Issue: Brittle Welds

A resistance spot weld that fractures with little or no plastic deformation in either the weld or the surrounding metal.

Description

The measurement of spot weld brittleness depends upon the test method. The size and shape of the welded structure under test, the rate of load application, the weld’s microstructure and surface conditions, and the ambient temperature all influence the amount and location of plastic deformation prior to fracture.

Each must be considered when evaluating welds. Brittle welds often fracture interfacially.

Detection

Brittle welds are frequently identified after testing by a visible granular fracture surface, with little or no stretching or yielding.

Significance 

Quality, Workplace Issues, Cost, Downtime, Maintenance, Throughput (cycle time; PPH), are all potentially affected by this condition.

Possible Causes

Strong Possibilities

• Hold time long
• Incorrect test procedure

Weak Possibilities

• Hold time short
• Incorrect electrode dressing
• Incorrect workpiece selected

Other Possibilities

• Prior working/heat treatment
• Rapid cooling caused by any combination of weld current, weld time, and electrode (or workpiece) cooling that permits rapid weld cooling
• Welding schedule not suitable to workpiece chemistry
• Workpiece chemistry not suitable for resistance spot welding