Materials Science and Engineering A 412 (2005) 235–240 Laser welding of a NiTi alloy: Mechanical and shape memory behaviour A. Falvo, F.M. Furgiuele ∗ , C. Maletta Department of Mechanical Engineering, University of Calabria, P. Bucci 44c, 87036 Arcavacata di Rende (CS), Italy Received in revised form 1 August 2005; accepted 18 August 2005 Abstract The present paper reports the effect of Nd:YAG laser welding on the mechanical and functional properties of Ni–51 at.% Ti alloy. The specimens were prepared from thin sheets and tested in martensitic conditions. The mechanical and shape memory effects of both welded and unwelded material (reference) were measured using standard tensile tests and thermo-mechanical cycles. Differential scanning calorimeter investigations were carried out to determine the phase transformation temperatures. Micro-hardness tests and microscopic investigations were performed in order to evaluate the extension of the heat affected zone and the melting zone. Finally, a systematic comparison of the results between welded and reference specimens was carried out. © 2005 Elsevier B.V. All rights reserved. Keywords: NiTi; Shape memory alloy; Nd:YAG laser welding; Shape memory effect 1. Introduction NiTi shape memory alloys (SMAs) have become well known in recent years for their special functional properties, namely shape memory effect (SME) and superelasticity effect (SE). These characteristics are due to the martensitic transformation and its reversion, which can be activated by thermal or mechan- ical loads. In particular, the SME is an apparent plastic strain phenomenon, due to a deformation mechanism of the crystalline structure known as detwinning [1]. This apparent strain can be recovered by a thermal treatment that activates the reverse martensitic transformation. The martensitic transformation can also be activated, under specific conditions, by mechanical loading; the martensite obtained in this way is known as stress-induced martensite. SE is a specific property, such as a large deformation capability, due to a stress-induced martensitic transformation. The large defor- mation can be recovered through a hysteresis loop by the reverse martensitic transformation that occurs during unloading [2]. Due to their high performance and great resistance to fatigue and corrosion, the use of near equiatomic NiTi alloys has grown recently in the field of microelectro-mechanical systems ∗ Corresponding author. Tel.: +39 0984 494827; fax: +39 0984 494673. E-mail address: furgiuele@unical.it (F.M. Furgiuele). (MEMS) [3]. Furthermore, good biocompatibility allows appli- cations in biomechanics, such as orthodontic wires and devices, stents and components in minimally invasive surgical devices [4]. Due to the low formability of these intermetallic alloys, a suitable joining technique must be used to obtain devices and components with complex geometries. Only a few welding tech- niques for joining NiTi alloys are reported in literature [5–11]. In friction welding and resistance butt-welding [5–7], the joint is subjected to a high compressive force during the welding pro- cess. This force closes any possible grain boundary crack and leads to an outward extrusion of the fusion zone, so that oxi- dation is well prevented. Unfortunately, this process requires a successive working to trim these extrusions. Tungsten inert gas (TIG) welding [8] causes marked degrada- tion in the mechanical properties of the joint due to an extended heat affected zone (HAZ). Laser welding is one of the most important joining techniques for this class of materials [9–11]. In particular, the Nd:YAG source is suitable for welding low thickness components due to its high precision and reduced HAZ [11]. Moreover, appropriate control of the process parameters can ensure good reproducibil- ity of the results. The aim of the present work is to evaluate the effect of the Nd:YAG laser welding process on the mechanical and functional properties of Ni–51 at.% Ti alloy in its martensitic condition, 0921-5093/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2005.08.209