Zirconium alloys are extensively used as a cladding material in nuclear power reactors due to its low neutron absorption cross section, excellent mechanical properties, and corrosion resistance. Zircaloy-4 is used to fabricate cladding tubes, end caps spacer grids and guide tubes in pressurized water reactors (PWRs). These claddings are produced by conventional casting route followed by subsequent thermomechanical processing such as cold pilgering, swaging, rolling, and extrusion. In the present work, influence of swaging process parameters on the microstructural evolution and tensile properties of Zircaloy-4 was investigated for increasing the production rate of end caps.
Zircaloy-4 was cold worked using swaging process and subsequently subjected to mechanical and microstructural characterization. The influence of feed rate (0.7, 1.25, 2 m/min) during swaging on phase analysis, tensile properties and fracture behavior of the alloy was investigated. The swaged samples were annealed at 732oC for 3 hours holding time to characterise the microstructural evolution and fracture behavior of the alloy. Tensile test results have shown improved tensile strength and ductility in case of 1.25 m/min feed rate. The three-point bend test was performed on swaged and annealed alloy to study its fracture behavior. The deformed and annealed Zircaloy-4 samples were characterized by SEM-EBSD to elucidate the fracture behavior as well as phase analysis of the alloy. Texture analysis was performed using EBSD on the samples subjected to swaging at different feed rate.
The hot deformation behaviour of Zircaloy 4 alloy was studied in the temperatures (650oC and 750oC) with strain rates (0.01 and 1 s-1). It was observed that flow stress of Zr-4 alloy found to be high at high strain rate (1 s-1) and low temperature (650oC). The true stress-strain curve of the alloy has shown the serrated oscillation characteristics. Electron backscattered diffraction (EBSD) was used to characterise the evolution of microstructural and texture features in hot deformed samples. Transmission electron microscopy (TEM) was used to characterise the sub grain boundaries, dislocation, and precipitates in hot deformed sample.
A three-dimensional finite element model was implemented in the Deform 3D software to simulate the rotary swaging (RS) process over a circular rod of Zircaloy-4 alloy. The simulation results based on 3D framework provided a detailed insight of residual stress and, flow curve over the rod during multiple pass swaging process and the results are compared with experimental data for different feed rates (0.7, 1.25, and 2 m/min) and die angles (2 to 14 degrees). The hole drilling method was used to determine the residual stresses on swaged zirconium alloy at different feed rates (0.7, 1.25, and 2 m/min). The experimentally measured residual stress from surface to center on swaged samples are used to validate the simulated residual stress. The combination of 1.25 m/min feed rate and 4o die angle provide the higher compressive residual stress at surface and lower tensile residual stress at center.