Challenges in CO2 Laser Cutting of Composite Materials

Abstract

Laser cutting is a manufacturing process that consists of cutting the material with a lot of energy generated by a laser and concentrated on a very small surface area. This technology is for companies specialized in the industry. The evolution of technologies around laser cutting is constantly evolving: diversification of materials, increase in the thickness of the cutting, finalization of the rendering. In this report, we will focus on the different parameters that affect cutting on composite materials, i.e. the heat-affected zone (respectively named HAZ) and the Kerf width of the cutting.

Keywords: CO2 laser cutting / Composite Materials / Heat Affected Zone / Kerf Width.

Introduction

I started my research by looking for theses and research papers that dealt with CO2 laser cutting of composite materials. Among all the composite materials that exist (wood / bone / fiberglass and carbon fiber / reinforced concrete / etc....), and allowed most of the research documents found they focus on Carbon Fiber Reinforced Plastics/Polymers (called CFRP).

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The CFRP has very interesting mechanical characteristics. Indeed, it has a high resistance to mechanical stresses and corrosion while having a very low density. These characteristics make it a very interesting material for many industries (aeronautics / defence / automotive / etc...)[2]. This material has always been used in industry but with different machining methods such as jet or shape cutting, which are now obsolete. CO2 laser cutting has a great potential in the cutting of this material because it would allow automating processes, with better quality, higher productivity, as well as a lower production cost than the machining, means seen previously [1,3].

CO2 laser cutting is a manufacturing process that is constantly evolving. Improvements are made in several aspects such as the type of materials that can be cut, accuracy and depth of cutting. Its operating principle is simple, the beam focuses on a specific part of the material to increase its temperature until it vaporizes. This area is called the heat-affected zone or HAZ. The material is cut by translating the laser cutting head with the help of a two-axis machine (up to 5 axes for the most complex shapes)[6].

Five main parameters influence laser cutting, namely, the nature and thickness of the material, the power of the beam emitted by the generator, the cutting speed, the nature of the gas (CO2 in our case) and the pressure of the gas used to assist cutting.

All these parameters are linked and must be maximized according to the material to be cut to limit thermal damage [6].

Figure 1: Diagram of laser cutting head [6]

Through this research report, we will see why CO2 laser cutting is seemed ideal for cutting our composite. First, laser cutting is considered to be the most efficient in the industry. It is mainly used to cut metals, polymers, and ceramics. Besides, the polymer matrix and carbon fibers absorb laser cutting radiation very well (k=10.6 lm) [3].

To address the different challenges of CO2 laser cutting, we will base ourselves on the needs of manufacturers. For an industrial company, the important thing in a cut is that it is of good quality. The parameters that ensure a good cutting quality are the HAZ and the Kerf width.

Figure 2: Schema of the quality features (Kerf width in entry and exit and HAZ width) [3]

The HAZ will affect the appearance of the cut while the Kerf width corresponds to the width of the cut and so to the loss of material. These two factors will be modified thanks to the parameters mentioned above: nature & thickness of the material, the power of the beam, the cutting speed, the nature, the pressure of the gas.

Our report will, therefore, test the different parameters to optimize our two factors, HAZ and Kerf width, which will allow us to have the best cutting.

Experience & Results

Experience

The purpose of our experience is, therefore, to test our different parameters to optimize them and thus to have the smallest HAZ and the smallest Kerf width. The parameters will, concern the CO2 laser cutting machine as well as our composite material CFRP.

First of all, the CFRP. In several of my references, I found the dimensions of the CFRP:

A 3 mm thick CFRP sheet, the fiber volume was 0.57 and the void volume was 0.02 [3].

Processing CFRP laminates with a thickness between 1.0 up to a thickness of 7.0 mm employing a CO2-Laser [1]. So we have our first parameter, the CFRP thickness, which will vary between 1.0mm and 7.0mm.

The other parameters will be defined by the C02 laser cutting machine. I found these parameters [6]:

• Cutting speed, from 2000 to 8000 mm/min.
• Laser power, from 200 to 600 W.
• Gas pressure, from 3 to 8 bars.

So that the values found in various references previously mentionned correspond to the figures given randomly by my student number, these values do not necessarily correspond to reality but come as close as possible to it. Here are the values we will take:

• Thickness: 10 to 25mm (parameter 2).
• Cutting speed: 0 to 182 cm/min (parameter 4).
• Laser power: 0 to 2 KW (parameter 3).
• Gas pressure: 31 to 155 decibar (parameter 1).

To know if the Kerf width is optimal and therefore that our cut is of good quality, we must subtract the W entry from the W exit that we see in Fig.2. since the W entry is supposed to be larger than the W exit, the closer the value of the subtraction will be to zero, the more optimal the Kerf width will be.

In the case of the Heat Affected Zone, we will focus on the aspect. For it to be optimal, it must be as small as possible so that the cutting is of high quality.

Figure 3: Methods for measuring the dimension of HAZ [4]

The observations of the different results obtained as well as the measurements will be assigned using an electron beam microscope [2].

Results

To be able to analyze the data from our experience, we use Expert Design. Depending on the variation of the different parameters we obtain different HAZ and Kerf width results.

Table 1: Expert Design Table with our four parameters and two responses (HAZ & Kerf width).

Kerf width analysis

Let's start by analyzing the Kerf width response. We will see below how our different parameters will influence our two responses, with a R^2 = 0.9896, That's mean our model is very precise and our ANOVA analysis will be significant.

Table 2: Fit statistics of the Kerf width analysis

With the help of the ANOVA analysis, we will be able to see which parameters among the gas pressure (A), the material thickness (B), the laser power (C) as well as the cutting speed (D) are significant. In Fig.6, we are able to see that the two main parameters that influe on the Kerf width are the thickness of the materials and the laser power. Now, we will focus on those two factors:

Table 3: ANOVA of the Kerf width analysis

To begin our analysis, let's start by studying the first parameter: the thickness of the CFRP (B). By varying the factors, namely gas pressure, laser power and cutting speed. It should be noted that to have the lowest Kerf width on the thinnest CFRP thickness, it is necessary to maximize the cutting speed as well as the laser power but to reduce the gas pressure to a minimum (Fig.7). But to have the lowest Kerf width on the thickest CFRP you have to maximize all parameters.

Figure 4: Influence of the thickness of the CFRP on the Kerf width

Then we will analyze the influence of the laser power on the Kerf width. Always by varying the factors, we find the best settings to obtain the smallest possible Kerf width. And unlike the previous parameter, the best setting for a low and powerful laser is the same, i.e. a reduced cutting speed and gas pressure and maximum CFRP thickness (Fig.8).

Figure 5: Influence of the laser power on the Kerf width

Now we will look at the interaction of these two parameters by varying the cutting speed and gas pressure. When we minimize the gas pressure and maximize the cutting speed for the thin thickness, we see that the powerful laser (in red) has a low Kerf width, approximately 59mm then the weak laser (in black) has a larger Kerf width with about 69mm (Fig.9).

Figure 6 : Influence of the thickness and the laser power on the Kerf width (low gas pressure and high cutting speed)

This trend is reversed as soon as the thickness is maximized. For the powerful laser to be a low Kerf width with thick CFRP it is sufficient to increase the gas pressure.

Heat Affected Zone analysis

Now let's move on to the analysis of the results obtained after combining our different parameters that influenced the width of the HAZ, we still have the same R^2 = 0.9896. As the data are the same as for the Kerf width, we obtain the same data for the ANOVA as well and therefore the same parameters: the laser power as well as the CFRP thickness.

So like for the Kerf width, let's start by the thickness. When we minimize the other three factors, we see that the thinnest thickness will have the largest HAZ unlike the thickest material which will have a smaller HAZ. However, when we maximize the parameters, the relationship remains the same, so we deduce that the thickness does not really influence the HAZ (or at least not alone).

Figure 7: Influence of the thickness of the material on the HAZ

Let's now move on to parameter C, the laser power. When we maximize the other parameters, we find that the laser with low power will make a larger HAZ, while the laser with high power will make a smaller HAZ (Fig.11). This trend is reversed when we minimize the parameters, the laser with a low power then has the smallest HAZ.

Figure 8: Influence of the laser power on the HAZ

Now we will see like for the Kerf width, the combination of our two parameters and by varying the other two factors. When these two parameters are reduced to a minimum, we see that the laser has low power (in black) creating a smaller HAZ than the powerful laser (in red) on the thin CFRP (Fig.12). But when the CFRP is at its maximum thickness then the powerful laser creates a smaller HAZ than the low-power laser.

Figure 9: Influence of the thickness and the laser power on the

HAZ (low gas pressure and speed cutting)

By increasing only the cutting speed, we can observe a reversal of this trend between the powerful and the weakest laser (Fig. 13). On a thin CFRP you get a low HAZ with the powerful laser and a high HAZ with the weak laser, and this is reversed on a thicker CFRP.

Figure 10: Influence of the thickness & the laser power on the

HAZ (low gas pressure & high-speed cutting)

Conclusion

To conclude this report, we were able to discover more about the challenges associated with CO2 laser cutting of composite materials. In this report we have only addressed one composite, the CFRP or Carbon Fiber Reinforced Polymer because it is the most advanced material in the research papers that I have studied. Due to the complexity of laser cutting, we understand why manufacturers are currently using other means of production such as jet cutting. However, laser cutting research has progressed a lot in recent years and we have been able to show throughout the report that if the machine is correctly configured and the thickness of our CFRP is correct we will be able to obtain a satisfactory cutting quality for potential customers. In addition, due to the automation, speed and quality of the cutting of the machines, it will be possible to develop their profitability. There is still a blocking point in our study, compared to the technical documents and theses found. For example, the maximum thickness of laser-cuttable CFRPs is currently 7mm[1], compared to the value given by our excel file which is 25mm, so this is an approximate study and requires further research.

References

1. A. Goeke, C. Emmelmann, "Influence of Laser Cutting Parameters on CFRP Part Quality" – 2010

2. Annett Klotzbach, Markus Hauser, Eckhard Beyer, "Laser Cutting of Carbon Fiber Reinforced Polymers using Highly Brilliant Laser Beam Sources" – 2011

3. A. Riveiro, F. Quintero, F. Lusquiños, J. del Val, R. Comesaña, M. Boutinguiza, J. Pou, "Experimental study on the CO2laser cutting of carbon fiber reinforced plastic composite" – 2012

4. J. Paulo Davim, Nuno Barricas, Marta Conceiçao, Carlos Oliveira, "Some experimental studies on CO2laser cutting quality of polymeric materials" – 2007

5. E. Abdulhadi, J.-M. Pelletier, M. Lambertin, J. Quesada, "CO₂ laser cutting of steel: heat affected zone investigation" – 1998

6. A.Lamikiz, L.N. López deLacalle, J.A.Sánchez, D. delPozo, J.M.Etayo, J.M.López, "CO₂ laser cutting of advanced high strength steels (AHSS)"- 2005

7. K. Jarosz, P. Löschner, P. Niesłony, "Effect of cutting speed on surface quality and heat-affected zone in laser cutting of 316L stainless steel" - 2016