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laser machining of CFRP is a potential processing method as an alternative of conventional mechanical machining. This paper studies the absorption behavior of CFRP surface by establishing an optical absorption model based on the interaction of laser and fiber-matrix microstructure in material. In addition to polarization dependence, laser absorptivity is also subjected to the influence of wavelength, fiber volume fraction, and power intensity distribution within the laser spot. It is found that effect of multiple reflections in the microstructure would increase the absorptivity by 30%. Experiments have confirmed that absorption shows little fluctuation along the scanning line due to large laser spot. Short wavelength and arbitrarily polarized laser source are advantageous in achieving an effective and consistent machining process. CFRP sample for validation experiment is obtained through the nanosecond Nd:YVO4 laser milling process. The experiment data are in good agreement with the calculated results.
© 2015 Optical Society of America
1. Introduction
Carbon Fiber Reinforced Polymer (CFRP) is getting more and more popular in the fields of automation, aviation, wind turbine, etc. due to its high specific mechanical properties. In Boeing 787 and Airbus 350, a total of more than 50% materials by weight are made of composites. Although the manufacturing of CFRP is near net shape, a large amount of holes need to be drilled in assembly and scarf repair may be required in service process [1]. However, the machinability of CFRP is not very good with conventional mechanical machining method. Dynamical machining force in drilling, cutting process results in some defects, such as delamination, and cracks, which may result in the rejection of a large amount of products in aircraft industry [2]. To solve the problems appearing in the field of mechanical machining, Laser machining of CFRP, including cutting, drilling and milling, has drawn the attention of researchers [3–5]. Laser processing is non-contact and controllable, which has achieved wide application in many difficult-to-machine materials, such as ceramics, hard alloy, and so on. Material removal is realized through thermal ablation with almost no machining force. Fischer et.al. developed a laser milling process in the repair work of damaged composite laminates [4]. By this process, high quality repair scarfs with designed geometries are obtained through the material removal layer by layer.
In CFRP made of PAN-based carbon fibers, continuous carbon fibers with diameters ranging from 7 to 10 μm [6] are distributed in polymer matrix. When it is irradiated with a comparable laser spot, fiber-matrix microstructure significantly influences the behavior of laser absorption. In laser processing, large heat affected zone (HAZ) where polymer burnout forms around the machining area seriously deteriorates the machined quality due to different ablation thresholds of carbon fiber and polymer. Numerical modeling is used to predict the ablation depth and HAZ by [7–9]. However, even though modeling in micro scale considering microstructure of CFRP is performed in some studies, laser absorption coefficient is usually assumed to be a constant ignoring the specific interaction between laser and material. Especially for laser milling, the interaction between laser and CFRP should be investigated locally to reveal the characteristics of energy input on the machining surface. When the laser beam moves along the scanning path, the absorptivity is fluctuant. The fluctuation effect plays an important role in the forming process of ablation morphology and machining defects. Li [5] observed the material removal between two scanning paths in multi-rings drilling strategy to reveal the remarkable influence of fiber microstructure on machining quality within the composites.
A limited number of researches show concerns with the absorptivity of CFRP. Carbon fiber consists of layered graphite. The optical properties of graphite, such as complex reflectivity and , have been measured in lots of experiments [10]. Graphite is uniaxial crystal. Mosteller et.al [11]. and Lekner [12] established analytical Fresnel models based on Maxwell's equations to calculate the reflectivity at the interface between incident medium and graphite. Freitag et.al [13]. studied laser absorption based on the Fresnel model in single carbon fiber and CFRP structure by ray tracing method, in which the polarization dependence phenomenon of laser absorption was found. However, some real processing conditions, such as laser spot diameter, power intensity distribution within a laser spot, specific scanning process are not considered.
In this paper, optical interaction of laser and CFRP is modeled. Based on Fresnel reflection, the absorptivity of CFRP is studied by ray tracing method. In order to get clear on the absorption behavior in laser machining, typical processing parameters, including moving laser, Gaussian distribution of power intensity are considered. Before validating the model, laser milling of CFRP by using 532nm nanosecond pulsed laser system was conducted on a galvanometer processing platform. Then the reflection measurement is performed on the surface of machined CFRP.
2. Experiment setup
The CFRP sample of laser machining was prepared using a 14.25W frequency doubled nanosecond pulsed laser milling system in Fig. 1. CFRP in the experiment consists of 60% volume fraction of Toray®T300 carbon fiber and 40% volume fraction of epoxy matrix. The lay-up is [0/90°]6s. In the machining, parallel scanning lines are hatched on the milling area to ablate a layer of material. In the next layer, direction of hatching scanning lines is changed by 90°. Focused plane of the laser beam is kept on the machined surface. Through cyclic layers ablation, a pocket with designed depth is manufactured.
Fig. 1 Schematic of experimental set-up, optimal processing parameters: scanning speed 1m/s, pulse width 10ns, hatching distance 20μm, spot diameter 30μm, repetition rate 50KHZ.
3. Absorptivity calculation of CFRP
This paper discusses a kind of widely used carbon fiber, named PAN-based carbon fiber. The cross section has two distinct regions: skin region and core region [6]. The skin region is composed of graphite layers, which are roughly parallel to the fiber surface. Graphite has uniaxial crystal with the optical axis perpendicular to the graphite layer. Therefore, optical axis of the carbon fiber is normal to the fiber surface.
When a laser ray is incident on the carbon surface, there appear two transmitted waves and one reflected wave. The coordinate system is defined in Fig. 2, in which z is along the fiber axis, while x, and y are along the radius direction. Incident ray, optical axis, and reflected ray are all in the x-y plane of incidence. Analytical Models in [11, 12] are adopted in calculating the absorptivity at the reflecting plane. Electrical field of incident wave is decomposed into s component, and p component, which are perpendicular and parallel to the plane of incidence, respectively.
According to [12], incident perpendicular component just generates the perpendicular reflected wave, while incident parallel component produces the parallel reflected wave. The reflectivity for the ordinary and extraordinary wave can be denoted as:
where where is the angle of incidence, is the refractive index of incident medium, is the complex refractive index for the ordinary wave, is the complex refractive index for the extraordinary wave. The optical properties of carbon fiber and matrix for typically used wavelength 355nm, 532nm, and 1064nm are listed in Table 1.
Table 1. Optical constants of CFRP [10, 13]
Laser used for CFRP machining has arbitrary polarization. The absorptivity A of a single fiber at the reflecting plane can be denoted as:
CFRP is represented by two rows of carbon fibers (circles in the figure) which are 7μm in diameter as shown in Fig. 3. The empty space among the fibers is filled with polymer matrix. Carbon fibers accounts for 60% volume fraction. As laser beam can be tightly focused in laser processing, the diameter of laser spot in this study is set as 16μm which covers up to the three fibers at the same time. Power intensity in the laser spot is shown as Gaussian distributed. There are different types in the interaction of laser with CFRP as shown in Fig. 3. In (a) and (b), light ray is reflected by less than two times. When the reflection number is more than there, it is assumed to be completely absorbed. A single ray in the laser beam is traced to calculate the local absorptivity. And then average absorptivitywithin a laser spot is the mean value of the local absorptivity of all the rays weighted by its power intensity.4. Results and discussion
4.1 Experiment validation
Figure 4 shows the local absorptivity within a laser spot. The type of intensity distribution within the laser spot influences the average absorptivity. The good absorption behavior showed by CFRP is beneficial in laser processing. Due to the fiber-matrix microstructure, it can be seen that absorptivity of CFRP varies along the X coordinate and shows polarization dependence. When power intensity within the laser spot is Gaussian distributed, average absorptivity of parallel component, p is 90.64%, which is 2.3% more than that of S component at the wavelength of 355nm. Local absorption curve presents periodic variation due to the symmetric material structure of CFRP. Select the area from 4μm to 8μm for discussion. In the area from 4μm to 5.2μm, light rays are trapped in fiber structure and completely absorbed. In the area from 5.2μm to 5.7μm, the light rays by multiple reflection move away from the fibers. In the area from 5.7μm to 8μm, light rays are reflected away by just one reflection, the absorptivity of which is relatively lower than the other two areas.
Measurement of absorptivity on the surface of machined sample was performed by using UV/Vis/NIR Spectrophotometer. The calculated absorptivity of S and P polarized wave are obtained at three wavelength 355nm, 532nm and 1064nm as shown in Fig. 5. The variation trend of calculated values agrees well with the experimental values. The absorptivity of CFRP is as high as 90% at UV light, and drops with the increase of wavelength. This is one of the reasons why UV laser source is widely used to process this kind of material [4, 5, 8, 14]. The experimental values are slightly higher than calculated values. There may be two reasons related to this deviation. The first one is that measured reflectivity is relatively small because not all the reflected light rays are detected by sensors due to the diffuse reflecting at the surface. The other is that carbon fibers in CFRP are not uniformly distributed as assumed in the model.
4.2Multiple reflections effect
The high absorptivity of CFRP owns to multiple reflection happened between two carbon fibers. This absorption process is different from what has happened on the homogeneous material surface where light is absorbed by one reflection. The absorptivity of interaction zone from 4μm to 8μm is compared with the situation which considers the reflection only once as shown in Fig. 6. For both S, and P component, two absorptivity curves coincide at the area where only one reflection happens. The enhancement effect is obvious with multiple reflections included. The average absorptivityfor S and P polarization are raised from 64.74% and 73.42% to 88.70% and 90.76% respectively. Therefore, microstructure composed of scattered carbon fibers effectively improves the surface absorption of laser. The absorption of CFRP is dependent on the volume fraction of carbon fibers.
Fig. 6 Absorptivity with and without multiple reflections at wavelength 355nm (a) S polarization (b) P polarization.
4.3Absorption fluctuation effect
In the laser processing of CFRP, focused laser spot moves on carbon fibers along the scanning path. As the diameter of laser spot is just as large as that of a few fibers, the absorptivity may vary at different places. Big absorption fluctuation should be avoided to ensure uniform ablation morphology in machining area. Figure 7 shows the absorptivity variation when the center of the laser spot moves from 8μm to 16μm. The curve of arbitrary polarization is the average of S and P polarization components. The absorptivity reaches the maximum when laser is incident above the fiber and the minimum when it is between the fibers. The amplitude of fluctuation is just 0.2%, which can be neglecting in machining. However, when the laser spot is further focused to the size of one carbon fiber, the fluctuation will be large enough to influence the consistency and accuracy of machining.
5. Conclusion
Laser machining is a potential processing method of CFRP due to its high efficiency and excellent machining quality. Different from homogeneous materials like metal, the microstructure of CFRP composed of fibers and polymer matrix has a remarkable impact on laser absorption. Interaction of laser with the special microstructure in CFRP determines the absorptivity behavior, which is affected by laser wavelength, laser polarization, and fiber volume fraction. CFRP surface has good absorption due to the effect of multiple reflections. The Existence of multiple reflections in fibers increases the absorption by 30%. The absorption model established in this paper provides a reliable way to evaluate the absorptivity on machined surface, which is useful for numerical simulation. It also provides some guidelines for laser source selection. CFRP has better absorption at short wavelength. To manufacture the designed shape, machining consistency, namely, uniform material removal on the milled surface should be ensured. The polarization dependence indicates that arbitrarily polarized laser is preferred in eliminating the influence of variable fiber direction in CFRP laminates. Meanwhile, the absorption fluctuation duo to moving irradiation should be limited as little as possible. As laser spot is generally as large as several fibers, this influence is proven to be of little impact.
Acknowledgments
This project is supported by National Natural Science Foundation of China (Grant No. 51575352).
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