Resistance spot welding of galvanized steel: Part I. Material variations and process modifications
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I.
INTRODUCTION
VARIATIONS in the spot weldability of galvanized steel sheet have inhibited its widespread use in the major fabrication industries. The acceptable welding current ranges of galvanized steel sheet can vary markedly from producer to producer. Hence, when a steel user mixes heats of steel from different mills, the same welding conditions may produce welds with both acceptable and unacceptable nugget diameters because the weld current lobes do not overlap. Tip life is another important aspect of weldability which varies with material and process changes. However, in the present study, we have focused on lobe width and reasons for its variations in position and width. In order to determine the cause of and possibly a solution for these variations in required welding current (lobe width), a research program has been established at MIT which concentrates on three main areas: (1) Variations in base material or coating which cause variations in weldability and welding current range, (2) optimization of the welding process parameters and use of parameter modification to provide the widest range of acceptable welding conditions for a given material, and (3) dynamic inspection of the weld current, voltage, resistance, electrode displacement, and force which can be used in a quality control scheme as well as provide information about the mechanisms of weld nugget formation and growth. Part I of this paper will present the results of the first two areas concerning the material variations and process modifications which affect the weld current lobe width of galvanized steel sheet. Part II will describe the mechanisms of 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
weld nugget formation and growth which explain these resuits. Although much work has been performed on the effect of material parameters, ~'2'3 process parameters, 4 and electrode composition 5"6'7 on electrode tip life, little work has been performed on their effects on lobe width, a topic which has recently become increasingly important, especially for galvanized steel. In this paper, the lobe is defined as the envelope of weld time and current conditions which result in a nugget with a minimum diameter of 0.56 cm (0.22 inch) and the absence of expulsion. The minimum acceptable diameter was chosen based on the thickness of the steels being welded. 8'9 Although the thicknesses varied somewhat (see Table I), they were usually about 16 gauge.
II.
MATERIAL CHARACTERIZATION AND EXPERIMENTAL APPROACH
A thorough characterization of each material was performed before any welding was attempted. Ten galvanized steels were studied, including four hot-dipped galvanized, two fully alloyed galvannealed coatings, and four Galfan coatings (Zn-5 pct A1). These steels are listed in Table I. All welding was performed on a single ph
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