Year

2017

Degree Name

Doctor of Philosophy

Department

School of Mechanical, Materials, Mechatronic and Biomedical Engineering

Abstract

Fabricating metal matrix composites with enhanced mechanical and physical properties are essentially related to the achievement of uniform distribution of nanoparticles within the metal matrix. This is a challenging task because these particles are susceptible to agglomeration during most fabrication processes which in turn diminish their surface energy.

This study presents a unique method for uniform dispersion of SiC nanoparticles within aluminium matrix by manipulating the ball milling speed and milling time to stimulate the formation of aluminium flakes during ball milling. This new flake metallurgy includes dual-speed ball milling process to take the advantages of low-speed and high-speed ball milling (LSBM and HSBM) processes. The achieved results have shown that by lowering the rotation per minute (RPM) during ball milling process, the formation of aluminium flakes is stimulated, thereby prompting the uniform confinement of these nanoparticles between the produced flakes. This, in turn, can impose more restriction on the grain growth of aluminium matrix during hot isostatic pressing process, prompting formation of low-sized grains instead of abnormal grain growth seen normally in the composites suffering from severe agglomeration of nanoparticles.

Two different forming processes were implemented to make the manufactured powders to produce the bulk composites. These forming methods are hot isostatic pressing and thixoforming processes. The obtained results have shown that utilising these processes to deform the powders produced by flake powder metallurgy can confer higher tensile properties especially tensile elongation on the produced composites compared to samples produced by deformation of the ball milled powders manufactured using conventional ball milling processes. It has been resulted from the lower crystallite size and porosity accompanied by uniform distribution of SiC nanoparticles within aluminium matrix in the composites manufactured using flake powder metallurgy route. The effect of flake powder metallurgy was significant in rendering the uniform distribution to the SiC nanoparticles that even crystallite size of thixoformed samples was restricted to the values lower than 200 nm which is significantly apart from 500 nm reported for hot isostatically pressed samples.

XRD results have also shown that the nanoparticles uniformly dispersed within the powders produced using the flake powder route have a profound effect on demining the crystallite growth in the hot isostatically pressed and thixoformed samples compared to the ones manufactured using conventional ball milling processes. The effect(s) of these uniform dispersed nanoparticles within powders produced using flake powder metallurgy route was significant even though crystallite size of the hot isostatically pressed and thixoformed samples to the value around 500 and 200 nm for hot isostatically pressed and thixoformed samples, respectively.

Results of the tensile tests implemented on the produced composites have shown that the flake-assisted thixoforming process has potential to produce the aluminium matrix composites with the enhancement in yield strength (12 %) and tensile strength (32 %) compared to the hot-pressed ones. This has also been confirmed with the results of the micro-hardness test applied on the produced composites, demonstrating the higher hardness for the composites containing uniform distribution of SiC nanoparticles. To delineate the main mechanism by which the uniform dispersion of SiC nanoparticles can render significant enhancement in tensile properties of the produced composites, a numerical strengthening model was built based on the Orowan, load bearing and thermal enhanced dislocation strengthen mechanisms. The devised numerical model has also shown the significant effect of the uniform distribution of SiC nanoparticles in enhancing the yield strength of the thixoformed samples compared to the hot isostatically pressed ones. Since the devised model was not highly successful in precise approximation of the tensile properties of the manufactured composites, the artificial neural network (ANN) was trained using the input data such as the ball milling time and speed, volume content of stearic acid and SiC nanoparticles to predict the tensile properties. Achieved results have shown that the devised ANN model has a good capability in envisaging the tensile properties due to the mean absolute percentage error lower than 0.9%, and correlation coefficient over than 0.97.

Additionally, it has been shown that the thermal conductivity of the composites containing uniform distribution of SiC nanoparticles is higher than those suffering from severe agglomeration of nanoparticles. This is attributed to the profound effect of agglomerated SiC nanoparticles in scattering the heat flow and higher porosities of the sample containing agglomerated SiC nanoparticles such as thixoformed and hot presses samples produced using ball milled powders. The thermal conductivity results have also shown that thixoformed samples have higher thermal conductivity compared to the hot isostatically pressed samples, attributed to the lower porosity content of the thixoformed samples. This study suggests more studies regarding the effect of nanoparticles in restricting the grain growth during reheating processes and manipulating other processing parameters rather than ball milling speed to achieve the flake powder morphology.

FoR codes (2008)

0910 MANUFACTURING ENGINEERING, 0913 MECHANICAL ENGINEERING, 091202 Composite and Hybrid Materials

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Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.