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There are two very distinct welding modes in laser welding, keyhole and conduction mode. The characteristics of keyhole laser welding; mainly high penetration, high productivity and high aspect ratio have led the industry to focus more on this mode. On the other hand, conduction mode does not have high productivity and has a low aspect ratio, but deep penetration depths can also be achieved using this mode. Despite these disadvantages conduction mode has advantages such as stability of the welding process, high quality, better control of the heat input and no spatter. These characteristics are not normally associated with laser welding, mainly due to keyhole welding being the usual operational mode. Conduction laser welding is a more complicated process than might be expected. This is because of the conflicting requirements of maximising penetration depth whilst maintaining very high quality. This means that thermal transfer needs to be maximised but vaporisation needs to be zero. The aim of this research was to fully understand conduction laser welding mode and therefore, achieve maximum penetration whilst maintaining high quality. The approach was to use power density, interaction time and beam diameter as process parameters in order to study and understand the transition between conduction and keyhole welding modes. The study included the differences between using a continuous wave laser system and a pulsed wave laser system. Most of the study was made in mild steel and was also extended to stainless steel and aluminium to include the effects of the material properties on the transition. For process optimisation the effect of system parameters, power and welding speed, on optimum beam diameter in conduction mode was also examined for aluminium and mild steel. This included a comparison between the use of a statistical empirical model and a finite element model for optimisation of the process. Finally a comparison of residual stress development in conduction and keyhole welding modes was made.

The aim of this work is to develop techniques and to optimize the process of bonding of cemented carbide metal cutting inserts. This replaces the common methods of clamping or brazing. The first stage was to-survey and test structural adhesives to select the most promising adhesive for metal cutting applications. This resulted in a choice of toughened epoxy heat cured adhesive. A comparison between bonded and brazed joints, from the damping capacity point of view was made. In this comparison adhesive bonding resulted in, a higher damping capacity when compared with brazing. The bond line thickness proved to be significant in increasing the damping capacity of the bonded joints. A comparison was carried out between the heat flow through bonded and brazed tools, and demonstrated pronounced heat insulation in the bonded tool, which depended mainly both on the thermal conductivity of the adhesive material and the thickness of the bonded layer. The effectiveness of the bond line thickness, from the points of view of thermal conductivity and developed thermal stresses was investigated. This showed that the thinner the bond line the better is the performance of the bonded joint. The effect of coolant on the temperature of the bond line as well as the tool tip was also investigated. Mixing metallic powder with the adhesive material in order to increase the thermal conductivity of bonded tools was investigated. A new apparatus for measuring low thermal conductivity is proposed. Extensive cutting tests were carried out in order to assess the performance of bonded tools with and without coolant, reground tools and bonded tools having copper powder mixed with the adhesive. The assessment of cutting performance when using bonded tools compared with brazed tools showed that not only a better surface finish could be obtained with bonded tools but also far less tool flank wear

Hyperbaric Gas Metal Arc Welding (GMAW) is an important technology for repair welding of deep sea pipelines and linking of existing pipeline networks to newer ones through tie-ins and hot-tap welding. With increasing water depth the process becomes susceptible to hydrogen assisted cracking due to the very fast cooling rate of the weld caused by higher habitat gas density and resulting higher thermal diffusivity. Maintaining sufficient heat in the welding zone is vital to avoid a potential cracking tendency especially as moisture pick-up may be difficult to avoid during hyperbaric welding operations. In addition to this, hyperbaric GMAW has a limitation of low heat input because it is operated at a short arc length or dip transfer mode to avoid process instability at high pressure. Also, the short arc length generates weld spatter that may affect weld quality. The research presented in this thesis, investigated the use of an industrial laser in conduction mode for the purpose of providing significant additional heat input to control the weld thermal cycles of GMAW. Advanced GMAW power sources such as the Fronius Cold Metal Transfer (CMT) and EWM ColdArc have also been investigated for reduced weld spatter generation. Studies were conducted to investigate the weld pool thermal cycles and resulting metallurgical phase formation in hyperbaric GMAW at different pressures ranging from 1 bar to 200 bar. This was followed by welding trials at one atmosphere to compare the process characteristics of traditional dip transfer GMAW with some advanced GMAW power sources such as CMT and ColdArc. The main experimental trials to investigate a laser assisted GMAW (CMT) process were performed at one atmosphere condition. A thermal model was developed using Abaqus software to predict the weld metal and heat affected zone thermal cycle in a laser assisted GMAW (CMT) process at one atmosphere and under high ambient pressures. Finally, investigation was carried out to evaluate the benefit of the laser assisted process in lowering diffusible hydrogen content from the weld metal. The hyperbaric GMAW experimental results showed that the weld pool cooling rate increases with pressure due to higher chamber gas density and resulting thermal diffusivity. But this effect is not prominent for thicker plates. Therefore, it was concluded that heat conduction through the steel thickness dominates convective losses to the chamber gas environment. It was also shown that the welding arc shrinks as pressure increases in order to minimise energy loss to the environment. This defined the weld bead profile; although it was found that beyond 100 bar pressure the weld penetration depth remained effectively unchanged. Apart from the hardness of the weld made at 1 bar, there was little difference between those at 18, 100 and 200 bar. However, all of the welds show hardness peaks greater than 350 HV10 recommended for offshore structures. It was observed that CMT produced the lowest weld spatter compared to the traditional GMAW and ColdArc. However, this advantage is constrained to low wire feed speed (3 to 5 m/min) beyond which it becomes relatively unstable. For the laser assisted GMAW (CMT) trials, it was shown that the laser serves as a spatially resolved heat source, reheating the weld bead and reducing the cooling rate. For the laser parameters investigated, over 200% reduction of cooling rate could be achieved when compared with GMAW alone. It was also demonstrated that the additional laser thermal input will extend the weld residence time at high temperature (over 300 °C). This will prolong the weld cooling time such that dissolved hydrogen can diffuse out before it comes to room temperature. The laser was shown to significantly reduce the weld peak hardness from about 420 HV0.5 to values below 350 HV0.5, which will be beneficial for hyperbaric welding. The model prediction of the weld thermal cycles was in good agreement with the experimental results. Therefore, it could be used to predict the weld metal and HAZ cooling rate of a laser assisted GMAW (CMT) process although the model would need to be calibrated for higher pressure data. It was also demonstrated that additional laser heat can reduce the weld hydrogen content to acceptable limits of 5 ml/100 g of weld metal even for high moisture content in the welding environment. In conclusion, the addition of laser heating to GMAW will reduce the weld cooling rate, extend the weld pool cooling time, and expel diffusible weld hydrogen. All of these would be immensely beneficial in terms of improving the quality and reliability of structures fabricated through hyperbaric GMAW.

Assembled structures are typically constructed by structural elements that are connected together by structural joints. For example, thousands of spot weld joints are used in a typical automotive structure in order to provide connections between layers of thin metal sheets used to form the structure. The spot weld joints also significantly contribute to the vehicles structural stiffness and dynamic characteristics; hence it is very important to have an acceptable FE model of the joints in order to evaluate the dynamic behaviour of such structures. It appears that most of the studies regarding spot weld joints have concentrated on spot welds made by the more conventional Resistance Spot Welding, while to the author's best knowledge, there is no reported works on modelling the dynamic behaviour of structures with laser welds, which is the main objective of this thesis. Existing elements available in commercial FE software are researched and a suitable element is chosen to represent the laser weld joints for its dynamic predictions. A set of laser spot welded structures are manufactured and FE model representing the structures is developed systematically, starting from modelling and updating the substructures to the development of the FE model of the welded structures. Experimental modal analysis is conducted in order to obtain the modal parameters from the test structures, which are then employed in validating and improving the correlation between the developed FE models and their experimental counterparts. Variability that exists in the test structures is also investigated and non-deterministic (or stochastic) model updating is carried out by using the perturbation method. Parameter selection for the stochastic model updating is studied first using two sets of very different structures: the first set consists of nominally identical (simple) flat plates, while the second set comprises of (more complicated) formed structures. The stochastic updating procedure is conducted with different combinations of parameters, and it is found that geometrical features (such as thickness) alone cannot converge the predicted outputs to the measured counterparts, hence material properties (for instance, Young's modulus and shear modulus) must be included in the updating process. Then, the stochastic model updating is also conducted on the welded structures, using two approaches of parameter weighting matrix assignments. Results from one of the approaches demonstrate good correlation between the predicted mean natural frequencies and their measured data, but poor correlation is obtained between the predicted and measured covariances of the outputs. In another approach, different parameter weighting matrices are assigned to the means and covariances updating equations. Results from this approach are in very good agreement with the experimental data and excellent correlation between the predicted and measured covariances of the outputs is achieved. Finally, the developed deterministic FE model of the welded structures is used in damage identification exercise, consisting of two parts: (I) identification of defects, and (2) identification of real damage in the welded structure. In the first part, a defective structure is selected from the set of nominally identical structures and FE model updating procedure is performed in order to quantify the defects in the defective structure. In this exercise, only the natural frequencies are employed in the identification procedure and the identified defects are found to be reasonable and in agreement with the findings from visual inspection conducted prior to the identification work. In identifying real damage in the welded structure, the identification procedure is conducted based on the natural frequencies and the mode shapes information of the damaged structure. The damage is characterised by the reductions in the Young's modulus of the weld patches to indicate the loss of material/stiffness at the damage region. Based on the updating results, it can be concluded that the identification procedure has successfully identified, localised and quantified the damage. The identification procedure also brings the predicted natural frequencies closer to their measured counterparts, with a very good correlation is achieved between the numerical and experimental modes.

This thesis describes the development of computational analysis techniques for weldments which contain three different material regions (Parent Material, Weld Material and Heat Affected Zone) and which exhibit different creep behaviour. The different strain rate behaviour of the three regions and the growth of creep continuum damage lead to local stress redistribution and to complex states of stress which can greatly enhance the accumulation of creep damage. A review of literature is presented which covers weldment design methodology and the associatedm etallurgy. The shortcomingso f design and assessmenct odes for pipe vessels, and the need for approximate methods of lifetime estimation, are revealed, indicating the need for further research in these areas. The development of the Continuum Damage Mechanics (CDM) method is presented, which is a physically based technique for the analysis of the creep behaviour of materials and engineering structures. Previous works are also reviewed showing that the CDM method can be used to accurately describe the behaviour of weldments providing the constitutive equations are available which describe the deformation and rupture of materials. Since the research is concerned with the analysis of the creep rupture of welded pressure vessels and pipes using the CDM method, previous research is reviewed regarding the creep behaviour of weldments and 'the determination of constitutive equations for different weldment material phases. A review is also presented of different solution techniques for solving systems of equations and for minimising the bandwidth and profile of matrix. The modification is described of the two-dimensional (2D) solver, Damage XX- 2D, to extend its capability to three-dimensional (3D), together with the techniques required to satisfy plastic incompressibility using a special brick arrangement of tetrahedral elements. The Three-dimensional CDM Finite Element Solver is known as Damage XX-3D. The 3D finite element theory and the co-ordinate transformation techniques are outlined for the solution of axi-symmetricengineering problems. The technique for the removal of failed finite elements is described for both of the two-dimensional and the three-dimensional analyses. The restart facility and the associated data output strategy are developed to help to minimise the loss of result data files, in the case of an accidental power cut to computers. These methods also allow a complete analysis to be divided into individual smaller analyses. Two numerical solution methods, with different storage schemes, for sets of linear algebraic equations have been developed and validated against results obtained independently using a commercial Finite Element package Abaqus (version 5.6-1). A damage averaging technique is developed to reduce the number of iterations required for the solution of three-dimensional problems which have large number of degrees of freedom; and also to preserve the symmetry of creep CDM solutions for axi-symmetric two-dimensional analyses. The creep CDM solutions obtained using Damage XX-3D are compared with the solutions obtained using Damage XX-2D Axi-symmetric analysis, and good agreement has been obtained for lifetimes and failure mechanisms. Applications of Damage XX-3D, are presented for the analysis of the high temperaturec reep behaviour of a CrossweldedT estpiece,t he Cylinder-SphereP ipe Intersection (Flank Section) subjected to an internal pressure, and Butt-welded ferritic steel pipe subjected to a combined internal pressure and a global bending moment. Finally, a three-dimensional Finite Element CDM Solver has been developed which is computationally fast and efficient, and which yields predictions which have been validated against independent solutions. IV

Stationary shoulder friction stir welding with Ti-6Al-4V of 7mm thickness was undertaken by TWI Ltd and six welds with varying traverse speeds and rotation speeds were studied in terms of microhardness measurement, optical microscopy, scanning electron microscopy (SEM), and electron backscatter diffraction (EBSD). Data were processed by Sigma Plot 10.0, HKL channel 5 and quantitative metallographic analysis including linear intercept method and point counting method. The weld zones were divided into four zones in this work, and they are the base material (BM) with equiaxed morphology, the heat affected zone (HAZ) with bimodal structure, thermo-mechanically affected zone (TMAZ) and the stir zone (SZ). Both the TMAZ and the SZ exhibit lamellar structure. Microhardness tests have been carried out on the cross sections, the normal section and the side sections for all the three welds with a constant rotation speed and on the cross sections for all the three welds with a constant traverse speed. Results showed that weld zones have slightly higher hardness than the BM; hardness at the advancing side (where tool rotation is as the same direction as the forward motion) is slightly harder than that at the retreating side (where tool rotation opposes the forward motion); an increased hardness was observed at the top surface regions of all the six welds and at the weld bottom of the two welds with 100 and 150mm/min traverse speed and microhardness distribution is most uniform in the four welds with a traverse speed of 150mm/min. Optical microscopy and scanning electron microscopy were used to investigate the cross sections on both horizontal and vertical lines revealing the microstructural evolution of the weld zones and the effect of traverse speed and rotation speed on the grain morphology of the bimodal structure and the lamellar structure, respectively. EBSD runs were collected from the cross sections of all the six welds for texture analysis with step sizes of 2μm and 3μm, and the texture analysis consists of the BM texture characterisation, the definition of TMAZ and HAZ, the representation of shear texture in the SZs, misorientation analysis of the welding edges of all the cross sections and two normal sections on the horizontal line. The BM was characterized by a classic rolling texture with equiaxed αp grains showing macrozones; the HAZ and the TMAZ both have developed a texture type containing three components: TD component, 45° component and WD component, using Ari-Gur and Semiatin's notation (1998); the SZ exhibited shear texture dominated by D1 (112)[111] simple shear texture component.

The work presented in this thesis is the result of an investigation into the closed loop velocity control of a d.c. arc, between parallel electrodes, subject to a transverse magnetic field. The velocity of the arc is in the order of 0.005 m/sec and has particular application to the processes of seam welding ,profile cutting and zone refining. Previous workers in this field have, over the past thirty years amassed a great deal of literature on the high velocity behaviour of electric arcs in magnetic fields. This has been used together with original experiments carried out by the author, to produce a mathematical model for a low velocity low power arc. A control system has been developed using an original arc velocity instrumentation technique the controller uses a hybrid form of both a digital computer and analogue components. By constantly monitoring the gain of the system, the controller is able to adapt itself to long-term changes in electrode surface conditions and to changes in electrode materials. Although it is not possible to control rapid variations in arc velocity, the feasibility of closed loop controlling the average velocity of electric arcs is demonstrated by output graphs of arc position versus time. These graphs indicate that average velocity can be controlled to within 10% of the input command indicating a suitability to the applications discussed above.

This study attempts to develop a set of parameters controlling the bead profile of deposits in powder melting, based on the spatial energy distribution of laser. Four parameters, identified as the laser material interaction parameters were used to study the bead profile formation in powder melting. The focus is put on control of the dimensional accuracy of powder deposits independently of the optical set-up and laser system. In the initial stage to understand the effect of welding parameters on the development of the fusion zone, a solid metal with homogenous and known thermal properties was used. The results indicate that for large beam diameters, typically used in cladding, power density and interaction time control the depth of penetration and beam diameter and interaction time controls the weld width. However, for small beam diameters, typically used in powder bed additive manufacturing, it was found that it is more difficult to achieve steady state conduction welds due to high conduction losses to the bulk material and rapid transition to keyhole regime. Therefore, with small beam diameters it is challenging to achieve pure conduction welds, which should guarantee good quality of deposits and low spatter. In the second part, the melting behaviour of solid material and powder for the same material type was compared. The build height in powder melting depends on layer thickness of the deposited powder and energy density, which needs to be provided to fuse the powder to the workpiece, which is equivalent to penetration in laser welding of solids. Similar to solid melting, the build width in powder melting is controlled by beam diameter and the interaction time. It was also found that with small beam diameters and large particle sizes it is more difficult to generate keyhole in the base plate, as compared to solid material. Therefore, despite the presence of spatter in the process, a full keyhole is often not generated. A set of parameters to describe the conduction welding process based on spatial distribution of laser energy has been developed. This enables achievement of a particular weld profile with various optical set-ups and potentially transfers of results between machines. However, more complex melting characteristics of powder requires some additional factors to be included to develop a similar model for powders.

The welding of C-Mn steels using the pulsed current MIG welding process was investigated. Following initial work on the basic fusion characteristics, the process was applied to the production of closed butt joint linepipe welding and narrow gap welding (in the flat and H-V positions). The initial stage of the work covered the determination of pulse parameters for controlled metal transfer with a variety of wires and shielding gases. Basic bead-on-plate fusion characteristics were investigated 1n all welding positions and quantified in terms of the major welding variables of current and travel speed. Welding position was shown to have no effect on fusion behaviour. Successful full penetration closed butt root welds were produced in all welding positions. Heat input was shown to be the factor controlling bead dimensions. Close control over welding parameters are needed to maintain consistent bead sizes. Complete joint simulations were also made. The vertical-up technique was shown to give superior fusion characteristics with the process producing excellent quality results in all positions. Narrow gap MIG welding was shown to be feasible without wire manipulation at gap widths below 12 mm in the flat position. A wire manipulation technique gave excellent fusion characteristics with gap widths greater than 12 mm. A computer program was devised as a guide to the selection of process parameters and possible fusion behaviour. Modelling considerations showed that H-V position narrow gap welding would be feasible only with gap widths less than 10 ... A short experimental programme validated the predictions of fusion behaviour by the model and indicated the direction for future development work.