Time‐Resolved Thermal Monitoring and Post‐Weld Crystallization Characterization of Ultrasonically Welded Glass Fiber/Polypropylene

This study presents an in‐depth investigation of crystallization behavior in polypropylene (PP) during ultrasonic welding (USW) of glass fiber‐reinforced thermoplastic composites using in situ and post‐weld characterization techniques. By systematically varying welding duration and monitoring thermal evolution via embedded thermocouples, interfacial cooling rates were linked to morphological development using analytical methods directly applicable to thin film characterization, including Differential Scanning Calorimetry (DSC), Wide‐Angle X‐ray Diffraction (WAXD), Raman spectroscopy, and Small‐Angle X‐ray Scattering (SAXS). Post‐weld interfacial characterization using DSC, WAXD, and Raman spectroscopy indicates that short welding durations (100–300 ms) largely preserve crystallinity due to limited melt disruption and rapid cooling. Prolonged welding (750–1000 ms) reduces crystallinity, decreases crystallite size, and promotes structural disorder as a result of longer thermal exposure. SAXS and Ozawa analyses show that slower cooling and residual heat can enhance crystallization kinetics and promote lamellar thickening under controlled conditions. However, this behavior does not result in higher interfacial crystallinity during actual welding, where the combined thermal and mechanical stresses are dominant. Overall, selecting welding conditions requires a balance between adequate thermal input for bonding and excessive exposure that can disrupt crystalline order.