Macro Processing: CFRP (LiM 2015)

Laser Cutting and Joining in a Novel Process Chain for Fibre Reinforced Plastics
Frank Schneider, Christoph Engelmann, Norbert Wolf, Wolfgang Moll, Dirk Petring

Laser transmission joining and cutting are presented as the final steps of a novel process chain comprising fibre spraying,
variothermal consolidation, joining and trimming of glass fibre reinforced parts to promote efficient manufacturing of
light-weight components in high volume production.
The transmission joining is demonstrated up to a material thickness of 2 mm of the transparent joining partner (glass fibre / polyamide with 60 wt.-% fibre content). With this joining technique closed cross sections can be generated to increase the stiffness of the component thereby supporting lightweight design. For the trimming, a CO2-laser was used operating either in continuous wave mode (cw) or in pulsed mode with a pulse width of a few hundred nanoseconds and an average power >1 kW. For both operating modes appropriate process regimes were identified for GFRP cutting with a focus on cw-mode for the component production due to higher productivity. Single-pass cutting with coaxial assist gas as well as multi-pass cutting has been investigated. Also some comparisons to cutting of CFRP and to cutting with fibre laser are provided. The selected demonstration part is a seat component for trucks, which was welded with a circumferential seam and trimmed at the edge of the seam in the welded zone.

Keywords: GFRP; CFRP; laser cutting; laser joining; process chain


Analytical Model for Laser Cutting of Carbon Fiber Fabrics: Maximum Cutting Speed and Heat Affected Zone
Alexander N. Fuchs, Thomas Woldrich, K. Manfred Heimhilger, Michael F. Zaeh

Laser cutting of carbon fiber reinforced plastics (CFRP) and carbon fiber textiles offers various advantages over conventional machining methods such as milling, water jet cutting, and ultrasonic knife cutting. The process is force and wear free, highly automatable and offers a high quality cut. Yet, it is not used on a large scale in industry. This is partially due to the anisotropic behavior of the material, which makes it hard to predict the cutting speed and the heat affected zone in relation to the angle between the fibers and the direction of the cut. In the presented study, an analytical model is developed, which predicts the heat affected zone and the maximum cutting speed for cutting non-crimp fabrics made from carbon fiber. The temperature distribution is modeled using a modified line heat source model which accounts for the anisotropic behavior of the fibers. The cutting speed is calculated by using an energy balance. The results of the analytical modeling are compared to those of an empirical analysis. The model will also be suitable for consolidated CFRP consisting of fiber and resin.

Keywords: Analytical Model; Laser Cutting; Carbon Fiber; CFRP; Non-Crimp Fabric


Ultrafast Lasers Jump to Macro Applications
Martin Griebel, Jan Langebach

Ultrafast Lasers have been proven for several micro applications, e.g. stent cutting, for many years. Within its development of applications Jenoptik has started to use ultrafast lasers in macro applications in the automotive industry. The JenLas D2.fs-lasers with power output control via AOM is an ideal tool for closed loop controlled material processing. Jenoptik enhanced his well established sensor controlled laser weakening process for airbag covers to a new level. The patented process enables new materials using this kind of technology. One of the most sensitive cover materials is genuine leather. As a natural product it is extremely inhomogeneous and sensitive for any type of thermal load. The combination of femtosecond pulse ablation and closed loop control by multiple sensor array opens the door to a new quality level of defined weakening. Due to the fact, that the beam is directed by scanning equipment the process can be split in multiple cycles additionally reducing the local energy input. The development used the 5W model as well as the latest 10W release of JenLas D2.fs and achieved amazing processing speeds which directly fulfilled the requirements of the automotive industry. Having in mind that the average cycle time of automotive processes is about 60s, trials had been done of processing weakening lines in genuine leather of 1.2mm thickness. Parameters had been about 15 cycles with 300mm/s respectively resulting in an average speed of 20mm/s and a cycle time even below 60s. First samples had already given into functional and aging tests and passed successfully.

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Effect of several gas ambiences on HAZ suppression in CFRP cutting with nanosecond laser
Yuji Sato, Masahiro Tsukamoto, Fumihiro Matsuoka, Kensuke Yamashita, Kenjiro Takahashi, Shinichiro Masuno

The laser cutting for a carbon fiber reinforced plastic (CFRP) is one of suitable way because of contact -free and high speed processing. However, a matrix material composed of CFRP was quickly decomposed before the carbon fiber deformation, which caused to form a heat affected zone (HAZ) which composed matrix evaporation zone (MEZ) and resin alteration zone (RAZ). The improvement of HAZ is the most important task in the laser cutting for CFRP. In this study, we have demonstrated that the CFRP plates were cut with a pulse fiber laser at pulse width of 0.4 ns, wavelength of 1064 nm and average power of 100 W under air and nitrogen (N2) gas flow.

Keywords: CFRP, pulse fiber laser, HAZ,


Analysis of potentially hazardous substances emitted during laser processing of carbon fiber reinforced plastics
Jürgen Walter, Christian Hennigs, Michael Huse, Michael Hustedt, Stefan Kaierle, Ludger Overmeyer

Lasers are promising tools to cut, ablate or even weld carbon fiber reinforced plastics (CFRP). Nowadays, it is possible to achieve very good product qualities (especially relevant for lightweight constructions) using specific laser processing strategies such as multipass cutting or short-pulsed ablation. The intention of these processing strategies is to minimize laser energy deposition within the material, because excessive heat causes defects such as pores, blowholes or delamination. Nevertheless, CFRP laser processing is connected with the emission of potentially hazardous substances, i.e. particles and fiber segments as well as inorganic and organic gases. This work describes a generalized approach to investigate hazardous emissions released during laser processing of CFRP. Measurement methods to quantify the emitted particles and gases in the exhaust air as well as in the air at the workplace are presented. These methods are applied to determine emission rates of specific material process combinations. The obtained values are implemented into a specific database to enable a fast access to the comparison with existing limit threshold values, maximum permissible exposures and adequate protective measures. The thorough analysis of various processes will help to assess the risks related to laser processing of CFRP in general and thus to create the framework, including standardization, for a safe handling of the emitted hazardous substances.

Keywords: laser; carbon fiber reinforced plastics; process emissions; measurement; hazards; risk assessment; occupational safety;
environmental protection


High Power UV Laser Processing of CFRP with Short ns Pulses and Pulse Splitting
Masayuki Fujita, Hiroshi Ohkawa, Toshihiro Somekawa, Takaomi Matsutani, Yoshinobu Maeda, Jim Bovatsek, Rajesh Patel, Noriaki Miyanaga

As the use of CFRP material in industrial applications increases, achieving high cutting speed andminimizing thermal damage remain the most challenging issues for laser-based processing. Among the various parameters in laser processing, we have focused on short-nanosecond pulses to investigate cutting speed and HAZ occurrenceusing a high power 355nm UV Quasar® laser from Spectra-Physics. The pulse width was varied from 2 ns to 10 ns and TimeShift™ pulse splitting with burst mode technology was also tested. In single pulse mode, the laser power was changed from 6 to 60 W by adjusting the repetition rate from 100 kHz to 1 MHz. To evaluate the effect of pulse splitting we compared ablation results achieved using a single 10-ns pulse with those achieved using two 5-ns pulses and five 2-ns pulses in a single burst. The samples used were PAN-based CFRP with thickness of 250 m. We measured the time to cut through the samples and evaluated HAZ for various parameter sets. It was found that shorter (single) pulse widths and pulse splitting were both effective at increasing cutting speed and reducing HAZ for the PAN-based CFRP.


Theoretical and experimental determination of the polarization dependent absorptance of laser radiation in carbon fibers and CFRP
Christian Wilhelm Freitag, Lukas Alter, Rudolf Weber, Thomas Graf

A detailed consideration of the fundamental mechanisms of interaction between laser radiation and carbon fibers is required to explore the potential of the laser as a tool for processing of carbon fiber reinforced plastics (CFRP). A key factor for laser materials processing is the absorptivity of laser radiation in the material at the laser wavelength that determines the fraction of laser energy coupled into the material. In the case of CFRP, the complex composite structure of the material requires sophisticated theoretical and experimental investigation of the absorptance. The absorptance was calculated modelled for a wide range of wavelengths and for an orientation of the electric field perpendicular and parallel to the symmetry axis of the carbon fibers. The model includes the nearly circular cross-section and the birefringent properties of carbon fibers as well as multiple reflections. For carbon fibers it was found, that the total absorptance is larger than 70% for wavelengths in the UV, VIS and NIR and drops to less than 40% for a wavelength of 10.6 μm (CO2-Lasers). The absorptance for light polarized perpendicular to the carbon fibers was shown to be larger than for light polarized parallel to the fibers. For CFRP the absorptance is increased to values around 80% for wavelengths in the VIS and NIR. Experimental measurements of the absorptance of carbon fibers and CFRP for laser radiation with a wavelength of 532 nm and 1047 nm were performed to validate the model predictions. Polarization states of the laser radiation perpendicular and parallel to the carbon fiber symmetry axis were investigated. The absorptance was measured to be about 90% for both wavelengths. The calculated higher absorptance for an orientation of the electric field perpendicular to the carbon fiber axis was verified.

Keywords: Micro Processing; Fundamentals and Process Simulation; CFRP; Carbon fibers; Absorptance


Metal meets Composite - Hybrid Joining for Automotive Applications
Christoph Engelmann, Daniel Meier, Alexander Olowinsky, Mathieu Kielwasser

Especially in automotive construction the bonding of dissimilar working materials is an important require ment. The combination of different working materials, such as thermoplastic composite materials and body steels, adapted to local loads should create new opportunities for further weight reduction. The presented approach consists of a microstructuring process to generate undercut grooves on the metal surface. In a second laser based process the thermoplastic composite material is melted and by external clamping the plasticized material is pressed into the generated structures and forms after curing a mechanical interlocking between both materials. In case of the presented PSA demonstrator a composite reinforcement bar has to be joined to a commercial vehicle door. At first suitable parameters for the surface treatment of automotive body steels P260 and XSG a re presented. For
the surface treatment a scanner based single mode fiber laser system is used, varying parameters like scan speed, laser power and number of iterations to generate microstructures in form of lines with undercut grooves.
Afterwards the results of a diode laser based simultaneous joining process are shown, whereby the parameters laser power and pulse duration are changed. To find out suitable joining parameters, which guarantee a homogeneous structure filling and high connection strength, flat shear tension specimen are joined and tested. The strength results are used to specify the bonding area between door and reinforcement bar to fulfill the requirements. At last all gained results are transferred to the demonstrator to join hybrid parts for crash tests at PSA.

Keywords: Joining; Hybrid Lightweight Components; Composite; Metal


Temperature monitoring independent of laser-beam-position during laser transmission welding of fibre reinforced thermoplastics
Hagen Dittmar, Verena Wippo, Peter Jaeschke, Helmut Kriz, Chris Beaver, Oliver Suttmann, Ludger Overmeyer

The heterogeneous heat conductivity in carbon fibre reinforced plastics (CFRP) critically influences process temperature during welding and increases the necessity for temperature control to secure weld quality. While infrared cameras are a powerful tool for monitoring process temperature they have a low temporal resolution and they are comparatively expensive. A potential alternative is sought by utilisation of a dynamically movable pyrometer measuring spot. In this work, heat development during bead-on-plate laser welding on carbon fibre reinforced polyphenylenesulfide (PPS) is investigated. The pyrometer spot is capable of measuring temperatures locally independent of the processing laser spot. Thus, temperatures are monitored at a high temporal resolution during welding in front of, behind, and aside the current weld area. Results of this investigation give rise to expectations that a dynamic pyrometer spot allows for precise temperature measurements at critical points of complex weld geometries such as curved weld seams. This will set up the basis for an automatic laser welding control, which will be capable of adjusting the laser welding power according to required temperatures for a supreme weld quality.

Keywords: laser transmission welding; composites; temperature monitoring; dynamic pyrometer spot


Cutting of CFRP with short-pulsed lasers at 1 µm and 10 µm wavelength and average powers of more than 1 kW
Margit Wiedenmann, Christian Freitag, Matthias Haug, Volkher Onuseit, Rudolf Weber, Thomas Graf

Simulations of laser processing of carbon fibre reinforced plastics (CFRP) show that laser intensities of more than 108 W/cm2 are needed to achieve thermal damage at the cutting edge of less than 10 μm (Weber et al., 2011). Today these high intensities are mainly achieved by pulsed laser systems with pulse durations below 1 μs. Furthermore an average power above 1 kW is needed to achieve industrial relevant cutting velocities in the range of 1 m/min. In this experimental study cutting of CFRP with two pulsed laser sources at the wavelength of 1 μm and 10 μm with an average power of more than 1 kW was investigated. The cuts were performed with fast laser scanners applying multiple passes to achieve complete separation of the processed parts. The cutting strategy was optimized in order to avoid heat accumulation from pulse to pulse and from scan to scan (Freitag et al., 2013). The 10 μm wavelength laser source was a pulsed CO2-laser prototype from TRUMPF with pulse duration of 170 ns and a repetition rate of 20 kHz. The peak intensity was 6·108 W/cm2. The achieved thermal damage was less than 50 μm with an effective cutting speed of 0.4 m/min. The 1 μm wavelength laser source was the IFSW-Kilowatt-Picosecond laser (Negel et al., 2014) with pulse duration of 8 ps, a repetition rate of 300 kHz, and an average power of more than 1.4 kW. The peak intensity was 7·1012 W/cm2. The minimal thermal damage was less than 10 μm with an effective cutting speed of 0.9 m/min. With these systems it could be shown that both short-pulsed laser systems with average powers of more than 1 kW are able to cut CFRP with industrial relevant cutting velocities. Furthermore with the correct process strategies the extent of the thermal damage is less than 50 μm.

Keywords: CFRP, laser processing, laser cutting, ablation depth, thermal damage, HAZ, short and ultrashort laser pulses;


Experimental and analytical description of the multi-wavelength remote-laser ablation process at fiber reinforced polymers
Andreas Fürst, Dominik Hipp, Michael Rose, Annett Klotzbach, Jan Hauptmann, Andreas Wetzig, Eckhard Beyer

To increase the acceptance of fibre reinforced polymers (FRP) in the industry, near net shape preforms with a minimum of material consumption are required. This should be accompanied by appropriate, fast and flexible processes. The remote laser processing expands the area of possible kinds of processing strategies, wherefore the laser can be a tool for the future. But the development of remote laser processing is accompanied with the understanding of the interaction between tool and material. Laser cutting processing of FRP is an ambitious process because of the inhomogeneity of both the reinforcement material and the polymer matrix material. The present paper shows an experimental set up for combining beam radiation with wavelengths of 1.07 μm and 10.6 μm. First results on carbon fiber reinforced polymers prove the increasing efficiency.

Macro Processing; Remote Laser Beam Processing; System Technology; CFRP; FRP


Remote Laser Cutting of CFRP: Increased Fatigue Strength as a Consequence of the Heat Affected Zone
Johannes Wolfgang Stock, Michael F. Zaeh

For the production of components made from carbon fibre reinforced plastics (CFRP), the cured parts need to be trimmed to the final outline. State-of-the-art technologies for this processing step are either milling or abrasive waterjet cutting. Although remote laser cutting is a wear free process, it is not yet used in industrial application. Recently, the heat affected zone (HAZ), resulting from the ablation of the material by the laser radiation, has been discussed as a quality criterion related to visible components.
To evaluate the influence of the cut edge on the fatigue strength, dynamic tensile tests were performed with an open hole specimen geometry. Noticeable higher values of the fatigue strength were observed using remote laser cutting compared to waterjet cutting. The specimens manufactured by remote laser cutting endured a higher average number of load cycles and showed a lower deviation of the fatigue strength than the control samples cut by a waterjet. Before fracture, only the laser cut samples showed a noticeable decrease of stiffness which could be relevant for purposes of structural health monitoring. Furthermore, the fracture patterns of the laser cut samples showed characteristics of a matrix based failure, whereas the waterjet cutting led to fibre fractures. This study confirms that remote laser cutting is suitable for structural CFRP parts which are exposed to dynamic loads.

Keywords: CFRP, laser Cutting, Machining, Fatigue, Strength, Tensile, Waterjet Cutting


Correlation between temperature field and heat affected zone during laser cutting of CFRP
Marten Canisius, Max Oberlander, Dirk Herzog, Matthias Schmidt-Lehr, Petter Ploog, Claus Emmelmann

Due to its ability for wear free 3D-processing and a high degree of automation, the laser remote cutting is a suitable technology for the mass production of carbon fiber reinforced polymers (CFRP) parts in the automotive industry. The heat affected zone (HAZ), which is typical for the laser process, is measured after the cutting process by a time-consuming and expensive preparation of cross sections. A missing in-situ measurement procedure for the cutting quality handicaps the laser processing of CFRP in an industrial application. The investigation deals with the question, if a relationship between the temperature field around the cutting kerf and the heat affected zone exists. The temperature field is measured by infrared camera while the corresponding HAZ is quantified conventionally by cross section preparation. The investigation shows that a positive correlation between the temperature field and the HAZ exists. Regarding the typical spreading of the HAZ during laser processing of CFRP, its prediction by measuring the temperature field is possible, independent of the laminates thickness, process parameters and cutting directions. Thus an in-situ measurement of the cutting quality is applicable. An algorithm to predict the HAZ reliable at areas wit h heat accumulation has to be developed.

Keywords: laser, remote, cutting, cfrp, temperature field, heat affected zone


Laser-remote-cutting of large-scale semi-finished carbon-fibre products using a solid state laser
Max Oberlander, Matthias Schmidt-Lehr, Dirk Herzog, Marten Canisius, Claus Emmelmann

Since carbon fiber reinforced polymers (CFRP) have excellent mechanical and weight-specific properties, and lightweight concepts become increasingly important, especially in the automotive and aircraft sector, the demand for automated, cost-efficient manufacturing processes such as laser cutting of CFRP and semi-finished products is quickly rising. This paper presents an approach of establishing a process of laser remote cutting large-scale 3D semi-finished carbon-fiber products using a high power continuous-wave (cw) laser. The result is a stable and fast process with a cutting speed up to 15 m/min. Flat and sealed cutting edges ensure a good handling as well as the possibility to drape and inject within an Resin Transfer Moulding (RTM) process.

Keywords: Macro processing, Cutting, System technology


Productive Laser Processing of CFRP
Volkher Onuseit, Torben Prieß, Christian Freitag, Margit Wiedenmann, Birgit Faisst, Roswitha Giedl-Wagner, Thomas Rettich, Rudolf Weber, Peter Middendorf, Thomas Graf

Laser processing of carbon fiber reinforced plastic (CFRP) is a very promising method to solve a lot of the challenges for large-volume production of lightweight constructions especially in automotive and airplane industries. Laser processes are very promising for tasks like cutting of dry fibers, trim cutting of parts after the curing process, or drilling of holes for riveting. The challenge for these processes is to reach both, the productivity and quality which is needed for large-volume production. In this paper processing with different laser sources including cw-lasers with high average power up to 6 kW and ps-lasers with average power from 30 W up to 1 kW will be compared in terms of productivity and quality for different applications. The main issue reducing the quality of laser-processed CFRP parts is the heat affected zone which results from heat conduction into the material. In the present paper, the influence of the heat affected zone on the mechanical strength of CFRP components will be discussed. It will be shown that damage-free laser processing of CFRP is possible when using high intensities above 108 W/cm² and avoiding any kind of heat accumulation in the processing zone. To reach this intensity it is favorable to use short and ultra-short pulsed lasers with pulse duration of several ns down to ps. However, the productivity of the process is dominated by the average laser power available which is actually below 150 W for commercial systems. Therefore it is necessary to increase the efficiency by using advanced process strategies.