Gas or Flux core Shielding
MIG vs. Flux-Cored: Which Welding Process Is Right for You?
Shielding gas affect the quality of the finished weld
GMAW as identified by the American Welding Society, is also popularly known as MIG (Metal Inert Gas) and uses a continuous solid wire electrode for filler metal and an externally supplied gas(typically from a high-pressure cylinder) for shielding. The wire is usually mild steel, typically copper colored because it is electroplated with a thin layer of copper to protect it from rusting, improve electrical conductivity, increase contact tip life and generally improve arc performance. The welder must be setup for DC positive polarity.
The shielding gas, which is usually carbon dioxide or mixtures of carbon dioxide and argon, protects the molten metal from reacting with the atmosphere. Shielding gas flows through the gun and cable assembly and out the gun nozzle with the welding wire to shield and protect the molten weld pool. Molten metal is very reactive to oxygen, nitrogen and hydrogen from the atmosphere, if exposed to it. The inert gas usually continues to flow for some time after welding to keep protecting the metal as it cools. A slight breeze can blow the shielding away and cause porosity, therefore welding outdoors is usually avoided unless special windscreens are erected. For most mild steel applications, CO2 will provide adequate shielding, but when you must have a flatter bead profile, less spatter or better wetting action, you may want to consider adding 75 to 90% argon to your CO2 shielding gas mix.
Why? Argon is essentially inert to the molten weld metal and therefore will not react with the molten weld metal. When CO2 is mixed with Argon, the reactivity of the gas is reduced and the arc becomes more stable. But, Argon is more expensive. In production welding, selecting the perfect shielding gas can be a science of its own. Attributes such as material thickness, welding position, electrode diameter, surface condition, welding procedures and others can affect results.
Common gas mixes for the home hobbyist and small fabricator would be:
- 100% CO2 -Lowest price, generally greatest penetration, and higher levels of spatter. Limited to short circuit and globular transfer.
- 75% Argon - 25% CO2 -Higher price, most commonly used by home hobbyist and light fabricator, lower levels of spatter and flatter weld bead than 100% CO2. Limited to short circuit and globular transfer.
- 85% Argon - 15% CO2-Higher price, most commonly used by fabricators, with a good combination of lower spatter levels and excellent penetration for heavier plate applications and with steels that have more mill scale. Can be used in short circuit, globular, pulse and spray transfer.
- 90% Argon - 10% CO2- Higher price, most commonly used by fabricators, with a good combination of lower spatter levels and good penetration for a wide variety of steel plate applications. Can be used in short circuit, globular, pulse and spray transfer.
- The best choice when cosmetic appearance is an issue since it provides lower spatter levels than flux-cored. The arc is soft and less likely to burn through thin material.
- The lower spatter associated with gas/mig welding also means no slag to chip off and faster cleaning time.
- Gas/mig welding is the easiest type of welding to learn and is more forgiving if the operator is somewhat erratic in holding arc length or providing a steady travel speed. Procedure settings are more forgiving.
- If you are skilled and get specific proper guns, shielding gas, liners, drive rolls, and electrode, gas/mig can weld a wider range of material including thinner materials and different materials such as stainless, nickel alloys or aluminum.
- Since a bottle of external shielding gas is required, gas/mig welding may not be the process of choice if you are looking for something that offers portability and convenience. Gas/mig also requires additional equipment such as a hose, regulator, solenoid (electric valve) in the wire feeder and flowmeter.
- The welder's first job is to prepare the surface by removing paint, rust and any surface contamination.
- Gas/mig has a soft arc which will not properly weld thicker materials (10 gauge would be the maximum thickness that gas/mig could soundly weld with the 115 volt compact wire feeder welder we are referring to or ¼" with the 230 volt input compact wire feeder welder.) As the thickness of the material (steel) increases, the risk of cold lapping also increases because the heat input needed for good fusion is just not possible with these small machines.
Keep cylinder upright and chained to support
• Keep cylinder away from areas where it may be damaged.
• Never lift welder with cylinder attached.
• Never allow welding electrode to touch cylinder.
• Keep cylinder away from welding or other live electrical circuits.
A WARNING -
BUILDUP OF SHIELDING GAS may harm health or kill.
• Shut off shielding gas supply when not in use.
BE SURE TO KEEP YOUR FACE AWAY FROM THE VALVE OUTLET WHEN “CRACKING” THE
VALVE. Never stand directly in front of or behind the flow regulator when opening the cylinder valve. Always stand to one side.
- Chain the cylinder to a wall or other stationary support to prevent the cylinder from falling over. Insulate the cylinder from the work circuit and earth ground.
- 2. With the cylinder securely installed, remove the cylinder cap. Stand to one side away from the outlet and open the cylinder valve very slightly for an instant. This blows away any dust or dirt which may1 have accumulated in the valve outlet.
3. Attach the flow regulator to the cylinder valve and tighten the union nut securely with a wrench.
NOTE: If connecting to 100% CO2 cylinder, make certain the plastic washer is seated in the fining that attaches to the CO2 cylinder.
- Refer to Figure AS. Attach one end of inlet gas hose to the outlet fining of the flow regulator and tighten the union nut securely with a wrench. Connect the other end to the SP-1 35T Gas Solenoid Inlet Fitting (5/8-18 female threads — for CGA — 032 fitting). Make certain the gas hose is not kinked or twisted.
Self-shielded Flux-Cored Arc-Welding process
FCAW per the American Welding Society, or flux-cored for short, is different in that it uses a wire which contains materials in its core that, when burned by the heat of the arc, produce shielding gases and fluxing agents to help produce a sound weld, without need for the external shielding gas. We achieve a sound weld, but in a very different way. We have internal shielding instead of external shielding. The shielding is very positive and can endure a strong breeze. The arc is forceful, but has spatter. When finished, the weld is covered with a slag that usually needs to be removed. A "drag" angle for the gun is specified which improves operator visibility. The settings on the wire feeder welder are slightly more critical for this process. Improper technique will have results that are magnified. This type of welding is primarily performed on mild steel applications outdoors.
The flux-cored process is only recommended on materials as thin as 20 gauge, a bit thicker than the 24 gauge we said for MIG. In general, this process is best for welding thicker materials with a single pass, especially if you need to weld outdoors such as to repair a tractor out in the field. A 115 volt flux-cored machine using an electrode such as .035" Innershield NR-211-MP will generally allow you to weld steel up to ¼"thick. Note that this is more than double the thickness maximum of 12 gauge with MIG on 115 volts. With the proper electrode on a proper machine, such as .045" Innershield NR-211MP, and a 230 volt input machine, you can weld steel up to 1/2" thick. Note that Innershield® NR-211-MP requires that the machine be setup for DC negative polarity.
- The Self-Shielded electrodes are optimal for outdoor procedures since the flux is built into the wire for positive shielding even in windy conditions. An external shielding gas and additional equipment are not needed, so setting up is simpler, faster and easier.
- The flux-cored process is most suited for applications with thicker materials as it is less prone to cold lapping.
- It is not recommended for very thin materials (less than 20 gauge).
- When flux-cored welding, machine settings need to be precise. A slight change in a knob position can make a big difference in the arc. In addition, the gun position is more critical in that it must be held consistently, and at the proper angle, to create a good weld.
- This process creates spatter and slag that may need to be cleaned for painting or finishing.
It should be noted that the same machine can be used to weld with both MIG and flux-cored processes though a special package is usually needed to change from one application to the other. Drive rolls, shielding gas, gun liners, contact tips and procedure settings need to be addressed when changing processes.
Another area that may cause the novice welder some concern is how to choose the best wire. Proper electrode diameter is related to plate thickness and the welder you have. A smaller wire makes it easier to weld thinner plate.
You must be careful to match the output voltage of your machine with the voltage of the electrode and the appropriate wire diameter and wire feed speeds to make sure you have a compatible system.