Mechanism of resistance microwelding of crossed fine nickel wires
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I. INTRODUCTION
RESISTANCE cross-wire welding is one of the electricresistance welding processes, in which two wires are joined to each other, usually at a right angle, by resistance heating under the effect of electrode force.[1] At a large or “regular” scale, cross-wire welded products (mainly of steels) include such items as stove and refrigerator racks, lamp shades, baskets, fencing, concrete reinforcing, etc.[1] At a small or microscale (usually with a wire diameter less than 0.2 to 0.5 mm), resistance cross-wire microwelding is commonly used in electronics and instrument components, mainly for electrical interconnections.[2,3] This latter area of resistance cross-wire welding is becoming increasingly important because of the rapid downsizing of manufactured products. In resistance welding in general,[1] the heat required to form a joint between metals is generated by the resistance to the flow of electric current through the workpieces and can be mathematically described by Q I 2Rt
[1]
where Q is the heat generation, I is the welding current, R is the resistance, and t is the duration of the current application (weld time). The resistance includes contact resistance at the electrode/workpiece and workpiece/workpiece interfaces and bulk resistance of the base materials.[4] Among these resistance components, the workpiece/workpiece contact resistance, which is influenced mainly by material properties (such as hardness and resistivity), surface characteristics (such as cleanliness and roughness), and electrode force, may be the most important factor affecting the process.[1,5,6] This is especially true in resistance microwelding because of the relatively low values of welding current, electrode force, and resistivities of nonferrous workpieces.[4,7] It is generally considered that, in resistance cross-wire welding, “set down,” an indication of the extent to which the wires
S. FUKUMOTO, Research Associate, is with the Graduate School, Himeji Institute of Technology, Himeji, Hyogo, Japan 671-2201. Y. ZHOU, Canada Research Chair in Microjoining (www.chairs.gc.ca), is with the Department of Mechanical Engineering, University of Waterloo, Waterloo, ON, Canada N2L 3G1. Contact e-mail: [email protected] Manuscript submitted February 9, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
are compressed into each other, can be used to evaluate joint strength.[1] For example, Moravskii et al.,[8] in an investigation of resistance cross-wire microwelding of various fine wires using a capacitor-discharge power supply, found that 30 to 35 pct setdown produced good joint quality, while Stroev et al.[3] observed an optimum setdown of 30 to 50 pct in a similar investigation, but on nickel lead-outs of 0.2 mm in diameter. It has been shown[2,3] that inadequate heat generation resulted in weak bonding with a low setdown, while weldmetal expulsion took place when the heat generation was too high, resulting a very high setdown but a low joint strength. When the joint was of high quality, the bond interface between the welded w
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