MIG Wire Selection: ER70S-6, ER70S-3, Stainless & Aluminum Wires

ER70S-6 vs ER70S-3: The Deoxidizer Decision

Both wires are solid carbon-steel MIG electrodes classified under AWS A5.18, and both deposit weld metal with roughly 70 ksi tensile strength (the '70' in the designation). The practical difference is chemistry: ER70S-6 carries higher levels of silicon (0.80-1.15%) and manganese (1.40-1.85%), which act as deoxidizers, while ER70S-3 runs leaner on both (silicon around 0.45-0.70%). Those extra deoxidizers are what let ER70S-6 tolerate mill scale, light rust, and dirty plate that would give ER70S-3 porosity and cold lap.

For 90% of general fabrication, repair, and hobby work, ER70S-6 is the right default. It wets out flatter, ties in at the toes better, and is far more forgiving of surface contamination, which is why it is the wire most distributors move by the pallet. Reach for ER70S-3 when you are welding clean, prepped material and want a slightly lower silicon-island deposit, common in code work, sheet metal on properly cleaned steel, or multi-pass applications where excessive silicon can be an inclusion concern.

One practical note: the higher silicon in ER70S-6 leaves more of those glassy brown silicon islands on the weld surface. On single-pass work they are cosmetic, but on multi-pass structural welds under D1.1 you should knock them off between passes because they can become trapped slag inclusions.

• Choose ER70S-6: Rusty, scaled, or oily steel; general fab and repair; anything where you cannot fully clean the joint; the safe all-around default.
• Choose ER70S-3: Clean, prepped steel; sheet metal on bright material; multi-pass work where lower silicon is preferred; some code procedures that call it out.
• Both meet: AWS A5.18 ER70S-X, ~70 ksi tensile, and qualify under AWS D1.1 for common prequalified carbon-steel procedures.

Wire Diameter by Material Thickness

Diameter is the single biggest driver of your usable amperage range, and matching it to material thickness keeps you out of burn-through on thin gauge and lack-of-fusion on heavy plate. The common solid-wire sizes run .023, .030, .035, and .045 inch. Smaller wire concentrates less heat and feeds a lower amperage floor, which is exactly what you want on auto-body sheet; larger wire carries more current and lays more metal per pass on structural steel.

A useful rule of thumb: .023-.024 for 24-gauge up to about 1/8 inch, .030 for roughly 1/8 inch and lighter general work, .035 as the do-everything size from about 1/8 to 1/4 inch, and .045 when you are running 1/4 inch and heavier or want maximum deposition on a machine that can push the amperage. On a typical 230V single-phase MIG at home or in a light shop, .035 ER70S-6 covers the widest range of jobs on one spool.

Do not chase thickness with a wire that is too large for your machine. A 140-amp 120V unit will never run .045 well; it does not have the amperage to melt it in cleanly, and you will get ropey, cold beads. Size the wire to both the metal and the welder.

Stainless Wires: Matching ER308L, ER309L, and ER316L

Stainless MIG wire (AWS A5.9) is selected by matching or over-alloying the base metal, and the wrong choice shows up as cracking or corrosion months later. ER308L is the workhorse for welding 304/304L stainless to itself, the most common austenitic grade in food equipment, railings, and general fabrication. The 'L' means low carbon (0.03% max), which resists carbide precipitation and preserves corrosion resistance in the heat-affected zone, so specify the L grade unless a procedure says otherwise.

ER309L is the go-to for dissimilar joints, most importantly joining stainless to carbon steel, and as a buffer or barrier layer before cladding. Its higher chromium and nickel content tolerates the dilution from mixing stainless with mild steel without cracking. If you are tacking a stainless bracket to a mild-steel frame, 309L is the wire that will not give you a brittle, cracked joint.

ER316L adds 2-3% molybdenum for pitting and crevice corrosion resistance in chloride environments: marine hardware, saltwater exposure, and many chemical and pharmaceutical applications. Match 316/316L base metal with 316L filler. Do not substitute 308L on a 316L job to save money; you lose the molybdenum protection that the part was specified for in the first place. Suppliers across the US stock all three in 2 lb and 25-30 lb spools, though 316L usually carries the highest price per pound.

• ER308L: 304/304L to itself. General stainless fab, kitchens, railings. The default stainless wire.
• ER309L: Stainless to carbon steel, dissimilar joints, and buffer/barrier layers before hardfacing or cladding.
• ER316L: 316/316L base metal, marine and chloride/chemical exposure where the molybdenum content matters.

Aluminum: ER4043 vs ER5356

Aluminum MIG wire (AWS A5.10) comes down to two dominant choices, and picking between them is about the base alloy, the service, and how the part will be finished. ER4043 is a silicon-bearing wire (about 5% Si) that runs smoother, wets out better, and is more crack-resistant on the 6xxx series extrusions and castings you see in most general fabrication. It is more forgiving for a welder new to aluminum and leaves a duller, gray bead.

ER5356 is a magnesium-bearing wire (about 5% Mg) with higher tensile and shear strength, better color match after anodizing, and it is the required choice for welding 5xxx-series base metals like 5052 and 5083. It is also stiffer, which makes it feed more reliably through a spool gun or push-pull setup, an underrated advantage when you are fighting the soft, birdnest-prone nature of aluminum wire.

Two rules keep you out of trouble. First, never weld a 5083 or other high-magnesium base metal with 4043 in elevated-temperature service; the combination can form a brittle magnesium-silicide phase. Second, if the part will be anodized and appearance matters, use 5356, because 4043 turns dark and mismatched. For aluminum feeding, most shops run a spool gun for anything past a couple of feet of gun cable, use a U-groove or knurled drive roll, a Teflon or nylon liner, and pure argon shielding.

Metal-Cored Wire: When Tubular Beats Solid

Metal-cored wire (AWS A5.18 for carbon steel, e.g. E70C-6M) is a tubular electrode filled with metallic powder and deoxidizers rather than a solid rod. Unlike flux-cored wire it produces almost no slag, so it runs like a solid wire but with a broader, more forgiving arc and higher deposition rates, often 15-20% more deposited metal per hour at the same wire feed speed. That efficiency is why production shops and robotic cells across the US have moved to it for repetitive structural work.

The trade-offs are real. Metal-cored wire runs in a spray or high-energy transfer mode that needs a mixed shielding gas (typically 80-90% argon), higher amperage, and it does not do out-of-position work as easily as short-circuit solid wire or self-shielded flux-core. It also costs more per pound. For a single-pass fillet welded in the flat and horizontal position at volume, the labor savings usually dwarf the wire cost. For a one-off repair or vertical/overhead work, solid ER70S-6 or a gas-shielded flux-core is often the better tool.

A common point of confusion: metal-cored is not the same as flux-cored (E71T-x). Flux-cored leaves a slag you must chip, tolerates wind and dirty steel better, and self-shielded versions need no gas at all, which is why they dominate field and structural erection work. Metal-cored trades that ruggedness for a cleaner, faster, gas-shielded deposit.

• Metal-cored (E70C-6M): High deposition, minimal slag, needs argon-rich gas and higher amps; great for flat/horizontal production fillets.
• Solid ER70S-6: Versatile, cheapest, works short-circuit for thin and out-of-position; the general-purpose choice.
• Flux-cored (E71T): Tolerates wind, rust, and field conditions; self-shielded needs no gas; leaves slag to chip.

Matching Wire to Shielding Gas

Wire and gas are a system; changing one changes how the other behaves. For carbon-steel ER70S-6 and ER70S-3, the two mainstream choices are 100% CO2 and a 75/25 argon-CO2 blend (often called C25). Straight CO2 is cheaper and gives deeper penetration but a harsher, spatter-prone arc and only supports short-circuit transfer. C25 runs smoother, spatters less, and gives a flatter bead, which is why it is the default for shop and hobby work. Typical flow rates run 20-25 CFH; bump it up in a drafty shop or with a large nozzle.

Stainless MIG wants a tri-mix, commonly 90% helium / 7.5% argon / 2.5% CO2, for short-circuit work because it keeps carbon pickup low and gives good wetting. For spray-transfer stainless, an argon-rich blend with about 1-2% oxygen or CO2 is common. Keep CO2 low on stainless; too much carbon migrates into the weld and hurts corrosion resistance. Aluminum runs on 100% argon for most thicknesses, with helium added on thick sections to boost heat input and penetration.

Getting the pairing wrong is a frequent shop mistake. Running aluminum on a CO2-containing gas will not work; the CO2 oxidizes the aluminum and you get filthy, porous welds. Likewise, running a spray-mode metal-cored wire on straight CO2 robs you of the smooth transfer it was designed for. When in doubt, ask your local gas distributor what blend pairs with the exact wire classification you are buying, they set this up every day.

Spool Sizes, Packaging, and Rust-Free Storage

Wire ships on standard spools: the 4 inch (roughly 1-2 lb) spools for compact 120V machines and spool guns, 8 inch (10-11 lb) spools for most home and light-shop welders, and 12 inch (30-33 lb) or larger spools for production. High-volume shops step up to 44-60 lb coils, 250-1000 lb drums, and reels. Buy the largest spool your machine accepts and your usage justifies; per-pound cost drops sharply with size. As a rough guide, .035 ER70S-6 runs about $2-4 per pound on an 11 lb spool and less on a 33 lb spool, while stainless and aluminum run several times that.

Rust is the enemy of solid steel wire, and a rusty or oxidized wire causes feeding problems, contact-tip wear, porosity, and erratic arc starts. Copper coating on carbon-steel wire helps conductivity and gives some corrosion resistance, but it is not a license to store wire in a damp shop. Once a spool has surface rust, it is scrap for quality work. Keep partially used spools sealed, not sitting open on the welder for weeks in humidity.

The practical storage routine: keep wire in a dry, climate-controlled space, ideally 40-60% relative humidity. When you pull a spool off the machine, drop it in a zip-top bag with a desiccant pack and keep the original box. For shops in humid climates or coastal areas, a small heated storage cabinet pays for itself. Aluminum wire is especially finicky; its oxide layer and softness mean you should keep it sealed until use and handle it with clean gloves, because skin oils and moisture cause porosity. Rotate stock first-in-first-out so nothing sits long enough to degrade.

• Small spools (4"): 1-2 lb, for 120V machines and spool guns; highest cost per pound.
• Mid spools (8"): 10-11 lb, the standard for home and light-shop 230V welders.
• Large spools (12"): 30-33 lb, best value for steady users; drums and reels for production.
• Storage: Dry, 40-60% RH, sealed with desiccant off the machine; scrap any wire showing rust.

Frequently Asked Questions