Interfacial Microstructure And Strength Of Transient Liquid-phase Bonding Of The TiNi Shape Memory Alloy And Stainless Steel

TiNi shape memory alloys(SMA)have a low density,a good biocompatibility,a super-elasticity,a good corrosion resistance and so on,which have great potential in applications in many fields such as aerospace,atomic energy,ocean development,instrumentation,and medical devices.The successful application of any new material not only depends on its inherent characteristics,but also depends on the processing(welding,mechanical processing,etc.).The single titanium material is difficult to meet the requirements of practical application in many cases.The combination of TiNi SMA and stainless steel,which has the advantages of titanium alloy and steel,can give full play to the complementary advantages of the two materials in the performance and economy.Transient liquid phase(TLP)diffusion bonding technique has the advantages of brazing and diffusion bonding,which can be used in the efficient connection of dissimilar material with a high strength and low brittleness under a low pressure.Therefore,in the present thesis,the foils of Cu/Ni,Ti/Ni and Ti/Cu were taken as the middle layers,respectively,and the TiNi SMA and stainless steel were bonded by the TLP.The effects of the bonding conditions,including temperature,time and pressure,on the microstructure and mechanical properties of the bond were studied.The main contents and conclusions are as follows:(1)The TiNi SMA and stainless steel were bonded by the TLP method at different temperatures by using Cu/Ni as a composite interlayer,and a compact bonding interface without defects such as hole,crack and so on was obtained.However,the joint exhibits a low shear strength and plasticity,which can’t meet the requirements in practical application.(2)The TiNi SMA and stainless steel were bonded by the TLP method at different temperatures and time by using Ti/Ni as a composite interlayer and a sound metallurgical bond without any defects was achieved.The width of the diffusion layer and the micro-hardness of the connecting layer increased with the increase of the joining temperature and time.The joint with the highest shear strength(about143.5MPa)was obtained at 1100℃ for 40 min.The fracture surface of the joint showed a typical brittle fracture characteristic.(3)The TiNi SMA and stainless steel were bonded by the TLP method at different temperatures and time by using Ti/Cu as a composite interlayer and a sound metallurgical bond without any defects was achieved.The joint obtained at 1100℃for 20 min exhibited the highest shear strength of about 163.1MPa.The fracture occurs at the TiNiCu compound layer near the TiNi parent material side.The joint also fractured in the brittle fracture mode during the shear test.(4)The TiNi SMA and stainless steel were bonded by the TLP method by using Ti/Cu as a composite interlayer under the pressure of 4GPa.In comparison with the joint obtained under normal pressure,the bond exhibited more sound.High pressure suppressed the formation of the intermetallics,leading to the decrease of the contents of the brittle phases in the bond.The joint exhibits relative high shear strength,which reaches 194.6MPa.Moreover,the plasticity of the joint was also improved.