Reaktives Diffusionslöten von Keramik an Stahl mittels Zr–Cu–Zr‐ und Zr–Ni–Cu–Zr‐Schichten für Anwendungen im Hochtemperaturbereich

Joints manufactured by transient liquid phase bonding feature comparable properties as diffusion weldements, but considerably lower process temperatures and pressures have to be applied. The liquid phase, which is hereby used, occurs due to interdiffusion between the base and/or the filler materials at a constant temperature, which lies below the melting temperature of the substrates. An essential requirement for this diffusion‐based melting is that the involved materials have low melting alloy‐constitution areas, such as eutectics. The aim of the study, presented in this contribution, is to evaluate an approach, in which an active transient liquid is created by suitable interlayers, in order to facilitate the wetting of ceramics. The potential of this attempt will be illustrated on zirconia/stainless‐steel‐joints for high temperature applications, such as solid oxide fuel cells. In such applications, the used materials have to withstand harsh conditions, e.g. high operating temperatures, oxidizing or reducing environments, which represent a demanding challenge for joining technologies, even at the latest state of research. In this study interlayers, consisting of Zirconium, as the active element, in combination with Copper and/or Nickel, have been investigated. These systems exhibit a wide range of alloy‐constitutions with low melting temperatures, which can be used for the formation of the transient liquid phase. For the application of the interlayers, physical vapor deposition as well as 75 µm‐thick Nickel‐foils have been used. The joining was carried out in high vacuum with changing holding times and temperatures. Additionally, the ratio of the thickness of the used interlayers was changed. Results of microstructural investigations, nano‐hardness measurements of the joining area as well as shear strength and fractography are presented.     

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].     

Aktivlöten von keramischen Segmenten für den Einsatz in verschleißkritischen Bereichen von Schmiedegesenken

Active brazing of ceramic inlays for the application in wear critical areas of forging dies The use of reinforcing ceramic segments in forging tools is investigated and has been successfully tested with model of dies recently. With reinforcing ceramic segments, however, the thermal widening of the steel tool is a major problem for forging dies. Further, only rotationally symmetrical ceramic inserts can be used as reinforcements which restricts the shape capabilities in tool design significantly. A considerably greater design flexibility is possible if the ceramic segments are brazed into the die body material. To this end, reactively brazed ceramic‐metal composites are to be developed and tested for feasibility in the forging process.     

Auftraglöten zum Verschleißschutz von Titanwerkstoffen

Wear Protection of Titanium using Surface Brazing Titanium and titanium alloys possess high specific strengths up to a temperature of about 600 °C in addition to an extraordinary corrosion resistance [1]. The low wear resistance constitutes a crucial impediment for a much broader use. Titanium materials are especially susceptible to friction fatigue and erosion. Coating techniques have to be developed in order to counteract this technical constraint. Surface brazing presents a promising approach. Hard metals mixed with brazing filler metals on a silver and titanium basis were brazed in a vacuum furnace and subsequently characterized. Wear resistance was quantified and optimized using ball on disc measurements.     

Betrachtungen zum Löten in Gegenwart und Zukunft

Consideration on brazing and soldering today and in future In all parts of industries using metals, the application of brazing and soldering is increasing. The origin of those technologies is assumed 6.000 years ago, and for a long time, they were limited to a few industrial fields only. In recent years, they turned out to become a source of innovation. Today, they are essential in different industries as microelectronics, electrical engineering, automotive industry, aviation industry, turbine industry or tool industry. Starting with defining brazing and soldering, especially in comparison to competitive joining techniques, some selected developments of the last decades are presented, as brazing of Aluminium, High Temperature Brazing and brazing of ceramic materials. Finally, an outlook is given concerning ongoing and future developments.     

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.     

Modellierung der Stängelkristallitbildung beim Hartlöten von Kohlenstoffstählen mit Kupfer

Simulation of columnar crystallite formation in brazed seams of copper‐brazed carbon steels When brazing steels of different carbon content with copper filler metal, columnar crystallites form on the carbon‐rich iron surface if the width of the brazing gap is smaller than 100 μm. Braze seams with such microstructures were described as early as the 1950ies and it was found out, that the strength of such a joint is significant enhanced, if this crystallites penetrate the entire seam. Extensive experimental investigations in recent years confirm, that the final average length of the crystallite increases superproportionally with decreasing brazing gap width and is almost inversely proportional to the difference in carbon content of the joined steels. Although many attempts to explain this phenomenon are known from literature, the mechanism of columnar structure formation has not been clarified properly until now. The aim of the present work was to develop an appropriate physical model, that describes the growth of crystallites as a function of carbon content in the base materials, the initial brazing gap width and the applied process parameters (temperature, time). The model is an appropriate tool for a general choice and development of filler metal‐base material combinations forming columnar crystallites in the braze seam.     

Verschleißfeste Beschichtungen von Aluminium‐Stahl‐Hybridstrukturen mittels thermischer Spritztechnik

Spraying of wear‐resistant coatings of aluminium‐steel‐hybrid‐structures By means of high velocity oxy fuel flame spraying of aluminium‐hybrid‐structures consisting of a Al Zn 5,5 Mg Cu 1,5 (ENAW7075) and a NiCrBSi‐coating as well as a Cr₃C₂ 25NiCr‐ coating are manufactured. The hybrid composite structures are analysed and compared with each other regarding to hardness, surface roughness, wear‐resistance and coating density.     

Ausgewählte Ergebnisse der Untersuchung der Prozesse des Schmelzens und der Erstarrung beim Schmelzschweißen und Schmelzlöten mittels der materiellen Modellierung

In the article a method for the material modelling is described under application of visually transparent materials of the thermal and chemical solidification processes when welding and soldering. All solidification processes carried out independently of the form of the primary crystals, first about the planar one, then cellular and in the end dendritically growth the solidification structure. It is determined by the chemical composition of the weld and soldering good, the weld, soldering, crystallization or solidification speeds and by the temperature gradients at the solidification front particularly. The vacancy concentration is made by the temporary formation of so‐called zipper primary grain boundaries in the solidification structure when welding. Among other things a preferential growth direction of the primary crystals explains itself by the fast growth of so‐called victim crystals which completes the known theories of the preferential growth.     

Einfluss der Beschichtungen beim stoffschlüssigen Lichtbogenfügen von Stahl mit Aluminium

Influence of the coating by bonded arc joining of steel to aluminium The firmly bonded joint between steel and aluminium has, so far, not been achieved successfully in joining technology. The problems of joining these metals thermally are caused by the low up to non‐existing solid solubility of the metals and by the development of brittle intermetallic phases. The quality of the joints depends on many influential factors which are, sometimes, also interacting. One of those influential factors is the coating of the steel sheets. Using the example of coatings which are common and others which are unusual in automotive engineering it has been tried to demonstrate the influence on the wetting length and wetting angle of those factors with, otherwise, constant boundary parameters. The results are part of a test series which is currently carried out at the Joining and Welding Institute, Aachen.     

Erforschung Ti–Co‐basierter,bioaktiver Auftraglötschichten auf oxidischen Hochleistungskeramiken in der Medizintechnik

The demand of prostheses and implants made from biomaterials grows as a result of the rising age of patients. For biomaterials, such as those found in joint‐ or hip‐prostheses, that are in direct contact with the organism, not only mechanical stability is required, but also biocompatibility as well as their ability to support bone regeneration. Taking this into account, a thin‐walled bioactive titanium cobalt‐based brazing coating on high‐performance oxide ceramics (Al₂O₃) has been developed. Here, the coating process offers an economical and at the same time technologically simple way for the coating ceramic materials. The biocompatible coating has been enhanced by addition of bioactive particles made of bioglass and calcium phosphates in order to improve bone formation. The reactions between the bioactive particles and the brazing alloys, as well as the particular melting behavior, were determined through thermo analytical methods. The structures of the brazing alloys enriched with bioactive particles were investigated through metallographical methods. The combination of three bioactive additives and two brazing alloys were analyzed in terms of their melting behavior and the resulting porosity, the parameters of the brazing process have been gradually optimized. The results show, that the combination of calcium phosphate particles and Ti–Co alloys effectively meet the requirements for a defined porous, biocompatible brazing coating.     

Löten von Diamant – Grenzflächenreaktionen. und Benetzung

Diamond Brazing – Interfacial Reactions and Wetting Diamond tools are increasingly gaining importance as cutting materials for various construction materials. The quality of synthetic diamonds, monocrystalline as well as polycrystalline or CVD‐diamonds has been significantly improved over the last years. Integrating these cutting materials requires adequate joining technologies that produce sound joints without exposing the temperature sensitive diamond to too elevated temperatures. The paper highlights current developments in the joining of synthetic diamonds to steel and cemented carbide. Owing to their covalent atomic bonding diamonds cannot easily be wetted and joined by employing conventional brazing alloys. Hence, active agents are needed to foster an interfacial reaction. Different active filler concepts are presented and discussed regarding their joint formation. The brazing temperatures influence not only possible diamond degradation but also the interfacial decomposition of the diamond due to the formation of corresponding reaction layers.Active brazing, monocrystalline     

Korrosionsuntersuchungen an mittels PVD mit Zn und Zn/Mn beschichteten Stählen

Corrosion Studies of Steels Coated by means of PVD with Zn and Zn/Mn Alternative methods for hot dip‐ or electrogalvanic deposition of zinc coatings on steel are gas phase depositions (PVD). They posess high flexibility with respect to alloy composition, and are environmentally harmless. However, a PVD‐coated steel must have at least the same corrosion resistance than steels with “classical” surface finishing. Therefore, the corrosion behaviour of Zn‐coatings and Zn/Mn/system‐coatings deposited by electron beam evaporation without and with ion beam assistance (IBAD) on low alloy steel, was determined by means of salt spray test and electrochemical potential/time measurements. At first the influence of chemical and irradiation pre‐treatment and ion bombardment during deposition on the corrosion resistance of the coatings was investigated. Than the effect of the Zn‐layer thickness was determined in comparison with an 8μm thick electrogalvanized reference coating. Finally Zn/Mn‐alloys, Zn/Mn‐multilayers and Zn‐coatings with Mn‐ or Zn/Mn‐surface layers (top layers) were investigated. By means of optimised pre‐treatment and ion bombardment conditions one obtains, considering the layer thickness, PVD‐Zn coatings with corrosion resistance comparable with the reference layer. The best Mn‐containing coatings are Zn‐coatings with Mn‐toplayer. They surpass the corrosion resistance of the reference layer considerably. Additionally it could be shown that in tendency the potential/time measurements agree very well with the results of the salt spray test.     

Innovative Lösungsansätze für das Fügen von AMCs

Innovative solutions for joining AMCs The application of aluminium matrix composite materials (AMC) in structural components requires appropriate joining technologies. One important aspect is the preservation of the properties of the composite during the joining process. The thermal load of the base material is particularly critical as a result of the process‐related heat input. In order to keep it low, adapted joining technologies and low‐melting fillers are essential. The combination of an ultrasound process and an induction heating offers various advantages. On the one hand, the ultrasound introduction achieves a destruction of the surface oxides and therefore a wetting of the joining surfaces without the use of flux. On the other hand, local inductive heating allows to realise very short joining times so that the heat input into the base materials to be joined can be reduced to a minimum. The application form of the fillers plays an important role for the achievable compound strengths. Selected test results will show the advantages and disadvantages of different variants.     

Löten und Härten im Vakuum zur Herstellung leistungsfähiger Gesteinwerkzeuge

Brazing and hardening in vacuum for manufacturing of high‐performance tools for construction applications High‐performance tools for applications in construction industry are used under severe service conditions. Huge amounts of construction materials have to be removed in short times with long service times. By sophisticated heat treatment processes or combination of materials construction tools are optimized for the dedicated application. Vacuum brazing is a proven technology to join different materials. Case studies will be presented where vacuum hardening of tool steel and vacuum brazing of hard metal to steel improve tool performance significantly.     

Fügeverfahren im Vergleich: Löten,Kleben, Mechanisches Fügen von Magnesium

Comparison of Joining Techniques: Soldering, Adhesive Bonding, Mechanical Joining of Magnesium Due to their light weight construction potential magnesium sheets are of increasing interest during recent years. During the production process appropriated joining technologies are necessary for sheets to obtain various knot, profile, and sheet structures. Thus studies to develop appropriated joining technologies are of high importance.     

Einfluss der Werkstoffzähigkeit auf das Festigkeitsverhalten unter mehrachsiger Beanspruchung am Beispiel von Schweißverbindungen aus Stahl und Aluminium

Influence of Ductility on the Multiaxial Fatigue Behaviour by the Example of Welded Joints of Steel and Aluminium The multiaxial fatigue behaviour of materials with different ductility under constant and changing principal stress directions is also applicable to welded joints of different materials. For this, welded flange tube connections of the fine grained steel StE 460 and the artificially aged aluminium alloy AlSi1MgMn T6 were investigated under constant amplitude combined bending and torsion. Out‐of‐phase loading, i. e. changing principal stress directions, of the steel joints led to a decrease of fatigue life, which is observed at ductile material states. However, for the aluminium joints out‐of‐phase loading resulted same behaviour as in‐phase loading, which indicates a semi‐ductile material behaviour. The results for the welded steel joints were evaluated on basis of local stresses by the integral hypothesis of the Effective Equivalent Stress EES (WVS). This hypothesis for ductile material states takes into account the life decreasing influence of out‐of‐phase loading by considering the interaction of the shear stresses on different planes. The fatigue behaviour of the aluminium welds is described by the critical plane based combination of shear and normal stresses (KoNoS), which is valid for semi‐ductile material states.     

Schwingfeste Auslegung von Schweiß‐ verbindungen aus der Magnesiumknetlegierung AZ31(ISO‐MgAl3Zn1) mit dem Konzept der fiktiven Ersatzradien von rf = 1,0 mm und 0,05 mm

Fatigue design of welded joints from the wrought magnesium alloy AZ31 (ISO‐MgAl3Zn1) by the local stress concept with the fictitious notch radius of r_f = 1.0 mm and 0.05 mm The investigations were carried out with three different types of MIG‐ and TIG‐welded magnesium joints of the alloy AZ31. The evaluation of the results showed that the local stress concept using the fictitious notch radius of r_f = 1.0 mm can be applied to magnesium welded joints from plates with thicknesses t ≥ 5 mm independently of the weld geometries (fully or partially penetrated butt welds, transversal stiffeners). Design curves are proposed for different stress ratios, i.e. R = ‐1 as well as 0 and 0.5, which allow the consideration of residual stresses as well as load induced mean stresses. The results permit also the suggestion of Δσ = 28 MPa as FAT‐value for the IIW‐Fatigue Design Recommendations. Further, the FAT‐value Δσ = 73 MPa for the fictitious radius of r_f = 0.05 mm to be applied to welded thin magnesium joints is derived, too. These FAT‐values are compared with already known data for steel and aluminium joints. A linear relationship between the FAT‐values and the Young’s modulus is determined.