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Title: Mechanical and tribological properties of aluminium matrix syntactic foams manufactured with recycled aluminium smelter waste
Author: Mehta, Bhavik Ajaykumar
ISNI:       0000 0004 9358 7014
Awarding Body: University of Liverpool
Current Institution: University of Liverpool
Date of Award: 2020
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Aluminium smelter waste (ASW) is a big contributor to landfill. Recycling ASW and forming more useful products has been of great interest. This study explores the feasibility of using ASW particles, which has a porous structure, for manufacturing a lightweight composite material known as a syntactic foam. Metal matrix syntactic foams (MMSFs) have many attractive properties, such as high strength, low wear and high energy absorption, and are suitable for a wide range of applications. However, no study has been conducted on MMSFs with ASW as a filler material. This thesis investigated the mechanical and tribological properties of Al matrix syntactic foams manufactured by melt infiltration with ASW particles of three particle size groups, designated as Small (125-250 µm), Medium (250-425 µm) and Large (425-1000 µm), and compared the ASW syntactic foam samples with a syntactic foam made with a commercially available ceramic microsphere powder (E-sphere; 250-500 µm). Compressive strength and energy absorption properties of the syntactic foams were measured by both quasi-static and drop hammer impact tests. Under quasi-static loading, the strength increased as ASW particle size increased. The failure mode was a more ductile deformation in the Large syntactic foam sample and a more catastrophic brittle failure in the Medium and Small syntactic foam samples. Heating the Large ASW particles before melt infiltration showed a transition from ductile to brittle failure, because the strength of the Large particles increased upon heating. Heating the Small and Medium particles before infiltration had no significant effect, because their non-heated counterparts were small enough to be heated during melt infiltration. T6 treatment of the syntactic foam samples led to an increase in strength and energy absorption capacity, with no change in deformation behaviour. The Large syntactic foam sample showed a transition from ductile to brittle deformation under drop hammer impact, indicating strain rate sensitivity. The ASW syntactic foam samples showed a higher yield strength and energy absorption capacity than the Esphere syntactic foam under both quasi-static and drop hammer impact loading, mainly because the ASW particles have a lower porosity. Flexural strength and energy absorption were assessed by three-point bending and Charpy impact tests. In three-point bending, the crack propagation occurred through the interface between the ASW particles and Al matrix and deviated away from the point of loading as particle size decreased. The flexural strength and energy absorption decreased with particle size. T6 treatment led to significant increases in flexural strength and energy absorption because of the increase in hardness of the Al matrix. In Charpy impact tests, all samples showed a brittle failure, with the impact toughness decreasing with decreasing particle size. The crack propagated straight through the Al matrix and particles alike. ASW syntactic foam had inferior flexural strength and energy absorption compared to E-sphere syntactic foam. Tribological properties were evaluated through dry and lubricated sliding wear tests. Under dry sliding conditions, the coefficient of friction (COF) had an initial sharp increase, followed by a decrease in gradient and finally a steady state as sliding distance increased. The wear surfaces showed presence of adhesive, abrasive and oxidative wear, with some presence of delamination. Small particle sizes led to decrease in average COF, increase in the amount of abrasive wear, decrease in surface roughness, and decrease in specific wear. Heating the Large ASW particles enhanced overall wear properties of the resultant syntactic foam. T6 treatment enhanced the wear properties due to the hardening of the Al matrix. The average COF of the ASW syntactic foams was higher than that of the E-sphere syntactic foam, which had predominantly abrasive nature of wear. The specific wear of the ASW syntactic foams can be higher or lower than the E-sphere syntactic foam, depending on the ASW particle size. Under lubricated sliding conditions, the wear type changed from predominantly adhesive to predominantly abrasive due to the lubricating effect, which led to an enhancement of wear properties when compared to the dry sliding wear behaviour. The porous particles acted as lubricant reservoirs and provided a constant supply of lubricant. The lubrication had a more significant effect on the wear behaviour than the sample characteristics.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral