How Does Titanium Metal Injection Molding Work?

Titanium metal injection molding starts by mixing fine powder with a binder, molding the feedstock into a green part, removing the binder, and sintering the part to develop its final density and properties. Powder type, binder system, debinding, and sintering conditions vary for commercially pure titanium, titanium hydride, Ti-6Al-4V, and TiAl. The process is studied for engine valves, turbine blades, watch cases, golf-club heads, consumer-electronics parts, and medical implants.

Commercially Pure Titanium and Titanium Hydride

Titanium metal injection molding (Ti-MIM) has been studied extensively for biomedical products and precision titanium components. Research commonly addresses powder type and particle size, binder systems, debinding, sintering schedules, pores, and impurity control.

Feedstock powders include plasma-atomized titanium, gas-atomized spherical titanium, titanium hydride, and hydride-dehydride (HDH) titanium. Some studies use atomized powder smaller than 45 μm, while others use fine titanium-hydride particles with a mean size of only a few micrometers. These choices help reveal how powder size and morphology affect feedstock behavior, molding quality, and sintering densification.

Binder systems may be water-soluble, paraffin-based, naphthalene-based, or multicomponent mixtures of paraffin wax (PW), polyethylene glycol (PEG), low-density polyethylene (LDPE), and polypropylene (PP). Titanium powder and binder are compounded into a uniform feedstock and injection molded into a green part.

Debinding may use solvent extraction, thermal treatment, or both. For water-soluble or otherwise soluble binder components, the principal soluble fraction can be removed first, followed by vacuum or protective-atmosphere thermal debinding to remove residual organics and limit carbon and oxygen contamination during sintering.

Vacuum sintering is common. Adjusting temperature, hold time, and heating and cooling schedules changes diffusion between particles, pore shrinkage, and microstructural evolution. Researchers also examine large voids caused by gas trapped during feedstock preparation or molding and fine pores caused by inadequate densification. Feedstock preparation, injection parameters, debinding, and sintering are optimized to reduce these defects.

Powders produced by different routes or with different morphologies may also be blended. For example, gas-atomized spherical powder can be mixed with HDH powder to balance powder loading, feedstock flow, sintering shrinkage, and shape retention.

Research on commercially pure titanium MIM therefore follows a connected process chain: powder selection and blending, binder design, compounding and injection molding, debinding, vacuum sintering, and pore and impurity control. Parameters at each stage are adjusted to study their effects on final microstructure, dimensions, and properties.

Ti-6Al-4V Powder Injection Molding

Ti-6Al-4V is the most widely used titanium alloy. It combines heat resistance, strength, ductility, toughness, formability, weldability, corrosion resistance, and biocompatibility, and accounts for more than half of titanium use.

Ergül and colleagues injection molded gas-atomized Ti-6Al-4V powder. The samples had a coarse acicular structure. Higher sintering temperatures and longer hold times increased density, tensile strength, and elongation. After 10 hours at 1,275°C, density reached 99%, tensile strength was 704 MPa, elongation was 6.37%, and hardness was 38.6 HRC. Other studies have noted that higher temperature and longer holds can coarsen grains and impair fatigue performance.

Germany’s GKSS Research Centre used gas-atomized Ti-6Al-4V powder with a paraffin-based binder. Green parts were solvent-debound in n-heptane, thermally debound in argon, and vacuum-sintered. Sintering temperature and cooling rate strongly affected final tensile strength, while debinding parameters had little effect on tensile strength and the process parameters had little influence on ductility.

Another research team compounded a mixture of 90% Ti-6Al-4V powder and 10% HDH Ti-6Al-4V powder with a paraffin-based binder at 150°C and injection molded it at 165°C. After debinding, the parts were vacuum-sintered for 3 hours at 1,230°C and hot-isostatically pressed for 3 hours at 920°C. The dense alloy reached a tensile strength of 1,030 MPa, a reduction in area of 12%, and an elongation of 21%.

TiAl Powder Injection Molding

TiAl intermetallics combine attributes of ceramics and conventional metals: low density, high service temperature, high-temperature strength, specific stiffness and elastic modulus, and good oxidation, creep, and fatigue resistance. They are considered promising lightweight high-temperature structural materials. Compared with pure titanium, TiAl requires a higher sintering temperature, absorbs impurities more readily, and demands tighter impurity control.

GKSS combined gas-atomized TiAl powder with a paraffin-based binder. Argon-atmosphere debinding, vacuum sintering, and hot isostatic pressing produced a dense alloy with low oxygen content. Flowing argon during sintering prevented aluminum evaporation from the TiAl surface, but carbon and oxygen contents were substantially higher than after vacuum sintering.

Kim and colleagues used MIM to produce Ti-48Al with 98.8% relative density, fine microstructure, and good properties. SHS-produced prealloyed powder was compounded with binder and molded at 120°C. After solvent debinding, thermal debinding was performed in argon or hydrogen. Hydrogen debinding deposited fine oxides on the original TiAl particle boundaries and significantly reduced mechanical properties after sintering. Presintering for 3 hours at 1,000°C followed by sintering for 30 hours at 1,350°C produced 98.8% relative density. Heating rate also affected density; about 3°C/min was considered suitable.

Osaka Yakin Kogyo produced γ-TiAl components with a nominal Ti-47Al-2.6Cr composition in atomic percent. The alloy powder was made by self-propagating high-temperature synthesis, then mixed with an organic binder, compounded, injection molded, and sintered. Density reached 97%, with high strength and ductility comparable to conventionally produced specimens.

Another team used inert-gas-atomized prealloyed powder to mold complex high-niobium TiAl parts. After sintering for 2 hours at 1,480°C, density reached 96.2%. The material had a uniform fine near-lamellar structure with a lamellar size of about 60 μm, compressive strength of 2,839 MPa, compressive strain of 34.9%, tensile strength of 382 MPa, and elongation of 0.46%.

Other Titanium Alloy Systems

Alloying additions such as zirconium, niobium, tantalum, palladium, and tin can improve mechanical properties, corrosion resistance, and biocompatibility. Tin and zirconium can also improve strength and high-temperature creep resistance.

Kyushu University researchers injection molded Ti-6Al-2Sn-4Zr-2Mo-0.1Si. Blending elemental powder with titanium caused poor diffusion of high-melting elements and high oxygen content, reducing density. Using titanium powder with master-alloy powder produced 96.4% density, 910 MPa tensile strength, and 14.1% elongation. Adding 4% Mo to form Ti-6Al-2Sn-4Zr-6Mo raised tensile strength to 1,010 MPa while maintaining 14.7% elongation.

MIM is also widely studied for biomedical titanium alloys. Ti-6Al-4V and Ti-6Al-7Nb implants can release aluminum or vanadium ions, whereas Ti-Mo alloys avoid these elements and combine mechanical performance, corrosion resistance, and biocompatibility for dental and orthopedic applications.

University researchers in Japan injection molded Ti-12Mo from titanium powder with a mean size of 38 μm and aluminum powder with a mean size of 0.6 μm. [TO VERIFY: source identifies aluminum powder in the Ti-12Mo feedstock] The debound parts were vacuum-sintered at 1,120-1,300°C. At 1,160-1,200°C, tensile strength reached 1,000 MPa, comparable with wrought alloy of the same composition. Higher temperatures coarsened grains and reduced strength. Cold isostatic pressing between presintering and final sintering increased final density by 10% and nearly doubled tensile strength.

Researchers at the Beijing General Research Institute for Nonferrous Metals used three titanium powder sizes to make Ti-Mo getters by powder injection molding. Powder size controlled specific surface area and porosity, which in turn controlled gas-gettering performance. Powder with a mean size of 35 μm produced high porosity and surface area and the best gettering performance. A powder loading of 40% by volume also produced relatively high porosity and surface area.

Porous Titanium MIM

Porous titanium and newer titanium alloy systems can combine a suitable pore structure with mechanical properties needed for orthopedic replacement implants. Their lower effective stiffness reduces mismatch with bone and the associated stress-shielding effect. Interconnected pores also allow body fluids and bone cells to penetrate the implant.

Powder injection molding of porous biomedical titanium alloys was first proposed in 2003. KCl and NaCl were common space holders, and porosity above 10% was achieved. Later studies added TiH₂ to titanium-aluminum-vanadium elemental powder as a foaming agent and activator. The resulting open-cell Ti-6Al-4V had a uniform pore distribution, pore sizes of 90-190 μm, porosity of 43%-59%, and an elastic modulus of 5.8-9.5 GPa.

Other researchers combined atomized spherical powder, HDH titanium powder, and a paraffin-based binder with NaCl or KCl space holders. Changing the space-holder content produced porous titanium with pore sizes and porosity suited to medical implants, while the composition met the stated Grade 3 commercially pure titanium requirements. At Central South University, HDH titanium powder and NaCl produced porous titanium implants with 42.4%-71.6% porosity and pores up to 300 μm. Adjusting NaCl content created interconnected pores, and the mechanical properties approached those of cancellous bone.