African Fusion June 2015

(a) Inclination angle = 0°

(b) Inclination angle = 45°.

Figure.1: The change in bridge profiles after contact between a pendent droplet and a weld pool; (a): At an inclination angle of 0°. (b): At an inclination angle of 45°.

it induces pool oscillation. The time required for breakup of the liquid bridge dominates short-circuiting frequency and dictates the welding current necessary for practical welding. In principle, therefore, only current modulation is available for use to achieve stable and smoothmetal transfer. When the wire is inclined to the pool, because electro-magnetic forces do not act axial-symmetrically, the bridge is deformed like a bow. And after the breakup of the bridge, the drop at the wire tip is detached from the solid wire tip and is propelled away as a spatter, as shown in Figure 1 (b). For several decades in Japan and Western countries, re- search and development in the short-circuit welding process has been focused on realising stable welding that is spatter free by means of actively controlling welding current alone. But in practical applications, this has never been completely satisfactory, because amultitude of welding variation, such as current, shielding gas and consumablematerial, which restrict the levels of control possible. The presentation and publication of the CSC process for aluminiumMIGwelding by G. Huismann [7] notes that control of wire feeding speed and direction was very effective for stabilising the short-circuiting process and reducing spatter. Simultaneous control of current and wire feeding was then established, which expanded the available range of applica- tions possible for the process. By using digitally controlled welding machines, the breakup of the liquid bridge could be achievedmechanically, via a precisemovement of thewire tip under the control of the feeder. Inaddition, currentmodulation could be used to widen the available range of welding condi- tions. For example, the combination of short-circuit transfer with pulsed transfer was developed for the GMAWprocess and used in practice[8]. To realise a new welding process, however, we need to start from the concept stage, for which advanced numerical simulation is an effective tool. Figure 2 shows the numerical simulation results for metal transfer in an argon gas shielded

is, essentially, a result of a lack of deep understanding of welding phenomena. Modelling and high performance control of GMAW processes The digitally controlledweldingmachine, as well as delivering very fast and precise current and voltage control, now also offers digitally controlledwire feeding, which has been consid- ered a weak point in GMAWprocesses for many years, limiting the use of the process for high performance applications. It is now possible, however, for GMAW processes to become a high quality welding process with an extremely stable arc – equivalent to TIG welding arc – and simultaneously, a higher productivity welding process because of its high deposition rate. But in order to ensure the stability of the gas metal arc is equivalent to that of the TIG arc, software is required to continuously control welding phenomena and to provide the optimum combination of current, voltage and wire feeding. This means that information about the dynamic behaviour of the arc, the electrode wire and the weld pool must be linked to the control variables of the digital welding machine. The computer technologies of today havemade rapid and significant progress. They can nowbe used to carry outmodel- ling and simulation of very complicated welding phenomena with high temperature and high luminescence. Some predic- tion about welding control phenomena are described here basedon knowledge obtained through numerical simulations. Figure 1 shows the change of a molten drop’s shape with time calculated using a 3D numerical model just after contact- ing the pool surface in the short-circuiting transfer process [6]. In the case of a torch inclination angle of zero degrees, the liquid metal at the wire tip flows into the pool due to the forces of capillary pressure and electro-magnetic forces, as shown in Figure 1 (a). When high current flows in the liquid bridge during short-circuiting, the depression in the pool oc- curs at the breakup of the liquid bridge and, subsequently,

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June 2015

AFRICAN FUSION