African Fusion July-August 2024

current, consequently stable droplets can be obtained to improve low spatter performance [5] . The other waveform control that has been developed is shown in Figure 4. After the first peak current is output, the current is rapidly reduced to prevent repelled transfer at arc reignition as shown in Figure 4 (1)-(2). After that, the secondary peak current is also out put to suppress instantaneous short circuiting between droplet at wire tip and molten pool – Figure 4 (3)-(4) [6] . In both cases, spatter generation at the time of reignition is suppressed by causing large molten droplets on the welding wire tip to dip into the molten pool in a periodic cycle aligned with the wire axis. A 100% CO 2 GMAW process has also been developed to control metal transfer in free flight without short-circuiting while moving the wire feed direction forward or backward [7] . Figure 5 shows a diagram of wire feed and current waveforms, along with metal transfer. For example, in Figure 5a-b, the droplets move toward the molten pool with acceleration by advancing the wire while forming droplets at the wire tip. As a result, when the wire feed direction is reversed backward, as in Figure 5c, the droplet at the wire tip moves toward the molten pool due to inertia, resulting in a wedge above the droplet (Figure 5d) and the droplet detaching without a short circuiting (Figure 5e). In this case, if the welding current at the time of droplet de tachment can be controlled to a low current of about several tens of amperes, spatter generation at droplet detachment can be suppressed.

Figure 6: Arc phenomenon of the buried arc realized by the low frequency modulated voltage control. Figure 6(B)-(E). However, in the rotating transfer, the heat input by the arc is concentrated in the direction of the sidewalls of the buried space, resulting in a relatively wide and shallow penetration. Therefore, in the low-voltage period, the drop transfer mode is used as shown in Figure 6(A), where the arc is directed downward to give heat input to the deeper part of the base metal to ensure deep penetration. Through this series of operations, a stable buried arc can be achieved. Figure 7 shows the results of welding joints with the same groove geometry with conventional CO 2 GMAW and high-current buried arc-controlled CO 2 GMAW, respectively. In this example, the comparison is made under welding conditions of 40 kJ/cm or less, which is the heat input limit in the Japanese Architectural Standard Specification for architectural steel frames. In conventional CO 2 GMAW, welding current, heat input, number of passes, and other construction conditions vary according to factory specifications and welding operators. However, a welding current of 300 A or less is generally applied. For example, a welding current of 280 A requires seven welding passes. On the other hand, with the buried arc control GMAW, welding can be completed in four passes, which reduces welding time by nearly 50%.

Figure 5: Droplet transfer phenomenon using inertia in the developed process. High efficiency and productivity High current buried arc welding process: Welding of thick plate welded structures requires considerable time for fabrication and weld strain removal heat treatment due to multilayer welding op erations. The high-current buried arc welding process has therefore been developed to save welding time. The buried arc is difficult to stabilise because it is prone to molten pool instability and irregular short-circuits, and this ten dency is more pronounced in the high-current range where deep penetration is expected. Until now, this has prevented the process from being put into practical use. In order to stabilise the buried arc, a low frequency modulated voltage control, based on external characteristics control has been developed [8] . Figure 6 shows an example of current and voltage waveforms with low frequency modulated voltage control. Here, a high and low voltage period is repeated periodically, and different metal transfer modes are utilised in each period. In the high-voltage period, the rotating transfer mode is enabled to stabilize the buried arc space with support from the sidewalls of the molten metal, as shown in

Figure 7: Comparison of multi-layer welding result between conventional GMAW and buried arc GMAW. Automatic pulsed waveform adjustment using rule-based AI: In high-speed welding using the pulsed GMA process for thin steel sheet welded structures such as automobiles, the arc length is shortened by lowering the set voltage to prevent the occurrence undercut, but this increases spatter generation. In particular, large amounts of spatter is generated when a short circuit occurs during a pulse peak current period. In this case, if the pulse parameters can be adjusted so that any short circuit occurs during a base current period of the pulse, the molten droplets in the weld pool can be detached from the wire tip by surface tension, suppressing the spatter. This has enabled a high-speed pulsed GMAW system to be ap plied in the auto industry, which can both suppress spatter and undercut. The system uses rule-based AI control that records the occurrence of short-circuits from time to time and automatically

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July-August 2024

AFRICAN FUSION