The Study Of Nugget Formation Mechanism And Properties In Resistance Spot Welding Of Galvanized Steel Sheets Based On Time Constriaints

In order to improve the life of the automobile body and increase thecorrosion resistance of the material of the vehicle body, steel sheets withprotected layer coated are often to be used, among which galvanized steelsheets are currently the most used. Some problems are caused forgalvanized steel sheets due to its low melting temperature, low hardnessand low electrical resistivity. This paper is aimed to study the resistancespot welding of galvanized steel sheets based on a time restriction, whichmeans as much as possible to shorten the welding time under thecondition of ensuring the strength not reduced, thus the efficiency of thebody assembly can be improved and automotive production costs saved.Meanwhile, many structural parts includes three-sheet or multiple-sheetjoints, and some structures need to add a strengthen board, so there willbe dozens of joints which are three even multiple sheets welded among the4000to6000joints of the automotive body. To research the nuggetformation mechanism and optimization of hard parameters for three-sheetresistance welding spots become particularly essential.The three sheets include a galvanized sheet (0.65mm thick),2uncoated sheets (0.65mm thick and0.70mm thick). Resistance spotwelding of two-sheet low carbon steel and three-sheet with a galvanizedsheet were studied using a method combining the numerical simulationwith process experimental method. The nugget formation processes weresimulated, and the dynamic resistance changing rules were analyzed. Thetensile-shear strength and metallurgical structure of the welding nuggetfusion were investigated. Finally, the nugget formation mechanism wasanalyzed. The optimization of hard parameters for the galvanizedcombination a+a and three-sheet combination a+b+c were done, and theoptimistic welding parameters were obtained. In addition, thetensile-shear strengths between different pieces of three-sheet weldingspots were investigated and explained.Through the study of two galvanized steel sheets’ spot weldingbased on a time restriction, research results show that the galvanized layergreatly reduces the weldability of the material, and it always needs longertime to produce a nugget at the interface containing zinc layer. Accordingto the orthogonal test results, the best-quality joints for two galvanizedsheets can be obtained when welding time is7cycle, electrode force is2.0kN and the welding current is9.5kA.After the research of three-sheet resistance spot welding including agalvanized sheet, the results show that the nugget firstly forms at theinterface of uncoated sheets, which experiences the horizontal and vertical expansion, and then to be a complete nuclear fusion. The shape ofnugget seems like a ellipse. The microstructure of the nugget zoneconsists of a large amount of bainite and a bit of pro-eutectoid ferrite andmartensite. The tensile-shear strength between upper sheet (galvanizedsheet) and lower sheet (uncoated sheet) is higher than that betweenmiddle sheet (uncoated sheet) and lower sheet, which is resulted from thedifferences of the nugget size at two interfaces. The tensile-shear strengthbetween middle sheet and lower sheet can be increased by25%withoutsplash when welding time is4or5cycle, and electrode force is3.0kN or3.2kN, and welding current is9.0～10kA.