Mechanism of Welding Surface Roughness on Weld Joint Performance in Thermoplastic Composite Structures

This study investigates the ultrasonic welding (USW) of glass fiber reinforced PA6 (GF/PA6) thermoplastic composites without the energy director (ED), and reveals for the first time the effect of surface roughness in USW without ED. Specifically, a coupled thermo‐mechanical finite element (FE) model was developed to analyze the welding parameters by revealing the evolution of the interfacial temperature field and molten zone. Following this, a particular emphasis was placed on the critical influence of welding surface roughness on joint properties. The results showed that using optimized welding parameters—a welding amplitude of 70% of the rated value and a welding time of 0.8 s—the lap shear strength (LSS) of GF/PA6 reached a high value of 27.6 MPa. On this basis, when the welding surface roughness was reduced from 16.3 μm to less than 9 μm through simple sanding, the diffusion and bonding of the molten resin and fibers were promoted, which further increased the LSS from 27.6 to 29.9 MPa.