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Physics Procedia 41 (2013) 415 – 420

Lasers in Manufacturing Conference 2013

High power laser cutting of fiber reinforced thermoplastic polymers with cw- and pulsed lasers F. Schneider*, N. Wolf, D. Petring Fraunhofer Institute for Laser Technology, Steinbachstrasse 15, 52074 Aachen

Abstract Glass fiber and carbon fiber reinforced polymers with thermoplastic matrix enable high volume production with short cycle times. Cutting and trimming operations in these production chains require the use of high average laser power for an efficient cutting speed, but employment of high laser power runs the risk to induce a wide heat affected zone (HAZ). This paper deals with investigations with cw and ns-pulsed CO2-laser radiation in the kilowatt range in single-pass and multiple-pass processes. Using multi-pass processing at high processing speeds of 100 m/min and above a reduced heat affected zone in the range of 100 μm to 200 μm could be achieved by the ns-pulsed radiation. With cw radiation at the same average power of 1 kW however, the HAZ was 300-400 μm. Also employing ns-pulses in the kW-range average power leads to heat accumulation in the material. Small HAZ were obtained with sufficient break times between subsequent passes.

2013 The Authors. Published by access Elsevier B.V. © 2013 The©Authors. Published by Elsevier B.V. Open under CC BY-NC-ND license. Selection and/or peer-review under responsibilityunder of the German Scientificof Laser (WLT e.V.) Selection and/or peer-review responsibility theSociety German Scientific

Laser Society (WLT e.V.)

Keywords: laser cutting; fiber reinforced polymers; CFRP; GFRP; ns-laser; heat affected zone

1. Introduction The use of glass or carbon fiber reinforced polymers (GFRP / CFRP) with thermoplastic matrix gains increasing importance in the automated production of fiber reinforced parts due to short cycle times in the production chain. Thermoplastic FRP can be reheated to press and shape the material, which is impossible with resin matrix material. This enables efficient production in huge lot sizes and production chains similar to

* Corresponding author. Tel.: +46-241-8906-426; fax: +46-241-8906-121. [email protected]

1875-3892 © 2013 The Authors. Published by Elsevier B.V. Open access under CC BY-NC-ND license. Selection and/or peer-review under responsibility of the German Scientific Laser Society (WLT e.V.) doi:10.1016/j.phpro.2013.03.096

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F. Schneider et al. / Physics Procedia 41 (2013) 415 – 420

those known from metal processing can be set up, e g. a production flow consisting of heating molding trimming. Molding and flexible production methods as tape laying [1] or fiber spraying [2] reach near net shape parts, but cutting holes and precise trimming remain as indispensable manufacturing steps. Therefore cutting processes, which feature a low thermal influence on the cut edge as an essential criterion for good cut quality and which employ a high average power to enable efficient production, are required. Several investigations show that the heat affected zone (HAZ) can be reduced by high intensities and short interaction times which minimize the thermal conduction by the fibers and consecutive matrix damage caused by the heat impact [3, 4, 5, 6]. With average power in the kilowatt-range short interaction times are typically realized by high processing speeds using scanner optics. Cutting 1kW and short pulse operation on a subprovide these features. To avoid heat accumulation in the material from pulse to pulse, the repetition rate of the laser and processing speed have to match [7]. With a repetition rate of 20 kHz the laser used for the following investigations allows an adequate pulse separation with standard scanner optics or machine axes. 2. Experimental In this study cutting results with single-pass and multi-pass processes are compared. The multi-pass process is a layer-wise removal of material in the direction of the top side of the workpiece till after several passes the groove reaches the bottom side and the material is completely cut. The speed for each pass is here referred to as the processing speed. The lasers used for the investigations are a CO2-laser in cw mode and a new high power short-pulsed CO2-laser with a pulse width of 500 ns at an average output power up to 1.5 kW with a repetition rate of 20 kHz. Processing speeds up to 5 m/s are tested, predominant speeds in these investigations are 2.5 m/s or below. The material under test is GFRP (roving glass/PA6, fiber volume 47%) in 1 mm thickness and CFRP (carbon/PA, fiber volume 45%) in 2 mm thickness, both twill fabric with 0°/90° orientation. 3. Results and Discussion Single-pass cutting of GFRP leads to a significant layer of molten glass on the cut edge at all tested cutting speeds up to the maximum cutting speed for complete separation (Fig. 1(a)). In contrast, with multi-pass processing at a processing speed of 150 m/min the amount of coalesced fibers on the cut edge is at least significantly reduced. The edge surface is regular and clean, as to be seen in Fig.1(b).


a) GFRP, 1 pass@2kW cw 15 m/min 20 m/min

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b) GFRP, 7 passes@2kW cw 150 m/min

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Fig. 1. (a) Cross sections of single-pass cuts in GFRP (1mm thickness) with varying speeds up to maximum cutting speed compared with (b) multi-pass cut with cw-CO2-laser radiation (cross section and cut edge).

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The ratio between processing speed and the number of passes needed for complete separation is approximately constant. Thus, the process efficiency is not affected by changing the processing speed and distributing the ablation process over different numbers of passes (Fig. 2). Multi-pass processing of CFRP at 100 m/min using a CO2-laser in cw operation mode leads to a significant heat affected zone (HAZ) with a width of 300-400 μm (Fig. 3, left), increasing with the number of passes, and an irregular ablation of fibers. Significant improvement in edge quality can be obtained with the short pulsed CO 2-laser. A peak power up to 200 kW at 1 kW average power leads to a reduced HAZ between 0.1 and 0.2 mm. In Figure 3 multi-pass cuts with 100 m/min processing speed at 1 kW average power in cw-mode (left) and pulsed with 500 ns pulse width (right) are compared. The HAZ is reduced roughly by a factor of two and the surface of the cut edge appears to be less carbonized with the ns-laser. This observation has to be investigated in more detail. The same is true for the result that the advantages of the pulsed mode compared to the use of cw lasers are less substantial with higher cw average power.

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Fig. 3. Cut edges of C/PA [(0,90)]9, 2.1 mm thickness, multi-pass cuts with 100 m/min processing speed. CO2-laser in cw mode (left) and pulsed, 500 ns pulse width (right).

As to be seen in figure 4, the HAZ can significantly be reduced by high processing speeds. Figure 5 shows the influence of the processing speed on quality by cross sections of the cut edge. Up to 50 m/min processing speed re-solidified matrix material can be observed. For the processing speed of 100 m/min and above the HAZ is below 200 μm. In this regime the HAZ is characterized by areas of excavated fibers that were orientated in cutting direction and separated from the composite because heat is transported by adjacent fibers perpendicular to the cut direction. This weakens or melts the matrix at the cut edge and fibers are detached. Re-solidified matrix in the HAZ inside the material is marginal at that speed.

Fig. 4. Dependence of the heat affected zone (HAZ) on the processing speed (left) and number of passes to reach a full separation cut versus processing speed (right) for C/PA [(0,90)]9 (2.1 mm thickness) . Parameters: CO2-laser, 1 kW average power, 500 ns pulses, cross jet gas assistance.


Fig. 5. Cross sections of cuts in C/PA with a variation of processing speed and details of the cross section for 50 m/min and 100 m/min. Parameters as in Fig. 4

The short-pulsed laser radiation with high peak power enables effective ablation of the carbon fibres. Remaining damage of the matrix material is induced by the huge amount of hot sublimated material in the kerf and potentially by scattered laser radiation. Even if heat accumulation is reduced by a cooling break between subsequent passes, repeated absorption of radiation with low intensity at the edge of the beam and escaping vapor having to proceed up to the groove entrance during each pass leads to thermal damage of the matrix material. The importance of the break time t between subsequent passes to reduce heat accumulation is shown in Fig. 6. The reduction from t=1.6 s to t=0.6 s results in a significant increase of the HAZ from below 200 μm to over 1 mm.

Fig. 6. Multi-pass processing with different time intervals between subsequent passes. Heat accumulation leads to increased HAZ. (C/PA [(0,90)]9, 2.1 mm thickness, 1 kW average power, 500 ns pulses, 100 m/min processing speed)

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4. Conclusion Cutting of FRP with ns-pulsed CO2-laser radiation in the kW-range was performed to achieve high cutting rates and a small HAZ. By multi-pass processing of CFRP with 2.1 mm thickness the width of the HAZ could be reduced down to 100 μm. This could be achieved by using high processing speeds of at least 100 m/min and sufficient break times for cooling in the range of 1.5 s. Cw-laser Cutting of CFRP at the same average power results in a significant wider HAZ, whereas for GFRP also with cw-laser radiation in multi-pass processing cut edges with high quality can be obtained. To avoid the accumulation of heat in the workpiece, processes with effective assistant gas flow have to be developed. At high laser powers as they are required for high cutting rates high evaporation rates occur. Only if the evaporation products are instantly removed a detrimental heating or even degradation of the cut edges and an excessive scattering of laser radiation can be prevented.

5. Acknowledgements This work is in parts funded by the German Federal Ministry of Education and Research (BMBF) within the programme (funding number 02PJ2073 ) and managed by the Project Management Agency Karlsruhe (PTKA), and in parts funded by the European Union withi - Integrated Process Chain for Automated and Flexible Production of FibreReferences [1] Brecher C, Kermer-Meyer A, Steyer M, Dubratz G, Emonts M. Laser-assisted thermoplastic tape laying, JEC Composites 2010, Paris (France 2010); no56, pp. 71-73 [2] Böttcher A, Pöhler M, Rösner A, New Process Chain for Fiber Reinforced Lightweight Components, Kunststoffe international 2012/05, pp. 36-39 [3] Dittmar H, Bluemel S, Jaeschke P, Stute U, Kracht D, Advantages and Challenges of CFRP Laser Machining with ns-Pulses, ICALEO 2012, Proceedings of ICALEO 2012 [4] Klotzbach A, Hauser M, Beyer E, Laser Cutting of Carbon Fiber Reinforced Polymers using Highly Brilliant Laser Beam Sources, Physics Procedia, Volume 12, Part A, 2011, pp 572-577 [5] S. Katayama, K. W. Jung, and Y. Kawahito, High power laser cutting of CFRP, and laser direct joining of CFRP to metal, in Proceedings of ICALEO2010 (LIA, Anaheim, CA), pp. 429 434. [6] Kwang-Woon Jung, Yousuke Kawahito, Seiji Katayama, Ultra-high speed disk laser cutting of carbon fiber reinforced plastics, J. Laser Appl., Vol. 24, No. 1, February 2012 [7] Weber R, Freitag C, Kononenko T, Hafner M, Onuseit V, Berger P, Graf T, Short-pulse Laser Processing of CFRP, Physics Procedia, Volume 39, 2012, pp 137-146