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

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 1137 - Cleaning and activation of strip galvanised steel semi-finished products for bonding by a combination of carbon dioxide snow blasting and adhesion promoter application In the areas of vehicle construction, renewable energies and the construction industry, there is an increasing desire, for economic and ecological reasons, to join strip galvanised steel components using adhesive bonding technology in a cost-effective and efficient manner and to use the joined components without subsequent surface coatings. The challenge for adhesive bonding is to achieve a durable bond on the surface designed for corrosion protection, which is contaminated during production and handling, and to prevent corrosive infiltration of the bond. Traditional pre-treatment methods carry the risk of damaging the very thin galvanised strip. Additional chemical activation of the surfaces by applying adhesion promoters as an additional process step represents an unacceptable technological or economic hurdle in many applications. On this basis, the research project investigated the suitability of combining CO2 snow jet treatment with simultaneous process-tolerant adhesion promoter application to create a single-stage process for the pre-treatment of strip galvanised steel sheets prior to bonding. Firstly, the basic effects of CO2 snow blasting and manual application of adhesion promoter were identified independently of each other. Based on this, the combined single-stage pre-treatment was validated. By correlating surface analytical characterisations with the results of destructive tests and resistance tests, a reliably controllable pre-treatment process for semi-finished strip galvanised steel products could be derived. For both electrolytically galvanised (ZE) and hot-dip galvanised steel surfaces (Z, ZM), a reproducible increase in strength and resistance of bonds with room-temperature curing adhesives can be achieved through single-stage pre-treatment. The key criteria for achieving durable adhesive bonds are the intensity of the CO2 snow jet treatment, which influences both the cleaning performance and physical activation of the surfaces, and the dosing quantity of the adhesion promoter as part of the simultaneous application. The results of the research project make it possible to provide a fully automated adhesive pre-treatment process for semi-finished galvanised steel strip at a reasonable cost, even for SMEs, without the need for subsequent expensive corrosion protection coatings, thus opening up a wide range of industrial applications for steel. A practical guideline also guides the optimisation of the pre-treatment process for the respective application and also shows methods for process-related monitoring and control of the production of long-term resistant bonds. The pre-treatment process developed is superior to traditional hand-guided processes, particularly in terms of process reliability. Publication: 2019 Authors: Univ.-Prof. Dr.-Ing. P. L. Geiß

P 1138 - Development of an externally stressed test method for determining the susceptibility of high-strength steels to cracking during resistance spot welding The ever-increasing demands for lower fuel consumption, reduced pollutant emissions and increased passive safety have presented the automotive industry with challenges. To meet these challenges, the consistent use of modern steel materials has proven to be an effective approach. However, the complex microstructure and the special mechanical properties of these materials cause an increased tendency for cracks to form, particularly in resistance spot welded joints. The risk of not being able to fulfil their client's specifications with regard to the quality of the spot-welded joints is particularly high for suppliers, who are often small and medium-sized enterprises, when new materials are introduced. With the help of the test method developed in this research project, this risk can be reduced without extensive time-consuming and costly preliminary tests. In addition to experimental tests based on the test method developed in the previous project (P921), numerical simulation is used for this purpose. The conditions that prevail during the occurrence of liquid metal-induced cracking, namely the temperature, stress and strain field, are analysed. Forming simulations are used to analyse the component condition along the process chain for case studies. Subsequent welding simulations of the case studies are checked for previously determined critical conditions. The results provided enable the assessment of the processing safety of material/component geometry combinations. The procedure is documented so that user-specific combinations can be analysed independently. Publication: 2020 Authors: Prof. Dr h.c. Dr.-Ing. E. Uhlmann

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