Hot Forming of Nitinol: Fundamental Investigations and Applications

Nitinol’s thermomechanical properties are well studied and understood below the so-called Martensite death (M_D) temperature above which Martensite cannot be induced by mechanical stress: Even at high stresses nitinol stays in the Austenite phase. In this talk we will present tensile tests performed well above M_D (150 °C - 650 °C) with nitinol dog bones laser cut from tube (OD: 10 mm, WT: 0.5 mm). Our investigations show that nitinol drastically changes its mechanical properties in this temperature range: the superelastic plateau shortens and finally vanishes. Furthermore, nitinol starts becoming ductile as the temperature exceeds M_D.

This behavior can be utilized to optimize the shape setting process being a key process for the fabrication of nitinol medical devices. In general, there are two paths to apply heat to the specimen: (1) locally heating the regions being deformed and (2) heating the entire specimen. For realizing path (1) both controlling the temperature distribution and precise mechanical actuation need to be synchronized and are crucial to the process outputs, namely well-defined geometry and proper phase transformation temperatures. For realizing path (2) temperature control is taken care of, e.g., by a salt bath oven leading to a homogeneous temperature distribution throughout the entire specimen. However, the main challenge is the precise actuation at such elevated temperatures.

To review the process output DSC, SEM and tensile testing has been carried out and the hot formed specimens have been compared to conventionally formed nitinol.