Manufacturing “shape shifting” wires from nickel and titanium alloys – like Nitinol? How can you measure the complex drawing dies this requires?

As you might imagine, the field of metallurgy isn’t static. Over time, new alloys are discovered and many soon find a role in manufacturing of one kind or another. We’re going to focus on one example of this in this blog. It’s particularly relevant to the wire manufacturing industry: Nitinol – an alloy of nickel and titanium.

Nitinol was discovered in the USA in 1959 and was immediately notable due to its unique properties (which we’ll look at more closely in the next section). It was the first example of a class that has come to be known as super-elastic Shape Memory Alloy materials (SMAs).

What are shape memory alloys?

SMAs share common properties (they are lightweight, compact, have a high force-to-weight ratio and are biocompatible among other characteristics). Intriguingly, when used in manufacturing they “remember” the form into which they were processed – so even though heat or pressure can change this temporarily, they will revert to their original form under the right conditions, even after undergoing significant deformation. There are no other alloys with similar properties, so they cannot be ignored.

You can see what this means in the picture below.

SMAs have three key particular characteristics:

  • The Shape Memory Effect (SME). This refers to the alloy’s ability to return to its original shape after being subjected to a particular stimulus (the most common is heat).
  • Super elasticity: This is the property that underpins SME and makes the alloy particularly useful in manufacturing applications that require significant flexibility and resilience.
  • Pseudo elasticity: The is the reversible-stress behavior that enables shape recovery to occur when a stress is removed, rather than involving a stimulus like heat. Pressure is the common example here.

Alloys with such properties are useful in multiple manufacturing situations and, indeed, shape memory alloys can now be found in numerous industries including biomedical (stents, orthodontic wires, surgical tools, and implants), aerospace (actuators, deployable structures, shape morphing components), automotive (engine components, actuators for active safety systems, smart materials for vibration damping) and many more ranging from robotics to consumer goods.

What about nitinol?

Nitinol is probably the most common and best-known shape memory alloy.

It’s made from nickel and titanium (the clue being in the name!) and it exhibits all the characteristics defined above, particularly both shape memory effect (SME) and super elasticity. This means Nitinol can be deformed into a different shape at low temperatures only to recover its original shape when heated above a certain transition temperature. It also means that it can undergo significant deformations and then return to its undeformed shape when the deforming stress is removed.

Nitinol in wire manufacturing

All this means that in industrial manufacturing, Nitinol has a wide range of applications including biomedical devices, eyeglass frames, actuators, couplings, and others. But for our purposes in this blog, we’ll focus on one domain where it’s commonly used: wire. Because of its unique properties, nitinol is used to make a number of different types of wire, foremost among them:

  • Orthodontic wires, for instance the wires used for dental braces. These wires function by exerting constant forces on teeth to move them into a desired position and Nitinol’s super elasticity makes it particularly suitable for this purpose, as its ability to retain shape memory enables controlled movement.
  • Stent wiring, the wiring used in stents, medical devices that are used to keep arteries and other canals open. Again, super elasticity features, because it allows a stent to be compressed for insertion, then to expand to its original shape once inside the body.
  • Guide wiring is another sort of medical application, used in procedures like catheterization. Flexibility and shape memory in guide wires mean a wire can navigate through blood vessels and other narrow passages with ease.
  • Actuator wires: Nitinol wires are used as actuators in a number of different mechanical systems and devices. When heated above their transformation temperature, they can change shape, exerting force or movement that can be utilized in robotics, aerospace, and other fields.
  • Seismic Dampers: When used in seismic dampers, Nitinol wires mitigate the effects of earthquakes in buildings and structures. Their super elastic properties allow dampers to absorb and dissipate energy during seismic events.

The picture below shows nitinol as used in a self-expanding stent, one common application. Alas, we can’t show you what it looks like inside a smartphone!

These are, of course, other examples or SMA wiring but those above should quickly confirm the importance and value of nitinol in wire manufacturing where its unique combination of properties makes it suitable for a wide range of uses in various industries.

The hot application

One use case stands out as driving considerable demand for nitinol wires. That’s Optical Image Stabilization (OIS) applications which are increasingly common in the smartphone sector. These are based on a micro-electromechanical gyroscope that is used to detect movement – permitting the movements to be compensated for in the resulting picture.

Nitinol’s shape-memory characteristics make it particularly useful because OIS applications require mechanical actuation and nitinol wires can therefore be deployed as actuators to move the optical elements like lenses or mirrors that address, and correct camera shake or vibrations.  When the OIS system detects motion, a nitinol wire is activated to drive stabilization of the image.

Why nitinol in OIS? Well, it’s shape memory properties and super elasticity are obvious reasons. The fact that nitinol wires are thin and lightweight make them easy to integrate into compact camera systems where bulk and weight matter. And nitinol wires are known for their durability and resistance to fatigue.

But it’s not easy!

So far, so good. On the other hand, though, the manufacturing of Nitinol wires does present several challenges because of the same unique properties of the alloy and the precision required to achieve desired wire characteristics. Let’s dig deeper to understand some of the issues:

  1. Because it’s a complex alloy, Nitinol’s balance between nickel and titanium has to be carefully controlled during manufacturing to achieve the desired outcomes. Variations in composition or processing can significantly affect the material’s behavior, including its shape memory effect and super elasticity.
  2. Nitinol undergoes a phase transformation in manufacturing which is critical for its shape memory effect and super elasticity. This must be closely controlled, including during heat treatment processes, to ensure consistent and predictable behavior in the final product.
  3. Nitinol wires, like other wires, are manufactured through a wire drawing process, which involves pulling the material through progressively smaller dies to achieve a desired diameter. Because of Nitinol’s unique properties, particularly its tendency to work harden and its sensitivity to temperature changes, the drawing process can be challenging and require specialized equipment and techniques.
  4. The surface finish of Nitinol wires is essential in applications like medical devices where smoothness is critical to minimize tissue irritation or damage. But achieving the desired surface finish while maintaining the material’s properties can be difficult and may require additional processing steps.
  5. Manufacturing Nitinol wires is often more expensive compared to traditional materials due to the complexity of the alloy and the specialized processes it requires, as we’ve noted. One ramification of this is that reducing waste is of paramount importance. Accuracy is manufacturing is of paramount importance.

Despite these challenges, though, Nitinol wires offer unique advantages so they must be overcome as the difficulties are outweighed by the benefits.

Conoptica has a solution for enhancing production efficiency through the measurement of drawing dies used for manufacturing nitinol wire

With respect to the wire drawing issue identified in point 3, above, Conoptica’s camera-based measurement systems provide a valuable tool to help ensure successful manufacture of wires from this complex material.

 As we’ve already seen, nitinol has unique properties which have the potential to adversely impact the drawing process so wire manufacturing using nitinol can be particularly difficult.

The OIS use case we mentioned earlier serves as a good example of why the challenge needs to be met. Demand is increasing with growing interest in applications that use innovations in metallurgy to create new products. Die measurement is key to their efficient production.

Why is this (die measurement) challenging? Because the pressure of the drawing process naturally generates heat, and the effect of heat on nitinol can lead to adverse results if the alloy reverts to its original form once the stimulus is removed. The point here – and the way to address this – is that precise measuring of the drawing dies used for nitinol wires – which are not “standard” dies – is absolutely critical for successful outcomes.

Conoptica is a leading source of high accuracy measurement systems, offering a variety of different measurement systems applicable to the medical manufacturing industry. They enable accurate measurement, so users can track consistency and take appropriate actions – avoiding excessive quality deterioration and enabling better production outcomes.

Conoptica is the market leader for measurement equipment in the wire & cable industry and has been providing high tech camera-based measurement solutions since 1993. We make sure that the metal working industry has access to key quantitative data about their products and tools.