Electron Beam Melting with Titanium Alloy for Orthopaedic Applications

Abstract

Electron Beam Melting (EBM) can create near 100% dense complex parts in Ti-6Al4V(Ti64) ELI, suitable for use in medical applications. While EBM Ti64 ELI currently meets the standards for surgical implants there are still challenges facing the technique with graded microstructures, anisotropic mechanical properties and defects/porosity found within manufactured products. The principal aim of the research reported in this thesis was to develop understanding of these issues in order to inform design and quality control practices in the production of representative orthopaedic medical device using EBM. A max volume Triflange revision hip prosthesis was chosen as it acts as a worst-case scenario using the EBM additive manufacturing (AM) method. Initially, a test specimen build was designed to analyse the effects of position within the build chamber, orientation, and support structures on the material properties of EBM Ti64 ELI. These build design variables were then correlated to Ti64 ELI material and mechanical properties. As the build height increased α lath widths increased by 45%, while hardness and yield strength decreased by 7% and 10% respectively. There was also a clear indicator of anisotropic mechanical properties, with vertical specimens having a higher yield strength than their horizontal counter parts. These results were validated by a second test specimen build. A 2D finite element thermal model has been developed to model the complex thermal history of EBM Ti64 ELI. The model incorporates temperature dependent material properties, applies phase transformation during the melting and cooling of layers through a simplified heating mechanism and utilises the element birth and death technique to apply new powder layers. The results show rapid cooling of the layers (of the order of 105 °C/s) and a maximum temperature close to 2200°C. Once an understanding of the effects of build design variables was established, a representative revision hip prosthesis component was produced twice to check that the observations from the specimen part build applied when representative geometries were being produced. The results show a close correlation with the coupon build design results, with the same relationships and data set ranges through the build volume found.

This research demonstrates a set of clear relationships through the build height of the EBM volume on a specimen’s material and mechanical properties. These relationships can give rise to quick and effective quality control techniques utilising the correlation between α lath widths, hardness, and yield strength.