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Finally, testing of welds will be extended to all periods of the life-cycle of the welded structures. In addition to the fabrication and service phase, they will also significantly contribute to life-cycle extension (repair) procedure of the critical components.

4.2 High energy density welding processes and material response High energy density (HED) welding simply refers to Laser Beam Welding (LBW) and Electron Beam Welding ( EBW) processes. The benefits offered by HED fusion welding are narrow, deep weld penetration, high welding speeds, low heat input (and hence low distortion and heat affect) and precision. EBW is somewhat mature and has various well-accepted applications including packaging (e.g. pressure sensors) and aerospace components. There have been attempts to broaden application to high production (e.g. automotive) manufacturing, but incompatibility of vacuum systems with the demands of large scale production have prevented these applications. In contrast, LBW has benefited from continual development of new laser sources with new capabilities and is somewhat more compatible with the demands of high production and large component manufacturing, so it enjoys a broader range of applications. Nonetheless, challenges of high equipment cost, lack of system portability, laser safety, lack of robustness to manufacturing environments (particularly, the need for cleanliness of optical surfaces) continue to hamper LBW applications. Smaller, more efficient laser sources or central laser generators with fibre distribution system would help with LBW portability. Practical, robust cost-competitive out-of-vacuum capability would promote EBW applications. Eliminating root defects in partial penetration LBW would assist its application, and laser systems with multiprocess capability (i.e. cut, drill, weld, machine interchangeably on the same system) would be more economical. The general need for off-line process planning for virtual manufacturing are the same as for other joining processes, but the models and needs for HED processes are unique. The continuously disappearing global resources in metals require and make it economically advantageous to join dissimilar materials according to function. Bi-metal segment saws need wear resistant teeth and a carrier for fastening. With continuous wear the teeth may be sharpened by grinding down to the holes of the rivets ( Figure 4.5 ). High speed steel costs today 10 times more than carbon steel. It is evident, that such a bi-metal design is cost effective and works as well as made from full HSS. The preferred process with minimum energy consumption for joining is electron beam welding.

Figure 4.5 Segment of a cold saw of high speed steel EB-welded to carbon steel in annealed condition. This saves 50% of the expensive high- speed steel (Reproduced courtesy: Probeam)

A similar application with the same underlying concept is EB welding of worm gears consisting of bronze and steel. The expensive bronze is only used where its lubricating characteristics and wear resistance are required at the outer circumference. There are many such examples already taking place today and with rising material cost, in the future, these applications of joining dissimilar materials will increase.

Figure 4.6 Combination of steel and bronze, 30 mm deep electron beam weld (Reproduced courtesy: Probeam)

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Through Optimum Use and Innovation of Welding and Joining Technologies

Improving Global Quality of Life