Enhancement of ultrasonic welding of CFRTP by introducing in-situ laser-etched groove-type energy directors and structural parameter design based on energy absorption theory

Ultrasonic welding (USW), as an efficient and environmentally friendly joining technology, has demonstrated significant advantages in joining carbon fiber reinforced thermoplastic composites (CFRTP). In terms of improving joint strength, energy directors (ED) are generally introduced as a crucial structure for promoting interfacial heat generation and improving joint quality. To address the drawbacks of complex manufacturing and increased costs associated with traditional external ED structures, novel groove EDs were fabricated in this study using in-situ laser etching on the surface of CF/PA66. Interfacial morphology, mechanical property and fracture morphology at different grid parameters were measured and the reinforcing effect of groove EDs has been verified. Based on acoustic wave propagation theory, the energy absorption mechanism of groove-typed EDs was analyzed and the influence of structural parameters on the interfacial heat accumulation were further elaborated. The results demonstrated that in-situ laser etched EDs enable effective concentration of welding energy, which not only reduces the randomness and dispersion of weld distribution but also significantly increases the melting range, thereby improving joint failure load by 82.3 % compared to the joints without ED. The thermal-mechanical coupling finite element analysis also verified that the heat was concentrated and produced faster at the interface, compared with the joints without EDs. The addition of grid EDs enhances interfacial energy absorption, and the joints exhibit cohesive fracture as the dominant failure mode. The interfacial strengthening mechanism of laser etched EDs can be concluded into two aspects: (i) local protrusions of the grid EDs enhance interfacial viscoelastic heat generation and energy absorption, realizing interfacial heat concentration and further expanding the weld area. (ii) Grooves of the grid structure promote and guide the flow and spreading of molten material, forming mechanical interlocks at the weld edges to further improve joint mechanical properties. This study provides a new process route and theoretical basis for achieving high quality ultrasonic welding of CFRTP.