Influence of focus position variation on laser ablation of thermoplastic CFRP as preparation for three-dimensional repair processes

The use of composite materials is well established in lightweight construction applications, and it is widely used in the aerospace industry. Currently, the focus is shifting from thermoset to thermoplastic composites (CFRTP) due to their advantages in sustainability, resource efficiency, and manufacturing. CFRTP enables out-of-autoclave processing and integral construction methods. The meltability of thermoplastics allows the welding of CFRTP parts. This offers new opportunities for efficient repair strategies. A promising repair process is the laser-based ablation of damaged materials, which ensures a stepwise material removal without compromising surrounding structures. Subsequently, the ablated area is filled by inserting thermoplastic repair patches. In this study, the laser ablation of polyphenylene sulfide-based CFRTP is investigated using a high-power nanosecond pulsed laser. The investigations serve as preparation for the development of a process strategy for three-dimensional ablation on curved surfaces. Continuous curvature leads to a shift in focal position if not adjusted. Real-time focus adjustment is complex and costly. Therefore, the focus position was systematically varied in discrete steps to analyze its influence on ablation volume, surface quality, and taper angle. The focus shift effectively causes a variation in the spot radius and changes the intensity profile. To account for these effects, the results are correlated with the resulting single pulse fluence as well as with the number of overlapping pulses acting on the same surface location. Optical measurement techniques, like laser scanning microscopy, were employed to assess the effect of different focal positions.