Additive Manufacturing of Porous Titanium Structures for Use in Orthopaedic Implants
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This dissertation explores additive manufacturing of porous titanium structures for possible use as scaffolds in orthopaedics. Such scaffolds should be tailored in terms of mechanical properties and porosity to satisfy specific physical and biological needs. In this thesis, powder metallurgy was combined with additive manufacturing to successfully fabricate porous Ti structures. This study describes physical, chemical, and mechanical characterizations of porous titanium implants made by the proposed powder bed inkjet-based additive manufacturing process to gain insight into the correlation of process parameters and final physical and mechanical properties of the porous structure. A number of processing parameters were investigated to control the mechanical properties and porosity of the structure. In addition, a model was developed based on the microstructural powder compaction to predict the porosity as a function of the developed sinter neck among the particles during the sintering process. The produced samples were characterized through several methods including porosity measurement, compression test, Scanning Electron Microscopy (SEM), Energy-dispersive X-ray spectroscopy (EDX), and shrinkage measurements. Additionally, a new method for manufacturing Ti implants includes encapsulated networks of macro-sized channels was introduced. Also, the influence of different orientations and numbers of channels within the additive-manufactured structures were investigated. The characterization test results showed a level of porosity in the samples in the range of 12-43%, which is within the range of cancellous and cortical bone porosity. The compression test results showed that the porous structure’s compressive strength is in the range of 56-1000 MPa, yield strength is in the range of 27-383 MPa, and Young’s modulus is in the range of 0.77-11.46 GPa. This technique of manufacturing porous Ti structures demonstrated a low level of shrinkage with the shrinkage percentage ranging from 1.5-12%. Also, the experimental results demonstrated excellent agreement with the developed model. Moreover, the novel method of fabricating the encapsulated channel show a reduction in the shear strength to 24-30% that is advantageous for bone implants. The results demonstrate that the channel orientation in the structure affect the shrinkage rate in the parts with vertically orientated channels, in which a relatively isotropic shrinkage in vertical and horizontal directions is achieved after sintering.