ER70S-6 is the best general-purpose solid wire for GMAW welding with 100% CO2. Its higher levels of silicon and manganese act as deoxidizers, counteracting the extra oxygen that CO2 introduces into the weld pool. This keeps porosity low and produces sound welds even on steel that isn’t perfectly clean.
Why ER70S-6 Outperforms Other Solid Wires
The two most popular solid wire classifications for MIG welding are ER70S-3 and ER70S-6. Both have a tensile strength of 70,000 psi, but the “-6” designation signals a higher concentration of silicon and manganese in the wire’s chemistry. Those elements serve a specific purpose: they scavenge oxygen from the molten weld pool before it can form gas pockets (porosity) inside the finished bead.
This matters because CO2 is a reactive gas. At welding temperatures it breaks apart, releasing oxygen directly into the arc. An argon-rich blend produces far less free oxygen, so a wire like ER70S-3 can handle it fine. But under straight CO2, that extra oxidation overwhelms a lower-deoxidizer wire. ER70S-6 has enough silicon and manganese to neutralize the oxygen and push it to the surface of the bead as a thin, glassy residue called silicon islands. If you see those small, dark specks on your weld, that’s actually evidence the deoxidizers did their job.
ER70S-3 is designed for clean steel with only light mill scale and works best paired with argon/CO2 mixed gas. ER70S-6 tolerates medium to high levels of mill scale and mild surface contaminants, making it a more forgiving choice in shops where parts aren’t always prepped to perfection.
What CO2 Does to Penetration and Spatter
One reason welders choose 100% CO2 in the first place is penetration. Gases with higher thermal conductivity, like pure CO2, produce a broader, deeper penetration profile compared to argon-based blends, which create a narrower, more tapered fusion zone. For structural work or thicker plate where you need to get deep into the joint, that’s a real advantage.
The tradeoff is spatter and transfer mode. With 100% CO2, true spray transfer isn’t achievable. You’re limited to short-circuit transfer at lower settings and globular transfer at higher voltages. Globular transfer does deliver deep penetration, but it throws off significantly more spatter than spray, which drives up cleanup time. Short-circuit transfer with CO2 is more controllable and works well for thinner material, out-of-position work, and root passes, though penetration is shallower.
Choosing ER70S-6 helps on both fronts. Its deoxidizer content produces a more stable arc and slightly better wetting action than a lower-deoxidizer wire under CO2, which can reduce (though not eliminate) the spatter that comes with globular transfer.
Typical Settings for ER70S-6 With CO2
Wire diameter determines your operating window. The two most common sizes for shop work are 0.035-inch and 0.045-inch.
- 0.035-inch wire, short-circuit transfer: 15 to 18 volts, 100 to 160 amps. Good for thinner material and positional welding.
- 0.045-inch wire, short-circuit transfer: 17 to 18 volts, 100 to 220 amps. Handles heavier plate while still allowing positional work.
If you push voltage into the upper 20s (around 28 to 31 volts), you’ll transition into globular transfer with CO2. This is where you get that deep-digging penetration on thicker joints, but expect more spatter and plan for cleanup. Fine-tuning voltage by a volt or two in either direction can make a noticeable difference in arc stability and spatter levels, so it’s worth running test beads on scrap before committing to a production weld.
When Metal-Cored Wire Makes More Sense
For higher-volume production work, metal-cored wire is worth considering as an alternative to solid ER70S-6. Metal-cored wire has a tubular construction filled with metallic powders instead of flux. It deposits more weld metal per hour than solid wire, allows faster travel speeds, and handles mill scale and rust even better than ER70S-6, all while producing very little spatter.
There’s a catch, though. Metal-cored wire needs a high-argon shielding gas (typically 90% argon, 10% CO2) to run in spray transfer, which is the mode that delivers all those productivity benefits. If you’re committed to straight CO2, you won’t get the clean spray arc that makes metal-cored wire worthwhile. The wire itself also costs more per pound than solid wire, so the math only works out when the faster deposition and reduced cleanup time offset the higher material and gas cost. For shops running short production runs or doing repair and maintenance welding, ER70S-6 with CO2 remains the more practical and economical choice.
Choosing the Right Wire Diameter
Wire diameter should match your material thickness and joint type. For sheet metal and light structural work up to about 3/16 inch, 0.030 or 0.035-inch wire keeps heat input low enough to avoid burn-through. For plate 1/4 inch and thicker, stepping up to 0.045-inch wire lets you run higher amperage and deposit more metal per pass, which saves time on multi-pass welds.
Thicker wire also handles the aggressive arc characteristics of CO2 a bit better. The larger diameter carries more current before the arc becomes erratic, giving you a wider usable range in globular transfer. If you’re welding heavy plate with CO2 specifically for the deep penetration, 0.045-inch ER70S-6 is generally the most efficient pairing.
Managing Silicon Islands
The glassy residue that ER70S-6 leaves on the weld surface is a normal byproduct of the deoxidizing process. Silicon in the wire reacts with oxygen in the weld pool, forming silicon oxide that floats to the surface. On single-pass welds that won’t be painted or coated, these deposits are cosmetic and harmless. On multi-pass welds, you should chip or wire-brush the silicon islands between passes because they can cause inclusions if welded over. The same goes for any weld that will be painted: the glassy spots don’t accept paint well and should be removed with a wire wheel or light grinding before coating.