Filler metal selection is one of the earliest decisions on a WPS — and one of the most frequently misunderstood when base metal strength is unusually high or when a structural engineer specifies something unexpected. The default assumption in AWS D1.1:2025 is that filler metal should match or overmatch the base metal's minimum tensile strength. But undermatching — intentionally using a lower-strength filler — is permitted in specific situations, and it is sometimes the right call.

Understanding when AWS D1.1 allows undermatching, and how to document it, matters for CWIs reviewing WPSs and QC managers managing the audit trail.

The matching requirement and where it comes from

AWS D1.1:2025 Table 3.1 is the filler metal matching table for prequalified WPSs. It pairs base metal groups (by ASTM grade) with approved filler metal classifications by process. The intent is that prequalified WPS combinations will produce weld metal that is at least as strong as the base metal in tension — either matching or overmatching.

The logic behind matching: for a complete joint penetration (CJP) groove weld loaded in tension perpendicular to the weld axis, the weld metal must carry the full base metal load. If the weld metal is weaker, the joint is the limiting element — and that changes the structural design assumptions. Structural engineers generally design to the base metal minimum tensile strength, assuming the weld is not the weak link.

This is why the default is matching. But the word "default" matters.

Rule library based on AWS D1.1:2025; verify against your governing edition.

When undermatching is structurally acceptable

Fillet welds in longitudinal shear

The most common legitimate application of undermatching filler is fillet welds loaded parallel to the weld axis — longitudinal shear connections. A typical example is a beam web fillet weld carrying shear from the beam to a shear tab or connection plate.

In this configuration, the weld metal is in shear, not tension. The effective throat area resisting load is based on the weld size and the fillet weld shear strength — which is calculated as a fraction of the filler metal's nominal tensile strength. If the Engineer of Record (EOR) designs the connection using the actual filler tensile strength (not the base metal strength), a lower-strength filler can be used with a proportionally larger weld size and still achieve the required connection capacity.

AWS D1.1 does not prohibit this. The EOR's design calculation is the authorization. The fabricator's job is to confirm that the WPS uses the filler the EOR specified, and that the weld size on the drawings matches what was designed.

PJP groove welds in compression

Partial joint penetration groove welds loaded purely in compression through the weld (bearing connections in column splices, for example) do not rely on weld metal tensile strength in the same way as CJP tension joints. When load transfer is primarily through compression bearing, the weld's role is stability and alignment rather than primary tension transfer.

Some structural engineers specify undermatching filler for PJP compression joints as an economy measure. Again, the EOR's calculation is the authorization. The fabricator should not assume this is acceptable without written EOR confirmation.

Dissimilar base metal joints

When two base metals of different strength grades are joined — say, A36 (Fy = 36 ksi) to A572 Grade 50 (Fy = 50 ksi) — the matching filler for the lower-strength material is technically "undermatching" relative to the higher-strength material. AWS D1.1 addresses dissimilar base metal combinations and generally allows the filler to be selected to match the lower-strength member, particularly for fillet welds or PJP welds where the higher-strength member's capacity isn't fully mobilized through the weld.

For full CJP groove welds between dissimilar base metals in tension-critical applications, the EOR may require filler that matches the higher-strength member, or may specify the lower-strength match with an accompanying weld sizing calculation. This is a design decision, not a fabrication decision.

See dissimilar base metal WPS under AWS D1.1 for the broader discussion of this joint type.

PQR implications

The moment a filler metal falls outside the prequalified groupings in Table 3.1 — including any undermatching selection — the WPS must be qualified by test (PQR). There is no "undermatching exception" to the PQR requirement.

This means:

  • A tensile test specimen from the PQR coupon will be tested at whatever strength the undermatching filler produces
  • The test report documents actual tensile strength achieved, not the base metal minimum
  • The WPS will reference the PQR, which establishes the actual qualified limits

For the PQR tensile test: pass/fail criteria are based on whether the test specimen meets the minimum requirements. If the EOR has specified undermatching and designed to it, the test result should be consistent with what was designed. If the PQR tensile specimen fails because the filler was too weak, that's a design problem to resolve before production welding starts.

See PQR tensile and bend test requirements for the test mechanics, and PQR vs. prequalified cost comparison if you're weighing the qualification path.

What the WPS document needs to show

The WPS is a procedure document, not a structural engineering justification. It needs to identify:

  • Filler metal specification — AWS A5.XX classification
  • Filler metal designation — e.g., E70XX for SMAW, ER70S-6 for GMAW
  • F-number — from D1.1 Annex C or the applicable AWS A5 standard
  • A-number — weld metal composition group (from D1.1 Annex C)
  • Minimum preheat and interpass temperature — established by the qualified procedure

The WPS should NOT contain a filler tensile strength statement that conflicts with what was actually used. If you used E70T-1C (70 ksi nominal) on an A572 Grade 50 base metal, the WPS simply documents E70T-1C. If you used E60XX (60 ksi nominal) as an undermatching filler, document E60XX.

Best practice is to include a reference note: "Undermatching filler authorized per EOR Design Memo [document number], dated [date]." This is not required by D1.1 but it makes the audit trail clear and protects the fabricator.

Common misconceptions worth correcting

"Any fillet weld can use undermatching filler." Not automatically. The structural design must support it. A 3/16 in fillet weld with E60 filler where the EOR designed for E70 is a problem, even if the geometry is the same. Confirm with the EOR before substituting.

"Undermatching means deficient welds." Undermatching is not a deficiency when it is intentional, engineered, and documented. It becomes a deficiency only when it is accidental — a filler substitution made without engineering review.

"I can use prequalified status with undermatching filler." No. Prequalified WPSs are limited to the filler-base metal combinations in Table 3.1. Undermatching selections require a PQR.

Connecting the WPS to the design intent

The WPS package should make it easy for an inspector or auditor to verify that the filler being used in production matches what the EOR specified and what the PQR tested. For undermatching situations, that chain — EOR memo → WPS → PQR → heat number on the filler spool in the shop — needs to be traceable.

Fillet weld sizing documentation, referencing the undermatching filler's design strength, belongs on the structural drawings or in the connection calculations — not on the WPS. But if an inspector asks why a lower-strength filler is being used, the WPS reference note should be enough to redirect them to the right document.

See fillet weld size requirements on a WPS for how weld sizing interacts with procedure documentation, and the WPS welding procedure library checklist for an audit-readiness overview. For shops looking to manage these documentation chains without paper files, WPS Welding's procedure management tools keep the WPS-to-PQR linkage traceable.