Melting of nitinol alloys
Commercial nitinol alloys at Memry are generally prepared by either a primary vacuum induction melting (VIM) followed by vacuum arc melting (VAR) or by a multiple VAR process. Materials prepared by the VIM/VAR process tend to have more uniform distribution of transformation temperatures along the ingot.
Conversion of nitinol ingots into mill products
Nitinol ingots are hot forged/swaged and hot rolled to bars and coils. Wires are subsequently drawn to finish sizes from large diameter hot rolled coils. Hollowed bar stock with 0.5-1.5 inch diameter is subsequently drawn down to finished tubing sizes.
The difference between an as-drawn wire and a straightened superelastic wire
An as-drawn wire is cold-worked directly coming off the drawing machine. This wire is not straight and usually exhibits some cast and twist. Wire in this condition is not useful without further processing. However, this as-drawn condition is typically preferable for downstream shape setting.
Shape setting a nitinol component
The material needs to be rigidly fixtured and constrained in the desired shape and heat treated. Typically for superelastic material, a heat treatment in the 500°C range is adequate. The length of heat treatment varies with the equipment used for the heat treatment and the thermal mass of the shaping fixture. In a molten salt bath for example, the heat treatment time is generally between 2 and 5 minutes, whereas in an air convection furnace the heat treatment time can be as long as 45 minutes or more for larger tools.
Heat-treating nitinol to yield two different independent shapes upon heating and cooling
The “intrinsic two-way effect” requires not just a heat treatment but also a costly, time-consuming and sometimes not very tightly controllable training procedure to get two different shapes upon heating and cooling. The training consists of a series of repeated cooling -> deformation -> heating cycles to strains of about 10 - 15%. Sometimes this procedure is also done using constant external loads.
In fact there are several different training strategies that can be used. These strategies are always based on the introduction of defects which cause the direct formation of oriented martensite during cooling of the part. The two-way shape memory has some disadvantages:
- expensive training procedure
- parts with intrinsic memory tend to show a certain amnesia
This effect can be introduced into the alloy but it has been demonstrated that using an external force (extrinsic two-way effect) rather than internal stresses is the more suitable and reliable way of solving the design problems with this material. Only in very few cases do we find strong arguments for the use of the intrinsic two-way effect.