The effect of cooling rate on resistance-welded CF/PEEK joints

The influence of cooling rate on the mechanical properties of resistance-welded continuous carbon fiber/poly-ether-ether-ketone (CF/PEEK) joints and crystallization behavior of PEEK in the heating elements (HEs) was investigated. 6 kinds of joints with different cooling rates were designed and prepared in resistance welding processes. A lap shear strength (LSS) improvement of 28% was reported for joints with lower cooling rates relative to joints without controlled cooling process, and the crystallinity degrees measured by differential scanning calorimetry (DSC) of lower-cooling-rate joints were also higher. The fatigue performances showed the same trend. The polarized optical microscope (POM) images showed that stainless steel (SS) wires in the HEs acted as heterogeneous nucleating agents, promoting high-density nucleation sites and accelerating formation of transcrystallinity at a cooling rate lower than 80 °C/min, and the transcrystallinity was hardly observed at higher cooling rates. The results of crystallization kinetics also showed that the Avrami exponent n reduced with cooling rate increasing, indicating that a two-dimensional growth of transcrystallinity dominated at lower cooling rates. It was suggested that qualities of resistance-welded joints were significantly affected by crystallinity and morphology which were both controlled by the cooling rate.