Microstructure and properties of NiTi/steel joints produced by laser welding

Fusion joints of nickel-titanium shape memory alloys (SMA) and austenitic steel are combinations of great interest in terms of system integration of the NiTi components. Laser welding of similar NiTi joints is an established process and is the most compromising one to join dissimilar NiTi/Steel combinations. The major problem in case of joining NiTi-SMA and austenitic steel is the formation of brittle intermetallic phases i.e. of the type FeTi, FeTi₂. These intermetallic phases tend to reduce the ductility of the joint to virtually zero. Hence the joint could not benefit of the shape-memory-effect. In case of thin wires we were able to increase the tensile strength of joint by shifting the laser spot onto the pseudo elastic NiTi-component.

In the present study laser welded NiTi/steel joints of sheet material of 1 mm thickness were investigated. The weldability was analysed concerning variations of the laser spot position and the influence of the carbon content of the steel part on the weldability and its effect on the mechanical properties of the dissimilar joints. The chemical composition, the microstructure, the microhardness and the formation of cracks and pores in the melting zone as well as in the heat affected zone were studied. In addition we investigated the mechanical properties of the joints and related them to the microstructure and the joining parameters. Microhardness tests of the different dissimilar joints showed an increase of the microhardness with increasing carbon in the steel component content