characteristics of titanium foam - HGP

Foam metal has become one of the top ten new materials due to its excellent performance. Among them, foam titanium is a new type of functional material developed in the 21st century, which combines the advantages of porous structure and titanium alloy. Compared with traditional dense titanium alloy, foam titanium has smaller density, excellent mechanical properties, and unique functional characteristics. It is widely used in aerospace, marine engineering, biomedical, and other fields. In comparison with foam aluminum, foam titanium has a higher melting point and better insulation properties. Titanium also has the characteristics of low density and good corrosion resistance, making it more suitable for application in harsh service environments such as aviation, aerospace, and military. Foam titanium has excellent biocompatibility, and its pores can transport nutrients for the growth of connective tissue, promoting cell growth and differentiation. Therefore, it is widely used in the production of artificial bones, teeth, and other bio-simulated structures. Foam titanium can also be used in the production of fuel cells and electrodes, and the presence of a large number of pores is beneficial to the release of energy in the electrochemical reaction process.

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1. Compressive and Energy Absorption Performance

The porous structure of foam titanium gives it excellent compressive performance. As the relative density increases, the compressive stress of foam titanium at room temperature also increases. Under the same strain, the energy absorption per unit volume increases as the porosity of foam titanium decreases. The maximum energy absorption efficiency of foam titanium is 0.27, and the ideal energy absorption rate is approximately 0.78, indicating that foam titanium with a porosity of 71% to 88% is suitable for energy absorption applications.

2. Biocompatibility

An ideal bone graft material should possess osteogenic, osteoconductive, and osteoinductive properties. The porous structure and elastic modulus of foam titanium are similar to human bones, allowing for good compatibility with human tissues. Muscles can grow into the pores, and bodily fluids can flow into them. Moreover, when foam titanium undergoes special treatments such as NaOH, CaCl2, H2SO4/HCl chemical immersion, and heat treatment, it can be deeply penetrated by newly formed bone tissue (bone conduction). For example, a porous titanium layer formed through sequential chemical and heat treatments in artificial hip joints can be penetrated by new bone in a biologically active manner, tightly fixing it to the surrounding bones. Although the thickness of these porous titanium layers is generally less than 1mm, if the pore size is appropriate, bones can grow into deeper regions throughout the porous structure. Foam titanium with a porosity of 50% and an average pore size of 300μm, prepared by sintering, can be used to create dog bone tissue that has already grown to the central part of the foam titanium pores after three months of chemical and heat treatment. For treated foam titanium, the contact area between the bone implant and dog bone tissue accounts for 35% of the implant area, while for untreated foam titanium, the contact area is only 11%. By customizing the porous structure, foam titanium can have excellent permeability and absorption performance, allowing for fluid transport and promoting bone growth, cell migration, attachment, and the enhancement of new bone tissue growth and vascularization. Therefore, foam titanium is commonly used as a scaffold for bone implants.

3. Shielding Performance

Foam titanium exhibits significant electromagnetic shielding effects, with better shielding performance at low frequencies. Porous foam metal can attenuate incident microwaves through reflection, scattering, and absorption, thereby reducing electromagnetic energy. The electromagnetic shielding efficiency of foam titanium decreases initially and then increases as the frequency of electromagnetic waves increases. The electromagnetic shielding performance of closed-cell foam metal is mainly related to factors such as reflection loss, absorption loss, multiple reflections within the pores, and eddy current loss. Among them, reflection loss and multiple reflections within the pores are dominant, while absorption loss and eddy current loss contribute more in the high-frequency range.

4. 4. Sound Absorption Performance

The sound absorption mechanism of foam metal mainly involves the damping attenuation of the material itself, the viscous dissipation generated by friction between the pore walls and the fluid inside the pores, and the interference sound absorption caused by sound wave reflection. Most metals have poor inherent damping ability, so foam metal mainly attenuates sound waves through friction, viscous response, and reflection mechanisms. In the frequency range of 200 to Hz, foam titanium with larger pore size exhibits better sound absorption performance when the frequency is below Hz, while foam titanium with smaller pore size exhibits better sound absorption performance when the frequency is above Hz. Higher sound frequencies may cause more air vibrations within the pores, resulting in greater viscous forces between the air and the pore walls. At this time, sound energy is mainly attenuated through viscous dissipation mechanisms. Therefore, when the sound frequency exceeds a certain value, foam titanium with lower porosity and pore size exhibits better sound absorption performance.

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The greater one's responsibilities, the more a person grows. The same principle applies to the human bone: The greater the forces it bears, the thicker the tissue it develops. Those parts of the human skeleton subject to lesser strains tend to have lesser bone density. The force of stress stimulates the growth of the matrix. Medical professionals will soon be able to utilize this effect more efficiently, so that implants bond to their patients' bones on more sustained and stable basis. To do so, however, the bone replacement must be shaped in a manner that fosters ingrowth -- featuring pores and channels into which blood vessels and bone cells can grow unimpeded. Among implants, the titanium alloy Ti6Al4V is the material of choice. It is durable, stable, resilient, and well tolerated by the body. But it is somewhat difficult to manufacture: titanium reacts with oxygen, nitrogen and carbon at high temperatures, for example. This makes it brittle and breakable. The range of production processes is equally limited.

There are still no established processes that can be used to produce complex internal structures. This is why massive titanium implants are primarily used for defects in load-bearing bones. Admittedly, many of these possess structured surfaces that provide bone cells with firm support. But the resulting bond remains delicate. Moreover, the traits of massive implants are different from those of the human skeleton: they are substantially stiffer, and, thus, carry higher loads. "The adjacent bone bears hardly any load any more, and even deteriorates in the worst case. Then the implant becomes loose and has to be replaced," explains Dr.-Ing. Peter Quadbeck of the Fraunhofer Institute for Manufacturing and Advanced Materials IFAM in Dresden. Quadbeck coordinates the "TiFoam" Project, which yielded a titanium-based substance for a new generation of implants. The foam-like structure of the substance resembles the spongiosa found inside the bone.

The titanium foam is the result of a powder metallurgy-based molding process that has already proven its value in the industrial production of ceramic filters for aluminum casting. Open-cell polyurethane (PU) foams are saturated with a solution consisting of a binding medium and a fine titanium powder. The powder cleaves to the cellular structures of the foams. The PU and binding agents are then vaporized. What remains is a semblance of the foam structures, which is ultimately sintered. "The mechanical properties of titanium foams made this way closely approach those of the human bone," reports Quadbeck. "This applies foremost to the balance between extreme durability and minimal rigidity." The former is an important precondition for its use on bones, which have to sustain the forces of both weight and motion. Bone-like rigidity allows for stress forces to be transmitted; with the new formation of bone cells, it also fosters healing of the implant. Consequently, stress can and should be applied to the implant immediately after insertion.

In the "TiFoam" project, the research partners concentrated on demonstrating the viability of titanium foam for replacement of defective vertebral bodies. The foam is equally suitable for "repairing" other severely stressed bones. In addition to the materials scientists from the Fraunhofer institutes IFAM and IKTS -- the Institute for Ceramic Technologies and Systems in Dresden -- physicians from the medical center at the Technical University of Dresden and from several companies were involved in developing the titanium foam. Project partner InnoTERE already announced that it would soon develop and manufacture "TiFoam"-based bone implants.