A Selection Guide of MIG Welding Wires

MIG welding wire consists of a spooled-up electrode that’s fed through a MIG welding “gun” and is heated to melt metal and join workpieces together. Sounds simple enough, right?

Well, it’s not quite that easy. There are a lot of different types of MIG welding wire, and in this article, we’ll explain some of the differences between them, what to consider when choosing welding wire, and the importance of choosing high-quality wire.

MIG Welding Wires

In Gas Metal Arc Welding (GMAW), you won’t be using a stick electrode or a filler rod. Instead, everything you need to deposit a weld comes from a spool of metal wire.

In this welding process, more popularly known as MIG, a gas tank (typically CO2 or argon) provides the shielding while the wire melts into the base metal. Nowadays, it is the most common process for structural welding and product fabrication.

When a welder pulls the trigger on his MIG gun, a wire feed 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.

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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.)

MIG Welding Wires

Wire Classification

There are two consumables to consider in the GMAW process – the gas and the wire.

Like stick electrodes, there’s a classification system for the different choices of MIG wire available that’s 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 affect 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 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.

Flux-Core vs. Solid MIG Welding Wire

There are two primary types of MIG welding wire – flux-core and solid MIG welding wire.

A flux-core wire is a metal electrode that contains a “flux compound” inside the electrode. When the wire melts and reacts with the welding arc, this forms a gas that protects the weld from oxygen, which can cause defects in the weld. This means that no shielding gas is required for this type of wire – though in some cases, shielding gas may be used alongside flux-core wire for even more protection.

In contrast, solid wire electrodes are exactly what they sound like – large reels of solid metal wire that do not contain any flux. This means they must be used with shielding gas. The most common shielding gas is 75% argon and 25% carbon dioxide. The welding gun delivers a steady stream of gas from a container, which surrounds the electrode and weld area to prevent oxidation and defects in the weld.

Depending on the application, both kinds of MIG welding wire (solid and flux-core) can be made from a variety of materials including aluminum, stainless steel, copper, and silver, to name just a few.

Considerations When Choosing MIG Welding Wire

Wondering what you should think about when you choose MIG welding wire? Neither type is superior compared to the other – rather, the right choice depends on your situation and preferences. Here are a few things to keep in mind when you pick MIG welding wire.

1. Desired Weld Cleanliness

As a rule, solid MIG welding wires used with shielding gases produce a cleaner weld with much less spatter compared to flux-cored wire. Spatter does not necessarily affect weld quality, but it may require additional grinding, polishing, and finishing to remove it before painting or other such surface preparations, so solid wire is more commonly used in these situations.

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.

Obviously, the wire feed mechanism on the welding machine will dictate which option is chosen. (A small non-industrial MIG machine uses a much smaller spool than you see in the photo below.)

MIG wire can also be ordered in “TIG cut lengths”. This refers to the three-foot strands used in TIG welding. Diameter sizes in this case are not given in decimals but rather as normal TIG rod sizes, such as 1/16 or 3/32.

2. Workpiece Material

Different types of wire are used for different materials. ER70S-3 wire is often used for mild steel that’s clean, oil-free, and rust-free, while ER70S-6 contains a deoxidizer and is used for mild steel that’s contaminated with corrosion or mill scale.

3. Environment

In very windy or challenging outdoor environments, flux-core wire is usually a better option. The gas used to shield solid MIG welding wire may be blown away while welding in windy conditions, so unless a windscreen is used, this results in a lower-quality weld. Since flux-core wire contains gases that emerge when it’s heated, the metal is shielded more effectively in those conditions.

4. Wire Thickness

Once a wire type is determined for your welding equipment, two additional pieces of information are needed in order to purchase this consumable.

The first is the wire diameter, which is usually given in thousands of an inch. The most common sizes for welding on sheet metal are 0.35 and 0.45.

A 035-inch diameter wire is a standard used for most welds, but it may not be adequate for very thick pieces of metal. Multiple passes may be required to create a strong weld.

5. Weld Unit Power

This is closely related to wire thickness. The higher the voltage and power of a MIG welding unit, the higher thickness it can accommodate. Using a thicker wire is not a good idea with lower-powered MIG welding units.

The lower overall amperage and output may result in a failure to melt the workpieces properly and create a quality weld. Always consult the manufacturer’s instructions to see the maximum recommended wire thickness.


No matter what type of MIG welding wire you buy, you should always make sure to invest in the high-quality wire. Compared to lower-quality welding wire, quality MIG wire is more forgiving, can produce a sounder weld even in less-than-perfect conditions, and still represents only a fraction of the overall cost of welding.

As you can see, the many variables involved in choosing the right wire will take a while to learn. It’s a good idea to focus on the most common rod classifications at first and absorb the rest of the arsenal over time.

That said, as an entry-level welder, however, you may find yourself having to stock consumables or replace empty spools on MIG machines for seasoned veterans. So, exercising due diligence correctly selecting and storing wire products is essential to preventing costly mistakes when a welding operation begins.

The American Welding Society has handbooks available for purchase, but the cost can be prohibitive. Check with your school or workplace to see if you can access a handbook and photocopy any info you need to place in your literature binder.