New developments in advanced welding

Future trends

There are a number of misconceptions and genuine limitations relating to GTAW and these must be addressed if the process is to retain its relevance in the future. The ‘basic’ GTAW process has been hampered by its low penetration and consequent poor productivity. As a production tool it tends to be used when quality or other overriding issues demand it. This chapter has argued that the process has much more to offer and has illustrated this with the detailed description of two of its many variants. It is suggested that this realisation that GTAW has ‘more to offer’ will be increasingly appreciated, particularly as fabrication operations become more integrated and mechanised. One of the historical impediments to the seamless integration of welding into production lines has been poor joint fitup and the consequent need for a degree of adaptability that was only available with manual intervention. This impediment is rapidly being removed as component tolerances improve, welding processes become more tolerant, and control systems are made more intelligent and responsive. This trend will suit the lower deposition welding processes such as GTAW and should renew the search for innovative ways of exploiting this very elegant process.

Many of the changes to GTAW over recent times have been forecast correctly to be in the area of the equipment used to implement the process (Lucas, 1990; Muncaster, 1991). This area covers power sources, control systems, monitoring, viewing and data acquisition (Muncaster, 1991). This trend is expected to continue in the future, with the increasing availability of significant computational power driving the process in the direction of greater adaptability and user friendliness. Occupational health and safety as well as environmental issues are also becoming more important and concerns about electromagnetic radiation and its potential to interfere with computerised equipment, metal fume and overall power requirements, will all lead to further changes in equipment and practices.

However, the opportunity for new variants is expected to continue and to produce some very productive processes. One example of this is the recent research into hybrid processes and particularly laser plus GTAW (Dilthey and Keller, 2001; Ishide et al., 2002). Hybrid welding refers to a situation where two processes (in this case, laser welding and GTAW) are coupled together to act at a single point. The coupling between the laser beam and the gas tungsten arc produces a number of synergistic effects that enhance the best features of each process. For example, the laser not only provides deep penetration but also stabilises the anode spot of the arc. As one result the gas tungsten arc then can be operated in the more efficient DCEN mode, even when welding aluminium. At the same time the arc broadens the weld pool at the plate surface, improves the laser to material coupling, and relaxes the very high joint tolerances required for laser welding. It also provides additional heat input and an improved weld profile with reduced notch angles. In one set of trials on a 2mm aluminium 3% magnesium alloy, Dilthey and Keller (2001) reported an increase in welding speed from 5m/min for the laser, to 8m/min with the hybrid process. The GTAW operated alone could only be operated in the ac mode at 2 m/min.

Another innovation, in GTAW is the newly reported guided GTAW or GGTAW process (Zhang et al., 2003). In this variant the main arc is established between a short, hollow tungsten electrode and the workpiece. However, a separately powered electrode positioned above the main electrode provides a lower current ‘pilot arc’. This arc is constricted in passing through the hollow main electrode. The result is two concentric arcs, the inner of which has a high energy density and is relatively stiff. The inner arc has the effect of stiffening or ‘guiding’ the main arc, hence the name of the process. This process is anticipated to have some advantages over both GTAW and plasma arc.

In summary, GTAW is a particularly elegant welding process because of its apparent simplicity and appeal to fundamental physical principles. It is also becoming far more productive and versatile than popular images of the process suggest. The likely scenario is that this process will continue to be developed in new and imaginative ways for many years to come.

3.3 References

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Heiple C. R. and Roper J. R. (1981), ‘Effects of selenium on GTA fusion zone geometry’, Welding Journal, 60 143s-5s Heiple C. R. and Roper J. R. (1982), ‘Mechanism for minor element on GTA fusion zone geometry’, Welding Journal, 61 97s-102s Howse D. and Lucas W. (2000), ‘Investigation into arc constriction by active fluxes for tungsten inert gas welding’, Sci. and Tech. Welding and Joining, 5(3) 189-93 Ishide T., Tsubbota S., Watanabe M. and Ueshiro K. (2002), Development of YAG Laser and Arc Hybrid Welding Method, Int. Inst. Welding Document Doc No. XII-1705-02 Jackson C. E. (1960), ‘The science of arc welding’, Welding Journal, 39(4), 129s-140s and 39(6), 225s-30s

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Lowke J. J., Kovitya P. and Schmidt H. P. (1992), ‘Theory of free-burning arc columns including the influence of the cathode’, Journal of Physics D: Applied Physics, 25 (11) 1600-6

Lu S.-P., Fujii H., Sugiyama H., Tanaka M. and Nogi K. (2002), ‘Weld penetration and marangoni convection with oxide fluxes in GTA welding’, Materials Trans. of Japan Institute of Metals, 43(11) 2926-31 Lucas W. (1990), TIG and Plasma Welding, Cambridge, UK, Woodhead Publishing Ltd Lucas W. (2000), ‘Activating flux - improving the performance of the TIG process’, Welding and Metal Fabrication, (2/12) 7-10 Lucas W. and Howse D. (1996), ‘Activating flux - increasing the performance and productivity of the TIG and plasma process’, Welding and Metal Fabrication, (1/12) 11-17

Lucas W., Howse D., Savitsky M. M. and Kovalenko I. V. (1996), ‘A-TIG flux for increasing the performance and productivity of welding processes’, Proc. 49th International Institute of Welding Annual Assembly, Budapest, Hungary, Int. Inst. Welding, Doc. No. XII-1448-96

Makara A. M., Savitskii M. M. and Kushnirenko B. N. et al. (1977), ‘The effect of refining on the penetration of metal in arc welding’, Automat. Welding, 9, 7-10 Matsuda F., Ushio M. and Sadek A. (1990), ‘Development of GTA electrode materials’.

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New developments in advanced welding

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