| Cu-Al composite components can give full play to complementary advantages of copper and aluminum to make up for the performance defects of a single metal component,thereby effectively improving the comprehensive performance of the component and reducing its cost.It can be used in the fields of electric vehicles,power devices,electronic information,etc.It has broad market prospects and it is of great significance for promoting the quality and upgrading of industrial manufacturing.Moreover,it can also help to achieve the strategic goals of “peak carbon dioxide emissions” and “carbon neutrality”.However,there are significant differences between Cu and Al in physical properties such as melting point,specific heat capacity,and linear expansion coefficient.Through traditional methods such as fusion welding,it is easy to form brittle intermetallic compounds,and there are obvious defects in their effectiveness and adaptability.How to realize an effective and reliable connection of Cu and Al has become a key issue that severely restricts the preparation and application of Cu-Al composite components.High-speed impact welding based on electromagnetic forming(also called electromagnetic pulse welding,EMPW)is a new welding process that is expected to solve the defects of traditional welding methods,and it has become a research hot area in recent years.At present,some progress in the research of EMPW technology has been achieved;however,there is still a big gap between China and foreign countries in this field.Besides,there is still a lot of research gap in the existing research on the multi-field simulation,transient process,and joining mechanism of EMPW.In view of the problems mentioned above,this work focuses on the transient process and joining mechanism of EMPW of Cu-Al plate.The major achievements are as follows:(1)Development and experiment of EMPW equipment for Cu-Al plate have been done.Based on the physical properties of materials and EMPW conditions,the threshold value of discharge current for EMPW of Cu-Al was determined through theoretical calculation and analysis.The multi-rod trigger vacuum switch(MTVS)was selected as the discharge switch,and a trigger source was developed for MTVS,based on Si C MOSFET and pulse transformer structure.After the comparison and selection of charge power supply,charge switch,an energy storage capacitor,and connecting cable,and the development of the welding coil,fixing device,and operating platform,two pieces of EMPW equipment with a fixed frequency of 28 k J and an adjustable frequency of 56 k J respectively were developed for different application scenarios.Based on these equipment,EMPW experiments of Cu-Al plates were carried out,and the effective connection of Cu-Al plates was achieved.The corresponding window of process parameters and welding effect was established,including the discharge voltage and welding gap.The macro morphology and mechanical properties of the Cu-Al plate EMPW joint were analyzed by peeling test and tensile test.(2)The dynamic behavior of the Al plate during the EMPW of the Cu-Al plate was studied,which revealed the formation mechanism of the macroscopic morphology of the Cu-Al plate welding joint.Considering the dynamic changes of circuit parameters,a three-dimensional space-time simulation model of EMPW of Cu-Al plates was established through COMSOL Multiphysics software,which sequentially coupled the electrical-magnetic-force physical fields,and the magnetic induction intensity,induced eddy current,Lorentz force,and plastic deformation during the welding process were analyzed.A comprehensive observation platform for experiments was built,and the entire moving process of the Al plate was captured.The influence factors of deformation angle and deformation speed of the Al plate were analyzed.The phenomenon of “cut top” and its influence in the deformation process of the Al plate was explained.(3)The evolution process of the Cu-Al bonded interface during the EMPW process was studied,which revealed the formation mechanism and movement characteristics of the metal jet.A smooth particle hydrodynamics(SPH)simulation model of the collision process between the Cu plate and the Al plate was established.Combined with the microscopic morphology analysis of the bonded interface,the evolution process of the bonded interface and its motion law were studied.The complete process of generation,persistence,and disappearance of the metal jet was captured.The formation mechanism,composition,and movement characteristics of the metal jet were revealed based on simulation analysis and its motion trajectory.The corresponding relationship between the metal jet and the length of the bonded interface was established.Based on the characteristics of the metal jet spot,a non-destructive assessment method of joint welding quality under atmospheric pressure was proposed.(4)The formation mechanism of micro-gap discharge of Cu-Al plates and surface defects of plates was studied,and the methods of suppressing micro-gap discharge were proposed.A kind of special spot was discovered for the first time in the welding process,and the difference between it and the metal jet spot was analyzed.The correlation between the special spot and the micro-gap discharge was confirmed through simulation analysis and experimental research.The conditions of micro-gap discharge,influence of electric field strength,and contact pressure density were studied.And the formation mechanism of the surface defects of plates was revealed.Two processes for suppressing the micro-gap discharge were proposed,including the strengthening of insulation strength and electromagnetic pulse driver welding,which were proved effective to reduce the surface defects of plates through experiments.In conclusion,this paper carried out in-depth research on the problems existing in the transient process and joining mechanism for EMPW of Cu-Al plates.Based on pulse power technology,EMPW equipment were developed to achieve effective Cu-Al welding;a three-dimensional multi-field time-space simulation of EMPW of Cu-Al plates with dynamic changes of circuit parameters was established;the formation mechanism of the welded joints was revealed through simulation and experiments;the relationship between the evolution process of bonded interface and the metal jet was revealed based on the complete process of the metal jet and an SPH model;the formation mechanisms of micro-gap discharge of Cu-Al plate and plate surface defects were discovered and analyzed;the micro-gap discharge suppression methods were proposed and validated.This paper is expected to provide the theoretical foundation and technical support for the industrial application of EMPW of Cu-Al plates. |
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