Evaluation of Electrodes of the Resistance Spot Welding with a Chromium Coating on the Welding of Zircalloy

Abstract

To weld zircalloy nuclear fuel assembly structures, there is a small parameter range for acceptable welds, since zircalloy has a higher melting temperature than the copper alloy of the electrodes. In order to increase the parameter range, a chromium coating was inserted and on the surfaces of the electrode and on other welding devices, once the melting temperature of the chromium is higher than that of zircalloy, as well as of copper. In order to compare and test the new methodology, samples have been welded with the conventional devices which do not have any coating or welding devices covered by chromium and the new coating devices. A comparison on the improvements and enlargement of the range of the control parameter were measured and predicted to assure that the welded region presents adequate quality. Metallographic tests were used to evaluate welding lens size together with corrosion tests to verify the integrity of the welding regions. The lens dimensions and areas are usual criteria for weld acceptance assuring the quality. In addition, computational simulation was used to analyze the welded region, allowing the use of an additional analysis tool to predict the effect of process parameters on the heat affect zone. The simulation was able to predict the size of the welding lens as a function of process control parameters. Both approaches indicated that an increase in the range of acceptable welding parameters and a consequent increase in the lens size of the weld were obtained for the new chromium coating electrodes. The results of experimental data and model predictions confirmed that the new methodology is more reliable and suitable for safety welding procedure approval.

Key-words:
Nuclear energy; Resistance spot welding; Zirconium