I. The Effects of Welding Current, Arc Voltage, and Welding Speed on the Weld Seam
1. Connect the current
When the welding current increases (with other conditions remaining unchanged), the weld penetration and reinforcement height both increase, while the weld width remains unchanged (or increases slightly). The reasons are as follows:
① After the current increases, both the arc force and the heat input on the workpiece increase, causing the heat source to shift downward and deepening the melt penetration. The melt penetration is nearly directly proportional to the current.
② After the current increases, the amount of wire melted increases nearly proportionally. Since the weld width remains approximately constant, the excess height increases accordingly.
③ After the current increases, the diameter of the arc column expands, but the depth to which the arc penetrates into the workpiece also increases. As a result, the range of movement of the arc spot is restricted, and consequently, the weld width remains nearly unchanged.
2. Arc Voltage
After the arc voltage increases, the arc power also increases, leading to a slight rise in heat input to the workpiece. At the same time, the arc length elongates and the distribution radius expands, resulting in a slight decrease in penetration depth but an increase in weld width. The excess height decreases because, although the weld width increases, the amount of wire melted slightly diminishes.
3. Welding speed
As the welding speed increases, the heat input per unit length decreases, and consequently, both the penetration depth and the weld width as well as the excess height all decrease. This is because the amount of weld metal deposited per unit length of weld seam is inversely proportional to the welding speed, while the weld width is approximately inversely proportional to the square root of the welding speed.
II. Connection Method
DC positive polarity: The workpiece is connected to the positive terminal of the welding machine, and the welding torch is connected to the negative terminal of the welding machine.
Reverse polarity DC: Connect the workpiece to the negative terminal of the welding machine, and connect the welding torch to the positive terminal of the welding machine.
In general, during melt-arc welding with a consumable electrode, the penetration depth and weld width are greater when using direct current reverse polarity than when using direct current straight polarity. This is because the workpiece (cathode) releases more energy. When using direct current straight polarity, the welding wire serves as the cathode, resulting in a higher melting rate of the welding wire.
During tungsten inert gas (TIG) welding, direct current with positive polarity provides the deepest penetration, while reverse polarity yields the shallowest. When welding aluminum, magnesium, and their alloys, it is important to remove the oxide film from the surface of the molten pool; therefore, alternating current is preferable. Reverse polarity can also be used when welding thin sheets. For welding other materials, direct current with positive polarity is generally recommended.
3. Weld Seam Formation Defects and the Causes of Their Formation
1. Lack of Penetration: In fusion welding, the phenomenon in which the root of the joint is not fully penetrated is referred to as lack of penetration. The causes include insufficient welding current, excessive welding speed, improper bevel dimensions, or misalignment of the welding wire with the weld seam center. When using short-circuit transfer CO2 welding with thin wires, this defect tends to occur easily due to the low heat input into the workpiece.
2. Burn-through: During fusion welding, the phenomenon in which molten metal flows out from the back side of the weld, creating a hole, is called burn-through. This defect can occur if the welding current is too high, the welding speed is too slow, or the gap and bevel dimensions are excessively large.
3. Undercut: The phenomenon in which the base metal along the weld seam is melted and forms a depression or groove is called undercut. Defects may occur during high-current, high-speed welding. When welding fillet welds with the web in a vertical position, undercut can also develop if the weld toe is excessively large in a single pass or if the voltage is too high. Improper operation during the welding of butt joints can likewise lead to undercut.
4. Weld Bead: The phenomenon in fusion welding where molten metal flows beyond the weld seam and solidifies on the base metal that has not been fused is called a weld bead. Weld beads are caused by excessive filler metal, which can be attributed to factors such as small gap and groove dimensions, low welding speed, low voltage, or excessive wire extension length.
