CFRP is Demanding Also on Laser Cutting

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CFRP is Demanding Also on Laser Cutting The processing strategy is essential for cutting composites with lasers Frank Schneider and Dirk Petring Composite materials as glass fiber and carbon fiber

In competition with other materials as high-strength steel or light metals, the market forecast for carbon fiber reinforced materials predicts a continuous growth around ten percent per year as it

1 CFRP is Demanding Also on Laser Cutting The processing strategy is essential for cutting composites with lasers Frank Schneider and Dirk Petring Composite materials as glass fiber and carbon fiber reinforced plastics (GFRP and CFRP) are among the most promising materials to achieve the demanding goals of lightweight design. In competition with other materials as high-strength steel or light metals, the market forecast for carbon fiber reinforced materials predicts a continuous growth around ten percent per year as it was achieved in the recent years [1]. This underlines the importance of CFRP in the lightweight material mix. Aerospace and automotive are the major growth drivers in this market. Even though the material costs take a large share for parts made of CFRP, efficient production technologies are in demand. Fig. 1 Continuous remote multi-pass cutting with scanner and robot. Independent of the big variety of fiber and matrix materials and production technologies, most of the technology chains need cutting operations, e.g. for preparing precursor material or for trimming the final part. The standard cut processing methods for applications as trimming, edging or hole-making in consolidated CFRP components are mechanical milling and drilling next to waterjet cutting. The high strength and the high E-modulus make CFRP a difficult material for mechanical cutting and induce high wear on the tools. Main quality issues are delamination and splintering due to poor cutting action with worn tools. This clearly motivates to establish laser cutting as a contactand wear-free method. Besides common process criteria in laser cutting, as meeting geometrical tolerances and achieving a profitable cutting speed, an additional focus in cutting CFRP is to avoid thermally-induced degradation of the cut edges caused by the strongly differing thermo-physical properties of fibers and matrix material. Depending on the kind of material, the thermal stability of the matrix is in the range of 15 3 C. Polyamide 6 as a common matrix material for thermoplastic based carbon composites has a melting temperature of 215 C and a decomposition temperature of 3 C. However, the carbon fibers must be heated up to more than 3,6 C for sublimation. In consideration of the high heat conduction of carbon fibers, strategies for gentle heating of the material in thermal cutting processes are required. A common approach is to reduce the material-laser beam interaction time to a minimum by splitting the energy that is applied to transform the material into small portions with a temporal distribution that avoids heat accumulation, well explained with fundamentally motivated, approximating scaling laws for minimizing the heat-affected zone (HAZ) in [2]. With ultrashort-pulsed lasers and a fast beam-deflecting device to avoid excessive pulse overlapping, the best edge quality in CFRP laser cutting is possible. However, present drawbacks of using ultrashort pulsed or short pulsed lasers are low processing speeds and high costs of investment and operation. Moreover, even with the average power of these lasers being meanwhile available in the range of 1 kw at least in the academic sector, an excellent cut edge quality with a HAZ of a few 1 µm is only possible if high scan speeds in the m/s range and appropriate cooling breaks are applied [3, 4]. Cutting with cw lasers To achieve effective cutting speeds in the range of meters per minute for composites up to some millimeters thickness, multi-kilowatt cw lasers are still the best choice. As with (ultra)short pulsed lasers, the appropriate method to process CFRP is multi-pass remote cutting, i.e. scanning the same path several times until the depth of the generated kerf reaches the thickness of the material. High scan speeds reduce the beam- Institute Meet us at AKL 18 Exhibition Booth 6 Fraunhofer Institute for Laser Technology ILT Aachen, Germany With nearly 5 employees and more than 19,5 square meters net floor space, the Fraunhofer Institute for Laser Technology ILT, founded in 1985, is one of the most important contracting research and development institutes of its sector worldwide. Its experts develop and optimize laser beam sources and laser processes. In close cooperation with its clients, it uses laser technology to solve tasks for production, measurement technology, environment, energy, medical technology and biotechnology, all done in real life situations WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Laser Technik Journal 2/18 61

www.photonicsviews.com Number of scans [-] 1 1 8 6 4 5 1 15 25 3 35 4 Fig. 2 Number of scans required for cutting of a CFRP-GFRP hybrid material in 2 1.2 mm thickness.

Minimal HAZ vary in the range 5 µm to 15 µm [5 8], depending on processing parameters and material properties as matrix and fiber material, fiber volume content and fiber layup.

After the scan, the temperature of the kerf decreases.

Intervals between the scans limit the width of the zone next to the cut edge, where the degradation or melting temperature of the matrix is exceeded.

processing, because in contrast to carbon fibers, the glass fibers have a molten phase and a lower heat conductivity, allowing a process similar to laser fusion cutting of metals.

At the ends of the part, a metal plate is joined to the composites during the molding process.

The GFRP is reinforced with unidirectional (UD) carbon fiber tapes in the load direction at the long sides of the roof bow.

2 Number of scans [-] Fig. 2 Number of scans required for cutting of a CFRP-GFRP hybrid material in mm thickness. material interaction times within a single scan. Minimal HAZ vary in the range 5 µm to 15 µm [5 8], depending on processing parameters and material properties as matrix and fiber material, fiber volume content and fiber layup. During the irradiation time, the kerf bottom and walls and the material surface is heated by the laser beam and also hot emissions generate heat that is conducted into the material. After the scan, the temperature of the kerf decreases. The directly following scan starts on a higher temperature level of the kerf than the one before, resulting in a heat accumulation from scan to scan. Intervals between the scans limit the width of the zone next to the cut edge, where the degradation or melting temperature of the matrix is exceeded. In contrast to CFRP, GFRP, in particular GFRP consisting of randomly oriented fibers, are better to be cut by conventional, gas-assisted single-pass cutting instead of fast-scanning multipass processing, because in contrast to carbon fibers, the glass fibers have a molten phase and a lower heat conductivity, allowing a process similar to laser fusion cutting of metals. Because of their high absorption both in glass and matrix, CO 2 lasers are beneficial [9]. compression mold glass fiber PA6 composite with 4 vol% randomly oriented fibers. At the ends of the part, a metal plate is joined to the composites during the molding process. This joint reaches its strength without adhesives thanks to a pretreatment by laser microstructuring of the metal surface leading to a mechanical interlocking between the two materials. The GFRP is reinforced with unidirectional (UD) carbon fiber tapes in the load direction at the long sides of the roof bow. At the edge of these tapes, the part has to be trimmed to its final dimension, requiring a cut through CFRP in the top half of the material and GFRP in the bottom half, each 1.2 mm in thickness. The cut path is parallel to the carbon fiber orientation. For the trimming, a 5 kw singlemode fiber laser with a fiber diameter of 3 µm is used. The beam movement is carried out with a prototype galvo-scanner from Scanlab, working with an image ratio of 1 : 2.1 and a maximum scan speed of 35 m/s. This setup (Fig. 1) is chosen to meet the high-speed scan requirements for high quality CFRP cutting. However, to cut the material in one setup in this application, also the GFRP layer is to be cut by the multipass scanner process. Under the premise to use the maximum laser power of 5 kw to achieve high throughput, the scan speed and the a b interval time from scan to scan are the most relevant parameters determining quality and processing time. At low scan speeds, the material removal per scan is higher and thus less scans for complete separation are needed. Fig. 2 shows that the number of required scans increases roughly linear with the scan speed. The lower limit of applicable scan speeds is marked by the increase of the HAZ and an irregular flank geometry with uncontrolled excavated material. Fig. 3a shows this with the extreme example at a cut speed of.3 m/s that allows a complete cutting with a single scan. The heating of the edge leads to softening of the thermoplastic matrix and protruding carbon fibers. The use of unidirectional material, which is cut in the direction of the fibers, promotes long protruding fiber bunches along the cut flank compared to material with a multiaxial layup. At 2 m/s scan speed, edge delamination and a rounding at the upper side of the carbon fiber layer is still visible (Fig. 3b). The speed range with the best results for the carbon layer is between 5 and m/s. The cross-section of the CFRP section in Fig. 3c demonstrates this with a high flank contour accuracy and a HAZ that is too small to be clearly identifiable in comparison with the anyway existing irregularities of the base material. Even with most critical inspection, the HAZ is below 5 µm. c 45 µm CFRP-GFRP car roof bow demonstrator For the demonstrator part described in the following, a material sandwich from GFRP and CFRP has to be trimmed. The demonstrator is a hybrid car roof bow, based with its dimensions and requirements on an original part from the BMW series. The base material is a 1 mm 1 µm Fig. 3 Cut edge (top) and cross sections of cuts with scan speed.3 m/s (1 scan) (a), 2 m/s ( scans, 5 ms interval time; b) and 1 m/s (24 scans, 5 ms interval time; c). 62 Laser Technik Journal 2/18 18 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

3 Temperature [ C] 1.69 s exceeding softening temp. 16 C.35 s exceeding softening temp. 16 C GFRP CFRP Last scan to complete the cut The next important parameter to adjust the edge quality is the interval time from scan to scan. The course of the temperature can be evaluated with thermographic videos of the surface and the kerf in top view. The videos discussed in the following are recorded with an IR camera at a resolution of µm/pixel and Hz frame rate. Fig. 4 shows the temperature in the kerf, averaged over a longitudinal, mm long evaluation line positioned in the middle of the cut path. Every scan generates a peak, followed by a period of cooling by heat conduction into the material and convective heat transportation with the cross jet flow. The minimum temperature that is reached in the cooling intervals rises over time due to heat accumulation. Shorter interval times Last scan to CFRP GFRP complete the cut Fig. 4 Temperature vs. time in the kerf for two interval times 15 ms and 55 ms, demonstrating the scan-to-scan heat accumulation. The yellow line marks the softening temperature of the PA-matrix. lead to a faster increase of the temperature, as the course of the minimal temperatures between two scans shows in Fig. 4. Assuming a low-threshold critical temperature already at the softening temperature of 16 C instead of the melting temperature (215 C) of the PA matrix, for interval times of 15 ms after 16 scans, the critical temperature is permanently exceeded in the cooling intervals. With a longer interval time of 55 ms, this happens only in short times after each single scan. The total integral time during which 16 C is exceeded is 1.69 s at 15 ms interval time and.35 s at 55 ms interval time. The corresponding HAZ in the carbon tape is below 5 µm for the long interval time compared to nearly 4 µm for the short interval time. The conservative HAZ definition, applied in this article, is the maximum distance from the cut flank at which in cross-sections optical or structural changes such as discoloring, small cavities or edge delamination are visible by optical microscopy. The heat propagation over time on a line perpendicular to the cut path can be seen in streak images in Fig. 5. Every horizontal line in the graph represents the temperature on this 2.6 mm long evaluation line at a time step of 5 ms between the lines. The kerf width at the surface is parameter dependent and varies in the range of 3 45 µm. The kerf width tends to increase during the processing, but already after the first scan it is within this range that significantly exceeds the beam diameter by a factor of up to 3 5. This can be explained by secondary material removal mechanisms due to the laser-induced hot vapor emissions and scattered laser radiation forming an expanded heat source outside the original laser beam. If the fiber orientation is parallel to the cutting direction as it is the case here, this also contributes to the already described kerf widening, because in parallel cuts, long fiber segments separate from the composite just due to softening, melting, or vaporization of matrix material without cutting the fibers. This detachment is supported by induced vapor pressure gradients in the kerf. Fibers perpendicular to the cutting direction are fixed in the residual matrix. In contrast to cuts crossjet specimen kerf measurement line kerf width Length [mm] kerf width Length [mm] Temperature [ C] Fig. 5 Temperature field perpendicular to the cut path over time for two interval times (15 ms and 55 ms, scan speed 1 m/s), showing the higher temperature load of the edges at short interval times. Bright white lines at certain time steps of the IR video streak images are caused when the Hz video frame captures the hot laser process itself accidentally. These lines have to be excluded from evaluation as the temperature measurement is falsified due to oversteer. 18 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Laser Technik Journal 2/18 63

www.photonicsviews.com along parallel oriented fibers, the HAZ is expanded in these less-widened kerfs. The elevated temperature regimes in Fig. 5 are limited to the kerf width.

However, the difference in heat accumulation for the longer and shorter intervals is visible. Fig. 6 shows application-specific HAZ vs.

4 along parallel oriented fibers, the HAZ is expanded in these less-widened kerfs. The elevated temperature regimes in Fig. 5 are limited to the kerf width. Due to low heat conduction perpendicular to the fibers, the lateral extension of the temperature field beside the cut edges is moderate. However, the difference in heat accumulation for the longer and shorter intervals is visible. Fig. 6 shows application-specific HAZ vs. scan speed maps, separately for the CFRP and GFRP sandwich layers at varied interval times t Inter on three different routes of constant total processing time t total. In this specific application case, the scan path length is mm. The interval time includes the single-pass processing time for this path length, the time for the return pass with laser switched off and the break time before starting the next processing pass. The interval times are chosen such that the total processing time t total = N Scan. t Inter is constant on the selected routes. As the number of required scans N Scan is proportional to the scan speed v Scan, the interval times allowed for a given total processing time route are proportional to t total /v Scan. Under this premise, optimal ranges for the scan speed in the CFRP and GFRP layers can be read from the HAZ vs. scan speed maps. A low scan speed of 2 m/s leads to a considerable heat impact during each single scan. a 1 HAZ [µm] Total processing time 3, b 1 HAZ [µm] Fig. 6 HAZ vs. scan speed maps for three routes of constant total processing times, evaluated separately for the CFRP (a) and GFRP (b) layer of the hybrid material. At very high scan speeds to the right of the minimal HAZ regimes, the allowed interval times are too short and heat accumulation leads to a significantly increasing HAZ. For the GFRP layer, Fig 6b reveals that the influence of the interval times is much less pronounced than for the CFRP layer in Fig. 6a. A possible explanation is residue of molten glass at the walls of the kerf, releasing sustained heat even during rather long cooling intervals. That leads to a higher slope in the rise of the minimum temperatures in Fig. 4, starting after the first half of the cutting time, when the depth of the kerf reaches the GFRP part. However, according to both HAZ vs. scan speed maps, a scan speed of 5 m/s and an interval time of 25 ms can be identified as the best choice to cut both material layers of the sandwich with acceptable quality in the most efficient way. Here, we do not yet consider further advanced methods such as applying special scan patterns, non-constant open or closed-loop controlled interval times and material dependent change Total processing time 3, Interval time [ms] of parameters during the process when reaching the GFRP layer. With the described simple approach, a reasonable compromise between quality and productivity can be selected according to the demands of the specific application. In the case described here, the available scan field for processing the demonstrator part is mm². The contour that has to be trimmed is an 8 mm line at both long sides of the part. Thus, superposed axis of the scanner and a robot, moving the scan field over the demonstrator part, are applied to cover the whole cut length. During the continuous movement of the robot the scanner processes repeatedly a section of the cut path in the scan field. Fig. shows the processing. The speed of the scan field movement can be determined such that the number of required passes is exceeded, once the scan field has completely passed by. Using a scan speed of 5 m/s, the resulting proceeding speed of the scan field is 3. m/min. Fig. Cutting process of the car roof bow demonstrator out of CFRP / GFRP hybrid material. Fig. 8 Cut edge of CFK-titanium stacked material, processed in a combination of remoteand gas-assisted fusion cutting. 64 Laser Technik Journal 2/18 18 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Stacked CFRP-metal cutting If the material combination consists not of two composites but of a composite and a metal, the material can be cut in a two-step working operation. Fig.

To provide accessibility for the metal cut through the CFRP kerf, the kerf is widened by multiple lateral scans next to each other.

5 Stacked CFRP-metal cutting If the material combination consists not of two composites but of a composite and a metal, the material can be cut in a two-step working operation. Fig. 8 shows a cut edge of a CFRP-titanium stack. In a first step, the CFRP is cut with a multipass remote process until the depth of the kerf reaches the metal surface. To provide accessibility for the metal cut through the CFRP kerf, the kerf is widened by multiple lateral scans next to each other. In a second step, the metal is cut in a single pass with a cutting gas flow from a coaxial nozzle positioned close to the CFRP surface. With an adequate lateral and axial focus position during the metal cut, a smooth transition from the CFRP to the metal part of the flank is possible and the increase of the HAZ of the CFRP by the heat impact from the metal cut is minimized, e.g., below µm for the CFRP edge and an increase below 45 µm at the CFRPmetal contact zone of a CFRP-titanium stack with 1 mm total thickness. Acknowledgement This work was funded by the German Federal Ministry of Education and Research BMBF in the frame of the project HyBriLight under grant number 13N1218. DOI: 1.12/latj.1812 [1] Composites Marktbericht 1 CCeV/ AVK, media/2996/ccev-avk-marktbericht-1. pdf [2] R. Weber, T. Graf, C. Freitag, A. Feuer, T. Kononenko, V. Konov: Processing constraints resulting from heat accumulation during pulsed and repetitive laser materials processing, Opt. Express 25 (1) [3] C. Freitag, M. Wiedenmann, J.-P. Negel, A. Loescher, V. Onuseit, R. Weber, M. Abdou Ahmed, T. Graf: High-quality processing of CFRP with a 1.1-kW picosecond laser, Appl. Phys., A Mater. Sci. Process. 119 (15) 4, [4] S. Bluemel, V. Angrickb, S. Basticka, P. Jaeschke, O. Suttmann, L. Overmeyer: Remote Laser Cutting Of Composites With A Fibre Guided Thin-Disk Nanosecond High Power Laser, LIM 15, 8th International WLT Conference on Lasers in Manufacturing, Proceedings, München, DE, Jun 22-25, 15 [5] R. Staehr, S. Bluemel, P. Jaeschke, O. Suttmann, L. Overmeyer: Laser cutting of composites Two approaches toward an industrial establishment, Journal of Laser Applications 28 (16) 223 [6] A. Klotzbach, M. Hauser, E. Beyer: Laser cutting of carbon fiber reinforced polymers using highly brilliant laser beam sources. Phys. Procedia 12 (11) 52 5 [] D. Herzog, M. Schmidt-Lehr, M. Canisius, M. Oberlander, J.-P. Tasche, C. Emmelmann: Laser cutting of carbon fiber reinforced plastic using a 3 kw fiber laser, Journal of Laser Applications 2 (15) S281 [8] K.-W. Jung, Y. Kawahito, S. Katayama: Ultra-high speed disk laser cutting of carbon fiber reinforced plastics, Journal of Laser Applications 24 (12) 1 [9] F. Schneider, N. Wolf, C. Engelmann, W. Moll, D. Petring: Laser Cutting and Joining in a Novel Process Chain for Fibre Reinforced Plastics, LIM 15, 8th International WLT-Conference on Lasers in Manufacturing, Proceedings, München, DE, June 22 25, 15 Conclusion Multi-kW single-mode fiber lasers provide the necessary power and power density to cut consolidated FRP in a cutting speed range of meters per minute. A multipass processing strategy with well-adjusted scan speed and interval times between the scans has been demonstrated to achieve high-quality cut edges with acceptable productivity. With the described processing maps, also cutting parameters for hybrid material consisting of CFRP and GFRP can be identified. Trim cuts of this kind of material are demonstrated on a car roof bow. Since the cut contour is more than 8 mm in length, the scan field ( mm) is moved continuously over the workpiece, demonstrating the applicability of multipass processes with fast scanning also for bigger components. Authors Frank Schneider is senior project manager in the group Macro- Joining & Cutting at Fraunhofer Institute for Laser Technology ILT in Aachen. He studied mechanical engineering at RWTH Aachen University and received his degree in He finished his PhD thesis in the field of process control for laser cutting processes in 4. The focus of his research is high-speed and thick section cutting, cutting of composites, process control and combi-processing. Dirk Petring is head of the group Macro- Joining & Cutting at Fraunhofer Institute for Laser Technology ILT in Aachen, Germany. In 1986 he set up ILT s Laser Cutting Group. He received his PhD at RWTH Aachen University in In 1998, Dr. Petring took over in addition the responsibility for the joining activities at ILT. In 6, he co-founded Laserfact GmbH. Current projects of Dr. Petring s group at ILT provide advanced high-power laser solutions for cutting and joining. Dr.-Ing. Frank Schneider, Fraunhofer-Institut für Lasertechnik ILT, Steinbachstr. 15, 54 Aachen, Germany, phone , fax , frank.schneider@ ilt.fraunhofer.de, 18 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Laser Technik Journal 2/18 65

INVESTIGATIONS ON THE THERMAL LOAD AND ACTIVE THERMAL LOAD REDUCTION DURING LASER PROCESSING OF CFRP

21 st International Conference on Composite Materials Xi an, 20-25 th August 2017 INVESTIGATIONS ON THE THERMAL LOAD AND ACTIVE THERMAL LOAD REDUCTION DURING LASER PROCESSING OF CFRP R. Staehr 1, S. Bluemel