Wire feed speed

Increasing the wire feed speed automatically increases the current in the wire. Wires are generally produced in 0.6, 0.8, 1.0, 1.2, 1.4 and 1.6mm diameter.

2.2 Voltage

The voltage setting is the most important setting in spray transfer as it controls the arc length. In dip transfer it also affects the rise of current and the overall heat input into the weld. An increase of both wire feed speed/current and voltage will increase heat input. The welding connections need to be checked for soundness, as any loose connections will result in resistance and will cause the voltage to drop in the circuit and will affect the characteristic of the welding arc. The voltage will affect the type of transfer achievable, but this is also highly dependent on the type of gas being used.

Figure 3 The effect of arc voltage.

•

•

•

2.3 Gases

Figure 4 Gas composition effect on weld bead profile.

For non-ferrous metals and their alloys (such as Al, Ni and Cu) an inert shielding gas must be used. This is usually either pure argon or an argon rich gas with a helium addition. The use of a fully inert gas is the reason why the process is also called MIG welding (metal inert gas) and for precise use of terminology this name should only be used when referring to the welding of non-ferrous metals.

The addition of some helium to argon gives a more uniform heat concentration within the arc plasma and this affects the shape of the weld bead profile. Argon-helium mixtures effectively give a hotter arc and so they are beneficial for welding thicker base materials, those with higher, thermal conductivity eg copper or aluminium.

For welding of steels – all grades, including stainless steels – there needs to be a controlled addition of oxygen or carbon dioxide in order to generate a stable arc and give good droplet wetting. Because these additions react with the molten metal they are referred to as active gases and hence the name MAG welding (metal active gas) is the technical term that is used when referring to the welding of steels.

100%CO2

CO2 gas cannot sustain spray transfer as the ionisation potential of the gas is too high. Because of this high ionisation potential it gives very good penetration, but promotes globular droplet, transfer also a very unstable arc and lots of spatter.

Argon +15 to 20%CO2

The percentage of carbon dioxide (CO2) or oxygen depends on the type of steel being welded and the mode of metal transfer being used. Argon has a

Ar Ar-He He CO2

Argon +1 to 5%CO2

Widely used for stainless steels and some low alloy steels.

Figure 5 Active shielding gas mixtures for MAG welding of carbon, carbon-manganese and low alloy steels

(Blue is a cooler gas mixture; red is a hotter mixture)

Gas mixtures - helium in place of argon gives a hotter arc, more fluid weld pool and better weld profile. These quaternary mixtures permit higher welding speeds, but may not be suitable for thin sections.

Stainless steels

Austenitic stainless steels are typically welded with argon-CO2/O2 mixtures for spray transfer or argon-helium-CO2 mixtures for all modes of transfer.

The oxidising potential of the mixtures are kept to a minimum (2-2.5%

maximum CO2 content) in order to stabilise the arc, but with the minimum effect on corrosion performance. Because austenitic steels have a high thermal conductivity, the addition of helium helps to avoid lack of fusion defects and overcome the high heat dissipation into the material. Helium additions are up to 85%, compared with ~25% for mixtures used for carbon and low alloy steels. CO2-containing mixtures are sometimes avoided to eliminate potential carbon pick-up.

Figure 6 Active shielding gas mixtures for MAG welding of stainless steels.

(Blue is a cooler gas mixture; red is a hotter mixture)

For martensitic and duplex stainless steels, specialist advice should be sought. Some Ar-He mixtures containing up to 2.5%N2 are available for welding duplex stainless steels.

Light alloys (aluminium magnesium, titanium, copper and nickel and their alloys)

Inert gases are used for light alloys and those that are sensitive to oxidation.

Welding grade inert gases should be purchased rather than commercial purity to ensure good weld quality.

Argon

Argon can be used for aluminium because there is sufficient surface oxide available to stabilise the arc. For materials that are sensitive to oxygen, such as titanium and nickel alloys, arc stability may be difficult to achieve with inert gases in some applications. The density of argon is approximately 1.4

Argon-helium mixtures

Argon is most commonly used for MIG welding of light alloys, but some advantage can be gained by the use of helium and argon/helium mixtures.

Helium possesses a higher thermal conductivity than argon. The hotter weld pool produces improved penetration and/or an increase in welding speed.

High helium contents give a deep broad penetration profile, but produce high spatter levels. With less than 80% argon, a true spray transfer is not possible. With globular-type transfer, the welder should use a 'buried' arc to minimise spatter. Arc stability can be problematic in helium and argon-helium mixtures, since argon-helium raises the arc voltage and therefore there is a larger change in arc voltage with respect to arc length. Helium mixtures require higher flow rates than argon shielding in order to provide the same gas protection.

There is a reduced risk of lack of fusion defects when using argon-helium mixtures, particularly on thick section aluminium. Ar-He gas mixtures will offset the high heat dissipation in material over about 3mm thickness.

Figure 7 Inert shielding gas mixtures for MIG welding of aluminium, magnesium, titanium, nickel and copper alloys.

(Blue is a cooler gas mixture; red is a hotter mixture)

A summary table of shielding gases and mixtures used for different base materials is given in below.

Summary of Shielding gas mixtures for MIG/MAG welding

Metal

Shielding gas

Reaction

behaviour Characteristics Argon-CO2 Slightly

oxidising

Increasing CO2 content gives hotter arc, improved arc stability, deeper penetration, transition from finger-type to bowl-shaped penetration profile, more fluid weld pool giving flatter weld bead with good wetting, increased spatter levels, better toughness than CO2. Minimum 80% argon for axial spray transfer. General-purpose mixture:

Argon-10-15%CO2. Argon-O2 Slightly

oxidising

Stiffer arc than Ar-CO2 mixtures, minimises undercutting, suited to spray transfer mode, lower penetration than Ar-CO2 mixtures, finger-type weld bead penetration at high current levels. General-purpose mixture:

Argon-3% CO2. Ar-He-CO2 Slightly

oxidising

Substitution of helium for argon gives hotter arc, higher arc voltage, more fluid weld pool, flatter bead profile, more bowl-shaped and deeper penetration profile and higher welding speeds, compared with Ar-CO2

mixtures. High cost.

Carbon steel

CO₂ Oxidising Arc voltages 2-3V higher than Ar-CO₂ mixtures, best penetration, higher welding speeds, dip transfer or buried arc technique only, narrow working range, high spatter levels, low cost.

He-Ar-CO2 Slightly oxidising

Good arc stability with minimum effect on corrosion resistance (carbon pick-up), higher helium contents designed for dip transfer, lower helium contents designed for pulse and spray transfer. General-purpose gas: He-Ar-2%CO2.

Stainless steels

Argon-O2 Slightly oxidising

Spray transfer only, minimises undercutting on heavier sections, good bead profile.