As a metal alloy of nickel and titanium, Nitinol exhibits the unique properties of shape memory and superelasticity. Innovations in the manufacturing and processing of Nitinol have led to a great expansion of Nitinol medical devices in a variety of therapeutic areas.
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Nitinol is a very abrasive material to machine due to its tough titanium oxide surface. Milling, turning, and grinding are possible with excellent results, but expect a lot of tool wear. Carbide tooling is highly recommended. During centerless grinding, ensure adequate cooling lubricant is present. Nitinol can be electrical discharge machined (EDM), water-jet cut, and laser machined with excellent results.
Cold work vs shape set anneal
Nitinol in the cold work condition (as drawn or as rolled) is material that has not been subject to a final heat treatment (shape set anneal). Nitinol wire and tube typically will have 30-40% cold work reduction during the last drawing steps. In sheet and strip products, that value is usually closer to 20% reduction. The amount of cold work a material has prior to its shape set anneal dictates the ultimate strength of the material. Once the Nitinol material has been shape set annealed it will exhibit the superelastic and shape memory properties. Generally the shape set anneal is a straightening process performed under controlled time, temperature, and pressure conditions. This process defines the final mechanical properties of the material until it is subject to further processing. Varying the parameters of the shape set anneal will affect these properties. This annealing process is continuous for wire and strip straightened on spools, or in discrete lengths for tubes and sheet and bar stock material. NDC produces wire, strip, sheet, and bar stock in both the cold work (as drawn) and shape set annealed (straight) condition. NDC’s tubes are in the shape set annealed condition.
Shape setting refers to the process used to form Nitinol. Whether the Nitinol is superelastic or shape memory, in the cold work or straightened condition, it is often necessary to form the material into a new “memory” shape. This is done by firmly constraining the material into its new shape in a fixture or on a mandrel and then performing a heat treatment.
The heating method can be an air or vacuum furnace, salt bath, sand bath, heated die, or other heating method. The temperature should be in the range of 500º-550ºC, with higher temperatures resulting in lower tensile strengths. Cooling should be rapid to avoid aging effects; a water quench is recommended. The heat treatment time should be such that the material reaches the desired temperature throughout its cross-section. The time will depend on the mass of the fixture and material, and the heating method. Times may be less than a minute for heating small parts in a salt bath or heated die. Times may be much longer (10-20 minutes) for heating massive fixtures in a furnace with an air or argon atmosphere. In these cases a thermocouple in contact with the material or fixture is recommended. In all cases, experimentation for the proper time and temperature will be required to determine the combination that gives the desired results.
Aging can be done to raise the austenite finish (Af) temperature of superelastic Nitinol components. Aging is done by heat-treating to about 475ºC for extended periods. Aging and shape setting can be done simultaneously by firmly constraining the material to its new shape in a fixture and heating to around 475ºC for up to an hour. Longer times result in higher Afs. Aging can also be done on material that was previously shape set. As with shape setting, aging times must be determined experimentally, because they depend on the processing history of the material, the heating method, and the temperature. It is advisable to perform a water quench after aging to sharply define the heating time.
Nitinol can have different surface finishes as a raw material and as a finished component. This choice of surface will depend on performance and cosmetic requirements for the particular application, as surface roughness, fatigue, corrosion, and other properties can vary depending on the surface condition of the material
Oxide surface—Current Nitinol producers make Nitinol with an oxide surface that varies in color from light amber to blue to a black color. This surface denotes material that was annealed in a controlled environment. Oxide surfaces are typically very smooth and may be more lubricious than other surface finishes.
Mechanically polished surface—Nitinol wire and strip is abrasively polished to a shiny, bright surface prior to straightening. This surface looks very similar to stainless steel and is very popular for its cosmetic appeal. A centerless grinding method is applied to Nitinol wire and tubes to achieve a bright surface.
Chemically etched wire—Nitinol wire that is run through an acid bath (pickled) prior to straightening. Discrete components can also be etched after shape setting in a bath. Etched surfaces are typically rougher than oxide surfaces and lend themselves to improved coating adhesion. An etched surface has a very thin oxide layer, and studies have shown better corrosion performance than the oxide or polished bright surfaces.
Sandblasted surface—Nitinol wire that is passed through a sandblasting apparatus prior to straightening. This surface has a slightly rougher texture and can aid polymer adhesion.
Electro-polished surface—Nitinol components such as stents and filters that are to be implanted are usually electro-polished as a final process step. This process creates an exceptionally smooth, uniform oxide layer that improves biocompatibility and reduces corrosion.
Coatings—Nitinol can be coated with polymers such as polyurethane and Teflon. Polyurethane extruded over Nitinol is common for cell phone antennas. Teflon extrusion and spray coating of Nitinol is becoming more common today. The high cure temperatures of PTFE materials (>300ºC) can affect the superelastic properties of the Nitinol; special care is required.
Platings—Nitinol has been successfully plated with various metals (gold, silver, nickel, copper) for various commercial applications. Plated Nitinol eyeglass frames are very popular today. Care must be taken to eliminate the potential for introducing excess hydrogen on the surface that could lead to embrittlement. Plating adhesion on high-strain medical components is a critical performance issue that must be resolved.
Joining to Nitinol
Soldering—Nitinol’s tough oxide layer does not promote good solder wetting. An aggressive flux (Indium Corp flux #2) is required to remove the oxide; then a standard Sn-Ag solder can be used to attain good results.
Welding—Welding Nitinol to itself is usually very effective if the weld is protected by an inert atmosphere and the heat-affected zone is minimized. Laser, TIG, and resistance welding are all processes that have been successful. Nitinol welded to dissimilar metals, such as stainless steel, does not give acceptable results since the outcome is a brittle intermetallic interface which cannot be stress relieved.
Other techniques—Nitinol can be bonded to other materials using medical-grade epoxies and adhesives. Mechanical techniques such as crimping and swaging are possible. Another mechanical technique is to use Nitinol’s shape memory or superelastic properties to join materials. A Nitinol tube connector can be expanded either mechanically or by cooling it to martensite and deforming it. The Nitinol connector is then inserted over another element and allowed to return to austenite, causing it to clamp down on the element.
Transition temperature hysteresis
Hysteresis is the temperature difference between a material’s phase transformations upon heating and cooling. This spread is typically around 20º-30ºC for Nitinol superelastic alloys used in medical applications. Various heat treatments can shift the hysteresis higher or lower, or widen it. A typical Nitinol hysteresis curve is shown below.
Austenite: Nitinol’s stronger, higher-temperature phase. Crystalline structure is simple cubic. Superelastic behavior occurs in this phase (over a 50º-60ºC temperature spread)
Martensite: Nitinol’s weaker, lower-temperature phase. Crystalline structure is twinned. Material is easily deformed in this phase. Once deformed in martensite, it will remain deformed until heated to austenite where it will return to its pre-deformed shape, producing the “shape memory” effect
As: temperature where material starts to transform to austenite upon heating
Af: temperature where material has finished transforming to austenite upon heating
Ms: temperature where material starts to transform to martensite upon cooling
Mf: temperature where material has finished transforming to martensite upon cooling
Measuring transformation temperatures
DSC—Differential Scanning Calorimetry measures the phase transformation temperatures in Nitinol by detecting the changes in heat flow in the material. DSC results are used to quantify the base alloy transformation properties.
Active Af measurement—It is important to understand the phase transformation temperatures for the component in its application. Measurement techniques such as Bend and Free Recovery (BFR) or Tube Crush are similar in their approach. A sample of the finished component (wire, tube, stent, etc.) is cooled below Mf, deformed to a defined strain amount, and then heated up until it returns to its original shape. As the material warms, a LVDT records the change in strain with temperature, and a plot is created.
Biocompatibility and corrosion resistance
The unique combination of shape memory and superelasticity properties, coupled with its biocompatibility response, has made Nitinol an excellent material for medical and dental applications. Products such as orthodontic arch wire, filters, stents, and bone anchors are all prime Nitinol applications. Data from in vivo and in vitro laboratory tests support the excellent biocompatibility and corrosion properties of this material. This performance is attributed to Nitinol’s passive titanium oxide layer, which protects the base material from corrosion and nickel release. In fact, many researchers report that Nitinol is more corrosion-resistant than stainless steel. Surface preparation is critical to good biocompatibility and corrosion performance, and the finished device maker must ensure that the device meets the required standards.
Nitinol is stable against permanent temperature-induced metallurgical changes as long as the exposure temperature is less than the annealing or aging temperatures. For the SE508 alloy, the aging temperature range is 200ºC-500ºC. Nitinol products have infinite shelf life under normal conditions. There are no indications that Nitinol is affected by humidity changes. In general, metallic materials are not humidity-sensitive.
How to specify Nitinol
A good place to start when developing material specifications is the alloy data sheets. Keep in mind that these values are typical properties taken from tensile tests at room temperature. Nitinol’s properties can vary with composition, thermo-mechanical processing, and finished component processing. Commonly specified material properties for superelastic applications are: UTS, upper plateau stress, permanent set, elongation, and Af temperature. For shape memory alloy applications, specified values are typically UTS, upper plateau stress and Af.