New High-Pressure Hydroform Tool Design Reduces Springback in Ultra-High Strength Steels

Flexform™ is a high-pressure hydroforming technique using a flexible rubber diaphragm between the liquid pressure medium and the sheet metal. No seal is thus needed between the metal sheet and the pressure medium. Only a lower tool half is required, as the diaphragm will act as a flexible upper tool half. Further, the flexible diaphragm allows for several tools to be used in one and the same forming operation. All kinds of sheet metal and thicknesses may be formed using the technique. Perforated sheets, pre-coated sheets or sheets with sensitive surface are in addition to more traditional materials also suitable for Flexform pressing. Likewise, it is known that the Flexform technique works well with such difficult-to-form sheet materials as ultra-high strength steel, titanium and aluminum. Further, the process may also be simulated with good accuracy using FEM. However, as with any other cold forming method, springback is obtained after forming. Even if high-pressure forming often leads to less springback than conventional stamping, material springback is still a factor to take into consideration and needs to be controlled in some way. A tool design and forming technique is shown which introduces a controlled stretch into the blank during forming, which, combined with the pressure exerted by the flexible diaphragm, contributes to significantly reducing the material springback. This paper will also present a technique to both form and to cut the part to final shape in one and the same forming cycle.