Fundamentals of MAG Welding {CO2 or Argon (+) CO2Arc Welding}

1698


C Sridhar, Director-Technical,
Advance Institute of Welding Technology,
Chennai, Tamilnadu

1.MAG Arc Welding                            

In CO2 or Argon (+) CO2arc welding, the welding wire wound in coil is fed into the welding torch by the feeding motor automatically. The welding wire that is electrified through the contact tip becomes the electrode to strike an arc between itself and the base metal. The arc heat melts the wire and the base metal to join two pieces of base metal. In this case, in order that the weld metal will not be affected by oxygen and nitrogen in the atmosphere, CO2 or Argon (+) CO2gas is supplied from the nozzle of the welding torch to shield the weld pool.

Its schematic is shown below.


Fig. 1 Schematic diagram of semiautomatic CO2 arc welding

2. Principles

Iron becomes brittle when it combines with nitrogen that exists much in the atmosphere. CO2 gas, therefore, is often used to shield the weld pool from the atmosphere. CO2 gas can be decomposed by the ultra−high temperature arc heat into CO and O near the arc

The decomposed O combines with molten iron to form FeO.

Sequentially, C that is contained in steel is easier to combine with O than Fe deprives O from FeO to generate CO gas, which is apt to left in the weld metal to form blowholes. A weld metal that contains blowholes cannot be deemed to be sound

To improve the soundness, a welding wire that contains Si and Mn that have stronger affinity with O is used; in this case, O in FeO combines not with C but with Si and Mn and floats up on the surface of the weld pool to form slag of SiO2 and MnO. Though slag is formed, the weld metal becomes sound without blowholes.

Besides Si and Mn that prevent blowholes, various other chemical elements are added to the welding wire in order to let the weld metal possess required strength, impact toughness, corrosion resistance and other properties.

3. Features

As compared with shielded metal arc welding (SMAW), MAG arc welding has the following advantages and limitations.

(1) Advantages:

a) As the diameter of the wire is small, the welding current density is high and thus the
Deposition rate is big.

b) Good concentration of the arc realizes deep penetration.

c) The deposition efficiency is high and formation of slag is little, which makes it unnecessary to remove slag after each pass.

d) The arc generation rate is high, thereby lowering the welding cost and making the process to be more economical.

e) Hydrogen in the weld metal is low, which contributes to good crack resistance and mechanical properties.

(2) Limitations:

a) Windbreak screen is needed against high wind at a velocity of 2m/sec. or higher.

b) Even if a long conduit cable is used, welder’s movable area is limited.

c) The price of the power source is high


If you compare such advantages and limitations with those of the Shielded Metal Arc Welding (SMAW) process, it is evident that MAG arc welding offers higher efficiency, lower welding costs and better economy. Such advantageous effects can be maximized in automatic welding, particularly in robotic welding

4. Comparison of usability between solid wire and flux−cored wire

Comparison of usability between solid wire and flux-cored wire
Item Type of wire
Solid wire Flux−cored wire
For high current For low current Slag type Metal type
Less−slag type Much−slag type
Penetration Deep Shallow Slightly shallow Deep Slightly shallow
Welding position Flat,
Horizontal fillet
All positions All positions Flat,
Horizontal fillet
Flat,
Horizontal fillet
Bead appearance Slightly rough Fair Good Fair Good
Amount of slag Less Less Much Less Much
Spatter Much Less Very low Less Very low
Arcing
characteristics
With sharp
sound
Good Very good Good Very good
Amount of fumes Fair Less Less Less Less

 

5. Molten droplet transfer

The welding wire can be melted and transferred to the base metal as droplets in three different modes:

(1)short−circuiting transfer, (2) Globule transfer and (3) Spray transfer.

Depending upon the mode, the appearance and shape of weld bead, quantity of spatter, and penetration can vary.

In CO2 (MAG) arc welding, the short−circuiting transfer and the globule transfer can be observed.

(1) Short−circuiting transfer:

The welding method that uses the short−circuiting transfer is called the Short Arc Welding or the Dip Transfer Welding. When a comparatively low welding current (200A or lower) is used in either CO2 (MAG) arc welding or MIG welding, the droplet transfers to the base metal after short circuiting with it, as shown in Fig. 2. It is suited for welding of thin plates, sheet metals and in all−position welding including vertical up, vertical down and overhead welding.


Fig. 2 Short−circuiting transfer vs. welding current.

(2) Globule transfer

This transfer mode is also known as the globular droplet transfer, which is observed in welding with a comparatively high welding current and the droplet as big as the wire diameter or bigger transfers to the base metal. Because of this, a slightly higher amount of spatter is emitted than in other modes of transfer. But it is used often for it is highly efficient. In high current CO2 arc welding, the mode of droplet transfer becomes this mode.


Fig. 3 Globule transfer (Globular droplet transfer).

(3) Spray transfer

In high current MIG/MAG welding with DC electrode positive polarity and inert shielding gas, the droplet becomes smaller than the wire diameter due to the effect of plasma flow on the arc column. This is why, the emission of spatter is little and the weld bead with good appearance can be obtained.


Fig. 4 Spray transfer.

6. Welding conditions and their effects

In CO2 (MAG) arc welding, the weld bead appearance and penetration change markedly by welding conditions. It is, therefore, necessary to select proper welding conditions that suit the purpose of use. The effects of the welding conditions are shown in the figure below.

Changes in welding parameters vs. bead shape
When an arc voltage is changed
(with current and speed kept constant)
Arc voltage : Low → High

When a welding current is changed
(with voltage and speed kept constant)
Welding current : High → Low

When a welding speed is changed
(with current and voltage kept constant)
Welding speed : High → Low

Changes in welding parameters vs. bead shape
When an arc voltage is changed
(with current and speed kept constant)
Arc voltage : Low → High
When a welding current is changed
(with voltage and speed kept constant)
Welding current : High → Low
When a welding speed is changed
(with current and voltage kept constant)
Welding speed : High → Low

 

7. Shielding gas flow rate and nozzle standoff distance

The shielding gas flow rate and the nozzle standoff distance affect markedly the occurrence of such defects as pits and blowholes.

Therefore, an appropriate gas flow rate and nozzle standoff distance must be determined considering the welding conditions. The relationships between the gas flow rate and blowhole, and between the nozzle standoff distance and blowhole are shown in the following tables. The last table shows the appropriate gas flow rates and nozzle standoff distances.

Relationship between gas flow rate and blowhole
Nozzle standoff distance
(mm)
Shielding gas flow rates
(ℓ/min)
Bead appearance X−ray test results
20 25    
20    
15    
10    
5    

 

Relationship between nozzle standoff distance and blowhole
Nozzle standoff distance
(mm)
Gas flow rates flow rates
(ℓ/min)
Bead appearance X−ray test results
10 20    
20    
30    
40    
50    

 

Rough standard for gas flow rates and nozzle standoff distance
Wire diameter
(mmφ)
Welding current
(A)
Nozzle standoff distance
(mm)
Gas flow rate
(ℓ/min)
1.2 100 10~15 15~20
200 15~20 20~25
300 20~25
1.6 300 20~25 20~25
350
400

8. Wire extension

The wire extension gives great influences on the appearance of the weld bead, penetration depth, arc stability and efficiency. If it is too short, larger amounts of spatter adhere onto the inside of the nozzle to hinder smooth shielding gas flow. The following table shows effects of the wire extension on various performances, and Fig. 5 shows the proper wire extension for the proper welding currents.

Effects of wire extension on various performances
Various
performances
Influences
Melting rate With the constant welding current, the longer the wire extension, the larger the melting rate.
Arc stability When the wire extension is excessive, the arc becomes unstable and spatter increases.
Penetration When the wire extension is excessive, penetration becomes shallow.
Porosity When the wire extension is excessive, the nozzle standoff distance becomes long; thus, the shielding effect becomes degraded, thereby causing higher tendency of porosity occurrence.
Others When the wire extension is too short, the nozzle hides the sight of the welding groove and the weld pool. Spatter adheres much on the inside surface of the nozzle, thereby causing deteriorated gas shielding. In addition, the contact tip and the nozzle will severely be damaged.


Fig. 5 Suitable wire extension vs. welding current.

9. Forehand welding and backhand welding

In MIG / MAG (CO2) arc welding, the torch can be manipulated by the forehand welding technique or backhand welding technique. Each welding technique has different characteristics; hence, you should choose the appropriate technique according to the application.

The forehand welding is adopted in many applications; by contrast, the backhand welding is more suitable in groove welding with a high current.

Forehand welding and backhand welding

Features of forehand welding:

  1. Easy to target the wire onto the welding line.
  2. Flat bead shape with low reinforcement.
  3. Consistent melt−through root pass bead.
  4. Comparatively large particles of spatter flown ahead.
  5. Shallow penetration due to the molten metal flowed ahead

Features of backhand welding:

  1. Not easy to see the welding line hidden by the sight of nozzle.
  2. Narrow bead shape with high reinforcement.
  3. Difficult to obtain a consistent melt−through root pass.
  4. Low spatter generation.
  5. Deep penetration due to the molten pool formed backwards.
  6. Easy to control bead width and reinforcement due to ease of seeing the bead shape…
Typical applications of forehand welding and backhand welding
Application Forehand
welding
Backhand
welding
Reasons
Thin plate, Flat welding × Easy to see the groove. Shallow penetration with flat bead.
Medium/thick plate, Flat welding Deeper penetration, better usability and fewer passes in backhand welding.
Horizontal fillet welding
(1−pass weld)
× Flat bead shape.
Horizontal fillet welding
(Multi−pass weld)
Backhand welding is suitable for filling passes and forehand welding for cover pass.

10. Influence of wind velocity

In gas−shielded arc welding like MIG / MAG (CO2) arc welding, poor shielding effect tends to generate blowholes. Be sure that the shielding effect deteriorates especially when it is windy. Fig. 6 shows an example of the X−ray test results of the weld bead deposited on a 9 mm thick plate.

It exhibits that there is a sharp increase of blowholes when the wind is over 2mt /sec. in velocity.In order to prevent blowholes, it is recommended to use partitions or windbreak screens for protective measures. Also, in order to get more efficient shielding, it is important to shorten the distance between the contact tip and the base metal and to increase the gas flow rate in the respective allowable ranges

11. Weld imperfections and preventive measures

Weld imperfections and preventive measures in MIG /MAG (CO2) arc welding
Weld imperfections Causes (Preventive measures)
1) Shortage of shielding gas flow rate.
2) Gas heater does not work (in case) CO2 used.
3) Purity of CO2 gas is questionable. Has more moisture in it. .
4) Inadequate gas shielding.5) Excessive gas flow rate.
6) Excessive nozzle standoff distance.
7) Much oil or grease is deposited on the base metal.
8) Much scale is remained on the base metal.
9) Much rust is remained on the base metal.
10) The shielding nozzle is clogged with much spatter.11) Too long arc length (Too high arc voltage).

 

  1) Too low welding current.
2) Too high welding current.
3) Too low arc voltage.
4) Too low welding speed.
5) Wrong targeting position of the welding wire.
6) Too small groove angle.
  1) Too high arc voltage.
2) Too high welding current.
3) Wrong targeting position of welding wire.
4) Rough manipulation of the welding torch.
5) Too high welding speed.
1) The center of the wire feed roller is deviated.
2) Inappropriate adjustment of the wire straightener.
3) Loosened contact tip.
  1) Too low welding current.
2) Too low arc voltage.
3) Too high arc voltage.
4) Too low welding speed (in groove welding)
5) Too high welding speed.
6) Too narrow groove angle.
7) Wrong targeting position of the welding wire.
  1) Too large tack weld or, welding is directed towards the preceding weld.
2) Place the workpiece cable connection away from the weld as far as possible (on a large weldment).
3) On a small weldment, place the workpiece cable connection at the start of the weld.
4) Keep the arc length shorter
5) Direct the welding torch toward the opposite direction of the magnetic arc blow.
6) Put tab plates at both the start and end of the weld line.
  1) Too high welding current.
2) Too narrow groove.
3) Too low arc voltage.
4) Wrong targeting position of the welding wire.
  1) Too large contact tip diameter for the wire diameter used.
2) Worn contact tip.
3) Loosened contact tip.
4) Irregular rotation of the wire reel.
5) Worn groove of the wire feed roller.
6) Inadequate pressure of the pressure roller.
7) Large resistance of the conduit tube.
8) Too long wire extension.
9) Too low or high arc voltage.
10) Too low welding current.
11) Inappropriate welding speed.
12) Unstable torch manipulation.
13) Nozzle is clogged with much spatter.
14) Fluctuation of wire extension.
15) Wrong targeting position of the welding wire.
16) Contaminated base metal with dirt, rust, paint and oil.
17) The position of the workpiece cable connection isInappropriate.

 

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