The service performance of zirconium-2.5niobium components is largely dependent on the microstructure and mechanical properties achieved through a specific thermo-mechanical process applied during manufacturing. Welding causes microstructural changes which can result in unfavorable changes of properties. The aim of the current study is to understand the transformation behavior of zirconium-2.5niobium during welding, the resulting microstructure and the effect on mechanical properties.
Autogenous bead on plate welding using varying parameters was conducted on Zr705 (zirconium-2.5niobium) alloy sheet of 1.5 mm thickness. The two most common fusion welding processes in reactor core fabrication, namely laser beam welding (LBW) and gas tungsten arc welding (GTAW), were applied. Dilatometry was performed on cooling from 1050°C at varying cooling rates ranging between 0.5-600°C/s. In addition, tensile and micro-Vickers hardness tests were done to characterize the mechanical properties.
Microstructural examination of the dilatometry samples cooled between the ranges of 0.5-50°C/s showed a Widmanstätten structure. At 0.5-10°C/s cooling rate the structure exhibited coarse hcp alpha phase structure with a large volume fraction manifesting as basket weave. Parallel plate structure and grain boundary allotriomorphs (GBA) were also observed. At intermediate cooling rates (10-50°C/s), basket-weave morphology was dominant and composed of finer intragranular alpha plates that randomly precipitated on a number of planes within the same parent β grain. At cooling rates of 150°C/s and higher, the dominant transformation product was martensite. The morphology exhibited acicular (αʹ) martensite plates. The martensite structure was finer at higher cooling rates.
The GTAW and LBW morphology were very similar. The heat affected zone (HAZ) and weld metal of both processes were characterized by an equiaxed grain morphology at low heat input. Increased heat input resulted in the equiaxed to columnar grain morphology transition at the weld metal. The microstructures of GTAW exhibited a basket weave structure in both HAZ and weld metal, with retained beta phase observed in HAZ of some welds. The LBW microstructures consisted of a mixture of martensite, retained beta and Widmanstätten structure in the HAZ, with a fully martensitic weld metal.
Based on the dilatometry and weld results, hardness and cooling rate have a positive linear relationship up to about 150°C/s, when the martensitic reaction occurs and the hardness plateaus. Laser beam welds had a higher tensile strength than the gas tungsten arc welds, this was due to the higher cooling rate and a much finer martensitic structure.