Design Optimization of Friction Stir Backward Extrusion Tools for Improved Material Mixing and Reduced Plunging Force: A Numerical Modelling Approach

Abstract The friction stir backward extrusion process has garnered considerable attention for extrusion of tubes and fabrication of bimetallic tubes, where the tool geometry plays an important role in determining material flow, heat generation, and plunge force requirements. However, existing tool design is limited to a tapered geometry, which restricts effective material flow and leads to non-uniform material mixing. In this study, a three-dimensional numerical analysis was performed using DEFORM-3D to understand the flow dynamics of various tool geometries. Three tool designs: a conventional tapered tool (Ttt), a tapered tool with a cylindrical pin (Ttp), and a hemispherical tool (Tth), were considered. The hemispherical tool resulted in a significant ∼2/3 reduction in peak plunge force compared to the tapered profile tool. This hemispherical tool exhibits improved material mixing due to the drag involved with the hemispherical surface, whereas the pinned tool results in a controlled rotation of material around the pin. Inspired by superior volumetric material flow by the hemispherical tool end and controlled flow around the pin, two hybrid tools (Htp-1 and Htp-2) were introduced with the combination of a hemispherical end surface and a conical pin of varying diameters. These hybrid tools result in controlled vortex-type flow that combines strong stirring with global dispersion. Among them, Htp-2 achieved the most balanced particle distribution, and the hemispherical-based tool geometries showed superior stirring and reduced force requirement, highlighting their potential for optimizing tube fabrication through the FSBE process.