纵向动态变焦下皮秒激光切割 CFRP 研究

Objective Carbon fiber reinforced polymer (CFRP) composites are important strategic materials. Owing to significant differences in thermophysical properties of resin and carbon fiber, it is extremely difficult to perform subtractive processing. Laser processing has the advantages of noncontact and easy automation, and is more advantageous in CFRP processing. However, in conventional laser processing, the efficiency and quality are often unbalanced, especially in thick-plate cutting. The removal of materials outside the depth of the focus range can only rely on the slow thermal accumulation of the laser pulse, which seriously affects the quality and efficiency of laser processing. We used the liquid-crystal spatial light modulator (SLM) to control the movement of the laser focus during processing by dynamically loading the Fresnel phase, which helps the laser energy act better on the interior of the material to achieve a more efficient and high-quality CFRP cutting. Methods The Fresnel phase can be generated by calculation to achieve an accurate change in focus. The SLM dynamic loading was controlled by the algorithm to achieve the change in focus during processing (Fig. 1). The longitudinal dynamic zoom method was adopted to shorten the laser action time at the same longitudinal position (Fig. 2). During the cutting of 1 mm thick CFRP sheets, the focal length was changed twice, with a longitudinal downward zoom of 0.33 mm each time. This study mainly compares the differences in quality and efficiency between this longitudinal dynamic zoom method and conventional single-focus processing, and seeks the optimal longitudinal dynamic zoom cutting parameters by optimizing the scanning width and contour spacing (Fig. 5). The processing effect was evaluated by the processing time, heat affected zone, cross-sectional taper angle, and roughness of the cross-section (Fig. 6). Results and Discussions Appropriate laser parameters were selected for CFRP cutting. The characterization data of the heat affected zone of the surface, height difference of the cross-section, and cross-sectional roughness under single-focus processing and dynamic zoom processing are given (Fig. 7). The total processing time of the fastest dynamic zoom is 159 s, which is 30.8% faster than that of the single-focus processing. With slight reductions in taper angle and the roughness of the cross-section, the heat affected zone can be reduced by 14.2%. The influence of the change in the scanning width and contour spacing on the machining quality and efficiency is further studied. Within the scanning width range of 0.1‒0.6 mm, the width of the heat affected zone gradually increases (Fig. 8), the cross-sectional gradient gradually decreases (Fig. 9), and the cross-sectional roughness first decreases and then increases (Fig. 10). From the comprehensive processing time, a scanning width of 0.3 mm is a relatively reasonable processing parameter. At this time, the transverse and longitudinal removal of the material by the laser approaches a balance. Under this parameter, the laser works efficiently and can complete the cutting at the fastest speed while improving the cutting quality (Fig. 11). Under the contour spacing of 1‒20 μm, the heat affected zone shows a trend of first decreasing and then increasing (Fig. 12), while the cross-sectional taper angle and roughness show only slight changes (Figs. 13‒14). This indicates that the change in contour spacing only affects the surface heat affected zone. At this time, a contour spacing of 10 μm is the optimal processing parameter. Conclusions In this study, a picosecond laser was used to conduct an experimental study on CFRP cutting. Combined with the iterative Fresnel phase of a liquid-crystal SLM, the longitudinal movement of the focus along the direction of light transmission was achieved, and a new longitudinal dynamic zoom processing method for the CFRP was proposed. Experiments show that compared to traditional fixed-focus cutting, this method can increase the cutting speed by 30.8% and reduce the heat affected zone of cutting by 14.2%, with almost no influence on the taper angle and roughness of the cross-section. It successfully demonstrates its advantages in processing quality and efficiency. Combined with the scanning galvanometer, the influence laws of the scanning width and contour spacing on the processing quality and efficiency were studied. With a scanning width of 0.3 mm and contour spacing of 10 μm, the processing time can be reduced from 159 s to 36 s, and the speed was increased by 77.3%. The heat affected zone width decreases by 48.8% from 13.22 μm to 6.77 μm. This longitudinal dynamic zoom processing method effectively enhances the laser processing ability of CFRP and can address the difficulty in balancing the quality and efficiency of the secondary processing of large-thickness plates.