A study of ultrasonic metal welding

Ultrasonic metal welding (USMW) has received significant attention in the past few years, and has become more reliable and suitable for a wide range of applications. In recent years, the technique has been extensively used due to the advent of component miniaturisation and improvements in producing lightweight components. There are a number of advantages for USMW, including greater efficiency and speed, longer tool life, higher accuracy and no filler or flux needed to be used. Thus the technique can be viewed as being environmently friendly. However, the technique is not inexpensive, primarily due to the high cost of welding tools. Therefore, the design and construction of a lateral-drive USMW system which is capable of joining thin metals is presented in this thesis. The fundamental aspect of this study is the design of an integrated spot welding horn, along with other welding components such as a stationary anvil, mounting holder, welding bed, as well as the relevant fixing tools and fixtures. High precision is required in the design of the components, and in particular the welding horn. Because the horn is responsible for transferring energy to the welding zone, specimens must be prevented from sliding during the joining process, and an appropriate clamping force must be applied which will ensure acceptable bonding. Many criteria have been examined to enhance the performance of a working horn. The horn excitation frequency has been matched to the transducer frequency, ensuring that the horn will be vibrated longitudinally close to 20 kHz, thereby allowing the tuned mode to be isolated from other non-tuned modes, which guarantees uniformity of the vibration amplitude at the horn working surface, high gain factor of 4.108, and the avoidance of any stress initiated at the points between connecting components. Examining of these criteria is essential in order to optimise the excitation of the horn and to transmit the energy with minimum dissipation. The analytical studies and the finite element (FE) modelling of the welding components were successfully simulated, from which the vibrational behaviour and dynamical characteristics of the system were precisely verified using experimental modal analysis (EMA). The welding stack (the horn connected to the transducer), welding components and fixtures were then set-up on the driving machine. The device was examined prior to welding to ensure the excitation at high vibration. Many tests were successfully conducted on the welding together of aluminium and copper in a number of different configurations using the ultrasonic metal spot welding system. Weld strength and quality were shown to depend on complex relations of process parameters such as clamping force, amplitude of vibration, welding time and input power. A series of weld combinations with different thicknesses and ii variations in metal conditions were studied. The results of the lap tested specimens suggest that the bond strength is sensitive to the relationships between clamping force and vibration amplitude. Overall, the weld strength results suggest that the Al-Al welds are stronger and more consistent in terms of weldability than the Cu-Cu welds. In the welding of dissimilar metals, stronger welds are produced when the aluminium specimen is placed on top and in contact with the horn tip, rather than the copper. The thickness and surface condition of the metals such as hardness, surface roughness and oxides, are significantly affect the weld strength. In welding of Al-Cu or Cu-Al, an increase in energy and time was necessary to generate an acceptable bond. The use of stepped amplitude profiling results in a pronounced increase in the weld strength improves consistency and enhances weldability. However, horn tip/specimen adhesion and specimen marking did not occur under certain conditions. The results of the FE simulation and experimental tensile tests, for the load displacement curves profiles, allow for good estimation of the maximum load and therefore weld strength. Weld quality of aluminium and copper specimens were observed through investigation of the deformed surfaces using Nomarsky optical microscopy and scanning electron microscopy (SEM). The results illustrate that good quality welds can be achived by joining specimens, regardless of the surface condition of the metal. The SEM confirmed that no mixing occurred by melting or fusion between intimate surfaces, which indicates that USMW occurs due to adhesion and cohesion mechanisms. Furthermore, xray diffraction confirms the percentage of morphology between Al and Cu, which indicates that largest weld formations are prevalent for those specimens that are softer and lower in hardness and surface roughness, regardless of the type of tempering.