基于超塑性钛合金缓冲层设计的Al2O3/不锈钢钎焊接头组织结构及应力调控机制

Residual stress is one of the key factors that influences the strength and reliability of ceramic/metal brazed joints, especially in those joints, which have a large mismatch in coefficients of thermal expansion between the ceramic and the corresponding metallic side. Considering the advantages of current stress-relief methods by adding different soft or hard metal buffer layers, a new idea has been proposed in the present work to relieve the residual stress in the Al2O3/stainless steel brazed joints using a superplastic titanium alloy interlayer with the intermediate thermal expansion coefficient. However, rapid grain growth in titanium alloy generally degrades its superplasticity during the brazing process. Therefore, the influences of the hydrogen content and brazing process parameters on the microstructure of the pre-hydrogenated Ti6Al4V interlayer will be systematically investigated in this work. Deformation behaviors of titanium alloy interlayer with a changing microstructure and complicated stress-strain state during brazing process will be evaluated by the micro-mesh etching method. In addition, the interfacial microstructure in the Al2O3/stainless steel brazed joints and its formation mechanisms during brazing will be investigated. Also, residual stress and the strength of brazed joints will be identified. The relationship among deformation behavior of the titanium alloy interlayer, microstructure and the strength of brazed joints will be clarified. The mechanisms of residual stress relaxation in the Al2O3/stainless steel brazed joints will be finally revealed in combination with fracture behavior analysis of the as-brazed joints. Based on the above-mentioned study, a novel and effective way to relaxing the residual stress in the ceramic/metal brazed joint will be highlighted to lay a foundation for the development of ceramic/metal joining theory.

残余应力是影响陶瓷/金属钎焊接头强度和可靠性的重要因素，尤其是制约热膨胀显著失配材料可靠连接的难题。本项目结合并发展现有软、硬金属层缓冲法的优点，提出添加热膨胀系数适中的超塑性钛合金来缓解钎焊接头残余应力的设计思想。针对钎焊热过程钛合金晶粒长大引起的超塑性降低问题，研究预置氢量和钎焊工艺参数对Ti6Al4V中间层显微组织的影响规律，并通过刻蚀微网格法研究钎焊过程复杂应力应变和组织变化下Ti6Al4V中间层的变形行为。研究Al2O3/不锈钢钎焊接头的界面组织结构及其形成机制。研究接头的残余应力及强度。在此基础上，建立钛合金中间层变形行为、接头组织结构与接头强度之间的关系，并结合断裂行为的研究，揭示Al2O3/不锈钢钎焊接头残余应力的调控机制。本项目将为缓解陶瓷/金属钎焊接头残余应力提供新途径，为丰富和发展陶瓷/金属连接理论奠定基础。

残余应力是影响陶瓷/金属钎焊接头强度和可靠性的重要因素，尤其是制约热膨胀显著失配材料可靠连接的难题。本项目针对Al2O3陶瓷与不锈钢的连接需求，提出采用低膨胀超塑性钛合金作为过渡中间层，利用其超塑性变形、塑性变形以及热膨胀调节作用来缓解钎焊接头残余应力的设计思想。研究发现，随着氢含量的增加，Ti6Al4V先后发生α→β和β→α+δ转变，β相含量增加并成为主相；钛合金流变应力降低，延伸率先增加后降低。温度的升高或保温时间的延长将使钛合金晶粒尺寸增加，但经控制降温除氢之后，Ti6Al4V中间层晶粒变细，β相含量降低至未置氢时的水平。800oC下，钎焊接头典型组织结构为Al2O3/Cu-Ti-Al-O层/Ag(s,s)+Cu4Ti+TiAl/Cu2Ti+Cu3Ti2/Ti6Al4V/Cu3Ti2+Cu2Ti/Ag(s,s)+Cu4Ti/不锈钢。其形成与元素的溶解、扩散、挥发和反应有关。钎焊过程中，Ti、Al、O等元素向液态钎料中溶解并在界面形成富集，Ti元素向Al2O3或不锈钢母材一侧扩散，钎料中Cu元素同时向母材和Ti6Al4V界面扩散，从而在钎缝形成多层结构。当温度升高到900或950oC时，Ag元素挥发，Ti6Al4V中的Ti元素加速溶解和扩散，Ag基钎料层和Cu-Ti层分层结构转而形成Cu-Ti连续过渡层。钎焊保温时间、降温方式、钎料中Ti元素含量、中间层厚度对钎焊接头多层结构影响不大。当Ti6Al4V中间层未置氢时，Al2O3/不锈钢接头剪切强度最低，残余应力主要集中在钎缝之中，使得断裂发生于Ti-Cu-Al-O层。随着氢元素的加入，残余应力逐渐向Ti6Al4V转移，陶瓷侧最大拉应力降低，钎焊接头强度均出现不同程度的提高。当中间层置氢0.3wt.%时，钎焊接头具有最高的剪切强度，达到127.2MPa。高强度接头断裂模式为陶瓷基体断裂或者陶瓷与钎缝混合断裂。Ti6Al4V本身热膨胀系数介于Al2O3和不锈钢之间，可减小两者之间的热膨胀失配。氢的引入增加了β相含量，促进了位错及层错的形成，提高了位错密度和位错攀移能力，进而促进晶界滑移，同时在钎焊冷却过程中通过共析反应细化了晶粒，从而显著降低其流变应力、提高延伸率，引起钎焊接头应力分布的改变和陶瓷侧拉应力的降低。上述成果有望为缓解陶瓷/金属钎焊接头残余应力提供新途径，为丰富和发展陶瓷/金属连接理论奠定基础。

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