Page 173 - CITS - Welder - Trade Theory

WELDER - CITS

Argon satisfactory on sheet metal argon-helium
Aluminium
copper, magnesium, Argon and preferred on thicker
nickel and argon helium sheet metal
their alloys
Less than 1/8 in.(3.2mm) thick; high welding speeds
Argon without melt
through; minimum distortion and spatter; good
Carbon steel 20-25%CO2
penetration
CO2 Deeper penetration; faster welding speeds;minimum
cost
90% helium No effect on corrosion resistance small heat affected
Stainless Steel zone; no
7.5% argon
undercutting; minimum distortion; good arc stability
2.5% CO2
The arc in a CO2 atmosphere is unstable and a great deal of spattering occurs. This is reduced by holding a short
arc. Deoxidizers like aluminium, manganese or silicon are often used.
The deoxidizers remove the oxygen from the weld metal. Good ventilation is required when using pure CO2. About
7-12 percent of the CO2 becomes CO (carbon monoxide) in the arc. The amount increases with the arc length.
A 25% higher current is used with CO2 than with argon or helium. This causes more agitation of the weld puddle,
hence entrapped gases raises to the surface of the weld, so low weld porosity.
Argon carbondioxide: CO2 in argon gas makes the molten metal in the arc crater more fluid. This helps to
eliminate undercutting when GMA welding carbon steels.

CO2 also stabilizes the arc, reduces spatter and promotes a straight line (axial) metal transfer through the arc.
Argon-Oxygen: Argon-oxygen gas mixtures are used on low alloy carbon and stainless steels. A 1-5 percent
oxygen mixture will produce beads with wider, less finger shaped, penetration. Oxygen also improves the weld
contour, makes the weld pool more fluid and eliminates undercutting.
Oxygen seems to stabilize the arc and reduce spatter. The use of oxygen will cause the metal surface to oxidise
slightly. This oxidization will generally not reduce the

strength or appearance of the weld to an unacceptable level. If more than 2% oxygen is used with low alloy steel,
a more expensive electrode wire with additional deoxidisers must be used.
The desirable rate of gas flow will depend on the type of electrode wire, speed and current being used and the
metal transfer mode.
As a rule small weld pools 10 L/min
medium weld pools 15 L/min
and large spray weld pools 20-25 L/min

Too much gas flow can be just as bad as not having enough. The reason being that if the gas flow is too high it
will come out of the MIG Torch.
Suggested gases and gas mixtures for use in GMAW spray transfer

CO2 gas cylinder and regulator: The shielding gas required for GMAW/CO2 welding is supplied from a gas
cylinder through an outlet valve and regulator.
Gas flow meter: It is a unit which has graduations marked on the glass tube. A flow rate adjustment valve fixed
to the flow meter controls the rate of flow of inert gas/CO2 gas to the welding gun in litre per minute. Fig 2.

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CITS : C G & M - Welder - Lesson 49 - 60 CITS : C G & M - Welder - Lesson 49 - 60