Effect of energy director on microstructure and mechanical properties of CF/PEEK joints obtained by ultrasonic welding

The ultrasonic welding was employed to connect carbon-fiber-reinforced polyetherether-ketone (CF/PEEK) composites. The effects of the shapes of energy directors (ED) on the microstructure and mechanical property of joints were mainly investigated. The results showed that the heat affected zone (HAZ) of the joints was larger when the rectangular or triangular ED were used, which was caused by the high viscoelastic deformation of EDs, rapid heating rate at the interface and high interface temperature. Thermal stress was larger at the interface between carbon fiber and resin in the HAZ, which easily resulted in cracks and other defects. In addition, the flow pattern of ED made void defects easily occurred. Therefore, the joints had poor mechanical property. When flat ED was used, the interface formed a good connection with ultrasonic action time of 0.9 s. The flow pattern of flat ED was better than that of the rectangular and triangular ED, so no void defects occurred in joints whose tensile-shear strength could reach 28 MPa.