On preventing thermal damage in high-temperature joining applications of thermoplastic composites with metals

This paper addresses the critical need to investigate the thermal limits of thermoplastic composites in thermal joining applications with metals. A numerical framework is employed to identify processing conditions that prevent thermal degradation in composite-metal joining, demonstrated through a case study of laser joining carbon fibre (CF) reinforced poly-ether-ether-ketone (PEEK) with a Ti6AL4V Titanium (Ti) alloy. The PEEK kinetics are integrated in the numerical solver and a coupled thermal-chemical analysis takes place that accounts for the heating rate effect on the material's thermal response. To validate the model, experiments are conducted where the two materials are joined under varying laser power. To assess the extent of thermal degradation, optical microscopy, scanning electron microscopy, and attenuated total reflection – Fourier transform infrared spectroscopy are used. To correlate thermal damage with mechanical performance, lap-shear tests are performed. A strong agreement is observed between simulations and experiments: the model accurately identifies 500 W as the critical power at which degradation initiates, leading to a 9 % drop in joint strength. Optimal joint performance is achieved just below this threshold, at 450 W, while higher powers cause severe thermal damage and porosities, reducing performance by up to 76 %. These findings demonstrate that the proposed methodology can effectively determine the thermal limits of CF/PEEK in rapid heating applications where critical temperature-time combinations are difficult to identify. The model can therefore be used to optimise a wide range of high-temperature - short-duration processes where thermal degradation is a potential issue. • A methodology is proposed for detecting the thermal limits of thermoplastic composites in thermal joining applications with metals. • The model accurately detects the critical parameters that first trigger the thermal degradation of CF/PEEK in laser joining with Titanium. • Optimal joint performance is found just below the identified degradation threshold. • Higher powers lead to porosities, significantly impacting joint performance. • The proposed numerical framework can be used to optimise a range of joining applications where fast heating is involved and thermal degradation is a potential issue.