On the formation of a centreline grain boundary during fusion welding

The formation of a centreline grain boundary during fusion welding constitutes a potential weldability issue and its presence may affect the life of a welded structure. In practice, it is therefore often appropriate to avoid it through the choice of suitable welding conditions. A numerical model for the formation of a centreline grain boundary is proposed. It involves the coupling of a semi-analytical heat transfer model and a treatment of dendritic growth in multicomponent alloys. It has been applied to the tungsten–inert gas (TIG) welding of IN718 superalloy. The thermal model and the predicted weld pool shape are validated against experimental observations. A weldability diagram is predicted, on which the conditions for centreline grain boundary formation are identified. The formation of a centreline grain boundary is found to be strongly dependent on welding power, and relatively independent of welding velocity. A weldability window arises if the formation of a centreline grain boundary is to be avoided.