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 s heet steel. Spot welds with expulsion are characteristic for a ''doubl e image'' near the fusion line, an equiaxed dendritic grain zone along the faying surface, and deep indentations on the outer surfaces. Micr ostructural examinations and tensile shear tests of spot welding speci mens were conducted to determine the correlation between microstructur e and strength. It was found that the equiaxed dendritic structure was associated with significant solidification cracking. Fractography on tensile specimens reveals that brittle failure is associated with an e quiaxed microstructure. Furthermore, the surface indentation will chan ge the stress at the nugget edge, and deep surface indentations are ex pected to promote premature failure.