Research On Weld Nugget Formation During Resistance Spot Welding Of Non-thick Al Alloys

Resistance spot welding (RSW) is one of the important parts in the weldingtechnologies, and it is a material joining method which is high efficient and easy to beautomotive. RSW has been widely used in the fields of automobile, aerospace and soon. The shift of nugget in the non-thick sheets of resistance spot welding is animportant issue. The shift of the nugget will lead to the smaller size of the bondingsurface and the less weld penetration rate of the thin pieces, which will greatly affectthe welding quality. The nugget formation mechanism of non-thick sheets as well asthe process parameters influence had been studied in this paper. A suitable spotwelding process for non-thick sheets was developed to ensure the welding quality andstrength.Firstly, a three-dimensional heat conduction model for resistance spot welding oftwo or three sheets was established, simulating the temperature field in the nuggetcross-section by conjugate gradient method. The molecular dynamics simulation wasused to calculate the interface of aluminum alloy and contact resistance. The resultsshowed that the nugget shift was mainly determined by the discrepancy of heating andcooling process of the weldment. For the non-thick two sheets, before the nugget wasformed, the cooling process was the dominant factor of high-temperature shift. Withthe time increasing, the high-temperature shifted from the interface to the thicker partbecause of the low cooling rate. When the nugget began to form, the heating processbecame the dominant factor for nugget shift, with the time increasing, the nuggetshifted to the thicker part because the larger resistance of the thicker sheet and lowcooling rate. For the equal-thickness three sheets combinations and the up-downsymmetrical three sheets combinations, there is no nugget shift happening, and thetemperature of two interfaces increased simultaneously and formed two nuggetsseparately, the nugget finally formed at the symmetry center of the thickness directionof the three sheets. This nugget finally became a large one, including the upper andlower interfaces and the intermediate sheet. For the non-thick three sheetscombinations, the nugget began to form at the symmetry center of the thicknessdirection of the three sheets. And the nugget shift to the thicker part. A digitalhigh-speed camera was used to visually monitor and observe the process of nugget formation. The main process contained information on the nugget formation whichwas shown by comparing the captured faying interface of two-sheet and three-sheetcombinations and verified the results of the simulation.The resistance spot welding process of0.5mm/1.0mm,0.5mm/1.0mm/1.5mmand0.5mm/1.5mm/1.0mm thickness combinations were investigated to study thenugget shift discipline under different welding parameters and conditions, the shiftdiscipline was further verified by the mechanical properties. A suitable spot weldingprocess for non-thick sheets was developed to ensure the welding quality and strength.For the0.5mm/1.0mm, the suitable current value was16kA to19kA, the welding timewas160ms, and the electrode was kept to0.1MPa. For0.5mm/1.0mm/1.5mm, thesuitable current value was15kA to19kA, and the welding time was120ms to220ms.And for0.5mm/1.5mm/1.0mm, the suitable current value was15kA to17kA, and thewelding time was120ms to220ms. And the electrode was kept to0.1MPa.