Research On Structure Optimization And Laser Brazing Welding Process Of Cu-Al Terminal

The manufacture process of Cu-Al terminals for new energy vehicle includes"forging rod-blind hole machining-friction welding-turning external circle",which results in spillage,flown,grooves and other defects at the joints,accompany with high process cost,serious material waste and low processing efficiency.Aiming at above problems,the structure and manufacture process of terminals are both optimized in this paper.With tube profile as raw material,the laser texture pretreatment on copper surface was put forward,and the process of laser-brazing-lap welding of copper and aluminum was researched.Meanwhile,the feasibility of the new process was analyzed by SYSWELD,so as to improve the material utilization and production efficiency,and to provide a useful reference for its wide application in new energy vehicles.The main contents are as follows:The structure and process of Cu-Al terminal were optimized,and their laser welding manufacturability were analyzed.Based on the connection mechanism between wire and terminal,the formula for calculating the diameter and lap length of base metal under optimized structure was derived,and the"copper out/aluminum in"terminal model with laser-brazing-lap welding was established.Through the analysis of laser welding manufacturability between copper and aluminum,two key technologies of the new process are revealed:1)Before welding,the copper surface should be pretreated so as to improve its laser absorptivity;2)The brittle and hard compounds exist at the interface,which leads to high resistivity of joints.It is necessary to optimize the laser welding process reasonably.The pretreatment technology of laser texture on copper surface was proposed,and the mechanism of infrared laser absorptivity on the textured surface was explored.Based on picosecond laser textures,parallel grooves textures along the welding direction were formed on the copper surface,and the effects of processing parameters on micro texture and surface quality were studied.The research shows that:texture depth,surface roughness and oxide are three important factors affecting the laser absorption.After three-step absorption of"light self-trapping texture-spatter particles on surface-CuO layer",the infrared laser absorptivity of the textured copper surface can be increased by nearly 90%.The laser-brazing-lap welding process of copper and aluminum was researched,and joints electrical and mechanical properties,as well as their formation mechanism,were analyzed.Based on the research about the influence of process parameters（welding power,welding speed,defocus distance）on the morphology,penetration and tensile-shear force of copper and aluminum laser-brazing-lap welding joints,the effects of Ag addition on the tensile strength,resistivity and microstructure were compared and analyzed.The research shows that:The addition of Ag is beneficial to improve the welding quality.With the increase of Ag layer thickness,the tensile strength and resistivity of joints decrease first and then increase.Compared to the direct joints of laser-brazing-lap welding,with the addition of 20μm silver foil,the melting zone is dominated byα-Al,and the two-elementary reaction of copper and aluminum is restrained.Al-Al₂Cu-Ag₂Al like rodents are formed on the copper side,and the tensile strength is increased by nearly 1/3.There are few kinds of compounds in the interface layer with uniform distribution,and the cracks caused by internal tensile stress are reduced,which is beneficial to the resistivity reduction.The fracture occurs at the interface between the melting zone and the aluminum side,which is a quasi-cleavage fracture.The temperature and stress field of Cu-Al terminals with laser-brazing-lap welding were simulated and analyzed based on SYSWELD.The 3D cone heat source was checked by the actual process parameters,and the molten pool morphology was formed with a width1mm/depth 1.52mm,in accordant with the experimental results.The simulation results show that the initial weld joint in the ring weld goes through two welding,and the temperature-time history curve has two peaks.The energy absorptivity of each point increases with time.The residual stress at the terminal is mainly tensile stress with the maximum stress at the weld seam 667.55MPa.The overall deformation of the weld position is up to 0.59mm,and the residual tensile stress at the weld seam can reduce the deformation to 0.3mm after cooling.The feasibility of Cu-Al terminals with laser-brazing-lap welding process is verified.