Micro-CT Based Morphological and Mechanical Characterisation of Open Porous Metallic Materials

Porous materials have very attractive properties because of their lightweight and unique geometry that lead to their shock or sound absorbing properties. Thus, porous materials are frequently used in many fields, such as lightweight sandwich manufacturing, packaging, crash worthiness and medicine. Most porous materials have a random morphology, however the request for porous structures with highly controlled properties, coming from different application areas within the industrial and scientific market, forced researchers to develop novel additive manufacturing (AM) production techniques like selective laser melting (SLM), selective laser sintering (SLS) or electron beam melting (EBM). A critical aspect in optimising these production techniques and their postproduction treatments is to allow control of the morphological and mechanical properties of manufactured porous structures on both meso- and microscale. Therefore, the main focus of this study was twofold, namely: i) optimisation of the porous structures towards novel engineering materials with fully customized morphological properties and ii) unravelling of the mechanical behaviour and failure of those porous structures subjected to the mechanical loading. Due to the large variety of porous structures, and their broad range of applications, this PhD study focuses on one specific production technique and material type. The prerequisite was that, within this specific manufacturing approach the porous structures with a well controlled macro-morphology can be produced based on the design input which can be modified according to the desired output. In practice a case study on open porous Ti6Al4V structures, produced by SLM, was performed.The dissertation consistsmainly of two parts: one on material functionalization (chapters 2 and 4) and one on material characterization (chapter 3, 5 and 6). In chapter 2 a protocol for the surface topology improvement has been developed and applied to Ti6Al4V porous structures produced by SLM. Topology changes were introduced by several surface treatments consisting of chemical etching followed by electrochemical polishing. In that way the surfaceirregularities, typical for porous structures manufactured by SLM have been eliminated.In chapter 3 a novel tool for roughness measurements has been developed and validated for a quantitative characterization of the surface topology. For the first time, the micro-computed tomography (μCT) has been applied for quantification of the materials’ surface texture. Validity of this surface roughness analysis has been given by comparison to physical roughness measurementsperformed by conventional systems showing that the novel μCT image based tool for surface roughness analysis can be applied for quantitative surface characterization.The unique properties of porous structures strongly depend on the morphological properties, thus their thorough characterization is required. Therefore, in chapter 4 a relationship between thestructural properties and the μCT based analysis of porous Ti6Al4V structures has been investigated to define the most optimal characterization conditions. In this study, a basic, but systematic protocol for determination of the best acquisition parameters such as spatial resolution has been developed regarding the μCT based morphological characterization of the complex porous structures. The findings of this studycan assist to increase the quality of 3D quantitative morphological analysis of any object in relation to its surface complexity as well to reduce the investigation time and costs by evolving towards a customised relationship of μCT settings versus morphological analysis level.In chapter 5, the surface modification protocol presented in chapter 2 has been developed further in order to manufacture Ti6Al4V porous structures with customized morphological properties. Application of the multi-factorial design of experiments led to a controlled, at both macro and micro level, morphological modification of the porous structures.This allowed to:i) eliminate the surface irregularities,ii) modify the surface roughness in a robust manner but, alsoiii) produce customized structures with desired global morphological propertiesAdditionally, the developed protocol can be applied for production of various porous structures with a final beam thickness that islower than the resolution of the selected manufacturing process. Finally, modification of the beam surface can be used for controlling the biological cell behaviour seeded on 3D porous structures. In that way, the most optimal surface properties for future designs and production of 3D structures for orthopaedic application can be looked for and validated experimentally.Finally, a proof of concept case study was performed by using an automated non-rigid image registration to assess strain through analysis of μCT images acquired prior-to and after compressive loading of SLM made Ti6Al4V open porous structures. Additionally, the evaluation of the potential and limitations of the proposed approach was assessed based on the simulated deformation of the phantom object. It was shown that the µCT based strain mapping, performed by combining the in-situ loading and non-rigid image registration of the µCT images, provides a valuable tool to identify and analyze the critical sections in the porous structure having a higher strain concentration, eventually leading to sudden failure. Additionally, the local strain analysis revealed larger strain concentrations at the beam geometry imperfections. Experiments with the phantom object confirmed potential of the proposed approach for the local strain analysis as the computed straincorresponded with the deformation artificially applied to the tested object. However, obtained strain results showed dependency upon the applied grid spacing of the B-spline transformation. Therefore, further development of the non-rigid image registration approach as a tool for local strain analysis is required, although a qualitative analysis of the localdeformation can already be performed to evaluate the volumetric changesin the porous Ti6Al4V structures.In conclusion, the work shows that combination of different tools was proven to be a valuable technique for thorough morphological characterization of complex porous structures, as well as their mechanical analysis. This resulted in production ofnovel porous Ti6Al4V structures with controlled morphological properties which can assist in more controlled evaluation of the combined effect of various functional properties. Furthermore, a novel characterization tool for surface analysis has been developed which can be beneficial forvarious research subjects dealing with surface engineering aspects.status: publishe