Thin Walled Steel Tube Pre-Bending for Hydroformed Components - Bending Boundaries and Presentation of a New Mandrel Design

摘要

For auto-body structures, thin-walled hydroformed and laser-beam-welded hollow steel sheet sections are recommended as a replacement for mainly spot-welded box-shape components. Such components have the same strength but a lighter weight. Pre-bending into the required shape prior to the hydroforming step is difficult to realize in the case of thin wall thicknesses in combination with small bending diameters and relatively large tube diameters. There are bending limits with respect to buckling or cracking which have still to be clearly defined. The boundaries for the cold rotary draw bending process are fixed in the so-called "bending diagram." This diagram in itself is insufficient. Based on simple equations in combination with numerical calculations, an analytical equation was drawn up to better define the boundary conditions. This also takes into account the influence of the material. Furthermore, a new and low-cost mandrel is presented which enables the realization of the pre-bending process under extreme conditions. One of the characteristic features of such a bending mandrel is the in-line calibration of the tube to clamp very thin walls and eliminate the ovality. For example, we are currently working on tubes with an outer diameter of 60 mm, a wall thickness of 0.5 mm and a bending radius of 99 mm.