102 research outputs found

    Development of a real-time ultrasonic sensing system for automated and robotic welding

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    This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.The implementation of robotic technology into welding processes is made difficult by the inherent process variables of part location, fit up, orientation and repeatability. Considering these aspects, to ensure weld reproducibility consistency and quality, advanced adaptive control techniques are essential. These involve not only the development of adequate sensors for seam tracking and joint recognition but also developments of overall machines with a level of artificial intelligence sufficient for automated welding. The development of such a prototype system which utilizes a manipulator arm, ultrasonic sensors and a transistorised welding power source is outlined. This system incorporates three essential aspects. It locates and tracks the welding seam ensuring correct positioning of the welding head relatively to the joint preparation. Additionally, it monitors the joint profile of the molten weld pool and modifies the relevant heat input parameters ensuring consistent penetration, joint filling and acceptable weld bead shape. Finally, it makes use of both the above information to reconstruct three-dimensional images of the weld pool silhouettes providing in-process inspection capabilities of the welded joints. Welding process control strategies have been incorporated into the system based on quantitative relationships between input parameters and weld bead shape configuration allowing real-time decisions to be made during the process of welding, without the need for operation intervention.British Technology Group (BTG

    Visualisation of shielding gas flows during high-value manufacture

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    This thesis is a collection of experimental and theoretical analyses of the behaviour of inert gases during material processing. The approach taken is the use of a combination of schlieren imaging and numerical simulations to understand the physical mechanisms associated with the gas flows in each process. The visualisations carried out experimentally were used to validate the models, while the models aided in the interpretation of the imaged refractive index gradients. For gas metal arc welding (GMAW), the variation of Ar input flowrate with varying torch angle, standoff and joint type was investigated. Magnetohydrodynamic (MHD) models of the arc and electrodes showed that air entrainment was determined by the interplay between the momentum in the shielding gas stream and the inwards pull of Lorentz forces which develop within the plasma jet. Good agreement was found between the images and model, showing that gas coverage decreased at values below 9 l/min. Torch angle and standoff were shown to not significantly influence coverage. Similar coverage was found to occur in bead on plate and fillet welds under the same conditions. Further experiments using flux-cored, gas shielded arc welding (FCAW-G) and 80% Ar / 20% CO2 gas allowed good quality welds to be deposited with flowrates as low as 3 l/min. These results supported the use of flow controllers in production welding units at BAE systems Govan, leading to cost savings and reduced environmental impact by locking the gas flowrate to 12 l/min. A study of gas tungsten arc welding (GTAW) using alternating Ar and He shielding gases as a method of arc pulsing was also investigated. The effects of pulsing frequency and input flowrate were investigated. When pulsing, it was found that alternating the gases resulted in He constriction close to the arc region due to the preceding Ar pulse. Comparison of weld macrographs showed that He can be used more efficiently through alternating technique compared to a premixed gas with the same He content. The schlieren system was used to analyse the flow of Ar from a trailing shield device and plasma arc welding (PAW) torch in the context of wire-arc additive manufacture (WAAM). Flow characterisation with changes in standoff and welding configuration showed that air entrainment can be minimised when using a trailing shield. However, increasingly tall parts were insufficiently covered due to interactions between fast jets from the torch and shielding gas streams. MHD modelling of the torch allowed the characterisation of heat transfer and O2 levels with varying input current or over different geometrical features. The final study in this thesis concerns the fluid-particle interactions in laser powder-bed fusion (LPBF). High-speed direct and schlieren imaging showed that differences in laser plume orientation arise from different process settings, even under the same energy input. It was shown that the denudation of the powder bed was caused by drag forces acting on particles, due to atmospheric gas flow induced by the plume. Numerical modelling was in good agreement with the experiments, indicating that evaporative phenomena are an integral part of the heat and mass transfer in LPBF

    Machine-human Cooperative Control of Welding Process

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    An innovative auxiliary control system is developed to cooperate with an unskilled welder in a manual GTAW in order to obtain a consistent welding performance. In the proposed system, a novel mobile sensing system is developed to non-intrusively monitor a manual GTAW by measuring three-dimensional (3D) weld pool surface. Specifically, a miniature structured-light laser amounted on torch projects a dot matrix pattern on weld pool surface during the process; Reflected by the weld pool surface, the laser pattern is intercepted by and imaged on the helmet glass, and recorded by a compact camera on it. Deformed reflection pattern contains the geometry information of weld pool, thus is utilized to reconstruct its 33D surface. An innovative image processing algorithm and a reconstruction scheme have been developed for (3D) reconstruction. The real-time spatial relations of the torch and the helmet is formulated during welding. Two miniature wireless inertial measurement units (WIMU) are mounted on the torch and the helmet, respectively, to detect their rotation rates and accelerations. A quaternion based unscented Kalman filter (UKF) has been designed to estimate the helmet/torch orientations based on the data from the WIMUs. The distance between the torch and the helmet is measured using an extra structure-light low power laser pattern. Furthermore, human welder\u27s behavior in welding performance has been studied, e.g., a welder`s adjustments on welding current were modeled as response to characteristic parameters of the three-dimensional weld pool surface. This response model as a controller is implemented both automatic and manual gas tungsten arc welding process to maintain a consistent full penetration

    Optimisation of welding parameters to mitigate the effect of residual stress on the fatigue life of nozzle–shell welded joints in cylindrical pressure vessels.

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    Doctoral Degree. University of KwaZulu-Natal, Durban.The process of welding steel structures inadvertently causes residual stress as a result of thermal cycles that the material is subjected to. These welding-induced residual stresses have been shown to be responsible for a number of catastrophic failures in critical infrastructure installations such as pressure vessels, ship’s hulls, steel roof structures, and others. The present study examines the relationship between welding input parameters and the resultant residual stress, fatigue properties, weld bead geometry and mechanical properties of welded carbon steel pressure vessels. The study focuses on circumferential nozzle-to-shell welds, which have not been studied to this extent until now. A hybrid methodology including experimentation, numerical analysis, and mathematical modelling is employed to map out the relationship between welding input parameters and the output weld characteristics in order to further optimize the input parameters to produce an optimal welded joint whose stress and fatigue characteristics enhance service life of the welded structure. The results of a series of experiments performed show that the mechanical properties such as hardness are significantly affected by the welding process parameters and thereby affect the service life of a welded pressure vessel. The weld geometry is also affected by the input parameters of the welding process such that bead width and bead depth will vary depending on the parametric combination of input variables. The fatigue properties of a welded pressure vessel structure are affected by the residual stress conditions of the structure. The fractional factorial design technique shows that the welding current (I) and voltage (V) are statistically significant controlling parameters in the welding process. The results of the neutron diffraction (ND) tests reveal that there is a high concentration of residual stresses close to the weld centre-line. These stresses subside with increasing distance from the centre-line. The resultant hoop residual stress distribution shows that the hoop stresses are highly tensile close to the weld centre-line, decrease in magnitude as the distance from the weld centre-line increases, then decrease back to zero before changing direction to compressive further away from the weld centre-line. The hoop stress distribution profile on the flange side is similar to that of the pipe side around the circumferential weld, and the residual stress peak values are equal to or higher than the yield strength of the filler material. The weld specimens failed at the weld toe where the hoop stress was generally highly tensile in most of the welded specimens. The multiobjective genetic algorithm is successfully used to produce a set of optimal solutions that are in agreement with values obtained during experiments. The 3D finite element model produced using MSC Marc software is generally comparable to physical experimentation. The results obtained in the present study are in agreement with similar studies reported in the literature

    Predicting weld distortion in the design of automotive components

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    This project was set up in order to investigate whether it may be valid to use a simplified approach to weld distortion prediction in order to make an assessment of the distortions occurring in a welded structure. Distortion may manifest as problems associated with production, which increase costs and reduce profit. In order to validate the proposed prediction approach an experiment was developed to gain detailed information about distortions occurring in a series of simple welded joints. The joints and the parameters used for the experiment were based on the processes and applications of the case study company. The experiment was set up with the aim of evaluating the general magnitude and directions of distortion in bead on plate and butt weld joints to develop a database of distortion. The method for inspection of the experimental samples was to scan the parts using a 3D laser scanner to collect a detailed resolution point cloud that could be analysed. From the experimental results a number of key factors relating to the welded joints were found relating to material thickness, weld speed and penetration for the four different modes of distortion occurring. The experimental results were compared with the published data and equations presented by other authors, and some general agreements found, however, some differences were evident. In order for a designer to adjust the shape and form of a components design to counteract the distortions to achieve a nominal tolerance, based on these results, it was necessary to develop some new models specific to the materials and process variables of the case study company. Using an approach based on DoE software techniques, response surfaces for the experimental results were generated. This allowed equations to be developed for each distortion mode, which a designer could use to make predictions in the design phase to reduce risk from distortion

    Arc-based sensing in narrow groove pipe welding

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    Big 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.EThOS - Electronic Theses Online ServiceBP Exploration and Pipeline Research Council International (PRCI)GBUnited Kingdo

    Thermomechanical factors influencing weld metal hydrogen assisted cold cracking

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    Hydrogen Assisted Cold Cracking is a phenomena that manifests itself in weldments when a critical hydrogen concentration is trapped within a susceptible microstructure and subjected to a threshold level of stress and poses a significant threat to pipeline girth weld integrity, in particular the root pass of girth welds. This thesis explores the thermomechanical factors influencing the formation of Weld Metal Hydrogen Assisted Cold Cracks (WMHACC) in High Strength Low Alloy line pipe steel when welded with cellulosic electrodes using the Manual Metal Arc Welding process. The overarching objective of this body of work is to delineate a safe boundary across which the characteristics of weld metal samples can be defined to enhance the understanding of the factors which influence the formation of weld metal hydrogen cracks. The significance of achieving this objective is the improved ability to predict the onset of the phenomenon, consequently facilitating the development of strategies, which can be assimilated by industry to minimise the presence of cold cracks increasing the safety and reliably of pipeline girth welds. To address the overarching objective and address the limitations identified though the literature review, in particular the limited number of studies focusing on welding in the Australian context, a three-part experimental and analytical program was developed and undertaken. Development of an Enhanced Weldability Test. To facilitate deposition of tests welds under high restraint in a range of heat inputs which reflected the desired test window, an enhanced weldability test, the MWIC, was designed and commissioned. The test’s geometry and characteristics were based on the well-established Welding Institute of Canada Weldability test, but was enhanced to allow for deposition of welds at very low heat inputs, of which no published cracking data exits. Additionally, the test was modified to allow for the extraction of critical welding data, facilitating its use as a research tool. Delineation of Safe Welding Envelope. Using the MWIC test, deposition envelopes were created for thick (20mm) and thin (10mm) sections of API 5L X70 line pipe steel welded with E6010 electrodes under high and low restraint. Welds were deposited at low heat inputs (<1kJ/mm) over a range of preheats and examined under magnification to establish the critical/ threshold preheats above which no cracking was observed. Characterisation of weld metal samples across the derived cracking boundary. The weldability test samples generated from weldability testing were characterised using a range of macroscopic, microstructural, and micromechanical techniques to establish the interrelationship between cracking and thermomechanical parameters. The influence of heat input on bead eccentricity was established and its consequent effect on the welds susceptibly to hydrogen cracking was proposed. The potential interrelationship between solidification and hydrogen cracks was discussed in relation to restraint levels. The inefficiency of the currently accepted hardness threshold of 350HV used to predict the onset of HAZ-HACC as a proxy for Weld Metal Hydrogen Assisted Cold Cracking susceptibly was established for all test cases. Additionally, the use of micromechanical properties as a proxy, in particular the use of the ratio of E/H was introduced as a means of quantifying susceptibly to WMHACC.Thesis (Ph.D.) -- University of Adelaide, School of Mechanical Engineering, 2017

    Modelling of the Welding Process using Bayesian Network and Applying Data Collected from Several Sources

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