Study On Numerical Simulation Of Residual Stress In Al₂O₃ Brazing Joints And Al₂O₃-Based Multiphase Ceramic Brazing Joints

Due to the high strength,high hardness,high temperature resistance and corrosion resistance,the ceramics have been widely used in military,electronics,communications,aerospace,machinery manufacturing and other fields.In many cases,it is necessary to connect the ceramic with itself and metal materials to achieve the purpose of manufacturing complex components and meeting service conditions,and brazing is one of the most common methods to achieve the connection.Due to the great differences between the ceramic and the metal brazing filler or the metal base,inevitably,a large residual stress will be generated in the brazed joint.The existence of large residual stress in the joint can easily reduce the ability of the joint to withstand loads and fatigue life of the joint,which affects the quality of the joint connection.Therefore,the research on the residual stress of ceramic brazed joints has important application value.However,it is difficult to characterize the residual stress of ceramic brazed joints by using current stress testing methods,and the complete residual stress distribution inside the joint cannot be accurately obtained.In recent decades,with the vigorous development of numerical simulation methods,it has become an important way to analyze the residual stress distribution characteristics of ceramic brazed joints and assist in improving the quality of ceramic brazed joints.In this paper,based on the special software SYSWELD for welding numerical simulation,and the"thermal elastica-plastic finite element calculation method"was used to study the residual stress distribution of ceramic brazed joints.On the basis of the research results of the effect of brazing material properties on residual stress,the influences of the joint structure and the second phase strengthening method on residual stress of brazed joints were studied respectively.First,taking the brazed joint of Al₂O₃/Ag-Cu-Ti/Al₂O₃ as the research object,the numerical simulation of residual stress was carried out.By designing different filler metal material models,under the same brazing process and joint structure conditions,the effects of brazing filler properties on the joint residual stress were studied.Afterwards,the material model of the filling material was simplified in different ways,and the influence of the simplified model on the calculation of the residual stress of the brazed joint was explored.The results show:（1）With the same extent variation,the coefficient of linear expansion has the most significant effect on the residual stress in X direction of the joint,the yield strength has the most significant effect on the residual stress in Z direction of the joint,by contrast,the elastic modulus has no significant effect on residual stress.（2）During the calculation of residual stress,the residual stress result calculated by using a material model that considers the change in yield strength with temperature is closer to the results obtained by using a model where all performance parameters change with temperature.Second,taking ZTA/Ag-Cu/TC4 dissimilar brazed joints as the research object,based on its actual joint structure,a finite element model of the joint dividing the reaction layer was established.And compared with the simplified model that taking original filler as the braze,according to the distribution of residual stress,the influences of the joint reaction layers on the residual stress were explored.The results show:the peak residual stress of the brazed joint appears at the position of the Ti₃（Al,Cu）₃O reactant layer on the interface close to the ceramic base material.Because the reactant is a brittle compound,the presence of high stress tends to cause cracks and cracks in this area that is a weak position to make the joint failure.By changing the thickness of the Ti3（Al,Cu）3O reaction layer,it was found that the decrease of the thickness of the reaction layer significantly reduced the peak X-direction residual stress of the joint.Therefore,during the actual brazing process,controlling or reducing effectively the amount of the reactant at the interface is beneficial to improve the reliability of the joint.Third,taking the Al₂O₃/Ag-Cu-Ti/Al₂O₃ brazing joint as the research object,the Al₂O₃ particles with a diameter of 40μm were added into the original brazing material to obtain a composite brazing filler of Ag-Cu-Ti+0.5vt.%Al₂O₃.Through the comparison of calculation results of the joint residual stress,the effect of the second phase particle strengthening method on the joint residual stress was analyzed.The results show that the addition of Al₂O₃ particles obviously increases the X-direction residual stress of the Al₂O₃ particles and the nearby matrix metal in the brazing joint,and he Al2O3 particles also have a large Z-direction residual compressive stress,while the residual stresses of the base ceramic and the other matrix metal part weren’t influenced by Al₂O₃ particles.During the cooling process,Al₂O₃ particles are susceptible to cracking and forming particles of smaller size,which is conducive to the formation of more dispersed reactants in the joint.With overall consideration,adding Al₂O₃ particles can improve the brazed joint structure and have no negative effect on the residual stress of the joint.It can be believed that adding an appropriate amount of Al₂O₃ particles can play a role in strengthening the brazed joint.