the need for a reinforcing medium (especially in the case of ns short-pulse lasers, usually in the form of a flowing water film). The biggest disadvantage is the very limited area rate for continuous processing of the component surface, which is often only a few square millimeters to square centimeters per second and thus prevents economically viable use in large-scale production despite excellent surface properties.
 

Project goals – Use of USP lasers

We are investigating the optimal energy input of the high average power available with the CAPS source into the component surface by means of adapted beam focusing and dynamic beam shaping and are striving for the realization of area rates in LSP treatment up to the range greater than 5 - 10 square centimeters per second by making the best possible use of 

Material innovations in battery research

The shift in the automotive industry towards zero-emission powertrains calls for sustainable battery storage with high energy density. The projected annual production volume of battery cells for automotive applications is 8 TWh in 2030, making research into new battery materials a challenge with huge economic implications.

Decisive for the performance characteristics of battery cells are the electrode materials used and their micro- and nanostructure. The Advanced Battery Technology Center (ABTC) at Fraunhofer IWS focuses on innovative thin film anodes based on silicon or lithium, which promise a significant increase in energy density compared to today's graphite anodes. A key challenge of these material systems is their volume change over the state of charge of the battery. The resulting layer stresses and constantly changing interfaces to the electrolyte system are the cause of degradation and a relatively short service life of the new battery cells. Adjustments to the composition, microstructure and interfaces have a strong influence on the performance characteristics in some cases. For example, various material innovations have already brought significant progress in terms of improved lifetime combined with high energy density. A good understanding of the complex (electrochemical, mechanical, structural) processes and mechanisms is essential for further development. Thus, there is a growing need for advanced analytical imaging techniques, both for the further development of currently used battery materials and for research into new material systems.

Further development by imaging methods

Using established methods, i. e. optical spectroscopic methods (IR, NIR, VIS, UV) and investigations with the electron beam (SEM, TEM), only the surfaces of the materials and edge regions are accessible. A major challenge in the imaging investigation of battery materials is the structural conditions. Typically, these are particle sizes of 2 to 5 µm with an internal substructure in the nanometer range. In the electrodes, these particles form a porous layer about 100 µm thick. In the case of thin film anodes, which are being researched at Fraunhofer IWS and others, these are layers with a thickness of 1 to 20 µm consisting of Si or Li and an internal microstructure.

Novel EUV / X-ray sources offer the potential for imaging analysis with information that was previously inaccessible due to a high penetration depth and specific material contrasts. Areas of application thus arise in research and development, as well as in production-related quality assurance. Building on initial results (EUV microscopic imaging of Si/C composite particles) in the CAPS partner project 2020/21, the methodology is to be extended and transferred to analytics on thin film electrode systems.