Resistance spot welding of precoated steel sheet: Computational heat-transfer analysis
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INTRODUCTION
RESISTANCE spot welding has played an important role in the automobile industry because of its high efficiency and excellent quality control in joining the bare steel sheets. It uses the electric resistance heating at the faying surface, i.e., the steel/steel contact surface, when a high electric current is applied. A large quantity of thermal energy is instantaneously released to melt the steel for joining. [1,2,3] In recent years, there has been increasing interest in the development and applications of precoated steel sheets in the automobile, housing, and construction industries. [4-9] The precoated steel sheet consists of a thin organic coating, i.e., a thin film of polymeric material coated on a steel sheet. For a typical surface-coating process, a low-viscosity polymer solution (usually thermoset type) is first coated onto the steel surface, then baked at elevated temperatures to allow complete curing of the polymer and thorough removal of the organic solvent. This prewelding surface treatment raises the cost; however, it offers several advantages that are often economically worthwhile. These include better corrosion resistance, better formability and deep-draw property, easier secondary coating, and an improved automated spot-welding process. In order to develop a precoated steel sheet suitable for the resistance spot welding, the organic coating has to be conductive and the polymer has to be stable at elevated temperatures, tl~ The former is accomplished by the addition of some conductive metallic powders such as zinc powders; the latter is accomplished by selecting a high-temperature thermoset polymer. If thermal degradation of the coating occurs during resistance spot welding, the degraded polymer would hinder the welding process, causing damage to the electrode and hence resulting in a low welding efficiency. In the past few years, resistance spot welding of bare CHING-CHOU CHEN, Student, and TZU-CHIEN J. HSU and NEW-JEN HO, Professors, are with Institute of Materials Science and Engineering, National Sun Yat-Sen University, Kaohsiung 804, Taiwan. Manuscript submitted February 15, 1993. METALLURGICAL AND MATERIALS TRANSACTIONS B
steel sheets and zinc-coated steel sheets has been extensively studied both experimentally and theoretically, t13,14,151 Finite-element modeling of the resistance spot-welding process of bare steel sheets was studied by Nied [13j who used the electrical-thermal-mechanical interaction and temperature-dependent material properties. A detailed review concerning the development of the mathematical modeling was also included in Nied's work. Gould tx41 examined the nugget development during spot welding using both experimental and analytical techniques. However, only a one-dimensional finiteelement thermal model was proposed. Four stages of nugget development were identified in Gould's study: (1) an incubation period before which no melting was observed, (2) a region of rapid nugget growth, (3) a region over which the rate of nugget growth progressively decreased, a
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