Nitinol shape memory wire: design limits for production

Two behaviours, two design routes

Nitinol is a nickel titanium alloy with a martensitic transformation. Above its transformation temperature it is austenitic and can recover large strains on unloading, which is superelasticity. Below the transformation temperature it is martensitic and can be deformed, then return to a remembered shape when heated, which is the shape memory effect.

The two behaviours are used for different parts. A superelastic part is a spring, a clip, a guide or an orthodontic component that flexes and returns at constant temperature. A shape memory part is an actuator, a clamp or a release that changes shape when heated. A design has to pick one route, because the strain limits, the test method and the production controls are different.

Transformation temperature is the first limit

The key temperature is the austenite finish temperature, usually written as Af. Above Af the material is fully austenitic and superelastic. Below it, the behaviour depends on the martensite start and finish temperatures and on the stress applied. The application temperature has to sit on the right side of Af for the behaviour the design needs.

A superelastic part usually needs an Af below the lowest service temperature, with a margin. A shape memory actuator needs an Af above the service temperature and below the activation temperature. The margin matters because Af moves with composition, with the heat treatment and with the lot. A supplier that quotes a single Af without a tolerance and a test method has not given a usable specification.

Strain limits: the headline and the working limit

The published recoverable strain for superelastic nitinol is often quoted as about 8 per cent. That is a single cycle, ideal condition figure. A part that has to survive many cycles cannot work at that strain. Functional fatigue limits are much lower, often below about 1 per cent for very high cycle counts, depending on the mean strain, the surface and the geometry.

For shape memory, the one way recoverable strain is often quoted at about 4 to 5 per cent, with two way training able to produce a smaller reversible movement. A design that needs more than that will either yield permanently or need a different mechanism. Treat these figures as indicative and confirm them with the wire supplier for the specific grade and condition.

Fatigue is a functional limit, not a material constant

Nitinol fatigue depends on the strain range, the mean strain, the temperature, the surface finish and the number of cycles. It also depends on the wire surface, because a drawing mark or an inclusion can start a crack in a material that is otherwise very tolerant of deformation. Two parts with the same drawing but different surface condition can have very different lives.

Test at the application strain and the application temperature, with the real surface finish and the real end conditions. A rotating bend test on polished wire is useful for comparing lots, but it does not prove a formed clip with a laser cut end. If the part is safety related, plan a test series rather than a single pass or fail.

Forming and shape setting

Nitinol wire can be bent and formed, but it springs back much more than steel and it work hardens. A bend that is formed cold may need a shape setting heat treatment to hold its geometry. Shape setting is usually done in a fixture at about 500 °C to 550 °C for a short time, followed by a controlled cool. The time and temperature set the Af as well as the shape.

The heat treatment is a process step with its own controls. The fixture has to hold the shape at temperature, the atmosphere has to avoid heavy oxidation, and the oxide has to be removed afterwards without damaging the surface. If the part is superelastic, the heat treatment also has to restore the Af, so the shape setting and the transformation temperature are set together.

• Form around a radius that suits the wire, not a steel spring radius
• Expect high springback and compensate in the tool
• Agree the shape setting temperature, time and atmosphere
• Remove oxide without scratching the surface
• Re-check Af after forming, not just on the incoming wire

Joining, corrosion and contact with other metals

Nitinol is difficult to weld and braze without affecting the transformation temperature. A welded joint creates a heat affected zone with a different Af, and the weld itself is usually weaker than the wire. Mechanical joints, crimps and adhesive bonds are common alternatives, but each needs its own validation.

The alloy is sensitive to corrosion in some environments, and it contains a high proportion of nickel. Coupling it to a dissimilar metal can create a galvanic couple, and a scratched surface can be a starting point for corrosion. If the part sits in a wet or salty environment, test the assembly, not just the wire, and state the cleaning and passivation route.

The production risks a prototype hides

A prototype is often made from a small quantity of wire with a known history. Production brings lot changes, and Af can move from lot to lot even within a supplier's specification. If the design has a narrow temperature margin, a lot change can turn a working part into one that does not transform at the right temperature.

The other risk is the process window. Shape setting, oxide removal and joining all have windows that are wider on a bench than on a production line. Ask for a control plan that names the Af check, the dimensional check, the surface check and the functional test, and agree the lot definition. A nitinol part without lot traceability is difficult to contain if a temperature issue appears.

What to put on the drawing and in the enquiry

State the behaviour, not just the alloy. Write superelastic or shape memory, give the Af with a tolerance and the test method, give the working strain and the cycle count, and give the shape setting condition. Add the surface finish, the oxide limit and any cleaning requirement. If the part is an actuator, give the activation temperature and the force at the working position.

Ask the supplier for the lot specific Af, the tensile result, the surface condition and the heat treatment record. Ask what happens to Af after forming and after any joining step. Those four answers separate a supplier who understands the material from one who is treating nitinol as a stiff stainless wire.