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Part 1
Magnesium alloys, as the lightest of all structural metallic materials, and Aluminium-based Particulate Metal Matrix Composites (PMMCs), offering a unified combination of metallic properties and ceramic properties, have attracted increased interest mainly triggered by the never ending demands for improved performance and cost reduction in the automotive, aerospace, electronic and recreation industries. Current processing technologies for PMMCs do not achieve a uniform distribution of fine-sized reinforcements and produce agglomerated particles in the ductile matrix which are detrimental to the ductility. At the same time, molten Mg alloys contain impurities and oxides and when cast conventionally, the final components usually exhibit a coarse and non-uniform microstructure, containing various casting defects and severe chemical segregation.
A grand challenge is to develop solidification processing technologies which can ensure a fine and uniform dispersion of any solid particles present on the melt, resulting in microstructures free of cast defects, so that the cast products can be either directly used in the as-cast state, or only require minimal thermo-mechanical processing. The key idea has been to adopt a novel melt conditioning process allowing the application of sufficient shear stress (τ) that would disperse particles and create uniform temperature and composition fields. The Melt Conditioned High Pressure Die Casting (MC-HPDC) process has been developed, where intensive shearing is directly imposed on the alloy melt which is then cast by the conventional HPDC process and is expected to offer unique solidification behavior, and an improved fluidity and die-filling during the subsequent HPDC process. This talk will present the effect of intensive shearing on the solidification behavior, microstructures and mechanical properties of Al-SiC PMMCs and Mg alloys. The potential of the MC-HPDC process as a physical route for the recycling of Mg-alloy scrap will also be discussed.
Part 2
Transcatheter aortic valve implantation (TAVI) eliminates some of the main risks associated with invasive surgical operations, hence representing a potential transformative step towards a more sustainable healthcare. It is, however, at an early stage of development and substantial procedural and design improvements are still required to enhance the safety and effectiveness of the treatment. A novel aortic valve suitable for TAVI is being developed at UCL, aimed at overcoming the main limitations experienced with currently available percutaneous devices. The valve consists of three polymeric leaflets attached to a self-expandable Nitinol wire stent. The stent design provides a high expanded/collapsed diameter ratio and enhanced anchoring through sufficient radial and axial forces. The device is fully retrievable and repositionable. The material selected for the leaflets is a biocompatible polymeric nanocomposite recently developed and patented at UCL, which exhibits superior mechanical and surface properties and higher resistance to calcification compared to other polymers experimented in heart valve applications. Moreover, the use of synthetic materials enables the reduction of the leaflet thickness, the use of consistent manufacturing techniques and eliminates the problem of the tissue dehydration in the crimped state. Prototypes of the device have been produced and their hydrodynamic performance is being assessed in a specifically adapted hydro-mechanical cardiovascular pulse duplicator system, reproducing physiologically equivalent aortic pressures and flows.The preliminary in vitro tests confirm the indications obtained from the numerical analyses, suggesting that the proposed device could contribute to the transition towards the adoption of TAVI procedures. |
