OBRABOTKAMETALLOV Vol. 25 No. 4 2023 technology Fig. 5. Welding pulse shape with controlled transfer (TC) Unlike MAG process sources, MAG-TC process sources operate on a constant current curve rather than a constant voltage curve. Thus, the source is capable of changing the electric current of the arc in a short period of time. Arc stability is maintained even with changes in electrode length and welding angle due to precise control of the welding current. Thus, as in the MAG process, the change in current to adjust the electrode elongation is eliminated, ensuring that there is no point reduction in the heat transferred [35]. Point A in fig. 5, corresponds to the base current (from 50 to 100 A), which has the function of maintaining the arc open and transferring heat to the weld pool. When a drop formed at the tip of the electrode touches the molten pool, creating a short circuit (point B), a current drop occurs. At point C, the current of the pinch effect of the drop is applied, which has the function of separating the drop from the tip of the electrode and placing it in the melt pool. At point D, the electronic control device of the welding current source monitors the electrical parameters of the arc and determines when the liquid bridge between the molten drop and the tip of the wire is about to break, in order to then reduce the current to values from 45 to 50, ensuring the restoration of the electric arc. After restoration of the arc (point E), the peak current, the function of which is to press down on the molten pool to prevent short circuit and heat the connection. The function of the tail is to control the rate at which the peak current decreases to the base current, acting as a rough control of the welding energy. The advantages of using the MAG-TC process for pipe root welding compared to MAG welding are that short-circuit control prevents lack of fusion, heavy smoke and spatter even when using CO2 as a shielding gas, which ensures good surface finish and weld strength [35]. Compared with the TIG process, the MAGTC process has a welding speed 4 times higher [35]. Compared to the ER process, the MAG-TC process has advantages mainly in terms of increased productivity, since there is no need to stop welding to change consumables and grind after finishing the root pass, since, unlike the ER process, the weld profile is flat. The finishing profile of the root pass with cellulose wires is convex, which leads to large losses of time during the roller grinding operation [35]. Another promising option, from the point of view of reducing the cost of welding works and increasing productivity, is butt resistance welding of pipes (BRW), which significantly increases work productivity. However, the disadvantage of the technology is the non-standard cutting of edges. To solve this issue, a hybrid technology of combining resistance welding and flux-cored welding (FCW) methods is possible. With BRW, it is difficult to obtain high impact strength of the joint on specimens with a sharp notch (Charpy). To obtain the required impact strength indicators for welded joints of BRW pipes, it is recommended to perform an additional technological operation – local heat treatment of the welded joint. Friction stir welding (FSW) is in the research stage, being introduction into traditional pipeline welding technologies. X80 pipeline steel plates were friction stir welded (FSW) under cooling conditions of air, water, liquid CO2 + water and liquid CO2, resulting in defect-free welded seams [26]. The microstructural evolution and mechanical properties of these FSW joints were studied. It has been shown that the impact toughness of the metal in the HAZ is 20–60 % higher compared to traditional welding methods [26]. Welding features The weld is formed by crystallization of the melt of the weld pool, containing both the main and filler (when introduced) materials. Welding thermal cycles cause significant changes in the mechanical properties of the base material. It is well known that the weld metals of steel differ from most parent steels in that it has
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