Tag Archives: CFRP

Approaching Photonic Serial Production: Laser-remote-processing of Automotive CFRP Components

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By: Dr.-Ing. Peter Jaeschke, Laser Zentrum Hannover e.V.

The efficient use of limited resources is one of the greatest challenges of our times. To address this, lightweight solutions and concepts are already being adapted for the transportation industry, in particular within the automotive and aerospace sectors. However, in order to broaden the use of lightweight materials, there needs to be suitable processing, testing and measuring techniques in place for a variety of materials, constituting a prerequisite for economic, flexible and automated high volume production. In this context, photonic technologies can provide solutions. Since the operating mode of the laser is both highly flexible as well as no-contact, and thus wear-free, it offers numerous benefits for the machining materials, especially as an alternative to conventional processing methods encumbered by high tool wear. Furthermore, the energy input, tailored to the respective manufacturing requirements, offers new possibilities for the processing of temperature-sensitive materials.

In the supporting measures “Photonic Processes and Tools for Resource-Efficient Lightweight Construction” within the framework of the program “Photonics Research Germany“, the German Federal Ministry of Education and Research (BMBF) is aiming at overcoming existing constraints regarding the wide use of lightweight materials in serial production. For the corresponding R&D activities, the BMBF is providing a total amount of approx. 30 Mio. €. The initiative “Photonic Processes and Tools for Resource-Efficient Lightweight Construction“ is coordinated by Laser Zentrum Hannover e.V., Hannover, Germany (Figure 1).

Figure 1. The German initiative “Photonic Processes and Tools for Resource-Efficient Lightweight Construction“ is supported by the Federal Ministry of Education and Research and co-ordinated by Laser Zentrum Hannover e.V. (Source: LZH)

Within this research initiative, nine co-operative projects under industrial leadership are working on the development of laser sources and optical components as well as system technology and applications. In addition to welding and the surface preparation for both metallic parts and hybrid materials, the laser based processing of composites, particularly continuous carbon and glass fiber reinforced plastics forms the core issues of the BMBF initiative.

In this context, the main R&D activities focus on composite processing. This is comprised of cutting and drilling RTM parts, robotically-guided 3-D scanning optics and CFRP reparation preparation using short pulsed laser radiation. Other examples include composite surface preparation for adhesive applications, direct bonding, and joining of metal-metal interfaces as well as composite-metal hybrids. In the field of laser material processing, continuous carbon fiber reinforced plastic (CFRP) based parts and components represent a relatively new material class, exhibiting outstanding mechanical properties at a low density. As a result, such composites have been identified to have significant potential in lightweight construction for a wide variety of industrial applications.

During the manufacturing process of CFRP parts, trimming, drilling and ablation steps are of particular importance. Another point is the layer-by-layer removal to prepare the repair or rework of defects. In this context, conventional machining techniques, such as milling, drilling, grinding or abrasive waterjet cutting, which are well developed for a wide variety of industrially established materials, suffer from high tool wear, insufficient quality or their complex setup and limited flexibility when it comes to the requirements of CFRP machining.

The main reason for this is the heterogeneous composition of CFRP. Combining both carbon fibers, either arranged as fabrics or non-crimped fabrics, with a polymer matrix, either thermoset or thermoplastic, produces a unified material with very different individual material properties, and as results presenting a very unique challenge from a material processing perspective. Furthermore, for cutting applications both components have to be processed simultaneously which causes enormous difficulties. In this regard, the processing of CFRP components brings many challenges.

Photonic processes, however, offer solutions for many of these: Including the high flexibility and, in particular, the contactless, wear-free mechanism of the laser that offers advantages for the processing of CFRP materials. For the processing of complex components or temperature-sensitive materials, the locally limited and to the given manufacturing requirements adjusted energy input offers new opportunities. An implementation of laser-based processes in serial production in industry, however, requires a thorough understanding of the process, a high degree of automation as well as the consideration of environmental and occupational safety aspects.

If CFRP is processed with NIR lasers, carbon fibers show excellent optical absorption and heat dissipation, contrary to the plastics matrix. Therefore heat dissipation away from the laser focus into the material is driven by heat conduction of the fibers. The matrix is heated indirectly by heat transfer from the fibers. To cut CFRP, it is required to reach the melting temperature for thermoplastic matrix materials or the disintegration temperature for thermoset systems as well as the sublimation temperature of the reinforcing fibers simultaneously. One solution for this problem is to use short pulse nanosecond lasers, as has been demonstrated in one of the joint research projects, HolQueSt3D.

Figure 2. Towards serial production: Laser-Remote-Processing of automotive CFRP components (Source: LZH).

Based on an existing lightweight part used in the automotive industry, LZH has developed remote cutting processes for three-dimensional composite structures (Figure 2). A newly developed high-power disc laser of the TRUMPF Laser GmbH serves as the basic process technology. This fiber-guided laser source emits at 1030 nm and is providing a maximum average output power of 1.5 kW. With a constant pulse length of 30 ns, the maximum pulse energy of 80 mJ is realized for a repetition rate of 18.8 kHz. For remote processing of the automotive part, the KMS Automation GmbH has designed a clamping system, custom designed to address the specific requirements of laser processing of CFRP components. One of these requirements is an integrated exhaust system for the process emissions. The impact of laser processing on the characteristics of the components as well as on possible subsequent processes, e.g. primer and painting steps, has been investigated by the partners Volkswagen AG and INVENT GmbH.

Another priority was the development of repair concepts for 2D and 3D components. For this purpose, the LZH developed process strategies for the scarfing of defective areas. Due to the flexible system technology, it is possible to remove large areas on complex free-form surfaces. After the laser based repair preparation, the TU Clausthal developed repair concepts that work without hardening in autoclaves, making a more flexible and cost-efficient repair possible.

Furthermore the detection and analysis of the process emissions as well as the development of a catalytic exhaust air treatment system matching the requirements of laser-based CFRP processing played an important role. Based on the emission measurement during the processing, the Jenoptik Automatisierungstechnik GmbH has developed a fully regenerative, continuously working exhaust air cleaning system. As the involved end user, the Volkswagen AG supported the development of the process during the whole duration of the project, and evaluated its suitability for serial production. By processing an existing component used in automotive industry, the suitability of the developed processes was proven at the end of the project.

The supporting measures “Photonic Processes and Tools for Resource-Efficient Lightweight Construction“ within the framework of the program ”Photonics Research Germany“ is funded by the German Federal Ministry of Education and Research (BMBF). The author would like to express his gratitude to the corresponding overall project management VDI Technologiezentrum GmbH for their support. Furthermore the author would like to thank all coordinators of the involved co-operative research projects for their engagement and their support of the co-ordination work as well as all partners of the HolQueSt3D-project for their excellent work in a constructive manner.

Laser Cutting of CFRP Using a 30 kW Fiber Laser

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By Dirk Herzog, Matthias Schmidt-Lehr, Marten Canisius, Max Oberlander, Jan-Philipp Tasche and Claus Emmelmann

Today, industrial usage of Carbon Fiber Reinforced Plastic (CFRP) is steadily increasing, with an amount of 67,000 t/year. Latest products such as the Boeing 787 and Airbus A350 in the aerospace sector, as well as the BMW i3 from the automotive industry, consist of more than 50 percent of CFRP in their structural weight. At the same time these products also have comparatively high production volumes, in the five-digit range per year in the case of the BMW i3. Therefore, a higher degree in automation and cost-efficiency is needed in production. Due to the highly abrasive carbon fibers, conventional machining processes result in short tool life and high costs.

For that reason laser cutting of CFRP as a wear-free alternative has become the focus of several research groups. Two different approaches are commonly chosen: Cutting by short- and ultra-short pulsed laser systems to reach a process regime of cold ablation, and cutting with continuous wave (cw) lasers at high cutting speeds. For the latter approach, it has already been shown that by increasing power and cutting speed, the heat affected zone (HAZ) can be reduced due to less time allowed for heat conduction. Continue reading →

Ultra-short Pulse Laser Processing of CFRP with Kilowatt Average Power

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By: Christian Freitag1,2, Margit Wiedenmann1, Jan-Philipp Negel1, André Loescher1, Volkher Onuseit1, Rudolf Weber1, Marwan Abdou Ahmed1, Thomas Graf1

In order to process Carbon fiber reinforced plastics (CFRP) with a satisfying productivity, average laser powers >1 kW are necessary. Usually high average laser powers are achieved using continuous wave (cw) laser systems but the appearance of thermal damage ranging from 50 µm to several mm was observed using cw lasers for CFRP processing. According to model predictions the absorbed intensity has to be larger than 108 W/cm² to achieve a thermal damage smaller than 10 µm. Today such high intensities are conveniently achieved with ultra-short pulse laser systems. However, the average laser power of such laser systems is usually too low for productive cutting processes. The IFSW thin-disk multipass amplifier allows for the first time ultra-short pulse laser processing of CFRP at an average laser power of 1.1 kW with pulse energies of 3.7 mJ.

Fig. 1. A sketch of the experimental setup is shown.

Experimental setup

A sketch of the experimental setup used in this study is shown in Fig. 1. The laser source is a thin-disk multipass amplifier for 8 ps pulses with a maximum used average output power of 1100 W. The laser has a constant pulse repetition rate of 300 kHz which gives a maximum pulse energy of about 3.7 mJ. The laser emits at a wavelength of 1030 nm with a beam quality factor M² < 1.4. A fast scanner system was used leading to a maximum feed rate of the laser beam of 30 m/s. The resulting focal diameter was calculated to be about 125 µm (1/e² intensity level). The CFRP samples used were Toray T700S-12k carbon fibers with a RTM 6 matrix which is a monocomponent resin. The samples were processed by ablating on a circular path with a diameter of 50 mm in a multi-pass process.

Heat accumulation effects as a limitation

The heat affected area, where the matrix material is vaporized leaving blank carbon fibers, is called matrix evaporation zone (MEZ). The extent of the MEZ for different feed rates of the laser beam after 15 and 50 scans can be seen in Fig. 2. For both number of scans the extent of the MEZ becomes larger with decreasing feed rate. This is a consequence of the pulse-accumulation effect. Each laser pulse contributes to the heating of the processed material. This accumulation results in additional matrix damage if the temporal delay between consecutive pulses is too short for the material to cool down to almost its initial temperature. To limit the influence of the pulses-accumulation effect, the number of pulses applied at one spot should be reduced by choosing a high feed rate up to complete separation of the consecutive laser pulses.

Beside the pulse-accumulation effect, the scan-accumulation effect can also be observed in Fig. 2.  For

Fig. 2. The extent of the MEZ is shown for different feed rates at 1.1 kW average laser power after 15 and 50 scans.

an increasing number of scans, here from 15 to 50 scans, the MEZ increases significantly especially for low feed rates. The scan-accumulation effect is a major damaging mechanism when cutting CFRP with high average laser powers using a multipass process. Like the pulse-accumulation effect it also contributes to the increase of temperature in the compound but on a slower time scale. A characteristic parameter of the scan-accumulation effect is the number of scans above which the scan-accumulation effect causes additional matrix damage. The scan-accumulation damage develops, if a critical number of scans Ncritical is exceeded. Below this threshold, matrix damage is primarily caused by single-pulse damage or by the pulse-accumulation effect as can be seen in Fig. 2 for 15 scans. Above the critical number of scans, the matrix damage is mainly caused by the scan-accumulation effect as it is the case for 50 scans.

Cutting CFRP with high quality and high productivity

Fig. 3. Images of a cut in CFRP. a) Top view of the cut. b) Microscope image of a cross section of the cut. c) Magnified microscope image of the right, inner part of the cut. CFRP has been cut with a thermal damage smaller than 10 µm.

A rectangular shaped CFRP part was cut with 1.1 kW average laser power. To avoid the pulse-accumulation effect, a feed rate of 30 m/s was chosen. The influence of the scan-accumulation effect was reduced by a long cutting contour of 640 mm which increases the temporal delay between two consecutive scans. However, the scan-accumulation effect could not be completely avoided. Therefore, after each 200 scans, which are for this contour length still below the critical number of scans, a break of about 1 minute was implemented. The duration of this break was not yet optimized and is certainly much too long. The laser could be used for other processes during this break to further improve the productivity.

A view from the top on the cut work piece can be seen in Fig. 3a). It is noted that the gap between the inner and outer part does not represent the actual ablated kerf width. In total about 2100 scans where necessary to completely cut the material. With the applied feed rate of 30 m/s the effective average cutting speed was 0.9 m/min. By further optimization of the cutting process e.g. by ablating multiple parallel lines to increase the kerf width, an additional improvement of the effective cutting speed may be achieved.

The achieved quality of the cut can be seen in Fig. 3b) in a cross section. The inner part of the cut rectangle is shown on the right side while the outer part can be seen on the left. In Fig. 3b) some damage in the range of 200 µm is seen on the outer part.  In Fig. 3b) and in the magnification of this part in Fig. 3c) it can be seen that the inner part of the cut has no measureable thermal damage.

Conclusion

A novel ultra-short pulse laser system with an average laser power of 1.1 kW, 8 ps and 300 kHz was used to process CFRP.

Ablation experiments in CFRP with different feed-rates revealed the impact of the pulse-accumulation effect on the formation of the matrix evaporation zone (MEZ). For lower feed rates and therefore higher pulse overlaps the MEZ increases. A very important influencing factor on the MEZ formation is the scan-accumulation effect. This effect can lead to a burning of the matrix material and therefore to vast thermal damage. A characteristic value for the scan-accumulation effect is the critical number of scans above which the extent of the MEZ starts to increase very rapidly.

To demonstrate the capabilities of the used innovative laser source, CFRP has been cut with an effective average cutting speed of 0.9 m/min and no measureable thermal damage on the inner part of the cut rectangle.

 

 

1 Institut für Strahlwerkzeuge IFSW, Universität Stuttgart, Pfaffenwaldring 43, 70569 Stuttgart, Germany

2Graduate School of advanced Manufacturing Engineering GSaME, Universität Stuttgart, Nobelstraße 12, 70569 Stuttgart, Germany

Additive Manufacturing with High-Performance Materials, Lightweight Structures by Laser Metal Deposition and Infiltration

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By Frank Brueckner, Mirko Riede, Thomas Finaske, André Seidel, Steffen Nowotny, Christoph Leyens, Eckhard Beyer

Laser Metal Deposition (LMD) is used for repair/redesign as well as for manufacturing of new parts. Thereby, wire or powder filler material is reabsorbed in the laser-induced melt pool resulting in a strong metallurgical bond with the subjacent substrate in combination with a low dilution. Among various applications, LMD is an attractive process for jet engines to improve performance and efficiency as well as to contribute to more sustainability. In addition to design methods, such an improvement can be realized by lightweight structures and high-performance materials. Figure 1 shows the specific strength as a function of the temperature of high-performance materials. Since PMC structures are very important in the first stages of a jet engine, TiAl, Ni-base superalloys as well as CMCs are more relevant in hot engine areas.

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High-Efficiency Laser Processing of CFRP

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By Rudolf Weber and Volkher Onuseit

The benefit of CFRP for lightweight construction in automotive and airplane industries is widely accepted. Impressive pictures of high-performance cars and airplanes with numerous high-tech looking carbon fiber parts are familiar to everybody.

However, industrial large-volume application of CFRP requires efficient and high-quality processing. And of course, the laser is a very promising tool. Its advantages and disadvantages have been discussed in numerous papers in the last few years. Continue reading →