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.

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.

For the last decades usage of aluminium alloys have been increasing tremendously. They have been used in aerospace industry widely and now aluminium alloys are becoming more and more popular in automotive and defense industries. Consequently / successful welding of aluminium alloys gains importance. In this study a research is carried out on eldability of plates having different thicknesses of composition 5754 aluminium and 6063 aluminium in T-fillet geometry using Gas-metal Arc Welding technique. It was aimed to have a successful joint without using pre-weld and post-weld heat treatments. During tests welding current and voltage were the varying parameters as welding speed was held constant. Macro-examinations were performed to see the penetration of the weld metal. It was seen that the type of filler wire greatly effects weld penetration. Hardness tests, tensile tests were done to compare the mechanical properties of the welded joints with different filler wires. Despite having better penetration in 4043 filler wire used weld joints, 5356 filler wire used weld joints had higher tensile strength and ductility. In the second part of the study, a dynamic loading machine was designed and manufactured to see the behavior of the fillet welds under dynamic loading. The amount of stress and strain given to the specimen on this machine was adjustable but can&rsquo / t be measured. The tests that were made with this machine aimed only to compare the number of cycles of specimens before fracture. For dynamic loading tests two groups of specimens were prepared with filler wire 4043 / each group having been welded with different heat inputs. It was aimed to see the effect of welding heat input on service lifes but no significant difference between cycle numbers of specimen groups having been welded with different heat inputs was observed. Microstructure examinations of these specimens revealed that coarsening the grains, grain boundaries, particles in PMZ and HAZ regions between Al 6063 base metal and weld zone made these areas more susceptible and favorable for crack propogation than Al 6063 base metal.