Efficient manufacturing of hybrid structures consisting of steel and CFRP with using residual heat / Effiziente Herstellung von Hybridbauteilen aus Stahl und CFK mithilfe von Restwärmeausnutzung

Abstract

Weight reduction through lightweight construction is a proved means of saving resources and increasing dynamics in the automobile industry. Due to their good specific strength, carbon fiber reinforced plastics (CFRP) are employed increasingly. In addition to the use of unalloyed CFRP components, the material can also be utilized in hybrid composites in order to effectively strengthen mechanically highly loaded components, e.g. structural components or chassis components. Here, too, weight reduction can be achieved by saving material in the metallic component. A time- and cost effective production of hybrid components can be attained if both the joining of the metal- and CFRP components and the manufacturing of the component can be can be performed in one process step. This hybridization offers the advantage of reducing the process steps of fabricating hybrid structures, leading to shorter cycle times. One way of producing hybrid components is prepreg pressing, a procedure whereby CFRPduroplastic- based prepregs are pressed into an already formed steel component; the prepreg is pressed in noncured condition. Only partial ageing of the matrix resin takes place in the heated mould. The epoxy resin also functions as an adhesive. Final curing can take place through heat at a later stage of the cathodic dip-painting at the automobile manufacturer. Thus, cycle times significantly shorter than five minutes are feasible. Prepreg pressing has been tested e.g. for idealized body structures (hat profile as a replacement structure for rocker panels or pillar segments). Essential to this were static and dynamic (crash-) requirements. Steel was used as the metallic material component; with regard to lightweight construction, the focus lay on high-strength, hot-formed steels. In order to make the prepreg pressing process more energy- and time efficient, a concept for using the residual heat of the hot-formed component has been developed. Hot-forming furnishes an ultra-high-strength steel component with high dimensional accuracy which can be taken from the tool while still warm. The approach to the production of hybrid components is making use of the residual heat that is available in any case. For this purpose, two tools with temperature regulation are used; the hot-forming process takes place in the first tool, hybridization in the second. Both tools are operated by the same press with the same cycle time, the challenge being to control temperatures so that the proportion of martensite of the steel is as high as possible and the resin is sufficiently pre-cured. JMAT Pro-simulations of the microstructure states and strength levels that occur have shown that even with the relatively long cooling times of 60 s, high strength levels can be achieved with the steel materials used. During the hybridization process, the resin is used as an adhesive, rendering an additional joining process unnecessary.