Introduction
Titanium and titanium alloys have become one of the most strategic material groups in today's engineering thanks to their high specific strength, excellent corrosion resistance, and biocompatibility. The main reason they are preferred in many fields, from the aerospace industry to medical implants and from the defense industry to the chemical industry, is that they offer high mechanical performance despite their low density. However, alongside these superior properties, machining titanium is quite costly. In conventional machining methods, up to 70–90% of the material can be lost as chips, which significantly increases production costs. [1]
At this point, Metal Injection Molding (MIM) technology emerges as an important alternative, particularly for the series production of small, complex-geometry parts that require high precision. With the MIM method, near-net-shape production becomes possible, the need for machining is greatly reduced, and production efficiency increases. However, because of titanium's high reactivity with elements such as oxygen, carbon, and nitrogen, the Ti-MIM process requires much more precise process control compared to stainless steel MIM. [2]
Key Advantages of Titanium Production with MIM
Metal Injection Molding is an advanced manufacturing method that combines plastic injection molding technology with powder metallurgy. Fine metal powders are mixed with suitable binder systems and shaped by injection molding. The binder is then removed (debinding), and sintering is carried out at high temperatures to obtain the final dense structure.
The main advantages of the MIM method for titanium are:
- Ability to produce complex geometries in a single operation
- Low material loss
- Suitability for series production
- High dimensional precision
- Reduction of machining operations to a minimum
- Cost advantage for small parts
In recent years, the use of MIM technology has increased significantly, particularly in medical implants, dental implants, surgical instruments, aerospace fasteners, and firearm mechanisms. [3]
Main Titanium Alloys Used in MIM
Not every titanium alloy is suitable for the MIM process. By far the most widely used alloy today is Ti-6Al-4V.
This alloy;
- high strength,
- low density,
- good fatigue strength,
- excellent corrosion resistance,
- high biocompatibility
can offer these properties simultaneously.
In addition, commercially pure Titanium Grade 2 and Grade 4 are preferred especially in medical implants, while beta titanium alloys are gaining importance in biomedical applications due to their low elastic modulus. However, the tolerance of beta alloys to carbon and oxygen contamination is lower, and process control becomes more critical. [4]
Factors That Make Titanium Sintering Difficult
The greatest challenge in producing titanium via MIM arises during the sintering stage.
Above approximately 600 °C, titanium;
- oxygen,
- carbon,
- nitrogen,
- hydrogen
can easily react with them.
The diffusion of even very small amounts of these elements into the material;
- reduced ductility,
- embrittlement,
- decreased fatigue life,
- crack formation
can cause these effects.
For this reason, in the Ti-MIM process, not only the sintering atmosphere but also the oxygen content of the powder used, the binder system, the debinding conditions, and even the material of the sintering setter can directly affect mechanical properties. Many studies in the literature have shown that even small increases in oxygen levels significantly reduce elongation values. [5]
Sintering Parameters
Sintering is the most critical stage determining the success of the Ti-MIM process.
When the parameters used in the literature are examined, some common points stand out.
The sintering temperature is mostly in the range of 1250–1400 °C. For Ti-6Al-4V, the 1300–1360 °C range is the most commonly preferred window. At lower temperatures, sufficient densification cannot be achieved, while at excessively high temperatures, grain growth accelerates and mechanical properties can be adversely affected. [6]
It is important to keep the heating rate under control, especially after debinding. Studies with HDH titanium powders have shown that controlled heating at approximately 1 °C/min reduces distortion and provides more homogeneous shrinkage. [7]
Vacuum or Argon?
The two atmosphere options most frequently encountered in the literature are:
- high vacuum
- and high-purity argon.
Sintering under high vacuum provides a significant advantage in terms of reducing oxygen contamination. However, in industrial production, many companies also successfully use high-purity argon atmospheres (up to 99.999%).
The primary objectives in atmosphere selection are to:
- prevent oxygen pickup,
- reduce carbon formation,
- prevent nitride formation,
- ensure homogeneous densification.
For this reason, maintaining atmosphere purity throughout the process is of great importance not only during the sintering stage but also during mixing, injection, and debinding. [8]
Powder Selection
For many years, it was thought that only gas-atomized spherical titanium powders were suitable for Ti-MIM. The main reasons were their high flowability and low oxygen content.
However, recent studies show that, with appropriate process optimization, HDH (Hydride-Dehydride) titanium powders can also be used successfully.
Advantages of HDH powders:
- much lower cost,
- easier availability,
- high production potential.
On the other hand, due to their irregular particle geometry, feeding behavior and shrinkage control must be managed more carefully. Current studies show that relative densities above 96% can be achieved with suitable sintering parameters. [9]
Application Areas
The application areas of Ti-MIM technology are expanding every year.
The medical sector is currently one of the largest markets. Dental implants, orthopedic screws, bone plates, and surgical instruments can be produced with high precision using the MIM method. [10]
Since weight reduction is critical in the aerospace industry, Ti-MIM solutions are being evaluated for small but complex fasteners, fittings, and various mechanical components. [11]
In the defense industry, mechanical parts requiring lightness, corrosion resistance, and high strength constitute an important application area for Ti-MIM.
In addition;
- automotive,
- chemical industry,
- marine,
- sports equipment,
- watchmaking,
- electronics
Ti-MIM applications are also becoming increasingly widespread in sectors such as these. [12]
Conclusion
Metal Injection Molding is recognized as one of the most important powder metallurgy technologies enabling the economical and series production of titanium alloys. Although it offers advantages such as lower material loss, the ability to produce complex geometries, and high dimensional precision compared to conventional manufacturing methods, process success depends largely on controlling oxygen, carbon, and nitrogen contamination.
Today, Ti-6Al-4V in particular continues to be the base alloy for Ti-MIM applications. However, studies on the use of HDH titanium powders reveal significant potential for reducing costs. In the coming years, thanks to advances in powder production technologies, optimized binder systems, and more precise control of sintering processes, Ti-MIM is expected to become a much more widespread production method not only in the medical sector but also in aerospace, defense, and high-performance engineering applications. [13]
References
[1-3] Dehghan-Manshadi, A. et al. Metal injection moulding of titanium and titanium alloys: Challenges and recent development. Powder Technology, 319 (2017), 289–301. DOI: 10.1016/j.powtec.2017.06.053.
[4-6] Ebel, T. Metal Injection Molding (MIM) of Titanium and Titanium Alloys. Handbook of Metal Injection Molding (2012). DOI: 10.1533/9780857096234.4.415.
[7-9] Fang, Z.Z. et al. Powder Metallurgy of Titanium – Past, Present and Future. DOI: 10.1080/09506608.2017.1366003.
[10-11] Friederici, V. et al. Metal Injection Moulding of Thin-Walled Titanium Parts for Medical Applications. DOI: 10.1179/0032589914Z.000000000154.
[12-13] Ebel, T. et al. Reduction of the Embrittlement Effect of Binder Contamination in MIM Processing of Ti Alloys. DOI: 10.1080/00325899.2017.1291085.