Selective laser melting of novel titanium-tantalum alloy as orthopaedic biomaterial
Sing, Swee Leong
Date of Issue2016-08-08
School of Mechanical and Aerospace Engineering
Selective laser melting (SLM) is an additive manufacturing (AM) technique that is capable of fabricating complex functional three-dimensional (3D) metal parts of high relative density with the complete melting and fusion of powders. As a powder bed fusion technology, SLM has the potential in expanding the materials library by formation of alloys that were previously difficult to achieve from metal powder mixtures that can be customised according to the application requirements. Titanium-tantalum (TiTa) is a potential material for biomedical applications due to its high strength to modulus ratio. However, it is still not widely used due to the difficulties in obtaining this alloy. SLM is chosen as the method to form this alloy due to its versatility in processing metallic materials and good results obtained from commercially pure titanium (cpTi) and Ti6Al4V. This research aims to develop TiTa as a potential material for biomedical applications. The study also paves the way for better understanding and control of the SLM process in porous lattice structure fabrication through statistical modelling. Firstly, the TiTa formation is studied to understand the forming mechanism and the effect of SLM processing parameters on the resulting density and macrostructure of the parts. The processing window and optimised parameters based on optimum relative density achieved are then presented. Secondly, the resulting microstructure of TiTa parts was examined and characterised to facilitate the understanding of the SLM forming process. The SLM TiTa parts exhibited a microstructure characterised by homogenous β titanium and tantalum matrix with randomly distributed tantalum particles. Thirdly, the mechanical properties of SLM TiTa parts is benchmarked against the more commonly used cpTi and Ti6Al4V. The TiTa Young’s modulus is 75.77 ± 4.04 GPa and has yield strength of 882.77 ± 19.60 MPa, ultimate tensile strength of 924.64 ± 9.06 MPa and elongation of 11.72 ± 1.13 %. It is found that SLM TiTa parts have lower Young’s modulus and comparable strength to Ti6Al4V and cpTi. Fourthly, the effect of SLM processing parameters on lattice structure properties is statistically modelled and analysed using regression analysis and analysis of variance (ANOVA). It is shown that laser power and layer thickness are dominant factors in affecting the properties of the lattice structures. Lastly, the properties of SLM TiTa lattice structures are also benchmarked against lattice structures fabricated using cpTi and Ti6Al4V. With 59.79 ± 0.68 % porosity, TiTa exhibits an elastic constant of 4.57 ± 0.09 GPa and yield strength of 151.93 ± 4.04 MPa which provides a higher strength to elastic constant ratio when compared to Ti6Al4V and cpTi. The same conclusions are drawn for compressive and tensile properties. Cell culture study using osteoblast-like SAOS-2 cells found that SLM TiTa has similar biological response to cpTi and Ti6Al4V. These favourable findings ascertained the feasibility of SLM TiTa as a biomaterial and contributed to the scientific knowledge that SLM processed mixed powder can produce desirable materials for actual applications.