緒 言自動車製造ではResistance spot welding of three steel sheets is used in automotive manufacturing. The determination of welding conditions for three steel sheets welding is more difficult than that for two steel sheets welding because in the former case, two interfaces are simultaneously welded. The welding condition to improve the weld strength of thin and thick sheets' interface is required for thin, thick, and thick sheets welding. In addition, avoiding weld spatter is an important technical concern for the three sheets welding.This paper discusses the triply coupled effects of elasto-plastic large deformation contact, electric current, and thermal conduction for the three sheets welding via triply coupled finite element analyses. The triply coupled effects of thin and thick sheets' interface and those of thick and thick sheets' interface are compared to discuss the special characteristics of the triply coupled effects of three sheets welding.The nugget diameter of the thick and thick sheets' interface is larger than that of the thin and thick sheets' interface because the electrical contact resistance and the base electrical resistance of high-strength steels are higher than those of general-strength steels. Furthermore, the chain of the coupled effects of electrical resistances, Joule heat generation, temperatures, and electrical resistances is strong. As the effect of welding current on temperature by the coupling effect for the thick and thick sheets' interface is stronger than that for the thin and thick sheets' interface, the possibility of spatter increases. Increasing the nugget diameter solely for the thin and thick sheets' interface using electrode force is difficult because the effect of the electrode force on the nugget diameter is approximately the same for both interfaces. On the contrary, material characteristics can affect the nugget diameter solely for either of the interfaces. The possibility of spatter increases for the welding condition with a sheet gap because the temperature of the contact edge for the sheets' interface increases.
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