Influence of the Oscillation Parameters Amplitude and Frequency on the Microstructure of Laser-Welded Thin Nitinol Foils

Laser welding has become well established for joining Ni-Ti-based shape memory alloys and extends the manufacturability of highly functional components with complex geometries. Published studies on the effect of laser welding on alterations to microstructure and properties of these alloys, however, mainly deal with conventional component dimensions and linear laser beam movement. In view of the increasing importance of microtechnology, research into joining of thin-walled Ni-Ti components is therefore of interest. At the same time, studies comparing oscillating and linear beam movement on other materials and the authors’ own work on Ni-Ti materials suggest that oscillating beam movement has a more favorable effect on alterations in material properties and microstructure. Therefore, laser welding of foils made of Ni55/Ti45 with 125 µm thickness was systematically analyzed using a fiber laser and circular oscillation. Amplitude A and frequency f were varied from 0 to 200 µm and 0 to 2000 Hz, respectively. Microstructural analysis showed that by increasing the frequency, grain refinement could be achieved up to a certain value of f. An increasing amplitude led to decreasing hardness values of the weld seam, while the influence of f was less pronounced. The analysis of the weld material using chip calorimetry (Flash DSC) revealed that the beam oscillation had fewer effects on the change in transformation points compared to a linear beam movement.