Salt bath heat treating to increase ductility, hardness, and conductivity in metals has been around for over a century. Using salt bath heat treating for shape setting an alloy is much newer to the industry.
Nitinol, an SMA (Shape Memory Alloy), is an alloy that you can train to remember a shape. This means that you can stretch, bend, or wind this alloy and after it is heated to a certain temperature it will spring-back to its original trained state. The process of shape setting the Nitinol is what defines the shape that the Nitinol will return to once heated.
Nitinol shape setting is the process of using elevated temperatures in conjunction with a rigid, fixed object or a mandrel to shape the material’s heated form for commercial or industrial purposes.
The Process of Shape Setting Nitinol
Nitinol is a highly shapeable alloy given the right treatment and conditioning. To perform shape setting, you must fix your nitinol material to a solid surface or object. The form must be stable enough that it remains in its desired shape once heat is introduced.
Submerging in a salt bath is typically done in temperature ranges of 455° to 565°C. These temperatures best facilitate an environment to allow deformations in the material to become stable, or memorized, but not hot enough to allow the alloy to re-crystallize.
The time needed to heat your material cross-section differs depending on the size and mass of the nitinol you are shaping. Smaller components only require a few minutes to reach the proper level of heat, whereas large scale objects take upwards of 20 minutes. Once the material reaches the appropriate temperature, it should be quenched in water to cool it quickly.
Shape Memory is one of the main attributes that make Nitinol, and other SMAs, so appealing to many industries. That makes the temperature at which a specific alloy remembers its trained shape the most critical property to specify when manufacturing Nitinol. The Austenite Finish, or Af, represents the temperature at which the Nitinol alloy will be finished transforming from its martensite state to austenite state, or when the Shape Memory recovery is complete.
Nitinol Material Highlights
Nitinol, available in sheet, foil, bar, and wire, is an alloy generally composed of about 50% nickel and 50% titanium. Subtle modifications to the percentage of nickel versus titanium can change the temperature at which the alloy turns from its Austenite to Martensite forms.
Tipping the balance to have slightly more Nickel in the alloy could similarly increase the strength of the Austenite form. While these subtle changes in composition make uniform properties difficult to reproduce, it opens the possibility for more specific attributes for specialized tasks.
Nitinol has three properties that facilitate it becoming ubiquitous in many different fields.
- Shape Memory: Nitinol, and other similar SMAs, automatically return to its base shape once the alloy is heated above a certain threshold, despite deformation at lower temperatures. When it reaches its transformation point, its crystalline structure changes to austenite, and the material reverts to the molded shape. As it cools, it enters its martensite phase and can be deformed again. In essence, nitinol changes its shape when you add heat.
- Superelasticity: Typical metals only allow for less than 1% of strain before they are deformed, Nitinol allows up to 8% of strain and reforms back to shape after. Nitinol distinguishes itself from other metals and alloys by being more than 15 times more kink resistant, elastic, and flexible than most metals or alloys.
- Biocompatibility: Nitinol has a natural fit in the medical field, as it does not produce a toxic or immunological response when exposed to the body or bodily fluids. Once inside the body, at 98.6°F the memory of our Shape Setting Nitinol can help with keeping veins open with stents, or help mending bones and recovery..
There are, however, challenges inherent to working with Nitinol. It is a relatively expensive material to manufacture compared to other options such as aluminum and steel. Because of the change to its crystalline structure during Shape Setting, coupled with its superelasticity, Nitinol can be difficult to machine; it wears out common tools quickly and requires special techniques to efficiently craft.
It also has relatively low fatigue resistance. Superelasticity makes the surface of nitinol prone to small cracks and imperfections. Extra attention to finishing processes is necessary to preserve nitinol components and their long-term integrity.
Applications of Shape Setting Nitinol
Nitinol is a highly desirable material due to its biocompatibility. It is a commonly sought manufacturing material for medical devices such as staples, stents, and wires.
Some applications that benefit from the Biocompatibility of Nitinol:
- Stents in veins that with the heat from the body expand to keep veins flowing freely.
- Staples that suture wounds inside the body.
- Bone plates and bone staples help keep bones positioned so that the fracture can heal and remain in place to help strengthen when time for rehabilitation.
Besides healthcare, nitinol is useful across a diverse set of industries. Despite its flexibility, the alloy is strong, especially in its cooled form, and the nickel titanium mix is naturally corrosion resistant.
Some more common applications Nitinol is used for:
- Antennae on old cell phones were unbreakable and kink resistant.
- Bendable eyeglass frames, especially around the ears and nose areas.
- Braces and other orthodontist devices needed to be tightened less, as the wire was giving constant pressure, wanting to reform to its original shape.
- Hydraulic Aircraft lines use Nitinol sleeves that are slightly smaller than the hose and are stretched, once they warm up they provide enough pressure that the connection is superior to a welded joint.
- Actuators in robotics can sometimes be made of Nitinol to use the Shape Memory aspect to exert force on small parts. Some computers turn off when over heated with the use of Nitinol actuators.
- Surgical equipment helps in feeding catheters, or other endoscopic procedures because the Nitinol will not bend, kink, or break. Some tools can be inserted through a catheter, warm up and remember whatever shape the Surgeon needs, perform the procedure, and then pull the formed tool back straight again through the catheter.
Automated Shape Setting Nitinol Systems
Ajax Electric has been building fully automated lines for Shape Setting Nitinol for over twenty years now. We offer a completely automated and self-contained system to Pre-Heat, Heat Treat, and Quench Nitinol parts. We have used many methods of transferring the work through the different stations and are happy to work with different budgets and customer specifications.
Our Work Handling Systems off the following:
- High speed transfer between the high heat and quench furnace to meet the critical metallurgical transfer time
- Critical parts located away from areas subjected to heat and fumes
- Process and System monitoring and alarming software for all temperatures in different stages, High and Low temperature settings, over travel conditions, and drive fault for any motors
- Programming to allow system to run automatically or manually
- PLC system and software with color touch screen display to interface with furnace and material handling system for setting alarms, water addition, cycle times, and much more
- Prewired NEMA 12 control panel wired to NEC standards with a disconnect handle extended through the front door, pushbuttons, pilot lights, relays, as needed
- All systems designed for operator safety with auto door lock out preventing access while in operation
Contact Ajax Electric to Learn More
Ajax Electric has been the trusted provider of quality heat treatment equipment for over 60 years. Our company prides itself on delivering quality products and excellent customer service and we would love to help you create your custom Nitinol application.
Ajax Electric Co. is pleased to offer state-of-the-art solutions and designs with the expertise and equipment to produce top-quality Nitinol components quickly and efficiently. To learn more about our cutting-edge salt bath technology and its uses in Nitinol alloys, contact us today.