Dual-shield welding is gas-shielded flux-cored arc welding (FCAW-G), a process that combines a flux-cored wire electrode with external shielding gas. The flux inside the wire provides slag coverage, deoxidizers, and additional shielding. The external gas (75/25 argon/CO2 or 100% CO2) provides the primary atmospheric protection. This dual shielding system produces weld quality and mechanical properties that match or exceed solid MIG wire at 2-3 times the deposition rate.
Dual-shield is the dominant welding process in structural steel fabrication shops across North America. It fills the gap between solid MIG wire (limited deposition rate on thick steel) and submerged arc welding (flat position only). With E71T-1 wire in 0.045" or 1/16" diameter, dual-shield deposits 8-20 lbs/hour in all positions on heavy plate.
How Dual-Shield Works
The process uses the same equipment as MIG welding: a constant-voltage (CV) power source, a wire feeder, a MIG gun, and a shielding gas supply. The difference is the wire.
Solid MIG wire (ER70S-6): Homogeneous metal wire. All shielding comes from external gas. No slag produced.
Dual-shield wire (E71T-1): Tubular wire with flux-filled core. Flux provides slag, deoxidizers, alloying elements, and supplemental shielding. External gas provides primary atmospheric protection.
When the arc melts the dual-shield wire, the flux compounds react. Some form a protective slag layer over the weld bead. Others decompose into gases that supplement the external shielding at the arc zone. Deoxidizers (silicon, manganese, titanium) scavenge oxygen and nitrogen from the weld pool, improving mechanical properties.
The slag coverage is a major advantage on multi-pass welds. It protects each pass from oxidation as it cools, controls the bead profile in out-of-position welding, and slows the cooling rate slightly for better metallurgy.
Gas Selection: CO2 vs 75/25
The choice of shielding gas significantly affects dual-shield weld characteristics.
100% CO2
CO2 is the cheapest shielding gas option. It produces deeper penetration and a more aggressive arc, making it popular for heavy fabrication where maximum penetration matters.
Advantages of CO2:
- Deeper penetration profile (finger-type penetration)
- Lower gas cost ($15-25 per 300 CF cylinder refill)
- Better for thick root passes where deep fusion is critical
- Higher heat input for the same wire feed speed
Disadvantages of CO2:
- More spatter (roughly 50% more than 75/25)
- Rougher arc with more turbulence
- Wider bead profile that may require more cleanup
- Higher fume generation
E71T-1 wire classified specifically for CO2 carries the suffix “C” (E71T-1C).
75% Argon / 25% CO2
The 75/25 blend is the most popular gas choice for structural dual-shield welding in shops. The argon content smooths the arc and reduces spatter.
Advantages of 75/25:
- Smoother, more stable arc
- 40-50% less spatter than CO2
- Better bead appearance and profile
- Lower fume generation
- Less cleanup time between passes
Disadvantages of 75/25:
- Higher gas cost ($25-45 per 300 CF cylinder refill)
- Slightly less penetration than CO2
- Slightly lower heat input at same parameters
E71T-1 wire classified specifically for 75/25 carries the suffix “M” (E71T-1M).
Which Gas to Choose
| Application | Recommended Gas | Reason |
|---|---|---|
| General structural fabrication | 75/25 | Cleaner operation, less cleanup, acceptable penetration |
| Heavy plate root passes | CO2 | Maximum penetration into root |
| Out-of-position work | 75/25 | Less spatter, better puddle control |
| Budget-conscious shop | CO2 | Lower gas cost, acceptable for production work |
| High appearance requirements | 75/25 | Smoother bead, less spatter to clean |
| Low-temp toughness required | 75/25 | Lower heat input preserves toughness |
Gas flow rate for dual-shield runs 35-45 CFH. Don’t exceed 50 CFH; excessive flow creates turbulence that pulls in air contamination.
Dual-Shield vs Solid MIG Wire
The main competition for dual-shield in shop fabrication is solid MIG wire (ER70S-6 with 75/25 gas). Here’s how they compare on heavy steel:
| Factor | Solid MIG (ER70S-6) | Dual-Shield (E71T-1) |
|---|---|---|
| Deposition Rate (0.045") | 4-8 lbs/hr | 8-14 lbs/hr |
| Deposition Rate (1/16") | 6-11 lbs/hr | 12-20 lbs/hr |
| Slag Produced | None | Yes, must be cleaned between passes |
| Spatter Level | Low (spray transfer) | Moderate |
| Penetration (CO2 gas) | Moderate | Deep finger-type |
| Out-of-Position Capability | Limited above 200A | Excellent in all positions |
| Contamination Tolerance | Sensitive to rust/scale | Better (flux deoxidizers) |
| Wire Cost per lb | $2.00-3.00 | $2.50-3.50 |
| Mechanical Properties | Good | Equal or better |
The key advantage of dual-shield is the deposition rate difference. On a multi-pass groove weld filling a 1" thick joint, solid MIG wire might need 25 passes. Dual-shield with 1/16" wire might need 15 passes, each deposited faster. The time savings compound on every joint.
Solid MIG wins on thin material (under 1/4") where the lower deposition rate is actually an advantage, providing more control. It also wins when appearance matters and slag cleanup between passes adds unacceptable time.
Structural Welding Applications
Dual-shield FCAW is pre-qualified under AWS D1.1 Structural Welding Code for Steel. This means welding procedures using E71T-1 or E71T-1M on pre-qualified joint designs don’t require individual procedure qualification testing, saving time and money on code work.
Common structural applications:
Moment connections: Complete joint penetration (CJP) groove welds connecting beams to columns. These are the critical connections in steel-frame buildings. Dual-shield handles the multi-pass groove welds efficiently.
Column splices: CJP groove welds joining column sections end-to-end. Often welded vertical-up in the field or flat/horizontal in the shop.
Bracing connections: Gusset plates welded to beams and columns with fillet and groove welds. High weld volumes make dual-shield’s deposition rate advantage significant.
Plate girders: Long, heavy fillet welds connecting flanges to web. Can be positioned flat for maximum dual-shield productivity.
For thick-plate technique details, see flux-core welding thick steel.
Equipment Requirements
Power Source
Dual-shield requires a constant-voltage (CV) power source with enough amperage for the wire diameter:
- 0.035" wire: 130-260A (200A machine minimum)
- 0.045" wire: 170-320A (250A machine minimum)
- 1/16" wire: 220-400A (350A machine minimum)
- 5/64" wire: 300-500A (450A machine minimum)
Industrial dual-shield welding machines include Lincoln Power Wave, Miller XMT/Invision, and ESAB Warrior. These multi-process machines output CC and CV power for both stick/TIG and MIG/flux-core.
Wire Feeder
Standard MIG wire feeders handle dual-shield wire with appropriate drive rolls. Use V-knurl drive rolls for 0.035" and 0.045" flux-core wire. For 1/16" and larger, follow the wire manufacturer’s drive roll recommendation.
Set drive roll tension just enough for reliable feeding without crushing the tubular wire. Over-tightening compresses the flux core and causes feeding problems. If the wire flattens visibly at the drive rolls, reduce tension.
Gun Selection
Standard MIG guns work for dual-shield up to their rated amperage and duty cycle. For heavy production:
- 0.045" wire, 300A: Standard 300-400A MIG gun with 15-foot cable
- 1/16" wire, 350A+: Heavy-duty 400-500A gun, often water-cooled for production work
- 5/64" wire, 450A+: Water-cooled gun mandatory at these amperages
Air-cooled guns work for intermittent welding up to about 350A. Above that, or for continuous high-duty-cycle work, water-cooled guns run significantly cooler and last longer.
Settings Overview
For detailed settings charts by wire diameter and material thickness, see flux-core welding settings. General ranges for dual-shield:
| Wire Diameter | Voltage Range | WFS Range (IPM) | Amperage Range | Stick-Out |
|---|---|---|---|---|
| 0.035" | 21-27 | 250-450 | 130-260 | 1/2"-3/4" |
| 0.045" | 23-30 | 180-340 | 170-320 | 5/8"-1" |
| 1/16" | 25-32 | 140-280 | 220-400 | 3/4"-1-1/4" |
| 5/64" | 27-34 | 100-220 | 300-500 | 1"-1-1/2" |
Advantages Over Self-Shielded FCAW
Dual-shield outperforms self-shielded flux-core in nearly every metric except wind tolerance:
- 40-50% less spatter with 75/25 gas
- Higher deposition rates at equivalent wire diameters
- Better mechanical properties, especially low-temperature impact toughness
- Less smoke and fume (roughly half of self-shielded)
- Thinner, easier-to-remove slag
- Better bead appearance
- Deeper penetration with CO2 gas
The only scenario where self-shielded beats dual-shield is outdoor welding in wind. Self-shielded wire doesn’t need gas, so wind can’t disrupt its shielding. Dual-shield’s external gas is just as vulnerable to wind as solid MIG wire. For a complete comparison, see self-shielded vs gas-shielded FCAW.
Common Dual-Shield Problems and Fixes
Porosity in the weld: Usually a gas coverage issue. Check gas flow rate (35-45 CFH), inspect for leaks in hoses and fittings, clean spatter from inside the nozzle, and verify the gas diffuser isn’t damaged. Also check for drafts in the shop that might disrupt shielding.
Excessive spatter: Voltage is the first thing to adjust. Also verify DCEP polarity, check contact tip condition, and confirm gas flow. See flux-core spatter reduction for a complete troubleshooting approach.
Worm tracking (herringbone marks on the bead surface): Gas trapped in the solidifying slag. Caused by excessive moisture in the wire or base metal, or painting/coating over hot slag. Use dry wire, clean the base metal, and let slag cool before coating.
Slag inclusions between passes: Incomplete slag removal between passes. Remove all slag completely. Grind out trapped slag visible at the surface. Wire brush to reveal any remaining pockets.
Cracking in heavy section welds: Hydrogen-induced cracking from rapid cooling. Ensure proper preheat per AWS D1.1 or the applicable code. Maintain interpass temperature. Consider post-weld heat treatment for very heavy, high-restraint joints.
Wire feeds erratically: Check drive roll tension (not too tight), liner condition (blown out with compressed air recently?), contact tip condition (worn bore?), and wire spool for kinks or tangles. Flux-core wire is softer than solid wire and more susceptible to feeding problems.