In GMAW shielding gas plays an important role for high-quality weld. We know that shielding gas protects the molten weld pool from outside contamination while mig welding.
Without using gas in GMAW, the weld quality can be compromised and a lot of spatter can be produced.
In this post I will describe how to select shielding gas for mig process. So lets start-
Factors Affecting Gas Selection
Choosing a correct shielding gas is critical for finished quality of weld. There are few factors that decide which gas to use for mig applications. The factors are-
1. Mig solid wire electrode Type
Mig wire is an essential part of mig welding. For choosing gas you need to consider what size of wire you are using in your project.
Typically to mig weld carbon steel, 0.030″, 0.035″ and 0.045″ wire is used with 98% Argon & 2% Oxygen.
For stainless steel mig welding, wire electrode size is 0.035″, 0.045″ & 0.062″ with 98% argon & 1% oxygen shielding gas mix.
2. Mig metal arc transfer mode
Common two types of arc transfer mode is used in mig welding. They are
a) Short circuit arc transfer &
b) axial spray transfer & pulsed spray transfer mode.
Different blends of gas is used in accordance with the transfer mode variation. For example-
- 75% argon and 25% Co2 gas mix is used to weld carbon steel in short circuit mode. The use of this mix of gas reduce the spatter and improve welding bead appearance. You can also use 80% argon + 20% Co2 mix for carbon steel. This will reduce the spatter more than the 75/25 mix. And if you decrease Co2 from the mix, less heat would be generated resulting in less risk of burn through. This is good option for welding sheet metal.
- For mig welding carbon and stainless steel in axial spray transfer mode, you should go for 98% argon+ 2% Co2 mix. The mix is very effective in welding high speed steel and also presents faster travel speed. For better metal puddle fluidity you can add additional 5% Co2 in the mix making it 95% argon and 5% co2 mix. The mix is better for pulsed spray transfer mode.
- If you use 92% argon & 8% co2 mix with both pulse and axial spray transfer, you will get more energy in mig process which in turn would increase the puddle fluidity rate.
- For broad penetration in axial spray transfer mode, you should go for 90% argon & 10% Co2 mix. The mix would eventually reduce the depth of finger like penetration.
- Increasing the level of Co2 results in increased sidewall fusion. If you go for 85%argon and 15% Co2, it would produce improved weld toe wetting in carbon steel welding. The highest amount Co2 you can use in mig welding is 82% argon & 18% Co2. This one penetrates the steel very deep and suitable for thicker metal.
- You can also use argon/oxygen mix for mig welding in axial transfer mode. For stainless steel application high argon blend is essential. 99% argon+ 1% oxygen or 98/2 argon/oxygen can be used. For welding heavy carbon steel use 95% argon and 5% oxygen for better result.
3. Mechanical properties of weld metal
Mechanical properties of weld metal helps in selecting gas for mig welding. Metal hardness, toughness, ductility, strength, stiffness, stress and strains are important factors that determine the shielding gas for GMAW process.
4. Thickness of metal
Thickness of material is important factor for gas selection. For thicker metal gas combination is different than for thinner metal.
Also different metal types like stainless steel, carbon steel, aluminum etc require different gas mixture.
For example, mig welding of thicker carbon steel will require 95% argon and 5% oxygen. For thinner carbon steel the mix should be argon/Co2 where co2 in the mix should be lower in percentage say 2%.
5. Joint design
Different joint designs require different mix of gas mixture. There are many types of joint like butt joint, lap joint, edge joint, tee joint and corner joint.
Combination of 80/20 argon/co2 is suitable for butt joint. For lap joint you can use binary or ternary gas mix for better result.
75/25 argon/co2 is good for tee joint and 75% argon and 25% carbon di oxide can be used for corner joint.
6. Welding position
Gas selection also varies based on welding position. There are four types position in welding.
Overhead, flat, horizontal & vertical. Each of them require different gas mix better contour and high quality.
7. Cost
You should go for a gas mixture that at one hand provides the best quality weld and on the other hand spend less amount of money. Costing should be streamlined for maintaining the economy of the project.
8. Fit up condition &
9. Penetration profile
Mechanical Properties Of Shielding Gas
There are three basic criterial for understanding the properties of shielding gas. They are
- Potentiality of ionization of the gas
- Thermal conductivity
- Chemical reactivity of gas components
There are two basic shielding gases for mig welding. They are Argon shielding gas and Helium.
They protect the molten weld pool and do not react with the pool. So to make them conductive like plasma, the gas must be ionized. For argon to ionize 15.7 eV level of energy is required while for helium, it is 24.5 eV. Here eV means electron-volt.
We can see that with lower energy argon can be ionized but for helium energy must be increased.
The thermal conductivity (ability of gas to transfer thermal energy) of gas components play an important roll in selecting proper gas. It affects the arc shape and temperature distribution in the region.
The argon has lower thermal conductivity than helium. It is 10% of the conductivity of helium. In this case helium gains more power to penetrate the material broadly and can reduce the depth of penetration.
And due to less conductivity the argon creates finger like penetration which is narrower than what helium can produce.
However, it is tested that argon holds better arc starting than other shielding gas.
Gas Combination
You can mix the gas in two ways.
1. Binary
In binary two gasses are mixed for welding purpose. They can be either of the following-
- Argon+ Helium
- Argon+Co2
- Argon+O2
Argon and helium mix is best for nickel alloy and aluminum alloy. The addition of helium provides more puddle fluidity and flatter bead. It also promotes high travel speed.
Helium reduces the finger like penetration profile while mig welding aluminum. It also reduces the hydrogen pores in case of welding 5xxx series of aluminum.
Normally 75% argon and 25% helium is standard mixture for improved penetration profile on copper, nickel and aluminum projects.
Most common blend of gas is argon+co2. It is suitable for welding carbon steel.
You can mig weld stainless steel with the gas bled but make sure co2 doesn’t exceed 4%. Co2 content in the blend increases the heat input. So it may thrive risk of burn through in axial transfer mode for thin metal.
However you can reduce the risk of burn through by reducing the co2 from the mix and using short circuit mode.
2. Ternary Blend
Ternary blend means using 3 gas in the mixture. For improved molten metal fluidity and reduced spatter in mig welding, you should use ternary gas mix. There are two common ternary blends.
They are-
- Helium+ Argon+Co2
- Argon+Co2+Oxygen
For welding stainless steel using short circuit mode, you may use 90% Helium + 7.5% Argon + 2.5% CO2. The blend allows higher travel speed on Stainless steel.
With 55% Helium + 42.5% Argon + 2.5% CO2 you can weld stainless and nickel alloy using short circuit mode. The mix also allows axial and pulsed transfer mode for welding stainless.
Blend with 38% Helium + 65% Argon + 7% CO2 is suitable for welding mild and low alloy steel using e short-circuiting transfer mode. The mix creates high conductive weld resulting in broader penetration profile.
Blend with 90% Argon + 8% CO2 + 2% Oxygen is suitable for welding carbon steel. Higher percentage of argon gas reduces spatter.
You may use short-circuiting, pulsed spray, and axial spray modes with the mixture.
What Gas To Choose (Gas Selection Chart)
I have compiled the shielding gas requirement based on material, electrode type and mode of arc transfer below.
| Base Metal | Arc Transfer Mode | Electrode Type | Shielding Gas |
|---|---|---|---|
| Carbon Steel | Short Circuit | ER 70S -3 | 100% CO2 |
| Carbon Steel | STT™ | ER 70S – 4 | 70 – 90% Argon + 10 – 25% CO2 |
| Carbon Steel | Axial Spray | ER 70S – 6 | 82–98% Argon+ 2%– 18% CO2 |
| Low alloy Steel | Short Circuit | ER80S-Ni1 | 100% CO2 |
| Low alloy Steel | STT™ | ER80S-D2 | 75 – 80% Ar. + 20 – 25% CO |
| Low alloy Steel | Axial Spray | ER90C-G | 95% Ar. + 5% CO2 |
| Low alloy Steel | Pulsed Spray | ER 110C-G | 95% – 98% Ar. + 2% – 5% O |
| Aluminum | Axial Spray | ER 1100 | 100% argon |
| Aluminum | Axial Spray | ER 4043, ER 4047 | 75% Helium + 25% Ar |
| Aluminum | Pulsed Spray | ER 5183, ER 5356 | 75% Ar. + 25% Helium |
| Aluminum | Pulsed Spray | ER 5554, ER 5556 | 100% Helium |
| Austenitic Stainless steel | Short Circuit | ER 308L Si | 98 – 99 Ar. + 1-2% O2 |
| Austenitic Stainless steel | STT | ER 316L Si | 55% He + 42.5 Ar. +2.5 CO |
| Austenitic Stainless steel | Axial Spray or GMAW-P | ER 316L Si | 98 – 99 Ar. + 1-2% O2 |
| Nickel alloys | GMAW-S or STT | ER NiCr-3 | 100% argon |
| Nickel alloys | GMAW-S or STT | ER Ni Cr Mo-4 | 90% He. 7.5Ar. + 2.5 CO |
| Nickel alloys | GMAW-S or STT | ER Ni Cr Mo-10 | 89% Ar. + 10.5% He. +0.5% CO2 |
| Nickel alloys | GMAW-S or STT | ER Ni Cr Mo-17 | 66.1% Ar. 33% He + 0.9 CO2 OR 75% Ar. + 25% He OR 75% He + 25 Ar |
| Duplex Steel | GMAW-S | 2209 | 66.1% Ar. 33% He + 0.9 CO2 |
| Duplex Steel | STT TM | 2304 | 90% He. + 7.5% Ar. + 2.5% CO2 |
| Duplex Steel | Axial Spray GMAW-P | 2304 | 75% Ar. + 25% He. OR 75% He. + 25% Ar OR 100% Ar. OR 100% He. |
| Aluminum Bronze | GMAW-P Axial Spray | ER Cu Al – A1/ ER Cu Al – A2/ ER Cu Al – A3 | 100% Argon |
| Copper alloys | GMAW-P Axial Spray | ER Cu (Deoxidized) | 75% Ar. + 25% He OR 75% He. + 25% Ar. OR 100% Ar. |
| Silicon Bronze and Brasses | GMAW-P Axial Spray STT TM GMAW-S | ER Cu Si | 100% Ar. |
I highly recommend to follow the chart for selecting proper gas for projects.
FAQ (Frequently Asked Questions)
1. Is CO2 or argon better for MIG welding?
Using 100% argon provides finger like penetration with wider bead profile. While co2 provides deeper penetration than argon.
So for welding thicker metal, Co2 gas is best and for thinner metal argon is suitable.
However using some blend of either binary or Ternary would get better result.
2. What gas do you use to MIG weld mild steel?
For mild steel argon and co2 mix is better option. Typically 90% argon and 10% co2 gas mix is recommended.
Or you can use 90% Helium + 7.5% Argon + 2.5% CO2 gas mix for better penetration and puddle fluidity.