钛及钛合金激光焊接头BCC→HCP相变机制及对组织性能影响

BCC→HCP phase transformation mechanism is complicated in laser beam welded joint of titanium and titanium alloy, but its effect on microstructure and mechanical properties is quite important. The cooling rate during phase transformation in laser beam welded joint is obtained by simulating the welding temperature field. Based on the relationship between the cooling rate during phase transformation and phase transformation mechanism, the phase transformation mechanisms in laser beam welded joints are estimated preliminarily. The microstructural characteristic of product phase in laser beam welded joint is revealed by characterizing the microstructure. Based on the relationship between the microstructural characteristic of product phase and phase transformation mechanism, the phase transformation mechanisms in laser beam welded joints are confirmed finally. The relationship between the parent phase and product phase in laser beam welded joint is established by reconstructing the parent phase boundary. Combined with the microstructural characteristic of product phase and phase transformation feature during laser beam welding, the microstructural formation mechanisms and evolution models in laser beam welded joints under different phase transformation mechanisms are clarified. According to the microstructural characteristic of product phase, the relationship between the microzone hardness properties and microstructural characteristic of product phase in laser beam welded joint is founded by evaluating the microzone hardness properties. And then the microzone strengthening mechanisms in laser beam welded joints under different phase transformation mechanisms are uncovered. The research in this project profoundly discloses the phase transformation mechanism in laser beam welded joint of titanium and titanium alloy, and detailedly expounds the effect of phase transformation mechanism on microstructure and mechanical properties. It lays the theoretical foundation for the preparation of high strength joint by controlling the microstructure, and is of great importance to promote the widespread use of titanium and titanium alloy in the industrial field.

钛及钛合金激光焊接头所发生的复杂BCC→HCP相变机制对组织性能产生重要影响。通过激光焊温度场模拟，捕获接头相变时冷却速度，基于相变时冷却速度和相变机制的相关性，初步预判接头所发生的相变机制。通过接头微观组织表征，揭示接头子相微观组织特征，基于子相微观组织特征和相变机制的相关性，最终确定接头所发生的相变机制。通过接头高温母相晶界重构，建立接头母相和子相的相关性，结合接头子相微观组织特征和激光焊相变特点，揭示不同相变机制作用下接头微观组织的形成机理和演化模型。通过接头微区硬度性能评价，结合接头子相微观组织特征，建立接头微区硬度性能和子相微观组织特征的相关性，揭示不同相变机制作用下接头微区的强化机制。本项目研究将深刻揭示钛及钛合金激光焊接头所发生的相变机制，详细阐明相变机制对接头组织性能的影响，为微观组织调控下高强度接头制备奠定理论基础，对促进钛及钛合金在工业领域中广泛应用具有重要意义。

钛及钛合金激光焊接头所发生的体心立方到密排六方（BCC→HCP）相变机制十分复杂却又非常重要，它不但直接控制了接头的微观组织，而且也间接影响了接头的力学性能。然而，少有学术报道系统关注钛及钛合金激光焊接头所发生的BCC→HCP相变机制及其对接头组织性能的影响，因而迫切需要开展相关研究工作。基于Fluent建立了考虑固态相变的三维激光焊准稳态流动导热模型，探究了固态相变对钛及钛合金激光焊温度场的影响，研究表明固态相变物性参数和相变潜热对温度场计算结果有明显影响，且考虑固态相变的计算结果与实测结果更加吻合。通过钛及钛合金激光焊温度场模拟和接头微观组织表征，探究了接头所发生的相变机制，研究发现TA2接头中发生了长程和短程扩散相变，随着焊速的提高，短程扩散相变比重增加，而TC4接头中发生了短程和无扩散相变，且随着焊速的提高，无扩散相变比重增加。基于Burgers取向关系构建并编译了自动重构钛及钛合金高温母相晶体信息的程序，对真空热处理后TA2试样进行了自动运算，获得了合理的高温母相晶体信息。通过接头微观组织表征和显微硬度评价，揭示了钛及钛合金激光焊接头的强化机制，结果表明TA2接头可主要归结于低角晶界位错强化，而TC4接头则联系于马氏体强化。对TA2激光焊接头进行了真空热处理，探讨了热处理对接头组织性能的影响，结果发现在625-700℃之间热处理时接头BM和FZ的组织性能没有发生明显变化，而HAZ则出现了再结晶现象，且随着热处理温度和保温时间的增大，再结晶面积逐渐增大，相应地导致了显微硬度的下降。开展了钛合金复杂曲面薄壁构件的激光焊工艺研究，通过仿形工装设计和焊中横向加载方式制备了具有良好焊缝成形的激光焊薄壁构件。项目研究成果为钛及钛合金激光焊接头微观组织调控下的高强度接头制备奠定了理论基础，对促进钛及钛合金激光焊技术在工业领域中的广泛应用具有重要意义。