In this study, the inertia welding process was studied by both an FEM model and three analytical models. The thermal analysis shows that there is a steep temperature gradient near the mating surface, which is the cause for the existence of a band of high hydrostatic stress near the weld line. The holding effect of this high static stress is the reason for the presence of the very soft material at the welding interface. The models were used to predict the displacement of the weld line (upset) with a lambda model to describe the constitutive relation of IN718 at high temperature. The results from the different models are in broad agreement. The shear stress induced by friction at the interface is found to enlarge the upset value; its effect must be taken into account if the upset is to be predicted accurately. The extrusion of the burr during the last second of the welding is a direct result of the quick stop of the rotating part due to the balance of the momentum, which is clearly explained by the analytical mechanical model put forward in this work.

1-3 Piezocomposites show significant improvements over conventional, monolithic transducers. However, a 10 MHz piezocomposite would require a pillar array structure with an aspect ratio of > 9 and a feature size of < 30 μm; which makes fabrication difficult and increasing the operating frequency challenging. Several processes have been developed to improve on the industry standard, dice and fill, but they remain laboratory based. Viscous polymer (VP) embossing has been demonstrated as a route to smaller feature sizes and higher aspect ratios, but the process is limited by the availability of suitable moulds, which must be replicated to make the process cost effective. This thesis reviews the micro replication and fabrication processes necessary to produce moulds for 1-3 piezocomposites, and characterises the replication process from hot embossing to a functioning 1-3 piezocomposite. It demonstrates that the Bosch process and electroforming can be combined to produce a master with a deeper array structure than has been previously demonstrated by these two processes. Piezocomposites have also been made which have pillar widths that are not possible by dice and fill, by filling hot embossed moulds that have aspect ratios of 14 and feature sizes of 30 μm.

This thesis is concerned with the porosity formation mechanism during electron beam welding of titanium-based alloys. During the welding of titanium alloys for aerospace engine applications, porosity is occasionally found in the solidified welds. Hence the key factors responsible for porosity formation need to be identified, and guidance to minimise porosity occurrence needs to be provided. The research conducted in this work is twofold. First, porosity formed in electron beam welded titanium samples is characterised to rationalise the porosity formation mechanism. Second, models based on sound physical principles are built to aid understanding of porosity formation, and to provide predictive capability. Porosity formed in electron beam welds is characterised using metallographic sectioning, high resolution X-ray tomography, residual gas analysis (RGA), scanning electron microscopy (SEM) and energy and wavelength dispersive spectroscopy (EDS/WDS) analysis. The results confirm porosity formed in electron beam welded titanium-based alloys is associated with gas dynamics; hydrogen is very likely to be responsible for porosity formation. A coupled thermodynamic/kinetic model is proposed to study the hydrogen migration behaviour during electron beam welding process, and then the effect of hydrogen on bubble formation is investigated via quantitative modelling, backed up by targetted experimentation

In centrifugal casting, molten metal is introduced into a mould which is rotated at high speed. The centrifugal force helps to fill thin sections but this benefit may be offset by the effect of the turbulent flow on the casting quality. In this research, the effect of direct and indirect gated mould designs on the quality and reliability of aluminium alloy investment castings made by centrifugal casting was investigated. The scatter in the ultimate bend strength and the modulus of elasticity was analyzed using the Weibull statistical technique, which showed that the Weibull modulus of both properties was significantly improved for the indirect gated cast test bars compared to the direct gated bars. A detailed microstructural characterization was carried out on the cast test bars, which included grain size, dendrite cell size and porosity. Scanning electron microscopy was used to examine and analyze the presence of defects on the fracture surfaces such as shrinkage pores, entrapped bubbles and oxide films resulting from surface turbulence during mould filling. The results indicated a clear correlation between the mechanical properties and the presence of casting defects. Water modelling experiments were carried out using purpose-built experimental centrifugal casting equipment and filling sequences recorded using a high speed video camera. The water modelling results showed that the general tendency for the direct and indirect gated mould designs was that the higher the rotational velocity, the lower the filling length and consequently the lower the filling rate. Subsequently, this information was used to validate the computer software ANSYS CFX. An excellent correlation was obtained between the experimental water modelling and simulation results for both direct and indirect gated moulds.

Friction Stir Welding (FSW) is known to result in a complex microstructural development, with features that remain unexplained, such as: the formation of the onion rings structure. Moreover, various microstructural factors have been suggested to control the strength in Al-Mg AA5xxx welds. The influence of the basemetal microstructural parameters (e.g. grains, intermetallic particles, stored energy) on the microstructure-property development has not been previously investigated, and is the subject of the present work. To rationalise the microstructural and local strength (hardness) development, especially within the heat affected zone (HAZ), a simple and rapid 3-D heat transfer model was established to predict the thermal fields associated with FSW. This numerical model utilises the alternating direction implicit method to simulate the transient thermal cycle based on the process parameters, thermo-physical and thermo-mechanical properties of the material. The model was fitted for the friction coefficient and contact conductance between the sheet and the backing plate using experimental torque and force data, as well as in-situ thermocouple measurements for AA2xxx and AA5xxx welds. The model predictions were consistent with the microstructural and microhardness development in the welds. Gleeble thermal simulations showed that the heating rate during welding affects the recrystallisation start temperature, which could delay or speed up recrystallisation. In the thermo-mechanically affected zone (TMAZ), the onion rings structure was studied in several AA5xxx and AA2xxx welds. This follows a thorough microstructural investigation of the basemetals sheets prepared by direct chill and continuous casting, to establish the influence of the microstructural heterogeneity in the basemetal on the onion rings formation and the microstructural development. Stereological studies of the intermetallic particle distributions in the basemetal and the welds revealed that there is a direct relation between the banding of constituent particles (Al(Fe,Mn)Si or Al6(Fe,Mn) in AA5xxx) or equilibrium phases (Al2CuMg or Al2Cu in AA2xxx) along the rolling direction, and the formation of the onion rings. A clear onion rings structure was defined by three microstructural features, which are: 1) the existence of fine and coarse grain bands, 2) grain boundary precipitates coinciding with the fine grain bands, and 3) coarse particle segregation in the coarse grain bands. Upon etching, these microstructural heterogeneities form the unique onion rings etching profile. The formation of the onion rings was rather independent of the process parameters and alloy type, as long as the intermetallic particles are banded regardless of their types. However, alloys with high area fraction of intermetallic particles (~> 0.02) were found to produce more pronounced microstructural heterogeneities, which resulted in a stronger etching intensity. The microstructural heterogeneities within the AA5xxx welds, especially the interaction between the dislocations and the fine Al6(Fe,Mn) dispersoids, indicated that establishing a structure-property model requires the incorporation of the various strengthening factors. Stereological studies of the grain size and intermetallic particle distributions in the TMAZ indicated that the hardness is a combination of various microstructural factors, with grain-boundary strengthening as the main factor, with additional contributions by Orowan strengthening by the Al6(Fe,Mn) particles in specific locations, as well as a minor contribution by solid solution strengthening which resulted from the dissolution of Mg2Si during welding. The high dislocation stored energy in the TMAZ, as measured by differential scanning calorimetry, was associated with the geometrically-necessary dislocations which resulted from the interaction with the intermetallic particles and grains, but do not contribute to the hardness.

In surface mount technology (SMT) electronics assembly, the solder paste is printed onto the PCB's surface through a stencil and the components are later placed over the solder paste deposits. Since 2007, the use of extremely small SMT components for assembly of SMT devices has been widespread, and achieving consistent print deposits for fine pitch (the distance between the leads of the components) components has become a real challenge. The majority of the defects at the printing stage, such as skipping and bridging, were found to be related to the quality of the solder paste. These defects are usually carried over to the reflow process, causing defective final products. Hence, it is important to monitor the quality of the solder paste. Conventional techniques for monitoring the quality of solder pastes during the production and packaging stage are usually based on the viscosity measurements of the solder pastes from the viscometer and rheometer. One of the potential limitations of viscometer- and rheometer-based measurements is that the collection and preparation of the solder paste samples can irreversibly alter the structure and flow behaviour of the sample. Due to the sample preparation process (removal, preshear/ pre-mixing), repeatability issues were often encountered when taking measurements using a viscometer or rheometer. Secondly, rheological measurements and the interpretation of rheological data comprise a very technical and time consuming process, which requires professionally trained research and development (R&D) personnel. Finally, the monitoring/inspection process usually employs random sampling technique from the production batch. Hence, measurement may not represent the actual quality of the whole production batch. This would mean that the conventional solder paste quality control that has been employed in the industry as a benchmark for printability (i.e. checking the viscosity of the paste before being despatched to the customers), would need to be re-evaluated for its feasibility and other possible methods of solder paste quality control would need to be considered. This has brought the ultrasound technique into context as it can offer a non-destructive evaluation of the quality of the solder paste in terms of viscosity. Also, it can be used at different stages of paste production and processing. It is for these reasons that materials suppliers that formulate and produce solder pastes, as well as solder paste consumers (especially contract electronics manufacturers), are keen to see the development of simple, easy-to-use and accurate techniques for the rheological characterisation of solder pastes. This thesis concerns the study of a non-destructive ultrasonic technique for characterising the rheological properties of solder pastes and, specifically, the use of through-mode microsecond ultrasonic pulses for evaluation of viscoelastic properties of solder paste materials. In this study, a wide range of flux systems and solder alloy particle distributions used in the industry are investigated to determine the correlation of the ultrasound attenuation and velocity to the viscosity of the solder paste and their correlation to paste printing performance. The work is part of a bigger study aimed at the development of an on-line quality control technique for paste manufacture based on both conventional rheological tests and ultrasound measurements. Results from the work on the comparative study of standard fluids and both commercial and newly formulated solder pastes and flux vehicle systems have been used to demonstrate the utilisation of the ultrasound technique for on-line, non-destructive measurement of the viscosity of non- Newtonian materials such as solder pastes. The study also found that the viscosity of the solder paste is governed by the intermolecular forces between the solder particles and the flux. The strength of these intermolecular forces depends on the probability of these particles rubbing up against one another while the paste is being sheared. Provided that the right correction factor for a particular shear rate is used, the ultrasound viscosity results obtained were found to be comparable to the rheometer viscosity results or to the viscosity provided by the solder paste manufacturer. The ultrasound technique produced consistent results and was also proven to work at low temperatures. The ultrasound technique may be used to help solder paste manufacturers to add the correct amount of flux or solder particles to their paste in order to reach a desired viscosity. Otherwise, it can be used as a quick go/no-go monitoring tool in the production line for predicting printing quality. Based on the foregoing, it can be concluded that the ultrasound technique is a viable alternative to using a rheometer.

The aim of this research was to acquire and formalise understanding of the Minimum Quantity Lubrication (MQL) technique in the surface grinding operation. The investigation aimed to show through experiment and theoretical study the effects of MQL on grinding process performance, measured in terms of tangential and normal forces, temperature and surface finish. A comparison of conventional, dry and MQL fluid delivery methods was performed. The experimental study was undertaken on a CNC grinding machine with integrated monitoring. A Taguchi methodology was employed to provide qualitative evidence of the strength of process parameters on performance indicators. The usefulness and promise of MQL was established. The study identified regimes of grinding where MQL can be employed successfully. This outcome is supported by results showing, in some applications, that MQL is comparable in performance to grinding under conventional fluid delivery. It was found that for some conditions MQL outperformed conventional fluid delivery. This was particularly so in the case of the tests with material EN8, (approximately 32 HRQ), where MQL was found to outperform conventional fluid delivery in almost all measures. As expected, not all conditions were in favour of MQL delivery and the reasons for this are discussed in detail in the thesis. A theoretical explanation for the efficient process performance is developed in relation to the experimental results obtained. The effects of variables such as DOC, dressing conditions, wheel speeds, workpiece speed and workpiece material are considered. It is reasoned that the MQL technique achieves efficient performance due to effective lubrication and effective contact region penetration by the fluid. Effective lubrication conditions were confirmed by highly competitive specific energy and grinding force measurements.

The world is experiencing a growing demand for miniaturised products. Micro-milling, using carbide micro tools has the potential for direct, economical manufacture of micro parts from a wide range of workpiece materials. However, in previous studies several critical issues have been identified that preclude the direct application of macro machining knowledge in the micro domain through simple dimensional analysis. The research presented in this thesis focused on some of the areas that require development of the scientific knowledge base to enable determining improved microscale cutting performance. In the mechanical micro machining of coarse grained materials, the programmed undeformed chip thickness can be lower than the length scale of the workpiece grains. Moreover, when the microstructure of such materials is composed of more than one phase, the micro cutting process can be undertaken at a length scale where this heterogeneity has to be considered. Driven by this challenge, the material microstructure 'size effect' on micro-machinability of coarse grain steel materials was investigated in this PhD. In this regard, a predominantly single phase ferritic workpiece steel material and another workpiece material with near balanced ferrite/pearlite volume fractions was studied over a range of feedrates. The results suggested that for micro machined parts, differential elastic recovery between phases leads to higher surface roughness when the surface quality of micro machined multiphase phase material is compared to that of single phase material. On the other hand, for single phase predominantly ferritic materials, reducing burr size and tool wear are major challenges. In micro machining the so called 'size effect' has been identified as critical in defining the process performance. However, an extensive literature search had indicated that there was no clear reported evidence on the effect of process variables on driving this size effect phenomenon. It is often assumed in literature that the un-deformed chip thickness was the main factor driving the size effect. This limit manufactures to only altering the feedrate to try and influence size effect. To explore the significance of a range of inputs variables and specifically, cutting variables on the size effect, micro cutting tests were conducted on Inconel 718 nickel alloy. Taguchi methodology along with signal processing techniques were applied to micro milling acoustic emission signals to identify frequency/energy bands and hence size effect specific process mechanism. The dominant cutting parameters for size effect characteristics were determined by analysis of variance. These findings show that despite most literature focussing on chip thickness as the dominant parameter on size effect, the cutting velocity is a dominant factor on size effect related process performance. This suggests that manipulating the cutting speed can also be a very effective strategy in optimising surface finish in micro machining and in breaking the lower limit of micro machining.In micro machining the lower limit of the process window is set by the minimum chip thickness. Identifying this limit is thus important for establishing the process window. Process windows are valuable guidelines for industrial selection of cutting conditions. Additionally, understanding factors that influence the value of minimum chip thickness is even more important for progressing micro machining capability to the nano-scale machining regime. For this reason, in this PhD study, acoustic emission signatures emanating from microscale milling of six different workpiece materials were characterised to identify the rubbing mode and this enabled the identification of the threshold conditions for occurrence of minimum chip thickness. The minimum chip thickness predicted by this novel approach compares reasonably well to the values that exist in published literature. Additionally, the decomposition of raw acoustic signal allowed the determination of energy levels corresponding to deformation mechanisms. The PhD work provides significant and new knowledge on the utility and importance of acoustic emission signals in characterising chip formation in micro machining. A novel method for determining the minimum chip thickness was developed, micro machining chip formation mechanisms were identified and the machinability of coarse grained multiphase material is presented.

No description available.

This research addresses the development of a laser metal deposition process with wire feeding and melt pool temperature control. The system consists of a2 kW fibre laser, a CNC table, a wire feeder and a temperature monitoring and control system. A study of the influence of the main parameters on the process and on the deposited bead geometry was performed. The parameters analysed were: laser power, traverse speed and wire feed rate. As a result of this study, a process window was established for metal deposition of stainless steel 308LSi (wire) on stainless steel 304 (plate). The influence of the parameters on the bead geometry (height and width) was analysed applying the Design of Experiments methodology, using a full factorial design 3k. The results are presented, together with important practical considerations for laser metal deposition with wire. A closed-loop temperature control system was developed: it controls the melt pool temperature by means of modifying the laser power. The melt pool temperature was measured by a two-colour pyrometer, whereas a single-colour pyrometer was used for monitoring the workpiece (upper layer) temperature. A model of the melt pool was derived from a heat balance equation. It was then utilized for the design of the controller in the discrete domain, using the root locus method. The control algorithm was developed in LabVIEW software and executed in a computer. The control system was implemented successfully and was utilized to build single-bead walls and cylinders of stainless steel 308LSi. The study performed on the parameters and the developed temperature controller proved to be very effective tools to facilitate the transition to the deposition of titanium alloy Ti-6A1-4V, requiring only minimum adaptations. Single-bead walls and cylinders were also built in this material. Stable and smooth metal deposition was achieved for both materials. During the experiments, several strategies for the automation of wire metal deposition of multilayered structures were developed. Finally, mechanical tests were performed. The mechanical properties of the deposited materials are comparable to those in wrought (annealed) condition and to similar alloys made by laser powder deposition systems. The system developed in this work provides a means to perform stable and smooth wire metal deposition, achieving good mechanical properties. It also facilitates the transition to deposit different materials. It has a flexible structure and can be expanded or adapted to be used in other wire metal deposition systems.