Tailoring the Crystallinity of Ultrasonically Welded Interfaces in Glass Fiber-Reinforced Thermoplastic Composites

Ultrasonic welding (USW) is a fast and effective method for joining thermoplastic composites, offering excellent bonding strength that results in lightweight, durable structures, making it a cost-effective alternative to traditional joining techniques. The crystallinity at the weld interface impacts the mechanical properties and chemical resistance of the joint. The crystallization mechanisms at the bonded interface remain inadequately understood for the USW process, especially given its rapid cooling rates. This study investigates the use of polypropylene (PP) and multiwalled carbon nanotube (MWCNT)/PP films for ultrasonic welding of glass fiber (GF)/PP adherends, focusing on how process parameters influence the crystallinity degree, crystalline phases, crystallite size and spacing, lamellar structure and anisotropy, and molecular changes at the welded interface. Differential scanning calorimetry (DSC), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and small-angle X-ray scattering (SAXS) were employed to gain a better understanding of crystalline structure at the interface. Four different sets of welding force and amplitude were tested: (1) 500 N, 38.1 μm, (2) 500 N, 54.0 μm, (3) 1500 N, 38.1 μm, and (4) 1500 N, 54.0 μm. The study demonstrated that despite fast cooling rates obtained during the process, higher welding force and amplitude significantly enhanced crystallinity, achieving 55% for welds with pure PP films and approximately 60% for MWCNT/PP films, compared to 35% and 41%, respectively, before welding. Notably, amplitude influenced the crystallinity at the welded interface more significantly compared to the force. SAXS experiments revealed that both pure PP and MWCNT/PP films exhibited isotropic structures prior to welding, but distinct anisotropy after welding. These findings suggest that strain-induced crystallization results from the welding process, with the degree of anisotropy correlating with the applied welding force and amplitude.