title: Optimisation of the selective laser melting process for the production of hybrid orthopaedic devices
Creators: name: Robinson, Joseph
Publisher: name: University of Liverpool Repository
Year: 2014

description: This thesis details investigations of residual stress in selective laser melting (SLM). SLM is an Additive Manufacturing (AM) process that builds parts by melting consecutive powder layers using a fibre laser. Residual stresses caused by high thermal gradients create deformation leading to cracking of the final components. This deformation and cracking limits the geometries that can be built and the materials from which they can be manufactured using the process. Previous research on residual stresses has shown contradictory results mainly due to the differences in measurement methods, processing parameters and materials used. In order to address this shortcoming this study focused on the use of Titanium and its alloys for the production of medical devices. The residual stress in final components was measured using several methodologies: deflection, hole drilling and EDM cutting followed by FEA (the contour method). These measurement methods allowed the comparison of commercially available scanning strategies to be investigated. Results showed that the chequerboard technique commonly cited in the literature as reducing residual stress had little benefit over the use of more standard rastered vectors scanned orthogonally to the previous layer vectors. Using this suite of techniques the principal residual stress was determined to be parallel to the scan vectors, contradicting a number of previous studies. A simple finite element model was developed enabling the comparison of measured profiles with analytical results. This model was then extended to allow the evaluation of new techniques aimed at reducing the levels of residual stress. Further experimentation showed that the use of increased bed temperatures reduced the residual stress in components even at small increases in temperature. Hatch angle rotation as a method for increasing part quality was also tested. Eighteen angles, specifically chosen, using analytical models were investigated to define the optimum angle. No statistically significant difference was found in density, surface finish or strength for any of the tested angles. To minimise residual stress it was concluded that unidirectional scan vectors should be avoided and that there was little difference between the other rotation angles. In order to measure precisely when and where the residual stresses were generated in the process an experimental apparatus was designed which allowed in-situ measurements of stresses and provided an understanding of the transient stresses in components as they are built. This residual stress dynamometer (RSD) offered state of the art spatial and temporal resolution. This experimental equipment allowed the conclusions drawn from the previous post process techniques to be confirmed on a layer by layer manner SLM has also been shown to be a viable technique for the production of hybrid orthopaedic devices that encompass both porous and solid volumes, this work considered the effect that optimisations on the solid volume of the part, to remove residual stresses, would have on the porous volumes. Techniques were developed which made the porous structures less sensitive to part orientation through the removal of broken links at the surface. Further additional features where then added to improve the roughness of the surface to increase initial fixation of an implant.