Elektronenstrahlschweißen von massiven metallischen Gläsern

Electron‐Beam‐Welding of bulk metallic glasses Because of their excellent mechano‐technological properties bulk metallic glasses form a promising, relatively new class of materials. Due to their low thermal stability the weldability of bulk metallic glasses is subject to narrow limits, in case the joining zone shall be prevented from crystallization. The paper at hand describes the status‐quo of the research work on electron beam welding of beryllium‐free, zirconium‐based bulk metallic glasses (Zr_52,5Ti₅Cu_17,9Ni_14,6Al₁₀) carried out at the Welding and Joining Institute at RWTH Aachen University. So far, high quality joints free from defects could be produced, however, it has not beenaccomplished to avoid the crystallization of the joining zone completely. Further research is in progress.     

Fügen von Magnesium und Magnesiumlegierungen

Welding of Magnesium and Magnesium alloys Magnesium is mainly connected by screws. In this paper the results of experiments with different welding processes will be presented. The following methods have been applied: TIG, MIG, Nd: YAG‐Laser and CO₂‐Laser welding, electron beam welding and High Power Diode Laser welding.     

Elektrodenauswahl für das WIG‐Unterwasser‐Schweißen

Selection of electrode for GTA‐Underwater Welding Reproducible good weld quality and economical benefit of underwater‐welding require a complete automation. For this purpose Gas Tungsten Arc‐welding (GTA or TIG) offers numerous advantages, especially for the root and the following hot pass run. Disadvantages of GTA‐welding are the low weld deposit rate and the limited lifetime of the tungsten electrode. Already small wear damages cause wide alterations of the arc under high surrounding pressure, so that a suited choice of the electrode enables to increase the productivity considerably. Therefore the influence of the electrode features on welding process, arc stability, arc ignition, weld geometry and electrode wear has been investigated. For quick and elementary selection of electrode an assessment catalogue was elaborated.     

Fortschritte beim Rührreibschweißen von Aluminium,Magnesium und Stahl

Progresses on the friction stir welding of aluminium, magnesium and steel Friction Stir Welding (FSW) represents an innovative welding process for joining light metal, especially, aluminium and its alloys. Friction Stir Welding offers an attractive alternative to conventional fusion welding processes because of the excellent properties (particularly ductility), reproducibility, robustness, and surface finish obtained with the process. Within the scope of this work the Friction Stir Welding‐Process with its possible joint configurations is explained. The focus of this work concentrates on weldability studies concerning cladded aluminium alloys, aluminium cast alloys, aluminium tailored welded blanks both from similar and dissimilar joints produced in aluminium, magnesium and steel. The mechanical properties of the welded samples will be discussed.     

Investigation on a new process chain of deposition or friction welding and subsequent hot forging

According to the state of the art most current forging parts and technical components are made of mono‐materials. Nevertheless, parts consisting of only one material increasingly reach their specific material and constructive limits in the established production processes. Through use of previously joined raw parts consisting of different materials, it is possible to produce application‐optimized hybrid parts. This paper describes the production chain of hybrid parts produced by combining two different joining processes with subsequent hot compression tests. The joining of various materials is realized by a deposition welding with a laser‐stabilized gas‐metal‐arc deposition welding (LGD) process and a conventional friction welding process. Subsequently, the hybrid samples are compressed under varying forming parameters such as temperature and deformation degrees. In order to characterize the joining zone, metallurgical investigations are carried out.     

Entwicklung des Reibnietens als neues Fügeverfahren für Kunststoff und Leichtbaulegierungen

Development of FricRiveting as a new joining technique for polymer and lightweight alloys The increasing demand on environmental consciousness, cost savings and high performance end products has been guiding scientists and engineers to a constant development of new materials and technologies. This class of lightweight structures are specially used in industrial fields such as transportation and modern civil engineering. Currently available joining methods for polymer‐metal structures (adhesive bonding and mechanical fastening) are usually application‐specific, presenting high operational costs, limited mechanical performance or are not environmental friendly. A new Friction Riveting technique for polymeric‐metallic joints was developed, demonstrated and characterized in this work, as an alternative, reliable, environmental compatible and economically viable spot joining process. In the simplest process variant a rotating cylindrical metallic rivet is inserted in one or more thermoplastic base plates. The high rotation speed and pressure increase friction and heat is generated. When the preset time is achieved the temperature highly increases and the rivet tip plasticizes. At this point rotation is decelerated and the axial pressure increased, so the plasticized rivet tip becomes deformed; after cooling it becomes anchored in the polymeric base plate. In this work case‐study joints on commercially available polyetherimide (PEI) and aluminium 2024‐T351 (Al‐Cu‐Mg alloy) were chosen for demonstrating proposed theories and mechanisms of FricRiveting. Sound friction riveted point‐on‐plate and single‐rivet overlap joints with elevated joint efficiencies in terms of base materials strength were obtained (tensile joint efficiencies of about 97 % of the rivet strength and shear joint efficiencies of about 70 % of the polymer strength) through tensile and lap shear testing at room temperature. Finally, the microstructural changes and properties were described for this case‐study joint. The feasibility of FricRiveting was demonstrated in this work by the presented technical and scientific results. From this work it can be suggested that FricRiveting has the potential to be established as a reliable, simple, cost effective and environmental friendly joining technique for polymer‐metal components.     

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.     

Friction stir welding of dissimilar metal joints

Friction stir welding is a solid‐state welding technology, which is suitable for joining dissimilar metals such as aluminium and copper. Because the solidus temperature is typically not exceeded, the formation of intermetallic phases can be reduced when compared to fusion welding processes. In friction stir welding, the intermetallic layer thickness, which determines the seam properties, is influenced by the welding temperature and is formed in correspondence with the Arrhenius law. It is typically in the range of a few hundred nanometers thick. In turn, the process temperature is determined by the process parameters, primarily the rotational speed and the feed rate of the machine tool. In this study, a temperature‐controlled friction stir welding process has been applied to lap joints of aluminium and copper. Welding experiments with various welding speeds and probe lengths were performed in order to assess the effect of the temperature‐time profile near the welding interface. The joints were investigated by tensile shear tests as well as optical microscopy and scanning electron microscopy.     

Process boundaries of collision welding at low energies

Collison welding is a promising material‐closed joining process that enables bonds with various advantages. It is already used as explosion welding to produce clad materials that cannot be joined otherwise. Other collision welding processes as electromagnetic pulse welding do not contain that amount of energy, but they can be used in mass production. In order to achieve a high process and product reliability, the process has to be designed accurately. But the process boundaries are not yet completely understood. In this paper, process windows for aluminium and copper joints, produced by a model test rig, are compared. Additionally, high speed observation and micro sections are used to enhance the knowledge about process boundaries and the influence of the jet.     

Joining of cemented carbides to steel by laser beam welding

Welding of dissimilar materials such as steel and cemented carbides (hardmetals, cermets) is particularly challenging e.g. because mismatches in their thermal expansion coefficients and thermal conductivities result in residual stress formation and because of the formation of brittle intermetallic phases. Laser beam welding of cemented carbides to steel appears as an attractive complementary technique to conventional brazing processes due to its high precision, high process speed, low heat input and the option of welding without filler. Here a laser welding process including pre‐heat treatment and post‐heat treatment was applied successfully to joining as‐sintered and nitrided hardmetals and cermets to low alloyed steel. The microstructure and mechanical properties of the welds are investigated by microscopy, X‐ray diffraction, microhardness measurements, and bending tests. The results reveal that the three‐step laser beam welding process produced crack‐free and non‐porous joints. Nitridation of the cemented carbides results in a significant reduction of the amount of brittle intermetallic phases. The mechanical properties of the joints are competitive to those of the conventional brazed steel‐cemented carbide joints.     

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.     

Elektronenstrahlschweißen von Weichenherzen für den Gleisbau

Electron Beam Welding of Switch Points for Railway Tracks By means of computer simulation a temperature curve has been evaluated, which avoids formation of martensitic structures and subsequent hardening effects.     

Physico‐chemical and mathematical modeling of phase interaction taking place during fusion welding processes

The quality of metallic materials depends on their composition and structure and these are determined by various physico‐chemical and technological factors. To effectively prepare materials with required composition, structure and properties, it is necessary to carry out research in two parallel directions: 1. Comprehensive analysis of thermodynamics, kinetics and mechanisms of the processes taking place at the solid‐liquid‐gaseous phase interface during welding processes. 2. Development of mathematical models of specific welding technologies. We have developed a unique method of mathematical modeling of phase interaction at high temperatures. This method allows us to build models taking into account: thermodynamic characteristics of the processes, influence of the initial composition and temperature on the equilibrium state of the reactions, kinetics of heterogeneous processes, influence of the temperature, composition, hydrodynamic and thermal factors on the velocity of the chemical and diffusion processes. The model can be implemented in optimization of various technological processes in welding, surfacing, casting as well as in manufacturing of steels and non‐ferrous alloys, materials refining, alloying with special additives, removing of non‐metallic inclusions.     

Fatigue performance of hybrid overlap friction stir welding and adhesive bonding of an Al‐Mg‐Cu alloy

The need for weight reduction and leaner manufacturing and assembly processes in aircraft construction has led to the pursuit of welding technologies. One such technology that has been considered for this application is friction stir welding (FSW). Since it is a solid‐state joining method, it creates high performing joints in a wide range of materials while avoiding overlap lengths and added weight from fasteners, crack stoppers, doublers, etc. However, the adoption of this technology to the assembly of large fuselage shell components is challenging, due to geometric tolerance management requirements. In this paper, a hybrid joining method is proposed for such application, involving FSW and adhesive bonding. Fatigue performance of single lap joints of AA2024‐T3 Al‐Mg‐Cu alloy was assessed and benchmarked against FSW overlap and adhesive bonded joints. Significant strength and ductility increase was achieved through the hybridization of the overlap FSW joints. Fatigue strength of the hybrid joints was also higher than FSW overlap joints, although not as high as adhesive bonded joints.     

Effect of the wall thickness on the forming behavior and welding result during magnetic pulse welding

Magnetic pulse welding is a promising technology for the joining of dissimilar metals. Since the input of thermal energy is significantly reduced compared to conventional fusion welding technologies, critical intermetallic phases can largely be avoided. Therefore, proper collision conditions are necessary. Those require a careful adjustment of the energetic and geometric parameters at the impact welding setup. The thickness of the accelerated joining partner (flyer) determines the necessary energy input for a successful weld. However, at the same time, it has an effect on the weld formation. This study utilizes a novel optical measurement system to explain these findings and to gain insights into the forming behavior of the flyer parts. It is shown that the collision angle depends on the flyer tube thickness and, thus, directly has an effect on the welding result.     

Service fatigue design of complex welded construction equipment

Many steel structures and components are fabricated from steel plates and steel castings. The dominating joining method is welding for all kinds of transport vehicles and this paper describes briefly main problems in connection with design and fabrication of fatigue loaded structures. Design and fabrication has developed during the last 30 years and today we have tools, methods and knowledge to produce more optimised structures. Some of the work has been performed as joint work within Nordic projects and within IIW (International Institute of Welding). FE‐modelling, improved design rules, knowledge about influence from defects and residual stresses, new manufacturing methods and scientific based weld quality rules are briefly discussed in this paper.     

Beanspruchungsgerechte Formgebung von Umformwerkzeugen durch formgebendes Plasma‐Pulver‐Auftragschweißen

Load adjusted shaping of forming tools using build‐up PTA welding For the improvement of the mechanical‐technological properties of the forming tools (in particular medium sized component‐geometries) the build‐up plasma deposition‐welding was established as a manufacturing process. It could be proven that the thermo‐mechanical characteristics of these tools are improved by using of cobalt and nickel basis alloys. With the embedding of carbides in the cobalt basis matrix the most endangered surface regions of the tools may to be better protected and the life times significantly increase compared with the up to now with conventional materials manufactured components. With manufactured and tested segments of a transverse‐rolling tool it was possible to validate whether the won knowledge is transferable into the practice.     

Untersuchung von Schmelz‐ und Rekristallisationsprozessen sowie Riss‐, Oxid‐ und Porenbildung beim Schweißen

Investigation of melting and re‐crystallization processes and of crack, oxide and pore formation during welding In the paper some known investigation methods of the primary re‐crystallization and of the structure of the crystallization front are discussed. All these methods have disadvantages, in some cases they cause a limited validity or are expensive. The presented new method is based on the micrographic recording of the melting and solidifying metal surfaces. The method overcomes the above‐mentioned disadvantages. It is able to investigate in the welding seams not only the primary re‐crystallization processes but also the secondary ones including the formation of cracks and pores and the appearance of non‐metallic inclusions like oxides. The facilities of the method are demonstrated on some examples of use.     

Laserstrahl‐Kurzpulsschweißen von Metallfolien

Laser Beam Short Pulse Welding of Metal Foils Joining of metallic foils by laser beam spot welding is applied in different fields of electronic production and micro joining technology. The main challenges hereby are the realization of weld spots with high quality, i. e. weld spots with high stability and smooth shape, as well as the stabilization of the process. By the application of short laser pulses usually used for drilling or cutting the time at which the melt pool exists can be reduced and therefore the probability of arising instabilities, too. Short laser pulses are pulses with a length lower than one millisecond; pulses with a pulse length lower than one nanosecond are called ultra short pulses. The attempt of welding metallic foils by short laser pulses provides also the advantage of a lower thermal influence of the material. And a more simple system technology is possible then because different manufacturing methods can be realized with one laser. The main intention of the shown results of some investigations is the qualification of the welding process. Hereby the reachable weld spot quality as well as the enlargement of the process window is of special importance.     

Einfluss der Schweißparameter auf das Ermüdungsverhalten hochfester Baustähle

High strength low alloy steels are used in many different engineering areas. A commonly used joining technique for those steels is fusion welding. Generally, these components have to withstand fatigue due to dynamic loading. Using thermal joining techniques affect the mechanical properties of the steel. This study focuses on the influence of the heat input on the microstructure of high strength low alloy steels (S690). Furthermore, the fatigue behaviour with special regard to crack initiation and crack propagation is characterized.