Resistance Spot Welding of Dissimilar Steels: Temperature Curves

Resistance spot welding was used to joint AISI 316L austenitic stainless steel and AISI 1020 low carbon steel. During the welding process, temperature measurements were taken to obtain the temperature curves. Because of the different chemical compositions of the carbon steel and stainless steels, their thermal conductivity values are also different. Electrical resistivity is also an important parameter when carbon steel is spot welded to stainless steel. Differences in the thermal conductivity and in the electrical resistivity of metals lead to an asymmetrical weld nugget in the dissimilar joints. Dissimilar resistance spot welding can be more complex than similar welding due to different thermal cycle experienced with each metal.


I. INTRODUCTION
The spot welding process joints two or more metal sheets together through fusion at certain point. It is a simple process that uses two copper electrodes to press the work sheets together and high current to pass through it. The growth of weld nugget is, therefore, determined by its controlling parameter such as current, weld time, electrode tips, and force. The Fig. 1 shows the schematic of the spot weld.

Fig. 1: Resistance spot welding
The generation of heat is due to resistance to current flow, this phenomenon is known as joule effect. The total thermal energy generated during the welding process can be calculated by Joule's law: The schematic of the electrical resistances of the spot welding process is shown in Fig. 2.

Fig. 2: Electrical Resistances
In dissimilar joints, one of the important features of the weld nugget is its asymmetrical shape such that fusion zone size and penetration depth of stainless steel side are larger than those of carbon steel side. Electrical resistance and thermal conductance control heat generation and heat dissipation which in turn, effect weld nugget formation and its growth. Differences in the thermal conductivity and electrical resistivity of two steel sheets lead to an asymmetrical weld nugget in dissimilar metal joints. Lower electrical resistance of carbon steel, and its higher thermal conductivity compared to stainless steel leads to smaller fusion zone in the former. The joint region consists of three distinct structural zones: fusion zone (FZ) or weld nugget, heat affected zone (HAZ) and base metal (BM). The Fig. 3 shows a typical geometric morphology of resistance spot weld. EXPERIMENTAL PROCEDURE The materials used in the present work were AISI 1020 carbon steel and AISI 316L austenitic stainless steel in the form of 1x 25 x 100 mm sheets. The nominal chemical composition of both steels is presented in TABLE 1.

Fig. 5: Spot Welding Machine used in this investigation
In the temperature measurement procedure, three thermocouples type K were used. The first thermocouple was placed in heat affected zone (HAZ), the second thermocouple 1cm distant from the (HAZ), the third thermocouple 2 cm distant from the (HAZ). Fig. 6 shows positioning of the thermocouples in the sheets.    Fig. 9 and Fig. 10 shown the coefficient of determination R 2 values obtained in the curves of temperature. The 6 th degree polynomial was used.  The TABLE 2 and TABLE 3 show values of the coefficient of determination R 2 in relation to the position of the thermocouple in the stainless steel and carbon steel sheets, respectively.

IV. CONCLUSION
In the dissimilar spot welding of AISI 1020 carbon steel and AISI 316 L stainless steel, the temperature curves obtained show that the materials undergo different thermal cycles, this is expected because the steels have different chemical compositions and thermal properties. That is, the results are in agreement with the theory. The coefficients of determination R 2 obtained in the 1020 carbon steel sheet and the 316L stainless steel sheet have values very close to 1, that is, the fit of the model was good. Except, the third thermocouple of the stainless steel sheet (R 2 = 0,7308). properties of spot welded dissimilar joints for stainless and galvanized steels.