Nickel-titanium alloys have many advantages that make them suitable for the medical device market, including shape memory, superelasticity, high fatigue resistance, corrosion resistance, and biocompatibility, all of which have brought revolutionary changes to the field of medical device manufacturing. To fully exploit the properties of nickel-titanium alloys, it is essential to select reasonable and application-oriented processing techniques to achieve optimal efficiency. This is particularly true for nickel-titanium alloys, as they are difficult to process due to their phase transformation and work hardening.

Although there have been independent studies on the processing of nickel-titanium alloys, there is a lack of important comparative information on the advantages and limitations of different processing techniques applied in the micromachining of nickel-titanium alloys. This paper will introduce the machinability of nickel-titanium alloys from various processing methods.

Applicability of Nickel-Titanium Alloys in Medical Applications

The properties of nickel-titanium alloys make them favorable for use as medical-grade materials. As shown in Figure 1, the stress-strain behavior of nickel-titanium alloys is very similar to that of bones and tendons, so the use of nickel-titanium alloys can shorten healing time and reduce trauma to surrounding tissues.

In addition, compared with materials such as stainless steel, the stiffness of nickel-titanium alloys (40-75 GPa) is closer to that of dense bone (12-17 GPa), so the use of nickel-titanium alloys can greatly reduce the stress shielding problem of implants. This means that the implant does not need to absorb most of the external force during use, which would otherwise weaken the bone.

In terms of biocompatibility, nickel-titanium alloys do not show cytotoxicity, neurotoxicity, genotoxicity, or allergic activity compared with clinical reference control materials (i.e., AISI 316 LVM stainless steel). However, due to the relatively high nickel content in nickel-titanium alloys, concerns about the release of nickel ions in the body still exist.

To address this, methods such as alloying and/or surface treatment of nickel-titanium alloys have been applied. For example, adding a third element and forming a thin film on the surface of nickel-titanium alloys help to improve their biocompatibility. These non-toxic and biocompatible third elements may include silver, niobium, zirconium, tantalum, and molybdenum, which help to enhance passivation, inhibit the release of nickel into the body, and improve pitting corrosion resistance.

 

 

The biocompatibility of nickel-titanium alloys is also related to the manufacturing method. When testing nickel-titanium alloys prepared by powder metallurgy (PM) and arc melting (AM), the results showed that alloys manufactured by PM are more resistant to pitting corrosion compared with those manufactured by AM.

 

Significance of Nickel-Titanium Alloy Processing for Its Medical Applications

Integration requirements such as surface composition and charge are crucial for cell adhesion and protein adsorption, many medical applications of nickel-titanium alloys demand precise processing of the material with tight tolerances and carefully controlled surface finish. For example, a surface roughness in the Ra range of 1-2 μm has been proven to be beneficial for the biomechanical anchoring of dental implants. Both in vivo and in vitro studies have shown that implant surfaces with microscale and submicroscale (nanoscale) topography can improve the interaction process between the implant surface and bone cells (also known as bone-implant contact (BIC)). An in vitro study on the ability of Staphylococcus epidermidis (ATCC35984) to adhere to the surfaces of different solid biomaterials with a surface roughness below 30 nm Ra demonstrated that surface topography is very important for reducing implant-related infections, as bacteria have stronger adhesion on rougher biomaterials. Similarly, regulating the titanium dioxide oxide layer formed on the surface of nickel-titanium alloys can improve osseointegration and enhance corrosion resistance.

An important reason why nickel-titanium alloy materials must be carefully processed for successful application is that they have lower resistance to fatigue crack propagation compared to other biomedical implant materials. More specifically, studies comparing the fatigue crack growth resistance of nickel-titanium alloys with 316L stainless steel, Ti-6Al-4V, pure Ti, and CoCr Haynes 25 alloys have shown that at a fixed load ratio ≈ 0.1, the fatigue threshold ΔKTH of nickel-titanium alloys is significantly reduced by 2 to 5 times.

Fatigue cracks depend not only on the selected processing method (for example, it has been reported that laser-processed surfaces can generate 5-15 μm cracks) but also on factors such as the selected processing parameters and the surface integrity of the subsequently processed samples. This fact, coupled with additional surface finish requirements that may necessitate expensive finishing processes, underscores the importance of critically examining existing manufacturing and processing techniques to highlight their advantages, limitations, and applicability to processing nickel-titanium alloys.

Turning, Milling, and Grinding of Nickel-Titanium Alloys

In general, turning, milling, and grinding can be performed on a macroscale or microscale. Although these two variants are kinematically similar, they differ significantly in several aspects where boundary conditions depend on the relevant application. Several characteristics that help define the macro and micro boundaries include:

1. 1Uncut chip thickness: A thickness less than 200 μm is considered microscale, but this boundary is constantly changing with advancements in processing technology. Currently, uncut chip thicknesses less than 100 μm are achievable.
2. Component size and precision: At least two dimensions should be in the submillimeter range (1-1000 μm) with Ra ≤ 100 nm.
3. Geometric dimensions of turning, milling, or grinding tools: (≈25-1000 μm).

Basic processing mechanics: On the microscale, the uncut chip thickness is comparable to the cutting edge radius or the workpiece grain size, so chip formation is mainly plowing rather than shearing. In addition, factors such as the cutting edge radius effect, flank tool-workpiece contact, negative rake angle (even if the nominal rake angle of the tool is positive), microstructure effects, and minimum chip thickness become crucial. In fact, in a phenomenon known as the “minimum uncut chip thickness effect,” no chip is formed if the uncut chip thickness is below a specific critical value (which varies depending on the workpiece material).

Table 1 lists typical achievable geometries and Ra surface roughness values for processing on the microscale. However, these values depend on the workpiece material. For nickel-titanium alloys, the achievable Ra surface roughness values are shown in Table 7. Since there is a lack of specialized research on turning, milling, and grinding nickel-titanium alloys on the microscale, this review is not limited to the microscale.

 

Nickel-Titanium Alloy Turning

The phenomena during nickel-titanium alloy turning are very similar to those in milling, i.e., although a large feed rate will significantly increase the Ra value, an extremely small feed rate does not necessarily result in a very low Ra value. Due to accumulated tool/material adhesion, increased flank wear leads to an increase in Ra value. Large burrs appear on the machined surface, and burr formation is divided into four stages: initiation stage, initial development stage, pivot point stage, and final development stage.

In terms of tools, uncoated cemented carbide is not suitable for machining nickel-titanium alloys, as it causes severe tool wear characterized by significant notch wear and rake face depression. In contrast, TiN coating can be used to reduce the width of the wear surface. Studies have shown that cemented carbide tools with multi-layer coatings (eight alternating layers of TiCN and TiAlN coatings, with a TiN base coating first and a TiN top coating last) perform better.

As shown in Figure 2, three distinct rotational speed ranges were observed when machining nickel-titanium alloys using TiCN/TiAlN multi-layer tools. At low rotational speeds (Range 1), with a speed of 20 m/min, extremely high cutting forces exist, followed by high tool wear manifested as severe notch wear.

Within this range, lubricants affect the cutting force. Cutting force and notch wear decrease with increasing cutting speed, and at a cutting speed of vc = 100 m/min, notch wear is no longer significant. In Range 2 (vc = 60-130 m/min), results show that cutting force is neither affected by cutting speed nor by lubricants. For higher cutting speeds in Range 3 (vc = above 140 m/min), both tool wear and cutting force increase significantly under dry machining conditions, highlighting the necessity of lubricants.

 

Studies on the effect of cryogenic machining on nickel-titanium alloys have shown that when machining room-temperature austenitic nickel-titanium alloys, compared with dry machining and minimum quantity lubrication (MQL), cryogenic machining can reduce the progressive wear of tools, thereby significantly improving the performance of cutting tools. However, the quality of all methods becomes comparable after 4 minutes of machining, and the surface roughness of the machined parts increases as the machining time extends. It has also been found that turning nickel-titanium alloys has a considerable impact on their phase transformation behavior, which can be attributed to mechanical stress and the heat-affected zone.

 

Milling of Nickel-Titanium Alloys

Although milling nickel-titanium alloys is feasible, it poses significant challenges due to the alloy’s characteristics of work hardening, high strength, and high specific heat, which lead to substantial flank wear during machining. Studies have been conducted on the millability of nickel-titanium alloys using a CNC milling machine with a general-purpose synthetic coolant and (Ti,Al)N/TiN coated cemented carbide inserts.

These inserts are cost-effective, and research has shown that cemented carbide tools with multi-layer TiCN/TiAlN or TiCN/TiN coatings perform better than PCD or CBN tools. After machining, significant flank wear, coating peeling, and a certain degree of notching were observed on the cutting edges.

Compared with traditional metals, machining nickel-titanium alloys significantly shortens tool life. A surface roughness of Ra = 0.4 μm was achieved at the minimum feed rate (50 mm/min). However, an interesting phenomenon was observed: when the feed rate was increased to 200 mm/min, the Ra reached the lowest value of Ra = 0.19 μm, while a further increase in feed rate caused the Ra to increase significantly to 1.66 μm.

When milling grooves with high feed rates, low spindle speeds, and worn tools, the burr height on the down-milling side was larger. It was also found that burr formation depends on whether the cutting mode is mainly tearing or chip formation, which is related to the feed rate. Despite poor machinability, the use of minimum quantity lubrication (MQL) can achieve good tool wear and shape accuracy; compared with dry machining, MQL can reduce nickel-titanium adhesion.

As shown in Figure 3, although burrs cannot be eliminated, geometries in the range of 50-100 μm can still be successfully milled. MQL helps to improve tool life. In addition, micro-milling with a cutting speed of 33 m/min, high feed rates (6-30 μm/tooth), cutting depths of 10-100 μm, and relatively high cutting widths (250 μm) can form better chips, improve workpiece quality, and extend tool life.

Researchers studied the high-speed milling characteristics of nickel-titanium alloys and evaluated the effects of cutting speed, cutting depth, feed rate, and tool wear. They found that increasing the cutting speed can improve the machining process, reduce work hardening, and enhance surface quality. The cutting force or load can be reduced by decreasing the cross-sectional area of chips.

 

Grinding of Nickel-Titanium Alloys
Traditional grinding, micro-grinding, and ultra-precision grinding (UP) are grinding terms that are often misunderstood and used interchangeably. As shown in Figure 4, these terms, along with polishing, can be distinguished based on material removal rate (MRR) and grain size.

Figure 4. Comparison between grinding and polishing

Grinding nickel-titanium alloys is a significant challenge due to their abrasiveness. However, material removal through grinding is feasible and has been applied in industry. Although surface grinders, sandblasters, and belt sanders are very useful and effective in certain manufacturing steps of nickel-titanium alloys, most forming and surface treatment of nickel-titanium alloy components adopt centerless grinding. Commercially, micro-scale grinding is mainly used for processing guide wires. Swiss-style centerless grinding has the capability to produce small and difficult-to-machine parts (such as nickel-titanium alloy needles) with a surface roughness Ra as low as 0.4 μm.

 

Water Jet Lavorazione (WJM) of Nickel-Titanium Alloys

Water jet machining (WJM) is a high-energy fluid jet technology that controls material removal through high-speed jets. If abrasives are added to the water jet, it is called abrasive water jet machining (AWJM). Both methods are more suitable for cutting thin plates compared to milling internal features (such as blind grooves). WJM and AWJM for nickel-titanium alloys are feasible and promising, as small structural surfaces without heat-affected zones (HAZ), white layers, cracks, or deformed structures can be machined. However, due to the strain hardening and yield strength of nickel-titanium alloys, the cutting quality faces the challenge of uneven depth. In addition, nickel-titanium alloys have good erosion resistance, which benefits from their elastic behavior that can release local strains.

 

Furthermore, the high water pressure involved can cause unexpected deformation of tiny nickel-titanium alloy components. An analysis of controlled-depth milling of nickel-titanium alloys with and without abrasives shows that the use of abrasives can improve the machining process, thereby achieving better depth control. This is because the material removal mechanism is micro-abrasion, including grooving, plowing, and further erosion caused by water droplet impact.

Therefore, for the effective machining of nickel-titanium alloys, it is recommended to use AWJM first, followed by WJM for cleaning. Although AWJM temperatures may be irrelevant for other materials, they have been reported to affect the machining process of medical-grade nickel-titanium alloys, as temperature changes can induce austenitic phase transformation, which is reported to have lower erosion resistance.

Conclusione

This paper elaborates on the process chain of nickel-titanium alloys and provides comparative data on the manufacturing and processing of nickel-titanium alloys. Various factors are considered in the comparison, including the advantages and limitations of the analyzed processes, among which the applicability analysis can offer quick guidance for process selection.