AWS A5.18 and A5.28 GTAW filler wire classification decoded for structural steel WPS — ER70S-6 vs ER80S-D2, essential variable implications under AWS D1.1 explained.
Why GTAW Filler Wire Choice Matters on a Structural WPS
Gas tungsten arc welding (GTAW) sees regular use in structural steel fabrication: open root passes on CJP groove welds, precision work on connections requiring full fusion without spatter risk, and hybrid WPS that use GTAW for the root before filling with SMAW or FCAW-G. In each case, the filler wire specification must be correctly identified in the WPS, and substitutions made on the shop floor without WPS authorization are a common CWI audit finding.
Unlike SMAW — where the coating provides shielding and flux — or FCAW-G where the flux is inside the wire, GTAW uses a separate solid filler rod fed into the weld pool by hand behind the tungsten arc. The AWS classification system for GTAW filler wire is logical and consistent. Understanding it helps CWIs verify that what is written on the WPS matches what is actually in the welder's hand.
Decoding the AWS A5.18 Designation System
AWS A5.18, Specification for Carbon Steel Electrodes and Rods for Gas Shielded Arc Welding, covers both solid wire GMAW electrodes and GTAW filler rods. The designation format is:
ER 70 S - X
- E = electrode (can be used as a wire electrode in GMAW)
- R = rod (used as a separate filler rod in GTAW)
- 70 = minimum tensile strength of the deposited weld metal in ksi (70 ksi = 483 MPa)
- S = solid wire (not flux-cored)
- X = chemical composition suffix
The composition suffix drives the practical differences between classifications. Common A5.18 GTAW filler metals for structural steel:
| Classification |
Composition Feature |
Common Structural Use |
| ER70S-2 |
Triple-deoxidized (Ti, Zr, Al added beyond Si + Mn) |
Pipe root passes; code-quality welds requiring high fusion certainty |
| ER70S-3 |
Moderate Si + Mn deoxidation |
General structural applications on clean base metal |
| ER70S-6 |
Higher Si and Mn content than S-3 |
Structural steel with mill scale; more robust deoxidation pool |
All three classifications meet the 70 ksi minimum tensile strength requirement and are prequalified filler metals for structural carbon steel applications under AWS D1.1:2025 Clause 5. Rule library based on AWS D1.1:2025; verify against your governing edition.
AWS A5.28 for Low-Alloy and Higher-Strength Structural Steel
AWS A5.28, Specification for Low-Alloy Steel Electrodes and Rods for Gas Shielded Arc Welding, covers filler metals for GTAW and GMAW on higher-strength base metals. Common structural grades:
| Classification |
Min. Tensile Strength |
Typical Structural Application |
| ER80S-D2 |
80 ksi |
A588 weathering steel; HSLA base metals up to 80 ksi class |
| ER90S-D2 |
90 ksi |
Higher-strength HSLA structural members |
| ER100S-G |
100 ksi |
High-strength structural applications; tested WPS only |
For most structural steel fabrication under AWS D1.1 — A36, A572 Grade 50, A992, A588 — an A5.18 ER70S-class wire overmatches the base metal minimum tensile strength requirement and is the standard first choice. An A5.28 wire becomes necessary when base metal minimum specified tensile strength exceeds the 70 ksi deposited strength, or when the engineer of record explicitly specifies matching-filler or minimum-undermatching criteria.
A514 and A517 quenched-and-tempered steels require a tested WPS (not prequalified) and typically use A5.28 high-strength classifications. Those are a different qualification path than the prequalified Clause 5 WPS applicable to standard structural carbon and HSLA steels.
Prequalified GTAW WPS Under AWS D1.1:2025 Clause 5
AWS D1.1:2025 Clause 5 permits GTAW as a prequalified process for groove welds on prequalified base metals from Table 5.6. A prequalified GTAW WPS must list:
- AWS A5 specification: e.g., AWS A5.18
- Filler metal classification: e.g., ER70S-6
- Filler wire diameter or diameter range
- Shielding gas type and flow rate: GTAW uses argon or argon/helium mixtures; pure argon is the standard for carbon steel structural applications
- Tungsten electrode type and diameter: listed on the WPS as they affect arc characteristics
- Joint geometry: per Clause 5 Figures 5.1 (CJP) or 5.2 (PJP) prequalified joint designations
Any deviation from the prequalified limits — base metal not in Table 5.6, filler metal not in the approved A5.18/A5.28 list, joint geometry outside Clause 5 ranges — pushes the WPS to the tested path under Clause 6 with a supporting PQR.
Essential Variable Implications on the Tested WPS Path
When using a Clause 6 tested WPS, filler metal selection has direct essential variable consequences under AWS D1.1:2025 Table 6.6. Two categories govern:
AWS A5 specification and classification: A change from one A5 specification to another (e.g., A5.18 to A5.28) or from one classification to another within a spec (e.g., ER70S-6 to ER80S-D2) is an essential variable. The PQR records the specific classification tested. Welding production with a different classification than what the PQR supports requires either a new PQR run or demonstrating the new classification is within the qualified range — which typically requires listing both in the WPS or requalifying.
In practical terms: switching from ER70S-3 to ER70S-6 on a tested WPS where only ER70S-3 is listed in the PQR requires updating the WPS and confirming the PQR range supports the change. Both are A5.18 carbon steel classifications depositing similar chemistry, but the filler metal change is still an essential variable that must be addressed on paper.
Filler wire diameter: A change in filler rod diameter outside the range qualified by the PQR is an essential variable. If the PQR was run with 3/32" diameter rod, switching to 1/8" requires either a new PQR test or verifying the original PQR qualified a diameter range that includes 1/8".
Shielding gas composition: A change in shielding gas from one type to another — for example, from pure argon to argon/helium — is an essential variable for GTAW under AWS D1.1:2025 Table 6.6. For a detailed walkthrough of GTAW essential variables, see GTAW TIG WPS essential variables under AWS D1.1.
ER70S-2 vs. ER70S-6: The Practical CWI Choice
Both ER70S-2 and ER70S-6 are prequalified and widely used for structural GTAW on A36, A572, and A992 base metals. The distinction lies in pool behavior and surface tolerance.
ER70S-2 is the more heavily deoxidized option. The additional titanium, zirconium, and aluminum deoxidizers beyond the standard Si + Mn package give it tolerance for base metal surface contamination — making it the preferred choice when weld soundness cannot be compromised and the base metal surface condition is uncertain. It is commonly specified for pipe root passes on code-quality structural connections.
ER70S-6 produces a slightly more fluid weld pool with better wetting characteristics on mill-scaled structural steel. The higher silicon content aids deoxidation and weld bead shape on surfaces that are clean but not laboratory-grade. For shop applications with prepared joint surfaces, ER70S-6 is the dominant choice because of its broader availability, lower cost, and consistent performance on structural carbon steel.
For the structural CWI, the key check is simple: what does the WPS say, and does the lot tag on the filler wire in the welder's kit match that classification? The filler metal certificate of conformance from the manufacturer confirms the lot classification and should be retained as part of the filler metal traceability record.
Hybrid WPS: GTAW Root with SMAW or FCAW-G Fill
A frequent structural WPS pattern for open-root CJP groove welds is GTAW for the root pass followed by SMAW E7018 or FCAW-G fill and cap passes. This multi-process WPS requires AWS D1.1:2025 to document all processes used, with essential variables listed for each process separately.
The GTAW root pass filler wire is chosen for root fusion quality; the fill process filler metal is chosen for deposition rate and economy. ER70S-2 or ER70S-6 root passes are routinely capped with E7018 or E71T-1C without metallurgical concerns for standard A572 Grade 50 or A992 structural applications.
For qualification guidance on these combinations, see multi-process WPS essential variable scope under AWS D1.1.
Documentation Checklist for GTAW WPS Packages
Before submitting a GTAW WPS package for review on an AWS D1.1 project:
For a complete WPS submittal documentation framework, including how to organize the WPS, PQR, and mill certs for submittal to the engineer of record, see WPS submittal package review checklist.
Managing GTAW filler wire lot traceability, WPS version control for multi-process programs, and welder qualification tracking across process types is streamlined in a platform built for AWS D1.1 compliance. See WPS software for structural fab shops.