Laser Welding of Nitinol Thin Foils: Mechanical Properties and Microstructure Depending on Process Parameters

Ni–Ti alloys are used as functional materials in numerous sectors such as aerospace, automotive engineering, medical technology, and consumer goods. Their properties in terms of shape memory effect and superelasticity offer a great potential for innovative smart products. However, forming and machining of these alloys into concrete products is challenging. Assembling plain structures by laser welding to complex product shapes offers an economical alternative in many cases, but can be associated with negative effects, such as reduction of strength, development of brittle intermetallic compounds, alteration of transformation temperatures, and modification of shape memory effects and superelastic behavior. Against this background, investigations on laser welding of Ni55/Ti45 foil with a thickness of 125 µm by a fiber laser were conducted. Supported by methods of design of experiments, optimal parameters were determined with respect to laser power, welding speed, focus position, and beam oscillation, and the welding results were analyzed concerning the microstructure and mechanical characteristics of the welded joints. The effect of laser beam oscillation was investigated for the first time for the welding of this alloy. Due to the very low thickness, the preparation of the foils for the microstructure characterization is quite demanding. Best results were obtained by ion milling. Fracture surfaces and the influence of the welding were also investigated.