Resistance Spot Welding Metallurgy of Thin Sheets of Zinc-Coated Interstitial-Free Steel
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TRODUCTION
INTERSTITIAL-FREE (IF) steel is one of the formable steels commonly used in autobodies due to excellent deep drawability, owing to its ultralow carbon (usually less than 0.005 wt pct) and nitrogen (usually less than 0.005 wt pct) contents and high r value.[1–5] Moreover, in these steels, microalloying of titanium, niobium, and vanadium is done in order to achieve optimum formability and also to compensate for the loss in strength due to their ultralow carbon content. Such stabilizing elements (Ti, Nb, and V) enable the formation of carbides, nitrides, or carbonitrides in order to scavenge C and N from solid solution.[5,6] Other SAJAD SALIMI BENI, MASOUD ATAPOUR, and ROUHOLAH ASHIRI are with the Department of Materials Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran. Contact email: [email protected]; [email protected] MOHAMMAD REZA SALMANI is with the Eng. & Q. of AutoSteel Division, SAIPA Group, Tehran, Iran. Masoud Atapour and Rouholah Ashiri contributed equally to this work. Manuscript submitted December 6, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
properties of these types of steels extending their application in the automotive industry are their stretcher-strain (St-St) pattern inexistence and antiaging property due to the negligible amount of solute C and N atoms.[7,8] Furthermore, the low yield strength of these steels leads to the disappearance of surface deflection (orange peel effect) at the outer face of the autobody.[8] Therefore, due to these distinct characteristics of IF steels, they are frequently used as rear and front door inners, side panels, spare wheel wells, oil pans, and rear floor pans.[4,8,9] Resistance spot welding (RSW) is the most widely used process in sheet metal joining for making autobody, electronic industries, home applications (like refrigerators), and rail vehicles.[10,11] 2000 to 5000 resistance spot welds are used in a typical modern automobile.[12,13] The most important advantages of the RSW process in contrast to other welding processes, such as gas tungsten arc welding, laser beam welding, and friction stir spot welding, which make it possible to be used widely for manufacturing the structures with complex geometry and high mechanical strength in the automotive industry, are high operating speed (low process time), no requirement for consumables (e.g., filler metals), high productivity with low cost, possibility
of joining thin sheets together, and simplicity of operation or automation.[11,12,14] Among the various materials that are used in the automotive industry, galvanized steel sheets are of particular importance due to their high corrosion resistance induced by the presence of zinc coating. However, the presence of zinc coating can lead to susceptibility to weld cracking,[15,16] premature wear of electrodes, expulsion, an increase in the instability of the welding process, difficulty in controlling the welding parameters, such as welding current, welding time, and electrode force, reducing the weldability of the galvanized s
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