Research On Preparation And Properties Of Porous Titanium And Its Alloys

Metallic foams are novel materials with extremely low densities and unique combination of excellent mechanical, thermal, electrical and acoustic properties. Due to the unique combination properties of porous materials and titanium alloys, the titanium-based porous materials can be used for structural and funtional applications. The potential application fields include catalyst substrate, sandwich core for aerospace vehicles and exchanger at elevated temperature up to 400℃. Furthermore, owing to the good corrosion resistance and wonderful biocompatibility, the porous titanium and its alloys can be used as medical implants, such as bone substitution to provide the porous structure to assist the growth of bone tissue. However, application fields of them are confined within some special areas until now because of economical or technical problems. And some research results only work in lab scale. So it's important to make a fully understanding of these materials including preparation method, structure analysis as well as prediction of mechanical properties.In the present study, the powder metallurgy with space holder technique was used to produce porous titanium and its alloys with controlable porosity and stucture. Porous titanium with the porosity in the range of 55%~75% was fabricated by changing processing parameters. The orthoganal experiment was chosen to investigate the effects of manufacturing parameters on porosity. The most important factor for porosity is compaction stress, and the following is the size of space holder material. The porosity decreases with reducing of space holder size or increasing of the content of binder and sintering temperature. Initially, increasing the compaction stress can lead to lower porosity; however, the porosity will come up again with higher compaction stress.The image processing software Image-Pro Plus was used to identify the structure feature, such as pore size, pore wall and pore shape. And the results indicate that mean pore size is 410μm, thickness of the pore wall and the mean sphericity is about 100~200μm and 0.72, respectively.The orthoganal experiment was also used to study the effects of manufacturing parameters on mechanical properties. The Young's modulus and plateau stress are in the range of 1.38-6.90GPa and 4.51-24.70MPa, respectively. Both of them increase with decreasing of porosity. The influence of different parameters on Young's modulus is not so obvious. While the plateau stress rises and then decreases with compaction stress, which behaviors oppostie to the effect of porosity.Because of the pressure loss, the density distribution of the compact is un-uniform, so the strength of porous titanium has the same distribution with density. As a result, the typical repture section of compressed samples has inverse V-shape.The effects of alloys on microstructure and properties of porous titanium were also studied by adding iron and nickel. The matrix composition of porous titanium-iron alloy isα-Ti phase and eutectoid ofα-Ti andβ-Ti. The volume fraction of eutectoid increases with the addition amount of iron. There are two kinds of effect which dominate the mechanical property, one is second phase strengthening and another is pore weakening on the pore wall. The Young's modulus increases from 3.01GPa to 4.27 GPa and 4.11GPa when adding 2wt.% and 4wt.% Fe, respectively, while the strength maintains at about 72MPa. When adding 6wt.% Fe, the Young's modulus and strength are much lower bacause of more pores formed by Kirkendall effect. Nickel playes a key rule as a kind of grain refining element for porous titanium. The eutectoid is formed when adding small amount of nickel, while intermetallic compound is formed when adding 6wt.% nickel. The compression strength are increased from 72 MPa to 112.59MPa and 96.31MPa, respectively, while the Young's modulus are 4.26GPa and 3.73GPa when additions are 2wt.% and 4wt.%, respectively. However, the compression strength and Young's modulus can be deteriorated if adding 6wt.% nickel.Sintering mechanisum were investigated by adding iron and nickel. According to the expansion/shrinkage curves of porous titanium, Ti-Fe and Ti-Ni alloys, the sintering process can be separated into two parts. At low temperature sintering, sintering rate is reduced by adding iron and nickel. While at high temperature sintering, iron and nickel can accelerate the process significantly. However, the sintering mechanism can not be explained by only one mechanism because the sintering process is affected not only by particles themselves but also by porous structure.Furthermore, different porous structures were modelled to investigate their effects on effective properties. Finite element method is uesd to predict effects of porous structure on effective Young's modulus. The Young's modulus increases as the relative density increases. Crack-like pores penpendicular to the loading direction have the greatest effect in reducing the modulus. Moreover, the random distributions of pores decrease the moduli as well. According to the real structure of porous titanium (alloys) made by space holder technique, two kinds of model-3D porous model and 3D powder model are used, as well as three different spatial distributions of pores are considered: simple cubic (SC), face-centred cubic (FCC) and body-centred cubic (BCC). The SC array typically yields the highest modulus, while the rest two appeares similar. Changing from spherical pores to ellipsoidal pores reduces the modulus, in accordance with the predictions in 2D model. For porous model, power law relation can be used to fit all the relation between modulus and relative density. Different with porous model, powder models have relative density limitation obtained when there is no overlap between particles. The results indicate that modulus can be expressed by a linear function of relative density.Finally the predictive model for mechanical properties was proposed according to the current experimental data. Two-scale model is used by defining different variables: macro-relative density and micro-relative density. The results show that the micro-modulus fit very well with experimental data, because all the particles on the walls are roughly spherical and of similar size so the geometry is reasonably represented by the powder model. While the macro-modulus follows the same trend but lower than the simulated values, because the space holder technique generates a very random structure of different pore size, shapes and distributions within the material.The results of stress distribution in the material with non-uniform density indicate that the stress distribution doesn't rely on density distribution. In this case the fracture of material only depends on their own strength.