P 1132 - Extension of the fatigue design of K-nodes with gap made of RHP and KHP to high-strength steels and thick-walled sections

P 1131 - Alloying concepts and process strategies for press-hardening steels with increased residual formability One effective way of reducing emissions from passenger cars is to reduce the weight of the vehicle. The greatest savings potential lies in the area of car body construction, where, for example, high-strength steels are now replacing conventional thin sheet materials. However, the use of high-strength steels to increase vehicle safety in the event of a crash is particularly challenging. One of the most efficient processes for manufacturing crash-relevant components, such as the B-pillar, is press hardening, in which forming and hardening are combined in a single process step. The material 22MnB5 (1.5528) is currently used in the industrial series production of press-hardened components, but its ductility in the hardened state is limited due to the alloy concept. An improved property profile consisting of strength and residual formability is being sought for increased vehicle occupant safety in the event of a crash. This project is therefore concerned with the development of new alloy concepts and process strategies to improve the residual formability of press-hardened steels. Three different alloying concepts were investigated: the dual-phase concept (DP), the grain refinement concept (KF) and the retained austenite concept (RA). Depending on their chemical composition, different heat treatment cycles were combined with press hardening for a total of 6 steels. The investigations focussed on the microstructure development and the resulting characteristic properties in the tensile and bending test. In addition, the coatability and further processability by welding were investigated. It was demonstrated that an improvement in the ductility of press-hardened materials in the RA concept can be achieved by complex Q&P structures (quench&partitioning). In contrast, the DP and KF concepts, which were also investigated, showed only a marginal improvement in the property profile. The best properties in the RA concept result for a material with 0.3 wt% C and 2.5 wt% Mn with tensile strength values of 1400-1500 MPa and elongation at break values of 14-19 % depending on the respective heat treatment. Further tests on this alloy show that the material is temper resistant and therefore suitable for stove enamelling in vehicle production. Joining can be carried out by both laser beam and spot welding. Hot-dip coating with both AlSi and zinc coatings is possible in principle. This project has thus succeeded in demonstrating possibilities for improving the material properties of press-hardened steels using a Q&P heat treatment strategy. The crash absorption potential of the materials produced was doubled compared to the reference material 22MnB5. Publication: 2021 Authors: Prof. Dr.-Ing. W. Bleck, Dipl.-Ing. A. Rüskamp, M.Sc. Y. Sparrer

P 1133 - Development of pre-hole-free hybrid joining processes for mixed structures with novel steels with tensile strengths greater than 1,800 MPa The statutory regulations to reduce greenhouse gas emissions are prompting the automotive industry to implement consistent lightweight construction. The reduction of CO2 emissions to an average of 95g CO2/km in passenger car traffic by 2020 has been agreed. The vehicle body, which accounts for over 30% of the total weight, offers the greatest potential for implementing lightweight construction, which can be achieved by using high-strength steels and other lightweight materials such as aluminium. In particular, the mixed construction of press-hardened 22MnB5 steel and aluminium in combination with adhesive bonding leads to economical lightweight construction solutions. Even higher degrees of lightweight construction can be achieved by using even stronger steels with strengths above 1800 MPa (referred to as "UHSS+" in this report). This allows a further reduction in material thickness to be achieved while essentially maintaining component performance. The prerequisite for a design made of UHSS+ in combination with aluminium structures is the applicability of a cost-efficient and robust joining technology. The aim of the research project is therefore to develop pre-hole-free hybrid joining processes for mixed structures with new types of steel with tensile strengths greater than 1,800 MPa. To this end, the processes of solid self-pierce riveting, semi-tubular self-pierce riveting, friction element welding and resistance element welding are being qualified for joining the aforementioned material combination by adapting the process parameters, the tools and the auxiliary joining parts. In the case of semi-tubular self-pierce riveting processes, numerically supported optimisations of dies are used to determine various options for improving the joint quality. The deformations in solid self-pierce riveting can be reduced through the use of upset area dies. For the element welding processes, on the other hand, the process parameters are adapted so that welds with reproducibly good mechanical strength properties are possible. In addition, local conditioning is used to modify the UHSS+ sheet materials so that joinability can also be guaranteed with conventional mechanical joining processes. The results presented for hybrid joining indicate that the processes shown can be combined well with adhesive bonding technology. The determined property profile of joints made of UHSS+ and the aluminium alloy EN AW-5182 shows that a reproducible joint quality can be achieved. Publication: 2021 Authors: Prof. Dr.-Ing. G. Meschut , Sc. V. Sartisson

P 1135 - Hot moulding process integrated surface structuring for hybrid components Due to increasing requirements in the areas of safety, comfort and lightweight construction, the demand for structural parts made of ultra-high-strength steels is growing in the automotive body-in-white. By using these steels, the sheet thickness of the components can be reduced and thus the weight lowered. However, the production of complex geometries proves to be problematic due to the high mechanical strength, as large loads act on the forming tools and the process accuracy is impaired by the springback behaviour. The press hardening process provides a solution to this problem. Press hardening is a non-isothermal process that combines the forming of the component and the hardening of the material. In this process, martensitic hardening of the sheet metal component can be achieved. However, stability problems with reduced sheet thicknesses limit the lightweight construction potential of press-hardened components. In other branches of industry, such as the aerospace industry, structural components are often made from fibre-reinforced plastic composites (FRP) or, in more recent approaches, in a hybrid composite of metal and FRP, while high-strength steels continue to dominate the automotive mass market. A targeted combination of metal and FRP leads to stress-adapted and cost-effective hybrid lightweight structures. In the load application area of such hybrid components, concentrated, three-dimensionally effective load collectives often occur, which are preferably absorbed by isotropically acting metallic components. The transition area or the interface from the metallic component to the fibre composite structure, where the forces have to be transferred, is critical. New surface pre-treatment processes can help to improve component performance by creating targeted interlocking effects. The aim of this project is to develop a process that introduces surface structuring into the hot forming process of high-strength steels in order to subsequently achieve improved bonding of fibre-reinforced plastics. The structuring is applied using the corresponding hot forming tool, which eliminates an additional work step compared to other structuring processes. For this purpose, a tool for structuring at sample body level is first developed, with which the samples are simultaneously structured and hardened. The mechanical properties of the hybrid structures made of FRP and 22MnB5 are then determined experimentally. Based on the knowledge gained, the method is then applied to an automotive body structure and validated. Publication: 2020 Authors: T. Tröster, K. Dröder

P 1136 - Use of friction stir welding for joining steels of different grades in conjunction with the simultaneous development of corresponding new types of ceramic tools The most important and first development objective of the project was the economic expansion of friction stir welding (FSW) for joining steel materials, including those with high strengths. A great deal of previous experience available at SLV Berlin-Brandenburg and experimental applications were incorporated into the project. Based on enquiries, mainly from the areas of shipbuilding, offshore, vehicle construction and special applications and requirements from the Project Advisory Committee (PA), steel joints of various grades were welded and analysed. These include, among others: S235, S355, S690, CrNi18-10 and H800. Blind seams, butt joints and lap joints were welded. For two steel grades, the butt joint was also welded on both sides to record greater thicknesses. In addition, it was examined whether the use of shielding gas was necessary and how this could be optimised (simple gas nozzle above the stirring tool, drag nozzle, etc.). This was dispensed with in the further course of the project. No significant advantages could be identified. Material tests such as tensile tests, bending tests, X-rays etc. confirm this tendency. The tested samples consistently showed quality values in the range of the base material. The second important aspect of the project was tool development. This is also the main reason why FSW on steels is not very common (except for university or institutional research applications). During the friction stir welding process on steels, the friction stir tools are exposed to extremely high temperatures ~1200°C (e.g. only ~500°C for aluminium), corrosive and abrasive wear. Tungsten-rhenium alloys are mainly used for FSW steel welding (WRe05, WRe25), but other special metallic materials such as tantalum-based materials and solid carbides are also used. Another variant is tools made of polycrystalline cubic boron nitride (pcBN). Conventional ceramic material systems offer a further, promising and much more economical option. These systems in particular were investigated in the research project by SLV Berlin-Brandenburg and FGK GmbH from Höhr- Grenzhausen. It was important to understand the interactions between the material to be welded and the ceramic tool. Derived from the tests and the subsequent laboratory investigations, a material system crystallised, silicon nitride. This material, chemical formula Si3N4, a non-oxide ceramic, has proven itself in all the steels tested. Oxide ceramics, on the other hand, although easier to produce, were not convincing. Only in the course of the project, and only towards the end, were improvements achieved in some oxide ceramics through special geometric and chemical adjustments. However, this is also an important result of the research project: Understanding the failure mechanisms and their effects on the various ceramic material systems. Ultimately, however, it can be said that the Si3N4 material is essentially ready for use. This was clearly demonstrated and confirmed by the results of the reference tests. This means that at least one material system has been verified for friction stir welding on steel. Publication: May 2021 Authors: R. Boywitt, Dr.-Ing. M. Emmel