A new process chain for forming individually curved sheet stringers

Sheet metal hydroforming is a forming technology for the production of complex parts. The forming operation is performed by a pressurized fluid. The inner contour of the part has no contact to the tool. This offers high potentials with regard to the forming of stringer-stiffened sheet metals. However, the hydroforming process requires cost-intensive tooling, which makes it inflexible in adapting to short-term changes of the part geometry or producing batches of similar but not identical parts. Incremental forming processes are highly flexible because they reduce or even spare the need of dedicated tooling. In order to manufacture individually shaped sheet stringer parts, the process chain of hydroforming can be extended with an additional incremental forming step. Large batches can be produced costefficiently by hydroforming, upon which single parts can be modified according to particular specifications by means of incremental forming. In this paper investigations of a newly developed process chain for manufacturing sheet stringer parts are described. Their focus was to analyze the feasibility of incrementally forming surface structures with stiffeners both out of flat sheets and out of pre-forms produced by hydroforming. The examined process chain consists of laser welding and hydroforming, followed by incremental driving of the stringers. During the incremental step, the stringers are shaped through local stretching of selected areas. Thus using a simple universal tool set sheets of one millimeter thickness with stringers in varying constellations have been formed successfully. The radii of such curvatures could be changed along a stringer by modifying selected process parameters creating complexly curved surfaces in the sheet.