1. Short-circuit transition welding
In CO2 arc welding, short circuit transition is the most widely used, mainly for thin plates and all-position welding.
The standard parameters include arc voltage, welding current, welding speed, welding circuit inductance, gas flow rate, and wire extension length.
(1) Arc voltage and welding current must be matched appropriately for a certain wire diameter and welding current (i.e., wire feeding speed) to obtain a stable short circuit transition process with minimal spatter.
| Diameter of welding wire (mm) | 0.8 | 1.0 | 1.2 | 1.6 |
| Welding current (A) | 100-110 | 105-120 | 120-135 | 140-180 |
| Arc voltage (V) | 18-21 | 18-22 | 19-23 | 22-26 |
(2) Welding circuit inductance has the following main effects:
a. Regulating the rate of increase in short circuit current (di/dt). If di/dt is too small, it may cause large particles to spatter onto the wire, resulting in wire breakage and arc extinction. If di/dt is too large, it may generate a large amount of small metal spatter.
b. Regulating the arc burning time to control the depth of base metal fusion.
c. Regulating the welding speed. Welding too fast can cause undercutting on both sides of the weld, while welding too slow may result in burn-through and coarse weld structure.
d. Gas flow rate depends on factors such as joint type, plate thickness, welding specifications, and operating conditions. Generally, the gas flow rate is 5-15 L/min for thin wire welding and 20-25 L/min for thick wire welding.
e. Wire extension length should be 10-20 times the wire diameter. During welding, the extension length should be kept within 10-20 mm range as much as possible. Increasing the extension length will decrease the welding current and reduce the depth of base metal fusion, while decreasing the extension length will increase the welding current and increase the depth of base metal fusion. This effect is more pronounced for wires with higher resistivity.
f. The polarity of the power source. In CO2 arc welding, using direct current reverse polarity generally results in less spatter, more stable arc, deeper base metal fusion, better weld shape, and lower hydrogen content in the weld metal.
2. Fine particle transition welding
(1) In CO2 gas, for a certain diameter of welding wire, when the current increases to a certain value and is accompanied by a relatively high arc voltage, the melted metal of the welding wire will fall freely into the molten pool in the form of small particles. This transition form is called fine particle transition.
During fine particle transition, the arc penetration force is strong and the depth of base metal fusion is large, making it suitable for welding medium-thick plate structures. Direct current reverse polarity is also used in fine particle transition welding.
(2) The current and voltage range for achieving fine particle transition.
| Diameter of welding wire (mm) | Lower current limit (A) | Arc voltage (V) |
|---|---|---|
| 1.2 | 300 | 34-35 |
| 1.6 | 400 | |
| 2.0 | 500 | |
| 3.0 | 650 | |
| 4.0 | 750 |
As the current increases, the arc voltage must also increase; otherwise, the arc will have a scouring effect on the molten metal, which will deteriorate the weld formation. Increasing the arc voltage appropriately can avoid this phenomenon. However, if the arc voltage is too high, spatter will increase significantly. With the same current, the arc voltage decreases as the wire diameter increases.
There is a substantial difference between CO2 fine particle transition and jet transition in argon arc welding. The jet transition in argon arc welding is axial, while the fine particle transition in CO2 is non-axial, and there is still some metal spatter.
In addition, the jet transition in argon arc welding has an obvious transition current change feature (especially when welding stainless steel and black metals), while the fine particle transition does not have this feature.
3. Measures to reduce spatter
(1) Choose the correct welding parameters.
Arc voltage has a certain regularity between spatter rate and welding current for each diameter of welding wire in the arc. In the low current range, spatter during short circuit transition is smaller, and in the high current range (fine particle transition), spatter rate is also smaller.
(2) Welding gun angle.
The least amount of spatter occurs when the welding gun is vertical, and the greater the inclined angle, the more spatter. The best forward or backward inclination angle for the welding gun is not more than 20 degrees.
(3) Wire extension length.
The wire extension length also has a significant impact on spatter. When the wire extension length increases from 20 to 30 mm, the spatter rate increases by about 5%. Therefore, the extension length should be shortened as much as possible.
4. Protective gas types
(1) CO2 gas is used as a protective gas in the CO2 arc welding method. A preheater should be installed in the gas supply because liquid CO2 absorbs a large amount of heat energy during continuous gasification, and the expansion of the gas volume after pressure reduction will also cause the gas temperature to drop.
In order to prevent moisture in the CO2 gas from freezing and blocking the gas path at the steel cylinder outlet and pressure reducing valve, the CO2 gas is heated through a preheater between the steel cylinder outlet and pressure reduction.
(2) The MAG welding method uses CO2+Ar gas as a protective gas, also known as physical gas protection. This welding method is suitable for stainless steel welding.
(3) The MIG welding method using Ar as a gas shield is suitable for aluminum and aluminum alloy welding.
5. Basic Operating Techniques
- Precautions
(1) Refer to the instruction manual for the connection methods of the power supply, gas cylinder, wire feeder, and welding gun.
(2) Choose the correct gun holding position:
a. Keep the body and welding gun in a natural state, and the wrist can flexibly move the welding gun horizontally or rotate it.
b. During welding, the minimum bending radius of the cable should be greater than 300mm, and the welding gun can be dragged at will.
c. During welding, maintain a constant tilt angle of the welding gun and have a clear and convenient view of the molten pool.
d. Keep the welding gun moving forward at a uniform speed, and adjust the forward speed of the welding gun according to the current, the shape of the molten pool, and the melting condition of the workpiece, striving for a uniform forward speed.
- Basic Operations
(1) Check whether all connections are correct, water, electricity, and gas are connected, and turn on the power supply. Adjust the welding parameters according to the standard.
(2) Arc ignition: CO2 gas shielded welding adopts collision arc ignition. When igniting the arc, there is no need to lift the welding gun, just ensure the distance between the welding gun and the workpiece.
a. Before igniting the arc, press the jog switch on the remote control box or the control switch on the welding gun to feed out the welding wire, and keep the extruded length 10-15mm.
b. Place the welding gun at the ignition point as required, at this time, the end of the welding wire does not touch the workpiece, and the height of the gun nozzle is determined by the welding current.
c. Press the control switch on the welding gun, and the welding machine will automatically provide gas, delay the power supply, maintain high voltage and slow wire feeding. When the welding wire short-circuits the workpiece, the arc will ignite naturally. When short-circuiting occurs, the welding gun tends to lift automatically, so when igniting the arc, press the welding gun down slightly to prevent the arc from extinguishing due to the welding gun being lifted too high and the arc being too long.
- Welding
After igniting the arc, generally use left-side welding method. During welding, it is necessary to maintain the appropriate tilt angle and nozzle height of the welding gun, and move it as uniformly as possible. When the groove is wide, the welding gun should be swung horizontally to ensure that both sides are properly fused.
During welding, it is necessary to judge whether the welding process parameters are suitable based on the actual welding effect. Adjust the welding process parameters by observing the molten pool condition, arc stability, spatter size, and the quality of the weld formation until satisfactory results are achieved.
- Arc termination
The arc must be terminated before welding ends. Improper arc termination can easily cause arc craters and defects such as cracks and porosity. Measures must be taken before welding ends.
(1) The welding machine has an arc crater control circuit. When the welding gun stops advancing at the arc termination point, this circuit is activated, and the welding current and arc voltage are automatically reduced until the molten pool is filled.
(2) If the welding machine does not have an arc crater control circuit or the current is too small to use the arc crater control circuit, stop the welding gun from advancing at the arc termination point, and repeatedly break and ignite the arc several times until the arc crater is filled before the molten pool solidifies. The operation must be fast. If the arc is ignited after the molten pool has solidified, defects such as lack of fusion and porosity may occur.
