Carbon fiber reinforced plastics (CFRP) are widely used in aerospace, sports, medical, transportation and other fields due to their advantages of high specific strength and high specific modulus. Taking Boeing 787 aircraft as an example, the mass fraction of composites in the aircraft structure has reached more than 50%. Composite structures of the aircraft are vulnerable to be damaged in harsh environments during long-term service, and composite structure repairing is an important part in aviation equipment maintenance. The committed step of composite structure repairing is the damaged zone removal at a small angle, usually 2°6°, which is difficult for mechanical processing. At present, it mainly relies on manual grinding, which shows low efficiency and poor controllability. Laser beam machining (LBM) has the advantages of high precision, high efficiency, and high controllability. It is suitable for difficult-to-machine materials including CFRP, and has excellent performance in laser cutting, drilling, welding, cleaning, etching and other applications. The structural characteristics and thermal-physical properties of CFRP materials result in a more complex physical process for laser removal of CFRP than that for metal materials. The existing studies mainly focus on the thermal process of laser removal of CFRP and the surface state evolution mechanism, but little attention is paid to the thermal-mechanical couple ablation process based on the non-homogeneous characteristics of the CFRP. In this paper, a 1064 nm nanosecond pulsed laser is used for laser removal of CFRP, and the effects of process parameters on removal efficiency, quality as well as the process optimization methods are studied. Based on the anisotropic heat transfer mechanism, the effect of laser scanning angle on material removal rate (MRR) is investigated and the influence of laser spot overlapping rate on thermal-mechanical ablation of CFRP is also discussed.
Multidirectional and unidirectional carbon fiber composite laminates are milled by a 1064 nm nanosecond laser. Fig. 1 shows the laser scanning area and path. The solid line represents the light-on state, and the dashed line represents the light-off state. The variations of ablation depth (h) with power (P), scanning speed (v), hatching distance (d), and scanning angle (θ) are tested, and the variations of the corresponding MRR with process parameters are also calculated. The macroscopic and microscopic morphologies of the sample surfaces are obtained by optical microscope and scanning electron microscope.
The laser peak power density shows a significant effect on the removal quality. Since the transmittance of the epoxy resin matrix in CFRP to the 1064 nm wavelength laser is about 80%, most of the laser penetrates the surface resin and directly acts on the carbon fiber during laser processing of CFRP, causing the carbon fiber to heat up, oxidize, and vaporize. Under the action of heat conduction, the epoxy resin around the carbon fiber is heated, ablated, and vaporized. The surface morphologies under different peak power densities are shown in Fig. 3. Since the axial thermal conductivity of the carbon fiber is about ten times the radial thermal conductivity, the preheating effect is more significant when the scanning direction is along the carbon fiber axial direction which results in a decrease in MRR with the increase of scanning angle (Fig. 4). The scanning speed and hatching distance determine the spot overlapping rate (αA) along the scanning direction and the spot overlapping rate (αB) perpendicular to the scanning path, respectively. As shown in Figs. 7 and 9, at a specific overlapping rate, the MRR peaks, which is caused by the thermal-mechanical ablation effect (Figs. 8 and 10). Multi-layer stepped removal of multi-directional carbon fiber composite laminates is carried out, and it is found that the ablation depth decreases with the increase of defocusing distance (Fig. 13). When the defocusing distance is shorter than the Rayleigh length of the focused beam, the precision of the removal depth is controlled within ±20 μm (Fig. 12).
In this paper, the influences of process parameters such as peak power density, scanning angle, and spot overlapping rate on MRR and removal quality are investigated, and the physical mechanism is disclosed in the removal process of CFRP with a 1064 nm nanosecond pulsed laser. Based on the anisotropic heat transfer mechanism, the influence of laser scanning angle on the material removal rate is studied. The smaller the scanning angle, the more significant the preheating effect, and the larger the ablation depth, 220 μm at 0° and 150 μm at 90°. The physical mechanism of the effect of spot overlapping rate on the thermal-mechanical ablation of non-homogeneous materials is explored, and MRR is improved by using this mechanism. The 18-layer stepped removal of the multi-directional CFRP laminate is carried out, and the precision is controlled within ±20 μm. The influence of laser defocusing state on MRR is discussed. This research provides a process optimization strategy for practical processing applications and improves the processing efficiency and quality.
