High strain and long duration cycling behavior of laser welded NiTi sheets

Abstract

The use of NiTi in complex shaped components for structural applications is limited by the material cost and machinability and adequate joining techniques have been investigated to minimize the thermal cycle effect on the superelastic and shape memory effects exhibited by NiTi. Laser welding is the most used joining process for this material. However, existing studies mainly address the functional properties of laser welded NiTi wires, and the superelastic cycling tests are limited to either a low number of cycles (maximum 100) or to low strains (below 6%). This paper discusses the results of the cycling behavior exhibited by laser butt welded 1 mm thick NiTi plates, when tested to high strains (up to 10%) and for a large number of cycles (600). The superelastic effect was observed and the microstructural changes induced by the laser welding procedure, namely the extension of the thermal affected regions, were seen to influence the evolution of the accumulated irrecoverable strain. Thus, it is possible, by controlling the heat input introduced during welding, to tune the maximum superelastic recovery presented by NiTi laser welds.Keywords: NiTi; Shape memory alloys; Laser welding; High strain cycling; Superelasticity. Graphical abstract 2 Highlights Cycling at high strains and for a high number of cycles was performed on laser welded NiTi sheets. This is the first study of this kind.  Process parameters influence the superelastic recovery. Lower heat input implies lower irrecoverable strain.  Welded samples were successfully load/unloaded 600 times, 110 to 130 MPa below their ultimate tensile strength.  Martensite in the thermal affected regions is responsible to lower the superelastic plateau of the welds.

Welding and Joining of NiTi Shape Memory Alloys: A Review

“…Early laser welding studies on NiTi shape memory alloys were performed using CO ?2 lasers [31,[125][126][127] while, nowadays, the primary choice for these studies are the infrared lasers, such as Nd:YAG [128][129][130][131]145,165,166] and fibre lasers [132][133][134] due to their low wavelength and increased material absorptivity and, thus, narrow welds [38,135].…”

“…Several research groups have been performing structural characterization of existing phases in the different zones of laser welds, especially by X-ray diffraction [196], due to the microstructural features size and the narrow dimension of the different heat affected zones [133,[141][142][143]166,168]. Fernandes et al [142] and Oliveira et al [166,[168][169][170] used synchrotron X-ray radiation to probe laser welds from the base material into the FZ, in transmission mode, with a reduced spot size of 200 × 200 µm and good spatial resolution.…”

Effect of laser welding parameters on the austenite and martensite phase fractions of NiTi

“…Among the different joining techniques available, fusion-based ones, such as tungsten inert gas (TIG) [2] and laser welding [3][4][5][6], have presented the best results in terms of the mechanical properties of the joints. Laser welding is by far the most used process for welding NiTi due to the reduced extension of the thermally affected regions, which is especially important for NiTi due to the deterioration of its functional properties (shape memory effect and superelasticity) at high temperatures.…”

Martensite stabilization during superelastic cycling of laser welded NiTi plates

“…Recently, the present authors analyzed the functional fatigue by superelasticity in laser welded NiTi plates [10], by performing cycling tests up to 10% for a total of 600 cycles. The evolution of the accumulated irrecoverable strain was found to increase significantly at the early stages of cyclic deformation, followed by a tendency to stabilize at a fixed value.…”

“…All samples were removed from the same welded plate and subjected to different mechanical solicitations: one sample as-welded (sample reference W0, where W = weld, 0 = number of mechanical cycles); one sample with a total of 4 load/unload cycles up to 10% strain (sample reference W4); one sample with a total of 600 load/unload cycles up to 10% as reported in [10] (sample reference W600).…”