Melt processing and characterisation of lightweight metal composites reinforced by nanocarbon

Lead Research Organisation: Imperial College London
Department Name: Dept of Aeronautics

Abstract

This research aims to prepare a new generation of high performance lightweight alloys, to fulfil the perpetual need of the aerospace and automotive industries for materials with enhanced specific properties. Magnesium alloys reinforced by multi-walled carbon nanotubes (MWCNTs) will be fabricated using melt processing routes that are compatible with mass production.
AZ91 is the most commercially available and commonly used magnesium alloy that has a high strength to weight ratio, fluidity for casting and recyclability. However, AZ91 is also known for its poor high temperature strength and creep resistance (time dependant deformation under load at an elevated temperature), that has limited its use in the automotive and aerospace industry. Increasing the applicability of magnesium alloys by increasing their strength and high temperature strength could lead to a step change in light weighting both land and aerial vehicles. For example, if the relevant properties of AZ91 could be increased to match that of today's aluminium or steel alloys, a 30 and 70% weight saving, respectively, could be realised. Adding nanocarbons to lightweight metals, to form metal matrix nano-composites (MMnC), has been shown to significantly increase mechanical properties such as specific strength, hardness and interesting, ductility which is typically a mutually exclusive property.
Carbon nanotubes (CNTs) have extraordinary mechanical, thermal and electrical properties. The high stiffness and strength of CNTs can be attributed to the sp2 carbon-carbon bonds and it is considered to be one of the strongest bonds in solid materials. They also have a high surface area, high aspect ratio and a low density, making them ideal reinforcements. Nanocarbons, however, are notoriously difficult to disperse homogenously in a solution, and indeed a metal matrix, due to strong inter-tube interactions that lead to large agglomerations being formed. The nanocarbon agglomerates can significantly reduce the positive effect of the strengthening mechanisms they offer due to tube sliding and acting as crack initiation sites, preventing current MMnCs from reaching their theoretical strength potential.
Ultrasonic treatment (UST) is widely used to disperse CNTs in solvents, due to high shear stresses generated by a 'sonotrode' probe that vibrates and introduces powerful sound waves into the liquid. The high-pressure cycles from the sound waves form micro sized bubbles that either float up and out of the liquid or implode generating shockwaves and high-speed jets. Additionally, UST is already used in industry for its beneficial effects during metal melt processing that include degassing the melt, thereby reducing porosity of casts, and assisting crystal nucleation that decreases grain size, improving material properties.
A fabrication process will be developed using a furnace facility with in-situ melt stirring and UST capabilities has been installed at Imperial College London to fabricate nanocarbon reinforced magnesium alloys. As UST is widely used to disperse CNTs in solvents, this research will use UST in combination with melt stirring to disperse CNTs in molten metal. It will also investigate the use of surface treated nanocarbons, that aims to promote a stronger interaction between the metal matric and nanocarbon interface, that will aid dispersion and wettability. An understanding of dispersion and active strengthening mechanisms will be formed through thorough characterisation of mechanical properties and microstructure.

Publications

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Studentship Projects

Project Reference Relationship Related To Start End Student Name
EP/N509486/1 01/10/2016 31/03/2022
2092974 Studentship EP/N509486/1 01/04/2018 30/01/2020 Daniel Markcoons