The weld nugget splits along the original welding plane, rather than pulling a button. The weld zone shows evidence of adequate fusion between the parent materials.
Interfacial separation may occur with heavy stack-ups or certain materials such as aluminum and high strength steels.
With stack-ups involving thick gages, the workpiece may be stronger than the weld nugget.
Therefore during peel testing, chisel testing, etc., the crack that starts at the edge of the nugget will meet less mechanical resistance by growing through the nugget, rather than through the thickness of the workpiece (see Fig. 1).
Normally during destructive testing the crack starting at the nugget’s edge travels through the thickness of the sheet, forming the button (Fig. 1, A). With thicker or stronger stack-ups, or with certain materials, the crack may encounter less resistance by passing through the nugget (Fig. 1, B).
Such welds may be acceptable if the fused area exceeds the minimum button size, and the surrounding material shows evidence that the weld had adequate strength, for example, the flanges may be distorted during testing. (Refer to the appropriate company standards). Extra care should be taken with interfacial separations in high strength steel, since these might be indicative of brittle weld failures (see Brittle Welds).
A fused area where a button would normally be after teardown indicates an interfacial separation (see photo).
Quality, Workplace Issues, Cost, Downtime, Maintenance, Throughput (cycle time; PPH), are all potentially affected by this condition. Special considerations are noted below:
Quality: Welds that separate through the interface may indicate excessive edge porosity or brittleness and should be checked against company standards.