School of Applied Sciences (SAS) (2006-July 2014)

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Browsing School of Applied Sciences (SAS) (2006-July 2014) by Supervisor "Apps, R. L."

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    The Characteristics and Structure of High Pressure (1-42 bars) Gas Tungsten Arcs
    (Cranfield University, 1982) Allum, C. J.; Apps, R. L.
    The last decade has seen a considerable growth in the exploitation of deep sea mineral reserves. Accompanying these developments has come the need for suitable underwater repair and uainten-nce techniques. one such technique involves the use of fusion welding processes in localised dry high pressure environments created around weldments. Pressure chambers at Cranfield have been employed to simulate this sitiu. ation. These have been used to investigate the influence of ambient pressure (1 to 45 bars) on gas-tungsten arcs. The study has been of a basic nature with emphasis on arc characteristics and arc structure. Special attention is given to the argon TIG arc although helium TIG arcs and constricted argon-tungsten arcs have also been examined. Arc characteristics are reported in terms of arc appearance and modes, electrical characteristics and weld bead studies. The investigation of arc structure is concerned with gaining an understanding of observed characteristics. This has involved experimental and theoretical developments. Experimental techniques used include, electrostatic probes, calorimetry and the measurement of total power radiated from an arc. Models developed here have successfully explained many of the experimental findings and special emphasis has been directed at understanding gas flow in the column and arc stability. Much of the work included in this thesis has implications for the development of underwater welding procedures. Results presented should also contribute to gaining a better understanding of arcs operating under normal ambient conditions. However, these aspects are not discussed in any detail as the main emphasis has been placed on documenting and explaining the influence of pressure on gas-tungsten arcs.
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    Statistical modelling of the narrow gap gas metal arc welding process
    (Cranfield University, 1990-04) Modenesi, P. J.; Apps, R. L.
    The J-laying technique for the construction of offshore pipelines requires a fast welding process that can produce sound welds in the horizontal-vertical position. The suitability of narrow gap gas metal arc welding (NG-GMA W) process for this application was previously demonstrated. The present programme studied the influence of process parameters on the fusion characteristics of NG-GMA welding in a range of different shielding gas compositions and welding positions. Statistical techniques were employed for both designing the experimental programme and to process the data generated. A partial factorial design scheme was used to investigate the influence of input variables and their interaction in determining weld bead shape. Modelling equations were developed by multiple linear regression to represent different characteristics of the weld bead. Transformation of the response variable based on the Cox-Box method was commonly used to simplify the model format. Modelling results were analysed by graphical techniques including surface plots and a multiplot approach was developed in order to graphically assess the influence of up to four input variables on the bead shape. Conditions for acceptable bead formation were determined and the process sensitivity to minor changes in input parameters assessed. Asymmetrical base metal fusion in horizontalvertical welding is discussed and techniques to improve fusion presented. At the same time, the interaction between the power supply output characteristic and the bead geometry was studied for narrow gap joints and the effect of shielding gas composition on both process stability and fusion of the base metal was assessed. An arc instability mode that is strongly influenced by arc length, power supply characteristic and shielding gas composition was demonstrated and its properties investigated. An optimized shielding gas composition for narrow gap process was suggested.