H. Gugel, W. Theisen, Ruhr-University Bochum, Bochum, Germany
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
Summary: The paper focuses on laser welded joints between pseudoelastic NiTi-alloys and austenitic stainless steel in terms of thin wires (diameter < 100 µm) and sheet metal with a thickness of 1 mm. The results can be summarised as follows:
After optimization of the welding parameters the dissimilar joints appear to be without any appreciable welding faults. In contrast to joints of similar NiTi/NiTi pairs, microhardness measurements indicated a large hardness increase in the fusion zone. EDX studies in the SEM reveal the presence of numerous precipitated intermetallic phases between nickel and titanium (e.g. Ni3Ti and Ni4Ti3) as well as between iron and titanium (e.g. TiFe2). Joints of pseudoelastic NiTi with 50.6 % Ni and stainless steel wires (X10CrNi18-10), exhibited a fracture stress of just over 600 MPa, which is above the stress plateau, but only when the laser spot was shifted to the NiTi side. The fusion zone is not always completely mixed and is hardened by a large proportion of intermetallic phases of the elements nickel, titanium and iron to such a high degree that low-deformation fracture occurs in tensile tests. Compared to laser welding of wires the sheet metal joints are basically similar concerning the microstructure in the melting zone as well as in the heat-affected zone. Additionally there is a strong influence of the carbon content in the steel. With increasing carbon content the amount of fine dispersed TiC solidifying from the melt is lifted resulting in high microhardness values in the fusion zone. Moving the laserspot-position to the NiTi-side has a positive effect on the mechanical properties of NiTi-steel sheet metal joints, too.
