droplets and molten pool

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The welding arc is not only a heat source but also a force source; during the droplet transfer process, the droplets and the molten pool are subject to various external forces. By employing certain process measures, the forces acting on the droplets and the molten pool can be altered, thereby ensuring the stability of the welding process, controlling the shape of the weld seam, and reducing welding spatter, so as to obtain high-quality welded joints. (1) Gravity: Gravity causes objects to always have a tendency to hang downward. During flat welding, the gravity of the molten metal facilitates the transfer of droplets. Under the influence of gravity, if the temperature is too high or the molten pool is too large, weld bumps and burn-through can occur. During vertical and overhead welding, gravity hinders the transfer of droplets to the weld pool; using short-arc welding can overcome the effect of gravity. (2) During surface tension welding, the surface tension of the liquid metal droplet hinders its transition; whereas in vertical welding, the surface tension of the droplet makes it less likely to fall, which facilitates its transition into the molten pool. The surface tension of the liquid metal in the molten pool causes it to strive to remain flat, which can prevent surface depression caused by gravity to a certain extent. At the same time, when the droplet makes short-circuit contact with the molten pool, the surface tension of the molten pool can pull the droplet into it, accelerating the short-circuit transfer of the droplet. (3) Arc gas thrust: During shielded metal arc welding, the melting rate of the electrode coating is slightly slower than that of the electrode core; this results in the formation of a sleeve at the melted end of the electrode. The gas-generation agents contained in the coating melt and produce a large amount of gas that is ejected from this sleeve. At high temperatures, this gas expands rapidly, creating a straight and stable gas flow along the axis of the electrode, which pushes the molten material into the weld pool. At any welding position, the arc gas blowout helps with droplet transfer. (4) Electromagnetic compressive force: In electrode tension welding, the electrode and the droplet at its tip serve as conductors; when a welding current flows through them, a magnetic field is generated around them, resulting in an electromagnetic compressive force that acts from the surroundings toward the center. The necking region of the droplet at the tip of the welding electrode has a high current density, resulting in a strong electromagnetic compressive force that facilitates the rapid separation of the droplet from the tip of the electrode and its transition into the molten pool. (5) Polar pressure: In the welding arc, polar pressure is the force that hinders the transfer of droplets. When direct current is applied in the forward direction, the pressure of cations hinders droplet transfer; when direct current is applied in the reverse direction, the pressure of electrons hinders droplet transfer. Due to the larger mass of cations, the pressure exerted by the cation flow is greater than that of the electron flow; therefore, fine particle transition occurs more easily when the polarity is reversed, whereas it is less likely to occur when the polarity is in the normal direction.