Producing efficiently with light

Topic of the month

Lasers play a key role in the manufacturing of the future. Whether tailored 3D print of lightweight construction components or precise bores using an ultra-short pulse laser, the range of applications stretches across all sectors of manufacturing. Bundled light is a core element of Industry 4.0, in which the virtual, digital world is connected to real manufacturing.

The laser is the universal tool in production: It cuts, hardens, welds, polishes, measures, produces microstructures, traces errors and removes material. In the process, lasers impress with their high precision and speed. In contrast to mechanical tools, bundled light works on a contact-free basis and does not wear out even when processing high-strength steels or hardened glasses for smartphones. The fact that lasers are so widely used in production technology today is thanks in part to Fraunhofer.

In the last few decades, scientists, particularly from the Fraunhofer Light & Surfaces group (see box on page 8 and interview on page 14), have provided crucial impetus both in the development of new lasers and for their integration into production. Through research and development on behalf of laser manufacturers and innovative users, they contributed to Germany's current leading position in this market.

According to the industry report of the associations Spectaris, VDMA and ZVEI as well as the Federal Ministry of Education and Research (BMBF), approximately 35% of the ray sources sold worldwide and 20% of laser systems for material processing come from Germany.

However, the potential of lasers is far from exhausted. Fraunhofer researchers work on next-generation lasers, readying them for use in production. An example is the high-performance ultra-short pulse laser (USP laser). It produces light pulses that are only a few picoseconds or femtoseconds short (trillionths or quadrillionths of seconds), but are very rich in energy.

For the sake of comparison: While a ray of light requires approximately one second to go from the earth to the moon, in a picosecond it travels only 0.03 millimetres. Important foundations for the development and use of ultra-short pulse lasers were laid by experts of the Fraunhofer Institute for Applied Optics and Precision Engineering (IOF) in Jena and of the Fraunhofer Institute for Laser Technology (ILT) in Aachen.

For instance, in 2009 scientists of the IOF demonstrated an ultra-short pulse laser with a capacity of 830 watts. In 2010, the ILT experts in Aachen surpassed the magic mark of 1kW with a femtosecond laser. Since then, 1.5kW has even been reached at the ILT with a scaled version of the femtosecond laser. However, the researchers at the ILT do not work just on performance enhancement, but they also develop tailored ray sources and new applications for ultra-short laser pulses.

But what distinguishes ultra-short pulse lasers from traditional systems? "Thanks to the expert selection of the pulse duration, the pulse energy and the right focus, the material can be heated so quickly and so strongly that it evaporates without melting," explains Professor Andreas Tünnermann, Chairman of the Fraunhofer Light & Surfaces group and Head of IOF in Jena. Material removal takes place precisely and only where it should, micrometre by micrometre.

Such "cold processing" is not possible with conventional lasers. The latter produce heat influence zones. For example, if a laser ray comes into contact with metal, the material melts partially and unevenness can form. The material must then be elaborately post-processed. This costs time and money.

Producing with light flashes

For a few years, experts have already been using ultra-short laser pulses to process even highly sensitive materials precisely and gently. However, for a long time the process was used mostly just in research laboratories.

The first industrial applications have emerged only in the last few years. Thus, in cooperation with Bosch, Trumpf and Friedrich Schiller University Jena, IOF researchers managed to make ultra-short laser pulses into a successful series-production tool. The work of Prof. Stefan Nolte, who works at the Friedrich Schiller University and at the IOF, was an important pillar.

The physicist researched the interaction between laser radiation and material, thus creating the scientific basis for processing almost all materials with the energy-rich, ultra-short laser pulses. Both industrial companies developed the technology further, thus making it possible to integrate it into manufacturing and system technology for industrial series production. For this, the experts received the Federal President's Future Award in 2013.

Now, USP laser systems with outputs of up to 1kW are available on the market. For many industries, they open the way to new products that previously were extremely difficult to manufacture or impossible to manufacture. The technology is used particularly where materials must be processed especially gently and precisely. For instance, extremely fine nozzles for direct petrol injection valves as well as more compatible stents are produced with the new lasers or hardened glass is cut for displays in smartphones. The key challenge now is to combine the available laser pulses with suitable process technology and thus to develop further applications.

A possible new area of use for USP lasers is structuring lightweight construction materials such as plastics or carbon-fibre-reinforced plastics (CFP). The modified surfaces absorb metal powder better. Thus, even lightweight construction materials can be coated using the highly efficient cold gas spraying process (cold spray technology). In this process, the material is applied to the base material in powder form at very high speed.

The coated plastics or CFP are interesting particularly for the aerospace industry as well as the automotive sector. However, they enable a large number of applications in the electronics industry as well. With the cold spray technology, a copper layer that dissipates heat without air can be applied to non-conductive housing. In the joint EU project "Efficient Manufacturing of Laser-Assisted Cold-Sprayed Components" (EMLACS), researchers of the ILT are working with French, Dutch and German partners on the development of a corresponding process.

Ultra-short pulse lasers are of particular interest for the processing of glass, as they minimise voltages and thus possible damage such as tear formations. However, a sufficient understanding of the interaction between ultra-short laser pulses and the absorption effects in transparent materials has not yet been acquired. The "Femto Photonic Production" project aims to close this gap.

The objective is to lay the foundations for the material processing of glass, sapphire and diamond. Based on these results, the optimal performance parameters for the different laser classes, adapted optics and system solutions are then to be derived for all relevant material classes and subsequently evaluated together with the industrial partners in experimental studies.

The results are of particular interest for the manufacture of displays, modern LEDs or performance transistors for managing large voltages or currents. In the research project, which was launched in October 2014, experts from the Fraunhofer Institute for Laser Technology (ILT) and RWTH Aachen University, Chair for Laser Technology, collaborate with the ray source manufacturers Trumpf, Edgewave and Amphos as well as system providers 4Jet, LightFab and Pulsar Photonics.

Completely new manufacturing opportunities are opened up by selective laser melting (SLM). Crucial foundations for this generative manufacturing process were laid by researchers at the ILT as early as the mid-1990s. Since then, they have continuously developed the process, which was patented in 1996. In SLM, the component is built up layer by layer with powder directly on the basis of the computer-generated construction data of the planned workpiece (CAD) – without using binding welding fillers.

The starting material is mostly a metal powder that is melted selectively with the laser ray by means of local heat entry according to the calculated surfaces of the CAD model. The whole process basically works in a similar fashion to a printer, but in three dimensions. The process is now used in manufacturing – for example in tool construction, medical engineering, the automotive industry and the aviation industry.

Generative manufacturing offers numerous advantages. Neither special tools nor moulds are required. In addition, hardly any waste is produced – the excess powder can generally be reused. The extent to which generative laser manufacturing protects resources in comparison to traditional processes is shown in the example of blade integrated disk (BLISK) turbine manufacturing. Previously, these high-quality parts have been cut out of a huge material block.

However, a great quantity of the expensive material is lost in the process. Moreover, layer-by-layer production with laser surface cladding – in which a laser beam is aimed at the focus of a powder ray on the surface of the component to be processed – offers almost unrestricted scope for design and construction.

The engineers can design a component in such a way that it fulfils its function in optimal fashion, without regard to whether it can even be manufactured. "With generative manufacturing, almost any complex geometries whatsoever can be realised, including with internal structures. Thus components can be designed in a functionally optimised manner, without having to take restrictions of previous manufacturing processes into account," emphasises Dr.-Ing. Wilhelm Meiners from the ILT.

This makes the process of interest particularly for lightweight construction. For instance, ILT researchers used the SLM process to develop parts including a very lightweight traverse link bearer for a sports car, on which the wheels are hung individually. Thanks to a hollow structure in the interior, it is simultaneously lighter and more stable than cast or cut components. At this year's LASER World of Photonics trade fair, Fraunhofer, in collaboration with Materialise, is demonstrating how efficient the 3D technology is with regard to plastics. There, under the umbrella of the UNESCO Year of Light, they are exhibiting the word "LIGHT" in two-metre-high letters.

What is special about it is that the letters consist of a complexly formed airy grid structure that was manufactured by means of 3D print on Materialise's patented mammoth stereolithography unit.

Previously, companies have used generative manufacturing with SLM above all for small metallic components. To enable large components to be printed out by means of selective laser melting as well, researchers at the ILT developed a new system concept. "Instead of relying on scanner systems in the SLM process, in our system we use multi-spot processing – i.e. a processing head from which five individual laser beams come," explains Florian Eibl, a scientist at the ILT.

The advantage: The melting process is thus parallelised, meaning that even large parts can be produced quickly and without additional effort. The new system concept was developed, designed and constructed in the excellence cluster "Integrative Production Technology for High-Wage Countries".

With the generative manufacturing process, even components under a high thermal load can be produced from nickel super alloys. In order that such hard-to-weld or even non-weldable high-performance materials can be processed with bundled light, researchers of the Fraunhofer Institute for Material and Ray Technology (IWS) in Dresden are combining laser-powder resurface welding with induction.

"Through additional heat brought into the component locally and precise process control, the formation of hot and cold tears can be suppressed," explains Dr.-Ing. Frank Brückner from the IWS. Nickel super alloys are used mainly in stationary gas turbines or jet engines. They enable use temperatures of above 700°C. With the new technology, other novel high-performance materials such as intermetallic compounds made of titanium and aluminium can also be processed.

Bundled light for Industry 4.0

For a few years, researchers from IWS Dresden have been developing processes and the necessary system technology in order to produce components directly using metallic materials in virtualised process chains. In the project "Additive-generative manufacturing – AGENT 3D", they are working on first designing products on the computer and then manufacturing them directly in an automated process, without further intermediate steps, as products ready for installation.

The aim is to develop additive-generative manufacturing into the key technology of Industry 4.0. To this end, a consortium has been formed with 75 partners from business and science. The research project is part of the programme "Zwanzig20 – Partnerschaft für Innovation" (Twenty20 – Partnership for Innovation) supported by the Federal Ministry of Education and Research (BMBF).

How light can be used as a tool in production that will be increasingly digitalised in future is being investigated in Aachen at the "Digital Photonic Production" research campus. Behind the term "Digital Photonic Production" (DPP) is the concept of controlling laser radiation photons with bits (computer data) and using them to combine atoms into materials – at any level of complexity and at as low unit numbers as desired, at permanently low unit costs.

"The laser is the only tool that works as quickly as a computer thinks," explains Christian Hinke, who leads the group for integrative production at the Chair for Laser Technology of RWTH Aachen University and coordinates the DPP Initiative, which is being strategically promoted by the BMBF over the next 15 years. One of the initiators and spokespersons of the DPP Research Campus is Professor Reinhart Poprawe, Head of the ILT.

The following areas of focus are being worked on at the DPP research campus: Selective laser melting, the use of ultra-short pulse lasers and selective surface processing with innovative semi-conductor ray sources, whereby the light is radiated perpendicularly to the level of the semi-conductor chip. With such vertical-cavity surface-emitting (VCSE) lasers, surfaces can be refined selectively – i.e. on a space-resolved basis – in a very efficient manner.

The ILT brings already existing activities into the research campus – for instance the EUR 10 million-strong Fraunhofer innovation cluster AdaM. In the cluster, the ILT works with organisations such as the Fraunhofer Institute for Production Technology (IPT) on generative manufacturing processes with which components for aircraft engines and gas turbines for energy generation can be manufactured.

A key objective of the DPP research campus is to connect basic research, applied research and industry more heavily with each other. For this reason, the parties involved are testing new forms of cooperation, such as the enrolment model. Here, companies take up residence on the university campus and together with the scientists from RWTH Aachen University and Fraunhofer, they research topics that go beyond the short-term interest in new products.

The companies do not just maintain small offices on the campus; their experts are also actively involved in research and education. This facilitates the knowledge transfer between science and business. The researchers find out what industry is interested in. And the companies can convert current research results more quickly into new products. Industrial groups such as BMW, MTU, Philips, Siemens and Trumpf as well as small and medium-sized companies such as Amphos, Innolite, ModuleWorks and SLM Solutions are involved at the research campus.

The Federal Ministry of Education and Research (BMBF) is supporting the project for a total of 15 years with up to EUR 2 million per year. Additionally, by the end of the year an innovation centre – financed by private investors with more than EUR 11 million – will be completed, in which interested cooperation partners from industry can rent office rooms and laboratories in direct proximity to the ILT.

With their work, Fraunhofer researchers contribute to making production using light as a tool fit for the challenges of the future.