Shielding Gases For MIG (GMAW) – The Basics

The purpose of shielding gases is to protect the molten weld pool from the nitrogen and oxygen in the atmosphere.It can also help protect against or lessen the effects of impurities (rust, mill scale) in and around the weld pool.
Shielding gases also effect the way a weld behaves. Depending on what the shielding gas is made up of, you can change the depth of penetration, width of weld, travelling speed and general weld appearance.

Argon (Ar) : Inert Gas that is heavier than air. It has no effect on the arc or welding performance except to provide a shielding from the air and air born impurities. Coupled with excellent arc starting capabilities, Argon (Ar) the best shielding gas of all GTAW (TIG) applications and can be used for some GMAW (MIG) of mild steel and stainless steels below 1.5mm material thickness.

Carbon Dioxide (CO2) : Active Compound that is heavier than air. Has a substantial effect on arc performance. The hotter more fiery arc when using straight Carbon Dioxide (CO2) causes high spatter levels, high travelling speeds and deeper penetration. This makes it excellent for shielded flux cored wire (FCAW) and some GMAW (MIG) applications. Because of it’s oxidisation effect, Carbon Dioxide (CO2) can not be used on Aluminium or amounts in excess of 3% can not be used on Stainless Steels.

Helium (He) : Active Gas that is lighter then air. Has an substantial effect on arc performance. It’s active nature allows for a hotter arc without the excessive penetration and spatter that is characteristic of Carbon Dioxide (CO2).Helium’s lighter than air nature,requires the consumer to use higher than usual flow rates. It’s also very costly to produce compared to other gases, which means the costs flow on to the consumer. Because of the cost, high travel speed and poor arc starting characteristics, Helium (He) on it’s own (not in a gas mixture) is generally only used for fully automatic GTAW (TIG) of Aluminium. 

Argon, Oxygen and Carbon Dioxide Mixtures : Nearly all mixtures of Argon (Ar), Carbon Dioxide (CO2) and Oxygen (O2) in various amounts are designed solely for use in the GMAW (MIG) of Steels.
Depending on the manufacturer the mixtures can change a fair amount but these are mostly design for general purpose work and are set up for use in all welding positions.

25% Carbon Dioxide in a balance of Argon : Mixture of Argon (Ar) and Carbon Dioxide (CO2) solely used for GMAW (MIG) and FCAW (Flux Cored Arc Welding) of steel. Excellent for all weld positions where deep penetration without excessive spatter are required.
GMAW (MIG) 10mm+ Steels
FCAW 6mm+ Steels

Very common mixture as it is one of the mixtures quoted by AWS and AS Standards

Gas Mixtures for Stainless Steels: Generally mixtures of Argon (Ar), Helium (He), Carbon Dioxide (CO2) and sometimes Oxygen (O2) designed solely for use with GMAW (MIG). Manufacturers again have their own blends for different reasons, but the common mixture for most Satinless Steels is a 2.5% to 3% Carbon Dioxide in Argon.

Gas Mixtures for Aluminium: Generally mixtures of Argon (Ar) and Helium (He). Solely for use with GMAW (MIG) and sometimes (depending on mixture) for GTAW (TIG) of Aluminiums. Most common mixture is 25% Helium in an Argon balabnce.

Gases ain’t Gases

So, the stupid bug is catching first the NSW state government now Qantas.

According to The Age article, Qantas service techs were fill the emergency oxygen tank with nitrogen. The mistake was notice when a engineer questioned the techs on what they were doing.

I can imagine the language he used when he realised what they were doing.

Though Qantas’s safety processes kick in straight away and all aircraft serviced by Qantas had been notified it still leaves you to wonder.

I personally know the equipment and processes very well and I’m left almost without words. The service tech would have had to go to allot of trouble to connect the wrong cylinders and I do mean allot trouble. They would have had to put in thought to come up with the solution they did and it makes you wonder why they didn’t let their thoughts go one more step…..*shakes head*

For those who don’t know the air you breath is made up of 21% oxygen and 79% nitrogen (thought other gases are present in very very small volumes).

Human beings NEED 21% oxygen not 18%, not 25% we need 21% it’s amazing what happens to the human body when you allow it to have too much or not enough oxygen.

By filling the emergency cylinder with 100% near pure nitrogen if the the plane had had an emergency and the pilots had taken a breath of the nitrogen they would have passed out almost instantly and without intervention from some one else (like removing the mask) it only takes a few minutes for them to be dead.

Let alone the fact that no one would have been flying the plane……..

Silly and deadly mistake

Thankfully the mistake was spotted, and notification sent and all appears to be safe.

Still the Qantas techs reponsible should be red faced

The Age – Probe after Qantas pumps wrong gas into jets

Hydrogen in Welding

Like oxygen, hydrogen is used as an additive in shielding gas mixtures to improve weld bead characteristics. Hydrogen has a low ionisation potential (15.5 eV) but a high thermal conductivity. It is also the lightest known element at 0.0070 as heavy as air.

Used in small amounts it increases weld penetration and weld fluidity. It also cleans the characteristic helium surface oxides.

However hydrogen just like oxygen can be present in weld metals because of poor cleaning of weld area or welding technique which results in cracking.

Oxygen in Welding

Oxygen is an active gas with an ionisation potential of 12.08eV. It is 1.1 as heavy as air.

It is not a shielding gas on it’s own but a component in shielding gas mixtures in low concentrations.

Oxygen contained in the atmosphere attacks the molten weld pool during arc welding. This causes loss of carbon, manganese, silicon and other elements so affecting the strength of the steel.

It may also incorporate itself into the weld pool causing brittleness.

However in a shielding gas small amounts generally no more than 5% is used.

Oxygen reduces the surface tension of the molten weld pool allowing better transfer of weld metal to parent metal.

Being an active gas it increases heat input which effect effects travelling speeds, weld penetration and edge wetting.

It also helps promote spray transfer in GMAW of steels.

Oxygen can be present in weld metals because of poor weld preparation and technique.

Rimming steel have a high oxygen content these are welded with consumables which adds aluminium to the weld pool.

Helium in Welding

It is 0.14 times as heavy as air. It is chemically inert with a high ionisation potential (24.58 electron volt). Helium is a rare gas found in association with certain natural gas streams in low concentrations. It is costly to produce, store and transport, because of its very low liquid boiling point (-269 degrees).

Helium is generally used alone in the aerospace industry, laser welding and DCEP mechanised GTAW of aluminium.

Because it is lighter than air it is necessary to use a high flow rate. It is possible to require two to three times as much helium as argon to obtain shielding effectiveness.

Because of it’s high ionisation potential helium and thermal conductivity arc is hotter translating to high travelling speeds, low bead, higher weld pool fluidity, wetting and greater depth of fusion.

In GMAW the transfer is globular. To produce spray transfer at least 20% argon must be add to a mixture.

Helium also can leave surface oxides when using DCEP. Alternating current produces a cleaning action, which would elevate this problem.

The arc column produced by helium spreads out in a bell shape with equal heat input over the entire area of arc impingement on the base material.

The high ionisation potential means arc starting can be difficult compared with argon.

Helium is used in high amounts in gas mixtures for thick sections of non-ferrous metals. It main advantage being in welding aluminium, magnesium and copper alloys.

Because of cost, oxidisation, spatter levels and arc starting problem helium is generally used in mixtures.

Argon in Welding

Argon is a chemically inert gas used alone or in gas mixtures for the welding and plasma cutting of both ferrous and non-ferrous metals.

Argon is 1.4 times heavier than air, which makes ideal for use as a shielding gas or in shielding gas mixtures.

Because Argon is very non-reactive in the arc (low ionisation potential 15.8 eV) allows it to carry electric current better than other shielding gases. This also helps greatly with the removal of oxides when used with DCRP (electrode positive).

Argon has a constricted arc column combined with a high current density cause the arc energy to be concentrated over a small area. This causes a deep finger penetration profile when used with GMAW.

Though it can be used alone on ferrous and non-ferrous metals with GTAW and GMAW, in most ferrous application it’s used in gas mixtures (O2, CO2, H2 and He) to improve weld bead shape, arc control, penetration and spatter reduction.

When welding with spray or pulsed-spray transfer with pure Argon on ferrous metals the arc becomes unstable and produces an irregular weld bead. To solve this small amounts of O2 (1-5%)

or CO2 (2-25%) are added to stabilise the arc, improve wetting, minimise undercut and reduce the depth of the papilla that is characteristic of the weld penetration obtained with spray transfer.

Brazing and Braze Welding Basics

BRAZING AND BRAZE WELDING

Brazing or Braze Welding like Soldering uses adhesion to joint metals rather than fusion. The main differences between Solder and Brazing processes being that Brazing and Braze Welding have a much higher melting temperature (anywhere from 450 degrees and up), the other difference being strength, brazed joints are allot stronger than solder and often match the strength of the metals being Brazed.

Brazing and Braze Welding are different only in joint preparation.

Brazing uses the same joint preparation as soldering requiring a close fit up of parts (0.04-0.20mm) relying on capillary action to join the parent metals. Because of this requirement care must be taken with joint design and joint preparation.

Braze Welding use different joint designs with larger “gaps” between the two parent metals being joined. Saving time in joint preparation, also allowing it to be used on thicker sections but using more filler material and heat to achieve proper adhesion .

 

Metals that can be brazed

Copper and copper alloys

Bronze

Brass

Mild steels

Cast irons

Stainless Steels

Aluminium and alloys

 

Fluxes

Cleaning is necessary to allow the capillary action to occur.

Wire brushing and degreasing of all surfaces to be brazed is a must for best results

Whether you use a flux coated rod or manually apply the flux yourself, flux is required to protect the molten metals and parent metals surface from impurities on the surface of the parent metals, protect against atmospheric impurities and allows good flow of filler material.

After brazing any excess flux should be removed (generally hot soapy water; check with manufacturer if unsure)

Most cleaners and fluxes are corrosive so care should always be taken with these items, always use correct safety equipment and MSDS (Material Safety Data Sheets) should always be kept .

Soldering Basics

Soldering

Soldering or soft soldering refers to a process of joining metals by adhesion of a filler wire that has a melting temperature lower than 450 degrees to a parent metal or metals that has a melting point higher than that of the chosen filler wire.

Soldering is employed when a leak proof or a low electrical resistance is required.

As the join is held or “glued” together not fused the joint is never as strong as the parent metal. This should be taken into account when designing the joint.

Solders can be made up of mixtures of tin, lead, silver, gold, copper and so on.

Because of the wide range of filler wire materials and low melting point of these materials a number of different plant can be used to achieve the temperature and control that is required to successfully solder most metals.

Plant types like electrical soldering irons, propane (LPG) torches, air/fuel torches (Proline, Turbo torch), gas heated soldering irons and normal Oxy/acet plant.

EASY TO SOLDER METALS

Platinum

Gold

Copper

Silver

Cadmium Plate

Tin Plate

Some precious metals require special low temperature solders.

LESS EASY SOLDER METALS

Lead

Nickel Plate

Brass

Bronze

DIFFICULT TO SOLDER METALS

Galvanised Iron

Tin-Nickel

Nickel-Iron

Mild Steel

Good preparation is required for these metals.

VERY DIFFICULT TO SOLDER METALS

Chromium

Stainless Steels

Special corrosive fluxes should be used.

FLUXES

Fluxes are used to:

• To chemically clean the surface of the parent metals.
• Help prevent oxidation of the joint metal and solder.
• Provide a wetting action to help smooth flow of solder over the parent metal.

The more difficult the metal is to solder the more active (corrosive) the flux. The easier the metals to solder the less active the flux.

Regardless of which material or flux used the joint should be cleaned by a degreaser and be free of dirt and rust. The flux residue should also be clean off after soldering.

Because some fluxes and cleaners are corrosive the correct safety protection should be worn (gloves, eye protection, apron and so on).

How to Solder

For use with Oxygen/Fuel torches only 

1. Surfaces of parent metals should be cleaned and joints prepared (joints should be neat, have minimum gap possible and good alignment).
2. Apply small amount of flux to each side of joint.
3. Apply a soft (low-pressure) flame to fluxed joint.
4. Slightly warm end of solder stick or rod and “dip” the warmed end into flux (be careful that it is not excessive the stick or rod should be just warm enough to allow the flux to adhere correctly).
5. Touch solder on surface of parent metal in a scratching movement. (The parent metal should be hot enough to melt the solder; torch should only be applied to maintain required temperature.)
6. Continue with steps 4 and 5.

Things to Note

• The heated parent metal should be hot enough to melt solder. The torch should only be used to maintain parent metal temperature not to melt solder. DO NOT OVER HEAT.
• Do not use too much solder besides making for an ugly joint appearance and a waste of solder it will not add strength.
• NEVER over heat this could result in parent metal distortion and possible burning of flux and solder.

• If solder does not adhere:
• Wrong solder or flux has been used (e.g. stainless steel requires a particular flux and solder).
• The joint is contaminated with oil, rust or dirt. Stop, clean and reflux.
• With Stainless steel this a sign of over heating.

Safety always comes first.