Material flow around a friction stir welding tool: Development of a thermo-fluid code

In Friction Stir Welding the heat generated through friction and plastic deformation softens the materials and allows them to be joined. The tool consists of two features, a pin and the shoulder. Several pin geometries have been proposed, for example a threaded cylinder, a cylinder with flattened sides, etc. The rotating pin forces the materials to flow around the pin and to mix. The shoulder applies a pressure to the material to constrain the plasticized material around the pin, and generates heat through friction and plastic deformation in a relatively thin layer under the shoulder surface. The material flow around the tool is very complex, and to date not fully understood. However one can distinguish two main parts: an extrusion of material around the rotating cylinder, and a vortex flow field near the probe due to the downward flow induced by the probe thread. The correct prediction of the resulting temperature field during welding is very important if the process is used industrially, as this would allow the evolution of the metallurgy, as well as the residual stresses to be predicted. A model which is able to predict the temperature field can also be used to optimise the operating parameters (tool rotating and advancing speed). Currently most numerical models rely on the measurement of the heat flux to be used as input data into the numerical model. This means that these models are not predictive. This paper presents a finite element model which does not require the heat input to be measured. The MORFEO (Manufacturing Oriented Finite Element toOI) finite element code developed at CENAERO is used for all simulations. The simulation results of the flow around simple tool geometries are presented. The predicted flow fields are compared to a first weld experiment conducted at INSTITUT DE SOUDURE. The first results are encouraging as a reasonable correlation between the experiment and the simulation is obtained.