Mechanical behavior and fatigue life of micro welded joints obtained by TIG spots in NiTi wires

Abstract

This work evaluates the effect of welding energy and cycling strain amplitude on fatigue behavior of welded joints obtained by micro Tungsten Inert Gas spots in NiTi SMA superelastic wires. For this purpose, a Dynamic Mechanical Analyzer operating in single cantilever bending mode is employed. The equipment monitors the evolution of the force level to maintain constant the strain amplitude as cycling proceeds. Fatigue lives of as-received and welded NiTi wires are compared for different strain amplitudes applied during mechanical cycling. In the evaluated range of strain amplitude, as-received NiTi wires showed fatigue lives between 10 3 and 10 4 cycles. The fatigue life of the welded joints is lower, although it remains in the same range observed for the asreceived wires.

Effect of welding current on the microstructure, residual stresses, mechanical properties and nano-mechanical behaviour of P-TIG welded TiNi binary shape-memory alloy

“…Similar microstructures have been reported in the literature regarding TIG welding of TiNi-based SMA. 23,33…”

“…Similar microstructures have been reported in the literature regarding TIG welding of TiNi-based SMA. 23,33 The average grain size of the FZ was measured using ImageJ software and is presented in Table 4. It can be observed that the grain size in the FZ decreased with the increase in peak current which could be due to the higher cooling rate.…”

“…Furthermore, during joining, various brittle phases are formed, which result in a significant loss of toughness of the welded joints. 12 Several welding techniques such as laser welding, [13][14][15][16][17] electron beam welding (EBW), 18 friction stir welding (FSW) 19 and tungsten inert gas (TIG) welding [20][21][22][23] have been used for the preparation of similar and dissimilar joints involving TiNi-based SMAs. Most of the research Corresponding author: Huzaifa Asif, Faculty of Materials and Chemical Engineering, Department of Materials Science, Ghulam Ishaq Khan Institute of Engineering Sciences and Technology, Topi 23640, Pakistan Email: huzi.asif@gmail.com on these alloys has been done on laser welding, and reductions of 37%-68% were observed in mechanical properties of weldments as compared to that of base alloy (BA).…”

“…The retention of martensite nuclei promoted the formation of martensite during uniaxial loading and the phase transformation caused a reduction of mechanical properties. 22 Araujo et al 23 studied the mechanical properties and fatigue life of TIG-welded TiNi-based SMA wires, with a diameter of 0.9 mm at different welding energies (1.96 J, 2.06 J and 2.16 J) and concluded that by increasing the welding energy, the fatigue life decreased. Furthermore, the fatigue life decreased with the increase of strain amplitude, which was attributed to grain coarsening, dendritic morphology, and the formation of impurity phases during solidification.…”

NiTi SMA Superelastic Micro Cables: Thermomechanical Behavior and Fatigue Life under Dynamic Loadings

“…Both samples also exhibited R-phase as an intermediate phase in the forward-phase transformation (austenite to martensite), indicated by the low enthalpy of forward transformation (4.7 J/g and 6.1 J/g) and of reverse transformation (5.6 J/g and 6.7 J/g) for the NiTi micro cable and the reference element, respectively. Also, the thermal hysteresis was small for both the NiTi micro cable and the reference element (6.8 °C and 6.1 °C, respectively) [ 16 , 17 ].…”