Reliability Research And Numerical Simulation Of Brazed Joints Between Copper And Stainless Steel

Stainless steel is becoming one of the alternatives to copper in refrigeration system because of the good corrosion resistance,cheaper prices and better weldability.However,when replacing copper with stainless steel in the refrigeration system,we are bound to face the Cu / stainless steel connection problems.In general,Ag-based brazing filler metal are often used to braze Cu / stainless steel due to the good brazing processing properties,but it is relatively expensive.For this reason,it has been attempted to braze Cu / stainless steel with silver-free Cu-based brazing filler metal.In this paper,Cu/stainless steel were brazed in high-temperature ammonia decomposition furnace with Cu-Sn alloy,and properties of the brazed joints were evaluated by microstructure observation and mechanical property tests.Microstructure observation shows that the interfaces of the brazed joints were well connected without any defects,some reticular and massive intermetallic compounds were formed at the stainless steel/brazing filler metal interface,and there is solid solution in the Cu / brazing filler metal interface region;Mechanical property tests show that the brazed joints were fractured on Cu matrix near the joint and the yield strength was significantly reduced due to recrystallization annealing of the Cu matrix caused by high brazing temperature.To further verify the reliability of the brazed joints,we conducted a 500000 cycles fatigue test and a pressure resistance test.The results show that Cu / stainless steel brazed joints are able to meet the actual requirements.Austenitic stainless steel often appears the phenomenon of stress corrosion cracking,so this paper simulated the residual stress of Cu / stainless steel brazed joints with finite element software ANSYS,and studied the effects of different brazing parameters such as brazing gap,lap length and brazing temperature on residual stress.The results show that there is residual tensile stress on the outer surface of the two tubes for the brazed joints of stainless steel lap outside the copper,but their tensile stress peaks can be reduced simultaneously by adjusting the brazing parameters;as the brazed joints of copper lap outside the stainless steel,althoughthe tensile stress on copper is higher than that of the former,this method can make the residual tensile stress on the outer surface become compressive stress,which can reduce the stress corrosion tendency of stainless steel.In order to study the effect of residual stress on the fatigue life of non-defective brazed joints and cracking brazed joints,the fatigue life of the joints was simulated by using FE-SAFE fatigue analysis software and FRANC3 D three-dimensional crack analysis software.The results show that the existence of residual stress can greatly reduce the fatigue lifes of the brazed joints,and the greater the residual tensile stress is,the shorter the fatigue lifes are;Residual stress can not only change the type of crack,but also affect the expansion of the crack,in which the compressive residual stress hindered the expansion of crack,resulting in the fatigue lifes increasing.