African Fusion July-August 2024

ing is shown in Figure 12. The arc, placed behind the laser beam, is attracted to and stabilised by the laser-induced plasma. The laser creates a plasma with good electrical conductivity in the atmosphere near the arc, and a laser-molten pool is formed near the molten pool of the arc on the base metal. These factors stabi lise the arc by facilitating smooth movement of the molten pool. This arc stabilising effect is very effective in high-speed welding. In particular, in hybrid welding [14] that combined the laser with ac pulsed GMAW. The ac GMA process supplies the weld deposition at low heat input, while the laser arc secures wettability and penetra tion of the weld metal into the base metal, creating an excellent synergy effect. Good bead formation is possible, even when large joint gap exist, as shown in Figure 13.

Figure 15: Extrude aluminium alloy welding result by Laser AC synchro feed GMA hybrid process. The Cold Spot joining process Resistance spot welding is widely applied in the assembly of automobile bodies and parts. In recent years, the ratio of ultra high strength steel plates (cold- or hot-stamped materials) with a strength of 1.0 GPa or higher applied to automotive parts has been increasing due to the need to reduce the weight of the vehicle body. Compared to mild steel plates, these high-strength steel sheets do not fit well with each other when clamped by electrodes. They have a narrow contact diameter, and the energizing area cannot easily be expanded. Therefore, the electrical resistance is higher and splash (spattering) is more likely to occur. In addition, to secure the proper nugget size, it is necessary to use construction measures such as high applied pressure, securing the precise energizing current, and extending the energizing time, which shortens the life of the copper electrode. Hence the need for the development of solid-state resistance spot joining or Cold Spot Joining (CSJ), which applies high pressure to the joint materials to reduce the joining temperature and com plete the joint at a lower temperature than the melting point [11, 16] . Figure 16 shows the principle of this process. First, a pressuris

Figure 13: High speed welding results of aluminium alloy lap fillet joint by laser- AC arc hybrid process. As shown in Figure 14, when a short circuit is detected, the wire feed direction is reversed, and the optimum short circuit release current is set to promote re-arcing. As soon as an arc is established, the wire feed direction is reversed again and the polarity of the GMA welding current is switched to EN for the prescribed EN arc period ratio. During this same period, the output polarity is again switched over to EP: to ensure heat input to the base metal and to promote cleaning of the metal surface. At the same time, the appropriate arc length is maintained by voltage feedback control. Figure 15 shows an example of lap fillet joint welding using an A6063 extruded aluminium alloy with a thickness of 2.0 mm at a welding speed of 10 m/min, which is being applied to welding EV battery cases and other applications [15] .

Figure 16: Principle of Cold Spot Joining (CSJ) process.

Figure 14: AC synchro-feed GMA process.

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July-August 2024

AFRICAN FUSION