MIG Shielding Gas Basic Explanation
MIG is a subtype of Gas Metal Arc Welding (GMAW) that utilizes a shielding gas to protect the weld from atmospheric contamination. This shielding gas is more than just a protective barrier; it's a medium that significantly influences key aspects of the welding process. It helps to maintain the arc, transfer heat, control penetration, and ultimately define the final weld bead appearance and properties.
For instance, Argon is an inert gas that ionizes easily, providing a very stable arc. In contrast, Helium has a lower density and a higher ionization potential, requires higher voltage to sustain the arc and higher gas flow rates to provide adequate shielding compared to argon. This creates a wider, hotter arc, which increases heat input and penetration. Using pure (CO2) as a shielding gas results in deeper penetration but causes more spatter and an unstable arc.
Physical Properties Compared to Standard Atmosphere
Establishing the correct MIG welding gas flow rate is critical for achieving a quality weld. A flow rate that is too low will result in porosity from inadequate shielding. Excessive gas flow also leads to porosity, as the resulting turbulence, often driven by the Venturi effect, pulls in atmospheric contaminants. This article will help you understand the internal and external factors that affect the ideal flow rate and guide you on how to set the right rate for different metals.
Key Factors Affecting Gas Flow Rate
Nozzle Types and Sizes
The welding nozzle's main function is to introduce and shape the shielding gas flow to protect the weld seam. Its design is critical for ensuring the gas exits as a smooth, stable laminar flow, which creates an effective protective barrier over the molten metal.
Nozzles come in various shapes like cylindrical, bottle-shaped, conical, and tapered, each designed for different applications and affecting the gas flow pattern in a unique way.
Welding nozzle diameter is another crucial factor. When you select a nozzle with a different bore size, you should adjust the flow rate to fall within the corresponding range, which can be found by consulting a relevant chart.
Impact of Welding Conditions on Gas Flow Rate
Working conditions and environment have a significant impact on the required shielding gas flow rate. The main distinction is between indoor and outdoor welding.
Indoor Conditions
In a controlled indoor environment with no wind, a flow rate of 10-15 CFH (Cubic Feet per Hour) is generally sufficient. If you are using fans or an exhaust system, the air movement will require a higher flow rate in the 20-30 CFH range to maintain an effective gas shield.
Outdoor Conditions
When MIG welding outdoors, shielding gas is easily dispersed by the wind. It is crucial to set up physical wind barriers to protect the weld area. In addition, you should use higher flow rates, typically in the 30-35 CFH range, to help maintain a stable gas shield against ambient airflow.
Mig Welding Indoor vs Outdoor
Set the Right Flow Rate on Different Metals
MIG welding is suitable for common metals like mild steel, aluminum, and stainless steel. The choice of shielding gas is typically limited to readily available inert and semi-inert gases, with Argon and Helium being the primary options. Their characteristics and effects on the arc have been discussed previously.
Mild Steel
Mild steel has a relatively high "tolerance" for shielding gases.
100% CO2 is a "aggressive", low-cost option that causes the most spatter. When used with Globular Transfer, it provides a high deposition rate.
Alternatively, a 90/10 Ar/CO2 mix is a good choice for Spray Transfer mode with MIG welding (e.g., when welding thicker plates). Read more about GMAW Metal Transfer Modes.
A mix of 75% Argon and 25% (C25) is an economical and practical choice for welding mild steel. This mixture provides a stable arc, minimizes spatter, and results in a smoother, more aesthetically pleasing weld with good penetration. A starting flow rate of 10-15 CFH is a good baseline. If you notice any porosity, increase the flow rate to the recommended range of 20-30 CFH.
Aluminum
Aluminum has a relatively low melting point of 660°C (1220°F), so you must weld with high travel speeds to avoid burn-through. This requires a higher shielding gas flow, typically around 30 CFH, to maintain adequate protection.
Aluminum is also prone to oxidation, making 100% Argon the preferred shielding gas. For welding thicker aluminum, a mixture of argon and helium can be used to increase heat input and penetration while also significantly decreasing porosity.
Since helium is much lighter than air, it can easily dissipate, requiring a higher flow rate for effective shielding. For example, a blend of 75% helium and 25% argon may require a flow rate as high as 50 CFH.
Stainless Steel
Stainless steel gets its corrosion resistance from its chromium content. However, at high temperatures, chromium oxidizes easily, forming a harmful oxide.
Common choices include:
- 98% Argon and 2% CO2
- 98% Argon and 2% O2
- 90% Helium/7.5% Argon/2.5% CO2
Due to the material's sensitivity to contamination, a higher flow rate is necessary to ensure adequate shielding. A range of 25-30 CFH is typically recommended for stainless steel. When using helium-based trimix gases, the flow rate must be increased to 40-50 CFH to compensate for the helium's lower density and prevent gas dispersal.
