Scholarly article on topic 'High-speed Observation of the Heat Flow in CFRP During Laser Processing'

High-speed Observation of the Heat Flow in CFRP During Laser Processing Academic research paper on "Materials engineering"

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{CFRP / laser / "percussion drilling" / "heat flow" / "high-speed imaging"}

Abstract of research paper on Materials engineering, author of scientific article — Christian Freitag, Volkher Onuseit, Rudolf Weber, Thomas Graf

Abstract We present fundamental experiments on the heat flow in carbon fibre reinforced plastic (CFRP) during laser percussion drilling with a picosecond laser system. With high-speed imaging, the growing of the matrix evaporation zone (MEZ) during laser processing can be observed with a high temporal and spatial resolution. By varying the pulse energy and the repetition rate of the laser beam, the influence of these parameters on the extension of the heat affected zone can be investigated. Based on these methods, the question, whether high pulse energy or a high repetition rate is more suitable for laser processing of CFRP, has been examined.

Academic research paper on topic "High-speed Observation of the Heat Flow in CFRP During Laser Processing"

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Physics Procedia 39 (2012) 171 - 178

LANE 2012

High-speed observation of the heat flow in CFRP during laser

processing

Christian Freitaga,b*, Volkher Onuseitb, Rudolf Weberb, Thomas Graf0

aGSaME Graduate School of Excellence advanced Manufacturing Engineering, Nobelstraße 12, 70569 Stuttgart, Germany bInstitut für Strahlwerkzeuge IFSW, Pfaffenwaldring 43, 70569 Stuttgart, Germany

Abstract

We present fundamental experiments on the heat flow in carbon fibre reinforced plastic (CFRP) during laser percussion drilling with a picosecond laser system. With high-speed imaging, the growing of the matrix evaporation zone (MEZ) during laser processing can be observed with a high temporal and spatial resolution. By varying the pulse energy and the repetition rate of the laser beam, the influence of these parameters on the extension of the heat affected zone can be investigated. Based on these methods, the question, whether high pulse energy or a high repetition rate is more suitable for laser processing of CFRP, has been examined.

© 2012 Published by Elsevier B.V. Selection and/or review under responsibility of Bayerisches Laserzentrum GmbH Keywords: CFRP; laser; percussion drilling; heat flow; high-speed imaging

1. Motivation

Carbon fibre reinforced plastics (CFRP) have a great potential in lightweight applications. By a load-capable design of components, a lot of weight can be saved by replacing steel or aluminum with CFRP for example in automotive applications. The laser as a well automatable, noncontact tool without wear has a great potential in processing CFRP. However, the very inhomogeneous properties of CFRP make this a challenging task. The main issue is the large matrix damage caused by the heat load of the material [1,2]. The heat flow along the carbon fibres plays an important role in this context and can be observed with high-speed imaging [3].

* Corresponding author. Tel.: +49-711-685-69759 ; fax: +49-711-685-66842 . E-mail address: Christian.Freitag@ifsw.uni-stuttgart.de .

1875-3892 © 2012 Published by Elsevier B.V. Selection and/or review under responsibility of Bayerisches Laserzentrum GmbH doi: 10.1016/j.phpro.2012.10.027

2. Observation of the percussion drilling process

Carbon fibres and matrix material have significantly different thermo-physical properties [4] [5]. Since the heat conductivity along the axis of the carbon fibres is two orders of magnitude higher than for the matrix material, heat will flow mainly along the carbon fibres. When the evaporation temperature of the surrounding matrix material is reached, the matrix material will evaporate. Because of the highly different evaporation temperatures for carbon fibres and matrix material, an area of stripped carbon fibres will appear. This area, where the matrix material has been evaporated leaving the blank carbon fibres, is defined as the matrix evaporation zone (MEZ).

Investigations on the influence of different laser parameters on the extent of the MEZ become possible with high-speed imaging. In order to avoid the influence of additional process parameters like the spatial pulse-to-pulse overlap, a percussion drilling process was used for the investigations presented in the following.

The experiments were performed using a pulsed laser system with a wavelength of 515 nm, a pulse duration of 8 ps and M2 < 1.3. With this laser system it is possible to change the repetition rate without influencing other laser parameters such as the pulse energy. The average power has been measured directly at the work piece. The focal spot diameter was 33 ^m. The material was CFRP with 60% of the volume consisting of carbon fibres arranged as a multilayer lattice. The matrix material was the thermoset material RTM6. The high-speed imaging system recorded the process with 3000 fps and a resolution of 1064x624 pixels. The scene was illuminated using an 808nm diode laser and a corresponding transmission filter in front of the high-speed imaging system.

A sequence of single frames out of a high-speed film of the percussion drilling process is shown in Fig. 1. The average power was 22.2 W with a pulse repetition rate of 800 kHz which leads to a pulse energy of about 28J Each frame shows a magnified section of the CFRP work piece surface. The laser beam, coming from the upper side of the image, hits the work piece in the center of the matrix evaporation zone. The MEZ can be identified as the area, where the stripped carbon fibres are visible allowing to observe the temporal evolution of the MEZ. The MEZ is surrounded by partly molten matrix material which is believed to be due to some amount of thermoplastic materials in the otherwise mainly thermoset matrix.

Fig. 1. Inclined view of the temporal evolution of the thermal damage during a percussion drilling process in CFRP with 22.2 W average laser power

The carbon fibres of the CFRP work piece get heated when laser processing starts. The heat flows mainly along the carbon fibres. When evaporation temperature of the matrix material is reached, the matrix material evaporates leaving blank carbon fibres because their evaporation temperature is much higher. Since the layer of the matrix material on top of the carbon fibres is usually very thin, it is removed very quickly when the evaporation temperature is reached. This gives a good means to observe the heat flow occurring along the fibers. The influence of the polyester yarns, which originally kept the carbon fibres together, can be observed at the frames for 50 ms and 100 ms. This complicates the observation of the heat flow. In the area of the polyester yarn, the matrix material layer on top of the carbon fibres is thicker. Therefore it takes longer to evaporate the matrix material and much more melting can be observed. If possible, only matrix evaporation zones without an influence of a polyester yarn have been investigated in the following studies.

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Comparison of the MEZ 100 ms after starting of the percussion drilling process for different average laser powers. On the left side, the reduction of the average power was achieved by lowering the pulse energy, on the right side by lowering of the repetition rate

2.1. Variation ofpulse repetition rate and pulse energy

By analyzing the high-speed videos, the extent of the MEZ can be measured as a function of the elapsed time after starting of the percussion drilling process. The MEZ has been measured on both sides of the bore hole for different sets of laser parameters. In the following studies either the pulse energy or the repetition rate was varied in order to investigate their influence on the extent of the MEZ. In Fig. 2 the surface of the CFRP work piece is shown 100 ms after the percussion drilling process started for different laser parameters. On the left side, the repetition rate was kept constant and only the pulse energy was lowered resulting in a reduced average power. On the right side of the figure, the pulse energy was kept constant and only the repetition rate was lowered. This allows investigating the influence of heat accumulation.

A vast extent of the matrix evaporation zone can be seen at full average power (top images). The extent of the MEZ is reduced by lowering the average power. However there is a significant difference depending on whether the reduction of the average power is achieved by lowering the pulse energy or by reducing the repetition rate. At the same average power the MEZ is significantly smaller for the reduced repetition rate as compared to result for reduced pulse energies.

For each set of laser parameters the extent of the MEZ was measured on both sides of the bore hole in the direction of the fiber orientation as a function of the elapsed time after starting of the percussion drilling process. The temporal evolution of the extent of the MEZ along the carbon fibres can be seen in Fig. 3(a) and (b).

50 100

Time in ms

S 1500

800 kHz 400 kHz 200 kHz 80 kHz 40

50 100

Time in ms

Fig. 3. (a) extent of the MEZ as a function of time for different pulse energies but with constant repetition rate of 800 kHz; (b) extent of the MEZ as a function of time for different repetition rates but with constant pulse energy of 28 ^J

It is seen, that the growth rate of the MEZ is high at the beginning of the process and saturates for longer interaction times. The point in time, when saturation is reached, depends on the laser parameters. For 40 kHz repetition rate and 28 ^J of pulse energy the saturation is already reached after 6 ms whereas for full average power saturation is not reached within the shown 150 ms. Irregularities in the development of the MEZ can be explained by the inhomogeneity of the material. Since CFRP is a compound, properties like the thickness of the top matrix layer may vary. This influences the growth of the MEZ.

2.2. Extent of the matrix evaporation zone at a fixed point in time

Fig. 4 shows the extent of the MEZ at a fixed point in time versus the average laser power. The extent was measured 100 ms after starting the percussion drilling process for different sets of laser parameters. The average power was changed either by varying the pulse energy at a constant repetition rate of 800 kHz or by changing the repetition rate with a constant pulse energy of 28 J It can be seen that in any case a lower average power leads to a reduction of the MEZ independent on whether the average power was lowered by changing the pulse energy or the repetition rate. However a comparison of the resulting plots clearly shows that after the same processing time with the same average power the extent of the matrix evaporation zone is significantly smaller for low repetition rate and high pulse energy then for high repetition rate and low pulse energy. At an average power of 5.5 W for instance, the pulse energy of 7 ^J at the full repetition rate of 800 kHz leads to an MEZ with an extent of 1540^m. Using the full pulse energy of 28 ^J and a reduced repetition rate of 200 kHz instead, the extent of the MEZ is reduced by about 60% to 640 ^m.

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Average power in W

Fig. 4. Extent of the MEZ 100 ms after starting the percussion drilling process versus average power. The average power was changed either by varying the pulse energy at a constant repetition rate (shown by the line "Changed pulse energy") of 800 kHz or by changing the repetition rate with a constant pulse energy of 28 ^J (shown by the line "Changed repetition rate")

2.3. Extent of the matrix evaporation zone at same amount of total incident energy

In the previous analysis, the extent of the MEZ was measured at a fixed point in time for different average powers, hence the amount of incident energy was not constant. In the following, the extent of the MEZ is investigated for the same amount of total incident energy. The MEZ was investigated for 0.11 J, 0.17 J, 0.22 J, 0.33 J and 0.44 J total incident energy. Again, different pulse energies with the same repetition rate of 800 kHz or different repetition rates with the same pulse energy of 28 ^J were used.

Fig. 5 (a) shows the extent of the MEZ as a function of the pulse energy for different amounts of total incident energy. To get the same amount of total incident energy for different pulse energies, the interaction time varies depending on the pulse energy. All graphs show a similar behavior. For very low pulse energies the extent of the MEZ is very small. But already with slightly increased pulse energy, the extent of the MEZ quickly reaches a maximum. In this case, the maximum extent of the MEZ was

reached with a pulse energy of about 7 J With a further increasing pulse energy, the extent of the MEZ decreases again by up to 35% for a pulse energy of 28 J Accordingly in order to achieve a small extent of the MEZ, either very small pulse energies can be used with long processing times or high pulse energies with a very short interaction time.

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W 1000

10 20 Pulse energy in ^J

200 400 600 800 Repetition rate in kHz

Fig. 5. Extent of the MEZ for different amounts of total incident energy. In (a) the extent of the MEZ was measured as a function of the pulse energy with a constant repetition rate of 800 kHz. In (b) the extent of the MEZ was measured as a function of the repetition rate with a constant pulse energy of 28 ^J

Fig. 5 (b) shows the extent of the MEZ as a function of the repetition rate for different amounts of total incident energies. To obtain the same amount of incident energy, the interaction time varies depending on the repetition rate. It is shown that the extent of the MEZ can be decreased significantly by lowering of the repetition rate. Comparing the extent of the MEZ for repetition rates of 800 kHz and 80 kHz, it is found that the extent of the MEZ reduced by up to 67% with the lower repetition rate. Hence to get a small extent of the MEZ a low repetition rate is favorable.

2.4. Extent of the MEZ parallel and perpendicular to the carbon fibres

So far only the extent of the evaporated matrix material along the direction of the carbon fibres has been investigated. As shown in Fig. 6 (a) we have also investigated the extent of the MEZ perpendicular to the carbon fibres. The ratio a/b of the extents of the MEZ parallel and perpendicular to the fibers is shown in Fig. 6 (b) with a logarithmical time scale for a percussion drilling process with a pulse energy of 22.4 ^J and a repetition rate of 800 kHz. At the beginning of the process the ratio is about 3.3. It stays constant for about 150 ms. Subsequently the ratio begins to decrease. After 150 ms the growth of the MEZ in direction of the carbon fibres reaches saturation but continues to grow perpendicular to the carbon fibres. Consequently the ratio begins to drop. At time scales that are relevant for material processing, the ratio between the extents of the MEZ parallel and perpendicular to the carbon fibres can be considered to be constant.

22.4 J 800 kHz

1 1 10 100 1000 Time in ms

Fig. 6. (a) The extent of the MEZ differs depending on the direction of the carbon fibres. The image was taken 54 ms after starting of the drilling process with 22.4 ^J pulse energy and 800 kHz repetition rate. (b) Ratio of the extent of the MEZ parallel "a" and perpendicular "b" to the carbon fibres for 22.4 ^J of pulse energy and a repetition rate of 800 kHz

3. Conclusion and Outlook

The extent of the matrix evaporation zone was observed by means of high-speed imaging. After a given processing time, the MEZ is smaller for lower average laser powers regardless on whether the power was reduced by decreasing the pulse energy or the repetition rate. However, when using the same average power, the MEZ can be reduced significantly by using high pulse energies and low repetition rates instead of low pulse energies and high repetition rates.

To remove a certain amount of material, a certain amount of energy is needed. For a fixed amount of total incident energy it is favorable to use either very low pulse energies and long processing times or high pulse energies with short interaction times in order to get a small extent of the MEZ. The formation of a maximum of the damage at low pulse energies can be observed. Furthermore it has also been shown for a constant amount of incident energy, that a low repetition rate is favorable in order to get a small MEZ. Heat accumulation seems to be a major factor when laser processing CFRP with pulsed laser systems.

Investigations on the extent of the MEZ perpendicular to the carbon fibres have shown that the extents of the MEZ parallel and perpendicular form a constant ratio until saturation effects occur.

Further investigations have to be made in order to understand why a low repetition rate with high pulse energies is favorable to a high repetition rate with low pulse energy. Heat accumulation seems to be a major influencing factor.

Acknowledgements

The authors want to thank the Deutsche Forschungsgemeinschaft (DFG) for financial support and the "Graduate School of Excellence advanced Manufacturing Engineering" GSaME of the University of Stuttgart for the support of this work.

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References

[1] R. Weber, M. Hafner, A. Michalowski, T. Graf, "Minimum damage in CFRP laser processing", Proceeding Lasers in Manufacturing 2011, Munich.

[2] R. Weber, M. Hafner, A. Michalowski, P. Mucha, T. Graf, "Analysis of thermal damage in laser processing of CFRP", Proceedings 30th ICALEO 2011, Orlando.

[3] C. Freitag, M. Hafner, V. Onuseit, A. Michalowski, R. Weber, T. Graf, "Diagnostics of Basic Effects in Laser Processing of CFRP", Proceeding of International Symposium on Laser Processing of CFRP and Composites 2012, Yokohama.

[4] C. Sheng, G. Chryssolouris, "Theoretical Model of Laser Grooving for Composite Materials", Journal of Composite Materials, 29 (1995), 96-112.

[5] A. Goeke, C. Emmelmann, "Influence of Laser Cutting Parameters on CFRP Part Quality", Physics Pro., 5 (2010), 253258.