POTENTIALS FOR LASERS IN CFRP PRODUCTION Paper # M1203

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1 POTENTIALS FOR LASERS IN CFRP PRODUCTION Paper # M1203 C. Loumena 1, M. Nguyen 1, J.Lopez 1,2, R.Kling 1 1- Alphanov, 351 Cours de la Libération, Talence, France 2- Université Bordeaux 1, Celia Umr 5107, 351 Cours de la Libération, Talence, France Abstract In lightweight construction, CFRP is a potential candidate to replace metals for many parts, if production costs can be reduced and quality can be improved. Indeed, composite materials combine high mechanical performances, low weight and good corrosion resistance. Laser processes, like surface activation and cleaning compete with conventional processes like abrasive jet machining, cryogenic cleaning and mechanical micro machining. In order to determine the specific advantages and drawbacks of different laser technologies we compare different laser sources and wavelengths on the application of surface ablation. Results are evaluated in terms of damage characterization and process speed and surface roughness. Introduction In comparison to metals, carbon fiber reinforced plastics (CFRP) are quite new materials finding applications in light weight construction. For example major aircraft manufacturers decided to increase the percentage of CFRP usage in their future versions to about 50% [1]. Due to their mechanical properties, among all the specific weight per tensile strength is superior to any other technical material used for construction. However, CFRP processing imposes some challenges as it is abrasive in machining and does not tolerate the input of heat as much as metals. Due to the composition of fibre layers, the delamination between sheets and therefore the failure of the whole structure can be induced by surpassing the temperature of degradation of the matrix material which is a polymer in most cases. The composition of fibres and matrix material is under rapid development and so is the processing as a response of the change of the material properties. Three groups of technologies are currently applied for CFRP processing: mechanical machining, abrasive waterjet cutting and laser ablation. Mechanical machining is a direct transfer of metal milling and drilling technology, where minor adoptions are required to the material properties: the tool temperature has to be controlled in order to avoid decomposition of the matrix material. The drawbacks are the high running costs as the tool wear is extremely high, the fiber pulls out and potential delamination due to mechanical forces. The waterjet solves two material specific problems in CFRP machining already: water acts as a coolant and prevents any thermal damage to the composite. In addition, the process is very cost effective as the running costs are very low, so the invest of the cutting machine quickly amortizes. For these reasons the process has successfully been integrated in aircraft manufacturing. However, the high pressure of the waterjet implies delamination forces to the fibre ply as well and any small defects in the laminate structure are charged with water and abrasive particles. Thus engineers are still seeking a force free alternative process while keeping the major advantages of the water jet. This has been introduced by the laser machining. In the early studies processing with CO2 lasers produced poor quality as cutting edges were burned and heat affected zones seemed to be excessive and unavoidable. Then, due to more fundamental research, the interaction of the anisotropic material with laser light is much better understood and the specific absorption behaviour has been taken into account to minimize the HAZ and to realize much higher quality machining [2]. Employing UV lasers improved the absorption behaviour of the polymer matrix which leads to a photochemical decomposition instead of a heat transfer through the fibre material [3]. Cutting with cw lasers with average power up to 16 KW demonstrated to reduce the HAZ in the kerf due to the shorter interaction time [4]. Advanced process strategies adapt to the anisotropic nature of CFRP and do not ablate the full volume. They only cut fibers at

2 the outline of each layer which are set free after the matrix is evaporated. Therefore, parts of the fibers remain undamaged and less energy is needed [5]. Up to now there are very few laser processes qualified for industrial production and a well determined control of the HAZ is still to be demonstrated. In addition high demands of process speed in combination with sheet thicknesses in the range of 1 15 mm still pose major challenges for the laser processing to prove their readiness for industrial use. In this regard, the ongoing studies at Alphanov investigate the ablation rates and surface quality of processing with different pulsed laser systems. The results then can be used as a calculation scheme for geometry specific applications in cutting, drilling and repair. Similar studies have already been carried out with a thermoplastic polymer as the matrix material [6] whereas this study focuses on an Epoxy based matrix. and a repetition rate range between khz. The beam diameter used is 22µm at is focus position. The workstation was made of a GSI LDS14 galvanometer scanning head with a 160mm F-theta lens for focusing laser beam onto the samples. The samples were mounted on controlled X-Y translational stages (Aerotech Pro Series) under the scanning head. We changed the pulse energy with a system composed by λ/2 plate and a polarizer cube in order to keep a constant pulse width. We also kept a circular polarization on sample by adding a λ/4 plate. The focused laser beam was characterized by a CCD camera (WincamD, Dataray Inc.). For fluence, we only consider the beam diameter at its FWHM (50%). Experimental Carbon Fiber Reinforced Plastic samples The CFRP samples of our experiments are composed by carbon fibers into an epoxy resin (Toho Tenax). The thicknesses of these samples are in a range of 500µm up to 750µm. The fibers orientation is 0/90 with a fraction volume between 60-65%. The minimum elastic modulus is MPa, a specific weight around 1,57 g/cm 3 and a thermal stability up to 125 C. Laser workstation In our experiments of CFRP processing, we used nanosecond lasers based on a new generation of Rod- Type Fiber (BOREAS series, Eolite Systems, ESI Group). All the specifications of BOREAS series lasers used are mentioned into the table below: Laser BOREAS IR80 BOREAS G30 Wavelength 1030 nm 515 nm Pulse Width Down to 12ns Down to 10ns Repetition Rate khz khz Pulse Energy Up to 2 mj Up to 0.3 mj Beam quality M² < 1.2 M² < 1.2 Beam diameter 50µm 32µm Table 1: BOREAS Series technical specifications We also used a new generation of compact femtosecond amplifier (S-Pulse HP², Amplitude Systemes) generating a laser wavelength of 1030nm with pulse duration of 500fs, a pulse energy up to 1mJ Figure 1: BOREAS Series technical specifications Sample characterization The inspection of irradiated samples was done with several metrological means. The characterization of the surface and the cross section was done with a scanning electron microscope SEM (FEI Phenom). The volume removed by laser processing was evaluated by a confocal microscope and by a microscope (Mitutoyo) coupled with an integrated dimension calculator. Procedure The goals of this study were to investigate the feasibility of CFRP processing by short and ultrashort laser pulses and to evaluate both the quality and the etch rate depending on the pulse energy, on the duration and on the wavelength used. For this purpose, we decided to etch a squared cavity, by scanning the laser beam onto the sample with a cross-hatch pattern (see below): Figure 2: Hatch and cross hatch processing

3 To determine the etching rate we measured the processing time until a defined depth was reached. In order to investigate the different laser/cfrp interaction processes, we had to set some experimental conditions like the 2D overlap, composed by longitudinal and lateral overlap. The longitudinal overlap is fixed by scan speed, the repetition rate and the diameter of the focused laser beam. The lateral overlap is fixed by the period of the cross-hatch. Therefore, we measured beam diameter at its focus point and we set up the scan speed related to the chosen repetition rate, in order to set these overlaps at 75%. The total volume removed by laser irradiation was evaluated by confocal microscopy or by using the formula below if the etching goes through the sample: V = (h/3)(b 1 + (B 1 B 2 ) 1/2 + B 2 ) Where h is the thickness of CFRP sample, B 1 the area etched on the top side and B 2 the area etched on the backside. Results and discussion In this part, we will summarize our latest results on CFRP processing. We will focus on the study of the etch rate, the heat affected zones and the roughness leaded by the different laser/matter interaction processes tested. Study of Heat Affected Zone Main issues [8] for laser processing of multiple component material such as CFRP are linked to highly different thermal and optical properties of resin and fiber. Indeed, absorption of CFRP is dependent of the wavelength of the laser beam. In the range of main laser ablation technologies, maximum of absorption is reached at 10.6µm (CO 2 laser) while minimum is reached close to 1µm wavelength (solid state laser). At 515nm, the absorption rises again but at a lower level than for 10,6µm. Therefore, for short wavelength laser ablation processes (<1,1µm), we can assume the fiber as full absorber whereas resin is transparent. For pulse width longer than 10ps, laser ablation is highly leaded by thermal process. Most part of laser energy is transferred to fiber as heat. As we can notice (cf. table 2) [2], fiber has anisotropic thermo-physical properties. Matrix Carbon fiber parallel to fiber axis Carbon fiber perpendicular to fiber axis Density ρ kg/m Heat Conductivity W/m.K 0, Heat capacity J/kg.K Evaporation temperature K Latent Heat kj/kg Structure damage temperature K Complex refractive 2,05+0,7i 3,1+2,1i Optical penetration depth nm ,3 angle of incidence 83,7 58,4 Table 2: Typical CFRP thermo-physical properties Heat transfer across materials of high thermal conductivity occurs at a higher rate than across materials of low thermal conductivity. Heat capacity characterizes the amount of heat required to change a substance's temperature by a given amount. Latent heat is the heat released or absorbed by a body or a thermodynamic system during a process that occurs without a change in temperature. A typical example is a change of state of matter, meaning a phase transition. These anisotropic thermo-physical properties of carbon fiber lead to anisotropic heat affected zones. Carbon fiber has huge heat conductivity along the fiber (ten times higher than perpendicular fiber axis). Therefore, heat is transferred preferentially along the fiber axis and evaporates the matrix resin when temperature rises beyond 800K. To illustrate this anisotropic effect, we process a circular drilling with each laser source, at the same pulse energy. We could compare the effect of the pulse width and the wavelength on heat affected zones. All the samples are imaged without cleaning.

4 For infrared femtosecond laser processing, we can notice a highly reduced heat affected zone along the fibers axis because of the ultra-short pulse width (500fs) is below than the time of heat transfer. The accumulation of pulses around the drilling hole could be the reason of small affected zones. If we focus on fibers, we can notice some dust on it. Femtosecond laser process usually causes pulverulent dust that can be removed easily. Figure 3: Drilling hole processed by IR nanosecond (top), visible nanosecond (middle) and IR femtosecond (bottom) We clearly see that heat affected zones are preferentially along the fiber axis. The heat transfer perpendicular to fiber axis is very limited (due to its low heat conductivity). These pictures tend to show that heat affected zones are linked to the wavelength and to the pulse width. Indeed, infrared (IR) nanosecond laser processing at 1030nm has slightly wider heat affected zones than visible nanosecond processing at 515nm and fibers seems to be more damaged. It could be explained by the transparency of resin matrix at 1µm wavelength. The temperature of fibers could reach a temperature higher than structure damage temperature threshold. For 515nm processing, part of incident energy is absorbed by resin, so heat accumulated by fibers is lower than for IR, and lead to reduce the temperature reached by carbon fibers. The fibers close to the edge seem less damaged and their sections more softened than for IR processing at the same pulse energy. Figure 4: Influence of fluence on Heat Affected Zone at low repetition rate (<20kHz) (top) and at high repetition rate (100kHz)(bottom) With these two graphics, we see a clear upward trend between fluence and the width of HAZ. Nevertheless, we can notice a very slight effect of the repetition rate for nanosecond laser processing. In general, high repetition rate increases the HAZ zone by ~50µm for the same fluence. It is due to the lower time between each pulse which can increase thermal effects. Finally, we can obtain small HAZ with nanosecond laser by using fluence close to the ablation threshold. For IR femtosecond processing, we can reduce the HAZ by a factor of 5 (up to 10) in comparison with IR nanosecond laser for fluence higher than 3J/cm². We clearly see the effect of pulse width for HAZ.

5 We also notice an effect of repetition rate on HAZ with femtosecond pulses. On low repetition rate (10kHz), HAZ begins to increase from 3J/cm² whereas for high repetition rate (100kHz), it begins at 0,5J/cm². The shorter time between each pulse at high repetition rate increases the heat accumulation, and so the HAZ on laminate sample. Reducing HAZ is a challenge for precise laser processing of CFRP. We can reach higher precision without delaminating, a limiting feature of conventional CFRP mechanical processing. Figure 5: Focus on honeycomb cutting shape with IR femtosecond laser on carbon/epoxy sample nanosecond (green dots) and IR femtosecond (purple dots) for low repetition rate. We can notice that for low fluence <1J/cm², all these laser technologies give the same etch rate per pulse, close to 250µm 3 per pulse. Under this fluence close to the ablation threshold, there is neither effect of the pulse width nor of the wavelength. Pulse energies do not transfer enough heating to increase the etch rate by thermal effects through fibers. From 1J/cm², the etch rate of each technologies highly differs. IR nanosecond has a logarithmic behaviour and rises over 10000µm 3 /pulse from 4J/cm² and 20000µm 3 /pulse from 15J/cm². Green nanosecond has smaller etch rate per pulse than IR one, and rises 3000µm 3 /pulse from 4J/cm². Finally, for IR femtosecond, the etch rate is still lower because we reach 2500µm 3 /pulse at 16J/cm². An ultra-short pulse width induces an etch rate 10 times lower than for short pulse at the same wavelength. Then, we will consider the etch rate per pulse versus fluence for high repetition rate (100kHz). Study of the etch rate In this part, we will focus on evaluating the etch rate of each laser technologies related to the incident fluence, the wavelength, the pulse width and the repetition rate. In order to compare these different laser/cfrp interaction processes, we consider the etch rate per pulse, and we split results for low repetition trials (<20kHz) and high repetition rate (100kHz). Figure 7: Etch rate per pulse vs fluence for high repetition rate We can notice an increase of the etch rate per pulse for IR femtosecond processing at 100kHz in comparison with 10kHz. For example, at 3J/cm², we get 230µm 3 /pulse for 10 khz and 300µm 3 /pulse for 100kHz. The shorter time between each pulse at high repetition rate could raise the heat accumulated by fiber and so increase the etch rate per pulse. Figure 6: Etch rate per pulse vs fluence for low repetition rate For nanosecond laser processing (IR and green) at high repetition rate, we can notice a decrease of the etch rate per pulse. In this graphic, we have plotted the etch rate per pulse versus fluence for IR nanosecond (red dots), Green

6 IR nanosecond (3J/cm²) Green nanosecond (3J/cm²) Etch rate at 10kHz ~8000µm 3 /pulse ~2500µm 3 /pulse Etch rate at 100kHz ~5800µm 3 /pulse ~1500µm 3 /pulse Table 3: Influence of the repetition rate on etch rate per pulse The etch rate per pulse could be saturated by the accumulation of heat induced by high repetition rate. It could reduce the etch rate efficiency of each pulse. Nevertheless, even if the etch rate per pulse is reduced in high repetition rate regime, we increase the mean etch rate because we have 10 times more pulses per second (5 times for green nanosecond). Here is the variation of the average etch rate reached at 100kHz in functions of the fluence. Figure 8: Average Etch rate vs fluence for high repetition rate Figure 9: Cutting edge with IR nanosecond processing Cutting edge quality Then, we will focus on the quality of the cutting edge in high pulse processing. We set up energy for all laser sources as the same level than the lowest high pulse energy used (green nanosecond 150µJ). We process a square etching with cross-hatch strategy through laminate. We cut up this with scissor in order to have a close look at the cutting edge with SEM (scanning electron microscope). With IR nanosecond laser processing, we can note a very smooth cutting edge. Fibers along the cutting direction (parralel) seem to be softly cut and welded together by the induced heat. Perpendicular fibers have same behaviour. No trace of resin remains on the cutting edge.

7 Figure 10: Cutting edge with Green nanosecond processing With green nanosecond laser processing, we also note a smooth cutting edge on perpendicular fibers. The main difference with IR nanosecond is on the behaviour of parralel fibers. The cutting side of these fibers seems to be very straight and not welded. Heat effects could be reduced and acoustic waves induced by nanosecond processing could break fibers. No trace of resin remains on the cutting edge. Figure 11: Cutting edge with IR femtosecond processing The cutting edge induced by femtosecond laser processing is very straight. We clearly see some layers of resin. Ultrashort pulses induce very short heat affected zones and remove fibers and resin without thermal damages. This cutting quality is to be weighted with the low etch rate reached. Holographic effect on CFRP Some operating conditions with femtosecond laser interaction may induce bright optical effects. The color vieries by changing the viewing angle with the laminate. In pictures below, we note that this effect is linked to nanometric ripples formation on carbon fibers.

8 Theses strcutures are currently studied on various metals for theirs advantages on silicon solar cells efficicency, on induced superhydrophobic surfaces, and surprisingly also arise on carbon fibers. wavelength. Generally, high fluences lead to extended heat affected zones, preferentially to fiber axis because of the highly anisotropic heat conductivity of carbon fiber. We can reach small HAZ by reducing the wavelength of the laser beam or by using ultrashort pulses as femtosecond. Smaller part of the input energy is converted into heat and transferred to the carbon fiber. We have also shown the influence of repetition rate on the pulse efficiency for nanosecond laser sources. Indeed, the decrease of the etch rate per pulse for high repetition rate (comparatively to low repetition rate) could mean an induced shielding effect due to heat accumulation. We have to improve our estimation of the removed volume before bear out or invalidate this hypothesis. Figure 12: Holographic effect on CFRP The period of theses ripples is closed to 800nm. We can also note smaller ripples on the fiber (<150nm). These nanometric waves are not only perpendicular to the fiber axis. Indeed, the direction of ripples is linked to the polarization of the laser beam as well known from literature. On the picture below, we can also note some ripples parallel to the fiber axis. Cross-sections analysis reveals neither delamination effects nor free-standing fibers on it. The quality of these cross-sections depends on the wavelength used. Laser processing leads to smooth surface on the edge cutting. Moreover, resin matrix with femtosecond laser pulses seems to be undamaged. Moreover, laser technologie is the only one enable to induce some bright optical effects as holographic effects. This could be used, for example, on anticonterfeint applications... Laser processing of CFRP is ahead of most of conventional techniques by avoiding all their drawbacks. Highest quality of the cutting processing leads to a great potential for industrial applications of laser technologies as well as for drilling or milling. Future works will focus on UV processing (Excimer and Rod-type fiber laser), on the use of protective gas (with cutting head) and finally on improvement of a polarizer converter on holographic effects. Acknowledgements Figure 14: Ripples parallel to the fiber axis Conclusions and outlook CFRP processing without any delamination forces while reducing heat affected zones and free-standing fibers are key challenges for laser technologies. In order to overcome anisotropic thermo-physical properties of CFRP, we used short and ultrashort pulses in the wavelength range between 515nm to 1030nm. We show that the length of heat affected zones is linked to the fluence of laser beam and also to its We acknowledge the European Commission, the French Ministry of Research and the Aquitaine Regional Council for support and funding. Meet the authors Charly Loumena studied physics and laser/matter interactions at the University of Bordeaux. Since 2008, he worked as a development engineer in laser micromachining at Optical and Laser Technological Center of the Route des Lasers Competitiveness Cluster ALPHANOV. The main topics are related to short (nanosecond) and ultrashort (femtosecond) laser processing in many fields such as thin film solar cells,

9 ceramic matrix composite, organic matrix composite. He has collaborated on few scientific and technical papers. Rainer Kling is head of the BU Micro Machining at Alphanov in France since Previosly he worked for Laser Center in Hannover, Germany as head of department Production and Systems Technology for almost 10 years. He has a broadbackground in micro material processing and is co-author of 3 books in the field of micromachining and remote welding. He earned his Ph.D. from University of Hannover in He has published over 50 papers and conference presentations. References [1] Berges DE. Hexcel Corporation Annual Report; (2004) [2] Weber R, Hafner M, Michalowski A, Graf, (2011): Minimum Damage in CFRP Laser Processing, In: Physics Procedia 12, pp [3] Voelkermeyer F, Hermsdorf J, Denkena B. and Kling R (2007): Novel UV laser applications for carbon fibre reinforced plastics In: Proceedings of APT, Bremen,September; pp [4] Jung K-W, Katayama S, and Kawahito Y (2010): High brightness laser cutting of CFRP In: Transactions of JWRI, Vol.39, No. 2 [5] Negaestani R, Sundar M, Sheikh M, Mativenga P, Li L, Li ZL, et al. (2010) Numerical simulation of laser machining of carbon fibre reinforced composites. In: Proceedings of the Institution of Mechanical Engineers, Part B: Journal of Engineering Manufacture Vol. 224, No. 7, pp [6] Romoli L., Fischer F., Kling R.(2012), A study on UV laser drilling of PEEK reinforced with carbon fibers, in Opt. Laser Eng Vol 50, No 3, pp [7] Hocheng H, and Tsao C-C (2005): The path towards delamination-free drilling of composite materials, In:, J. of Materials Processing Technology 167, [8] C. Emmelmann, M. Petersen, A. Goeke, M. Canisius, (2011), Analysis of Laser Ablation of CFRP by Ultra-Short Laser Pulses with Short Wavelength, Physics Procedia 12,