A numerical analysis of an energy directing method through friction heating during the ultrasonic welding of thermoplastic composites

The ultrasonic spot welding of fiber-reinforced thermoplastic laminates received a wide interest from researchers mainly in the fields of aerospace and automotive industries. This study investigated a new technique for focusing the ultrasonic vibration energy at the desired spot between two mating thermoplastic composite laminates. In this investigated method, no additional energy directing protrusions between the mating laminates were required to focus the vibration energy. It was found that by welding the laminates amid an ultrasonic horn and an anvil in which the prior had a larger contact surface with the laminate as the latter, it was possible to generate a localized friction heating. In the initial phase of the welding, the friction heating softened the interfacial layers and thus caused the focusing of the majority of the cyclic ultrasonic strain energy in the weld spot center. The assumption for the presence of the friction and its influence on the heat generation was investigated by means of finite element method (FEM) mechanical dynamic analysis. Microscopic analysis of the weld spot eventually delivered the proof for the melt initiation by friction at a ring around the weld spot and subsequent spot growth by viscoelastic heating.