# Research of a Multi-Frequency Waveform Control Method on Double-Wire MIG Arc Welding
## 2 Principle of Double-Wire MIG Arc Welding  
Double-wire MIG arc welding facility is composed of two separated welding power sources and corresponding two wire feeders. Currently, the most commonly employed is the Tandem double-wire technology, whose schematic is shown in Figure 1. This technology put the two separated wires in a specially-designed welding gun in a predetermined pattern. During the working process, two wires are fed by two individual separated welding power sources, and insulated from each other. Then, the two power sources collaborate via a center control system. Apart from the wire feed rate, all of the other operational parameters are independent from each other, including the diameters and materials of the wires, whether or not feeding the wires by impulse signals, and so on. Under the circumstance, the electrical arcs can be controlled in an optimal pattern, and the mutual interference between two arcs can be as low as possible, under the precondition that the arcs were steadily stricken [16].  
Figure 2 shows the welding touch and corresponding welding pool of a double-wire MIG arc welding. Figure 2a is a typical structure of the tandem welding gun. Two electrical nozzles are separated and independently feed the corresponding wires. Because the welding gun is so heavy and has a very high rate, double-wire MIG arc welding operation is usually realized by means of robotics or other relative automatic welding facilities. Figure 2b shows the welding pool of the double-wire MIG arc welding, which can clearly show the working process. The two welding wires are leading wire and trailing wire. The front one is the leading wire with a large value of current and is used to backing welding to increase the welding penetration, while the other is the trailing wire and used to optimize the cover of the welding seam. During the process, the leading wire and trailing wire are working in the same welding pool. The heat generated by one wire can affect the other. In other words, the heat from leading wire can melt the trailing wire, while the heat from the trailing wire can increase the melting rate of the leading wire. Hence, this is a mutually promoting process. Under the circumstance, the deposited amount of the welding operation can be further increased, while the heat supplying to the parent metal can be decreased. This conduction can benefit the final quality of welding product under the same power supplied.  
Figure 1: Schematic figure of double-wire metal inert-gas (MIG) arc welding.  
Figure 2: (**a**) Welding gun; and (**b**) welding pool of double-wire MIG arc welding.  
During the pulse welding process, the welding current is switched between high-level value and low-level value, and then a welding arc with the form of current pulse is generated [17]. Two wires can induce two welding electrical arcs working during the process. Then, the pulse current generates a continuous welding seam which links all the welding spots. Each spot in the welding seam is generated by the welding current in peak value. High-level current is used for striking arc, and named as peak current in practice. Then, the current is switched to low-level, named the background current. The base value current is used to hold the arc and avoid the arc unsuccessfully striking again. In double-wire MIG arc welding process, the pulse currents are generated between leading wire and trailing wire, and the center control system adjusts both of the two waveforms to realize predetermined welding schedule.