Enhanced Performance of Ultrasonic Welding of Short Carbon Fiber Polymer Composites Through Control of Morphological Parameters

Ultrasonic welding (USW) is one of the joining technologies that can be applied to short carbon fiber thermoplastic composites. In this study, the USW of Nylon 6 reinforced by short carbon fibers created using injection molding is used to investigate the USW process without energy directors. In addition to process parameters and performance parameters, a new category of parameters is introduced to characterize the behavior of base materials to control USW without energy directors. These parameters, named morphological parameters, are the degree of crystallinity (DoC) and the ratio of the crystalline phases of Nylon 6 (α/γ ratio). One method of controlling the morphological parameters is annealing. A design of experiments is carried out using 5 replicates and 7 annealing temperatures above the glass transition temperature (Tg) and below the melting temperature (Tm) of Nylon 6 to investigate the influence of annealing on the morphological parameters. The DoC and α/γ ratio are measured for each replicate by utilizing differential scanning calorimetry and X-ray diffraction. The results show that the DoC becomes uniform and the α/γ ratio increases after annealing. Consequently, the variation in weld strength decreases and the average weld strength increases by controlling the morphological parameters through annealing.