Porous Titanium by Electro-chemical Dissolution of Steel Space-holders

Among the various methods existing to fabricate porous titanium for structural or biomedical applications, the spaceholder approach is particularly useful as it allows a close control of pore size, shape and fraction. In this method, a blend of pure or alloyed titanium powders and space-holder materials are cold-pressed into a preform which is sintered in vacuum. During the early stage of sintering, the space-holder is removed by evaporation or decomposition, creating porosity in the titanium. Space-holders in powder form used to create titanium foams have included carbamide (urea), magnesium, ammonium hydrogen carbonate and polymers. There is considerable interest in creating titanium foams with elongated pores for applications such as filters, heat exchangers and biomedical implants. Elongated pores in titanium have been created by the anisotropic expansion of entrapped argon in solid titanium. Using the spaceholder method, polymer/titanium preforms were extruded, thus producing, in-situ, elongated space-holders. In another recently demonstrated method, the elongated spaceholder consists of ice dendrites which are formed in-situ by directional solidification of a water/titanium slurry and are sublimated before titanium is densified. Alternatively, if the space-holder can be drawn ex-situ into wires, these wires can be incorporated into the titanium powder preform in an initial step, before being removed by evaporation or decomposition during densification. This approach produces pores with regular shapes and has, to our knowledge, been demonstrated in titanium foams only with magnesium wires (180– 450 lm in diameter). Here, we examine steel as a novel space-holder for creating porosity in sintered titanium. This approach differs from the existing methods reviewed above because the steel spaceholders are removed after densification by electrochemical dissolution, rather than during or before densification by evaporation or decomposition, as in previous studies. Furthermore, steel is easier to shape in sub-millimeter wires than the Mg wire space-holders used in the only previous study using metallic wires. However, this approach must address the interdiffusion between Ti and Fe expected to occur during densification. Here, we demonstrate the creation of intricate pore shapes replicating faithfully steel spaceholders in the shape of both spheres and wires. We also show that Fe-Ti interdiffusion can be inhibited by the in-situ formation of a TiC reaction layer, and that, alternatively, if an interdiffusion layer forms, it can be removed by dissolution, resulting in increased porosity.