The Influence of Projection Welding Parameters of M10 Nuts to Galvanized Steel Sheet
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THE INFLUENCE OF WELDING PARAMETERS FOR PROJECTION WELDING OF M10 NUTS TO GALVANIZED STEEL SHEETS
Projection welding belongs to the group of resistance welding technologies.
The basic parameters of the welding process are welding current and current flow time. The projection welding of fasteners to metal sheets is not as well understood as resistance spot welding of sheets, therefore, complex studies are still needed for the broader application of nut welding in the automotive industry. This research aims to evaluate the effect of resistance projection welding parameters (steel nuts on galvanized steel sheets) on joint properties. The hard welding mode (high welding current, clamping force, and short welding time) provided 2 times more strength of the weld joints than the soft welding mode. When using the soft welding mode, a higher concentration of Zn from the sheet's metallic coating was measured in the transition area between the welded materials at the folding sites. When applying the hard welding mode, only a slight increase in Zn concentration was observed at the inner limit of the weld joint.
In addition to the basic process parameters, which are welding current and current flow time, which will affect the amount of heat per ratio, another important process parameter is the clamping force F (N).
Suitable resistance welding parameters can usually be obtained from a diagram that consists of the dependence of welding time on electric current for a material and joint condition combination. The operating window includes conditions for hard welding mode (high current and clamping force in short welding time) and soft welding mode (low current and clamping force in long welding time). The basic starting points for determining suitable resistance welding conditions are primarily information on the chemical composition and thickness of the adhered materials.
The influence of the type of projections, the amount of welding current, and the clamping force in the projection welding of steel nuts with dual-phase (DP) steel sheets were described by authors Wang and Zhang. In their work, they documented the significant influence of the type of projections on the heat distribution during heating; on the other hand, only minor importance of the clamping force on the weld joint dimension. Other authors also studied the impact of projection configuration and dimensions, respectively the application system of the clamping force on the resulting joint properties. For this, several simulation models were designed and tested to gain new insights into the heating and deformation process in welded joints.
In addition to the basic data needed to select resistance welding parameters, there are many other factors that will significantly influence the selection of appropriate process variables. As the results of published works show, the surface quality of the adhered materials will also play an essential role in resistance heating. For example, Wang in his work pointed out that the type of surface treatment of galvanized steel sheets will have a great impact on projection welding by resistance.
Although resistance projection welding of nuts on galvanized steel sheets is widely used in industry, only a few research articles have been published. The weldability of fasteners to metal sheets is not as well understood as resistance spot welding of plates, so complex studies are still needed for the broader application of resistance nut welding in the automotive industry.
From the results of projection welding of M10 steel nuts to galvanized steel sheets, the following conclusions can be drawn:
- The hard welding mode (high welding current, welding force, and short welding time) provided 2 times more strength of the weld joints than the soft welding mode.
- When using the soft welding mode, the formation of strips inside the joint was observed. They were formed during the heating and deformation of the projection. In the hard welding mode, discontinuities were observed due to metal spatter from the projection during rapid heating. The occurrence of extruded metal was observed on the outside of the joint. In the soft welding mode, a lack of fusion was observed on this side, which had a negative impact on weld strength.
- Even though in no case were the conditions for the formation of fusion welded joints achieved, in the case of the hard welding mode, during the tensile test, in most cases the joint was interrupted by the rupture of the heat-affected zone (evaluated according to STN EN ISO 14 329). When using the soft welding mode, the fracture surface had a mixed character - it formed locations of brittle cleavage fracture and ductile shear fracture.
When using the soft welding mode, a higher concentration of Zn from the sheet's metallic coating was measured in the transition area between the welded materials at the strip locations. The cause of these locations was the insufficient heating of the metallic surface of the galvanized sheet, below the evaporation temperature of Zn. When applying the hard welding mode, only a slight increase in Zn concentration was observed at the inner limit of the weld joint.
The use of the hard welding mode also manifested in the narrower heat-affected zone from the side of the welded DP 600 sheet. The course of hardness and structure of the individual heat-affected zones corresponds to already published results of the evaluation of resistance weld joints of dual-phase steels.
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Source: Pavol Sejč, Judita Belanová, Zuzana Gábrišová, Branislav Vanko Faculty of Mechanical Engineering of the Slovak University of Technology in Bratislava, Nám.Slobody17, 812 31 Bratislava. Slovak Republic.