Laser Brazing Technology Of T2 Copper/304 Stainless Steel And Microstructure And Properties Of Joint

The composite structure of copper and stainless steel has the characteristics of high strength and high corrosion resistance of stainless steel,and also has an excellent electrical and thermal conductivity of copper,making copper/steel composite structure is widely used in nuclear power equipment,aerospace,petrochemical,automotive parts and other high-end industrial fields.The use of copper/steel composite structure to replace all-steel components or all-copper components can take into account the performance advantages of copper and steel materials,but also to save materials,reduce manufacturing costs,is one of the effective ways to meet the different performance of use under specific conditions.In terms of chemical properties,the lattice type of copper steel and atomic radius is close at high temperature,this facilitates the solder bonding of copper and steel.However,due to the large differences in physical properties of copper and steel,the coefficient of thermal expansion and thermal conductivity differences make copper and stainless steel welded joints are prone to welding cracks,which greatly reduces the use of copper and stainless steel welded joints performance.Therefore,it is of great theoretical importance engineering application value to carry out research on laser brazing of copper and stainless steel as a metal.In this paper,laser brazing technology is used to weld T2 copper and 304 stainless steel in the case of pre-coated Cu40 Mn Ni brazing metal respectively,and the welding process,microstructure and mechanical properties of T2 copper and 304 stainless steel dissimilar metal laser brazed joints are systematically studied.The results show that the choice of welding process has a significant effect on the laser brazed joints of T2 bronze and 304 stainless steel.There are two connection modes for joint connections with different welding parameters.One is the "fusion welding mode",where the molten brazing metal and the partially melted 304 stainless steel and T2 copper together form the joint;the other is the "fusion-brazing welding mode",where the molten brazing metal and the partially melted stainless steel base material form the joint,and the copper side forms the brazed connection.The selection of the appropriate laser defocus position,welding current,welding speed and laser beam bias angle helps to improve the formation and welding quality of the laser brazed joint.The amount of laser defocusing determines the size of the spot acting on the joint,which in turn affects the laser power density;the welding current determines the laser output power,which in turn affects the form of the welded joint;the welding speed mainly affects the heat input of the laser beam to a single "welding spot" during the welding process,which in turn affects the heat input to the entire welded joint,and also affects the length of weld cracks in the joint;The laser beam bias angle affects the metallurgical reaction of the joint in two ways: the laser bias angle improves the T2 copper of absorption rate of laser light,thus improving crack defects in the joint;secondly,the laser bias angle affects the amount of stainless steel melted in the joint,resulting in an increase in the corrosion resistance of the welded joint as the laser bias angle decreases and the amount of stainless steel melted increases.This paper presents an in-depth analysis of the forming mechanism and crack formation mechanism of T2 copper/304 stainless steel laser brazed joints based on the characteristics of copper/steel dissimilar metal solidification,combined with Cu-Fe phase diagram.Copper and steel welding does not produce intermetallic compounds,because laser brazing cooling speed fast and copper high thermal conductivity,can make the copper and iron liquid phase to reach the sub-stable miscible gap,and then occur once the iron-copper liquid phase separation,so that the fusion zone distribution of a large number of Cu-rich phase and Ferich phase.In the process of continuous cooling of the joint,the critical nucleation work of the Fe-rich liquid phase is smaller than that of the Cu-rich liquid phase,Fe-rich liquid phase nucleates in the copper-side interface area where the subcooling degree is larger firstly,and the copper atoms solidly dissolved in the Fe liquid phase precipitate out due to the decrease in temperature,and secondary liquid phase separation occurs.Because Fe-rich liquid phase firstly nucleates and grows,low melting point of copper liquid flow filling the crack between the Fe-rich liquid phase grain boundary,the contraction stress generated during the cooling process of the joint is greater than the bonding force between the copper-iron liquid phase,it will crack at the crack,and eventually form macro cracks to reduce the mechanical properties of the joint.In the experiment of laser brazing corrosion resistance between T2 copper to 304 stainless steel,the Cu-rich phase and the Fe-rich phase formed a primary cell with steel as the cathode and copper as the anode due to the potential difference,resulting in local galvanic coupling corrosion,which accelerated the corrosion rate of copper.