Successful medical applications of NiTi alloys depend on tight control of the entire manufacturing process, as defects can be transferred to the final product. There are several standards in place in this regard, including ASTM F2063, which not only limits the oxygen and nitrogen content in medical-grade nickel-titanium alloys to 500 parts per million (ppm), but also nickel used in the production of medical-grade equipment. The maximum size of inclusions in titanium alloy melts is limited to 39 μm. This section will look at the different Nitinol manufacturing steps and methods, highlighting their importance, their advantages and disadvantages, and their suitability for processing medical-grade Nitinol.
01.Casting/smelting process
Due to its high titanium content, molten nitinol is highly reactive and must be processed in a vacuum. Casting processes are the most common for manufacturing NiTi alloys and include vacuum induction melting (VIM), vacuum arc remelting (VAR), electron beam melting and plasma arc melting (PAM). Among these four methods, nickel-titanium alloys are mainly manufactured by multiple VAR or VIM first and then VAR. This section briefly discusses these methods, while Table 1 highlights their advantages and limitations. Additionally, as shown in Table 2, since this is a review of NiTi alloys for medical grade applications, suitability analysis was also performed based on sensitivity to carbon and oxygen, uniformity, and chemical composition, as these factors would Affects the quality of the alloy and thus its performance.
Table 1. Advantages and limitations of Nitinol casting/melting manufacturing methods.
Table2. Comparison of methods based on suitability for processing NiTi alloys for medical applications.
01.1. Vacuum induction melting (VIM)
The VIM consists of a molten graphite crucible housed within a steel shell and connected to a vacuum. When eddy currents are introduced into the graphite crucible and metal charges, electrodynamic forces are generated that aid in stirring and mixing of the melt. VIM is the most widely used process for commercial production of NiTi alloys. Compared with other vacuum melting processes, it provides greater flexibility and better control of uniformity and alloy composition through independent control of time, pressure, temperature and mass transfer through melt stirring. However, since graphite crucibles are used, they are susceptible to carbon contamination. Typical carbon impurity levels are between 300 and 700 ppm, although with careful control, ingots with carbon levels between 200 and 500 ppm are possible.
01.2. Vacuum arc remelting (VAR)
In vacuum arc remelting, consumable or non-consumable electrodes are continuously remelted using an arc in a vacuum environment. VAR melting produces alloys of extremely high purity and can therefore be used to improve the cleanliness and structure of VIM ingots. However, the entire ingot is not melted simultaneously and multiple melts may be required to achieve the desired uniformity.
01.3. Plasma Arc Melting (PAM)
In the plasma arc melting process, the input elemental metal is placed in a copper water-cooled crystallizer and then conveyed by a spiral beneath the argon plasma burner. This method eliminates contamination caused by the use of vacuum induction furnace crucibles. Therefore, the nickel-titanium alloy produced by PAM Company has higher purity and better corrosion resistance than the nickel-titanium alloy produced by VIM Company. It also has much smaller inclusions, as shown in Figure 3 . However, it has lower uniformity and requires multiple PAM torches to achieve similar uniformity to VIM.
Figure 3. PAM (a) and VIM (b) SEM images of hot-rolled and fully annealed Ni50.8Ti49.2 rods. Arrow points to typical inclusion gr4.
01.4. Electron beam melting
In this method, a round ingot prepared in a vacuum induction furnace is melted by electronic heating with a much higher vacuum (10^(-2) Pa) than VIM (10 Pa). Coupled with the absence of a crucible, the risk of further carbon contamination is eliminated and melt quality depends on ingot quality. EBM is highly pure with oxygen content as low as 70 ppm (4-10 times lower than VIM).
01.5. Summary of melting process
During the melting process, great care must be taken to ensure that factors such as inclusions and high carbon/oxygen content that can negatively affect the alloy are minimized. For example, studies have found that the presence of inclusions not only negatively affects the final product but may also affect the machining process. For example, studies have shown that inclusions can lead to shorter tool life when turning NiTi alloys compared to inclusion-free alloys. It is well known that non-metallic inclusions such as carbides (TiC) and intermetallic oxides (Ti4Ni2Ox) can cause fatigue failures when entering NiTi alloy medical devices during the melting process. Inclusions also affect the susceptibility of electropolished NiTi alloys to pitting corrosion, with the size of the inclusions having a greater impact than the number of inclusions.
02.Powder metallurgy process (PM)
Powder metallurgy processes include traditional metallurgy processes and additive manufacturing (AM) processes. Traditional powder metallurgy processes include conventional sintering (CS), hot isostatic pressing (HIS), spark plasma sintering (SPS), metal injection molding (MIM) and self-propagating high temperature synthesis (SHS). On the other hand, additive manufacturing PM processes include selective laser melting (SLM), laser engineered net shaping (LENS), electron beam melting (EBM), and selective laser sintering (SLS). The advantages and limitations of these processes are shown in Table 4.
Table 4. Advantages and limitations of powder metallurgy NiTi alloy manufacturing methods.
Although the casting process is more popular for making nickel-titanium alloys, especially for medical applications, powder metallurgy has proven that it has the potential to rival or even outperform casting in some areas, including where segregation does not occur. Higher alloy compositions are obtained at lower temperatures, resulting in isotropic physical and mechanical properties. In fact, the rapid solidification (RS) associated with powder metallurgy sometimes improves physical and mechanical properties. This is very important because it has a knock-on effect. For example, a uniform and fine microstructure improves machining properties, while the ductility provided by powder metallurgy improves cold and hot working properties such as rolling, extrusion and forging. Generally speaking, improvements in material properties will impact product shelf life. In order to improve the uniformity of the alloy, alloy powder sintering is more popular than raw metal powder sintering. Powder metallurgy can also be used to control phase transition temperatures.
Commercially, PM has been used to produce porous NiTi alloys. In this regard, different methods such as HIP, MIM and SHS fulfill the main prerequisites for porous NiTi implants. These requirements include: open and interconnected porosity between 30% and 80%, pore size between 100 μm and 600 μm, high strength (at least 100 MPa at 2% strain), low Young's modulus ( Young's modulus close to that of cancellous bone (<3 GPa) or cortical bone (10-20 GPa)) and high recovery strain (more than 2% recovery after 8% loading).
However, there are some problems that hinder the full utilization of medical-grade nickel-titanium alloy powder metallurgy materials. First, oxygen control is a serious challenge, as typical powder metallurgy NiTi parts have oxygen levels as high as 3000 ppm. Although it can be reduced to 1500 ppm with careful handling, the impact of this oxygen level on ductility and fatigue is still a concern. Furthermore, due to the large exposed surface area created by high porosity, nickel leaching is a serious problem due to its ability to cause harmful effects such as cell allergy, genotoxicity, and cytotoxicity. Furthermore, pores not only reduce the corrosion resistance of NiTi, but also affect nickel release, which is two orders of magnitude higher in untreated porous NiTi made with SHS than in solid NiTi.
In addition, sintered alloys produce alloys with a higher brittle oxide content (Ti4Ni2Ox:0 < x ≤ 1). Last but not least, the densification process of Ni-Ti alloy powder is difficult, mainly due to the diffusivity difference between nickel and titanium and the highly exothermic nickel-titanium alloy forming reaction and Ni3Ti, Ti2Ni liquid eutectic Capillary effect caused by presence.