Characteristics and fracture morphology of Ti-Ni type shape memory alloy and its laser weld joint.

Abstract

Mechanical properties and fracture behaviour of Ti-50.7at%Ni shape memory alloy and its laser weld joint have been investigated. The base metal aged at 673K for 3.6ks exhibited super elasticity and higher tensile strength than the solution base metal. However, the aged material showed less fracture strain and more brittle fracture surface than the solution material. Butt welding of 3mm thick Ti-Ni alloy was performed using 10kW CO2 laser. The laser welded specimens showed a successful shape memory effect and super elasticity. These properties were identical with those of the base metal. However, the tensile strength and the fracture strain of the laser welded specimen were less than those of the base metal. The fracture occurred at the columar crystal boundary at the center of the weld metal. The boundary was perpendicular to the direction of the loading and, moreover, oxide inclusions crystallized at the boundary. These facts seem to cause low strength of the weld joint. The fracture strain of the welded specimen, nevertheless, was more than 6% which is the maximum recoverable elongation in polycrystal Ti-Ni base metal. Laser weld is, therefore, applicable to the joining of Ti-Ni shape memory alloy when the alloy is used as a functional material.

Welding and Joining of NiTi Shape Memory Alloys: A Review

“…In the former the functional properties of the material suffer a degradation during cyclic solicitation, while the latter the material fails, during high cyclic loads like any other engineering material. Several studies report the cycling behaviour of NiTi welds [31,125,126,132,137,140,144,166]. However, a significant disparity in the reported experiments is observed.…”

“…The shape memory effect on laser welded NiTi was reported in early studies [125,126]. Full recovery of the imposed strain was achieved when the samples were heated to the austenitic phase.…”

“…Evidence of the shape memory effect in laser welded NiTi was investigated by either tensile [31,125,127,137,144] or bending tests [168]. In the former, the samples are pulled, in the martensitic phase, up to a certain strain and unloaded to a 0 stress condition, presenting a significant deformation.…”

“…Laser welding of NiTi shape memory alloys dates back to 1989 when Araki et al [125] welded 3 mm thick Ni-rich NiTi plates, using a CO ?2 laser. Due to the high sensitivity of NiTi to high temperatures, laser welding was considered to be an adequate process to minimize the thermal affected regions and to preserve its functional properties: shape memory effect and superelasticity.…”

“…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].…”