Spot Welding Process Of Galvanized Steel&Failure Of Spot Welding Research

With the continuous increasing quality requirements for automobile, the corrosionresistance of vehicle has been demanded to increase from3~5years to8~10years. So,the materials of vehicle body frame must be changed, and galvanized steel sheet is theoptimum candidate. At present, the car body of the joint venture brand are mainly madeof galvanized steel sheet, and the car manufacturers of the domestic brands also beginto use galvanized steel sheet in view of cost and quality controlling. The first two partsusing galvanized steel sheet are the outer-body panel and interior enforcement panel, thecomponents include outer side plate, A column reinforcing plate, B column reinforcingplate and so on. The selected galvanized steel sheet is mainly hot dip galvanized steel,also some enterprises produce the outer panels with electric galvanized steel sheet inview of the better electrophoresis paint effect. The thickness of the zinc layer on thegalvanized steel sheet is more than4times of that on the electric galvanized steel sheet,so the welding parameters of this two kinds of sheet shold be obviously different fromeach other. So, to study this helps to guarantee the vehicle uality..In this thesis, hot galvanized steel was selected as the research object. To treat thewelding quality problem occurred during welding processing, welding current, weldingpressure effect were investigated. The experimental results show that, the weldingprocess of the galvanized steel sheet is nearly same to that of the common low carbonsteel sheet, and the welding quality are needed to meet the vehicle safety requirementsof performance. The welding processing involvs pressing the sheets by electrode,pre-heating and heating by electrical current and cooling. As for the galvanized steelsheet, the zinc layer must be paid attention to and the pre-heating electrical currentshould be carefully slected so as to reduce the adverse effects of the zinc layer on thewelding qualit. When the welding current and the welding time is kept constant, the sizeof the nugget decreases with the increasing electrode pressure. When the electrode pressure and welding time remains unchanged, the maximum tensile strength of thewelding joint increases with the welding current, but the size of the nugget alsoincreases. Welding splash increase with the welding current which lead to the lower-strength joint. When welding the galvanized steel sheet, the current through theelectrodes in contact with the metal effectively heat the sheet and melt it, and the heattransfer to the arround of the nuclear fusion. When the welding current is turned off, theheat generated by the welding area is insufficient to compensate for the loss of heat,welding spots begin to solidify at once. The highest temperature at different spots areobviously different from each other, which results in different microstructures of thejoint. At the same time, by energy spectrum analysis of nuclear fusion experiment, itwas found that from the center to the parent material, the content of Zn increasedgradually and eventually reaching and value of the parent material. Finally, theexperimental results are futher proved by analyzing the spot welding defect casesoccurred during welding.