Correlation between the mechanical cycling behavior and microstructure in laser welded joints using NiTi memory shape alloys

Dissertação para obtenção do Grau de Mestre em Engenharia de Materiais

The demand of emerging joining techniques for shape memory alloys has become of great importance, as their functional properties, namely shape memory effect and superelasticity, present unique solutions for state-of-the-art applications. Literature shows that significant efforts have been conducted on laser welding of these alloys, although very limited results concerning mechanical properties are reported. In this study, the main objective was to correlate the microstructure of similar NiTi welds with its mechanical behavior. A set of cycling tests was used in order to analyze the superelastic behavior of the welds. Also, shape memory effect was evaluated by means of a bending and free-recovery test. X-ray diffraction analysis allowed to identify the existing phases in the base material, heat affected zone and fusion zone. SEM images of fracture surfaces were also analyzed. It was observed the superelastic behavior of the welds during the mechanical tests. Also shape memory effect was shown to exist on welded samples even after cycling. X-ray diffraction showed a microstructural gradient across the samples. The welding parameters influence the mechanical behavior under cycling of the welds. In particular, there is a specific range of values of heat input introduced during welding that allow obtaining a good mechanical behavior under cycling of the welds.

FCT/MCTES - projecto ‘Joining micro to small scale systems in shape memory alloys using last generation infrared lasers’ (PTDC/EMETME/ 100990/2008)