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In welding, there is no single universal welding process that is used for all applications. The process used must be varied depending on the type of material being welded, whether or not that material is ferrous or non-ferrous, magnetic or non-magnetic, and what the end use application of the metals is. These considerations will also influence the type of gas used in the welding process.
What Welding Processes Use Gas?
Two of the most common types of welding use gases in their process.
Gas Metal Arc Welding (GMAW or MIG Welding) uses a wire welding electrode on a spool that is automatically fed onto the two weld piece ends to be joined together and an arc created by an electrical current between the metal and the wire shielded by a shielding gas to heat the wire to the melting point.
Gas Tungsten Arc welding (GTAW or TIG welding) uses a tungsten electrode and shielding gas to heat the filler metal placed on two weld piece ends to be joined together. A constant current power supply produces the energy to create an arc through a column of ionized gas and metal vapors (plasma).
What Purpose Do Gases Serve In MIG And TIG Welding
Shielding gases are commonly used in gas metal arc welding (MIG) and gas tungsten arc welding (TIG) to protect the weld area from oxygen and water vapor. Oxygen and water in the atmosphere can make the welding process more difficult and cause a deterioration in the quality of the weld.
Shielding gases are typically denser than air so they can wrap around the weld and will require low flow rates to distribute the gas. They fall into two categories- inert (noble gases) or semi-inert.
Helium and argon are the only two noble gases used in welding. They are sometimes used in 100% pure form but can also be mixed or blended with other semi-inert gases such as carbon dioxide hydrogen or nitrogen to improve weld characteristics on certain types of metals.
The use of shielding gases provides several advantages in the welding process:
- It Protects the welding arc.
- It is possible to weld at higher temperatures without the material oxidizing.
- There is very little loss of alloying elements as the metal transfers across the arc.
- Side effects such as burn-in and spatter can be reduced.
- Individual gases can be adjusted to the requirements of the material being welded.
Why Gas Blends Are Important
Shielding gases protect the liquid metal pool created in the welding process from atmospheric contamination. They essentially remove potentially reactive gases such as oxygen and nitrogen as well as water vapor from the vicinity of the weld. They can also stabilize the arc and enhance the metal transfer mode.
In addition, the shielding gases can change the basic mechanical properties of the weld area through their interaction with the base metal and filler metal. This affects the strength, hardness and corrosion resistance.
The various shielding gases used in welding have specified properties and functions. Those most commonly used are in this table.
|Hydrogen (H2)||Better bead surface, higher arc temperature, good ignition, higher weld speed||Stable, concentrated arc|
|Argon (Ar)||Inert noble gas, improves oxide breakdown, improved control of the weld pool, confined penetration||Stable arc|
|Helium (He)||Inert noble gas, supplies more heat input, improves penetration and fluidity of weld pool||Unstable, wandering arc|
|Nitrogen (N2)||Can build nitrides in high temperatures||Wandering arc|
|Carbon Dioxide ( CO2)||Used in small ratio to oxidize and stabilize the arc, improves wettability of the weld bead,deep weld penetration||Unstable arc|
|Oxygen (O2)||Used in small ratio to oxidize and stabilize the arc, improves weld pool fluidity & weld penetration||Never used alone|
Quite often, blends of two or three gases are used to improve the efficiency and quality of the weld for specific base metals and filler metals. By adjusting the composition of the shielding gas, you can influence the distribution of heat to the weld. This, in turn, can influence the shape of the weld metal cross-section and the speed of welding.
The increase in welding speed can be substantial. As labor costs make up a considerable amount of overall welding costs, this can relate to a potential for significant savings.
Gases Used In MIG Welding
Many types of gases and gas blends can be used in MIG welding. The choice of which to use largely depends on the type of material you are welding.
Carbon Steel & Low Alloy steel
Argon mixed with CO2 in a 75%/25% blend will increase the weld penetration and bead wetting characteristics and can be used to join carbon steel and low alloy steels.
Tri-mix blends of argon, helium and CO2 in a 90%/7.5%/2.5% ratio are ideal for welding stainless steel because it provides good arc stability and depth of fusion.
Welding of aluminum is typically done with 100% Argon. However, if the aluminum being welded is thicker than ½ of an inch there may be Helium added to the mix to increase the heat penetration.
Either 100% pure argon or a blend of argon and helium in a 60% /40% mix are the most common gases used to weld nickel alloys to improve penetration and weld pool flow.
Pure argon or a blend of argon and helium in a 75% /25% mix are the most common gases used to weld copper alloys. This helps to improve penetration and weld pool flow.
Gases Used In TIG Welding
Unlike MIG welding, relatively few gases can be used for TIG welding.
Pure Argon is probably the most popular type of shielding gas for TIG welding because it can be used on every type of metal including aluminum.
Pure argon provides good arc stability and requires less cleaning action as the arc is narrow and concentrated so you can get a precise weld without a lot of bead rollover. It will typically produce a completed weld with a brighter, shinier surface appearance.
Argon and helium, where helium is 50% of the mix, allows for hotter welds while working which can create a wider heat profile and better conduction of heat. It is probably the second most common type of shielding gas for TIG welding and it is used for some specialized applications. The extra heat comes at a price though, so you’ll have to carefully monitor the weld to make sure you are not burning through the metal.
Pure helium or high percentages of helium (He-90%, Ar-10%) shielding gas are used primarily for TIG welding with direct current electrode negative (DCEN). Often designed as seam welders, the combination of GTAW - DCEN. The high heat input from the gas used can provide fast welding speeds and outstanding penetration.
Finally, there’s straight hydrogen. In some case, welders may opt to work with hydrogen because it increases the heat input while welding. It creates a hotter, wider bead that penetrates deeper into the metal. This is particularly advantageous in working with stainless steel.
Safety With Your Welding Gases
Since gases are invisible their presence is not readily identifiable. But they do have the potential to asphyxiate, burn or harm users. The following safety precautions and tips should be noted by weld operators.
Inspect equipment for leaks at all connections using approved leak-test solution.
Inspect hoses for leaks and worn places.
Replace bad hoses.
Protect hoses and cylinders from sparks, flames and hot metal.
Use a flint lighter to ignite the flame.
Stand to the side (away from the regulators) when opening cylinder valves.
Open cylinder valves very slowly to keep sudden high pressures from exploding the regulators.
Only open the acetylene cylinder valve 1/4 – 3/4 turn; leave wrench in place so the cylinder can be quickly closed in an emergency.
Open and light acetylene first, then open and adjust oxygen to a neutral flame.
Close the acetylene torch valve first when shutting off the torch ( a “pop” might occur as the oxygen “blows out” the flame, but this eliminates the possibility of the flame burning up the acetylene line).
When finished, close cylinder valves, bleed the lines to take pressure off regulators, then neatly coil hoses and replace equipment.
Have a fire extinguisher easily accessible at the welding site.
The shielding gas used during the MIG or TIG welding process is a key component in its success. In order to achieve proper penetration, uniform beading and good welding results the correct shielding gas must be used when welding using the MIG or TIG welding methods.
Using incorrect shielding gas results in poor penetration, irregular bead shapes, excessive splatter, overheating, lack of arc control and quick burning of the electrode. It’s important to learn about the proper gases to use and take due caution while using them.