15,575 research outputs found

    Arc pressure and weld metal fluid flow whilst using alternating shielding gases Part 2 : arc force determination

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    The transient variation of the shielding gas present in the alternating shielding gas process produces a dynamic action within the liquid weld metal. Flow vectors opposite in direction have been reported due to the various forces acting on the weld metal when argon and helium are present, however no data has been provided to substantiate this claim. This part of the study evaluates the various forces acting on the liquid weld metal when using argon and helium and their effects discussed. It was determined that argon produces a greater vertically downward force in the central region than helium for both the arc force and Lorentz force. While helium produces a greater radially outwards force at the pool surface than argon due to plasma shear stress and Marangoni convection. In addition, the buoyancy force, i.e. the vertically upward force in the central portion of the weld metal, was greater for helium

    Arc pressure and weld metal fluid flow whilst using alternating shielding gases Part 1 : arc pressure measurement

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    As part of an ongoing process to fully evaluate the effects of an alternating shielding gas supply on gas shielded welding processes, a comparison between the arc pressures generated using argon, helium, alternating shielding gases and pulsed GTAW has been conducted. Arc pressure variation and peaking are two of the fundamental phenomena produced during the alternating shielding gas process and are said to help create a stirring action within the liquid weld metal. However, there is no published data on arc pressure measurements during an alternating shielding gas supply and, consequently, these phenomena are based solely on theoretical assumptions. The experimental measurements made have shown that alternating shielding gases produces considerably higher arc pressures than argon, helium and pulsed GTAW due to a surge at weld initiation. The transient arc pressure measurements made when using alternating shielding gases are also considerably different from the theoretical assumptions previously reported

    Derivation of forces acting on the liquid weld metal based on arc pressure measurements produced using alternating shielding gases in the GTAW process

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    As part of an ongoing process to fully evaluate the effects of an alternating shielding gas supply on the gas tungsten arc and gas metal arc welding processes, a comparison between arc pressures produced using argon, helium, alternating gases and GTAW-P has been conducted. The alternating shielding gas process is reported to create a dynamic stirring action within the liquid weld metal as a result of three independent phenomena: a) variation in weld pool fluidity, b) arc pressure variation, and c) arc pressure peaking. These effects have been the basis of previous advantages associated with the process, however these phenomena have not previously been verified and are based solely on theoretical assumptions. Arc pressure measurements are presented which allowed for the numerical derivation of various forces acting on the liquid weld metal in order to estimate the flow vectors present when each shielding gas is present

    Evaluation of gas metal arc welding with alterating shielding gases for use on AA6082T6

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    Studies have been carried out to determine the effects of implementing alternating shielding gases for 6082T6 aluminium alloy welding. Alternating shielding gases is a newly developed method of supplying shielding gases to the weld area to enhance the efficiency of the standard Gas Metal Arc Welding (GMAW) process. This method involves discretely supplying two different shielding gases to the weld zone at a pre-determined frequency which creates a dynamic action in the weld pool. Several benefits have been identified in relation to supplying shielding gases in this manner including increased travel speed, reduced distortion, reduced porosity and, in the case of specific alternating frequencies, marginal improvements in mechanical properties. All in all, this method of shielding gas delivery presents attractive benefits to the manufacturing community, namely the increased productivity and quality in addition to a reduction in the amount of post-weld straightening required. However, the literature available on this advanced joining process is very scant, especially so for aluminium alloys. For this reason, an evaluation has been carried out on the application of alternating shielding gases for the GMAW process on 6082T6 aluminium alloys

    A potential solution to GMAW gas flow optimisation

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    A number of self-regulating shielding gas valves have been developed to synchronise the shielding gas flow rate to the welding current being used in the gas metal arc welding process (GMAW). These valves make claims to reduce the shielding gas consumption by up to 60%. One such system, the RegulaÂź EWR Pro, has undergone detailed evaluation in an effort to fully understand the benefits that could be obtained. This electromagnetically controlled system necessitates around an extremely fast response valve, which opens and closes continually throughout the welding process. This creates a pulsing of the shielding gas, further reducing consumption whilst maintaining optimal shielding gas flow. The unit has been identified to reduce the initial gas surge at weld initiation and results in a virtually instant decay of gas flow at weld termination. These particular characteristics have been found to be ideally suited to saving shielding gas when carrying out intermittent or stitch welding. It was established that the use of this valve generated deeper penetration in fillet welds, which in turn has highlighted the potential to increase the welding speed, therefore further reducing gas consumption. In addition, a computational model has been developed to simulate the effects of cross drafts. The combination of reducing the gas surge and slow decay with faster welding has been shown to meet the drive for cost savings and improving the carbon footprint

    Artificial neural network optimisation of shielding gas flow rate in gas metal arc welding subjected to cross drafts when using alternating shielding gases

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    This study implemented an iterative experimental approach in order to determine the shielding gas flow required to produce high quality welds in the gas metal arc welding (GMAW) process with alternating shielding gases when subjected to varying velocities of cross drafts. Thus determining the transitional zone where the weld quality deteriorates as a function of cross draft velocity. An Artificial Neural Network (ANN) was developed using the experimental data that would predict the weld quality based primarily on shielding gas composition, alternating frequency and flowrate, and cross draft velocity, but also incorporated other important input parameters including voltage and current. A series of weld trials were conducted validate and test the robustness of the model generated. It was found that the alternating shielding gas process does not provide the same level of resistance to the adverse effects of cross drafts as a conventional argon/carbon dioxide mixture. The use of such a prediction tool is of benefit to industry in that it allows the adoption of a more efficient shielding gas flow rate, whilst removing the uncertainty of the resultant weld quality

    Culture, chronology and change in the Later Neolithic of North Mesopotamia

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    The aim of this thesis is to examine the spatial, temporal and social patterning of the late Neolithic of north Iraq. In traditional terms, this covers the Hassuna and Halaf cultures. UndeqJining much of the analysis is a new chronology for the period which fits the available evidence better than has been achieved previously. This chronology emphasises the continuities as much as the changes and stress has been laid on making it general and able to accommodate regional variations. Important new information on the transition between the I-lassuna and the I-Ialaf was obtained by the excavation of one site, Khirbet Garsour, and the detailed surface collection of others in the North Jezira Project survey. Instead of this transition being very abmpt, it is argued that it is a smooth change in north Iraq with considerable cultural continuity. The spread of a single ceramic style over central and northern Iraq and northern Syria is proposed as occurring late in the Hassuna/Samarran sequence rather than several hundred years later in the Halaf.In chapter 6, it is argued that the period saw a progressive degradation of the environment in the main areas of settlement, which may have had an important influence on potential subsistence strategies. Chapter 7 presents new information on the sites from the North Jezira Project survey in north Iraq. Site distributions are analysed on as fine a chronological scale as possible and an emerging settlement hierarchy by the end of the Halaf is suggested. This chapter also considers how space was used within sites and suggests that major changes in the composition and relations of social groups may have occurred during this period. Chapter 8 evaluates evidence for long and short distance exchange systems using the examples of obsidian and pottery. It is suggested that exchange of raw materials was already taking place in a sophisticated manner even at the start of the period. There is evidence that these exchange systems were becoming more complex and transferring larger quantities of goods by the end of the Halaf and that new types of products are being included in the exchange. Chapter 9 looks at the burial evidence and suggests that, although there is some evidence for competition, there is little indication of social hierarchies. Chapter 10 re-examines the Burnt House at Arpachiyah and suggests that it indicates not just social and political control but bureaucratic means of administering it. Certain types of pottery were probably restricted to specific prestige contexts in the late Halaf.t is suggested that the traditional culture group is not well suited to describing spatial entities in this period. Instead, stylistic analysis may be an important future method and new techniques for the analysis of decoration are proposed. Finally, the scale of social development is discussed and it is suggested that significant developments in social organisation of long term significance took place in this period

    Imagining machines: Time & image in the shorter poems of William Carlos Williams

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    A discussion of time and image in the shorter poems of William Carlos Williams. The particular focus is Williams\u27 short poems published between the Great War and World War II. The relationship between the form of the poems, and Williams\u27 theories on poetry, as expressed in Spring and All, and The Wedge is examined. In turn, Williams\u27 theories are viewed in the context of Imagism, Objectivist poetics, and the modernist machine aesthetic. Williams\u27 application of these theories is seen to create a poetic form that generates images in the reader\u27s imagination in synchrony with the moment