Weldability of a zirconium alloy comparing resistance and pulsed laser methods

Abstract

The current study intends to characterize the microstructure resulting from the autogenous welding of zircaloy-4 and its influence on the mechanical behavior. For comparison purposes, laser beam welding (LBW) and electric resistance welding (ERW) were used for joint tubes to sleeves suitable for fuel elements of the Brazilian power reactor Angra 1. The tubes measured 14.15 mm external diameter with a wall thickness of 1.76 mm and the sleeves measured 15.61 internal diameter with a wall thickness of 0.64 mm. Under conventional ERW, the process parameters were chosen to produce an approximately 2 mm wide weld bead appropriate for the thermo-mechanical loading of the component. However, laser methods could be an alternative for automation and reproductiity. The microstructure in the fusion zone (FZ) for the welding processes was characterized by α martensite for LBW and α Widmanstätten for ERW. It was verified tin segregation in specific regions of the ERW coupons, however, without affecting the tensile strength or the hardness of the coupons. The maximum load for ERW coupons were about 8 kN, higher than LBW case (3 kN), although both above the standard requirements. The relative low value of LBW tensile strength was due to the gap between the tube and the sleeve, as conceived for ERW, and not optimized for the laser process. Loss of coolant accident (LOCA) shown a white ZrO2 coating on the LBW surface as a result of massive oxidation of the resolidified material. Keywords: Zirconium alloys, Electric resistance welding, Laser welding