Joining of zircaloy-4 of dissimilar thickness using electron beam welding

Abstract

Electron beam butt welding of zircaloy-4 plates of dissimilar thickness (with different thickness ratio, defined as the thickness of one side plate with variable thickness to that of another reference plate of fixed thickness) was performed in order to investigate the effect of plate thickness on the evolution of cooling rate, microstructure and its correlation to mechanical properties. Faster cooling rate on thicker plate results in lower peak temperature, finer grain size both in fusion zone and heat-affected zone with more but uniform porosity distribution than those in the thinner plate. The size of HAZ was higher on thinner plate compared with that of thicker plate. Although the width of fusion zone remained unaffected, the depth of penetration decreased with increase in thickness ratio at same heat input rate. Fusion zone consisted of columnar grains with lamellar widmanstatten features, while HAZ consisted of equiaxed grain with randomly oriented small acicular α-Zr phase in β-Zr phase. A small amount of fine second-phase particles of Zr(Fe,Cr)2 was identified in the fusion zone, which affected the tensile properties marginally. Weld failed in the base metal during tensile test, and YS and UTS are found to be comparable to that of base metal. However, the ductility and notch toughness of the joint was found to be inferior to those of base metal, which deteriorated with increase in thickness ratio. At constant heat input, porosity was found to be higher in dissimilar joint than its similar counterpart, which might be one of the reasons for inferior ductility and notch impact strength in addition to unfavourable microstructural features.