Browsing by Author "Richardson, Ian M."

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    Development and application of advanced plasma welding techniques for high strength aluminium alloys
    (Cranfield University, 1999-04) Tzevelekos, P; Richardson, Ian M.
    The main aims of this study are to generate a methodology for the optimisation of welding procedures for plasma welding of thin sheet aluminium alloys and to investigate weld quality modelling. Emphasis is focused on the recognition and evaluation of the consecutive stages of welding procedure development and on the formulation of a generalised procedural methodology that is potentially applicable to other processes and materials. The materials under investigation were 1.6 mm thick sheet 6013, 2024 and 7475 alloys, representing the major medium and high strength heat treatable aluminium groups used in the aerospace industry. Initial experimentation generated procedures relating to specimen and equipment pre-weld preparation. Bead on plate and square butt joint trials were performed in the downhand position. The plasma keyhole mode was operated autogenously while filler wire addition was employed for the melt-in mode. Arc monitoring techniques were used to log the arc voltage and welding current values. The effect of background and control parameters on process performance and joint quality was studied and used to generate operating envelopes and reveal optimum welding conditions. Geometrical data from the melt-in joints of all alloys were employed to build joint geometry prediction statistical models. Numerical algorithms, based on the information generated by the statistical models, were used to create joint geometry optimisation techniques. Calorimetric experiments and x-ray examination of joints revealed the relationship between major operating parameters and arc efficiency and the incidence of porosity, suggesting desired welding conditions that were incorporated in the optimisation process. Finally, two software tools for Joint Geometry Prediction and Welding Procedure Optimisation were developed, incorporating the knowledge and information created during the modelling and optimisation stages. These provide the end user with a means of process parameter selection and visualisation of the influence of parameter variation on weld bead geometry.
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    Orbital plasma welding of small bore tubes
    (Cranfield University, 1997) Tazedakis, A. S.; Richardson, Ian M.
    This work was primarily motivated by the industrial need for control of problems associated with the Gas Tungsten Arc Welding (GTAW) of small bore titanium and austenitic stainless steel tubes. These include: pore creation and entrapment in the weld zone, and variability of the fusion zone geometry. The primary aim of this study was the development of a low current orbital plasma welding capability using a structured approach which could lead to defect minimisation. The methodology should also have the potential to be used in a number of different conditions, extending the use of plasma welding in both melt-in and keyhole modes for the orbital welding of small bore tubes. The project originally involved the modification of a totally enclosed orbital GTAW welding head for low current welding operations. It was established that for the current range required for small bore and small to medium thickness tubes, the use of a solid copper torch was sufficient to provide the required heat absorption. A stable arc was produced even for very low current values (down to 7A) while arc voltages were within the operating range of a standard GTA welding power source. Procedural (i.e. off line) control was adopted for identification and optimisation of welding parameters. Since no procedure was available for the proposed welds it was necessary to generate the parameters required for the production of consistent weld profiles. Simultaneously, an expert system has been developed for the determination of optimum process parameters based on empirical models, developed using statistical techniques. Parameter combinations were selected based on physical as well as statistical relevance, providing a measure of confidence when predicting the required weld bead output characteristics. The approach also indicates the influence of the major input parameters on weld bead geometry and defect formation, such as undercut. Two quality acceptance criteria were employed during this investigation, weld bead dimensional accuracy, and the type and seriousness of defects present (penetration / burn-through, porosity and undercut). Off line programming was utilised to control heat build up and to ensure welds were obtained with the desired geometry and minimal defect levels. The end result was the development of a prototype system for low current orbital plasma welding (in both melt-in and keyhole mode) of small bore tubes in a totally enclosed head. Tolerant procedures for low current orbital melt-in and particularly keyhole welding have been generated and a systematic methodology for the prediction and optimisation of welding procedures based on predetermined criteria has been developed.
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    Pool oscillations and cast variations - penetration control for orbital tig welding of austenitic stainless steel tubing.
    (Cranfield University, 1997-03) Woodward, Neil J.; Richardson, Ian M.
    Pool oscillations in tungsten inert gas welding pools have been used in a closed- loop control system for orbital welding of ultra high purity tubing, determining a target level of penetration by altering the welding current in real-time. The technique is ideally suited to this application since it is does not contravene the cleanliness requirements for the inner bore and can be implemented outside the small orbital heads that are commonly used. The results presented in this thesis show how clear pool oscillation signals in extremely small molten pools can be monitored by optimising the welding conditions and signal processing of the arc voltage signal. As an indicator of the likely variation in cast behaviour present particularly in austenitic stainless steels, a 'time-to-penetrate' characterisation was made of the materials, using the time of the transition from the Mode 1 to the Mode 3 oscillation behaviour as the measured variable. By applying the test across a range of welding currents, significant insight was obtained into the cast and associated penetration behaviour. Late transitions indicated casts that exhibited significantly different responses to the more usually applied welding procedures, especially at the lower levels of welding current (highlighting their potentially more problematic penetration behaviour). It was shown that the established theoretical models were difficult to apply with certainty to moving weld pools, and consequently a fuzzy logic model was used in the control strategy. The closed-loop system comprised a user-interface PC, a control rack and commercial welding power source - control signals were applied every 2 to 3 Hz. Mode 3 pool oscillations were found to offer a more than satisfactory sensitivity to the inner bead width created for the various casts of 1.65 mm wall thickness materials studied.
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    Revealing internal flow behaviour in arc welding and additive manufacturing of metals
    (Nature Publishing Group, 2018-12-21) Aucott, Lee; Dong, Hongbiao; Mirihanage, Wajira; Atwood, Robert; Kidess, Anton; Gao, Shian; Wen, Shuwen; Marsden, John; Feng, Shuo; Tong, Mingming; Connolley, Thomas; Drakopoulos, Michael; Kleijn, Chris R.; Richardson, Ian M.; Browne, David J.; Mathiesen, Ragnvald H.; Atkinson, Helen
    Internal flow behaviour during melt-pool-based metal manufacturing remains unclear and hinders progression to process optimisation. In this contribution, we present direct time-resolved imaging of melt pool flow dynamics from a high-energy synchrotron radiation experiment. We track internal flow streams during arc welding of steel and measure instantaneous flow velocities ranging from 0.1 m s−1 to 0.5 m s−1. When the temperature-dependent surface tension coefficient is negative, bulk turbulence is the main flow mechanism and the critical velocity for surface turbulence is below the limits identified in previous theoretical studies. When the alloy exhibits a positive temperature-dependent surface tension coefficient, surface turbulence occurs and derisory oxides can be entrapped within the subsequent solid as result of higher flow velocities. The widely used arc welding and the emerging arc additive manufacturing routes can be optimised by controlling internal melt flow through adjusting surface active elements.
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    Spectroscopic determination of temperature distributions for a TIG arc
    (Cranfield University, 1993) Thornton, M. F.; Richardson, Ian M.
    Argon TIG arc temperatures have been measured for a wide range of arc parameters using the 'Fowler-Milne' spectroscopic method. Prompted by widespread disagreement amongst temperatures published by previous groups, a detailed investigation has been carried out into those experimental and theoretical aspects of the measurement process that may have led to incorrect results. The tests have included the variation of experimental parameters, the choice of Abel inversion procedure, and the calculation of argon species number densities and partition functions. The existence of equilibrium within the TIG arc has been investigated by determining temperatures with a number of argon emission lines. Significant differences 'in derived values of the temperatures were observed within 1mm of the tungsten cathode tip, smaller variations were observed over the remainder of the arc. The results indicate that large scale departures from a Boltzmann distribution amongst argon atom excited states exists close to the tungsten tip, deviations decrease with increasing distance from the tip but do not disappear completely. lt is believed that the breakdown of equilibrium within the argon TIG arc may help to explain the disagreement in temperatures obtained by past groups that have used spectroscopic methods. Despite evidence for non-equilibrium within the arc. results from previous groups suggest that derived values of .the temperatures are substantially correct in the main body of the arc. The detailed temperature maps provide useful information on temperature changes with arc parameters for the purposes of modelling and understanding of the arc.