The thermal behavior of the weldment during the resistance spot welding is studied to clarify the weld nugget growth and to predict the dynamic resistance which was known as the monitoring signal of weld quality.
The temperature distribution is governed by an axi-symmetric heat conduction due to the local resistivity of base metal, the interface resistance and current flow. The current flow is obtained from the voltage potential field formation which is represented as a Quasi-Laplace equation, and also the voltage distribution enables us to predict the dynamic resistance which varies during the weld cycle. Some nonlinearities of the dependence of electrical and thermal properties of the material on the temperature are taken into account and the melting phenomenon is also included in the analysis.
The voltage potential equation and heat conduction equation resulting from the analysis are numerically solved by F.D.M. in the case of mild steel for various welding currents and welding cycles. Some preliminary experiments precede the numerical calculation to determine interface resistivity. The calculated temperature distribution in the weldment is compared with the visually obtained isothermal lines based upon the microscopic feature and its calculated dynamic resistance is compared with the measured one.
A series of experiments was carried out to verify the analytically obtained temperature distribution in the weldment. Experimental temperature distribution was obtained from the picture of weld metal structure which was taken by an optical-microscope. The two distribution have relatively small differences, in spite of many assumptions. The dynamic resistance is also analytically predicted based upon the voltage distribution, and this was compared with the measured one.
