This thesis presents simulations of nanometre-scale ploughing friction and wear behaviour when a pyramidal diamond indenter is ploughed through the surface of bcc and fcc metals and semiconductors. Parallel molecular dynamics (MD) simulations of nanoindentation followed by nanoscratching using Newtonian mechanics have been employed to investigate the different friction mechanisms occurring at the atomic scale. Three models have been developed to carry out our investigations on nanofriction, namely the steady-state model, the spring model and the finite temperature model. Each model allows the study of distinctive aspects of atomic-scale friction. For instance, the steady-state model was employed to study the behaviour of the friction coefficient, contact pressure and scratch hardness of a silver surface as a function of depth. The effect of indenter orientation has also been investigated with results showing a diverse range of pile-up behaviour. The work material undergoes both elastic and plastic deformation during the scratching and we have studied these to analyse the origins of friction. The spring model and the finite temperature model have been employed to investigate the stick-slip phenomenon at a low temperature of 0K and at room temperature (i.e. 300 K), respectively. The dynamics of the indenter and the substrate, including the behaviour of the different forces in action and the coefficient of friction, at particular stick and slip events have been studied. The variation of the sliding speed and indentation depth and their effects on the occurrence of the stick-slip events has also been investigated. Some qualitative comparisons have been made between the results from the simulations and experiments where possible. Due to available computer power, feasible indentation depths and scratch lengths were an order of magnitude smaller than experiment, while simulation times were several orders of magnitude shorter. The MD simulations, however, gave a good description of nanoindentation and nanoscratching and correlated well with the experiments.

Selective Laser Melting (SLM) is a laser based Solid Freeform Fabrication (SFF process which uses laser energy to melt a thin layer of metal powder. This process is repeated to produce a 3-dimensional metallic part. SLM is capable of producing intricate parts which are otherwise difficult to produce with conventional manufacturing techniques. As compared to traditional manufacturing processes, SLM can also produce parts with higher density. Before a material is processed using SLM, suitable processing parameters are first identified. Over the years, different materials have been processed using the SLM process. However, very little work has been done on SLM of bio-compatible precious metals such as gold and its alloys. Gold and its alloys have been used for manufacturing of dental crowns for centuries. The SLM process could be used to produce intricate metallic substructures for porcelain fused to metal dental restorations. This research work was focused on understanding the processing parameters for SLM of 24 carat gold powder. The gold powder was analyzed for Particle Size Distribution (PSD), apparent density and tap density before identifying suitable processing parameters for SLM. The gold powder particles were found to be spherical in nature but smaller particles stuck to each other and formed larger powder agglomerates. From the apparentdensity experiments, the gold powder was found to be cohesive and non-flowing in nature which hindered powder flowability during the powder deposition process with the existing system. This issue was resolved by designing a new powder deposition system which could allow the gold powder to flow evenly over the substrate. The tap density of the gold powder was found by Constant Weight Tap Density (CWTD) and Constant Volume Tap Density (CVTD) techniques. The difference in results from these two techniques was negligible. Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) of gold powder showed it to be more than twice as reflective as other commonly processed metal powders such as stainless steel and H13 tool steel. This analysis proved useful in understanding the laser processing of gold powder. Due to the high cost and small quantity of material available for this work, a very small build platform was designed to optimise material utilisation and reduce wastage. Single scans were performed on a single layer of gold powder to identify the good melting region. Five different regions i.e. balling, good melting, unstable melt, weak sintering and very little sintering were observed in the processing window. The balling phenomenon was observed at low and high scan speeds, which was due to the melt pool instability at these parameter settings. The size of droplets (balling) also increased with decreasing scan speed and increasing laser power which was due to an increase in the break up time of the molten metal. In the good melting region, the gold powder was found to be completely melted and continuous beads were successfully produced. The unstable melt region showed the melt pool spreading unevenly in different directions whereas in the weak sintering and very little sintering regions the gold powder did not melt completely. Single layers were produced on a layer of gold powder, which showed the parameters in the good melting regions to be suitable for multiple layer parts manufacturing. Gold cubes were produced using the suitable processing parameters identified from single scan and single layer experiments and then analyzed for their internal porosity. The porosity in the gold cubes was found to be at a minimum for parameters obtained in the good melting region. The internal porosity was found to be mostly inter-layer porosity; this indicated less heat transferred to the region between the two layers which could be associated with the high reflectivity of gold. The inter-layer porosity in gold cubes was further reduced by reducing the layer thickness. This could be due to the thinner layers requiring less energy to melt and be fused to the previous layers. The hatch distance had a negligible effect on the inter-layer porosity of gold cubes. The reduction in hatch distance increased the energy delivered but it was still not enough to completely melt the gold powder and fuse it to the previous layer. A pre-scan technique was also tested to be used for pre-heating the powder bed. However, due to the rapid drop in temperature, this technique was not found suitable to be used as a powder bed pre-heating technique. The gold cubes were checked for their mechanical properties i.e. hardness and modulus. The hardness of gold cubes was found to be higher than expected for 24 carat gold. The modulus was found to be less than 24 carat gold. This variation in the mechanical properties of gold cubes could be due to the rapid heating and cooling of material during the laser processing or presence of internal porosity in these gold cubes. After single scans and single layers manufacturing, gold dental parts (premolar and molar) were also manufactured using the optimum processing parameters. These gold dental parts were also analyzed for their internal porosity, which was found to be less than that observed in gold cubes. This difference in porosity could be due to the difference in structure of gold cubes and premolar part, where the latter was a thin wall structure.

The demands for compact, light weight and Iow cost electronic products have resulted in the miniaturisation of solder interconnects to a sub-millimetre scale. With such a reduction in size, the solder joints cannot be assumed to behave in the same way as bulk solder in terms of reliability due to the fact that their material behaviours are influenced by the joint size and microstructure. The complexity of their reliability assessment is furthermore compounded by the demand for the replacement of traditional SnPb solder alloys with lead-free alloys, due to the presence of the toxic and health hazardous element (Pb) in the former alloy. However, these new lead-free alloys have much less history of industrial applications, and their material and reliability data is not as well developed as traditional lead-based alloys. In addition, most previous reliability assessments using finite element analysis have assumed a uniform distribution of temperature within the electronic assembly, which conflicts the actual temperature conditions during circuit operation. Therefore, this research was undertaken to analyse the effect of solder joint size on solder material properties from which material models were developed, and to determine the effect of an actual (nonuniform) temperature distribution in an electronic assembly on the reliability of its solder joints. Following a review of lead-free solders and potential lead-free alloys, lead-free solder microstructures, and the reliability issues and factors affecting the reliability of solder joints, the practical aspects of this research were carried out in two main parts. The first part consisted of substantial work on the experimental determination of the temperature distribution in a typical surface mount chip resistor assembly for power cycling conditions, and the stress-strain and creep behaviour for both Sn3.8AgO.7Cu solder joints and reflowed bulk solder. This also included building material models based on the experimental data for the solder joints tested and comparison with that for bulk solder. Based on the comparison of the material properties, two extreme material models were selected for the reliability study. Size and microstructure effects on the solder material properties were also discussed in this part. The second part comprised of extensive finite element analysis of a surface mount chip resistor assembly and reliability assessment of its solder joints. The simulation began with elasto-plastic analysis for 2D and 3D chip resistor assemblies to decide upon the kind of formulation to be used when the full complexity of both plasticity and creep is considered. The simulation was carried out considering the determined non-uniform temperature distribution and idealized or traditional uniform temperature condition. The solder joint's material properties were modelled using the two material models determined from the experimental results. The effect of temperature distribution during thermal cycling and of the selected material models on the solder joint reliability was demonstrated using finite element analysis and subsequent fatigue life estimation. In summary, this research has concluded that the material behaviour of the solder joint is different from that of bulk solder due to the effect of its size and microstructure. The anisotropic behaviour of the solder joint cannot be ignored in reliability studies, since it has a significant effect on the solder joint's fatigue life. The research also showed the significant effect of an actual (non-uniform) temperature distribution in the electronic assembly on the solder joint fatigue life.

Mathematical models have been devised expressing the levels of controllable welding factors as a function of the joint geometry, such that acceptable weld beads are produced. Weld beads were required to be both geometrically acceptable and mechanically sound despite changes in the root face thickness (0.5 mm - 2.5 mm) and root gap (0 - 1.5 mm). The equations are intended to form part of an adaptively controlled robotic arc welding system. Simulation was used to develop the adaptive expressions. The study was applied to the root weld bead of the closing seam of railway bogie side frames fabricated from structural steel. The self shielding flux cored electrode arc welding process was used to weld single J preparations orientated in the horizontal-vertical position. Single sided full penetration welds with underbeads were required. The weld bead geometry was defined in terms of ten responses. Mathematical models derived from factorially designed experiments were used to relate the weld bead geometry, incidence of porosity and the occurrence of electrode stubbing to a function of upto seven factors. A data base of almost 1000 test welds was generated, in which each test was characterised by 76 pieces of information. Analysis of variance was used to determine which factors most influenced each of the responses. Multiple regression enabled an expression for each response to be derived as a function of the welding factor levels. The weld bead geometry model consisted of ten equations, each a function of upto six factors, whilst the soundness model related the optimum welding voltage to a function of three factors so that porosity and electrode stubbing would not be encountered.

Resistance welding is used very extensively in industry for a wide range of applications. Knowledge and measurement of the dynamic characteristics of resistance welding equipment is important in the design of the equipment and in optimization of welding procedures using finite element software. This is especially true for projection welding where accurate measurements of effective lumped mass and damping of the welding head as well as its maximal acceleration and velocity are required for accurate modelling. This thesis describes a new concept where a mechanical model of the welding head is used together with the imposition of a mechanical load step function with simultaneous measurement of resulting head motion to calculate effective lumped mass and damping factor. Two test systems were devised to implement the step function. In the “free fracture test”, a metal or ceramic bar is loaded to its breaking point and resulting welding head velocity is measured. This data allows accurate calculation of machine parameters. The second test uses the explosion of a small metallic element to impose a step function, when the welding current causes the metallic element to explode. The final version of this test “the exploding button test” uses a small cylindrical element fabricated from welding filler wire, with the advantage that both button geometry and material can be controlled. The exploding button test has proved to be very effective, can easily be used for in-situ measurements and avoids the vibrations associated with the free fracture test. These test were applied to evaluate a range of resistance welding machines. Finally, an innovative projection geometry was developed to significantly increase projection weld quality and this design has now been used extensively in industry. The techniques developed in this thesis have been shown to be practical and effective and have enabled much better understanding of machine kinematics. The measurements provide essential data for modelling of projection welding and in guiding the development of resistance welding machines and procedures.

Arc welding is the most widely used set of joining technologies in industry today. The automotive tier supplier network and light manufacturing are significant users of arc welding, particularly gas metal arc welding (GMAW) and pulsed GMAW (GMAW-P). For sheet metal welding the majority of welds are single pass fillet welds on T-butt, lap, or edge joints. A fundamental problem and limitation to the use of higher travel speeds in GMAW is the phenomenon of weld bead humping, a weld profile defect with a wavelike profile to the weld bead that has peaks and troughs in the longitudinal direction.

No description available.

A series of Friction Stir Welds (FSW) has been produced in order to optimise tool designs and weld/process variables to minimise flaws in the weld and obtain the best possible microstructure for superplastic Forming (SPF). Therefore the main goal is to produce friction stir welds which do not fail during subsequent SPF processes. The friction stir welds have been created using novel tools which are oversized for the material thickness used; this creates a wider weld region of fine equiaxed grains which are suitable for SPF. These original tools have been compared to tools which are already in mainstream FSW production. The welds created for this investigation also represent an evolution of friction stir welding by starting with a milling machine; a very basic piece of engineering workshop equipment. This was then replaced by a modified milling machine with force monitoring capabilities and finally using a state of the art dedicated FSW machine for the final welds. Room temperature properties are not usually a good indicator of high temperature response; however in this thesis the room temperature properties are closely linked to the FSW microstructure and have been used to assess the suitability of the weld structure for subsequent superplastic forming operations. The welds created for this thesis have been completed using hot and cold welding conditions, evaluated for room temperature properties and microstructural stability. The results have then been used to assess the welds and select the most suitable structure for cone testing, which is used to test the welds‘ performance during SPF. Friction stir welds were then recreated and cone tested which reveals the different levels of deformation occurring across the entire weld section and the unaffected parent material. Specimens in the as-welded, post-weld annealed and post-SPF have been analysed using standard microscopy techniques and Electron Back-Scattered Diffraction (EBSD). Welds in Aluminium Alloy (AA) 2004 with excellent room temperature properties have been created and shown to be capable of superplastic deformation achieving strain greater than 200%. Welds in AA5083, although producing excellent room temperature properties are unable to deform superplastically due to the difference in strengthening mechanisms employed by the different alloys. AA2004 contains Al3Zr which effectively pins the microstructure allowing grain boundary sliding to occur, AA5083 lacks this grain refinement element and so suffers from abnormal grain growth leading to early failure.

The successful prediction of abrasive wheel behaviour should greatly simplify the complex problem of wheel selection. This investigation examines the influence of abrasive surface topography on the actual mechanics of the metal removal process. The characterisation of abrasive surfaces using statistical properties of the cutting profile is discussed. A study is undertaken which examines the influence of abrasive grit geometry on the mode of metal deformation. In these tests, every consideration is given to reproducing conditions realistic of a practical grinding operation. A simplified plunge grinding operation is then considered to determine the influence of various process parameters on the mechanics of metal removal. Finally, a computer simulation of the above grinding operation, using Monte Carlo simulation techniques, is presented.

No description available.