Shape Training of Nitinol Wire using Three-Dimensional Printing (3DP) Fixtures

The presented research focuses on work done at the University of Washington on process development for the training of nitinol shape memory alloy wire using Three Dimensional Printing (3DP). Fixtures are created using the commercial stainless steel printing system produced by Ex One. Superelastic nitinol wire is set by restraining the wire in a fixture and thermal processing. A two dimensional test array was designed and fabricated to examine the effects of fixture curvature on the final wire shape. Three dimensional coils and spheres were created to demonstrate the potential of this process for more complicated shapes. Introduction Nitinol belongs to a larger class of materials known as shape memory alloys due to the ability of the metal to retain a mechanical memory of a pretrained shape. It is composed of a nearly equal atomic ratio of nickel and titanium with occasional impurities added to affect the material properties. The shape memory effect is the result of two distinct and temperature sensitive crystal structures possible in the material. The higher temperature austenite state is a highly ordered crystal structure while the lower temperature martensite phase is less structured [1]. Nitinol in the austenitic state converts to the martensitic phase through physical deformation or cooling. The trained shape of a nitinol part is the natural shape of the part when fully in the austenite phase. The nitinol must be raised above the austenite start temperature (As) to return to the set shape. The As can range from -50 to 95 ̊C depending on metal processing. Alloys with an As below 5 ̊C are considered superelastic due to their ability to spring back from severe deformation (strains up to 7 – 8 %) at room temperature. Deformation at room temperature results in localized conversion of the crystal structure to the martensitic phase in the strained regions, but the crystal structure reverts back to the austenitic phase upon unloading and the metal returns to the trained shape. If the As is above the working temperature the part requires heating to return to the set shape. For certain alloys this can be done with body temperature (~30 ̊C), but the more common alloys transition between 70 and 90 ̊C. Higher temperature alloys are typically used as actuating wires and not trained to complicated shapes. For actuation the wire can be elongated at lower temperatures (elongation strain affects cycle life) and the wire contracts when heated. The force necessary to reset the wire to the initial condition after cooling is approximately 20% of the force generated during actuation. The shape setting of the nitinol occurs when the metal is annealed in the range of 500 550 ̊C. A fixture is used to restrain the wire at this temperature to establish the austenitic or ‘set’ shape. The fixture and wire are held at this temperature briefly and then quenched or rapidly cooled in air. Traditional training involves an often iterative process of custom fixture generation. Most fixtures are metal which typically involves a