| dc.description.abstract | The feasibility of using friction stir welding (FSW) to join nickel-titanium (NiTi) shape memory alloys (SMAs), both to themselves and to other materials, has been explored. The effects of the FSW process on the microstructure, mechanical properties, and shape memory effect have been investigated. The results indicate that friction stir welding is a suitable method for similar welding of NiTi SMAs, giving a joint efficiency of 93% when the NiTi was tested in its martensitic phase, and a joint efficiency of 84% at elevated temperatures, with the NiTi in its austenite phase. In both cases, fracture was observed to be of a ductile nature. Due to the grain refinement experienced in the stir zone (SZ) of the weld, the material in this region showed more strain hardening than the base metal. The martensitic plateau characteristic of SMAs when tested in their martensitic phase was observed in the welded region, indicating that detwinning is still the dominant mode of martensite deformation at low strains, even within the weld itself. The transformation temperatures within the stir zone, measured by differential scanning calorimetry (DSC), showed little to no change when compared to the base metal. Significant scatter was seen in the microhardness values measured on the cross section of the weld, even at distances far from the weld. Finely dispersed tungsten particles of about 75 nm were seen deposited in some welds, a result of tool wear.
FSW of NiTi to Inconel 625 and Ti-6Al-4V proved difficult, but dissimilar welds between NiTi and 304 stainless steel were achieved, with a joint efficiency of 80% at room temperature and 100% at high temperatures, when the NiTi was in its austenite phase. Additionally, the use of a permanent stainless steel backing strip resulted in a joint efficiency of 100% even at low temperatures, when the NiTi was martensite. The SZ was found to consist of refined grains averaging 8 µm in size, compared to 20 µm in the stir zone of the NiTi similar welds. Although welding parameters were not optimized for steady-state defect-free welds, defect-free sections were produced in the transient portion of the weld. | en |
