Effects of TiC composite coating on electrode degradation in microresistance welding of nickel-plated steel
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6/10/03
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Effects of TiC Composite Coating on Electrode Degradation in Microresistance Welding of Nickel-Plated Steel S.J. DONG and Y. ZHOU Electrode degradation has been studied during series-mode microresistance welding of thin-sheet nickel-plated steel to nickel. The main focus of the study was the effects of a TiC metal matrix composite coating. The results indicated that electrode degradation was caused predominantly by material loss due to pitting (as a result of the fracturing of local bonds between the electrode tip and sheet) and also by microscopic extrusion or plastic deformation (as a result of the softening of electrode tip regions). The composite coating improved tip life by about 70 pct, mainly because the TiC particles contained in the coating discouraged local bonding between the electrodes and sheets, and probably also improved the resistance to surface extrusion. It was also found that the use of an oxidedispersion-strengthened copper alloy (Cu-Al2O3) improved tip life by only about 15 pct compared to the conventional precipitation-strengthened Cu-Cr-Zr electrode alloy.
I. INTRODUCTION
MICRO- or small-scale resistance welding is a group of microjoining processes (such as resistance spot, parallel gap, series, and seam welding). These processes are commonly used for applications in electronic and medical packaging, such as lead /pad interconnections and hermetic sealing.[1–4] There are differences between micro-resistance welding and “large-scale” (regular) resistance welding, although the principles of the two processes are similar. For example, lower electrode force (pressure) used in microresistance welding results in a relatively smaller contact area and higher contact resistance at the faying interfaces, which, in turn, results in lower welding current required to initiate and form a weld.[5 – 8] In resistance welding, a weld is formed between two metal sheets through the localized melting and coalescence of a small volume of the material(s) at the faying interface due to resistance heating generated by the passage of electric current.[5] However, the welding current will also degrade the electrode tip surfaces due to the resistance heating at the electrode/sheet interfaces. Little work has been published on electrode tip degradation mechanisms and engineering solutions in microresistance welding. In large-scale resistance spot welding of Zn-coated steels for automotive applications, the primary mechanism limiting the electrode life is identified to be growth of the electrode tip face diameter.[9,10,11] Enlargement of contact face diameter results in reduced current density/heat generation and hence undersized welds between the sheets. A number of damage processes that could contribute to the electrode degradation during large-scale resistance spot S.J. DONG, Associate Professor, is with the Department of Material Engineering, Hubei Automotive Industries Institute, Hubei, People’s Republic of China 442002. Y. ZHOU, Canada Research Chair in Microjoining (www. chairs.gc.ca
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