Nitinol's shape memory and superelastic properties make it ideal for medical devices such as stents, but only when the device is made from one piece of material. But what if you could get a better part by joining two nitinol components? How could that be done? First of all, nitinol can be soldered or welded, but with some caveats — and difficulty.

“An oxide forms on nitinol, and it must be removed to get solder onto the base metal. And that is difficult because the oxide is tenacious,” says Darel Hodgson, president of Nitinol Technology Inc, Mountain View, Calif. “However, a few nonstandard fluxes do a respectable job when soldering the material to itself. Brazing and welding, on the other hand, generally anneal out some of the optimum properties for shape memory alloys,” he says.

But Hodgson adds that engineers who know what they are doing can avoid the problems caused by oxidation by laser or tig welding the material in a clean atmosphere, such as a vacuum or inert gas. “Nitinol welds tend to be brittle, although not weak. So weld only when other fastening methods won't do and don't place welds in high-stress areas,” says Hodgson.

Joining nitinol to nitinol is best done by mechanical joints: Let the shape memory effect of one part grab the other. “Here's one way,” says Hodgson, “Make a hole in one part slightly smaller than the other pieces you want to join, chill the material with hole below the transformation temperature, and jam a tapered mandrel into the hole to make it bigger. Pull the mandrel out and stick in the piece. As the chilled piece warms, the shape-memory effect will shrink the hole around the second piece and mechanically join the two. There are permutations to this theme.”

Orthodontic devices, for example, call for snapping a wire onto a clamp on a tooth. “In this case, designers use tooth clamp because nitinol is so springy or elastic. It bends or deforms more than common metals,” says Hodgson. “You can bend the tooth-mounted bracket with a fair amount of deflection to let the guide wire snap into place, so it's almost locked and cannot pop out without a large applied force. Nitinol arch wires have more elasticity than steel. When the dentist tightens the wire, it stretches further and holds tension longer than steel would,” he says.