Effects of expulsion in spot welding of cold rolled sheet steels

The effects of expulsion on microstructure and tensile shear strength of spot welds have been investigated for a high-strength cold rolled sheet steel. Spot welds with expulsion are characteristic for a “double im-age” near the fusion line, an equiaxed dendritic grain zone along the faying surface, and deep indenta-tions on the outer surfaces. Microstructural examinations and tensile shear tests of spot welding specimens were conducted to determine the correlation between microstructure and strength. It was found that the equiaxed dendritic structure was associated with significant solidification cracking. Frac-tography on tensile specimens reveals that brittle failure is associated with an equiaxed microstructure. Furthermore, the surface indentation will change the stress at the nugget edge, and deep surface inden-tations are expected to promote premature failure.