The service environment of composite paint layer on civil aircraft skin is severe, making local and overall paint removal become necessary steps for routine maintenance of civil aircraft. Currently, manual grinding and chemical stripping are main technical means for paint removal of civil aircraft. Manual grinding is inefficient and easy to damage aircraft skin. Although chemical stripping has higher efficiency, the serious waste pollution can not meet increasingly stringent environmental protection requirements. Therefore, laser cleaning technology with low cost, high efficiency and minimal pollution has entered vision fields of researchers. However, the interaction and coupling effects among ablation, plasma shock (shielding), and thermal stress mechanisms during the cleaning have not been deeply understood at present. In this paper, effects of different laser overlap rates in y direction on cleaning quality are investigated using high-repetition-rate pulsed fiber laser cleaning system. By analyzing macro-micro morphological characteristics and composition changes of cleaned surface, mutual effect and coupling behaviors of different paint removal mechanisms and thermal degradation principle of residual paint layer are elucidated. We hope that the above researches can provide theoretical and experimental support for industrial application of laser-cleaned aluminum alloy composite paint layer.
In this paper, the laser cleaning experiment of composite paint layers of 2A12 aircraft aluminum alloy is performed using the high-repetition-rate pulsed fiber laser complete processing equipment under process conditions of four different laser overlap rates in y direction (ηy=0%, 20%, 40%, 60%). Firstly, optical microscope (OM) is used to observe the states of cleaned surfaces and sections. Then, morphological characteristics of macro- and micro-structures as well as compositions and element contents on cleaned surface are further investigated by scanning electron microscope (SEM) and energy dispersive spectrometer (EDS). Furthermore, Fourier transform infrared spectroscopy (FT-IR) is adopted to study the change of functional groups in acrylic-polyurethane composite paint layer. At last, the composition change and content of cleaned surface are analyzed by X-ray photoelectron spectroscopy (XPS).
Laser cleaning experiments are carried out with y-direction overlap rates as variables, in which composite paint layer systems exhibit various removal degrees. Overall, with the y-direction overlap rate increases, effects of laser plasma shock and shielding are weakened and the ablation and gasification of composite paint layer are strengthened (Figs.4 and 6). Obviously, there are ablation-plasma shock and shielding-ablation circular paint removal mechanisms during the cleaning process [Figs.6(a) and (b)]. Main components of the crater surface formed by ablation include thermal degradation product of paint layer, ash, fixed carbon, topcoat colorants, and functional oxide particles (Fig.7). By analyzing the action characteristics of different paint removal mechanisms, it is found that the residual paint layer appears delamination and fragmentation, which is mainly caused by plasma shock and thermal stress [Figs.6(a)-(d)]. The changes of functional groups and components in residual paint layer show that the free radicals in acrylic resin chain segments are replaced and rearranged (Fig.8 and Table 8), which is due to the thermal effect of ablation and plasma shock.
Laser paint removal experiments on 2A12 aluminum alloy surfaces are performed under different laser overlap rates in y direction. Results demonstrate that the topcoat is removed completely and the primer is removed partially when the overlap rate is 20%; the residual primer exists barely with the destroyed oxide film and exposed aluminum alloy substrate when the overlap rate is 40%; the paint layer is completely removed and the substrate is damaged when the overlap rate is 60%. The research shows that there are three paint removal mechanisms of ablation, plasma shock (shielding),and thermal stress during the paint removal, in which the ablation leads to the stripping and deposition of the β-type copper phthalocyanine in primer and functional oxidation particles in composite paint layer and plasma and thermal stress cause physical damage of the paint layer. The appearance of crater-spacing-crater cleaning feature on cleaned surface demonstrates that the paint removal mechanisms of ablation and plasma shock (shielding) occur alternatively. At the same time, the heat affected zone forms on the surface of residual paint layer, which induces thermal stress paint removal during the cleaning. The thermal degradation mechanism is revealed by analyzing the changes of functional groups and components in residual paint layer.
