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Aluminum - Silicon alloys are recognized as appropriate materials for high performance cast components used in transportation powertrain applications. A combination of excellent wear resistance, good thermal conductivity and low density make these materials good candidates for engine bore applications. It is well accepted that the tribological properties of these alloys are dictated by the presence of hard eutectic silicon particles and their distribution in the soft aluminum matrix. Three near-eutectic aluminum-silicon engine bore alloys manufactured by different processing routes such as sand casting, chill casting and spray compaction were investigated to determine the influence of solidification on evolution of microstructure of these alloys and to establish correlation of microstructure with tribological properties. The spatial distribution of the silicon particles in aluminum matrix is analyzed using various image analysis techniques and contact distribution studies. The chill cast alloy shows large columnar primary aluminum dendrites interspersed with coarse silicon particles. The sand cast and spray compacted alloys show better spatial distribution of refined silicon particles. Microstructures generated under different solidification modes are found to have varying morphologies. The crystallographic orientations of the dendritic and eutectic aluminum as well as that of the eutectic silicon were studied using electron backscatter diffraction (EBSD). The eutectic silicon nucleating in chill cast alloy is found to exhibit strong orientation relationship with the aluminum matrix. The crystallographic orientation relationship shows that the solidification modes of the eutectics in these three alloys are different, from alloy to alloy, due to their different solidification rates and due to the addition of grain refiners and modifiers. The hardness values of the aluminum matrix and silicon particles of these alloys were found using nanoindentation and micro indentation tests. Preliminary wear studies were carried out on etched and unetched test alloys in dry reciprocating sliding. The results show that of the three test alloys, the alloy in which eutectic regions nucleate heterogeneously from the primary aluminum dendrites gives the best wear resistance and the highest hardness. The very low friction coefficient recorded for the etched alloys is accounted for by the insitu formation of a thin sheet of tribofilm on the protruding silicon particles. The physical and chemical natures of this protective film are being investigated.

Lightweight aluminium-silicon alloy is an attractive material for making engine cylinders in automobiles. It imparts good power to weight ratio to the engine. High silicon containing aluminium alloys are used in current engine block castings where the bore surface is etched or honed to partially expose the silicon particles to provide the primary contact between the piston ring and certain regions of the piston and the cylinder. Piston reversal near the top dead centre however causes starvation of lubrication which leads to wear. To explore the wear behaviour of etched aluminium-silicon alloys under nominally dry conditions and extreme lubricated conditions, a host of mechanical and spectroscopic techniques are used here to characterize mechanical and chemical changes caused by wear. In the absence of complex chemical transformations on the wear surface in dry condition, allows a close examination of surface and subsurface microstructures. Given this understanding of the wear under dry condition, we explore the effect of boundary lubrication, where chemical transformations leading to surface modifications are involved. In dry sliding tribology of aluminium-silicon alloy slid against a steel ball four stages of wear are identified; ultra-mild wear, mild wear, severe wear and post severe oxidative wear. In the ultra-mild wear regime silicon particles bears the load. Transition to mild wear occurs when the protruded silicon particles disappear(by sinking and fracture) under higher pressure and sliding. The sinking of silicon particles under normal loading is further investigated using a naoindenter. It is found that the resistance to sinking of such particles into the matrix increases with the unexposed surface area to the buried volume of the particles. In that sense, small particles are seen to provide the stiffest resistance to sinking. While in ultra-mild wear regime the basic energy dissipation mechanism is sinking/tilting, in mild wear regime the subsurface is either in an elastic or an incipiently plastic state. Subsurface plasticity in mild wear regime leads to a grain refinement, fracture of silicon and nucleation of cracks at silicon-matrix interfaces but does not promote large scale flow of the matrix. Transition to severe wear occurs when the contact pressure exceeds the plastic shakedown limit. Under this condition gross plasticity leads to a severe fragmentation of silicon particles and the fragmented silicon are transported by the matrix as it undergoes incremental straining with each cyclic contact at the asperity level. A large reduction in the inter-particle distance com-pared to that in a milder stage of wear, gives rise to high strain gradients in the severe wear regime which contribute to the enhancement of dislocation density. The resulting regions of very high strains at the boundaries of the recrystallised grains as well as within the subgrains lead to the formation of microvoids/ cracks. This is accompanied by the formation of brittle oxides at these subsurface inter-faces due to enhanced diffusion of oxygen. We believe that the abundance of such microcracks in the near surface region, primed by severe plastic deformation, is what distinguishes a severe wear regime from that in the mild wear. The transition from severe wear to post severe oxidative wear is thermally induced and it transfers the metal to metal contact interaction to metal to ceramic interaction. A thick oxide layer is abraded and spalls while the metal underneath continues to flow and delaminate. A study of lubricated tribology of ultra-mild and mild wear regime of aluminium-silicon alloy shows that the initial stages of sliding friction is controlled by the abrasion of the steel pin by the protruding silicon particles of the aluminium-silicon disc. Thegeneration of nascent steel chips helps to breakdown the additive in the oil by a cationic exchange that yields chemical products of benefits to the tribology. The friction is initially controlled by abrasion, but the chemical products gain increasing importance in controlling friction with sliding time. After long times, depending on the contact pressure, the chemical products determine sliding friction exclusively. In the mild wear chemically induced low friction is achieved in short periods of time whereas in ultra-mild wear regime it takes very long time to reach this low friction state. While the basic dissipation mechanisms are the same in the ultra-mild wear and mild wear regimes ,the matrix remains practically unworn in the low pressure ultra-mild wear regime. In the higher pressure mild wear regime at long sliding times a small but finite wear rate prevails. Incipient plasticity in the subsurface controls the mechanism of wear.

This thesis describes the processing and characterisation of Al-70Si alloys manufactured by gas atomised spray forming at Sandvik-Osprey (Neath, UK) and Oxford University using a newly commissioned spray forming pilot-plant facility. Spray formed Al-70Si (CE7) provides an attractive balance of thermophysical properties making it suitable for thermal management applications. Microstructural characterisation of CE7 was conducted using optical microscopy, image analysis, electron probe micro analysis (EPMA) and electron backscatter diffraction (EBSD). Microscopy revealed an interpenetrating network microstructure consisting of fine, randomly oriented polycrystalline primary Si interpenetrated by large, α-Al grains devoid of eutectic Si. Mechanical testing and thermal cycling simulated a service environment and revealed for the first time crack initiation, growth and blunting mechanisms, the effect of intermetallic phases on the bulk mechanical properties, and anisotropy effects resulting from macrosegregation of Al during solidification. A relationship between the inter-phase interface length and the fracture toughness has been proposed and methods of interface length refinement have been investigated, including chill casting and spray forming. Spray formed CE7 modified with separate additions of B, P, P+Ce and Sr have been microstructurally and mechanically characterised and compared with binary CE7. While alloy additions were effective in refining primary and eutectic Si in chill cast alloys, spray formed alloys showed little change in interface length. Particle injection of Si-Al powder was effective in refining the scale of the spray formed microstructure, and improving mechanical properties. The deleterious effect of intermetallic phases on bulk mechanical properties has been demonstrated and highlighted the importance of melt cleanliness and materials control during manufacturing.

The compression behavior of a newly developed near eutectic Al-Si based cast alloy with three different microstructures has been investigated in the present work. Microstructures with modified and unmodified Si particles and matrix with different tempers are investigated. The main objective of this work is to understand the effect of heat treatment and modification on the fracture behavior of the alloy under compression. This alloy is subjected to compressive loading at different strain rates and temperatures during the operation of the engines. Hence, the effect of strain rates and temperatures is also considered. The compression tests are carried out at different strain rates from quasi-static to dynamic viz., 3*10-4 to 102/s and three different temperatures RT, 100°C and 200°C. Microstructure of the alloys studied predominantly consists of eutectic colonies of α-Al and Si with a few interspersed α-Al dendrites. Modified alloy has more globular Si particles than unmodified alloy. Heat treated alloys are found to have hardening precipitates S’ & Al7Cu4Ni and 3-7 atomic layer thick zones, which may be precursors to S’ phase. A variety of large intermetallics, viz., plate like particles Al4.5FeSi, Chinese script like particles Al19Fe4MnSi2 and bulky phase Al3NiCu are also observed in the alloys. Mechanical behavior of the alloys is found to be different for different microstructures. Modification improves strength and ductility. Heat treatment improves strength of the alloy at the expense of ductility. A transition in mechanical behavior is observed after a particular strain rate for all the alloys studied. This transition strain rate is dependent on heat treatment, Si particle size and temperature. This transition can be explained on the basis of dislocation-precipitate and dislocation-Si particle interactions. Work hardening behavior of the alloys depends on the matrix microstructure in the unmodified alloys, and both matrix and particles play a role in the modified alloy. A statistically robust quantitative micro structural analysis has been carried out after compressing the samples at various strain rates and temperatures. The unique contribution of this work is the understanding of combined effect of strain rate and temperature on Si particle fracture characteristics in the alloy with different microstructures. From the fracture characteristics of Si particles, it is concluded that both dislocation pile-up mechanism and fibre loading are responsible for particle fracture in the modified alloy, whereas the fibre loading mechanism alone is sufficient to explain the particle fracture characteristics in the unmodified alloy. Si particles in the modified condition are found to cleave along the lowest surface energy planes {112} & {110} and the particles with orientations {112} & {111} are more prone to fracture. In addition to Si particle fracture, elongated Fe rich intermetallic particles are also seen to show peculiar fracture behavior. The Al4.5FeSi intermetallics with (100) as the plane of the plate cleave along (100) planes. This is a novel finding in this work and could have immense implications on the role of Fe impurities in the fracture behavior of these alloys. Moreover, since these cleavage fractures are seen to be more than 200 microns in size (which implies that the real penny shaped crack would be even larger) their role cannot be assumed to be negligible, as was previously thought. The load sharing between the Al matrix and eutectic Si particles are simulated by microstructure based finite element modeling. The program OOF (Object-Oriented Finite element analysis) is used to generate the finite element meshes for real microstructures with different Si morphology. The experimentally obtained stress – strain properties of the alloy is given as an input to describe the plastic behavior of the Al matrix, in the finite element simulation. This analysis helps to understand the effect of particle size, shape, orientation & clustering and matrix temper on the stress transferred to the Si particles. Combination of Electron Back-Scattered Diffraction (EBSD) and frequency shift, polarized micro-Raman technique is applied to validate the stress states in Si particles with {111} orientations. The stress at fracture of Si particles is also estimated from Raman technique. Even though the alloys with different microstructures show different mechanical behavior, the sequence of fracture mechanisms is found to be same for all the alloys. The failure occurs in three stages: cracking of Si particles at low strains, micro-crack formation along the fractured particles, micro-crack coalescence and propagation leading to final failure. Thus, the proposed analysis links various deformation mechanisms ranging from nano precipitate-dislocation interactions to micro short-fiber theory of load sharing by eutectic silicon along with coupled effect of strain rate and temperature. In addition, negative strain rate sensitivity is also observed in the lower strain rate regimes (3*10-4, 10-3& 102/s) at RT and 100°C for all the three alloys, and serrated flow is also observed in the same strain rate and temperature regimes. Some of the features of serrated flow can be explained by the dynamic strain aging model and some other features by precipitate shearing.

Aluminium is one of the metals playing a prominent role in automobile industry after cast iron. Because of its light weight property and good mechanical properties. When aluminium reinforced with silicon carbide showing good tribological properties and improved strength. Aluminium silicon carbide needs some good wear and frictional properties to use it as break disc. Aluminium reinforced with 15% and 20% silicon carbide and casted in two different ways, liquid casting and stir casting. Four different composites are compared in this paper. Hardness test was carried out on the samples. Increase in the Vickers hardness with increase in silicon carbide reinforcement for both the castings is observed. Rockwell C hardness is showing decreasing trend with increase in SiC reinforcement. The scratch resistance of the surface under micro level was analysed with the help of nano scratch test. The SiC particles in the aluminium matrix are resisting the indenter from deep deformation of the surface. Frictional forces are dropped whenever the indenter met the SiC particles. In other cases, SiC particles are deforming the aluminium matrix in the form of broken particles. The plastic deformation of aluminium is observed, and material is piled up on sideways of groove at high load.Sliding wear behaviour of the composites are investigated by means of reciprocating pin on plate wear rig. The test was carried out at load of 20N for five different sliding duration. Aluminium with 20% silicon carbide of liquid casting is used as a base metal. The worn-out surface of the samples is analysed in SEM. The metallography of the worn-out samples is showing some deep grooves and abrasion of the material. Wear debris from both the surfaces are forming into a cluster of layers. These layers are protecting the surface from wear in some areas were observed. Composition of tribo layer formed during the test was investigated with the help of EDS analysis. The tribo layer are rich in aluminium and silicon elements because both the samples are made of aluminium silicon carbide.

Due to recent legislation aimed at reducing carbon emissions into the environment through weight reduction, the automotive and aerospace industries are using light alloys such as aluminium silicon (Al–Si) and aluminium zirconium (Al–Zr) instead of steel due to their excellent mechanical properties and low weight to strength ratio. In order to further improve mechanical and metallurgical properties in these alloys, grain refinement is usually used in industry. However, the current and most widely used grain refiner Al–5Ti–B is unable to refine Al–Si alloys with silicon content greater than 3 wt.%., and Al–Zr alloys due to poisoning of the refiner by silicon and zirconium. The Al–5Ti–B refiner also contains larger Al3Ti particles and agglomerates of TiB2 which affect its efficiency and suitability in industrial applications where thin sheets are required. In this study, a new technique which improves the microstructure and efficiency of the Al–5Ti–B refiner has been developed. This involves the reaction of potassium tetrafluoroborate (KBF4) and potassium hexafluorotitanate (K2TiF6) salts at shorter reaction time before ultrasonic processing during solidification. This leads to the formation of a new Al3Ti morphology and de-agglomeration of TiB2 particles which enhances its grain refinement efficiency by 20%. Secondly, through phase diagram analysis of Al grain refining systems and crystallography studies, it was observed that Al3Ti and Al3Nb display similar lattice parameters with atomic misfit of 4.2% and would undergo a peritectic reaction with α-Al at low contact angles. Based on this, and using the duplex nucleation theory and poisoning by Si and Zr, a new quaternary grain refiner containing aluminium, titanium, niobium and boron (Al–4Ti–Nb–B) has been developed. This novel grain refiner has been found to be efficient in Al–Si alloys and Al–Zr, both at laboratory and industrial scales, and to improve the mechanical properties of the alloys despite the presence of Ti in the alloy. It was observed that the addition of Nb to an Al–Ti–B system leads to the formation of solid solution phases of Al3Ti1-xNbx, Al3Nb1-xTx, and (Ti1-xNbx)B2 which prevents poisoning by Si and Zr. Experimental simulations showed that Al3Nb1-xSix rather than Ti(Al1-xSix)3 are formed in Al–Si alloys, and Al3(Ti1-xNbx) and (Al3Ti1-xNbx)B2 phases are formed in Al–Zr alloys rather than Al3(Zrx,Ti1-x), B2(Zrx,Ti1-x) or ZrB2 phases. A new grain refining mechanism, ‘The Quad Nucleation Theory’ based on four nucleation events in Al–4Ti–Nb–B has been proposed. Other newly developed quaternary and ternary novel grain refiners capable of refining aluminium silicon alloys are also presented in this thesis. This includes a novel method of refining Al–Si alloys using phosphorus and niobium.

Friction Stir Processing (FSP) has the potential for locally enhancing the properties of Al-Si alloy castings, for demanding applications within the automotive industry. In this thesis, the effect of FSP has been examined on three different cast Al-Si alloys:i) A Hypoeutectic Al-8.9wt%Si Alloyii) A Hypereutectic Al-12.1wt%Si Alloyiii) A Hypereutectic Al-12.1wt%Si-2.4wt%Ni AlloyThe influence of different processing parameters has been investigated at a fundamental level. Image analysis of particle size distributions and growth method of tessellation were used to quantify the level of particle refinement and the homogeneity of the second phase spatial distribution. Stop-action experiments were also carried out, to allow the microstructural changes around the tool during FSP to be studied. Two computer models have been explored, in order to predict the temperature distribution and the material flow behaviour. Furthermore, the stability of the microstructure of the friction stir processed material was studied after being heat treated at elevated temperatures. The changes in particle size and grain structure were examined, hardness measurements were taken across the PZ, and tensile testing were carried out at room and elevated temperatures.After FSP, the microstructure of the cast Al-Si alloys was greatly refined. However, differences in microstructure have been observed throughout the PZ, which tended to be better refined and distributed on the advancing side, than the retreating side of the PZ. Changing the processing parameters also influenced the size and spatial distribution of the second phase particles. By studying the changes in microstructure around the tool from the stop-action experiments, and comparing the results to the thermal distribution and material flow behaviour predicted by the computer models, it has been shown that the flow stress, pitch, and temperature of processing, all needed to be considered, when determining the effects that FSP have on the microstructure. FSP caused very little changes to the hardness of the material, while tensile properties were greatly improved, due to the elimination of porosity and refinement of large flawed particles. In terms of the stability of the microstructure after FSP, particle coarsening and abnormal grain growth has been observed during high temperatures heat treatment. Furthermore, the Al2Cu phase was found to dissolve into solid solution at elevated temperatures, so GPZs and solute clustering can then develop within the alloy during natural ageing.

Aluminium silicon alloy has been found to be advantageous in many automobile components like pistons, cylinders, brakes and clutches. The main objective in using these alloys is to obtain lightweight and low friction at a reasonable cost without sacrificing reliability and durability. Out of all the tribological components piston skirts, piston rings and cylinder liners, have to face the most hostile of environments in an internal combustion engine. Wear mechanism of these components have been identified as abrasion, scuffing and corrosion. Narrowing down the line of interest, cylinder wear is more important than ring wear to both the engine manufacturer and the user, as cylinders are more expensive to replace than piston rings. Wear of piston ring and cylinder combination have been studied using a wide range of techniques. It is difficult to predict the tribological performance of these parts in an engine, even with the most well designed laboratory tests, due to chemical, thermal and mechanical complexities in the operating environment. Therefore, a good correlation is sought from the wear behaviour of test bed engines and laboratory tests. This should form the basis of further development particularly in terms of efficiency, weight eduction and wear life improvement of the components. Many ASTM bench-wear tests are used to study wear, some of the common tests being ball-on-disc and pin-on-disc testing. From these tests, a large database of wear information can be achieved and they offer rapid and low cost means of comparison. The only drawback is that the real components are not tested. However, since the bench tests can never simulate the engine environment completely, engine tests are always required for final verification. This thesis work reports preliminary studies of machining damage and wear in actual engine bore to set a bench mark, followed by a set of unidirectional sliding bench tests to study the wear of aluminium alloy under lubricated conditions, to classify the different wear regimes in boundary lubrication zone under different pressure conditions, and to study the effect of a surface modification technique, etching, which improves wear properties. The investigation is divided into four parts. 1. Study of subsurface damage in an actual cylinder surface as introduced by prior machining and actual worn case: A study of the microstructure of bores, processed through a range of machining variables; feed and speed, are investigated in this part of the thesis. This work suggests that the first step of rough machining may be responsible for the microstructure of the finished bore even though subsequent processing steps are intended to remove all prior damages. This also includes some observations of worn surface of an actually run engine, locating the various worn spots and studying the cause of this damage 2. Bench wear test in pin-on-disc under dry and lubricated condition with varying load and lubricant: After setting a benchmark on wear in engine using actual worn cylinder bore, a set of bench tests were carried out on aluminium alloy. Here, steel pins are slid on aluminium silicon alloy discs in the boundary lubrication regime in the presence of one drop of oil. The effect of pure hexadecane and engine oil containing additives on friction and wear are analysed and the data are discussed in terms of the formation of a mechanically mixed layer at the interface. 3. Ultra-mild Wear in Lubricated Tribology of an Aluminium Alloy: To study the different wear regimes in boundary lubrication zone, flat faces of cylindrical steel pins were slid on an eutectic aluminium silicon alloy under lubricated condition in the 1-100 MPa mean contact pressure range and 0.2 m/s sliding speed. Two transitions in wear rate were observed, at 10 MPa and 70 MPa. The wear rate in the 1-10 MPa regime was found to be very small and within the measuring instrument resolution and also insensitive to contact pressure. The regime is designated ultra-mild wear. Lack of plastic flow, minimal fragmentation of silicon particles and the presence of undistorted voids on the fractured and unfractured silicon particles in the subsurface suggest that the state of stress in the near surface region is elastic. Contact mechanical calculations demonstrate that at contact pressures less that 13.7 MPa the system is likely to shakedown to an elastic state. 4.Ball-on-disc wear tests for etched and unetched samples: In the fourth part of the thesis, comparative studies have been done between the as polished and chemically treated samples. Formation of grooves in a ball-on-disc experiment is observed on etched and unetched flats as a function of normal load and sliding distance. The groove is initially formed by plastic flow, and then expanded by micro-abrasion as the ball continues to slide on the groove. However etching causes surface hardening of the alloy, but, more importantly, creates a surface topology that reduces the peak contact pressure, which inhibits further plastic flow in the subsurface.