NiTiNb激光焊接接头退火前后的显微组织和力学性能分析

Microstructure and Mechanical Property Analysis about NiTiNb Laser Welding Joint around Heat Treatment

采用Nd:YAG脉冲激光焊机实现了200μm厚的NiTiNb形状记忆合金的激光焊接。焊接接头在850℃进行了退火处理。借助于光学显微镜、扫描电镜、精密拉伸机对接头的显微组织和力学性能进行了分析。结果表明,未热处理焊接接头的显微组织存在分层现象。熔合区的晶粒外延生长,焊缝中心为细小的等轴晶。焊接接头的抗拉强度为母材的90%。焊接接头断口韧窝小而浅,表现为延性断裂。热处理后焊缝的等轴晶消失,粗晶区和偏析区交替分布。母材和焊接接头都出现了应力诱发马氏体现象,表现为伪弹性。焊接接头的屈服强度仅为母材屈服强度的60%,母材和焊接接头的显微硬度低于热处理前的显微硬度。

Laser welding of 200μm thickness NiTiNb shape memory alloy is achieved by Nd：YAG pulse laser.The welding is annealed at 850℃.Microstructure and mechanical properties are analyzed by optical microscope,scanning electron microscope and precision tensile machine.The results indicate that a layered microstructure occurs before heat treatment.Grains in the fusion area are epitaxial growth and the weld center are full of small isometric crystals.The tensile strength of the welding joint reach 90% of the matrix.Small and light dimples are observed on the welding joint fracture,which means it is ductile fracture.All the isometric crystals in the welding joint vanish after heat treatment,and coarse grain region and segregated spot have alternate distribution.Stress induced martensite occurs in both matrix and welding joint,which is called pseudo-elasticity.The yield strength of the welding joint with heat treatment is only 60%of the matrix.The micro-hardness of the matrix and welding joint with heat treatment is lower than those of the unheated ones.