Abstract
D.Phil. (Mechanical Engineering Science)
Metal-matrix composites have received attention, owing to their attractive properties, including their good strength-to-weight ratio, adequate wear-and-corrosion resistance, amongst others. The introduction of Metal matrix composites has been very beneficial to the material-selection process. Among the identified base metals for metal-matrix composite applications, aluminium is attractive, owing to its reduced weight, adequate stiffness and corrosion resistance. These improvements have led to its incorporation into several applications, particularly in aerospace, marine, and in other structures. The reinforcing phases of metal-matrix composites are responsible for improving the properties of the monolithic material through several mechanisms, including load transfer, resistance to local application of the load, strong interfacial bonding and adequate wettability between the reinforcing phases and the matrix. The need for costreduction and environmental sustainability has led to the consideration of certain waste materials for the reinforcement of metal-matrix composites. The waste material currently incorporated into the production of aluminium-matrix composites are classified under agricultural and industrial waste. This work presents the use of fly-ash and eggshells, as reinforcements for the fabrication of binary and ternary aluminium-matrix composites. This investigation analysed the effect of the stand-alone reinforcements on the properties, including the microstructure, density, porosity, tensile strength, compressive strength, microhardness, and the corrosion resistance of aluminium. The second phase of the analysis studied the effect of both reinforcements on the aluminium-alloy matrix. This part of the study considered the effect of the hybrid reinforcements on the mechanical properties, the microstructure, vi corrosion resistance and wear resistance. The machinability studies of the fabricated cast composite were also considered. The machinability studies analysed the effect of the hybrid reinforcements on indicators, including surface roughness, tool wear, materialremoval rate, cutting temperature, built-up edges and chip morphology. Experiments were designed via the L9 and L16 orthogonal array, which selected the cutting speed, feed and depth-of-cut, as the input parameters for the machinability studies. In addition to conducting the experiments, single and multi-objective optimization processes, using Taguchi and Taguchi-based grey relational analysis, respectively was conducted on the tool wear, surface roughness, material-removal rate and the cutting temperature. The effect of fly-ash and carbonized eggshells on AA 1050 fabricated via stir-casting was conducted. The investigation of the density, tensile strength, micro-hardness, showed that the reinforcements improved these properties in comparison to that of the base metal. The corrosion-resistance analysis also investigated the two reinforcements, improved by reinforcing the aluminium alloy. The application of fly-ash and carbonized eggshells, as hybrid reinforcements on LM6, improved its tensile strength, compressive strength and micro-hardness, while the wear-and-corrosion resistance investigation also indicated that reinforcing the aluminium alloy with the hybrid reinforcement is beneficial in both regards. The machinability studies of the hybrid reinforcements on AA 6082 revealed that the cutting speed was the most influential factor in the surface roughness and on the tool wear, while machining the cast composite. It was also indicated that the increased surface roughness and tool wear were due to the presence of the hard-reinforcing particles in the aluminium matrix. The analysis of the material-removal rate and the cutting temperature showed that the depth-of-cut was the most influenced factor in both responses. The builtvii up edge formation during the machining of the cast aluminium-matrix composites was at a maximum at the lowest cutting speed and at the highest depth-of-cut. The analysis of the chip morphology indicates that the turning of the cast composites resulted in the formation of c- shaped chips, helically shaped chips and ribbon-shaped chips. From the study, it was concluded that reinforcing aluminium and its variants with fly-ash and carbonized eggshell – either as single reinforcements, or as a hybrid – is capable of improving its overall properties; and this could find a use in a variety of applications and highly recommended.