New High-Pressure Hydroforming Tool Design Reduces Spring Back in Ultra-High Strength Steels 2019-01-1084

Flexform™ is a high-pressure hydroforming technique using a flexible rubber diaphragm between the liquid pressure medium and sheet metal. No seal is 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 surfaces in addition to traditional materials. The Flexform technique works well with difficult-to-form sheet materials such as ultra-high strength steel, titanium, and aluminum. However, as with any other cold forming method, spring-back is obtained after forming. High-pressure forming often leads to less spring-back than conventional stamping. Material spring-back 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, combined with the pressure exerted by the flexible diaphragm, contributes to significantly reducing the material spring-back. For spring-back sensitive materials, such as ultra-high strength steel and titanium, reductions of 50 % may be achieved. The implication is that the blank size must be increased since the stretch must be applied in an area outside the part surface. The part must therefore be cut to final shape after forming. In addition to the spring- back issue, 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. The combination of forming and cutting in one and the same forming cycle, showcases a significant productivity increase. Actual forming results will in this paper be compared with FEM simulations.