Scholarly article on topic 'Wavelength and Pulsewidth Dependences of Laser Processing of CFRP'

Wavelength and Pulsewidth Dependences of Laser Processing of CFRP Academic research paper on "Materials engineering"

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{CFRP / "cutting efficiency" / "UV laser" / "ultrashort pulse laser" / "nanosecond laser"}

Abstract of research paper on Materials engineering, author of scientific article — M. Fujita, H. Ohkawa, T. Somekawa, M. Otsuka, Y. Maeda, et al.

Abstract As the use of CFRP material becomes widespread in various industries, achieving high-speed cutting with less pulse energy and minimal thermal damage is one of the important issues for laser-based processing. Among the various parameters in laser processing, we have focused on wavelength from UV (266nm) to NIR (1064nm) and pulsewidths from 100 fs to 20ns in order to investigate cutting efficiency in terms of ablated mass per irradiated laser energy and corresponding heat affected zone (HAZ). Samples used in our experiments were uni-directional CFRPs with thickness from 140μm to 250μm or 1.3 mm-thick cross CFRPs. We measured time to cut the samples and ablated volume in order to estimate cutting efficiency in mg/kJ. Also we observed SEM images of the processed samples to evaluate HAZ. In general, shorter wavelength and shorter pulsewidth resulted in higher cutting efficiency and less thermal damages. These results could contribute to optimize the machining of CFRP composite materials.

Academic research paper on topic "Wavelength and Pulsewidth Dependences of Laser Processing of CFRP"

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Physics Procedia 83 (2016) 1031 - 1036

9th International Conference on Photonic Technologies - LANE 2016

Wavelength and pulsewidth dependences of laser processing of

M. Fujitaa'c'*, H. Ohkawab, T. Somekawa3, M. Otsukab, Y. Maedab, T. Matsutanib, N. Miyanagac

aInstitute for Laser Technology, 2-6 Yamada-Oka, Suita, Osaka, 565-0871 Japan bDepartment of Electrical Engineering, Kindai Univ., 3-4-1 Kowakae, Higashiosaka, Osaka, 577-8502 Japan cInstitute of Laser Engineering, Osaka Univ., 2-6 Yamada-Oka, Suita, Osaka, 565-0871 Japan

Abstract

As the use of CFRP material becomes widespread in various industries, achieving high-speed cutting with less pulse energy and minimal thermal damage is one of the important issues for laser-based processing. Among the various parameters in laser processing, we have focused on wavelength from UV (266 nm) to NIR (1064 nm) and pulsewidths from 100 fs to 20 ns in order to investigate cutting efficiency in terms of ablated mass per irradiated laser energy and corresponding heat affected zone (HAZ). Samples used in our experiments were uni-directional CFRPs with thickness from 140 ^m to 250 ^m or 1.3 mm-thick cross CFRPs. We measured time to cut the samples and ablated volume in order to estimate cutting efficiency in mg/kJ. Also we observed SEM images of the processed samples to evaluate HAZ. In general, shorter wavelength and shorter pulsewidth resulted in higher cutting efficiency and less thermal damages. These results could contribute to optimize the machining of CFRP composite materials.

© 2016 The Authors.PublishedbyElsevier B.V. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.Org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of the Bayerisches Laserzentrum GmbH

Keywords: CFRP; cutting efficiency; UV laser; ultrashort pulse laser; nanosecond laser

* Corresponding author. Tel.: +81-6-6879-8732 ; fax: +81-6-6879-8732 . E-mail address: mfujita@ilt.or.jp

1875-3892 © 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.Org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of the Bayerisches Laserzentrum GmbH

doi:10.1016/j.phpro.2016.08.108

1. Introduction

Carbon fiber reinforced plastic (CFRP) is well-known composite material, which has attractive features like high-durability, high-strength and light-weight. These features have been stimulating the CFRP to be applied for a wide variety of applications in aerospace and automobile industries. To further spread the use of CFRP, high-quality machining processes must be established. Laser machining of CFRP had been studied by various kinds of pulsed lasers, such as UV nanosecond lasers, Li et al. 2008, Dittmar et al. 2012, IR nanosecond fiber laser, Takahashi et al. 2015, Staehr et al. 2015, green and IR nanosecond and IR femtosecond lasers, Loumena et al. 2013, UV to IR picosecond lasers, Wolynskia et al. 2011, IR picosecond and microsecond lasers, Janssen et al. 2015, high power femtosecond laser, Mans et al. 2014, where laser cutting speed, thermal damages and possible changes of strength were investigated. The difficulties of laser machining of CFRP arise from the differences of thermal properties of carbon fiber (CF) and polymer, Tagliaferri et al. 1985. When laser processing parameters are optimised for polymer, which has low decomposition temperature, it is difficult to ablate CF. On the contrary, if the laser intensity is so high to ablate CF, there will be serious thermal damages in polymer.

We also have investigated processing of CFRP with various pulsed lasers to study mechanical and thermal damages, Fujita et al. (2010, 2012, 2013, 2014, 2015). Experiments were performed with wavelengths from UV (266 nm) to NIR (1064 nm) and pulsewidths from 100 fs to 20 ns. For laser-based processing, achieving high-speed cutting with less pulse energy and minimal thermal damage is one of the important issues. Here, we roughly summarized the experimental results to clarify the wavelength and pulsewidth dependences of cutting efficiency in terms of ablated mass per irradiated laser energy and corresponding heat affected zone (HAZ). We evaluated time to cut the samples and ablated volume in order to estimate cutting efficiency in mg/kJ.

2. Experimental Conditions

First of all, it has to be noticed that the discussions below were based on results in four different series of experiments done in more than five years. Although experimental setup and parameters such as average laser power ranged from 0.1 Wto 44 W, pulse rep. rate from 1 kHz to 1.55 MHz, scanning speed from 17 mm/s to 37,200 mm/s, were diversified, every consecutive laser-matter interactions could be considered to be almost independent, i.e. little effect from the plasma generated by the previous pulse. The scan speed was set to be high enough for irradiated spot not to overlap consecutively or pulse interval was long enough for the plasma to disapper. Hence we believe it would give quite rough but reasonable comparisons.

Table 1 shows experimental conditions. All the experiments was done in air and no assist gas was used. Detailed experimental setups (A to D) are described below.

Table 1. Experimental conditions.

Pulsewidth Wavelength (nm) Power (W) Rep. rate (kHz) Spot diameter (^m) Scan speed (mm/s) Setup

a 100fs 800 0.2 1 100 17 A

b 35 ps 266 2.0 100 60 1500 B

c 200 ps 800 0.4 1 100 17 A

d 2 ns 355 45 1550 24 37200 C

e 10 ns 355 43 200 24 4800 C

f 10 ns 532 3.0 10 18 1500 D

g 20 ns 1064 3.0 20 27 1500 D

In the experimental setup A, Ti:Sap. laser pulses were used to make a straight line groove on 1.3 mm-thick cross-type CFRP samples, Fujita 2010. Ablated volumes were estimated from the groove depth and cross section.

In the experimental setup B, laser pulses with wavelength/pulsewidth of 266 nm/ 35 ps were used to cut 250 ^m-thick uni-directional CFRP samples, Fujita 2014. We measured time to cut off a CFRP disk of 20mm in diameter,

which was mounted on an off-centered high speed motor. Ablated volumes were estimated, assuming kerf was straight and the width was the same as the laser spot diameter.

In the experimental setup C, nanosecond UV laser pulses were used to cut through 250 prn-thick uni-directional CFRP samples, Fujita 2015. A high-speed galvo was used to cut through a straight line perpendicular to the orientation of CF. Ablated volumes were estimated by measuring kerf width on front and back surfaces.

In the experimental setup D, nanosecond laser pulses with wavelength of 532 and 1064 nm were used to cut 140 prn-thick uni-directional CFRP samples. We measured time to cut off a CFRP disk of 20 mm in diameter, which was mounted on the high speed motor. To evaluate the kerf width, small part of the sample was masked so that the disk did not come off. Ablated volumes were estimated by measuring kerf width on front and back surfaces.

3. Experimental Results

Figure 1 shows SEM images of CFRP samples cut by various lasers listed in Table 1. All the SEM images are taken in the same scale. Rough depedndencies of morphology of pulsed laser ablated CFRP on wavelength and pulsewidth can be seen. Here, we evaluated HAZ as the width where carbon fibers were exposed. Generally, shorter wavelength and pulsewidth resulted in smaller HAZ appearance.

§ 600

§ 400

100fs 1 ps 10ps 100ps 1ns 10ns

Pulsewidth

Fig. 1. SEM images of CFRP samples cut by various lasers listed in Table 1. All the SEM images are taken in the same scale.

It is noted that polymer layer near the kerf seems to be thermally damaged in Fig.l (f), even though significant exposure of CF cannot be seen at the edge of the kerf. This is due to overdose of 532 nm radiation, to which

polymer is transparent. As we measured time to cut off the samples, it seemed that the bottom layer of polymer remained so long even after CFs were removed. The bottom polymer layer was inferred to be ablated by weak multiphoton absorption of the 532 nm radiation. Although 1064 nm radiation is also transparent for polymer, thermal effect ofheated CFs seemed to help polymer to be evaporated.

4. Estimations of cutting efficiency

We estimated the cutting efficiency in terms of ablated mass per irradiated laser energy. The ablated mass was estimated from thickness of CFRP samples and cross section of the kerf, and the irradiated laser enegy was calculated by laser power and time to groove or cut through the samaples. In order to simplify the estimations we assumed;

• Average density of CFRP is 1.5 g/cm3.

• Volume fraction of carbon fiber is 50 %.

• Energy to evaporate polymer can be ignored.

Table 2 shows estimated values of cutting efficiency in terms of ablated mass per irradiated laser energy (mg/kJ). We also added estimated values from literatures. Table 2 (h) was calculated from the value of 2 mm3/s/ 150 W, Mans 2014. Also Table 2 (i) - (k) were calculated from the value of 1 mm3/min./W, Wolynskia 2011. The theoretical limit, i.e. all the irradiated laser energy is absorbed and consumed to evaporate CFs, is 45 mg/kJ, Weber 2012.

Table 2. Estimated values ofcutting efficiency in terms ofablated mass per irradiated laser energy (mg/kJ).

Pulsewidth Wavelength (nm) Power (W) mg/kJ

a 100 fs 800 0.2 31

b 35 ps 266 2.0 15

c 200 ps 800 0.4 15

d 2ns 355 45 14

e 10 ns 355 43 5.4

f 10 ns 532 3.0 2.5

g 20 ns 1064 3.0 2.7

h 8ps 1030 150 20

i 10 ps 355 10 25

j 10 ps 532 14 25

k 10 ps 1064 23 25

Figure 2 shows dependencies of the cutting efficiency on wavelength and pulsewidth listed in Table 2. Areas of each circle are proportional to the cutting efficiency. Generally, shorter wavelength and pulsewidth resulted in larger cutting efficiency. Several notes have to be mentioned. Compared Table 2 (h) to (a) and (k), the value seemd to be a little bit small. This is due to that the experiments were performed with 150 W laser with high rep.rate of 3 MHz. At such high power and high rep.rate, thermal effects of pulse accumulation as well as scan accumulation occur, Freitag 2014, and hence the cutting efficiency becomes lower. This becomes more significant for kW level average power. 11 mg/kJ would be calculated from the reported value of 8 mm3/s/ 1.1 kW, Freitag 2014, which is not shown in Table 2. Also compared Table 2 (b) to (i), the efficiency is clearly small. This might be due to that irradiated intensity of (b) was 2xl010W/cm2, which was 2 orders of magnitude lower than (i). According to the literature, Weber 2012, higher intensity is advantageous to suppress HAZ.

5. Conclusions

We have investigated processing of CFRP with various pulsed lasers to study the wavelength and pulsewidth dependences of thermal damages and roughly estimated cutting efficiency in terms of ablated mass per irradiated laser energy. Generally, shorter wavelength and pulsewidth resulted in smaller HAZ appearances and larger cutting efficiencies. Efficient laser-matter interactions seem to be beneficial for reducing thermal effects to the CFRP samples. These results could contribute to optimize the machining of CFRP composite materials.

« 400

10Ofs 1 ps

10ps 1OOps 1ns Pulsewidth

Fig. 2. Dependencies of the cutting efficiency on wavelength and pulsewidth listed in Table 2. Areas of each circle are proportional to the cutting efficiency.

Acknowledgements

A part of this research was supported by a grant from AMADA foundation. The 35 ps- 266 nm laser system was supplied by Spectronix Corporation (Ibaraki, Japan). The nanosecond UV laser system, Quasar®, was supplied by Spectra-Physics, aNewport Company (Santa Clara, USA.). We greatfully acknowledge these generous supports.

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