Evaluation of a production route for cold-drawn Nitinol wires
This master thesis has summarized articles and patents found that have been evaluating and optimizing cold-drawing of nitinol wire. The nitinol wire of interest is where the nitinol is used as a core-wire in guidewires. The super-elastic and linear-elastic property of nitinol is attractive in this application. It has been found that cold-drawing of wire is performed in a number of steps, depending on how thin the target wire should be. The process starts with pre-annealing followed by cold-drawing with inter-annealing in between each pass. After the final drawing pass a final annealing in combination with mechanical straightening should be done at a lower temperature than the inter-annealing. This process was tried out and it showed that with the existing equipment at one certain production plant of wire-drawing of nitinol following parameters showed superelastic and high-strength properties: pre-annealing at 550 °C for 10 min in air, drawing down to 55 % area reduction, inter-annealing at 675 °C for 15 min in hydrogen, drawing down to 61 % area reduction, final annealing at 450 °C for 2 min and 30 s. In order to realize a production line of nitinol wire more work must be done. Recommended is to perform a design of experiment at the final annealing times and temperatures in order to optimize the superelastic and high-strength properties of the nitinol wire to best suit as core wire for guidewires.
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