Browsing by Author "Yapp, David"
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Item Open Access Advances in Gas Metal Arc Welding and Application to Corrosion Resistant Alloy Pipes(Cranfield University, 2010-03) da Costa Pépe, Nuno Vasco; Yapp, DavidAccording to recent estimations, the construction of pipelines will continue to increase during the next thirty years, in particular as a result of oil and gas discoveries in remote locations. Significant advances in welding technology during the last ten years have potential to provide improvements in productivity, quality and structural integrity of pipe girth welds. In this thesis, several new processes Lincoln STT, Lincoln RapidArc, Fronius CMT, Fronius CMT-P and Kemppi FastROOT have been compared the first time to the GMAW-P to understand how these new waveforms operate for pipe welding. The process setting parameters have been analysed to understand their effect on metal transfer and arc stability control, and on bead shape characteristics. Although all waveforms present similar burn-off ratios, individual waveforms differ considerably, and especially the arc voltage waveform. This leads to considerable differences in the mechanism of metal transfer and the stability of the processes under similar experimental conditions. Understanding of these new waveforms in terms of the effect of setting parameters in the mechanism of metal transfer, process stability and melting phenomena provides a basis for assessing the potential of these processes for a range of applications, and in particular application to CRA pipe root welding Since the arc energy is the overall energy delivered from the power source at the contact tip of the torch, and part of that energy is not absorbed by the workpiece, research was performed to measure the process efficiency associated with some of these waveforms and process setting conditions. The study led to a better understanding of the potential errors in calculating process efficiency. The results obtained show that all the short-circuiting waveforms analysed (i.e. CMT, STT and FastRoot) had a similar process efficiency of 90±3%, while pulse spray waveforms (GMAW-P, CMT-P and RapidArc) are characterized by lower process efficiency, approximately 78±3%. The application of these waveforms to the welding a narrow groove pipe with a “J” groove design was investigated. These analyses were focused on the variation of bead shape characteristics and welding quality performance based on the analysis of the conditions that result in lack of penetration and top bead defects, such as lack of side wall fusion or undercutting. It was observed that RapidArc and CMT-P are able to satisfy the quality requirements, i.e. full penetration and absence of defects for the specific conditions described in this thesis. High welding speeds (up to 1m/min) were achieved with these processes, four times the typical speed 0.25m/min. Finally, the shielding gas plays an important role in terms of quality and weld bead performance. This led to an optimization of the shielding gas composition used, based on mixtures of carbon dioxide, argon and helium. Statistical modelling was undertaken to optimize the shielding gas mixtures using RapidArc and CMT-P waveforms. In parallel, a new purging shielding gas device was designed to achieve a weld root free of oxidation.Item Open Access Arc-based sensing in narrow groove pipe welding(Cranfield University, 2006-08) Gil Teixeira Lopes, Agostinho; Yapp, DavidBig gains in productivity are found in tandem and dual tandem pipeline welding but require highly skilled operators who have to control the position of the torch very accurately for long periods. This leads to high demands on the skills and stamina of the operators of mechanised pipeline welding systems. There is a very strong motivation to fully automate the welding process in order to reduce the required skills and to improve consistency. This project focuses on the use of through-the-arc sensing for seam following and contact-tip-workpiecedistance (CTWD) control. A review of literature reveals very little development work on arc sensing for Pulsed Gas Metal Arc Welding (GMAW-P) in narrow grooves. GMAW-P is often used to achieve optimum properties in weld quality and fusion characteristics and also positional welding capability, all of which are important factors for pipeline welding. The use of through-the-arc sensing for narrow groove pipe welding applications poses specific challenges due to the steep groove sidewalls and the use of short arc lengths, producing very different behaviour compared to V-groove arc sensing techniques. Tandem welding is also quite different from single wire techniques with both wires working in close proximity producing mutual interferences in arc signals. An investigation was conducted in order to assess GMAW-P arc signals and it was found that improved consistency, higher sensitivity and less noise was present in voltages in the peak current period (peak voltages) used for torch position control. As a result of this investigation, a CTWD and cross-seam control system was developed and tested for single and tandem GMAW-P, using a 5º narrow groove. The test results have revealed accuracies for both controls of better than 0.2 mm. CTWD control was developed by following the existent welding procedure voltage average and cross-seam control by peak voltage comparison between maximum torch excursions. Experiments were also performed to evaluate the influence of torch oscillation frequency on arc voltage behaviour and sensitivity, along with weld bead characteristics and fusion profiles. The resultant arc signal sensitivity was consistent with the results found in the literature for conventional GMAW. For GMAW-P, although no data was available from the literature for comparison, the results have shown no increase in sensitivity with the increase of oscillation frequency with the welding setup used. Bead profile analysis performed at different sidewall proximities indicated that optimum wire to sidewall proximities can be found between 0 mm and +0.2 mm, measured from the outer edge of the wire to the sidewall corner. Accurate control is required since +1 mm proximity produced poor sidewall fusion and no signal differentiation for control recognition of groove width. This work showed that negative proximities or wire proximity beyond the sidewall produce wire burn back and hence very long arc lengths, resulting in poor depths of penetration and shallower beads, with major undercut defects. In addition, this work has also shown the importance of torch oscillation width control, in order to produce accurate cross-seam control. A method is proposed to achieve torch oscillation width control by a continuous peak voltage comparison between centre and sidewall torch positions, using the optimum values of wire to sidewall proximity found and the resultant peak voltage value. This control will also provide a clear indication of actual groove width. Clearly this data can also be used to implement a system which adapts welding parameters to groove width.Item Open Access Conduction mode: broadening the range of applications for laser welding(2012-11-14) Assuncao, D. E.; Ganguly, Supriyo; Yapp, David; Williams, Stewart W.; Mustafa, KocakConduction laser welding opens up a range of innovative applications for laser welding. This relatively novel mode of laser processing expands the application potential significantly beyond what is normally achieved today. The main reason for this could be attributed to the different characteristics of conduction process when compared to keyhole laser welding. An example is the higher stability of conduction which results in welds of higher quality and better control of the welding process. Despite the advantages of conduction laser welding, it is yet to be exploited significantly for industrial applications and there are very few applications for which this mode of operation is used. This paper is aimed at presenting different varieties of applications for conduction laser welding using a fibre laser. This ranges from high quality aluminium welds to laser brazing of stainless steel to metal foams. The objective of this paper is to highlight the main features of conduction laser welding process and exemplify some conduction laser welding applications.Item Open Access Effect of welding thermal cycles on the heat affected zone microstructure and toughness of multi-pass welded pipeline steels(Cranfield University, 2012-09) Nuruddin, Ibrahim K.; Ganguly, Supriyo; Yapp, DavidThis research is aimed at understanding the effect of thermal cycles on the metallurgical and microstructural characteristics of the heat affected zone of a multi-pass pipeline weld. Continuous Cooling Transformation (CCT) diagrams of the pipeline steel grades studied (X65, X70 and X100) were generated using a thermo mechanical simulator (Gleeble 3500) and 10 mm diameter by 100 mm length samples. The volume change during phase transformation was studied by a dilatometer, this is to understand the thermodynamics and kinetics of phase formation when subjected to such varying cooling rates. Samples were heated rapidly at a rate of 400°C/s and the cooling rates were varied between t8/5 of 5.34°C/s to 1000°C/s. The transformation lines were identified using the dilatometric data, metallographic analysis and the micro hardness of the heat treated samples. Two welding processes, submerged arc welding (SAW) and tandem Metal Inert Gas (MIG) Welding, with vastly different heat inputs were studied. An API-5L grades X65, X70 and X100 pipeline steels with a narrow groove bevel were experimented with both welding processes. The welding thermal cycles during multi-pass welding were recorded using thermocouples. The microstructural characteristics and metallurgical phase formation was studied and correlated with the fracture toughness behaviour as determined through the Crack Tip Opening Displacement (CTOD) tests on the welded specimens. It was observed that SAW process is more susceptible to generate undesirable martensite-austenite (M-A) phase which induce formation of localised brittle zones (LBZ) which can adversely affect the CTOD performance. Superimposition of the multiple thermal cycles, measured in-situ from the different welding processes on the derived CCTs, helped in understanding the mechanism of formation of localised brittle zones. Charpy impact samples were machined from the two X65 and X70 grades, for use in thermal simulation experiments using thermo mechanical simulator (Gleeble). The real thermal cycles recorded from the HAZ of the SAW were used for the thermal simulations, in terms of heating and cooling rates. This is to reproduce the microstructures of the welds HAZ in bulk on a charpy impact sample which was used for impact toughness testing, hardness and metallurgical characterisation. The three materials used were showing different response in terms of the applied thermal cycles and the corresponding toughness behaviours. The X65 (a) i.e. the seamless pipe was showing a complete loss of toughness when subjected to the single, double and triple thermal cycles, while the X65 (b), which is a TMCP material was showing excellent toughness in most cases when subjected to the same thermal cycles at different test temperatures. The X70 TMCP as well was showing a loss of toughness as compared to the X65 (b). From the continuous cooling transformation diagrams and the thermally simulated samples results it could be established that different materials subjected to similar thermal cycle can produce different metallurgical phases depending on the composition, processing route and the starting microstructure.Item Open Access High-Speed GMAW and Laser GMAW Hybrid Welding of Steel Sheet(Cranfield University, 2009-03) Harris , I. D.; Yapp, DavidArc 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. Cont/d.Item Open Access An Integrated Approach to the Determination and Consequences of Residual Stress on the Fatigue Performance of Welded Aircraft Structures.(2006-02-01T00:00:00Z) Edwards, L.; Fitzpatrick, M. E.; Irving, Phil E.; Sinclair, I.; Zhang, X.; Yapp, DavidAlthough residual stress in welded structures and components has long been known to have an effect on their fatigue performance, access to reliable, spatially accurate residual stress field data has been limited. Recent advances in neutron and synchrotron X-ray diffraction allow a far more detailed picture of weld residual stress fields to be obtained that permits the development and use of predictive models that can be used for accurate design against fatigue in aircraft structures. This paper describes a fully integrated study of the three- dimensional residual stress distribution accompanying state-of-the-art fusion welds in 2024-T4 aluminum alloy, and how it is affected by subsequent machining and service loading. A particular feature of this work has been the development of techniques allowing the nondestructive evaluation of the residual stress field in the full range of specimens used to provide the design data required for welded aircraft structures and the integration of this information into all aspects of damage tolerant design.Item Open Access Interaction time and beam diameter effects on the conduction mode limit(Elsevier Science B.V., Amsterdam., 2012-06-30T00:00:00Z) Assunção, Eurico; Williams, Stewart W.; Yapp, DavidLaser welding has two distinctive modes: keyhole and conduction mode. Keyhole mode is characterized by deep penetration and high welding speeds, while conduction mode has higher quality welds with no defects or spatter. This study focuses on the transition from conduction to keyhole mode by increasing power density and using different beam diameters and interaction times. Based upon the results it was possible to evaluate that there is a transition mode between conduction and keyhole mode. The results show that the transition between conduction and keyhole mode is not defined by a single power density value. This transition has a range of power densities that depend on the beam diameter and on the interaction time. This study allows the identification of the power density that limits conduction mode, based on parameters such as beam diameter and interaction time instead of a single power density value independent of these parameters.Item Open Access Investigation of conduction to keyhole mode transition(Cranfield University, 2012-07) Goncalves Assuncao, Eurico; Williams, Stewart W.; Yapp, DavidThere 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.Item Open Access Measuring the process efficiency of controlled gas metal arc welding processes(2011-07-31T00:00:00Z) Pepe, Nuno; Egerland, Stephan; Colegrove, Paul A.; Yapp, David; Leonhartsberger, Andreas; Scotti, AmeiricoThe thermal or process efficiency in gas metal arc welding (GMAW) is a crucial input to numerical models of the process and requires the use of an accurate welding calorimeter. In this paper, the authors compare a liquid nitrogen calorimeter with an insulated box calorimeter for measuring the process efficiency of Fronius cold metal transfer, Lincoln surface tension transfer and RapidArc, Kemppi FastRoot and standard pulsed GMAW. All of the controlled dip transfer processes had a process efficiency of ∼85% when measured with the liquid nitrogen calorimeter. This value was slightly higher when welding in a groove and slightly lower for the RapidArc and pulsed GMAW. The efficiency measured with the insulated box calorimeter was slightly lower, but it had the advantage of a much smaller random erroItem Open Access Mechanical characterisation and modelling of resistance welding(Cranfield University, 2011-07) Van Rymenant, Patrick; Yapp, DavidResistance 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.Item Open Access Real time evaluation of weld quality in narrow groove pipe welding(Cranfield University, 2012-10) Marmelo, Patricia C.; Yapp, DavidWith the growth in pipeline installations all over the world, there is a great demand for highly productive and robust welding systems. Mechanised pipe welding has been developed over the last 50 years and the present focus is towards development of automated pipeline welding systems. Pipeline welding automation is aimed at reducing costs and improving the installation quality. To attain fully automated pipe welding systems there is a need to rely on sensors and controls systems to mimic human like capabilities, such as visual inspection, in real time. The key aim of this work is to develop and evaluate methods of automatic assessment of weld bead shape and quality during narrow gap GMAW of transmission pipelines. This implies that the measured bead profile will be assessed to determine whether the bead shape will cause defects when the subsequent pass is deposited. Different approaches have been used to conquer the challenge that is emulating human reasoning, all with different objectives in mind. In spite of extensive literature research performed, very little information was found concerning the real time determination and assessment of bead shape quality and none of it was reported to be applied successfully to the pipeline industry. Despite the continuous development of laboratory laser vision systems commercial ones have been on the market for decades, some specifically developed for the welding application. Laser vision sensor systems provide surface profile information, and are the only sensors which can satisfactorily measure bead profile on a narrow groove. In order to be able to use them to automatically assess weld bead shape and quality, a deep understanding of their characteristics and limitations needs to be achieved. Once that knowledge was attained it was then applied to determine the best sensor configuration for this purpose. After that the development of human like judgment algorithms were developed to accomplish the aim that was set. Empirical rules were obtained from an experienced welder regarding the acceptability of bead shapes and were then applied in the developed system with good results. To scientifically evaluate and determine the rules to use in this system, further experiments would be required. The output of the system developed showed very accurate, reliable and consistent results that were true to the external measurements and comparisons performed. The developed system has numerous applications in the pipeline industry and it could easily be implemented on commercial systems.Item Open Access Tandem gas metal arc pipeline welding(Cranfield University, 2007-04) Liratzis, Theocharis; Yapp, DavidEnergy consumption has grown by 2% per year worldwide over the past ten years. In 2005 worldwide 900,000 barrels of oil and 7.6 billion cubic metre of natural gas were produced daily. The exploitation of fields to meet the increased demands in energy requires the presence of adequate infrastructures. High strength pipeline steels(X100) have been developed to operate at higher pressures allowing a greater volume of fuel to be transported. Additional advantages arising from the reduction in wall thickness contribute to reduction in construction costs and steel volume. Cont/d.Item Open Access Variable Polarity (AC) Arc Weld Brazing of Galvanized Sheet.(2003-07-01T00:00:00Z) Joseph, Andy; Webb, Chris; Haramia, Mike; Yapp, DavidRequirements for improved durability in domestic consumer goods have invariably led to the application of more corrosion resistant materials. The basic material used by many industries, including automotive, ventilation and heating, construction, appliances, and furniture is sheet steel. Zinc-coated or galvanized sheet steels are the obvious corrosion resistant material choice for these industries. Steel sheet can be coated with zinc (galvanized) through either electrolytic or hot-dip processes, with coating weight between 30 g/m 2 and 400 g/m 2 (approximately 4 à µm to 55 à µm thick) depending on the process conditionItem Open Access Variable polarity plasma arc welding of high strength aluminium alloys(Cranfield University, 2004-12) Serrano, G. L.; Yapp, DavidThe VPP A W process has been evaluated and optimised for the joining of high strength aluminium alloys AA-2024 T351, AA-7150 W51 and AA-7150 T651. The initial experimentation was performed with conventional AC-GTAW power supplies with limited results. Failure of the control logic to stabilise the welding process or arc extinction became an issue. Not until a suitable VPP AW power source became available were sound welding conditions developed. Still, a great effort was placed in optimising the process. Factors such as the welding torch, the electrode composition, the plasma nozzle geometry, the pilot arc current, the electrode positive duration and amplitude, the shielding gas quality and the effect of gas backing had to be thoroughly studied to achieve sound welding conditions. Subsequent to the optimisation process, operational envelopes were generated to determine the most favourable welding conditions. Based on those, reliable techniques were developed to produce sound welds on butt welded assemblies. Further on, a special plasma welding torch was developed to weld complex structures such as dual stringer panels. A number of these panels were manufactured and surpassed the damage tolerance requirements of Airbus. Hardness profiles indicated a dip in hardness in the far heat affected zone. Microstructural observation of this area revealed significant coarsening of the precipitates related to the relatively high heat input conditions of the VPPAW process. Studies which involved the reduction of the heat input were carried out. Variables such as the addition of filler wire, the use of helium as shielding gas, the combined effect of both, and the use of an edge preparation were assessed. Temperature measurements were performed at different positions along the HAZ in order to relate the thermal history with the microstructural degradation by means of cooling curves and TTT diagrams. Similar investigations were performed under low heat input welding conditions with DC-GT A W helium shielding. I Variable polarity plasma arc welding of high strength aluminium alloys. Tensile tests were performed on selected specimens in order to asses the effect of temper condition, post weld heat treatment and heat input. The results of which indicated that low heat input conditions improved the tensile properties of the far heat affected zone. With regards to temper condition it was observed that the W51 temper was more advantageous. It was also noticed that VPPA welds offered similar 0.2% proof stress and ultimate tensile strength than that of MIG welds, together with, although still very low, improved elongation properties. This piece of research has involved the welding of high strength aluminium alloys which were considered difficult to weld or even unweldable in some cases. Prior attempts were performed with MIG welding but micro voids acting as initiation sites for cracks limited the damage tolerance of the assemblies. On the contrary VPPAW has proved its ability to produce extremely high quality welds where gas porosity or micro voids were almost inexistent.Item Open Access Welding with high power fiber lasers - A preliminary study.(Elsevier Science B.V., Amsterdam., 2007-01-01T00:00:00Z) Quintino, Luísa; Costa, A.; Miranda, R.; Yapp, David; Kumar, V.; Kong, C. J.The new generation of high power fiber lasers presents several benefits for industrial purposes, namely high power with low beam divergence, flexible beam delivery, low maintenance costs, high efficiency and compact size. This paper presents a brief review of the development of high power lasers, and presents initial data on welding of API 5L: X100 pipeline steel with an 8 kW fiber laser. Weld bead geometry was evaluated and transition between conduction and deep penetration welding modes was investigated.