Effects of cryogenic pre-cooling on the drilling performance of unidirectional CF/PEEK composites

Drilling is essential for composite structures as it ensures joint strength and assembly accuracy. The temperature sensitivity of thermoplastic carbon fiber-reinforced polyetheretherketone (CF/PEEK) significantly impacts drilling quality. However, research on how temperature control during processing affects its machinability remains limited. This study introduces cryogenic pre-cooling (CP) as a method to control drilling defects in CF/PEE and compares its performance with conventional room-temperature (AT) processing. The study examines chip morphology, axial force, and exit damage, with particular emphasis on the effects of temperature changes on fiber cutting angles, exit damage, and hole wall integrity. The results indicate that CP reduces the material’s ductility and makes CF/PEEK more prone to brittle fracture under tool compression, thereby significantly shortening chip length. Although CP increases the maximum drilling force by 19.1% compared to AT conditions, however, it reduced the maximum drilling temperature by up to 88.84%, lowers the maximum exit damage area factor by 80.5%, and decreases the maximum exit delamination factor by 2.47%. We have obtained the optimal process parameters for low-temperature drilling, improved the dimensional accuracy of the drilled holes, and minimized surface defects. This study provides practical guidance for high-precision, low-damage machining of CF/PEEK aerospace structural components.