Vacuum Brazing of NiTi Shape Memory Alloys Using Ag26.5Cu3Ti

Abstract Nickel-titanium alloys (NiTi) are known for their unique mechanical properties, in particular the shape memory effect and pseudoelasticity. The phenomenon of pseudoelasticity is based on the diffusion-free transformation of martensite into austenite upon mechanical loading and its capability to return to its original shape upon unloading. Thermal joining processes such as welding and brazing can affect the mechanical properties of NiTi due to thermal impact as well as microstructural changes in the joining zone. This can lead to an impairing or even absence of the pseudoelasticity. The aim of this study is to investigate whether NiTi can be successfully brazed without losing its characteristic shape memory properties. To achieve this, controlled heating experiments were first conducted to simulate brazing cycles and assess how different temperature profiles and cooling rates influence the base material. The NiTi wires (diameter 1.6 mm, length 150 mm) were subjected to quasi-static and cyclic tensile tests in order to identify changes in material behavior after thermal exposure. Although it is well known that thermal exposure generally has a detrimental effect on the functional properties of NiTi, the present study demonstrates that by a high fast cooling rate with argon in a vacuum furnace, the shape memory properties can be successfully preserved after heat treatment. Based on the results of the simulated brazing tests, joints were produced using the active braze alloy Ag26.5Cu3Ti and examined with regard to their microstructure and their cyclic and quasi-static strength. The results provide new insights into the feasibility of brazing NiTi while maintaining its functional properties through optimized thermal management during the brazing process.