Effect Of Nb Addition On Microstructure And Properties Of TiNi Laser Welded Joints

TiNi shape memory alloy has broad application prospects in aviation,aerospace,biomedical,micro-electromechanical systems,robots,automobiles and other industries.However,the microstructure of the TiNi joint weld metal changes significantly compared to the TiNi alloy base material.The shape memory properties（phase transition temperature,shape recovery rate）and mechanical properties of the weld metal are significantly different from those of the base metal.The practical application value of the NiTi shape memory alloy welded structure is limited.In this study,the shape memory performance（phase transition temperature,shape recovery rate）and mechanical properties of the weld metal of TiNi laser welded joints are improved by the"preset Nb wire+heat treatment"composite method.The TiNi alloy was laser welded using a preset Nb wire of different diameters.The laser power was changed to determine the TiNi/Nb/TiNi laser welding process.The effects of Nb addition on microstructure,joint forming mechanism,shape memory property and mechanical property of TiNi/Nb/TiNi laser welded joint were studied by optical microscope,scanning electron microscope,energy spectrum analysis,micro-area X-ray diffraction,differential scanning calorimetry and other testing methods.TiNi/Nb/TiNi joints filled with 0.10mm Nb wire was carried out to heat treatment at different temperatures to study the effects of heat treatment on microstructure,shape memory properties and mechanical properties.Through this"filling 0.10mm Nb wire+600℃ heat treatment"composite method,the weld metal which very similar to the shape memory property of TiNi base material and excellent in mechanical properties is obtained.The relevant conclusions are as follows:TiNi base metal and TiNi joint weld metal no filled with Nb wire are mainly B2phase and B19’phase.The weld metal of TiNi joint filled with Nb wire is mainly B2phase,B19’phase,NbTi phase,Nb phase and（TiNi,Nb）solid solution.The martensitic transformation process of TiNi base metal and each joint is B2→R→B19′,and the reverse phase transformation process is B19′→B2.The A_s of weld metal filled with 0.10mm Nb is-9.35℃ which similar to that of TiNi base metal,and the A_f point of the two is also relatively similar.The change of weld metal phase transition temperature is mainly related to the addition of Nb element to change the Ni/Ti atomic ratio in the TiNi matrix of weld metal,and the change of weld stress field caused by the generation of NbTi phase and the dissolution of Nb atoms in the TiNi matrix.The phase transition temperature of the weld metal filled with 0.10mm Nb wire is similar to that of the TiNi base material,and this joint is selected for heat treatment.As the content of Nb in the weld metal increases,the microhardness of the weld metal increases.Tensile strength and elongation of the joints filled with Nb wire are higher than those no filled with Nb wire,and all joints are quasi-cleavage fracture.The tensile strength and elongation of the joint filled with 0.10mm Nb wire is 743MPa and21%respectively,reaching 89%and 64%of TiNi base metal respectively.Nb atoms are dissolved in TiNi phase,which improves the strength of the joint.After heat treatment,the weld metal mainly consists of B2 phase,NbTi phase and white precipitated phase（TiNi,Nb）solid solution.During the heat treatment,part of NbTi phase is dissolved in TiNi matrix.A small amount of Ni₄Ti₃ phase is precipitated from weld metal after heat treatment at 600℃.After heat treatment at 600℃,the phase transition temperature and phase transition temperature range of weld metal are very similar to that of TiNi base metal:A_s=-7.08℃,A_f=38.67℃,M_s=18.77℃,M_f=-51.54℃,A_s-M_s=-25.85℃.The change of weld metal phase transition temperature after heat treatment is mainly related to the solid solution of NbTi in TiNi matrix,the precipitation of Ni₄Ti₃ phase and the stability of martensite phase.Its shape recovery rate reaches 90.2%of TiNi base metal,with good shape recovery.The average microhardness of weld metal after heat treatment is higher than that of as-welded metal.The average microhardness of weld metal treated at 600℃ is similar to that of TiNi base metal,reaching 254 HV_0.2.After heat treatment at 600℃,the elongation of joint is 25%higher than that of as-welded joint,and its tensile strength reaches 83%of TiNi base metal,which is related to Ni₄Ti₃ phase precipitation.