A novel approach for fabricating tubes with variable wall thicknesses in hollow sinking

Abstract Variable wall-thickness tubes are increasingly required for lightweight, functionally optimized structures and heat-transfer components, yet conventional hollow sinking with a constant speed ratio between the feeding and drawing sides produces an almost uniform wall thickness along the tube axis and cannot generate a taper. This study proposes a simple hollow sinking process for fabricating variable wall-thickness microtubes without an inner tool by controlling the speed ratio between the feeding and drawing sides, that is, by increasing the drawing speed during processing while keeping the feeding speed at the die entrance constant. Austenitic stainless-steel (SUS304) tubes with an outer diameter of 2.0 mm, an inner diameter of 1.82 mm, and an initial wall thickness of 0.09 mm were drawn under these conditions, where the drawing speed on the exit side was continuously increased while the feeding speed was fixed. The resulting wall-thickness distributions and surface roughness were evaluated experimentally and by finite-element analysis, which consistently showed that the increasing speed ratio enhances the axial elongation per unit time beneath and just after the die, thereby geometrically introducing a taper slope on the inner wall. Using this method, tubes with a maximum wall-thickness difference of about 40% between thick and thin sections were fabricated while maintaining acceptable outer and inner surface quality. These results demonstrate that variable-thickness microtubes can be intentionally produced in hollow sinking through simple control of the speed ratio between the feeding and drawing sides, extending the conventional geometric framework for straight tubes to tapered tubes.