Investigation of induced magnetic force on liquid nugget during resistance spot welding

This paper investigates the source, magnitude and direction of magnetic force on the liquid nugget during resistance spot welding (RSW). High speed photography was used to observe the nugget formation process during half-sectioned RSW of steel (ferromagnetic substance) and aluminium alloy (paramagnetic substance). The induced magnetic force acting on half-sectioned and regular nugget was afterwards analysed and calculated. The results show that in the case of steel RSW, the magnetised workpieces generated a very strong magnetic field around the spot welding zone, while weak magnetic field appeared in the case of aluminium alloy RSW. This strong magnetic field causes a strong convection in the liquid nugget of steel even when the welding current is low. This strong convection will promote that the dimensions of nugget in the steel RSW become larger and closer to rectangle (observed on the cross-section) than in the aluminium alloy RSW.