Ti6Al4V/Nb/NiTi异质接头脉冲激光焊诱发共晶反应成形机制及调控机理研究

The main problem in dissimilar welding of Ti6Al4V and NiTi alloys is formation of brittle intermetallic phases，resulting in the formation of transverse cracks in the brittle weld metal and deterioration of mechanical properties. In this work, the bonding method of pulsed laser welding induced eutectic reaction is proposed to solve this problem. Niobium is used as an interlayer for joining NiTi and Ti6Al4V. The laser is focused near the Ti6Al4V/Nb interface, so that Ti6Al4V alloy and Nb sheet partially melted. Meanwhile, the unmelted Nb interlayer absorb significant amount of energy from the welding pool and transferred it to the NiTi side, making eutectic bonding occur at the NiTi/Nb side of the joint. The mechanism of deep penetration/thermal conductivity co-pool welding, interface reaction and growth kinetics of interfacial microstructure and the influence of matching relation between fusion welding and eutectic reaction interface microstructure on properties of joint are systematically studied by the combined method of experimental and theoretical analysis. The mechanism and technology of pulse laser welding induced eutectic reaction bonding method are obtained，and the eutectic reaction interface model and mathematical model between reaction layer thickness and the welding parameters are established. Then，the optimized interface microstructure is achieved and high quality joints are obtained. A new method and the relevant theoretical foundation are established for solving the formation of brittle intermetallic phases during dissimilar metal welding process.

针对Ti6Al4V/NiTi异种金属焊接时易形成脆性金属间化合物问题，本项目提出能够避免焊缝中金属间化合物形成的脉冲激光焊诱发共晶反应的连接方法，即选用与母材反应不产生金属间化合物的高熔点Nb箔作为中间层，将熔化控制在Ti6Al4V/Nb界面，靠近NiTi侧的Nb箔未发生熔化，依靠热传导提供的热量在NiTi/Nb界面发生NiTi-Nb共晶反应实现连接。本项目拟采用试验和理论分析相结合的手段，对熔化焊界面的深熔/热导共熔池焊接机理，共晶反应界面形成机制及组织生长动力学，熔化焊/共晶反应界面微观组织状态匹配关系对接头性能影响机制等关键科学问题进行研究，掌握脉冲激光焊诱发共晶反应连接方法机理和工艺，建立共晶界面反应模型及反应层厚度与工艺参数关系数学模型，获得综合性能优良的焊接接头。研究成果为解决异种金属焊接易形成金属间化合物问题探索新的解决途径并提供相关理论依据。

本项目针对Ti6Al4V/NiTi异种金属焊接时易形成脆性金属间化合物问题, 利用脉冲激光焊诱发共晶反应方法获得了无任何金属间化合物形成的接头。该方法采用高熔点、高导热性的Nb作为中间层，通过一次焊接形成了Ti6Al4V/Nb熔化区和Nb/NiTi共晶反应层2个连接界面。未熔化的Nb阻隔了Ti和Ni的扩散，抑制了TixNiy脆性金属间化合物的形成。通过研究熔化区和共晶反应层的温度场、组织结构特点、微区力学性能等，揭示了熔化焊界面的深熔/热导共熔池焊接机理，共晶反应界面形成机制及组织生长动力学，熔化焊/共晶反应界面微观组织状态匹配关系对接头性能影响机制。随着热输入的增加，熔化焊界面的焊接模式由深熔/热导共熔池转为全深熔焊模式，共晶反应层厚度明显增加，接头的拉伸断裂位置由未熔Nb转为Nb/NiTi共晶界面。当Nb/NiTi共晶反应层厚度与未熔化Nb比值约为0.7时，接头的综合力学性能达到最佳。研究成果为解决异种金属焊接易形成金属间化合物问题提供了新的解决途径并丰富了相关理论。