Strategic Method to Enhancing the Formability of Nitinol Foils via Micro-incremental Sheet Forming Processes and Evaluation of Structure–Property

The Nitinol alloy foils were deformed incrementally using Single Point Micro Incremental Forming (SPMIF), in which the foils are deformed with the help of a hemispherical end tool. No specific die is required, and the forming limits are primarily influenced by local deformation. The present study investigates the formability, mechanical properties and fracture behavior of Nitinol alloy foils under the SPMIF process. Formability studies were conducted with varying spindle speeds. At higher forming speeds, the formability is greater compared to lower spindle speeds due to the presence of shear strains during incremental forming. The electron backscattered diffraction studies showed nearly 73% of reduction in grain size when the Nitinol foil was formed at highest spindle speed due to frictional heat and promotion of dynamic recrystallization. The Mechanical properties of Nitinol foil deformed with higher forming speeds (200 rpm) showed higher values of 995 MPa and 337 Hv when compared to its counter parts. Fracture behavior was investigated using the void coalescence method. The parameters for void coalescence, such as the void length-to-width (L/W) ratio, void size (µm), d-factor, and ligament thickness, were measured. The relationship between spindle speed and void coalescence parameters were correlated to understand the forming behavior of Nitinol foils.

Graphical abstract