| Joining of metal foils is a necessity in modern industry,for example,the assembly of tabs for lithium battery manufacture,cell connection of solar sail and consolidation of foil and substrate for ultrasonic additive manufacturing.Most of the time,the joining objects are of thin thickness,multiple layers and dissimilar materials.Ultrasonic welding is an ideal solid state joining method for dissimilar thin metals.Compared with traditional fusion welding method,it can avoid the defects caused by intermetallic compounds(IMCs).Especially,ultrasonic welding is the most popular joining method in lithium manufacturing process.There are two main evaluation criterions for ultrasonic weld joint: joint strength and interfacial contact resistance.They are closely related to material flow and deformation,macro/micro interfacial friction during welding process.It is of great importance to research the interface friction behavior and material deformation mechanism for in-depth understanding of joining mechanism,which is essential for optimization of welding process.However,it is now lack of study about the material softening mechanism with the effect of ultrasonic energy and the effect of ultrasonic vibration on stick-slip behavior between contact interfaces.Meanwhile,as ultrasonic welding is a transient process and the structure is closed,the difficulty is added to obtain inner structure deformation and interface friction behavior through experimental measurement.This dissertation focus on ultrasonic welding of cathode(0.2 mm Cu)and anode(0.2 mm Al)in lithium battery manufacture.The interface contact behavior and material deformation in joint area has been investigated by combining the basis theories such as materials mechanics,contact mechanics and friction physical principles,and numerical simulation and experimental observations.The main research work of this dissertation is as follows: building ultrasonic vibration assisted compression experimental system to study the material deformation behavior and temperature increment;material constitutive model is proposed considering ultrasonic softening effect and decoupling thermal softening effect at the same time;Based on the theories of contact mechanics and interface friction dynamics,the law of stick-slip behavior between contact interfaces is deduced;experimental observation is further carried out the characterize contact and deformation quantitatively.Combining with the above analysis,ultrasonicthermal-mechanical coupled numerical model is built up to further study transferring process of ultrasonic energy,material flow and plastic deformation,macro/micro friction and heat generation process.Finally,the relationship between joint quality and material flow as well as contact interface deformation is built up.The optimization strategies of sonotrode geometry are proposed to guide practical application of ultrasonic welding.The main content of this dissertation is as follows:1)Effect of ultrasonic energy on mechanical behavior of metallic materialsTension tests of copper and aluminum with various temperatures are carried out to obtain the thermal softening effect.Meanwhile,ultrasonic-assisted compression experiments are carried out.The relationship between specimen deformation,temperature increment and compression force,ultrasonic energy is constructed.By decoupling the effect of thermal softening,constitutive model considering ultrasonic softening effect is proposed.The proposed model is numeralized and compared with experimental results.Conclusions can be drawn that the accuracy of modified ultrasonic softening constitutive model decoupling thermal softening effect is improved by 20% compared with traditional model.2)Study of interface contact behavior during welding processEffect of ultrasonic vibration on the interface contact behavior is analyzed based on the theory of friction mechanics.Then the boundary condition for stick-slip is deduced and introduced into the situation of two plates under ultrasonic vibration of single ridge.Heat generation from interface friction and temperature increment is calculated.The contact interface is divided into two kinds of areas: plastic deformation and interface friction,based on the distribution of contact pressure.Based on above analysis,dynamic interfacial friction model is established to explore energy transferring process across contact interfaces of sonotrode and specimens.Then the evolution of relative motion between sonotrode and specimens is obtained quantitatively.The results show that increment of plastic deformation area leads to joint formation between specimens.The welding process can be divided into three stages in view of the state of relative motion.Accurate boundary conditions are achieved for the establishment of numerical model.3)Numerical analysis of coupled thermal-mechanical processBased on the above studies,ultrasonic-thermal-mechanical coupled finite element model is established.Thermal and mechanical characteristics during welding process,including plastic deformation and material flow,state of stick-slip of contact interface,heat generation from interface friction and temperature increment,are studied systematically.Conclusions can be drawn as follows: plastic deformation concentrates within the area under sonotrode tips.Mechanical interlocking is formed under the combined effect of compression stress and cyclic shear stress,which is beneficial for joint strength.Meanwhile,specimen upwarping between sonotrode tips is found because of eccentric load,resulting in interfacial void.Optimized sonotrode geometry with round surface is proposed to solve the problem of excessive thickness thinning under outer ridges of sonotrode.Guidance is provided for optimization of sonotrode geometry and process parameters based on the numerical and experimental analysis.4)Sonotrode optimization based on specimen deformation and interface frictionWith the combined analysis of experimental and simulation results,the effect of ridges number and curvature of sonotrode on surface condition,deformation of joint area,joint strength and failure mode is studied quantitatively.Optimized sonotrode with ideal ridge number and surface curvature is proposed for ultrasonic welding of two and four layer plates,which is the typical condition in lithium battery manufacture.The results show that thickness thinning of specimen under sonotrode tips is relatively uniform when the ridge number is 9 and surface curvature is 225 mm.Meanwhile,the faying interfaces come into intimate contact and joint strength reach the maximum value.In summary,the present dissertation has studied material flow and plastic deformation behavior,macro/micro friction and heat generation from contact interfaces thoroughly.Based on specimen deformation and evolution of interface friction during welding process,the optimize design for sonotrode is proposed.These studies can provide guidance to improve quality of ultrasonic welding joint,especially for multilayer thin metal plates. |
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