| Spot welds were produced using a high strength cold rolled sheet steel 0.08 in. (1.93 mm) thick. The welding parameters were systematically varied to examine their effects on nugget formation, microstructure, and mechanical properties. Following welding, selected spot welds were cross sectioned and mechanically polished for metallographic examination. Other spot welds were submitted to mechanical tests.;It is observed that welding current and weld time are more significant parameters than applied electrode force in affecting expulsion. Spot welds with expulsion are characteristic for a "double image" near the fusion line, an equiaxed-dendritic grain zones along the faying surface, and deep indentations on the outer surfaces. Microstructural examinations and mechanical 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. Fractography on tensile shear specimens show that brittle failure is associated with an equiaxed microstructure. Furthermore, the surface indentation will change the stress at the nugget edge, and deep surface indentations are expected to promote premature failure.;A criterion for expulsion in resistance spot welding is proposed incorporating welding parameters, material properties, welding set geometry, and welding machine condition to the occurrence expulsion.;For this sheet steel, an electrode force of 1600 Lbs., a weld time of 18 cycles, and a current of 12 KA turns out to be a critical setting for the production of a maximum nugget size, and occurrence of expulsion. Also, under this welding condition, spot welds show optimum strength and fatigue. |
