GMAW (Gas Metal Arc Welding) uses a continuously fed solid wire electrode that melts into the weld joint as you work. The specific wire type depends on the base metal you’re welding, but the most common by far is ER70S-6, a copper-coated carbon steel solid wire. Beyond carbon steel, GMAW wires come in aluminum, stainless steel, and metal-cored varieties, each matched to a specific base metal and shielding gas combination.
How AWS Wire Classifications Work
Every GMAW wire carries a code from the American Welding Society that tells you exactly what it is. Take ER70S-6 as an example. The “E” means it functions as an electrode, and the “R” means it can also serve as a filler rod. The number “70” indicates the wire produces a weld with 70,000 psi tensile strength. The “S” tells you it’s a solid wire, and the final number identifies its specific chemical composition.
This naming system is consistent across all GMAW wire types, so once you understand the pattern, you can read any classification. An optional “H” designator sometimes appears at the end, indicating the wire meets standards for low diffusible hydrogen, which reduces the risk of cracking in the finished weld.
Carbon Steel Wires
Carbon steel solid wire falls under the AWS A5.18 specification, and two classifications dominate the market: ER70S-3 and ER70S-6. Both produce welds with 70,000 psi tensile strength, but their chemical makeup makes them suited to different situations.
ER70S-6 contains higher levels of silicon and manganese, which act as deoxidizers. This makes it more forgiving on material that has light mill scale, rust, or surface contamination. It flows well, wets out smoothly at the toes of the weld, and produces a clean bead. For general fabrication, auto body work, and most hobbyist projects, ER70S-6 is the go-to wire.
ER70S-3 has less deoxidizer content, which means it works best on clean, well-prepared material. It produces slightly less silicon island residue on the surface of the bead. If your base metal is freshly ground or chemically cleaned, ER70S-3 gives you a quality weld with less post-weld cleanup.
Aluminum Wires
Aluminum GMAW uses two primary filler alloys: 4043 and 5356. They are not interchangeable, and the choice depends on the base alloy you’re joining and the service conditions the finished part will face.
4043 is an aluminum-silicon alloy that feeds smoothly and produces good-looking beads with less spatter. It’s the standard choice for welding cast aluminum and 6000-series alloys. One important limitation: if the finished part will be anodized, 4043 typically turns dark gray during the process, creating an obvious color mismatch with the surrounding base metal.
5356 is an aluminum-magnesium alloy that produces stronger welds and provides a much closer color match after anodizing. It’s the preferred wire for 5000-series alloys and structural applications where higher shear strength matters. However, 5356 should not be used in service temperatures above 150°F (65°C) for sustained periods, as it can become susceptible to stress corrosion cracking in that range.
Stainless Steel Wires
Stainless steel GMAW wires follow their own classification system, and the wire you choose needs to match the base metal’s alloy composition. The three most common are ER308L, ER309L, and ER316L.
ER308L is the standard wire for joining 304 and 308 stainless steels, the most widely used stainless alloys in food equipment, architectural work, and general fabrication. It can also be used on 321 and 347 stainless. The “L” in the name stands for low carbon, with the carbon content held to a maximum of 0.03%. This low carbon level prevents carbide precipitation, a problem that occurs during welding when carbon atoms cluster along grain boundaries and rob the surrounding area of corrosion resistance.
ER309L is designed for welding dissimilar metals, particularly joining stainless steel to carbon steel. ER316L matches 316-series stainless, which contains molybdenum for extra corrosion resistance in marine and chemical environments.
Metal-Cored Wire
Metal-cored wire looks like solid wire from the outside but has a tubular construction filled with metallic powders instead of a solid cross-section. It falls under a different AWS classification (the “C” designation for composite) but runs on the same GMAW equipment.
The reduced solid cross-section of metal-cored wire concentrates electrical resistance, which generates more heat at the tip. This gives you higher deposition rates than solid wire, meaning you’re laying down more weld metal per minute. The arc also has greater energy density, producing deeper penetration. Users frequently describe the puddle as flowing smoothly, wetting out to the toes of the joint without the spatter issues that solid wire can cause in spray transfer.
The trade-off is positional capability. Metal-cored wire is generally limited to flat and horizontal positions unless you’re using a pulsed transfer mode. For vertical and overhead work with conventional settings, solid wire or flux-cored wire is the better choice.
Matching Wire Diameter to Material Thickness
GMAW wire comes in diameters ranging from 0.023 inches (0.6 mm) up to 0.045 inches (1.2 mm), with 0.030 and 0.035 inches being the most common for general work. Thinner wire runs at lower amperages and suits thinner material, while thicker wire handles heavier plate.
For steel plate up to about 3 mm (roughly 1/8 inch), a 0.8 mm (0.030 inch) wire running between 90 and 200 amps covers most fillet and butt weld situations. Once you move to plate thicknesses of 5 mm and above, stepping up to a 1.0 mm (0.035 inch) wire at 185 to 240 amps gives you enough heat input to achieve proper fusion. Thinner material below 1 mm calls for the smallest available wire diameter and currents as low as 80 to 90 amps to avoid burning through.
Shielding Gas Pairings
The wire type dictates which shielding gas you need, and using the wrong combination produces poor results or outright defective welds.
- Carbon steel wire pairs with either 100% CO2 or a 75% argon/25% CO2 blend (commonly called C25). Pure CO2 costs less and penetrates deeper but limits you to short circuit and globular transfer modes, which produce more spatter. The C25 mix gives a smoother arc and allows spray transfer on thicker material.
- Aluminum wire requires 100% argon, typically run in spray transfer mode. For thicker aluminum sections, blending 25% to 75% helium with the balance in argon increases heat input and improves penetration.
- Stainless steel wire commonly uses a tri-mix of 90% helium, 7.5% argon, and 2.5% CO2. This combination works primarily in short circuit transfer mode and keeps the weld puddle manageable while protecting the chromium content from oxidation.
Choosing the right wire for GMAW comes down to three decisions: what base metal you’re welding, how thick it is, and what shielding gas you have available. Start with the base metal to pick the wire alloy, then match the diameter to your material thickness and amperage range, and finally pair it with the correct gas. Get those three factors lined up and the process runs the way it’s designed to.