Page Outline
This page introduces the heat treatment, machining, joining, and finishing as four of Nitinol manufacturing methods, and there is a schematic diagram for a better understanding.
Classification of Manufacture Techniques
The broad categories of manufacture methods and their subcategories are described in Table.1. All of the four major categories (heat treatment, machining, joining and finishing) of manufacturing Nitinol are covered.
Table 1. The rough manufacture techniques [1]
Heating Treatment
There are 2 common commercial heat treatment process of nitinol, shape setting and aging.
- shape setting: Before processing nitinol into complex shape, it is usually formed as wire, tube or sheet (like Figure 1). This process is done by constraining the nitinol into a designed shape using specific fixture. Then performing a shape set temperature for several times that depend on the requests. The heat treatment temperature is above 500oC to shape set nitinol [2]. Higher temperature reduces process time and improve shape retention. This step is finished off by a water quench to prevent further aging and reduce process variability [2].
Figure 1. An example of nitinol wire component.
- Aging can be used as a method to change the required properties of nitinol material. The matrix of Ni is depleted to form Ni rich precipitates. This reduction of Ni raises the phase transformation temperature of the material. The aging temperature range normally is 425-475oC [2]. Raising the temperature leads to over aging that is negative effects of mechanical properties of nitinol.
Finishing
Blasting: Including grit blasting and micronblasting. Blasting is a technology to remove oxides, surface marks and processing contamination. Normally, Grit blasting is applied in large scale process like nitinol wires or tube, whereas micronblasting is applied in smaller scale components for post-processing [2].
Mechanical tumbling: An operation when there are edges and rounding sharp features need to be removed. But not effective in removing large amounts of material [2]. Tumbling makes surface and round sharp edges smooth.
Chemical processing
- Etching (picking): Common practice for surface oxide or relatively large volume of material removal.
- Electropolishing: Another removing surface material technique by electrochemical process (Figure.5).
Figure 5. Sketch of electropolishing system.
- Passivation: Passivation is aimed to change the surface composition for reducing nitinol reactive or improve resistant to adapt to certain environments [2]. Passivation is utilized when the surface resistant to electrochemistry corrosion need to be enough to make sure components does not degrade or fail.
Joining
Welding: For joining nitinol components, laser, plasma and resistant welding processes are usually utilized in nitinol manufacture. A common require in the welding process is that a large amount of heat is needed to create the joint [2].
- Laser: CNC pulsed laser system has the advantages of high speed, non-constant and low heat.
- Plasma: Plasma welding is an arc welding process for forming atraumatic balls on the ends of nitinol tubes or wires (Figure.3).
Figure 3. A sample of plasma ball.
- Resistance welding: Higher electrical resistance at the faying surface produce more heat and mechanical load to form a weld nugget or a fusion zone [2]. Normally be utilized to butt welding the large nitinol components.
Crimping: Crimping is typical choice when the termination of Nitinol wires or tubes ends need to be welded (Figure.4).
Figure 4. A crimping process samples.
Soldering and adhesives: A ideal method of joining nitinol to dissimilar materials. UV and heat curable are available options for adhesives [2]. However, soldering and adhesive are not good choice for implantable medical device.
Machining
The cold work state nitinol is hard and the titanium oxide layer on the surface makes it tough [2]. Traditional machining techniques like turning or milling is not advised. Laser cutting is a commonly used machining technique because of its low thermal input and high precision properties. Nitinol component is cut by high intensity focused light and high pressure assist gas. The molten nitinol expel through the kerf depending on the laser spot size. Grinding, wire EDM, water jet cutting, stamping are also techniques that utilized to machining nitinol to different shape (Figure.2).
Figure 2. Sketch of laser cutting.
References
[1] Elena villa (2015), Shape Memory Alloy Engineering ,(4),80.
[2] Bethel, Menlo Park, New Hartford (2017), Introduction to Nitinol. Accessed on 5 June 2020, from: https://www.memry.com/.