Heat transfer and fluid flow during electron beam welding of 304L stainless steel alloy

A numerical model for three-dimensional heat transfer and fluid flow in keyhole mode electron beam welding was developed and applied to 304L stainless steel welds made at different power density distributions achieved by varying the focal spot radius at a fixed input power. The model first calculates keyhole geometry based on energy balance on keyhole walls and then solves the three-dimensional temperature field and fluid velocities in the workpiece. Since the energy balance and, consequently, the keyhole penetration are affected by the keyhole wall temperatures, the variation of the keyhole wall temperature with depth has been considered. A modified turbulence model based on Prandtl's mixing length hypothesis was used to calculate the spatially variable effective values of thermal conductivity and viscosity to account for enhanced heat and mass transfer due to turbulence in the weld pool. Unlike models available in literature, the model proposed in this work considers the physical processes like variations of keyhole wall temperatures with depth and the resulting influence on calculation of keyhole depth and fluid velocities along the keyhole wall, and three-dimensional heat and mass transport. Thus, the model can be applied to materials with a range of thermophysical properties. The model was used to study the fluid flow patterns in the weld pool and their effects on the calculated weld geometry. The calculated weld dimensions agreed reasonably well with the measured values. Peclet number calculation showed that convective heat transfer was very significant. The influence of convection was illustrated by comparing the calculated weld pool geometries in the presence and absence of convection. The vapor pressures and wall temperatures in the keyhole increased with increase in the peak power density.