African Fusion March 2016

Spot welding: Cu-Cr electrode caps

Investigating the electrodes under the welding process of similar and dissimilar materials in resistance spot welding Nachimani Charde, University of Malaya, Malaysia This paper presents an experimental investigation into copper-chromiumelectrode caps in the resistance spotwelding process. Apair of circular electrode caps of equal sizewas used to produce up to 900 spot welding nuggets between carbon steel sheets, stainless steel sheets and between carbon and stainless steel. The electrode caps were examined throughout the process for deterioration and mushrooming.

T his research presents an ex- perimental investigation of the Resistance Welder Manufactur- ers Association’s (RMWA’s) class two (copper-chromium) electrode caps using carbon and stainless steels in the resistance spot welding process. A pair of circular electrode caps of equal size (5.0 mm diameter) was used for up to nine hundred spot welding cycles. The electrode caps were sharpened, when necessary, between welding processes using an electrode dresser, typically after approximately four hundredcycles. The degrading factors of chromium- copper electrodes influence the weld geometries of carbon and stainless steels weld nuggets, directly impacting on the bonding strength. When the electrode caps used were subjected to a micro-structural analy- sis, several cracks were found in their internal structures. The internal cracks only appeared in the movable-upper electrode cap due to the continuous heating and contacting effects from the pneumatic pressures, as compared with the static-lower electrode in the 75 kVA spot welder. Mushroom growth of the electrode cap tips is another problem affecting weld quality, as it reduces the resistance during the weld- ing process.

In this experiment, the mushroom growth seemed to be higher on the upper side electrode than the lower side. With the increased diameter of the electrode tip due to the mushrooming effect, the weld geometries become ir- regular, which leads to inconsistency in its appearances and to weld rejections. Introduction In joining carbon and stainless steels using the spot welding process, class two alloys are recommended for the electrodes by the Resistance Welder Manufacturers Association (RMWA) [1]. The grounds for this recommendation is that these alloys have superior resis- tance, heat tolerance and higher corro- sion resistance [2]. Without the addition of alloying elements, pure copper is intrinsically soft and fails prematurely in demanding applications [3]. Amixture of substances is, therefore, a good choice for themanufacturing of electrode caps so as to produce superior quality, spe- cificallywith respect to theirmechanical and electrical properties. With this consideration in mind, copper-chromium-based electrode caps were practically tested to weld ap- proximately nine hundred weld joints of carbon and stainless steel sheets. Figure 1 shows the copper and chro-

steelmeltingpoint falls between 1 426 to 1 540 ºC and the stainless steel melting point falls between 1 400 to 1 450 ºC. The copper and chromiumsolubility phases areof the eutectic type. The face-centred cubic (FCC) structure will be formed in the copper, while body-centred cubic (BCC) crystals form in the chromium on solidification of copper-chromium alloys. Fundamentally, thewelding process is governed by its process parameters – welding current, weld time, electrode tip diameters and electrode force [6]. These parameter variations establish the corre- sponding heat growth for anymaterials, which in turn give the bonding strength. While welding, the heat produced in the enclosed areas of the electrode tip will cause the tips’ deterioration. Another factor that obviously affects this deterioration is the electrode pressing force, which was primarily supplied via pneumatic pressure in this research. Every time the electrodes are pressed to hold theweldablematerials together, the impact effects of the electrode tips on the basemetal subject the electrodes to fatigue. In this experiment the mushroom growth, degradation as well as the de- terioration is examined for the copper- chromiumelectrode caps using a 75 kVA spot welder. Part of this research has been previously published for the simu- lation, tensile shear strength, hardness distribution and themetallurgical analy- sis and, therefore, such information is excluded in this paper but relevant references are given in [7]. Experimental procedure Base metal plates with a thickness of 2.0 mm were prepared in rectangular shapes to a size of 200×25 mm. The chemical composition of the stainless steel sheets was found to be: C–0.046, Cr–18.14, Ni–8.13, Mn–1.205, Si–0.506, S–0.004, N–0.051 and P–0.030. The

mium phase diagram for copper-based al- loys [4]. It shows that the chromium is easily soluble in the liquidus of copperwhenheated above the 1 076 ºC and below 1 860 ºC. Once the compound is solid- ified, it requires equal amounts of heat to re- melt it again [5]. This is significant in the welding of the carbon and stainless steels because the carbon

Figure 1: Copper and chromium phase diagram (Chakrabarti DJ, 1984).

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March 2016

AFRICAN FUSION