2009 Volume 49 Issue 9 Pages 1384-1391
Varying nugget sizes are obtained in different coated and bare automotive steel grades applying identical spot welding parameters. The strength of sheet surface asperities and surface topography together seem to influence contact conditions under applied pressure and welding temperature. The low fractal roughness and easily deformable asperities of soft steels decreases both static and dynamic contact resistance. Contrarily, high fractal roughness and stronger asperities of higher strength steels resist successive yielding of the asperities. This, in turn, produces more heat for melting and therefore larger nuggets. The heat efficiency of welding increases with yield strength as favorable contact resistance conditions are set in. Significantly, the iron–zinc intermetallic phases in the steel coating consume part of the latent heat of fusion and decrease the energy efficiency. Continuous modification of the surface with progressive melting of the intermetallic layers also contributes to this effect. For longer weld times in coated steels, the drop in contact resistance at the early stages of welding is compensated later, which improves overall heat efficiency.