Investigation of the inbuilt properties of vacuum brazed NiTi shape memory alloys

• Applicant: Professor Dr.-Ing. Wolfgang Tillmann
• Subject Area: Joining and Separation Technology ; Synthesis and Properties of Functional Materials ; Mechanical Properties of Metallic Materials and their Microstructural Origins
• Term: since 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 492991758

The main objective of the present research project is characterized by a reliable and reproducible production of NiTi shape memory joints combined with the highest possible pseudoelasticity by vacuum brazing. Currently, laser welding is preferably used to join these functional materials or to produce NiTi/steel joints, whereby an irreversible expansion results in the joining zone and large area connections cannot be produced. Vacuum brazing can significantly reduce the width of the joining zone, which is intended to minimize the irreversible amount of expansion. Furthermore, brazing filler materials (Nb, Nb1Zr, Cu, Au65Cu, Ag26.5Cu3Ti) are preferably used in this research project, which enable pseudoelastic properties of the fusion zone due to the metallurgy of the resulting joint microstructure. Due to the necessity of a total base material heat treatment during vacuum brazing, a solution annealing process of the base material is carried out together with the brazing process (830-1180 ° C). As a result, previously precipitated phases (Ni4Ti3) which impair pseudoelasticity are brought into solution in the Ni mixed crystal and suppressed in precipitation by rapid pressure gas quenching during cooling. The comparatively slow vacuum furnace process results in grain growth of the originally ultra-fine-grained base material, which can also result in irreversible expansion of the components. In this research project, a work-hardened NiTi base material state is used for brazing for the first time. As a result of the initial recovery processes, an inhibition of grain growth and thus a significantly better pseudoelasticity is expected. The preparatory work demonstrates a high degree of pseudoelasticity for the regular as well as for the work-hardened initial state after the intended heat treatment cycle during brazing. The research project addresses NiTi/NiTi and NiTi/steel connections likewise. For NiTi/steel joints, normally a formation of brittle FeTi phases occurred at the interface of the joining zone and the steel. Thus, oüberhauptnly a few published works achieved a pseudoelastic behavior of these connections at all. The formation of these brittle phases was successfully avoided by using a diffusion barrier made of the high melting refractory metal tungsten. A central objective of the research project is characterized by a correlation of the microstructural joining properties (joining zone width, microhardness, elastic modulus, phase condition, grain size) with the pseudoelastic behavior of the heat treated base materials and the brazed connections. This is the first time that systematic research will discover the variables of the heat treatment process as well as of the brazing filler metal and the joint design to optimize the pseudoelastic behavior of NiTi shape memory alloys.

• DFG Programme: Research Grants