Modelling and simulation studies on electron beam welding of Nb-1Zr-0.1C alloy

Refractory metals and alloys are candidate structural materials for next generation of high temperature nuclear reactors like Compact High Temperature Reactor (CHTR) and fusion reactors. These alloys are difficult to weld because of embrittlement induced by oxygen/nitrogen ingress from surroundings and excessive grain growth. Therefore, electron beam (EB) welding is preferred for welding of these alloys to minimize oxygen/nitrogen ingress and also to minimize the grain growth by minimizing the heat input. It is desirable to understand the thermal cycles experienced by the material in the different regions - the fusion zone and the heat affected zone (HAZ) to get a better insight of the microstructural evolution in these regions. It is also desirable to understand evolution of the stress-field during EB welding for better implementation of clamping during welding. However, it is not possible to get temporal evolution of the thermal field and the stress-field during EB welding during to constraints imposed by the process. However, modelling and simulation studies of the EB welding process can provide a reliable quantitative estimate of temporal evolution of the thermal field and the stress-field during EB welding of the material. Modelling and simulation studies were performed for EB welding of Nb-1Zr-0.1C (in wt. %) alloy to get temporal evolution of the thermal-field and the stress-field during EB welding. These studies were performed using a finite element based welding and heat treatment simulation and solution software package - SYSWELD. This commercially available software package is from ESI, France and is extensively used for modelling and simulation of welding and heat treatment. A three dimensional (3D) mesh model of the plate to be welded was prepared and temperature dependent material properties were used in these computations. The heat source was modelled as a 3D Gaussian volume and thermal solutions were obtained by solving the heat diffusion equation in the solid. Heat loss from the plate surface was modelled as radiation loss. Non-linear structural model was sequentially coupled to the thermal solutions to obtain stress-field and the residual stresses. The computation showed a narrow fusion zone and a narrow HAZ, a characteristic of high power density weld joints. Temporal solution showed very rapid heating and cooling of the material in the fusion zone and the HAZ. The computations also showed that high tensile residual stresses exist in the fusion zone and the HAZ, which was counterbalanced by a low compressive residual stresses present in the parent metal, over a much wider region. (author)