What is Nitinol?
Nitinol is a nickel-titanium alloy with shape memory and superelastic properties, as the name alludes to. Nitinol’s capacity for deformation at one temperature and recovery of its original, undeformed shape upon heating over its transformation temperature is known as shape memory.
In the case of superelasticity, the undeformed shape recovers without heating at a restricted temperature range just above its transformation temperature. The material has a great deal more elasticity than regular metal between 10 and 30 times more. It was first found in 1968 while looking for mining minerals. Nevertheless, because of the challenges involved in its production and processing, nitinol didn’t become useful in the market for many years.
Since the mid-1990s, when its utility in a wide range of applications resulted from its being a shape memory’s alloy and its superelastic qualities, nitinol’s many uses have largely all been realized. It is currently widely utilized for a variety of medical components, including reinforcement coiling and braiding for catheter shafts and tubing, formed retrieval baskets, braided and formed stents, and many other uses. Due to its form memory, it is a fantastic choice for many in-body applications, including braided stents and catheter reinforcement. These medical gadgets are likewise made of nitinol since it is biocompatible, or compatible with the human body, and won’t injure human tissue as a result.
Large cast ingots are typically used in the production of nitinol, which may then be further treated to create ultrafine wires that are just a little smaller than human hair. The process’s last steps, known as wire drawing, are required for exact control to retain the process’s mechanical and physical qualities.
Extremely thin tolerances are maintained for ultra-fine wire, which has a very small diameter (thickness). Modern online gauging techniques are used in the engineering of fine wire goods to achieve exceptional dimensional accuracy and dependability. Usually, nitinol is manufactured in tiny and flat wire forms with micro-scale diameters and widths before painstakingly winding the material to assure tolerance uniformity and downstream performance. The mechanical qualities of nitinol wire in this form make it an excellent option for medical devices that may be inserted into the body to carry out surgical procedures or treat diseases, and they are a major factor in the choice of metal for their fabrication.
Characteristics of Nitinol:
When it comes to creating and manufacturing this wire, several qualities will influence the end application: Size, tolerance, and mechanical attributes including tensile strength, elongation, and surface quality are examples of physical characteristics.
Size and tolerance are two aspects of physical make-up. Size: Nitinol is extremely abrasive on wire drawing dies and must pass through these dies several times to reach the tiny diameters necessary in the medical industry. Tolerances: When it comes to the diameter, tight tolerances must be maintained, but it’s also crucial that they remain constant along the entire length of the wire. This is advantageous for the drawing die’s overall use as well as for minimizing wear and tear.
Tensile strength, elongation, and condition are mechanical properties. Nitinol ultra-fine wire is typically processed under two conditions: cold working and direct annealing. The preference is based on the production procedures used by the OEM for medical devices. At the following manufacturing tier, the majority of the wires will be heat set into a superelastic condition.
Producing a high-quality product while cutting costs during production requires carefully managing each of these factors throughout the wire and making sure that all of its dimensions are uniform. Precision machinery is needed to maintain these characteristics since even little variations generate problems, such as useless components or surplus scrap, during the coil reinforcement manufacturing process.
Properties of Nitinol Wires:
The most distinctive features of this alloy are popularly known as its form memory and superelasticity. When heated above its transformation temperature, the shape memory property of this metal enables it to “remember” and maintain its previous shape. It occurs because the crystal structures of nickel and titanium are dissimilar. The amazing elasticity of this pseudo-elastic metal is also about 10 to 30 times more than that of any other metal.
Here are some of this alloy’s fundamental mechanical and physical characteristics:
Physical Features comprise:
- Wires containing nitinol metal have a dazzling silvery appearance.
- The Density of this type of alloy is 6.45 gm/cm3.
- Nitinol Wires have a melting point which is approximately 1310 °C.
- Their resistivity ranges from 76 ohm-cm at low temperatures to 82 ohm-cm at high temperatures.
- In terms of Thermal Conductivity, this metal has a thermal conductivity of 0.1 W/ cm-°C.
- Heat Capacity: It has a 0.077 cal/gm-°C heat capacity.
- Latent Heat: The latent heat of this substance is 5.78 cal/gm.
- It has a magnetic susceptibility of 2.5 at low temperatures and 3.8 emu- gm at high temperatures.
Technical/Mechanical Features include:
- Nitinol Wires possess an ultimate tensile strength that ranges from 754 to 960 MPa.
- Elongation to Fracture Typically ranges around 15.5 percent.
- Average Yield Strength at high temperature is said to be 560 MPa; whereas at low temperature is known to be 100 MPa.
- Elastic modulus is around 75 GPa at high temperatures and 28 GPa at low temperatures.
- The Poisson’s Ratio is roughly 0.3.
Applications of Nitinol Wire in Medical Applications:
Nitinol is finding several uses in the medical industry since it is compatible with the human body and has the exceptional capacity to respond to extreme strains. The ultra-fine wire used to make stents, orthodontic wire, catheters, and other surgical implant devices are eventually braided into these goods. Medical device manufacturers must choose an alloy and a supplier that offers materials with undeviating, consistent mechanical properties, ensuring that those properties are suitable for processing medical wire. This is because end uses like these require materials with specific mechanical properties. Within the high-tech medical device business, manufacturing sources and production techniques are both crucial components of supply chains.
The choice of braid- or coiled-enforced catheters for a given application should be made by medical device manufacturers. These days, many OEMs opt to employ nitinol for coiled catheters in very delicate procedures or other situations where accuracy is crucial, such as cardiovascular or brain surgery. The benefit of this is nitinol’s extreme flexibility. It will behave nearly like a spring in that it can be stretched to a great extent before snapping back into place. As it frequently includes bending and twisting through arteries, this is quite helpful in the medical industry.
Nitinol is biocompatible. The development of a passive titanium oxide layer on Nitinol implants after the proper surface preparation steps of passivation and electropolishing creates a barrier that stops corrosion and the release of harmful Ni ions into the bloodstream. For Nitinol orthodontic archwires to exert consistent stress as the teeth move, the Nitinol material must be able to accommodate a wide strain when loaded. When utilizing traditional materials, this means fewer visits to the dentist for uncomfortable retightening or archwire changes. In contrast to the stiffness of titanium or stainless steel, Nitinol is more compliant with bone and other structural components of the body, making it a good material for orthopedic implants like bone staples and plates. Endoscopic applications have also made use of superelasticity and high strain accommodation. The idea of articulating laser-cut Nitinol shafts for endoscopy has been used to access challenging areas of the body.
Stents:
Nitinol is also highly utilized in stents. Stents are ‘scaffold-like’ devices that hold or sustain open blood arteries. Making self-expanding stents, especially for peripheral vascular applications, is one of the best-known uses of nitinol. The iliac, femoral-popliteal, and infra-popliteal arteries are examples of lower extremity peripheral arteries. When a person bends their knee, walks, or runs, the peripheral artery stents they have inserted are subjected to tremendous mechanical stress. Due to its unique superelasticity and stress hysteresis characteristics, nitinol can withstand these external forces better than other materials. These stents are ideal for peripheral arteries’ convoluted artery courses since they are kink-resistant. Peripheral vascular stents come in four different varieties: bare metal, drug-eluting, coated, and bioabsorbable.
Based on how the stent is deployed, stents can be divided into two categories: self-expanding and balloon expandable stents. The diameter of a self-expanding stent is made bigger than the vessel diameter. After that, it is compressed into a delivery tube. The stent recalls its former shape after being detached from the catheter and expands outward, giving rise to the phrase “self-expanding.” Balloon expandable stents are created in a crimped condition, which are then expanded to the vessel wall by inflating a balloon. In both situations, the stent expands to contact and support the vessel wall. the application of Nitinol hysteresis and the stress-strain curve to the assembly of stents into catheters, their deployment during surgery, and to their behavior inside body vessels.
Laser tube cutting or wire braiding are two methods for making stents. Laser cutting the tube into a strut pattern is the first step in the creation of a stent. Sand or bead blasting is used to deburr and remove slag from the laser-cut pattern. After that, the frame is extended or shape-set using progressively bigger mandrels in a series of steps until the stent is the right size. To clean up flaws and smooth out the inner diameter, the inner surface can be honed. The stent is then passivated and electropolished. Some stents are created by photo etching or laser cutting sheets, which are then formed into cylindrical shapes. Despite not being used as frequently as tubing for stents, sheets can be rolled to a thickness of just 0.001 inches.
The task of drawing tubing to very small wall thicknesses would be extremely challenging. Additionally, the sheet offers excellent control over thickness tolerance. On drawn tubes and wires, it is more challenging to maintain concentricity and tolerance. When producing stents, concentration (management of consistent wall thickness) and surface finish of the tube inner diameter are essential for good production. The deployment of stents and laser processing both depend on uniform wall thickness.
Laser cutting equipment is designed and configured to cut tubes with walls that are defined for a particular application. A thinner than nominally stated tube wall will cause a bigger than necessary slot to be cut into the material, which could cause burns or damage to the back wall. On the other hand, a tube wall that is thicker than necessary could cause cuts to be missed. When the stent is crimped for assembly into a catheter, the stent wall can buckle or collapse due to this non-uniformity, harming the stent. Inconsistent tube wall thickness can lead to inconsistent strut thickness.
Additional applications of nitinol wires include catheters, biopsy needles, surgical equipment, and several more. As the use spreads, the industry must deal with difficult applications that put the material’s capabilities to the test. The chemical composition of the material is closely regulated during melting, and recent advancements in melting have produced materials with reduced inclusion sizes, which are essential for enhancing the material’s fatigue life. Thermomechanical processing allows for the optimization of the product’s performance and the customization of its features to fit specific functional needs. Increased yields are made possible by the superior concentricity and surface uniformity of tube inner diameter. Physiological problems are being treated using products with small form factors since they cannot be treated with traditional materials.
How are Nitinol Wires processed?
A binary base intermetallic compound is a nitinol shape memory alloy. The melting process and processing technologies are directly related to its qualities, particularly its shape memory effect.
Smelting: Currently, melting is used to create usable nitinol shape memory alloys. The phase transition temperature of the alloy is between -50°C and 160°C, with a nominal NI concentration of 53%–57% and Ti. The alloy based on NI-TI intermetallic compounds can have its phase transition temperature further adjusted, broadening its scope of use, by adding various alloying elements.
Vacuum non-self-consuming button furnace smelting is used to create a limited quantity of NI-TI alloy test material, and vacuum induction smelting is used to create qualified NI-TI alloy ingots. The first smelting process is the name given to this smelting technique. Three more smelting methods were then developed to increase the furnace’s capacity, lessen the entrance of impurity carbon, and accommodate industrial-scale manufacturing. These processes are-
Vacuum induction primary melting method(VIM)- The idea behind this smelting procedure is to first prepare a small quantity of ni-ti mother alloy using known ingredients, place it in a graphite crucible in a vacuum induction furnace, energize and melt the ni-ti alloy bath, and then gradually add the ni and ti fragments per the composition ratio to the bath while it is melting in a safe environment. The main benefits of this procedure are the lower cost, lower cost of smelting, and more homogeneous composition of the alloy. However, the fact that smelting takes place in a graphite crucible presents a challenge. With ni or ti, graphite is quite soluble at high temperatures. As a result, impurity carbon contamination is unavoidably introduced.
Vacuum self-consumption + vacuum induction melting method(VAR+VIM)- A vacuum self-consuming furnace is used to refine this smelting process into a basic alloyed ni-ti mother alloy, which is then homogenized and remelted in a vacuum induction furnace. As long as the smelting temperature and smelting duration are properly regulated, this technique may effectively manage the carbon content. The carbon concentration is typically less than 0.1%. In the smelting process, it is also discovered that as the furnace ages (with no contact between the liquid steel and the crucible), the carbon content does not rise. The crucible’s inner wall generates a smooth and compact shell layer, which may be the cause since it prevents carbon atoms from diffusing between the liquid and solid phases of ni-ti.
Vacuum coagulation filling + vacuum self-consumption smelting method and vacuum coagulation shell one-time smelting precision casting method- The vacuum self-consuming furnace smelting technique releases heat energy through an arc discharge between the poles, melting the self-consuming electrode and forming alloy droplets that are then poured into a copper crucible that has been chilled by water to create ingots. To ensure that the bath’s high temperature and electromagnetic stirring effect fully alloy and homogenize nitinol alloys, the vacuum condensed shell furnace smelting employs the positive polarity connection method to cause high-speed electron flow to impact the bath and release heat energy.
The main benefit of this technique is that there is no introduction of impurities carbon because the smelting does not take place in a carbon crucible, which enhances the material and increases the furnace capacity of nitinol alloy.
In the end, after melting, to give the material the appropriate qualities, a combination of cold working (measured as the percentage reduction in cross-sectional area) and heat treatment (thermomechanical processing) is essential. Nitinol alloys work to harden extremely quickly during cold working fabrication procedures like drawing or rolling. After a certain amount of cold work, if the material is not annealed, the strength increases to the point where further deformation causes the fracture strength to be attained, which leads to failure.
The Nitinol component’s final shape is determined via heat treatment. Temperatures of 450–550°C with the proper dwell periods will produce a straight, flat, or shaped component if the Nitinol has a decent degree of cold work (in the order of 30–50%).
Setting a shape in cold-drawn material is referred to as “shape setting,” which is a broad word. Using customized fixtures, curved items can be produced in addition to flat sheets of material or straight wires. The wire’s end is restrained in a fastening in the form of a circle. The fixture containing the wire or even a tube is put in a furnace and heated for a short period to about 500°C. The fixture containing the part is quenched in water after being taken out of the furnace. When the nitinol product is taken out of the fixture, the desired shape is visible.
Challenges of Nitinol Wires:
Finding the ideal way to connect one wire to another can be difficult when working with nitinol. The wires can be rigid or extremely elastic depending on their phase, which makes them challenging to bind or weld. One technique entails mechanically crimping wires together with the aid of other materials, such as stainless steel. The final product can then be created by TIG welding these crimps to other parts.
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