【作者】 董旭；

【导师】 张杰；

【作者基本信息】 哈尔滨工业大学 ， 材料学， 2020， 硕士

【摘要】 导弹是国防领域的重要武器,导弹天线罩是其重要组成部分,其需要具备介电性能好、抗热震、热膨胀系数低等特性,多孔Si₃N₄陶瓷作为天线罩体能够满足上述要求。然而多孔Si₃N₄陶瓷脆性较大,难以直接与导弹弹体进行装配,因此需要在天线罩端部钎焊金属环,通过金属环实现与导弹弹体装配。Invar合金具备热膨胀系数低的优点,本课题选其作为金属连接环。大尺寸结构件连接面积大,残余应力随构件尺寸增大而急剧上升,由于多孔Si₃N₄陶瓷易破碎,钎焊过程极易因残余应力过大导致多孔Si₃N₄断裂。本课题通过改变中间层结构、中间层厚度、钎料层钎焊宽度及搭接位置等变量,探究钎料结构对接头残余应力的影响。通过中间层结构设计,将钎料装配形式设计为Ag-Cu-Ti钎料/Cu中间层/Cu泡沫层/Cu中间层/Ag-Cu钎料的ABA型结构和Ag-Cu-Ti钎料/Cu中间层/Cu泡沫层/Ag-Cu钎料的AB型结构,Cu泡沫选用1.5mm和2mm两种厚度,Cu层选用30μm、100μm、150μm,将12种中间层组合的试样进行连接,以确定模拟计算中钎焊接头的组织。使用ABA型结构钎料进行钎焊时,Cu中间层能有效屏蔽Ti元素与Invar合金反应生成Ni3Ti和Fe2Ti,同时Ag元素在化学势作用下能以扩散方式穿过Cu中间层,使Cu泡沫变成Ag-Cu固溶体泡沫,使其熔点降低发生坍塌。使用较厚的Cu中间层可以阻碍Ag的扩散,较好地保留Cu泡沫的三维网状结构。使用AB型结构钎料进行钎焊不能较好保留三维网状结构。对Cu泡沫进行建模,并模拟仅用Cu层和Cu泡沫作为中间层时的应力变化情况,确定Cu泡沫建模的适用性以及模拟计算的目标。设计的模型由五部分组成:多孔Si₃N₄环、Ag-Cu-Ti钎料、中间层（上述ABA型及AB型）、Ag-Cu钎料、Invar合金环。分析模型冷却过程的残余应力,综合实际组织样钎焊结果以及模拟计算结果可知:中间层结构对实际钎焊结果影响比较大,虽然在模拟计算中,相同中间层厚度的ABA型及AB型中间层并无过于明显的区别,但是从之前组织样钎焊结果可知,去掉靠近Ag-Cu钎料一侧的Cu层之后,Ag-Cu钎料将溶解Cu泡沫,使之产生较严重的坍塌,不利于缓解钎焊接头的残余应力。将中间层分为整块结构、三等分结构、四等分结构和五等分结构后,由模拟结果可知,相同结构的整块钎料残余应力大于等分钎料,而等分钎料之间的残余应力差距并不明显,三等分与五等分钎料轴向应力小于四等分钎料,径向应力略大于四等分钎料。对于30μm、100μm、150μm的Cu层厚度,Cu层厚度越大,钎焊接头残余应力越小。针对钎料层,设计了40mm和30mm两种宽度的钎料进行有限元模拟计算,结果表明,对于整块钎料钎焊,采用40mm宽度钎料钎焊过程中产生的残余应力均大于30mm宽度钎料,其中40mm宽度钎料与30mm宽度钎料上方位置钎焊产生的轴向应力曲线较为一致;对于不同等分份数钎料,30mm宽度钎料轴向应力均小于40mm宽度钎料,轴向应力极值相差超过50MPa,径向应力极值相差约75MPa,等分份数对于不同宽度钎料的残余应力值影响不大。模拟计算了40mm宽度钎料与30mm宽度钎料在钎缝中放置不同位置时的残余应力,发现采用ABA结构的30mm宽度钎料、150μm厚度的Cu层,在远离多孔Si₃N₄边缘的下方位置进行钎焊,得到的接头残余应力最低。将钎料整块、三等分、四等分以及五等分后,接头轴向应力的极大值分别为:193MPa、250MPa、230MPa以及235MPa;径向应力的极大值分别为:195 MPa、250 MPa、230 MPa和240 MPa。更多还原

【Abstract】 Missile is an important weapon in the field of national defense.Missile radome is an indispensable part of missile.The missile radome needs to have the characteristics of good dielectric property,good thermal shock resistance and low thermal expansion coefficient.Porous Si₃N₄ ceramics meet the above requirements very well.However,in order to apply porous Si₃N₄ ceramics to missile radome,it needs to be fixed by metal ring.The metal ring selected in this project is Invar alloy,mainly due to its small expansion coefficient and other advantages.Due to the brittleness and fragility of porous Si₃N₄ itself,and the large connection area of largescale structural parts,the residual stress increases sharply with the increase of brazing area,so how to reduce the residual stress of joints becomes the key.And the connection area of large-scale structural parts is large.The residual stress increases rapidly with the increase of brazing area,so how to reduce the residual stress of joints becomes the key to the project.This project will focus on exploring the influence of macro factors on the residual stress such as the structure of the interlayer,the thickness of the interlayer,the width of the solder layer and the position of the overlap.First,design the interlayer structure in advance.The solder is designed to be ABA type（Ag-Cu-Ti/Cu foil/Cu foam/Cu foil/Ag-Cu）and AB type（Ag-Cu-Ti/Cu foil/Cu foam/Ag-Cu）.Cu foam selects two kinds of thickness of 1.5mm and 2mm and the Cu foil selects 30μm,100μm and 150μm.Twelve kinds of interlayer structure samples were connected to determine the structure of brazed joint in the subsequent simulation calculation.Then modeling the Cu foam,and the stress changes were simulated only when Cu foil or Cu foam were used as interlayer.After that,the applicability of Cu foam modeling and the objective of simulation calculation are determined.The designed model consists of five parts: porous Si₃N₄ ring,AgCu-Ti,interlayer（ABA type and AB type above）,Ag-Cu and Invar alloy ring.Analyse the residual stress of the models after cooling process,the brazing results of the actual organization samples and the simulation results of the models show that the interlayer structure has a great influence on the actual brazing results.Although there is no obvious difference between the ABA type and the AB type interlayer with the same thickness of the interlayer in the simulation calculation,from the previous brazing results,it is known that after removing the Cu foil which is close to the Ag-Cu side,the Ag-Cu will use up the Cu foam and make it collapse more serious,which is not conducive to relieving the residual stress of the brazed joint.After dividing the interlayers into the whole structure,the 3 equal parts,the 4 equal parts and the 5 equal parts,it can be seen from the results that the residual stress of the whole solder is higher than that of the other equal solders with the same structure.However,the difference between the residual stress of the different equal solders is not obvious.The axial stress of the 3 equal parts solder and the 5 equal parts solder is lower than that of the 4 equal parts solder,and the radial stress is slightly higher than that of the 4 equal parts solder.The results show that for the thickness of 30μm,100μm and 150μm Cu foil,the larger the thickness of Cu foil is,the lower the residual stress is.According to the requirements of the project,for exploring the brazing width of the filler metal layer,the solder with 40 mm width and 30 mm width was designed for finite element simulation.The results show that for the whole solder,the residual stress of the solder with 40 mm width is higher than that of the solder with 30 mm width,and the axial stress of the 40 mm width solder is closer to 30 mm width solder brazed upper position than that of other positions.For different equal parts of solder,the axial stress of 30 mm wide solder is lower than that of 40 mm wide solder,the difference between the extreme value of axial stress is more than 50 MPa,and the difference between the extreme value of radial stress is about 75 MPa.The results show that the number of equal parts has little effect on the residual stress difference of different width solders.Based on the results above,the residual stress curves of the 40 mm wide solder and the 30 mm wide solder of the three positions of the upper,middle and lower parts are sorted out.It is found that the residual stress is the lowest when the Cu foil with the width of 30 mm and the thickness of 150 μm is brazed away from the edge of porous Si₃N₄,which is said the same as lower position.The maximum value of the axial stress of the whole structure,the 3 equal parts,the 4 equal parts and the 5 equal parts is 193 MPa,250MPa,230 MPa and 235 MPa respectively;the maximum value of the radial stress is 195 MPa,250MPa,230 MPa and 240 MPa respectively.更多还原