Papers by Author: Friedrich Wilhelm Bach

Abstract: Due to its low density and good mechanical properties Magnesium holds a high potential for design applications. The investigations discussed herein focus on the use of extruded magnesium sheets as semi-finished products e.g. for subsequent processing by roll forming. Special interest is given by the hcp-structure of magnesium which reduces forming abilities. Extrusion experiments using AZ31 were carried out to investigate the influence of different die geometries (S = sharp-edged and R= radius), billet temperatures (TB= 350°C and 390°C) and extrusion speeds (vext= 2.31 m/min and 9.24 m/min) on mechanical properties, grain structure and texture of said alloy. Two types of sheets with the dimensions of 80 x 1 mm and 80 x 2 mm respectively were produced using the institutes 10 MN extrusion press make SMS MEER. Sheet material with a thickness of 1 mm rolled from cast billets has been used for reference. Microstructural analysis focused on recrystallization behavior and grain size as well as grain distribution within the sheets. Tensile tests in extrusion or rolling direction as well as 45° and 90° to the same showed a strong influence of the specimen orientation on the mechanical properties. Additional Erichsen cupping tests demonstrated the stretch-forming capacity of the different test set-ups.

Abstract: When manufacturing joints of dissimilar materials thermal technologies as welding reach their limits. Impact welding by electromagnetic forming is a promising alternative because undesired heating of the parts and related disadvantages are avoided. In this process impact parameters need to be adjusted to each specific joining task, but cannot be settled directly. Thus, a two-step methodology is suggested for the process design: First the influence of the impact parameters and the surface preparation on the joint properties is investigated using a model experiment. Joint properties are characterized by metallographic investigations. Parallel to this, the influence of the adjustable process parameters and the equipment on the workpiece acceleration and the impact properties is analyzed. Then the results of both investigation paths are combined and conclusions regarding a target-oriented adjusting of the impact parameters via the process parameters are drawn. In the paper first results considering the model experiment and the analysis of the electromagnetic expansion process are presented and joints manufactured by electromagnetic expansion are characterized.

Abstract: In modern, highly loaded structures of metallic materials, particular standards are required both regarding the strength and deformation properties of individual components as well as in terms of the load carrying capacity of the bonded joints between individual components. This specific subject matter is taken up in the subproject C4 "Setting of Gradient Material Properties and Quality Control of High Tension 3D NVEB-Weld Joints" within the framework of “Collaborative Research Centre” CRC 675 "High strength, locally modified components and structures" in order to correspondingly and individually adapt a component's properties to the loading profile. For this purpose, components having specifically calibrated strength properties are joined into high quality structures. Although still quite new but powerful, and due primarily to its very favourable welding characteristics, the non-vacuum electron beam welding technology is, in this case, to be further developed with respect to an improvement in beam positioning and weld-seam defect inspection. On the other hand, a well-defined, local heat-treatment is to be carried out on individual, work-hardened components to be able to introduce targeted and structured local, load-oriented microstructural changes into the component: thereby locally changing the component's strength and deformation properties with a view to retarding crack growth.

Abstract: Current research activities at the Institute of Materials Science of the Leibniz Universität Hannover are engaged with the development of a new nitriding-process. This process has the convenience that a nitridation of surfaces is possible under atmospheric conditions so that the device treatment may be performed fractional. With the aid of the transmitted plasma-arc, diffusion able nitrogen is applied at the workpiece surface. This atomic nitrogen diffuses into the device and constitutes special- and iron-nitrides which raises the abrasion- and corrosion resistance.

Abstract: In recent years, suspension plasma spraying (SPS) was investigated regarding the spraying of functional coatings with deterministic abilities. In this contribution the possibilities are discussed to achieve oxide ceramic coatings based on titania by SPS, which show a reduction of their coefficient of friction under dry sliding conditions. Besides the possibility to mix different feedstock suspensions in the process, the achievable low thickness and homogenous microstructure of suspension-sprayed coatings shall allow an operation in fields, where high demands towards the adhesion, cohesion and near net shape are made.

Abstract: In order to produce optically cooperative surfaces to use triangulation measuring systems, the methods of surface and thin-coating technology are resorted to. Among these number physical methods such as PVD technology, (electro-)chemical treatment as, for example, electrochemical etching and electroless coatings as well as mechanical treatments of the components by means of glass-bead and corundum blasting. The processes mentioned are suitable for producing optically cooperative surfaces since, owing to the corresponding test parameters, a change of the component's surface occurs at the sub-micrometer scale. Currently, no indication in the literature is provided that such processes are employed for manufacturing optically cooperative surfaces vis-à-vis triangulation methods.

Abstract: Continuous process brazing in shielding-gas furnaces is tailor-made for manufacturing mass production components by means of brazing technology. Which brazing tasks can actually be carried out by a shielding-gas furnace, depend on many ancillary conditions. In particular these are, besides the component size and joint geometry, the base materials and brazing filler metals as well as the material specific process parameters which are to be maintained in a continuous furnace in order that a process assured brazing can be guaranteed. In this context, the activation of the component's surfaces play a central role for wetting with the braze melt. Within the scope of this contribution, the variables and ancillary conditions concerning this matter are presented and discussed using the brazing process on stainless steels as an example.

Abstract: In the case of welding of T-joints in a special structure, the joining is realized through a total penetration of deck plates. For the deck plate’s thickness over 6 mm, high power MAG welding process should be applied. To help experimental optimizing of this welding technology, a 3D quasi-stationary numerical model was established to predict the penetration and weld form of the high power MAG welding on a thick plate. In the analysis a new volumetric heat source model was put forward which considers the heat directly from the arc und that from transferred droplets separately. Because the weld pool surface under the arc was strongly pressed, the droplet heat source in the model was located under the workpiece surface. The size of the droplet heat source model was determined on the base of physical principles and available experimental data. Using a commercial finite element software the weld form inclusive penetration under different welding parameters was then simulated. Through comparison with the experimental results the presented FE-model was verified.

Abstract: Modern metallic materials used in high loading structures are increasingly being placed under stringent standards with regard to the mechanical strength and deformation properties of their individual parts as well as with regard to the loading capacity of their junctions. In order to determine the strain profile of the individual structural components, the mentioned topics will be taken from the SFB 675 “High tensile, locally manipulated structural components and structures” from the subproject C4 “Setting of gradient material properties and qualification of high-tension 3D-NVEB weld joints”. To this end, the strengthened individual structural components will be heated with an electron beam at defined locations. This is done in order to observe the load related local micro-structure changes and consequently the targeted, structured local changes in the strength and deformation properties of the material. A delay in the crack growth will also be sought after. Additionally, components with specifically designed tensile strength will be welded to high quality structures. At this the young but efficient non vacuum electron beam welding method will be preferably developed, qualified and used because of its good welding properties regarding to an improved beam positioning, process control and weld joint defect detection.

Abstract: The investigation of the basic principles for the production of foamed out sections using magnesium foam for support structures by including the foaming process into the cold forming of sections to produce indiviually locally strengthened components is the subject of this research project. To absorb tensile stress, the metal foam will be strengthened with three-dimensional branched struts of high-tensile materials. The quantification of the influence of locally introduced foaming elements on e.g. stiffness alterations and the influence of the resonance frequency of the total structure will be effected by destructive but particularly also by nondestructive tests.