P 1135 - Hot moulding process integrated surface structuring for hybrid components

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

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 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 1132 - Extension of the fatigue design of K-nodes with gap made of RHP and KHP to high-strength steels and thick-walled sections Fatigue-stressed structures are increasingly being manufactured from high-strength steels in order to realise lightweight construction. In truss construction, the K-node with gap is the favoured node configuration. To facilitate the application and design of such nodes made of high-strength steel grades and with greater wall thicknesses, the existing design rules must be adapted and extended. The test programme carried out for this purpose comprises a total of 55 fatigue tests on K-nodes made of rectangular hollow sections (RHP) and circular hollow sections (KHP) of different dimensions made of S355, S500 and S700. K-knots of smaller dimensions are tested at TNO in Delft, Netherlands, on a test rig with individual cylinders. K-knots with large dimensions are tested at KIT in Karlsruhe on a specially manufactured test frame. The experimental tests are analysed according to the nominal stress concept of EN 1993-1-9 (2010) by creating Wöhler curves. In particular, the tests on small KHP K nodes with a welded gap show that the fatigue strength depends not only on the node dimensions and geometric parameters but also on the weld design and geometry. This is not taken into account in the design rules, but can be used for the design of fatigue-compliant structures. For example, the fatigue strength of KHP K nodes with a welded gap can be increased by two notch classes from 71 to 90. In contrast, the small RHP K-nodes investigated are manufactured with weld start and end points in the corners of the strut profiles, which means that the weak point of the weld seam coincides with the hot spot of the geometry. The test results of these test specimens therefore represent a lower limit for the fatigue strength. The fatigue strength of K-nodes depends heavily on the wall thickness of the components and their ratio value <span class="fontstyle3">t</span>. The classification for the nominal stress approach is therefore rightly based on the inverse wall thickness ratio 1/<span class="fontstyle3">Τ = t0 / Τi. The results of the fatigue tests on large RHP and KHP K-nodes made of S355 and S700 with 20 mm wall thickness fit well into the existing notch classes and can be categorised according to CIDECT Design Guide 8 or EN 1993-1-9 (2010) without wall thickness reduction. In parallel, numerical calculations are carried out to extend the scope of validity. The finite element models are calibrated using strain measurements in verification tests. Volume elements are used for this purpose and the weld seams are also modelled in order to enable an exact representation of the local stresses in the weld seam. A parameter study is carried out to extend the parameter range under consideration and to identify the influence of individual geometric parameters such as chord slenderness 2<span class="fontstyle3">y, wall thickness ratio <span class="fontstyle3">t, width or diameter ratio ß or strut inclination angle O<span class="fontstyle3">. For RHP K-nodes, 72 different node configurations are calculated, each with 4 separate load cases. KHP K-nodes are calculated in 16 different dimensions with 3 load cases each, so that a total of 336 FE models are created, calculated and analysed within the parameter study. In addition, stress concentration factors are determined on the basis of the strain measurements in the test. These are then used together with the results of numerical parameter studies to validate or adapt existing SCF formulae. For RHP K-nodes, a re-evaluation of previous and new data for the load case "Axial loading of the strut (AX)" is carried out and suitable formulas with a slight modification of existing CIDECT formulas are provided. For RHP K-nodes with gap under in-plane strut bending (IPB), no formulae are given in CIDECT. A new set of equations has been developed for this purpose. The evaluation of the structural stresses of KHP K-nodes under axial loading of the struts shows that the SCFs according to CIDECT formulas often provide very conservative values compared to the SCFs according to DNV GL-PR-C203 (2016) and from FEA. Since DNV GL-PR-C203 (2016) has a wider range of application and provides SCF formulas for the IPB and out-of-plane bending (OPB) load cases, it makes sense to use these formulas. Only the formula for SCFs for the belt under IPB is modified so that the SCFs are on the safe side. As the fatigue strength of hollow section connections is heavily dependent on the non-dimensional parameters, this should be taken into account at the planning stage by selecting suitable parameters. Through careful planning and execution, fatigue-resistant hollow section structures can be realised, even when using high-strength steels. Publication: March 2020 Authors: Dr.-Ing. S. Herion, Dr.-Ing. Hrabowski, Prof. Dr.-Ing. T. Ummenhofer, Dipl.-Ing. P. Ladendorf, Dr. ir. R. Pijpers