Co-authors​ :

 Daiki TANABE (JAPAN), Takeshi EGUCHI , Kazuaki NISHIYABU (JAPAN) 

Carbon fiber reinforced thermoplastic (CFRTP) composites which can be manufactured by press molding, hybrid injection molding and automated tape laying are attracting attention recently in aircraft and automobile applications. The joining process is a necessary step to manufacture complex geometry parts and large-scaled structures using CFRTP. There are several types of fusion joining methods for CFRTP such as ultrasonic welding, resistance welding, induction welding and so on, and these heating principles are completely different [1]. The ultrasonic welding has some advantages such as very short welding time, low power consumption and ease of automation. The ultrasonic welding parameters for CFRTP has been investigated to optimize the welding conditions [1]. The method has been proposed in which an energy director made of thermoplastics is inserted into the welding interface [2]. Since the energy director is rapidly heated during ultrasonic welding, there is a concern that the reliability of the welding interface may decrease due to thermal deterioration of thermoplastic [3]. Furthermore, when the welded part becomes damaged, the damage progresses rapidly at the welding interface under high loads, so it is necessary to improve the strength and reliability of the welded part.
Our research group has been manufactured CFRTP round bars by pultrusion process and developed a CFRTP rivet fastening process using them [4]. CFRTP rivets can prevent weight increase and electrolytic corrosion compared to metal rivets. Furthermore, carbon fibers are continuously oriented on the shaft and head of the rivet. Our previous studies have shown that damage progresses of CFRTP rivets in stages and high shear strength can be obtained.
This study aims to development of “Hybrid Joining” process that combines ultrasonic welding and rivet fastening in order to improve the joining strength and reliability of CFRTP welded parts. The materials used for Hybrid Joining process was UD-CF/PEEK laminates, the energy director consisting of PEEK film and CF/PEEK round rod as the rivet fastening. The ultrasonic welding device consists of a servo press unit that can precisely control load and displacement, and a 20kHz ultrasonic heating unit. The ultrasonic welding parameters such as oscillation time, ultrasonic power, and sonotrode displacement, were investigated and evaluated to optimize the welding conditions. One end of CFRTP rivet head was fabricated by near-infrared heating and then press forming using the servo press. After ultrasonic welding, the welded part was perforated and riveted using near-infrared heating and a servo press. Then, the tensile shear strength of the welded part was evaluated using a single lap tensile shear test. In order to investigate the joint interface after welding, the condition of the welding part was evaluated by cross-sectional observation and microscopic observation.