Inertia friction welding is an important industrial joining technique for the production of axisymmetric components. Two parts, one rotating and the other stationary, are brought together under axial load and rotational kinetic energy stored in a flywheel is transformed into thermal energy and plastic deformation through friction at the interface between the work pieces. The process is quick and repeatable and generates good quality welds with a small heat affected zone (HAZ) One of the main objectives of this research was to produce a modelling tool that can be used to represent the welding of high strength aerospace alloys with particular reference to shaft applications. The commercial software DEFORM-2D was used as it contains a 2.5D modelling capability suitable for this application and can be easily used by industry. The aim of the process modelling tool is to reduce development time and cost by the use of a process modelling tool which would mean fewer development welds are required for new material combinations and geometries. Initial models created were based on the nickel-based superalloy, Inconel 718 and the capability was then extended to the high strength steels, AerMet 100 and S/CMV, which are suitable for aero-engine shaft applications. Material data required to run weld models was defined and a test programme commissioned in order to obtain the properties for the high-strength steels. Microstructural investigations, including continuous cooling and isothermal tests were also carried to determine phase transformation information that was relevant to the welding process. This included the presence of the "bainite nose", and the volume change associated with the martensite transformation on cooling. The latter was shown to have a significant effect on the residual stresses developed in as-welded components. The volume changes are shown to act as a stress relief of up to 1000MPa in the HAZ of the weld. Experimental testing, which included thermal imaging and thermocouple measurements, was carried out in order to gain more insight into the inertia friction welding of the high strength steels. This testing also included some tests using novel welding techniques to attempt to reduce the post-weld cooling rate and the effects of these techniques on the cooling rate are presented. These tests also provided data for validation of the weld model. The research concludes that DEFORM-2D can be used to model the IFW process between high-strength aerospace materials for aero-engine shaft applications and typical results show an error of ±15% with respect to the final upset value.

The effect of friction stir welding (FSW) on the toughness properties of two aerospace aluminium alloys has been investigated. Two typical aerospace alloys, high strength AA7449 and medium strength AA6013 have been studied in detail. The mechanical properties have been characterised via hardness testing, toughness testing and tensile testing incorporating strain analysis via digital image correlation. A notched 5ar test has been used to produce a profile of toughness across each of the welds and, in AA7449, through the depth of the welded plate. Each fracture surface was examined via FEGSEM to determine the mode of fracture and the microstructure was characterised, via optical microscopy and FEGSEM, such that the microstructural changes caused by FSW could be linked to the variations in toughness.

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This thesis firstly contains a review of previous literature concerned with both the laser welding and cutting of titanium alloys. In addition a brief introduction to both laser physics and titanium metallurgy is also included in the literature review section. The experimental part of the thesis can be divided into three sections which are as follows: (1) Measurement of focussed and raw laser beam diameter was carried out using a Laser Beam Analyser. In addition the laser mode used for both welding and cutting were evaluated. (2) 1, 1.7 and 2.7mm thick Ti–6Al–4V alloy was laser welded using both a 400W and a 2kW CO2 laser in the continuous wave mode. The effect of variation in the main laser parameters upon weld morphology, microstructure, mechanical properties and oxygen contamination was evaluated. In addition further. welding studies were carried out using the 400W laser in the pulsed mode. The effect of variation in pulse parameters upon weld bead morphology was studied. It was concluded from the laser welding section that using a 2kW CO2 laser it was possible to produce full penetration welds in up to 2.7mm thick Ti–6Al¬–4V alloy sheet. The use of pulsed welding was found to enhance welding efficiency with a greater weld penetration being achieved than with an equivalent power continuous wave output. (3) Ti–6Al–4V alloy sheet up to 2.7mm thick was laser cut using a 400W CO2 laser. The effect of variation in cutting speed and cutting gas pressure upon surface condition was studied. From detailed SEM examination of laser cuts a mechanism for inert gas assisted laser cutting was postulated. The effect of laser cutting upon the adjacent microstructure and oxygen contamination of the cut edge was also studied. As well as conventional laser cutting a series of laser cuts were also produced using the "Dross Jet". The effect of this device upon oxygen contamination and gross adhesion to the underside of the cut was studied. It was found that by using the dross jet in conjunction with inert gas assisted laser cutting that it was possible to produce cut edges with levels of oxygen contamination equal to that present in a guillotined cut edge. In addition the best laser cut edges were found to be as smooth as a typical guillotined edge. It was concluded that inert gas assisted laser cutting is an excellent process for cutting thin section Ti–6Al–4V alloy. The final section of the thesis aims to compare the experimental results from the laser welding section with results generated by an existing mathematical model. The results show that the model can be used to predict weld penetration depths and HAZ widths to a reasonable degree of accuracy.

Residual stresses can be defined as the self-equilibrating internal stresses that remain in a body in the absence of external forces or thermal gradients. They can arise from kinds of manufacturing processes and thermal treatments, where welding is a common process that generates residual stresses. Residual stresses are known to affect the fatigue, creep and brittle fracture properties of engineering components as well as their structural stability, wear and corrosion behaviour. Techniques that can accurately measure and predict residual stresses are therefore very important. The purpose of this project is to develop and improve the measurement of residual stress using deep hole drilling and neutron diffraction techniques. This is done using experimental methods and finite element analysis methods. Based on these developments, a series of measurements were undertaken to obtain through thickness distributions of residual stress in a variety of welded components. The deep hole drilling residual stress measurement technique is a semi-destructive, mechanical strain relief technique, which the strain components are measured during stress relief from the removal of a small amount of material. Neutron diffraction, a non-destructive technique, is based on Bragg's law and the components of strains are obtained from measurement of lattice spacing of polycrystalline material. The standard deep hole drilling, a newly developed incremental deep hole drilling techniques and the neutron diffraction technique were applied to a variety of metal welds to characterise the through thickness residual stresses generated by different welding methods, validate and develop the application range of these techniques. The finite element method was adopted for a ring weld specimen to simulate the welding process to obtain the residual stress field and to simulate the deep hole drilling techniques. An over-core deep hole drilling mcthod, a development from previous deep hole drilling techniques, was examined in this study. The extraction step in the over-core deep hole drilling technique allowed a hybrid procedure to combine the deep hole drilling and neutron diffraction techniques and increased the accuracy and reliability of residual stress measurements.

La mise en forme des composites MTP reprend des techniques déjà utilisées pour la mise en forme des pièces thermoplastiques simples comme le thermoformage ou le soudage. Cette thèse a proposé l'étude du procédé de soudage laser des composites à matrice thermoplastiques unidirectionnelles. La technique du soudage laser présente des avantages spécifiques pour des applications industrielles par rapport à d’autres technologies conventionnelles : méthode sans contact, précise et flexible. C’est un procédé facile à automatiser et à contrôler, un soudage rapide (quelques secondes), et une absence de vibration durant le procédé de soudage. Le procédé de soudage laser implique deux pièces composites. Une pièce semi-transparente à la longueur d’onde du laser et l’autre absorbante à la même longueur d’onde. La puissance du faisceau laser est transmise à travers le matériau semi-transparent et est absorbée à l’interface des deux matériaux. Le contact entre les pièces provoque le chauffage à l’interface (un transfert de chaleur par conduction a lieu entre les deux matériaux). Ainsi, la fusion des deux matériaux se produit (la liaison entre les deux parties se produit lorsque T>Tfusion dans ce domaine pour les polymères semi-cristallins et T>Tg pour les polymères amorphes). Cependant durant le procédé quelques difficultés apparaissent : les matériaux sont hétérogènes et anisotropes, cela entraine la divergence du trajet optique du faisceau laser à chaque interface fibre-matrice (phénomène de réfraction) causée par la multiplication des interfaces fibres matrice dans le matériau. Ce qui a en effet une influence sur la distribution de puissance à l'interface de soudure. La puissance de soudage du laser est réduite par cet effet de réfraction. L’obtention d’un joint de soudure de bonne qualité est conditionnée par une bonne compréhension du comportement du matériau sous l’irradiation laser, basée sur une identification, une modélisation des phénomènes optiques et thermiques impliqués. / The composite forming uses technologies being used for the simpler thermoplastic parts forming as thermoforming or welding. This thesis proposed study of transmission laser welding process in unidirectional thermoplastic composites. Transmission Laser Welding technique presents specific advantages for industrial applications over other conventional technologies: the method is accurate, flexible, small heat affected zone, easy to automate and control and non-contaminant, absence of vibration during the welding process (contrary to the ultrasonic welding, friction welding), fast welding speed for welding plastic parts with an acceptable welding time. Transmission Laser Welding of composites involves two joining parts: one semi-transparent to the laser wavelength and the other part is absorbent in the same wavelength. The two parts are positioned together before the welding. The laser beam energy is transmitted through the semi-transparent material and is absorbed within the surface of the second materials. The bonding between the two components the two parts allows the heating of the semi-transparent part by thermal conduction. Thus, melting and fusion of the both materials interface occurs (the bonding between the two parts occurs when T > Tmelt in this area). However, some difficulties are experienced during this process: heterogeneous and anisotropic materials; problems of the laser beam transfer caused by the multiplication of fiber-matrix interfaces in materials. This is highly correlated to the energy arriving at the welding interface. Obtaining of a good quality of welded seam is conditioned by a good understand of material behavior under laser irradiation, based on a clear identification, modeling of optical and thermal phenomena involved.

Environmental and commercial pressures have forced the aerospace industry to look at alternatives to riveting for the manufacture of aluminium aircraft structures. This resulted, at the end of last century, in an extensive study by Airbus into the possiblities of using CO2 lasers, which led to the process being implemented for a (small) number of stringer-to-skin fuselage panels in the newer Airbus models. Since this initial commercial success, new laser sources have become available that are more suitable for the welding of aluminium than CO2 lasers, in the form of Nd: YAG and Yb-fibre lasers. Both produce a wavelength that is absorbed more efficiently by aluminium alloys than the CO2 laser wavelength, resulting in an improved keyhole stability, as demonstrated in the late nineties for Nd: YAG lasers. In addition, Yb-fibre lasers have become available at output powers higher than available for Nd: YAG lasers, allowing thicker sections of aluminium to be welded in a single pass. However, despite their claimed advantages, no efforts were made to demonstrate the potential of these lasers for (aluminium) aircraft manufacture. For this reason, the author initiated a series of studies in 2001, with the overall aim to develop procedures to laser weld both thin (3.2mm) and thicksection (12.7mm) aerospace aluminium alloys using these fibre-delivered lasers to a weld quality, in particular related to weld metal porosity, suitable for aerospace service. The focus in this research was on weld metal porosity, because this is a particular problem when laser welding aluminium, either in the form of fine (hydrogen) porosity or larger porosity associated with an unstable keyhole behaviour. The benchmark weld metal porosity for this study was obtained from the stringent weld quality classes defined in BS EN 13919-2 and AWS D17.1. The approach to this research was in three parts, with work in the first aimed at demonstrating that a 3kW Nd: YAG laser was capable of producing low-porosity welds in 3.2mm thickness 2024 aluminium alloy, and thus can be considered for replacing the CO2 laser currently used for the stringer-to-skin fuselage application. Prior to the final part of the research, in which a 7kW Yb-fibre laser was used to demonstrate that these benchmark porosity levels could also be achieved in thicker section (aerospace-grade) aluminium, a comparison study was carried out to quantify the difference in welding performance between the Nd: YAG and the Yb-fibre laser. At an output power of 4kW focused in a 0.4mm diameter spot, the Yb-fibre laser was capable of a 30% higher welding speeds in 4mm (5083) aluminium alloy, or a 20% increase in depth of penetration for welding speeds between 1 and 15m/min, compared with the Nd: YAG laser. This improvement in welding performance, together with an output power of 7W, produced full penetration in 12.7mm thickness (aerospace-grade) AI-Zn-Mg-Cu aluminium alloy using the Yb-fibre laser autogenously, or in a hybrid configuration with a MIG arc. Both the autogenous laser and hybrid laser-MIG process were capable of producing welds with a weld metal porosity in line with the BS EN 13919-2 and AWS D17.1 benchmark conditions, at welding speeds of 0.55 and 0.75m/min, respectively. At these production rates, the 248 metres of stringer incorporated in a typical aluminium wing structure can be welded in 7.5 and 5.5 hours, in case of autogenous laser and hybrid laser-MIG, respectively, compared with 37.6 hours currently needed for the riveting process.