Synthesis, Microstructure and Mechanical Properties of Titanium with Controlled Levels of Porosity
Porous titanium (Ti) products have many applications due to their excellent corrosion resistance, high strength-to-weight ratio, high specific surface area, etc. However, the major limitation is the expensive cost of porous Ti products because of the high raw materials costs and the difficulty of processing. The novelty of this research work is concerned with a study of a ceramic casting technique, slip casting, to create porous Ti products and the effect of porosity on the mechanical properties and gas permeability of Ti compacts. An optimised Ti slurry was developed from 43 vol.% of Ti powder, 0.3 dw.% of dispersant, 0.8 dw.% of plasticiser and 0.8 dw.% of binder, mixed with a balance of distilled water, which produced a viscosity of 290 cP. It was then poured into a plaster mould to form compacts with a green density of 45%. Thermal debinding was carried out at 320°C with an argon flow for 2 hours, followed by vacuum sintering different samples at 1000°C and 1200°C for 0.5 hours, respectively. The porous sintered compacts had satisfactory tensile strength with some plastic deformation. The increase in oxygen and carbon content during processing was minor. The results from this investigation suggested that slip casting is a potentially low-cost, simple manufacturing route for porous Ti products. Porous Ti compacts with porosity in the range of 12.3 vol.% to 35.3 vol.% were fabricated by slip casting. The mechanical properties, fracture morphology, gas permeability, pore size analysis and pore shape factors for the porous Ti compacts were determined for different porosity levels. A decreasing porosity level resulted in less open porosity and gas permeability, reduced pore size, and an increased tensile stress and elongation. The mechanical properties of porous Ti compacts produced by slip casting were comparable with more conventional press and sintered materials at the same porosity levels. Theoretical models for tensile stress and ductility as a function of porosity were examined and incorporated into the results and differences between the theoretical models and experimental results are discussed. The pore shape factor analysis showed that tensile loading would stretch the pores in the compacts to produce more irregular pores, which were acting as linkage sites to allow the propagation of cracks. Additionally, a novel interconnected pore characterisation method using ammonium meta-tungstate solution is presented. By using backscatter scanning electron microscopy, the interconnected pores can be directly observed. The packing behaviour and sintering behaviour of Ti compacts fabricated with different particles size distribution were characterised and studied, in terms of porosity, pore size analysis, tensile properties and gas permeability. Two different particles size (avg. 14 μm & avg. 56 μm) of pure Ti hydride-dehydride powder were used in five volume proportions (20:80, 40:60, 60:40, 80:20 & 100:0). A theoretical model predicted that the green density reaches a maximum when the volume fraction of fine particles is 0.35. It was found that although experimental results showed similar behaviour there was no well-defined maximum green density. The sintered compacts showed that an increase in the volume fraction of fine powder particles reduced the porosity, permeability level and pore size, and increased the tensile properties. The relationship between permeability and porosity level was non-linear and this was caused by the differences in pore diameters in the compacts. The capillary tube model was used to discuss the relationship between the permeability, pore diameter and porosity. Using this information, a graded porosity compact was designed and fabricated by slip casting. A future study on this graded porosity compacts is recommended to undertake in more depth.
Titanium, Porosity, Slip Casting, Sintering, Permeability, Mechanical Property
Networked Digital Library of Theses and Dissertations