Abstract
In resistance spot welding of thin sheet-thick sheet-thick sheet joint, when the sheet thickness ratio is large (sheet thickness ratio = total thickness of sheet joint/thickness of the thin sheet positioned on the out side of the joint), how to stably secure the nugget between the thin sheet and the adjoining thick sheet is a key issue. If the sheet thickness ratio is so large, nugget formation between the thin sheet and thick sheet is extremely difficult. In order to control of the nugget (position of formation, shape, etc. of the nugget) during welding for three sheets joint with a high sheet thickness ratio, optimum welding process was investigated. The developed "2-step force, 2-step current" welding process was suitable for high sheet thickness ratio joint and relaxed the constraints on the sheet thickness ratio. In Step 1 (first part of welding period) of the welding process, a nugget is reliably formed between the thin sheet and thick sheet by applying conditions of low electrode force, short welding time, and high current. In the subsequent Step 2 (second part of welding period), a nugget is formed between the two thick sheets by apply high welding force and a long welding time. In the weld results of a three sheet joint (0.7mm+2.3mm+2.3mm; sheet thickness ratio: 7.6) using mild steel GA (0.7mm) as the thin sheet and 780MPa high strength GA (2.3mm) in the two thick sheets, "2-step force, 2-step current" spot welding process showed the wide available welding current range.
