One of the most important components in MIG welding is the shielding gas used to protect the weld pool from atmospheric contamination. Selecting the proper gas or gas mixture for your specific welding application ensures strong, clean welds free of defects. This article provides a guide to the common MIG welding gases available and the advantages of each based on the metal being welded.
Why Shielding Gas Matters
Inert shielding gas flows through the MIG torch nozzle to displace air and prevent it from contacting the arc and molten metal. Air exposure can cause issues like porosity, cracking, oxidation, and inadequate fusion. The appropriate gas will depend on the base metal, metal thickness, welding position, speed, and desired mechanical properties.
Pure Argon – Best for Non-Ferrous Metals
Argon is an inert noble gas that provides stable arc initiation and a consistent plasma column. Being heavier than air, it offers dense coverage over the weld. Argon is commonly used for welding aluminum, magnesium, copper, nickel alloys, and other non-ferrous metals since it offers deep penetration and low spatter.
100% Carbon Dioxide – Low Cost for Mild Steel
Carbon dioxide (CO2) is less expensive but more reactive than inert argon. The CO2 decomposes into oxygen and carbon monoxide, which can cause mild steel welds to be oxidized or porous. However, the increased weld puddle agitation creates a wider, more penetrating bead profile. The higher heat input also boosts deposition rates.
Argon-CO2 Mixes – Versatile Blend
By blending argon and CO2, you can achieve many of the benefits of both gases. The argon stabilizes the arc while the CO2 adds penetration. Mixes in the 20-25% CO2 range are common for steel since they offer a good balance of bead profile, oxidation resistance, and cost. Specialty argon-helium and argon-oxygen mixes are also used.
Matching Gas to Wire and Joint
For thin sheet metal, pure argon prevents burn-through. On thick plate welds, higher CO2 levels give better sidewall fusion. For automated work, argon-helium allows very high speeds. Pick welding wire diameter to match gas – smaller wires typically pair with lower current argon mixes. Consider joint fit-up too when selecting gas.
Other Gas Usage Tips
- Use 100% argon for aluminum to prevent sooty, porous welds.
- Choose the lowest acceptable CO2 content for mild steel to limit oxidation.
- For stainless steel, a 90/10 or 80/20 argon/CO2 mix prevents chromium oxidation.
- Flow rates around 20 CFH are typical, but windy conditions require more gas flow.
- Keep hoses, torch neck, and other passages free of debris to ensure smooth flow.
The Perfect Gas for Your Needs
With the variety of shielding gases and mixes available for MIG welding, it can be daunting to pick the ideal one. But understanding the properties of each, and matching gas choice to your metal type, thickness, and bead profile needs will lead you to the perfect solution. The right gas leads to pretty welds! Consult with your welding gas supplier for personalized recommendations.
Comparing Gas Delivery Options
Shielding gas can be supplied to MIG welders in cylinders, high pressure bulk tanks, or onsite generators. Here’s a comparison:
Cylinders: Available in a range of sizes from 20 to 300 cubic feet. Individual cylinders offer portability and the ability to switch gases quickly. But constant cylinder change-outs can reduce productivity.
Bulk Tanks: For high volume usage, bulk tanks can store hundreds of cubic feet of gas. They use pressures of 300 psi and higher to compact the gas. This saves on cylinder costs and downtime, but lacks flexibility.
On-Site Generation: Gas generators produce their own shielding gas using air separation membranes. An economical choice when very large volumes of gas are required. But upfront generator costs are high. Oxygen monitoring is required to ensure purity.
Evaluate costs, change-out frequency, storage space, purity needs and other factors to pick the best delivery method for your shop. Blending cylinders with a bulk or generator system can provide flexibility plus economy.
Diagnosing Poor Shielding Gas Issues
If weld defects start cropping up, inadequate shielding gas may be the culprit. Here are some common signs:
- Heavy spatter, sooty deposits on metal: Indicates too much turbulence allowing air entrainment. Check gas flow rate, leaks, drafts.
- Porosity: Air is contaminating weld zone and forming trapped bubbles. Confirm gas coverage over full length of weld.
- Blackish discoloration: Too much oxidation from excessive air exposure during welding.
- Poor fusion: Not enough gas flow leading to cold welds with lack of penetration.
Pay attention to any changes from previous good welds, like suddenly increased spatter. This likely signals a problem with shielding gas delivery. Promptly addressing issues ensures optimal weld integrity.
The Importance of Gas Flow Rate
The rate of gas flow, measured in cubic feet per hour (CFH), is a critical factor in shielding performance. Flow rate affects coverage area and turbulence. Too low of a flow allows surrounding air to contaminate the weld. Excessive flow can also be problematic by causing turbulence and air entrainment.
As a general rule, start with a flow of about 20 CFH when beginning a weld, then reduce to 15-18 CFH once the arc is stable. For robotic MIG welding, 25-30 CFH is typical. Increase flow rates by 5-10 CFH when there is draftiness in the work area that disrupts the shielding gas column.
Monitor flow with a dedicated flowmeter built into the regulator. Calibrate it regularly for accuracy. Keep torch angles less than 20 degrees and provide adequate gas postflow after completing the weld to ensure full coverage. Match flow rate to wire feed speed and arc voltage settings as well.
Effects of Moisture and Oil Contamination
It’s critical to prevent moisture, oil and other contaminants from getting into the shielding gas supply. Moisture in the gas line can cause instability in the arc voltage and reduce weld penetration. Porosity and soot-like contaminants in the weld bead are also common signs of gas contamination.
Oils used in air compressors can vaporize and get pulled into the gas delivery system, requiring the use of specialized oil-removing filters. Other tips to ensure contaminant-free gas:
- Inspect cylinders, regulators and hoses for damaged seats, worn O-rings, loose connections.
- Close valve on empty cylinders to block moisture-filled air being sucked back.
- Store cylinders upright and generators/bulk tanks indoors away from water sources.
- Drain moisture traps regularly and change filters per manufacturer.
With vigilance and proper procedures, critical shielding gas purity can be maintained from the supply source through to the torch nozzle.
Conclusion
MIG welding depends on inert shielding gas to protect welds from deficiency-causing oxygen and nitrogen. While more expensive gases like argon produce attractive results, affordable carbon dioxide blends allowacceptable weld quality on many common metals. By considering all factors from base metal to joint design, welders can narrow down options to the ideal MIG welder gas for their specific application. Quality work depends on the quality of the gas shield.
