Modeling the frictional boundary condition in friction stir welding

Material flow and frictional heating in friction stir welding are investigated using a three-dimensional numerical model. Two mechanical boundary conditions are investigated, including a sticking constant velocity and a slipping variable shear stress model. For the constant velocity model, material in contact with the tool is set at a velocity equal to some fraction of the tool rotational speed. The variable shear stress model is formulated such that it degenerates to Coulomb friction under low forces, but as forces at the tool increase, shear stresses at the tool/material boundary are limited according to Tresca friction. The variable shear condition model permits areas of significant slip, while shear stresses at other regions approach the flow stress of the material. The boundary models are compared with experimental data from flow visualization experiments and thermocouple measurements from plasticine welds. Results show that the variable shear model is superior to a sticking condition. With the variable shear model, the peak temperature in the weld compares well with thermocouple measurements. The variable shear model predicts that material simply extrudes around the tool, while the constant velocity model shows a region of material that rotates with the tool. Maximum velocities with the variable shear stress model are only 9% of the rotational speed of the tool and agree well with experimental findings. Additionally, the variable shear model shows a region of diminishing shear stress, velocity, and pressure at the back advancing side of the pin, suggesting formation of an internal void. The limited deformation, low velocities, and suggestion of void formation agree well with flow visualization studies using plasticine under identical operating parameters.