Alkoholatkorrosion von Aluminium (EN AC‐48100), Einfluss von Temperatur sowie Ethanol‐ und Wassergehalt

The corrosion behaviour of the aluminium alloy EN AC‐48100 (Al‐17wt%Si‐4wt%Cu‐Mg) was investigated in ethanol fuels with different contents of water (0.05 vol.‐%, 0.2 vol.‐%, 0.35 vol.‐%), at different temperatures (20°C, 60°C, 80°C), and with different contents of ethanol (10 vol.‐%, 25 vol.‐%, 50 vol.‐%, 85 vol.‐%). A interdependency of the corrosion with the temperature and the water content in the fuels was determined. At a temperature of 80°C in an ethanol fuel with less than 0.05 vol.‐% water, strong corrosion starts soon after immersion. An influence of the ethanol content to the degree of corrosion was determined for fuels with ethanol contents below 25 vol.‐% notably. Based on the observations and measurements, a theory for the multi stage character of the corrosion mechanism of aluminium alloys in dry ethanol fuels was developed.     

Verschleißmechanismen von CrAlYN PVD Schichten bei erhöhten Temperaturen

Wear mechanisms of CrAlYN PVD coatings at elevated temperatures Over the past years there have been developments in machining operations to replace the use of liquid cooling by dry cutting operations. This results in higher loads for the tools and therefore demands the continuous development of surface coatings. CrAlYN‐coatings designed at the Institute for Materials Technology (IfW) in Darmstadt, Germany, show beneficial tribological properties especially at high loads and high temperatures. Samples of different yttrium content have been distinguished and compared to an industrial TiAlN coating. Even though the configuration of the CrAlYN coatings is not yet as optimized as that of the TiAlN, which leaves plenty of room for improvement, the CrAlYN shows comparable and partly superior properties to the TiAlN coating. An increasing yttrium‐content of the CrAlYN leads to a more amorphous microstructure and to finer and denser morphology with a smoother surface. The TiAlN on the other hand exhibits a porous, columnar structure. This results in advantages in the wear behavior as well as in the oxidation resistance. Furthermore, a distinct improvement of the adhesion of the layer to the carrier is determinable, which results in higher sustainable loads at the scratch test.     

Verhalten von Hochtemperatur-Lötverbindungen bei schwingender Belastung und erhöhten Temperaturen

Fatigue behaviour of High Temperature Brazed Joints tested at elevated temperatures Fatigue testing, on a systematic and statistical basis, was carried our on high temperature brazed joints produced in the base metal NiCr20TiAl joined with BNi-5 and BAu-4 filler metals. The single-stage tests were conducted with various temperatures of 500 and 700 °C. Microscopic examination of the fracture surfaces (SEM) yielded information about crack initiation and propagation and the fracture behaviour of the joints.     

Untersuchungen zur Haftreibung zwischen faserverstärktem Borosilicatglas und Kalk-Natron-Glas bei erhöhten Temperaturen

Investigation of static friction of fiber reinforced borosilicate glass mated to soda lime glass at elevated temperatures Tribological tests were carried out on a soda lime glass mated to SiC- and C-fiber reinforced borosilicate glass. The static coefficient of friction and the transition to sliding were measured using a ring-on-block geometry at temperatures between 25 and 500 °C. The fiber orientation (parallel or antiparallel) refering to the direction of sliding was varied. The static friction coefficient decreased with increasing temperature for both the SiC and the C-fiber reinforced glass owing to increasing tendency for adhesion and resulting material transfer. At temperatures up to 300 °C, there was no difference between the static and the kinetic coefficient independent of the material, whereas the static friction coefficient was lower for parallel than antiparallel fiber orientation. At test temperatures above 300 °C and antiparallel fiber orientation, stick-slip effects occurred under the experimental conditions used. The tribological mechanisms could be explained by SEM (scanning electron microscopy) studies of the worn surfaces.     

Verhalten laserschockverfestigter und festgewalzter Randschichten der Ti‐Legierung Ti‐6Al‐4V bei schwingender Beanspruchung unter erhöhten Temperaturen

Residual stress stability and near‐surface microstructures in high temperature fatigued mechanically surface treated Ti‐6Al‐4V It is well known that mechanical surface treatments, such as deep rolling, shot peening and laser shock peening, can significantly improve the fatigue behavior of highly‐stressed metallic components. Deep rolling is particularly attractive since it is possible to generate, near the surface, deep compressive residual stresses and work hardened layers while retaining a relatively smooth surface finish. In the present investigation, the effect of deep rolling on the low‐cycle and high‐cycle fatigue behavior of a Ti‐6Al‐4V alloy is examined, with particular emphasis on the thermal and mechanical stability of the residual stress states and the near‐surface microstructures. Preliminary results on laser shock peened Ti‐6Al‐4V are also presented for comparison. Particular emphasis is devoted to the question of whether such surface treatments are effective for improving the fatigue properties at elevated temperatures up to ～450 °C, i.e., at an homologous temperature of ～0.4 T/T_m (where T_m is the melting temperature). Based on cyclic deformation and stress/life (S/N) fatigue behavior, together with the X‐ray diffraction and in situ transmission electron microscopy observations of the microstructure, it was found that deep rolling can be quite effective in retarding the initiation and initial propagation of fatigue cracks in Ti‐6Al‐4V at such higher temperatures, despite the almost complete relaxation of the near‐surface residual stresses. In the absence of such stresses, it is shown that the near‐surface microstructures, which in Ti‐6Al‐4V consist of a layer of work hardened nanoscale grains, play a critical role in the enhancement of fatigue life by mechanical surface treatment.     

Schwingfestigkeit der keramikverstärkten Kolbengußlegierung GP-ALSi 12 CuMgNi bei erhöhten Temperaturen

Fatigue Strength of Fibre-Reinforced Squeeze Cast Piston Alloy GP-AlSi 12 CuMgNi Under Elevated Temperatures On specimens made of squeeze-cast piston alloy GP-AlSi 12 CuMgNi without reinforcement and Al₂O₃-fibre-reinforced, thermal fatigue was investigated. Several mechanical properties such as tensile and compressive yield strength, incremental-step-method based monotonic and cyclic stress-strain curves in the temperature range of 20 to 400°C were determined. Moreover, fatigue tests in the range of 5 · 10⁴ and 2.5 · 10⁷ were carried out for various stress ratios in the same temperature range on unnotched specimens and S-N-curves were designed. Haigh-diagramms show the mean stress sensitivity of the materials. Obviously the fibre reinforced material is superiour at all regarded temperatures. The considerable gain of material strength obtained by fibrereinforcement entails even better conditions for the use of newly developed pistons under increased thermal load.     

Einfluss der Oberflächenbehandlung auf das Korrosionsverhalten von Magnesiumlegierungen

For the coating of magnesium alloys, especially those that should be used in medical technologies, fluoride coatings are tested and used. These coatings should enable a predictable degradation behavior for future implant materials. Presently the pre‐ and post‐treatment processes vary, so an optimum to achieve defined thicknesses of the fluoride layers is still not established. The present investigations are based on the process parameters known from the literature, they are discussed and compared. Different pre‐treating times of the base material (pure magnesium and a magnesium‐aluminum‐calcium alloy as a comparison) in 200 g/l sodium hydroxide combined with 96 hours of immersion in 40 % fluoride acid are examined.     

Korrosionsverhalten binärer Magnesium‐Zink‐Legierungen in salzhaltigen Medien

Aluminum is a prevalent alloying element to improve the corrosion resistance of magnesium alloys. Recent investigations found out that alloying small amounts of zinc can decrease the corrosion rate of binary magnesium‐zinc alloys. Based on these results low‐alloyed MgZn‐alloys have been analyzed in gravimetric and electrochemical measurements using varied chloride containing media. It was detected that small amounts of zinc have positive effects on the corrosion behavior; in low as well as high concentrated sodium chloride solutions. The overall tendencies of the results from the gravimetric and electrochemical investigations can be correlated. This was underlined using micro‐tomography analyses.     

Korrosion durch Biokraftstoffe – Schutz durch Beschichtungen auch bei zyklischer Beanspruchung

Corrosion by biofuels ‐ Protection by coatings also under cyclic loadings The influence of corrosion on fatigue design must be considered in numerous component parts in the automotive industry. By using appropriate coatings the negative influence of corrosion under static and cyclic loading can be suppressed. Using the aluminum cast alloy AlSi7Mg0,3 T6 as an example, selected material‐coating‐systems were to be characterized under mechanical‐thermal‐corrosive complex loadings and characteristic properties for the fatigue design under cyclic loadings were to be determined. Conditioning tests were carried out with uncoated, anodized and chemical‐nickel‐plated specimen at different temperatures in the media fuel E5 (fuel with 5 % ethanol added) and E10 (fuel with 10 % ethanol added) and followed by SEM examined. The cyclic fatigue tests with chemically‐nickel‐plated and tempered aluminum in the fuel E5 at 100 °C no reduced influence on the fatigue strength, within the scattering, was discerned. This result agrees with the conditioning tests in the fuel E5 at 100 °C, where no corrosion was observed. Despite the coating of the specimens salt spray fog leads to a clear reduction in the fatigue strength. Using a damage accumulation calculation, it could be shown, that the real damage sum of the callipers and specimens, despite different types of coating, are comparable.     

Production and properties of micro‐porous glas bubble zinc and aluminium composites

Micro‐porous syntactic foams were produced by means of integration of glass bubbles into aluminium and zinc matrices. Preforms of glass bubbles were pressure infiltrated with the alloys AlSi9Cu9 ans ZnAl4Cu using squeeze casting. The preforms were sintered thermically without the use of bonding agents. Using the combination of different sintering steps syntactic foams with locally different densities could be produced. The mechanical properties of the foams were tested indicating a high compression strength of the foams and a very good compression energy absorption. Furthermore, corrosion behaviour and behaviour at higher temperatures were investigated.     

Neuartige Sandwichverbunde – Herstellung,Umformverhalten, Fügen und Korrosionsverhalten

New Sandwichmaterials – Production, Forming, Joining and Corrosion Behaviour New sandwich structures with sheet metal of CrNi‐steel and core layer made of polypropylene were examined with regard to the processing and corrosion characteristics. Practicable solutions for bonding sandwich structures are represented with selected examples of bonding variants. Joining of the sandwich structures was carried out by laser welding and in form of hybrid junctions by adhesion and welding. It has to be taken into account that construction units are transformed before joining. The joint if possible does not lie in the transforming range if it is joined first. In the 3 roll bending test tubing elements up to a process‐bound diameter of 70 mm were manufactured. As expected the corrosion resistance is affected negatively by the oxide scale due to welding at the fusion lines. The characteristics of the basic material were regained after removing the oxide scale. A sensitization to intergranular corrosion was not detected. With correct subsequent treatment the finished sandwich structures are sufficient for the appropriate requirements.     

Aluminium alloys exposed to borderline solutions – a methodology to evaluate the susceptibility to corrosion fatigue with respect to corrosive de‐icers

In addition to high mechanical loads certain components for the automotive industry are exposed to corrosive environments, especially during winter, when corrosion promoting de‐icers are essential to sustain road traffic. The underlying research work contributes to the evaluation of the corrosion fatigue performance of aluminium alloys relevant for automotive application generally, while the present text focuses on wrought alloys. Aluminium alloys are of special interest because of the opportunity to reduce expenses for an additional corrosion protection and to support lightweight construction of vehicles. Components exposed to simultaneous mechanical and corrosive service loads as well as corrosion tests of different scale level (immersion and cabinet testing, long term outdoor exposure of components, usage on test carriers) were analysed to evaluate typical forms of corrosion. Based upon the prevailing damaging mechanism of material conditions with known differences in susceptibility to specific forms of corrosion, like intergranular corrosion or pitting, methods were derived to evolve service relevant types of corrosion during laboratory tests.     

Verhalten geschweißter Magnesiumlegierungen unter mechanisch‐korrosiver Komplexbeanspruchung

Corrosion and corrosion fatigue of welded magnesium alloys In addition to the prevalent use of magnesium cast alloys a high potential for lightweight constructions is offered by magnesium‐wrought alloys, in particular in the automobile industry. The use of rolled and/or extruded magnesium alloys (profiles and sheet metals) requires suitable and economic join technologies like different welding procedures in order to join semi finished parts. Thus, the realization of lightweight constructions asks for high standards of materials‐ and joining‐technologies. In this context, the mechanical properties as well as the corrosion behaviour of the joints are of large interest. During welding of magnesium alloys, influences concerning the surface, the internal stresses and the microstructure occur. These influences particularly depend on the energy input and thus, on the welding procedure as well as the processing parameters, which all affect the corrosion behaviour of the joints. Sheets of magnesium alloys (AZ31, AZ61, AZ91) were joined with different welding procedures (plasma‐, laser beam‐ and electron‐beam welding in the vacuum and at atmosphere). The corrosion behaviour (with and without cyclic mechanical loading) of the welded joints was investigated by different methods such as corrosion tests, polarisation curves, scanning electron microscopy and metallography. Furthermore, substantial influencing variables on the corrosion behaviour of welded joints of magnesium alloys are pointed out and measures are presented, which contribute to the improvement of the corrosion behaviour.     

Herstellung,Mikrostruktur und Korrosionsverhalten der konventionellen und ultrafeinkörnigen Legierung EN AW‐6082

Fabrication, microstructure and corrosion behaviour of the conventional and ultrafine‐grained AA6082 The effect on the corrosion behaviour of the commercially available AlSi1MgMn alloy (AA6082) with ultrafine grains in comparison to coarse grains, i.e. the conventional material state was investigated. The results of the electrochemical experiments are presented in correlation to the microstructure before and after the corresponding examinations. The quantification of the induced corrosion damage as well as the corrosion characteristics shows the reduced susceptibility of the material with ultrafine‐grained microstructure in contrast to the coarse‐grained initial state.     

Diffusionslöten von Aluminium mittels PVD applizierter Lotzusatzwerkstoffe

Diffusion brazing of aluminium by PVD applied filler metals Diffusion brazing of aluminium and aluminium alloys precoated with filler metal components enables fluxless wetting and obtains braze joints of high strength at moderate brazing temperatures. Previously deposited components of filler metals on the base materials as thin film, using Arc‐PVD‐process lead during a subsequently diffusion brazing process to the formation of a local liquid phase (transient liquid phase). The liquid phase is formed from the deposited thin film material and the base material and is solidified isotherm due to diffusion procedures. In doing so braze joints of higher melting point than brazing temperature can be realised. In this work, vacuum brazing of the two systems, Al‐Cu and Al‐Cu‐Si have been investigated. Cu and Al‐Cu‐Si were deposited on the base material using Arc‐PVD‐process. The base materials were pure aluminum and EN‐AW6060. Metallographic and scanning electron microscope analyses proved that the braze seam area after the completed diffusion brazing process shows similar structure and composition as the base material.     

Entwicklung einer neuartigen wirtschaftlichen,eisenbasierten Beschichtung zur Erhöhung der Lebensdauer von Gussbauteilen unter dem Gesichtspunkt der Korrosionsbeständigkeit

Cast irons are widely used to produce parts in rotating machines, e.g. in pumps, due to their excellent casting properties, machinability and good mechanical properties as well as their high economic advantages compared to other materials. However, they have to be replaced by more expensive materials in combination with more complex processes to meet the increased requirements of corrosion and erosion resistance. A more economical approach to increase the corrosion and erosion resistance of components of cast irons is to prevent the parts from corrosive and erosive attacks by means of a protective coating. In this research work, a novel economical iron based protective coating system based on a FeCrMnBC alloy was developed via atmospheric plasma spraying and investigated in terms of its corrosion behavior. The results show a significant improvement compared to uncoated substrate EN‐GJL‐250. This novel coating exhibits a high potential to protect parts of cast irons.     

Corrosion behaviour of cold‐rolled aluminium AA8015‐alloy in natural seawater at 0.18 μm surface roughness

Aluminium is one of the most produced and used metals globally, second to ferrous metals. Its good corrosion resistance is one of the reasons for its heavy usage in typical applications, such as in marine applications. Electrochemical corrosion study of cold‐rolled aluminium AA8015‐alloy at 0.18 μm surface roughness in natural seawater was explored. The aluminium AA8015‐alloy utilized in this study was cold rolled in a reversible Achenbach cold rolling mill in four pass schedules to a thickness gauge of 1.2 mm. A surface roughness of 0.18 μm with three cold mounted samples was achieved on an automated grinding/polishing machine using 320 grit, 800 grit, and 1200 grit SiC paper. Electrochemical corrosion experiments were conducted on the samples in natural seawater using a computer‐controlled potentiostat in an open polarization cell set‐up at room temperature. The corrosion behaviour on surface morphologies of the samples was observed by high mega pixel camera and scanning electron microscope. Findings reveal asymmetric polarization curves for all the samples and energy dispersive X‐ray spectrometry elemental analysis shows the existence of insoluble substrate complexes formed on the surfaces. Consequently, the scanning electron microscope analysis confirms localised corrosion in the mode of pitting.     

Löten von Aluminiummatrix‐Verbundwerkstoffen mit modifizierten Löttechnologien

Soldering of aluminium matrix composites by modified technologies The correct procedure is for the soldering of aluminium‐matrix‐composites very important. Two basic facts must be given. On the one hand the oxides on the surface must be destroyed and removed for wetting. On the other hand the thermal influence on the basic material must be kept down. Otherwise the intended profile of the composites will change. That’s why the work temperature of the solder must be less than 300 °C. The tin‐based solders offer large potentials. However the low strength and creep stability of these solders have a harmful effect. For improving the characteristics oft the solder reinforcement with ceramic particles is meaningful. Presented were selected results of the particles containing solders. Thereby the compound strength and the creep behavior are regarded.     

Flussmittelfreies Hartlöten unter reaktiver Prozessgasatmosphäre – ein alternatives Verfahren zum Fügen von Aluminiumwerkstoffen

Flux‐free brazing under reactive process gas atmosphere – an alternative process for joining of aluminium materials Due to the high affinity of aluminium towards oxygen, joining of aluminium materials has ever been a challenge. In particular the efficiency of the process and the abandonment of fluxes during controlled atmosphere brazing have been within the focus of current research projects. The specific addition of reactive gases to the inert process gas atmosphere offers a suitable possibility of aluminium brazing without the use of fluxes. Under the application of hydrogen chloride the activation of the brazing and the workpiece surfaces is to be initiated, thus leading to dissolving the oxide layers. Moreover, the strongly reducing gas silane is used, which specifically removes oxygen and water residuals from the controlled atmosphere. Through a suitable controlled atmosphere brazing process the combination of both, reductive and activating additions, is to be used and tested upon their influence on the gas mixtures and materials used [1–5].