Thermal degradation of thermoplastic composites in high-temperature-short-duration processing: An investigation for composite-metal joining

Abstract

The present work examines the thermal degradation of poly-ether-ether-ketone (PEEK) and carbon fibre (CF) reinforced PEEK in high-temperature - short-duration processing. The first part of the study examines the effect of rapid high-temperature processing on PEEK, CF-PEEK, and the interface of CF-PEEK. First, a new peak at 1711 cm−1 is detected with ATR-FTIR that can serve for characterising the extent of thermal degradation that occurs in PEEK and CF-PEEK in these conditions. In addition, the analysis shows that a decomposition of 1% of the materials’ PEEK matrix content does not significantly affect their crystallinity content, especially at faster heating rates. Furthermore, a significant improvement is captured in the IFSS of CF-PEEK which can be up to 25% at faster heating rates and is attributed to the thermal residual stresses that are formed at CF-PEEK in these conditions. Then, the laser heating of PEEK and CF-PEEK is examined and a composite/metal laser joining case study is also investigated. Results show that short-time laser heating acts as an annealing process that triggers recrystallisation and hardening phenomena that can improve the surface properties of PEEK. A further increase in the heating duration results in surface carbonisation and char layer formation, significantly affecting the material. Then, a coupled thermal-chemical numerical model is developed that can identify the processing conditions that can reach high temperatures at laser-heated CF-PEEK without triggering the decomposition of the PEEK matrix and the model is used to optimise a laser joining case study between CF-PEEK and Titanium. Interestingly, a good agreement is reached between the numerical and the experimental results, and the developed model can be used to optimise several high-temperature and laser applications of CF-PEEK where thermal degradation is an important damage mechanism such as composite-metal laser joining, induction welding, and composites machining.