Resistance spot welding of galvanized steel: Part II. Mechanisms of spot weld nugget formation
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I.
INTRODUCTION
IN Part I of this paper, results were presented on the effect of material variations and process modifications on the resistance spot weldability of zinc-coated steel sheets. For the purposes of this paper, weldability is defined as the acceptable welding current range as determined by the welding lobe as defined in Part I. Materials examined include hotdip galvanized material with a mostly free zinc coating, galvannealed material with a fully alloyed Fe-Zn coating, and the uncoated base metal substrates. Material variations studied include zinc coating integrity, composition, thickness, surface roughness, oil, and amount and type of Fe-Zn intermetallics. Process parameter modifications studied include upsloping and downsloping of the weld current, preheat current, postheat current, applied force, and electrode tip geometry. Part II presents the mechanisms of spot weld nugget formation and growth which will explain some of these previous results. In-process weld monitoring experiments were performed which are presented in support of these mechanisms. These experiments include dynamic resistance and displacement traces of hot-dip galvanized, galvannealed, and uncoated material. In order to determine the role of zinc coating location on the faying or electrode interfaces, dynamic inspection and lobe width data are presented for hotdip galvanized material where the coating was removed from either the faying surface or the electrode sheet interfaces. In order to explain the process modification results, SEM, nugget growth studies, and dynamic inspection data are also presented for welds made with upsloping or downsloping of the weld current.
II.
EXPERIMENTAL PROCEDURE
Scanning electron microscopy (SEM) photos were taken of a developing weld nugget so that the physical phenomena occurring at various stages of formation and growth could be monitored. The developing nuggets were examined by stopping the weld sequence at various times during a weld and S.A. GEDEON is Welding Research Scientist, United States Army Materials Technology Laboratory, Watertown, MA 02172-0001. T.W. EAGAR is Associate Professor. Massachusetts Institute of Technology, Cambridge, MA 02139. Manuscript submitted August 15, 1985. METALLURGICALTRANSACTIONS B
breaking the two sheets apart, or cross sectioning them, to view what was happening at the faying or electrode interfaces. By doing this, a stop-action sequence can be observed of the developing weld nugget. An Energy Dispersive X-ray Analysis (EDAX) unit attached to the SEM was used to examine the movement of the zinc and Fe-Zn alloy layers away from the weld area. This was also used to determine the amount of copper alloyed with the zinc on the electrode-sheet interface. The effect of zinc on either the faying or electrode interfaces was investigated by stripping the zinc with HC1 and water from one side of each of the steel sheets being welded. Dynamic resistance curves of the faying and electrode-sheet interface contact resistances were studied by connecting the voltage le
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