Experimental and Computational Investigation of Ultrasonic Welding of Short Carbon Fiber Reinforced Plastics

Abstract

Carbon fiber reinforced plastics (CFRP) have been used in various industries due to its beneficial mechanical properties for light-weighting. Joining of CFRP is one of the most critical barriers for high volume application because the joining technologies used for metallic structures are not feasible for CFRP. Ultrasonic welding (USW) is a friction welding technology in that parts to be joined are clamped together under a trigger force and subjected to a high frequency vibration through the oscillation of a horn, leading to heating and melting of the material for bonding. USW is fast, clean, and suitable for automation, thus is attractive to the automotive industry for high volume production. Ultrasonic welding generally uses energy directors on the surfaces of the parts to be joined by concentrating the energy to help melting. Despite the positive role of energy directors on weld formation, their manufacture involves additional costs related to tools and equipment. Moreover, their presence at the interface between the specimens modifies the composite structure locally, leading to a non-uniform distribution of fibers in the weld area. Hence, the current industrial practice seeks to eliminate the use of energy directors. However there has been limited research of ultrasonic welding for CFRP without energy directors. Therefore, this dissertation focuses on understanding the mechanism of ultrasonic welding of CFRP without energy directors. In this dissertation, three research topics are addressed: (1) Influence of morphological parameters on welding process and weld performance of CFRP by ultrasonic welding: The research investigates the influence of degree of crystallinity and the crystalline phase weight ratio of CFRP (here, α/γ ratio for Nylon 6) on welding performance experimentally. An annealing process is used to control the degree of crystallinity and the crystalline phase weight ratio with various annealing temperatures and heating speeds. With the changes of morphological parameters, the mechanical properties and viscoelastic properties are measured to investigate the influence of morphological parameters on the ultrasonic welding process and its performance. (2) Process modeling of ultrasonic welding of carbon fiber reinforced plastics without energy directors: A finite element method (FEM) model is created to investigate the heating phenomena of ultrasonic welding process of CFRP without an energy director. The mechanical properties of CFRP are measured to create the FEM model in ABAQUS. The model is validated with experimental results of temperature and weld area evolution in ultrasonic welding of CFRP. Different morphological parameters will be used in FEM model to investigate the impact of morphological parameters in ultrasonic welding of CFRP. (3) Dynamic analysis of ultrasonic welding of CFRP according to boundary conditions: A dynamic FEM model is created to investigate the influence of natural frequency in ultrasonic welding of CFRP by varying the weld locations. The dynamic FEM model is used to calculate the natural frequencies near the vibration frequency with respect to the entire ultrasonic welding process. The dynamic FEM model results will be compared with the process FEM model results of ultrasonic welding in terms of heat generation depending on the effect of natural frequency. This dissertation provides new understanding and insights to improve the process stability and quality in ultrasonic welding of CFRP without energy directors.