Fracture behaviour of nickel-titanium laser welded joint

NiTi shape memory alloys (SMAs) have seen growing use in recent years in many branches of engineering, due to their special functional properties namely shape memory effect (SME) and superelastic effect (SE) [1]. These characteristics are due to the martensitic transformation and its reversion, which can be activated by thermal (SME) or mechanical loads (SE). Unfortunately, due to the low formability of these intermetallic alloys, a suitable joining technique must be used to obtain devices and components with complex geometries. Laser welding is one of the most important joining techniques for this class of materials and in particular, the Nd:YAG source is suitable for welding low thickness components due to an accurate control of the welding parameters, such as power and welding rate, resulting in a reduced Heat Affected Zone (HAZ) [2-3].

For these reasons, some research activities have been carried out in the last few years to evaluate the effects of laser welding on the mechanical and functional properties of NiTi alloys. Furthermore, there are a very few works in literature concerning the fracture behaviour of these alloys. This topic is particularly interesting because the stress singularity around the crack tip activates the Stress Induced Martensitic (SIM) transformation and, consequently, change the toughness capability of the material.

In this work, the effects of Nd:YAG laser welding on the fracture behaviour of NiTi sheets are analyzed by experimental investigations.  In particular, Single Edge Cracked (SEC) specimens are made by Electro Discharge Machining (EDM) from both NiTi sheets and welded ones. Finally, systematic comparisons of the results are illustrated.

References

[1] K. Otsuka, X. Ren, Progress in Materials Science 50 (2005) 511.

[2] A. Tuissi, S. Besseghini, T. Ranucci, F. Squatrito, M. Pozzi, Mater. Sci. Eng. A273–275 (1999) 813.

[3] A. Falvo, C. Maletta, F. Furgiuele, Mater.Sci.Eng. A412 (2005) 235.