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

Abstract

This paper addresses the critical need for a comprehensive investigation into 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, an experimental investigation takes place where the two materials are joined with a varying laser power. To assess the extent of thermal degradation, the produced joints are examined with optical microscopy, scanning electron microscopy, and attenuated total reflection – Fourier transform infrared spectroscopy. To correlate the resulting thermal degradation with their mechanical response, lap-shear tests are performed. A good agreement is found between the two investigations: the model accurately identifies 500 W as the critical threshold where thermal degradation initiates (α ≈ 1.2%), leading to a 9% drop in joint strength. Optimal joint performance is achieved at 450 W - just below the degradation threshold - while higher powers result in severe thermal damage and porosities, causing performance losses of up to 76%. These findings demonstrate that the proposed methodology can effectively determine the thermal limits of CF/PEEK in fast heating applications where the exact temperature-time combination that would lead to thermal damage is elusive. Therefore, the model could be used to optimise a range of joining applications where high-temperature - short-duration processing is applied and thermal degradation is a potential issue.