How do welding current, voltage, and welding speed affect the weld bead?

As the welding speed increases, the heat input decreases, leading to reductions in both penetration and weld width. The excess reinforcement also decreases, because the amount of filler metal deposited per unit length of weld is inversely proportional to the welding speed, while the weld width is approximately inversely proportional to the square root of the welding speed.

Here, U denotes the welding voltage and I denotes the welding current. The current affects the penetration depth, while the voltage influences the weld bead width. Ideally, the current should be set high enough to achieve full penetration without burn-through, and the voltage should be adjusted to minimize spatter. By fixing one of these parameters and adjusting the other, the welding current can be optimized. The magnitude of the welding current has a significant impact on both weld quality and welding productivity. Welding current primarily affects the depth of penetration. If the current is too low, the arc becomes unstable, penetration is shallow, and defects such as incomplete penetration and slag inclusion are likely to occur, resulting in low productivity. Excessive current can easily lead to defects such as undercut and burn-through in the weld, as well as increased spatter.

Therefore, the welding current must be properly selected; typically, it can be determined using empirical formulas based on the electrode diameter, followed by appropriate adjustments according to the weld position, joint configuration, number of weld passes, and workpiece thickness.

Arc voltage is determined by arc length: a longer arc results in higher arc voltage, while a shorter arc leads to lower arc voltage. The magnitude of the arc voltage primarily affects the weld bead width.

During welding, the arc should not be excessively long; otherwise, arc instability will increase spatter and, due to atmospheric contamination, porosity in the weld may occur. Therefore, short-arc welding is preferred, with the arc length generally kept no greater than the diameter of the electrode.

Short-circuit transfer is the most widely used mode in CO2 arc welding, primarily for thin-plate and all-position welding. The process parameters include arc voltage, welding current, welding speed, inductance of the welding circuit, gas flow rate, and wire extension length, among others.

(1) For a given wire diameter and welding current (i.e., wire feed speed), the arc voltage must be properly matched to the welding current in order to achieve a stable short-circuit transfer process with minimal spatter.

The parameters for short-circuit transfer with welding wires of different diameters are shown in the table:

Welding wire diameter (mm): 0.8, 1.2, 1.6

Arc Voltage (V) 18 19 20

Welding current (A) 100–110 120–135 140–180

(2) Welding circuit inductance; the primary function of the inductance is:

a Adjusts the rate of rise of the short-circuit current, di/dt; if di/dt is too low, large spatter particles are ejected, leading to abrupt breakage of the welding wire and arc extinction; if di/dt is too high, a large amount of fine metal spatter is generated.

b Adjust the arc burning time to control the base metal penetration.

c Welding speed. Excessive welding speed can cause edge blow-out on both sides of the weld, while insufficient welding speed can easily lead to burn-through and coarse grain structure in the weld.

d The gas flow rate depends on factors such as the type of fitting, plate thickness, welding parameters, and working conditions. Typically, for fine-wire welding the gas flow rate is 5–15 L/min, while for coarse-wire welding it is 20–25 L/min.

e. Welding wire extension length. The appropriate welding wire extension length should be 10 to 20 times the diameter of the wire. During welding, it is best to keep the extension within the 10–20 mm range; if the extension increases, the welding current decreases and the penetration into the base metal diminishes, whereas a shorter extension results in higher current and greater penetration. This effect is more pronounced for welding wires with higher resistivity.

(1) In CO2 shielding gas, for a given wire diameter, once the welding current is increased to a certain value and accompanied by a relatively high arc voltage, the molten metal from the wire begins to detach in small droplets and fall freely into the weld pool; this transfer mode is known as fine-particle transfer.

During fine-particle transfer, the arc exhibits strong penetration and results in deep weld bead fusion, making it suitable for welding medium- to thick-gauge plate structures. Direct-current reverse polarity is also employed during fine-particle transfer welding.

(2) As the welding current increases, the arc voltage must also be increased; otherwise, the arc will scour the molten pool metal, degrading weld bead formation. Appropriately increasing the arc voltage can prevent this phenomenon. However, if the arc voltage is too high, spatter will increase significantly. At the same current, arc voltage decreases as the wire diameter increases.

(1) Select process parameters appropriately: In the welding arc, there is a definite relationship between spatter rate and welding current for each wire diameter. In the low-current range, short-circuit transfer results in relatively low spatter; similarly, as the current increases into the large-current range (fine-particle transfer regime), the spatter rate remains low.

(2) Welding gun angle: The amount of spatter is minimal when the welding gun is held vertically; the greater the tilt angle, the more spatter there is. The forward or backward tilt of the welding gun should ideally not exceed 20 degrees.

(3) Welding wire extension length: The extension length of the welding wire has a significant impact on spatter; increasing the extension length from 20 mm to 30 mm results in an approximately 5% increase in spatter. Therefore, the extension length should be shortened as much as possible.

4. Different shielding gases require different welding methods.