Effect of CO2 laser welding on the shape-memory and corrosion characteristics of TiNi alloys

Abstract

A CO 2 laser has been employed to join binary Ti 50 Ni 50 and Ti 49.5 Ni 50.5 shape-memory alloys (SMAs), with an emphasis on the shape-memory and corrosion characteristics. Experimental results showed that a slightly lowered martensite start (M S ) temperature and no deterioration in shape-memory character of both alloys were found after laser welding. The welded Ti 50 Ni 50 , with an increased amount of B2 phase in the weld metal (WM), had higher strength and considerably lower elongation than the base metal (BM). Potentiodynamic tests revealed the satisfactory performance of laser-welded Ti 50 Ni 50 in 1.5 M H 2 SO 4 and 1.5 M HNO 3 solutions. However, the WM exhibited a significantly higher corrosion rate and a less stable passivity than the BM in artificial saliva. On the other hand, the pseudoelastic behavior of the laser weld was investigated only for the Ti 49.5 Ni 50.5 alloy, to facilitate tension cycling at room temperature. The cyclic deformation of Ti 49.5 Ni 50.5 indicated that the stress required to form stress-induced martensite ( m ) and the permanent residual strain ( p ) were higher after welding at a given number of cycles (N ), which were certainly related to the more inhomogeneous nature of the WM.

Welding and Joining of NiTi Shape Memory Alloys: A Review

“…In the former, the hardness in the FZ increased compared to the base material, while in the latter, the opposite behaviour was registered: lower hardness of the FZ. Nevertheless, [31,138] did not observe a significant change when measuring the hardness across the different regions of the weld.…”

“…Additionally, wires with different transformation temperatures were successfully joined and the welded joints exhibited the stress-induced martensite transformation in two steps, where each plateau corresponded to the original ones in the base materials. However, as later was studied by [30,31], the presence of contaminants degraded the functional properties of the welded joints.…”

“…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].…”

“…This is the reason why, sometimes, it is reported that no HAZ is observed in NiTi laser welds [31,146].…”

“…A more in depth study of the tensile behaviour of the thermal affected regions of Ni-rich laser welded NiTi, was performed by Hsu et al [31]. Within the gauge length of tensile specimens, two-thirds of the volume fraction of the analysed weld corresponded to the FZ, while the remaining was the HAZ.…”