Papers by Author: Wolfram Fürbeth

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Authors: Christian Thyssen, David Holuscha, Jens Kuhn, Friederike Walter, Wolfram Fürbeth, Wolfgang Sand
Abstract: Bioleaching and biocorrosion are based on similar biochemical processes. Microbe-surface interaction, biofilm formation and concomitant extracellular polymeric substance (EPS) production gained increasing interest in the past decades. Nowadays it is generally accepted that biofilm formation and an accompanying formation of manganese oxides by manganese oxidizing bacteria such as Leptothrix spp. account for one type of pitting corrosion of stainless steel (SS). However, little is known about biofilm formation, EPS composition of manganese oxidizing microorganisms and their influence on microbiologically influenced corrosion. Consequently, we studied biofilm formation of Leptothrix discophora, the biooxidation of manganese in biofilms on floating filters as well as biofilm formation on stainless steel and the involved corrosion processes. Cells were visualized by epifluorescence (EFM) or confocal laser scanning –microscopy (CLSM). Additionally, the influence of biofilm formation and biooxidation of manganese by L. discophora on the open circuit potential (OCP) and pitting potential (Epit) of stainless steel was measured using a 3 electrode setup. L. discophora grew well in biofilms on floating filters and on SS coupons and incorporated in both conditions Mn2+ in the form of MnO2 from the bulk phase into the biofilm. OCP measurements of actively manganese-oxidizing biofilms on stainless steel showed a significant ennoblement of ≥200 mV.
Authors: Benjamin Strass, Guntram Wagner, Christian Conrad, Bernd Wolter, Sigrid Benfer, Wolfram Fürbeth
Abstract: Fusion welding of dissimilar metals is in the most cases difficult or even impossible as a result of different melting points and the development of undesirable brittle intermetallic phases. This often leads to joint strengths considerable below the tensile strength of the base materials. By using Friction Stir Welding (FSW) it is possible to reduce the development of the intermetallic phases of Al/Mg-joints significantly but not to avoid them completely. Hence a hybrid welding system at the WKK of the University of Kaiserslautern was developed called “Ultrasound Supported Friction Stir Welding (US-FSW)” with the aim to shatter the brittle interlayer lines and to scatter fragments in the welding area during the FSW process. Pre-investigations have shown that for Al/Mg-US-FSW-joints the strength can be increased up to 30% in comparison to conventional FSW. Moreover for the reliable detection of nonconformities in the weld during a post-process inspection by suitable non-destructive testing (NDT) methods is necessary. Also there is a strong need for better process monitoring and control by in-process NDT methods. Furthermore the corrosion behavior of the basic materials and hybrid-joints was investigated by electrochemical methods indicating an increased corrosion of the Mg alloy in the area of the Al/Mg-butt weld.
Authors: Marco Thomä, Guntram Wagner, Benjamin Straß, Bernd Wolter, Sigrid Benfer, Wolfram Fürbeth
Abstract: The innovative joining process of friction stir welding (FSW) offers a wide range of advantages for welding similar as well as dissimilar materials. Even for the field of poorly weldable material combinations like aluminum to steel with their strongly differing physical properties the method of FSW proved its capability for realizing dissimilar joints with tensile strengths up to more than 80 % of the aluminum base material. Trying to improve this value and other properties of the joints several approaches for hybrid friction stir welding processes were tested in the scientific community, whereas the ultrasound enhancement of FSW (USE-FSW) looked as one of the most promising reaching good results. To gain a deeper knowledge of the influence of the ultrasound on the friction stir welds different investigations were carried out in this paper. Therefore the method of USE-FSW was applied on two dissimilar aluminum/steel-joints with varying carbon content of the steel in this work. The material combinations AA6061/SAE1006 and AA6061/SAE1045 were welded successfully with and without additional power ultrasound. Afterwards a comparison between FSW-and USE-FSW-joints was carried out regarding the microstructure of the nugget and interface (IF) by light-microscopy as well as scanning electron microscopy. Furthermore the mechanical properties were characterized in a first step.
Authors: Florian Feil, Wolfram Fürbeth
Abstract: We developed multilayered, purely inorganic coatings for the corrosion protection of AZ magnesium alloys. Polymeric acid-catalyzed sols form relatively dense coatings, but any direct contact to the reactive magnesium substrate has to be avoided. However polymeric sols based on SiO2, B2O3, Al2O3, ZrO2 and up to 5% of lanthanide salts can be used to seal samples with prime coat based on aqueous nanoparticle dispersions. Without organic network modification, these sealings have to be kept thin to avoid cracks. However if the coating process with aqueous dispersions and polymeric sols is alternated, a kind of lamellar sandwich structure can be formed which stays crack-free up to several layers. The performance and the protective properties of these coatings were studied with different methods (EIS, salt spray tests and electron microscopy).
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