A Study On The Mechanical Behaviors And Connection Mechanisms Of Light Alloys During Ultrasonic Welding

This dissertation aims to address the high demands for light alloy joints and the existing problems involved in the fields of aeronautics,aerospace,vehicle manufacturing,etc.Through ultrasonic welding,an efficient approach to solve the difficult problem of mechanical properties of light alloy welding spots,and combining the current research of the domestic and international scholars on the ultrasonic welding,multidisciplinary scientific and technological research methods(e.g.,materials sciences and mechanics)are adopted to investigate the mechanical behaviors and connection mechanisms of several typical light alloys during ultrasonic welding.The detailed work of this dissertation is listed as follows:This dissertation has studied the influence weights of welding parameters and the interaction between theses parameters on the joint strength.The research objects include non-reactive,immiscible,soft/hard typical magnesium alloys(AZ31)and titanium alloys(Ti6A14V),and reactive,soft/hard typical aluminum alloys(AL6061)and steels(A36).Comprehensive experiments of ultrasonic welding have been conducted.The primary effects of welding parameters on the strength of welding joints have been obtained.Then the variance analysis has been conducted on the experimental results.Meanwhile,the comprehensive experimental data of homogenous aluminum alloys(AA6022-T4)during ultrasonic welding have been studied by variance analysis.As a result,the influence weights of welding parameters and the interaction between theses parameters on the joint strength have been obtained with respect to these three typical light alloys during ultrasonic welding.This dissertation has established a mathematical relationship model between welding parameters and joint strength for magnesium/titanium and aluminum/steel alloys during ultrasonic welding.The artificial neural network technology through the genetic algorithm have been used to train experimental data of ultrasonic welding for magnesium/titanium and aluminum/steels.The mathematical relationship model between welding parameters and joint strength has been established.The accuracy of the model has been verified by supplementing experiments.Therefore,the analytical model are used to obtain the detailed influence of the welding parameters on the joint strength.This dissertation has carried out the microscopic study on the connection mechanisms of ultrasonic welding joints.The research objects include magnesium alloy/titanium alloy,magnesium alloy/pure copper,and homogenous aluminum alloy ultrasonic welding joints.The ultrasonic welding connection mechanisms have been investigated for these three typical light alloys,using fracture,microstructure and connection mechanism analysis of the joints.The influence weights of welding parameters have been explained scientifically according to the connection mechanisms of different light alloys.The dissertation has investigated the influence of welding parameters on joint fatigue performance.Fatigue tests have been conducted,using magnesium alloys/titanium alloys,and homogenous aluminum alloys as research objects.The failure modes and the mechanisms of fatigue fracture have been observed and analyzed.The Swellam model and the Paris formula have been used to predict the fatigue life of the joints.Further,the influence of welding parameters on the fatigue life of the joints has been obtained by computational models and experimental results.The dissertation has established a model to analyze the thermal process of ultrasonic welding.Ultrasonic welding experiments of magnesium alloys/titanium alloys have been firstly carried out.The temperature history curve in the interior of the titanium alloys has been obtained by the measured distribution of temperature.The temperature variation has been analyzed.According to the experimental results,the analysis model of the thermal process of ultrasonic welding has been established through the finite element analysis software ABAQUS,based on the experimental results.According to the temperature history in the experiments and the temperature in the typical regions,the accuracy of the model has been verified.Then the mechanical parameters,e.g.,temperature and plastic strain distribution fields,during welding have been analyzed.