Investigation on energy director-less ultrasonic welding of polyetherimide (PEI)- to epoxy-based composites

In ultrasonic welding of thermoplastic composites an energy director (ED) (i.e. neat thermoplastic film), is used between the two adherends to be welded, to promote frictional and viscoelastic heating. For welding of thermoset composites (TSC), a thermoplastic coupling layer is co-cured on the surface to be welded as typical procedure to make the TSC “weldable”. This study focuses on investigating whether a polyetherimide (PEI) coupling layer by itself has the potential to promote heat generation during ultrasonic welding of CF/epoxy and CF/PEI samples, without the need for a separate ED, and if so, what thickness should that coupling layer be. The main findings were that welding without a loose ED resulted in overheating of the CF/PEI adherend and/or coupling layer due to the inability of the latter to promote heat generation efficiently. However, welding of CF/epoxy and CF/PEI samples with the use of a loose ED resulted in high-strength welds.