Thermomechanical fatigue of post-weld heat treated NiTi shape memory alloy wires

Highlights

• Thermomechanical fatigue of base metal and Nd:YAG laser welded NiTi wires.

• The welded wires had greater plastic strain after a smaller number of cycles.

• This deformation caused a loss of actuation stability, stroke, and cycle life.

• A post-weld heat treatment increased the actuation stability and stroke.

• This increased the cycle life of the welded wires by an order of magnitude.

Abstract

Integration of NiTi shape memory alloys (SMAs) into a growing range of applications will rely on the development of welding and joining techniques. Successful adoption of welded NiTi SMA actuators into these applications requires the characterization of the joint fatigue properties. Few investigations have been attempted to characterize the mechanical fatigue properties of NiTi joints, and to the authors’ knowledge there have been no previous investigations on the thermomechanical fatigue properties of these joints. The current work investigated the thermomechanical fatigue properties of Nd:YAG pulsed laser welded, and post-weld heat treated NiTi wires. The welded wires maintained over 86% of the base metal ultimate tensile strength; however, they had reduced actuation stability and stroke, and had significantly reduced cycle life. Use of a post-weld heat treatment successfully increased both the actuation stability and the cycle life by an order of magnitude compared to the welded wires.

Introduction

Advanced engineering designs are increasingly relying on shape memory alloys (SMAs) to facilitate light weighting, miniaturization, medical breakthroughs, and revolutionary applications [1], [2], [3], [4]. In these designs, traditional motors and actuators are being replaced with NiTi SMAs, with this replacement forecasted to significantly increase over the next decade [5]. Mechanical joining techniques are generally used to integrate NiTi SMAs into these systems, but these techniques have limitations in joint geometry and maximum load. These limitations can be overcome with metallurgical bonding, which has a number of advantages that include high strength connections to control systems [6], and reduction of cost by joining NiTi with less expensive materials [7], [8], [9]. The counterpoint to these advantages are the difficulties that can be encountered during joining of NiTi, which include solidification cracking, formation of brittle intermetallics and oxides, and degradation of the material properties [10], [11].

Nd:YAG laser welding was chosen for the current study because it is a superior method for joining SMAs, achieving NiTi joint efficiencies of 80% with minimal heat input [12]. The low heat input limits the deterioration of the strain of actuation, strength and ductility [13], [14], while preserving the functional properties (i.e. the shape memory effect and pseudoelasticity) of NiTi [15]. Characterization of the joint properties to date has focused on quasi-static tests, but integration of SMAs into actuator applications requires characterization of fatigue properties. There have been a handful of investigations on the effect of welding on the mechanical fatigue of pseudoelastic NiTi [16], [17], [18]; but to date there have been no investigations on the thermomechanical fatigue of welded SMAs. The sole investigation on a small number of thermal cycles of a welded SMA involved a two-way shape memory effect actuator, which concluded that the effect on the actuator performance was minimal [19]. The same cannot be concluded for the cycle life, which has been shown to be greatly affected by coarse grained microstructures like those in the heat affected and fusion zones of the welded wires [16], [17], [20], [21], [22]. Furthermore, mechanical fatigue investigations of welded NiTi reported a reduction in the mechanical cycles to failures compared to the base material as a result of the alteration of the microstructure [23]. A post-weld heat treatment strengthened the microstructure and improved the cycle life of the weld [16], [20]. It is hypothesized that a similar result would occur for thermomechanical fatigue of welded NiTi; however, one must carefully chose the post-weld heat treatment as it can promote phase transformations that inhibit either the shape memory or pseudoelastic effects at a given temperature of operation.

The objective of the current work was to perform the initial investigation on the effect of welding and a post-weld heat treatment on the thermomechanical fatigue of NiTi SMAs. In this study it was shown that the welding process reduced the properties and fatigue life of the thermomechanically cycled actuators compared to base metal, and that the post-weld heat treatment improved the properties and cycle life of the welded wires.