CAPS

Fraunhofer Cluster of Excellence Advanced Photon Sources (CAPS)

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New generation of ultrashort pulsed lasers and their application

Ultra-short pulsed lasers promise amazing possibilities. However, the technology has so far been limited by low laser power. The Fraunhofer-Gesellschaft has founded the Cluster of Excellence Advanced Photon Sources CAPS in 2018 to solve this restriction. Under the leadership of the two Fraunhofer Institutes for Laser Technology ILT in Aachen and the Applied Optics and Precision Engineering IOF in Jena, the participating research groups concentrate on developing laser sources and process technology for outputs of up to 20 kW. The goal is international technological leadership in laser systems that achieve the highest performance with ultra-short pulses (USP) and research into their potential applications. The new systems are to exceed all previous USP lasers by an order of one magnitude in average laser power. At the same time, the necessary system technology and possible applications in industry and research are being developed.

The Cluster of Excellence Advanced Photon Sources CAPS is designed to research not only beam sources but also process technology and applications. Partners from research and industry are invited to participate. The cluster acts as a "virtual institute" in which experts from the 13 Fraunhofer Institutes FEP, IAF, IIS, IKTS, IMWS, ISE, ISIT, ITWM, IWM, IWS, IZI as well as ILT and IOF offer their focused expertise.

We conduct research in these application areas

USP laser processes for high-performance processing of fiber-reinforced plastics for lightweight applications in multi-material design (CAPS FRP Processing)

Test part cut with ultrashort pulse laser and textured on the surface.
© Fraunhofer IWS
Test part cut with ultrashort pulse laser and textured on the surface.

Economical and automated manufacturing processes of FRP for use in lightweight applications

The use of modern materials, such as fiber-reinforced plastic composites (FRP), enables weight savings due to the excellent stiffness-to-mass ratio, reduces the consumption of fossil raw materials and reduces exhaust emissions from e.g. motor vehicles. The operating range of electric vehicles is increased, making them more attractive.
 

Production and further processing of pultruded fiber composite components

The transition from conventional materials to new lightweight materials is being inhibited due to manufacturing challenges that have not yet been solved. The properties and machinability of composites differ significantly from metals, so established metalworking manufacturing processes must be replaced by technologies 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) as well as for the production of reliable multi-material components, which firmly connect metallic elements with FRP components.
 

Technology gap

For multi-material components that also contain metal components, there is currently no satisfactory separation process. For the joining of FRP and metal components, load introduction elements are often integrated into fiber composite components. This can be done, for example, by additive processes. Bonding strength of the supplemented material are insufficient without surface pretreatment of the interface. Laser-based pretreatment in the form of selective fiber exposure or defined structuring of the FRP has shown promise in preliminary studies in combination with plastic injection molding and thermal spraying. However, the process speeds achieved to date are too low.

In the pultrusion process, metallic layers can also be integrated directly into the composite part. This improves the crash behavior of components and enables the subsequent use of conventional fasteners (screws, rivets). A prerequisite, however, is a strong material/form-fit connection in the interface between metal and FRP. Currently, the metal inserts are pretreated in a complex process involving powder coating or a subsequent, additional patching process. Here, too, a laser-based pretreatment process of the metal integrated into the process chain would be an attractive alternative to ensure the required bond strength between the metal layer and the pultruded component.
 

Use of USP lasers

As an alternative to mechanical processing, quasi-cold material removal with ultra-short pulse lasers offers the potential to overcome existing challenges, e.g., surface pretreatment of pultruded components for subsequent printing of tethering elements or integration of a laser-structured steel strip into pultruded components. The laser power available with the CAPS sources also opens up the possibility of multiplying the still too low process speeds for structuring but also for efficient cutting of metal-reinforced pultruded components.

The stated goal of this project is to significantly increase the marketability of hybrid materials and associated manufacturing processes, thereby revealing the potential of very high average power USP lasers for these applications.

High-rate functionalization of metal foils with an organic coating in roll-to-roll processes

Fraunhofer IWS developed picosecond DLIP setup with DLIP heads for wavelengths in the UV, IR and green range (from left to right).
© Fraunhofer IWS
Fraunhofer IWS developed picosecond DLIP setup with DLIP heads for wavelengths in the UV, IR and green range (from left to right).

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.

Structuring of electrical sheets for energy transfer using multi-kW cw lasers

Heat resistant laser treatment of grain oriented electrical sheet. Inserted structural defects maintain heat resistance.
© Fraunhofer IWS
Heat resistant laser treatment of grain oriented electrical sheet. Inserted structural defects maintain heat resistance.

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. 

Fundamental studies on laser shock peening with USP lasers and highest area rates for targeted influencing of residual stresses and material properties on component surfaces

Measuring set-up for the radiographic measurement of residual stresses using the Sin2ψ method.
© Fraunhofer IWM
Measuring set-up for the radiographic measurement of residual stresses using the Sin2ψ method.

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