When a welder pulls the trigger on his MIG gun (shown below), a wirefeed machine advances the wire out through a brass nozzle. This allows for pinpoint accuracy and an unobstructed view of what's happening inside the joint. Welding out of position is a lot easier than with SMAW. And since the spool holds about a mile's worth of wire, you don't have to stop and reload very often. (The term MIG, incidentally, stands for "metal inert gas". However, since CO2 and O2 are reactive gases, it's more accurate to say MAG - metal active gas - when these gases are used.)
At any rate, there are two consumeables in the GMAW process - the gas and the wire. Like stick electrodes, there's a classification system for MIG wire managed by the American Welding Society. The American Society of Mechanical Engineers also has a code, but it's nearly identical. The AWS code for solid steel wire is known as AWS A5.18. Here's what the classification number for a common wire for mild steel, ER70S-6, indicates:
ER - Electric Rod
70 - This two or three-digit number represents the minimum tensile strength of the weld metal, measured in pounds per square inch (PSI) multiplied by 1,000.
S - Solid wire.
6 - This number (with sometimes a letter added) indicates chemical additives used in the wire which may effect the polarity setting on the machine.
The 6 in this case indicates more deoxidizers have been added to the wire, which is helpful when welding on dirty or rusty steel. The other general purpose carbon steel wire type is ER70S-3. This one doesn't have the added chemicals, so is used primarily on new or clean steel.
The most commonly used aluminum MIG wires are ER5056, a soft wire with good ductility, and ER5356, which is harder and has a high tensile strength.
Stainless steel MIG wire includes designations like ER308, ER316 and ER308-L. The L stands for low carbon, which provides extra corrosion resistance.
Once a wire type is determined for your welding equipment, two additional pieces of information are needed in order to purchase this consumeable. The first is the wire diameter, which is usually given in thousands of an inch. The most common sizes for welding sheet metal are 0.35 and 0.45. The table below is commonly found in consumeable spec sheets and is worth copying for reference. It tells you how to set the controls on your equipment and which gas you need, depending on the welding process.
In making a MIG wire purchase, your last decision involves the quantity of wire and how it's housed. For instance, Lincoln Electric offers ER70S-6 as a 44-pound spool or a 1,000-pound drum. Their product sheet offers a few specs for buyers to chew on. Obviously, the wirefeed mechanism on the welding machine will dictate which option is chosen. (A small non-industrial MIG machine uses a much smaller spool.)
MIG wire can also be ordered in "TIG cut lengths". As the name suggests, the three-foot strands are used in TIG welding. Diameter sizes in this case are not given in decimals but rather as normal TIG rod sizes, such 1/16 or 3/32. See this example for details.
Using "cored" wire allows a MIG welder to skip the tank of CO2 or argon and weld without the gas. That's because the wire core contains ingredients that do the job of shielding the weld pool. This is particularly helpful when welding out of doors, since a stiff breeze is enough to disperse a compressed gas. The process is formally known as Flux-Cored Arc Welding (FCAW).
Like stick rod coatings, the core of a flux-cored wire provides the shielding gas. Flux-cored wire also enables a welder to use higher amperages and larger diameter wires than solid wire. The process is ued extensively in structural welding outdoors. The deposition rate increases, along with work productivity. While cored wire costs significantly more than solid MIG wire, the labor savings generally makes up for the additional expense -- and then some.
Despite the shielding additives, flux-cored wire is skinny enough to shoot out of a MIG gun. And the slag that comes with those ingredients is a fraction of what appears in the SMAW process. Regular flux-cored wire is often referred to as Self-Shielded or Innershield, which is a brand name sold by Lincoln Electric. (Hence, when no gas tank is used, the process is formally known as FCAW-S.)
Mercifully, the AWS standard for carbon steel flux-cored wire (AWS A5.20) varies only slightly from the MIG solid wire code. Instead of ER, an E (for electrode) begins the number in the case of all flux-cored wire. And instead of an S for solid wire, you'll see either a T for tubular, or C, indicating a "composite" wire. E70C-6 is an example of a general purpose flux-cored wire. However, where you see 70, there's a difference from wire code. Only the first digit is used to indicate tensile strength, multiplied by 10,000 (instead of 1,000 in the case of a MIG). The second number indicates the welding position. A zero designation means the wire is only good for horizontal or flat welding.
One of the most frequently used flux-cored wires in building construction is Lincoln Electric's Innershield 232, which conforms to AWS E71T-8. The digit 1 indicates the wire can be used in all positions. The 8 signifies low hydrogen, which calls to mind the common stick electrode E7018. NR-232 is popular because its chemical composition meets seismic requirements for earthquake zones like California. On the downside, the wire is more difficult to work with than wires that don't have the same level of seismic toughness.
The chart below lists recommended parameters for NR-232 and is excerpted from Lincoln's welding guide for Innershield wires. ("NR" said aloud sounds like "inner", which makes it easy to remember.) The number .068 in the chart is the wire's diameter. Notice the polarity is DCEN. The designation CTWD stands for "contact tip to work distance", which affects the amount of current moving through the joint. The "deposit rate" helps the purchaser calculate how much wire will be needed for a project.
When a compressed gas is involved, the welding process is known as either Gas-Shielded or Dual-Shielded Flux-Cored Welding, since both the flux ingredients and compressed gas produce the shield. The formal designation is FCAW-G.
Here's a more complicated example of a flux-cored wire, E71T-1C JH8, which breaks down like this:
E - Electrode
7 - Tensile strength measured in pounds per square inch (PSI), multiplied by 10,000; in this case, 70,000 PSI. Note the difference with MIG, which uses two numbers multiplied by 1,000.
1 - All-position welding capability
T - Tubular wire
1 - This is a wire usability specification. The options range from 1 to 14). The 1 here indicates that the wire has a rutile slag system (which means the chemical additives are acidic). Rutile coatings provide good weldability (low spatter, good arc quality and weld puddle control), but the mechanical properties are not considered as robust as a basic slag system.
C - This letter indicates that the wire requires CO2 shielding gas. (M would indicate an argon/CO2 shielding gas blend. )
JH8 - This optional code designates the maximum amount of diffusible hydrogen the wire can contain. (Editor's note: Not sure what the "J" means...) In this example, less than 8 ml of hydrogen is permissible for each 100 g of weld metal. The lower the number, the less hydrogen allowable in the wire, and therefore a lower chance for hydrogen-induced cracking in the final weldment.
For a more in depth explanation of what the classification E71T-1C JH8 represents, examine this Lincoln Electric product page. Be sure to click on the tabs for "mechanical properties" and "typical operating procedures".
As you can see, the many variables involved in choosing the right wire will take awhile to learn. It's a good idea to focus on the most common rod classifications at first. As an entry level welder, however, you're likely to find yourself restocking supplies and replacing empty spools on MIG machines. So exercising due diligence in selecting and storing wire products can prevent costly mistakes when a welding operation begins.
For a step-by-step tutorial of wire classifications, check out this online guide. The American Welding Society does have handbooks available, but their literature is extremely expensive. Check with your school or workplace to see if a copy is available.
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For more info on stainless steel and aluminum wire classifications, as well as selecting the right gas , wire diameter and welding parameters, read this comprehensive guide (PDF) from AirGas.
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Intro to MIG Welding
Delta School of Trades
How to Read Electrode Numbers in Wirefeed Welding Wisconline.com
Understanding Flux-Cored Wires
Innershield Wire Catalog
Metal Cored Wires: Advantages & Disadvantages
Benefits of Gas-Shielded Flux-Cored Welding Welding Design & Fabrication
Filler metal product approvals: What do they mean? The Fabricator