Nitinol Continuously Flat-Rolled Sheet and their Properties

The continuous rolling of Nitinol alloys is a metalworking process with the ability to produce large quantities of sheet with uniform properties. Great advantages of continuous rolling in comparison with other manufacturing methods are the cold work and heat treatment steps and their ability to influence the properties of the product and keep them in a very tight window over the width and the length of the process. Those tightly controlled properties are key-requirements to use the continuous rolled Nitinol material for subsequent automated processes like stamping in progressive dies or deep-drawing. It is also required for efficient reel-to-reel Laser- or EDM cutting.

The primary objective of this work is to evaluate and obtain the properties of Nitinol continuously flat-rolled sheets and optimization of the process parameters by fatigue evaluation. In order to determine the fatigue properties, bending tests with a fully reversed bending angle of ±180° and a sheet thickness of 0.2 mm were conducted at room temperature. A minimum bending radius of 7 mm and a maximum bending radius of 25 mm were used for this study. The superelastic Nitinol materials used for testing were manufactured with two different grain sizes, and different final heat treatments. Additional to fatigue testing, differential scanning calorimetry measurements and tensile testing have been conducted. The mechanical, thermal, and fatigue properties of the manufactured materials were compared and rated. The extended data evaluation shows the repeatability of fatigue properties of the continuously processed sheets under severe cyclic bending conditions.