A Novel Ceramic Precursor Route for the Direct Production of Hierarchically Structured Titanium Alloy Foams

Abstract

Titanium alloys find extensive use in the biomedical field, including applications in the form of a porous structure as a scaffold material for bone repair. Scaffold materials have demanding mechanical and biocompatibility requirements, which vary depending on the orthopaedic application. These requirements are determined by both the porous macrostructure of the foams and the strut wall microstructure. Therefore techniques are needed to characterise these structural features and relate them to the mechanical and physical properties. In this thesis new methods were developed to both manufacture titanium alloy foams and characterise them. Non- destructive X-ray micro-computed tomography (μCT) methods were employed to characterise the pore and interconnect size. The pore and interconnect size dominates the flow properties (permeability) of open-foam structures. Thus, μCT data was meshed and computational fluid dynamics analysis was performed to predict permeability. Direct finite element modelling, continuum micromechanics and analytical models of the foam were employed to characterise the elasto-plastic deformation behaviour. Pore anisotropy was quantified and related to the yield stress anisotropy, allowing identification of initial pore collapse. These results were validated against experimental measurements. Finally, the conventional production method of porous titanium is achieved through a costly multi-step powder metallurgical (PM) route. A new, potentially low cost, method was developed to produce porous titanium with properties similar or better than the existing titanium foam from a ceramic precursor via an electrochemical route. Two steps were involved: (1) preparing the ceramic precursor foam via a gel-casting route; and (2) reducing the oxide electrochemically via the FFC (Fray, Farthing and Chen) Cambridge process. The results of this preliminary study are very promising, with the foams produced via this method demonstrating mechanical and physical properties comparable to conventionally manufactured foams.