Parametric studies on pulsed Nd:YAG laser cutting of carbon fibre ...

Journal of Materials Processing Technology 89 – 90 (1999) 198 – 203

Parametric studies on pulsed Nd:YAG laser cutting of carbon fibre reinforced plastic composites Jose Mathew a,*, G.L. Goswami b, N. Ramakrishnan c, N.K. Naik d a b

Department of Mechanical Engineering, R.E.C., Calicut, Kerala 673 601, India Atomic Fuels Di6ision, Bhabha Atomic Research Centre, Mumbai 400 085, India c Department of Mechanical Engineering, I.I.T., Powai, Mumbai 400 076, India d Department of Aerospace Engineering, I.I.T., Powai, Mumbai 400 076, India Received 15 September 1998

Abstract Carbon fibre reinforced plastic (CFRP) composites are found to be cut satisfactorily by a pulsed Nd:YAG laser at the optimum process parameter ranges. Predictive models have been developed based on important process parameters, viz. cutting speed, pulse energy, pulse duration, pulse repetition rate and gas pressure. The responses considered are the heat-affected zone (HAZ) and the taper of the cut surface. The optimisation of process parameters was done using response surface methodology (RSM). The thermal properties of the constituent material and the volume fraction of the fibres are the principal factors that control the cutting performance. The effect of the process parameters on the output responses is also discussed. © 1999 Published by Elsevier Science S.A. All rights reserved. Keywords: Composites; Fibre reinforced plastics; Laser cutting; Response surface methodology

1. Introduction Fibre reinforced plastic (FRP) materials are one of the most widely used composite materials for structural applications, particularly for aerospace structures. Properties such as high specific strength, specific stiffness and ease of tailoring to a specific need make them attractive. As the cost of these materials is continually declining, they are finding ever-increasing applications in all fields. Even though near-net manufacturing of composite materials is possible, drilling will remain an unavoidable operation, particularly in assembly. Drilling of FRP composites using conventional tools produces problems such as delamination, fibre pull-out and tool wear [1,2]. Laser cutting, being a non-contact process, does not involve any mechanical cutting forces and tool wear. However, as laser cutting is based on the interaction of a laser beam with the composites, defects that are thermal in origin may arise if proper care is not * Corresponding author. Fax: +91-495-287250. E-mail address: [email protected] (J. Mathew)

taken regarding the selection of the cutting parameters ranges [1–7]. Most of the researchers who did experimental analysis with composite materials have used the method of varying one-factor-at-a-time to determine the effect of process parameters on the responses [1–7], but usually this technique is not only inefficient but also unsuccessful, as it fails to determine interactions [7–9]. It is felt that a systematic study with statistically planned experimentation using response surface methodology (RSM) will give more insight into such situations. RSM allows all main effects as well as interactions to be evaluated with the minimum number of experiments. Also, mathematical models correlating the output with the input process parameters can be obtained.

2. The workpiece material Woven fabric carbon fibre reinforced plastic (CFRP) composites of 2 mm thickness were used for conducting the studies. The reinforcing material used was T300

0924-0136/99/$ - see front matter © 1999 Published by Elsevier Science S.A. All rights reserved. PII: S 0 9 2 4 - 0 1 3 6 ( 9 9 ) 0 0 0 1 1 - 4

J. Mathew et al. / Journal of Materials Processing Technology 89–90 (1999) 198–203

carbon fibre. The fibres had a fabric form, with 50% fibre in the warp and weft directions. The matrix is epoxy resin LY-556 and the hardener is HY-951, both manufactured by Ciba-Giegy. The laminate was made by matched-die moulding. The nominal fibre volume fraction was 0.4. Typical thermal properties of the CFRP constituents are given in Table 1.

199

3. Methodology A cause–effect diagram showing the various factors influencing the quality of a laser cut FRP material is shown in Fig. 1. Based on the cause–effect analysis and based on initial experiments [7,8], the input parameters selected which are controllable and which influence the

Table 1 Thermal properties of CFRP composite constituentsa Material

r (g/cm3)

K (W/mK)

cp (J/kg K)

a (cm2/s) 10−3

Tv (°C)

H (J/g)

Carbon fibres (T300) Resin (epoxy)

1.90 1.2

50 0.10

710 1100

380 1.20

3300 400

43 000 1100

a

Density, r; conductivity, K; specific heat, cp; thermal diffusivity, a; vaporisation temperature, Tv; vaporisation energy, H.

Fig. 1. A cause – effect diagram showing the various parameters affecting the quality of the laser cut surface of FRP composite [7]. Table 2 Coded levels and actual values of the independent variables Coded levels of variables

−2

−1

0

1

2

Cutting speed, 6 (mm/s)= X1 Pulse energy, E (J) =X2 Pulse duration, tp (ms)=X3 Pulse repetition rate, fp (Hz) = X4 Gas pressure, p (kg/cm2) =X5

0.6 1.4 0.2 30 4

0.7 1.6 0.4 35 5

0.8 1.8 0.6 40 6

0.9 2.0 0.8 45 7

1.0 2.2 1.0 50 8


200

Table 3 Experimental design matrix for rotatable, uniform precision CCD with coded, actual values and with experimental resultsa Ex. No.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 a

Actual variablesb

Coded variables

Responses

x1

x2

x3

x4

x5

X1

X2

X3

X4

X5

HAZ (mm)

Wt (mm)

Wb (mm)

−1 1 −1 1 −1 1 −1 1 −1 1 −1 l −1 1 −1 1 0 0 0 0 0 0 −2 2 0 0 0 0 0 0 0 0

−1 −1 1 1 −1 −1 1 1 −1 −1 1 1 −1 −1 1 1 0 0 0 0 0 0 0 0 −2 2 0 0 0 0 0 0

−1 −1 −1 −1 1 1 1 1 −1 −1 −1 −1 1 1 1 1 0 0 0 0 0 0 0 0 0 0 −2 2 0 0 0 0

−1 −1 −1 −1 −1 −1 −1 −1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 −2 2 0 0

l −1 −1 1 −1 1 1 −1 −1 1 1 −1 1 −1 −1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 −2 2

0.7 0.9 0.7 0.9 0.7 0.9 0.7 09 0.7 0.9 0.7 09 07 09 0.7 0.9 0.8 0.8 0.8 0.8 0.8 0.8 0.6 10 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8

1.6 1.6 2.0 2.0 1.6 1.6 2.0 2.0 1.6 1.6 2.0 2.0 1.6 1.6 2.0 2.0 1.8 1.8 1.8 1.8 1.8 1.8 1.8 1.8 1.4 2.2 1.8 1.8 1.8 1.8 1.8 1.8

0.4 0.4 0.4 0.4 0.8 0.8 0.8 0.8 0.4 0.4 0.4 0.4 0.8 0.8 0.8 0.8 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.2 1.0 0.6 0.6 0.6 0.6

35 35 35 35 35 35 35 35 45 45 45 45 45 45 45 45 40 40 40 40 40 40 40 40 40 40 40 40 30 50 40 40

7 5 5 7 5 7 7 5 5 7 7 5 7 5 5 7 6 6 6 6 6 6 6 6 6 6 6 6 6 6 4 8

0.929 0.903 1.146 1.010 0.988 0.753 1.040 0.850 1.297 0.877 1.213 1.080 1.523 1.366 1.531 1.290 0.830 0.860 0.870 0.880 0.820 0.780 1.750 1.187 1.040 1.380 1.040 1.410 0.897 1.390 1.198 1.010

0.300 0.302 0.356 0.326 0.255 0.279 0.340 0.333 0.346 0.244 0.339 0.274 0.239 0.263 0.318 0.248 0.215 0.229 0.221 0.200 0.218 0216 0.261 0.211 0.258 0.335 0.285 0.221 0.291 0.311 0.255 0.246

0.148 0.132 0.197 0.172 0.094 0.105 0.137 0.121 0.085 0.060 0.147 0.109 0.096 0.122 0.152 0.145 0.097 0.085 0.100 0.120 0.092 0.066 0.117 0.136 0.106 0.176 0.162 0.093 0.121 0.121 0.154 0.139

The

response

Heat affected zone;

surface

models

obtained

the

analysis

are:

HAZ=14.4696−19.1992×X1−4.3589×X2−5.2267×X3−0.0305×X4+11.30S8×X 21+1.2108×X 22+1.3046×X 23+0.1×X3 (1)

× X4 Top kerf width;

from

Wt = 4.1126−0.7253×X1−1.9905×X2−0.8550×X3−0.0576×X4−0.1402×X5+0.8057×X 21+0.5188×X1×X3−0.0253×X1 ×X4+0 5795×X 22+0.3077×X 23+0.0009×X 24+0.117×X 25

(2)

Bottom kerf width; Wb = 1.4838−0.0217×X1−0.6156×X2−0.81×X3−0.0131×X4−0.1132×X5+0.1987×X 22+0.0191×X3×X4+0.0093×X 25 (3) where, HAZ, Wt and Wb are in mm; X1 is cutting speed (mm/s); X2 is pulse energy (J); X3 is pulse duration (ms); X4 is repetition rate (Hz); X5 is gas pressure (kg/cm2). b X1, cutting speed (mm/s); X2, pulse energy (J); X3, pulse duration (ms); X4, pulse repetition rate (Hz); X5, gas pressure (kg/cm2).

responses such as the heat-affected zone (HAZ) and cut taper, are cutting velocity, pulse energy, pulse width, repetition rate, and assist gas pressure. The parameter ranges selected [9] are shown in Table 2. A central composite design (CCD) with uniform precision was used for the experimental design. Such a design will help to analyse the main factor and two-factor interactions without any confounding. The CCD considered here consists of an L16 orthogonal array of resolution IV (rows 1–16; Table 3). The presence of pure

quadratic terms is checked by replicating the centre points (rows 17–22; Table 3). The values of the quadratic terms are found with the additions of axial points (rows 23–32; Table 3). Based on these experiments, response surface models for the HAZ and kerf width at top (Wt) and bottom (Wb) were developed. The overall model adequacy was checked with F ratio and lack of fit was also tested [9]. All these checks were performed with the help of ‘sas’ and ‘statgraphics’ software.


201

4. Experimental details and results A pulsed Nd:YAG of JK make with a 300 W capacity (average power) was used in this experiment. The laser was operated in the TEM00 mode and the focused spot diameter was 0.1 mm. The specimen was mounted on a CNC X-Y table. The table was covered with a special enclosure and the chemical by-products were driven out with the help of a vacuum pump. Argon gas was used as it was found to be more effective in obtaining a better quality cut [8,9]. Samples obtained from each specimen were examined through photomi-

Fig. 5. Effect of RR and cutting speed on the kerf width (pulse energy = 1.8 J; pulse duration=0.6 ms; gas pressure= 6 kg/cm2).

Fig. 2. Effect of pulse duration and RR on the HAZ (cutting speed =0.8 mm/s; pulse energy =1.8 J; gas pressure= 6 kg/cm2).

Fig. 6. Effect of RR and pulse energy on the kerf width (cutting speed = 0.8 mm/s; pulse duration =0.6 ms; gas pressure =6 kg/cm2).

Fig. 3. Effect of cutting speed and pulse energy on the HAZ (pulse duration= 0.6 ms; pulse repetition rate=40 Hz; gas pressure= 6 kg/cm2).

crographs. The preliminary cutting studies showed that the overall values of quality parameters such as the HAZ and the cut kerf width are not much different in the warp and the weft direction because the fabric used is a closely knitted one. The HAZ is characterised by the presence of fibres debonded from the matrix and thermal degradation of the fibres and the matrix Table 3 gives the CCD along with the responses measured. The fitted quadratic models are given below. Some of the fitted responses are plotted (Figs. 2–6) and are given below to explain the process behaviour.

5. Discussion Fig. 4. Effect of pulse energy and RR on the HAZ (cutting speed = 0.8 mm/s; pulse duration =0.6 ms; gas pressure= 6 kg/cm2).

The large difference in thermal properties of the carbon fibre and the epoxy matrix in CFRP composites

202


creates difficulty in securing good quality cuts with continuous mode lasers [5 – 7]. However, the high beam intensity and better focusing behaviour of pulsed Nd:YAG lasers gives a smaller thermal load which helps by obtaining good quality cuts with proper parameter combinations [6,7]. In CFRP composites, the laser power requirements strongly depends upon the carbon fibres and their volume fraction.

carbon fibres have a higher thermal conductivity than that of the matrix, thus heat is transmitted through the fibres faster, resulting in a wider HAZ. The greater the cutting speed, the less the interaction time and the less will be the HAZ. However, at higher speeds some of the laser radiation is deflected and the efficiency of the process may become reduced (Fig. 3).

5.3. Influence of pulse energy on the HAZ 5.1. Influence of repetition rate and pulse duration on the HAZ The pulse repetition rate (RR) and the cutting speed are the two most important parameters affecting the HAZ, the latter being found to be directly proportional to the RR. At high RR, the laser behaves like a continuous wave and the cut surface hardly has any time to cool down. The HAZ is also a function of power intensity. When the pulse duration is higher, the peak power will be lower, thereby leading the power intensity to drop. Hence, higher pulse duration at lower RR gives a smaller HAZ (Fig. 2).

5.2. Influence of cutting speed on the HAZ The cutting speed controls the interaction time. The time that elapses before the vaporisation condition is reached for carbon fibres is larger than that for the resin. Hence, it takes time to cut the composite, which is at the expense of more HAZ because of the burning of the matrix: the HAZ is thus inversely proportional to the interaction time. Further, in CFRP composites, the

Fig. 7. An enlarged view of the cut surface of CFRP woven fabric composite obtained under the optimum cutting conditions.

The pulse energy is another factor that controls the HAZ. In general, the HAZ is directly proportional to the pulse energy. In laser cutting, the cut surfaces are generally covered by chars originating from the decomposition of the fibres and the matrix [4]. The presence of such carburised residues is related to both the power and the cutting speed/interaction time. For a high P/6 (power to speed) ratio, the charred material forms a thick layer which covers the cut surface completely. With reduction in the power and cutting speed, the amount of charred material tends to increase the range for a good quality cut. In the present experiments it was found that for a P/6 ratio of between 2 and 4, the HAZ is the minimum, and hence it can be taken as the optimal cutting ratio for the material used (Fig. 4).

5.4. Effect of parameters on kerf widths and taper The kerf width at top (Wt) and bottom (Wb) are two major parameters to be assessed to evaluate the quality of a laser cut surface. The kerf width gives an idea of the amount of overcut at the top and the bottom as well as the taper of the cut surface With the adopted specific powers (105 –107 W/cm2), a ‘key-hole’ plasma column forms, which consists of the decomposition products of the material, and which behaves like a black body for laser radiation. The incident radiation that falls into the keyhole loses some power by absorption and reflection from the plasma, thus the energy absorbed by the material decreases with the depth along the keyhole. The divergence of the beam beyond the focal plane, which in the present case coincides with the top surface of the laminate, is another factor which leads to a decreasing power density along the keyhole. Furthermore, the energy absorbed is higher at the centre of the beam path due to higher power density. From the above reasons, the kerf width should be expected to decrease continuously from the top to the bottom of the composite. In the experimental results, RR and pulse energy were found to be the most influencing factors affecting the kerf widths. The top kerf width was found to decrease with an increase in RR up to the middle of the experimental range; and later it was found to increase (Figs. 5 and 6), whereas the bottom kerf width shows a decreasing trend with an increase in RR. Correspond-


ingly, the taper is found to be at a minimum at the middle range of RR and at higher pulse duration, higher cutting speed and lower pulse energy. Increase in cutting speed decreases the kerf width. Too high a cutting speed may result in a no-cut situation. Also, increase in pulse energy leads to more material removal and increase in kerf width. An enlarged view of the cut surface of the composite obtained with the optimum cutting parameters is shown in Fig. 7.

6. Conclusions The large difference in the thermal properties of the carbon fibre and the matrix material in CFRP composites creates difficulty in securing good quality cuts through continuous mode laser cutting. However, the high beam intensity and better focusing behaviour of pulsed Nd:YAG lasers gives a smaller thermal load during cutting. This helps in cutting the CFRP composites with minimal defects. RSM was found to be an effective and powerful method in finding optimum cutting parameters of pulsed Nd:YAG laser cutting of CFRP. Repetition rate, cutting speed, pulse duration and beam energy are the parameters that were found to have an influence on the HAZ. The gas pressure was also found to have an influence on the cut kerf width. Most of the factors in the middle range of the experimentation conducted (cutting speed= 0.5 – 0.8 mm/s; pulse energy= 1.5– 2 J; pulse duration= 0.4 – 0.7 ms; pulse repetition rate=35 – 45 Hz and gas pressure 5–7

.

203

kg/cm2) have been obtained as the optimal parameter ranges. This type of study can also be effectively conducted for other materials of similar types. Depending on whether it is a roughing or a finishing operation, the parameters can be adjusted for either a higher cutting rate or a better cut quality.

References [1] W. Koenig, C. Wulf, P. Grass, H. Willerchied, Machining of fibre reinforced plastics, Ann. CIRP 34 (2) (1985) 537 – 548. [2] R. Komanduri, Machining fibre reinforced composites, Mech. Eng. 27, April (1993) 58 – 64. [3] V. Tagliaferri, Laser cutting of reinforced materials, in: N.P. Cheremisinoff (Ed.), Handbook of Ceramics and Composites, Marcel Dekker, New York, 1990, pp. 451 – 467. [4] A. DI Ilio, V. Tagliaferri, F. Veniali, Machining parameters and cut quality in laser cutting of aramid fibre reinforced plastics, Mat. Manuf. Process. 5 (4) (1990) 591 – 608. [5] K.A. Fenoughty, A. Jawaid, I.R. Pashby, Machining of advanced engineering materials using traditional and laser techniques, J. Mat. Proc. Tech. 42 (1994) 391 – 400. [6] W.S. Lau, W.B. Lee, Pulsed Nd:YAG laser cutting of carbon fibre composite materials, Ann. CIRP 39 (1) (1990) 179–182. [7] J. Mathew, N. Ramakrishnan, N.K. Naik, G.L. Goswami, Laser cutting of FRP composites: a review, (1998), in preparation. [8] S. Ghosh, J. Mathew, P.C. Bhatt, N. Ramakrishnan, N.K. Naik, G.L. Goswami, Parametric studies on cutting of AFRP composites using Nd:YAG laser, Proceedings of the Symposium on Laser Applications in Material Science and Industry, Chennai, India, February 1997, pp. 187 – 192. [9] J. Mathew, N. Ramakrishnan, N.K. Naik, G.L. Goswami, Pulsed Nd:YAG laser cutting of CFRP composites: parameter optimisation by response surface methodology, (1998), in preparation.