The influence of welding parameters and processes on welds

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I. The influence of welding current, arc voltage, and welding speed on the weld 1. Welding current When the welding current increases (with all other conditions remaining unchanged), the penetration depth and bead height of the weld increase, while the bead width remains unchanged (or increases slightly). The reasons for this are as follows: ① As the current increases, both the arc force on the workpiece and the heat input increase; as a result, the location of the heat source shifts downward, leading to increased penetration depth. The penetration depth is nearly proportional to the current. ② As the current increases, the amount of wire that melts increases almost proportionally; since the weld width remains nearly constant, the bead height increases. ③ As the current increases, the arc column diameter grows, but the depth to which the arc penetrates into the workpiece increases, limiting the range of movement of the arc spot; as a result, the weld width remains nearly constant. 2. Arc voltage: As the arc voltage increases, the arc power rises, resulting in increased heat input to the workpiece. At the same time, the arc length increases and its distribution radius expands; as a consequence, the penetration depth decreases slightly while the weld width increases ； The remaining height decreases because the melting width increases, while the amount of wire that melts decreases slightly. 3. Welding speed: As the welding speed increases, the linear energy decreases, and both the penetration depth and width, as well as the bead height, decrease. This is because the amount of weld metal deposited per unit length of weld is inversely proportional to the welding speed, while the weld width is inversely proportional to the square root of the welding speed. II. Welding method: Direct current positive polarity: The workpiece is connected to the positive pole of the welder, while the welding torch is connected to the negative pole of the welder ； DC reverse connection: The workpiece is connected to the negative pole of the welder, while the welding torch is connected to the positive pole of the welder. In conventional GMAW, when using DC reverse polarity, both the penetration depth and the weld width are greater than those with DC positive polarity, as the energy released by the workpiece (cathode) is higher in this case. When connected in direct current positive mode, the welding wire acts as the cathode, resulting in a higher melting rate for the wire. In TIG welding, the penetration is greatest with direct current polarity and smallest with reverse polarity. When welding aluminum, magnesium, and their alloys, there is the issue of removing the oxide film from the surface of the molten pool; using alternating current is preferable, and reverse polarity can also be employed when welding thin sheets. DC positive polarity is generally used for welding other materials. III. Weld seam formation defects and causes of these defects 1. Underwelding: During fusion welding, the condition in which the root of the joint is not fully welded through is called underwelding. The reasons for this are a low welding current, an excessively high welding speed, inappropriate groove dimensions, or the wire not being aligned with the center of the weld. This defect is prone to occur in CO2 welding with fine wire short-circuit transfer due to the low heat input to the workpiece. 2. Penetration: During fusion welding, the molten metal flows out from the back side of the weld, resulting in a hole – this phenomenon is known as penetration. Excessive welding current, too low welding speed, or overly large gap and groove dimensions can all lead to this type of defect. 3. Undercutting: The phenomenon of melting and forming depressions or grooves in the base material along the weld seam is called undercutting. Defects may occur during high-current, high-speed welding. When welding fillet welds with the web in a vertical position, undercut can occur if the weld leg size for a single pass is too large or the voltage is too high; it can also arise from improper handling during butt weld joints. 4. Weld beads: The phenomenon in which, during fusion welding, the molten metal flows onto the unmerged base material outside the weld seam, forming metal beads, is called weld beads. Weld beads are caused by an excessive amount of filler metal, which is related to factors such as small gaps and groove sizes, low welding speed, low voltage, or a large wire protrusion length.