| The requirements of lightweight and long-life requirements for mechanical equipment involve extensive use of lightweight materials.Among all the joining technologies for various types of lightweight materials,ultrasonic welding has gradually received increasing attention due to its environmental friendliness,high efficiency,low energy consumption and suitability for automated production.Ultrasonic welding technology is widely used for connections between dissimilar metals,composite materials,composite materials and metals.Accurate predictions of structural dynamics and thermodynamic behavior are the keys to understanding the ultrasonic welding process.From the point of view of nonlinear dynamics,the ultrasonic welding process is periodically nonlinear since the horn vibrates periodically,and stick,slip,and separation occur at the welding interface.Hence,the study of ultrasonic welding can be fulfilled from the area of nonlinear dynamics.The Harmonic Balance Method is an effective approach for predicting the nonlinear dynamic behavior of periodic systems.In this thesis,a frequency domain method based on the generalized harmonic balance method is developed.Based on the frequency domain method and considering the characteristics of the thermal-structural coupling of the lightweight material ultrasonic welding,a methodology for comprehensively analyses and prediction of structural dynamics and thermodynamic behavior is proposed.Combined with experimental verifications and comparisons,the methodology was successfully applied to the analysis of Cu-Al ultrasonic metal welding,AA2024/carbon fiber composite(CF/PA6)ultrasonic metal welding,and was extended to the research on CF/PA6 ultrasonic plastic welding.The main work of the thesis includes:1.Aiming at the difficulty in solving nonlinear vibration of workpieces in ultrasonic vibration welding,a frequency domain method is developed as a combination of the analytical generalized harmonic balance method and the Newton-Raphson iteration for period-m solutions of second-order nonlinear systems.The complex-step derivative approximation is introduced to make the evaluation of Jacobian matrices fast and accurate for the nonlinear system.The periodic solutions for a periodically forced Duffing oscillator,a buckled,nonlinear Jeffcott rotor system and a nonlinear short bearing rotor system were analyzed through the present method,and the stability and bifurcation were evaluated through eigenvalue analysis.The results from the present method were found in good agreement with the existent analytical solutions as well as the numerical results by the fourth-order Runge-Kutta method.2.Based on the frequency domain method,a new methodology is proposed to analyze the structural dynamics and thermodynamic behavior of ultrasonic metal welding between dissimilar metals.The welding process is decoupled into a quasi-steady-state structural vibration and a transient thermal dynamics.A nonlinear contact model is used to accurately simulate the nonlinear interactions at the welding interface.The present frequency domain method is employed to solve the nonlinear dynamics efficiently with the assistanceof finite element method.The friction heat generation is taken as the heat source,and the heat conduction process of the workpiece in the welding is analyzed.Through analyzing the friction force of the welding surface,it is shown that nonlinear contact at most moments exists on the welding surface.Comparative studies between experiments and numerical simulations of Cu-Al ultrasonic metal welding are presented to 12 parametric groups which demonstrate that the methodology is capable to simulate the nonlinear thermal-structural dynamics.These results offer insights into the relationship between the temperature and the welding parameters(e.g.clamping force and vibration amplitude).3.A new methodology for analyzing the structural dynamics and thermodynamic behavior of metal and composite ultrasonic metal welding is constructed.The research on ultrasonic metal welding of metal and composite materials is still limited,especially in numerical part.Based on the full consideration of the nonlinear contact model,thermal-structural coupling and frictional heat generation,the viscoelastic behavior of materials is analyzed for structural vibration and heat generation.The influence of the loss modulus of the material is taken into account in the structural damping factor,and the viscoelastic heat generation is added to the heat generating portion.The proposed frequency domain method is used to efficiently solve the structural vibration,and the structural displacement response obtained is converted into the strain response of the structure.With the attention to friction at the center of the welding surface,the influence of different welding parametric combinations on the nonlinear contact of the interface is illustrated.Experiments of ultrasonic metal welding with AA2024 and CF/PA6 were carried out using various welding parameters.Comparative studies between experiments and numerical simulations are presented which demonstrate the capability of the methodology.The influence of different clamping force and amplitude combination on the process of temperature rise at the welding interface is analyzed.4.The structural dynamics and thermodynamic behavior of composite ultrasonic plastic welding are studied.The methodology proposed in the previous section is extended to the ultrasonic plastic welding system.The contact and separation states of the interface are analyzed by calculating the normal displacement and normal pressure on the welding surface.The effects of various welding parameters on the temperature change,including amplitude,pre-pressure,horn down speed and welding time are analyzed in detail.In addition,changes in the frictional heat generation and viscoelastic heat generation during the welding process are also presented,as well as the contact condition at the interface between the workpieces. |
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