Multi-species transport CFD simulation and experimental verification for material flow properties in dissimilar friction stir welding

Abstract

A thorough understanding of thermal history and material flow behaviour associated with the tool–material interface substantially eradicates defects from the dissimilar weld joints. Therefore, the authors performed a three-dimensional coupled thermal-fluid flow analysis based on computational fluid mechanics (CFD) for dissimilar friction stir welding of 304 stainless steel (304 SS) and 6061-T6 aluminum alloy (Al6061). A steady-state multi-species transport model coupled with a mixture model was established for the first time, where the CFD solver transported species mass fractions of the materials. The developed model can capture the transversal/horizontal material features and embedded steel fragments/strip in the weld joints, which were also detected in macrographs from experiments. The calculated temperature fields were also fairly agreed with experimentally measured temperatures. The highest strain rate and flow velocity were attained on the outside shoulder edge caused by the lowest viscosity on the Al6061 side. The rotational speed of 300 rpm could not extrude the steel from base 304 SS due to insufficient flow velocity, whereas the steel fragments were intermixed with the Al6061 matrix at 600 rpm. At 875 rpm and 1200 rpm, the continuous steel strips were extruded from 304 SS and deposited in Al6061 due to relatively higher heat generation and flow velocity. Furthermore, the velocity vectors revealed the discontinuous material flow at 300 rpm and 600 rpm, and it was turned into almost uniform at 875 rpm and 1200 rpm. The rotational speed of 875 rpm produced the sound weld with a maximum joint efficiency of 75.6%.