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In the electric resistance spot welding process shown in Fig. 1, electrodes 1 and 7 press against workpieces 3 and 5. A current is then passed through these components. Because of the electrical contact resistance, heat will be generated at electrode/workpiece interfaces 2 and 6 and faying surface 4. The heat at the faying face melts the workpieces to form a nugget, 4. To prevent melting at the electrode/workpiece interface, water is circulated in the cooling chamber of the electrodes -- Fig. 1. For a reduction of the problem size in this investigation, only one electrode was considered. This approach requires knowledge of the power input (determined experimentally) at the electrode/workpiece interface. Both the ambient air and initial water temperatures were assumed to be 20°C (68°F). If the water does not boil, the physical properties can be assumed to be temperature independent. A complete conjugate heat transfer solution can be performed in two separate steps: 1) a steady-state mechanical solution assuming an imposed coolant flow rate and pressure at the inlet of the water pipe, and 2) a transient thermal solution. This approach uncoupling the mechanical and the thermal solutions, using the assumption of temperature-independent material properties substantially reduces the computational time. The welding cycle, i.e., the time to make one spot weld that encompasses several cycles of alternating current, was taken as the duration of the transient solution time.
The accurate thermal simulation of a spot welding electrode cap could permit critical design parameters to be identified for improved electrode life. In this study, a parametric model has been developed to predict the transient thermal behavior of a typical spot welding electrode cap. The model employs the technique of conjugate heat transfer analysis to avoid the problem of estimating a value for the heat transfer coefficient that arises with conventional heat transfer analysis. Using experimental values for the input power, the predicted maximum tip surface temperature was 905 K. Traces of aluminum melting at the cap/aluminum interface are often observed in practice in the spot welding of aluminum. Since aluminum alloys have melting points of ∼900 K, the simulation closely predicts the tip surface temperature. The analysis indicated that convective and radiant heat losses were not important. A simple linear relationship between the maximum temperature and the input power was found. For very short heating times, no significant changes were found in the maximum temperature reached for a decrease of the coolant flow rate from 3.79 L/min (1.00 gal/min) to 2.24 L/min (0.75 gal/min), or for a decrease of the cap depth -- the distance between the tip working surface and the cooling surface -- from 9.00-6.35 mm. The overall behavior is typical to that of components with a slow thermal response, but a fast heating rate.
1. Murphy, A. J., Yeung, K. S., & Thornton, P. H. (1999). Transient Thermal Analysis of Spot Welding Electrodes. Welding Journal (Supplement), January, 1s-6s.
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