Currently the joining of cured thermoset composite materials for aerospace applications occurs in a suboptimal manner. The structure is joined by mechanical fastening, optionally in conjunction with an adhesive. The joining is sub-optimal because the fasteners require holes in the thin fibrous structure. These holes are stress concentrations, therefore the number of plies and thus the weight of the structure is increased to accommodate them. Adhesive joints do not require holes but these joints still have challenges in the field of non-destructive testing (NDT), which precludes their use for the joining of structural parts in aerospace. Consolidated thermoplastic materials have an additional joining technique that can be used, namely fusion bonding. This technique uses the property that distinguishes thermoplastics from thermosets, they have the ability to melt or soften under increased temperature. In this molten or softened state themolecular chains of the thermoplastic resin possesses increased mobility. If the thermoplastic is in intimate contact with another miscible thermoplastic while it is in this heated state the molecular chains can diffuse into each other and intermingle, creating a bond. This bond does not possess the stress concentrations associated with mechanical fastening, or the phenomenon of kissing bonds which inhibits certification in adhesive bonds. In industry fusion bonding is solely applied to unreinforced thermoplastic materials, fusion bonding of thermoplastic composites is only applied in research. To enable fusion bonding of cured thermoset composites a thermoplastic interlayer can be co-cured, which is a thin thermoplastic film that provides a fusible surface to the thermoset structure, it can thus be used to fuse the thermoset structure to another fusible part. The interlayer can be surface treated before cure so that they can form an interfacial adhesive bond with the thermoset laminate. This approach makes the joint susceptible to the same challenges faced by adhesive bonding. Therefore this research investigates the use of a material combination that is initially miscible, so that they can form an interphase during cure instead of an adhesive interface, the interphase is a gradual transition from onematerial to the next . The fusion bonding process itself also poses some challenges. Fusion bonding of aerospace grade thermoplastics typically require temperatures far above the maximum temperature allowed by the co-cured thermoset laminate. A solution to this problem is the use of a fast and concentrated method of heat generation that can keep the elevated temperature experienced by the thermoset material limited and short in time, preventing thermal degradation. Ultrasonic welding is a fusion bonding technique that is capable of creating fusion bonds with heating times in the order of 500ms, proven to be short enough to prevent thermal degradation for the materials used in this research. Another important issue foreseen and addressed in this research is the question if the interphase between the interlayer and the thermoset material can withstand the aggresive welding process. Previous experience with surface treated interlayers showed that some interlayer/ surface treatment/thermoset material combinations can detach at the interlayer/thermoset interface after welding. This research shows that the interphase is affected in some locations, but the fusion bond is nonetheless maintained through the entire weld overlap. The apparent lap shear strength of the joints created with an interphase forming interlayer is comparable to the joints created with adhesive interface forming interlayer. Both joint types retain their strength when exposed to prolonged exposure to moisture, simulating a worstcase scenario in amanufacturing setting. Exposure to a cleaning solvent significantly affected the interphase forming joint type, attributed to the amorphous structure of the thermoplastic used. This research shows that when selecting the right material as interlayer for creating a fusion bonding capable thermoset laminate, a joint can be created that could avoid the certification challenge experienced by adhesive bonding, while simultaneously using aerospace grade materials without thermally degrading the joint during fusion.