Herstellung verbundverstärkter Aluminiumprofile für ultraleichte Tragwerke durch Strangpressen

Manufacture of Extruded and Continuously Reinforced Aluminum Profiles for Ultra‐Lightweight Constructions Due to similar specific properties of aluminum and steel regarding stiffness and mechanical strength only minor achievements as to their application in lightweight space‐frame structures can be attained by substituting one material for the other. Only the usage of carbon fibre materials or high performance steels promises a weight reduction as to the application of pipes and profiles in ultra‐lightweight structures. At the Chair of Forming Technology of the University of Dortmund a process has been developed which is suitable for the manufacturing of continuously reinforced profiles by means of a modified direct extrusion process. Starting from conventional aluminum billets, various continuously reinforcing elements are being embedded in the wall thickness of the profile matrix. For this process, which is being investigated in the context of the Collaborative Research Centre SFB/TR10, new extrusion dies have to be developed. During experimental studies on a 2.5 MN laboratory direct extrusion press first reinforced profiles were manufactured successfully. In order to improve the understanding of the process and to predict the efficiency of new tool concepts, FEM simulations were carried out simultaneously. Reinforced straight profiles produced with the help of this method cannot be bend due to possible damages to the microstructure. However, in combination with the process of Rounding During Extrusion, which also has been developed at the Chair of Forming Technology, these reinforced profiles will be producible with a curved contour as well.     

Entwicklung der Konnektivität der Phasen in einer kurzfaserverstärkten Aluminium‐Kolbenlegierung unter Kriechbelastung

Developing connectivity of the phases in a short fibre reinforced aluminium piston alloy subjected to creep The evolution of the micro‐structure during creep of an AlSi12CuMgNi piston alloy with 15 vol% of Al₂O₃ short fibres is investigated by means of synchrotron micro‐tomography. The results reveal a 3D morphology of the rigid phases in the composite: the eutectic‐Si, the short fibres and the Fe‐ and Ni‐rich intermetallic particles, which form an interconnected hybrid reinforcement. The connectivity of these phases increases during creep exposure at 300 °C due to the diffusion induced ripening of Si and of the intermetallic particles. The hybrid reinforcement reaches almost complete percolation after 6400 h of creep exposure. The fibre orientation analysed by three‐dimensional Fast Fourier Transformation does not indicate any reorientation of the fibres along the load direction. The formerly observed strengthening effect during creep exposure is attributed to the increasing load carrying capacity of the interconnected hybrid reinforcement. The analysis of creep damage during secondary creep stage shows the increase of the void volume fraction by a factor of 2 with respect to the void content from processing, while the number of voids per volume remains practically constant. The voids are located at interfaces of the rigid phases and not within the α‐aluminium matrix.     

Analyse der Mikrostruktur und des Eigenspannungszustandes seilverstärkter Aluminiumstrangpressprofile

Analysis of the microstructure and the residual stress state of aluminium extrusions reinforced with ropes Closed profiles of the aluminium wrought alloy EN AW‐6060 have been in‐situ‐reinforced during the extrusion process with ropes built by wires drawn of the nickel‐based alloy Inconel 601 (2.4851). These profiles have been investigated with non‐destructive and semi‐destructive testing methods regarding the residual stresses evolving during cooling from extrusion temperature to room temperature. The results obtained via X‐ray diffraction and hole drilling method have been compared with simulations using a simplified model of the compound investigated. Thereby, compressive residual stresses appearing during the cooling process have been detected parallel to the rope axis nearby the rope‐matrix‐interface because of plastic flow of the matrix material. Additionally, the microstructure of the extruded composite has been analysed focussing on the contingent formation of intermetallic phases at the rope‐matrix‐interface and on the influence of the reinforcements on the longitudinal weld.     

Bohrbearbeitung von kurzfaserverstärktem Aluminium

Drilling of Short-Fiber-Reinforced Aluminium The new material-group of fiberreinforced metals requires a specific cutting technology because of the extreme abrasion of the used fibers. For these first fundamental drilling-tests a composite of aluminium matrix and short nondirectional Al₂O₃-fibers was used. Neither in regard to the cutting forces nor to the drilling-quality there were any difficulties found. As far as boring roughness and -diameter is concerned the results are better than for not reinforced aluminium. Main problem is the tool wear that already increases at small contents of fibers. Using coolant a further increase of tool wear was found. Decisive improvements were achieved by taking polycristalline diamond tipped drills that had an 50 to 100 times longer tool life compared with the carbide drills.     

Charakterisierung faserverstärkter Leichtmetalle

Metal-matrix composites combine the properties of metals with those of ceramic fibres. Pistons of highly stressed Diesel engines are reinforced in their bottom zones by the implantation of a preform of fibres. Investigations concentrate on the nondestructive detection of inclusions and defects of fibre distribution. Due to the very small dimensions of the defects test methods capable of responding sensitively to such small defect dimensions have to be applied. Good prerequisites to solve the task of defect detection are provided in first line by ultrasonic inspection within the high-frequency range in connection with scanning methods. Natural defects up to a size of 120 μm could be detected by ultrasonics. With artificial test defects, the limit was reached at 100 μm. Eddy-current inspection, another test procedure, was applied for the detection of fibreless zones, as there exist differences of conductivity between fibreless and fibre-reinforced zones. The use of scanning methods with special probes allows to depict fibreless zones up to a size of 100 μm on the specimen surface.     

Simulation der geometrischen Oberflächen‐ wandlung beim Drücken optischer Bauteile

Simulation of the surface roughness alteration in spinning of optical components Forming technology is often used to manufacture work pieces with sophisticated surface topology as well as part shapes. Today, the production of the required surface qualities is still based on expert knowledge and “trial and error” techniques. Thus, in this paper an approach will be shown to compute surface quality by finite element analysis.     

Dynamisch-mechanische Analyse naturfaserverstärkter Kunststoffe

Dynamic-mechanical analysis of natural fiber reinforced plastics Recently natural fibers are increasingly used as reinforcement in plastics. These materials are distinguished by their high tensile strength and stiffness as well as their low density at the same time. For technical applications the material behaviour in addiction of temperature is very interesting. Useing the example of flax fiber reinforced polypropylen it is shown, that the dynamic-mechanical analysis can be a meaningful test to describe the behaviour of natural fiber composites by different temperatures.     

Untersuchungen zum Erosionsschutz kohlenstoffaserverstärkter Duroplaste

Investigations on erosion protection of carbon-fibre-reinforced thermosetting plastics A double layer system of polyurethane rubber and a metal foil was described by calculation and experiment. There is a significant increase in erosion resistance of a carbon-fibre-reinforced plastic component by application of this system and the experimental results correlate with theoretical estimations. Among the investigated materials surface layers of titanium and a titanium aluminium alloy respectively possess the best resistance in the range of all angles of incidence. The polyurethane rubber also exhibits a very good erosion behaviour under conditions of normal impact as well as at shallow impact angles at room temperature. So the rubber takes care of a good emergency running quality after a destruction of the metal foil. It was found that the relative erosion rate of the protective metal layer decreases with increasing thickness of the rubber. A thickness of about 0,5–0,7 mm of the protective metal layer is sufficient in practice. The titanium foil should have a thickness of about 100–200 m?m for the design life because if the metal foil is too weak it will undergo destruction rather fast under laboratory conditions. There is no change of the well-known erosion mechanisms with the formation of walls around the places of normal impacts as well as grooving and machining at shallow impact angles by application of the double layer system. An increase in temparature up to 80 °C doesn't cause any change of the erosion rate of the investigated metal foils in the double layer system but will influence the erosion of the polyurethane rubber alone. The galvanic deposition of nickel is another way for the use of the double layer system especially for components with a complex geometric shape.     

Thermisches Spritzen vanadiumkarbidverstärkter Eisenbasisschichten

Thermal spraying of vanadium carbide reinforced iron based HVOF sprayed cermet coatings for wear protection like WC‐Co(Cr) and Cr₃C₂‐NiCr have found a broad range of applications in the past. By using chromium containing matrices, they exhibit good corrosion resistance along with outstanding wear resistance. In present research in the area of powder metallurgy and PTA welding iron based alloys with high content of chromium and vanadium have been developed, revealing similar properties and therefore being a cost efficient alternative to established cermet coatings. HVOF sprayed coatings of these iron based alloys are investigated regarding their economic applicability and their corrosion and wear properties.     

Theoretische und experimentelle Analyse des Ausrichtungsverhaltens plättchenförmiger Verstärkungskomponenten beim Strangpressen von Verbundkörpern mit Glasmatrix

Theoretical and Experimental Analysis of the Orientation-Behaviour of Reinforcing Platelet-Components during the Extrusion of Composites with a Glass-Matrix By embedding platelets consisting of materials with high modulus of elasticity and strength in the extruded glass-melt, composites with increased Young's modulus and strength compared to non-reinforced glass can be produced. This improvement of the mechanical properties is primarily caused by the orientation of the platelets, which is a result of the laminar shear-flow of the highly viscous glass-melt within the deformation zone and within the die channel. To acquire statistical statements about the plateletorientation within the extruded compound rods theoretical research methods are preferentially employed over experimental ones. By the systematic variation of individual extrusion-parameters a systematic optimization of the manufacturing process concerning the reinforcing effect of the embedded particles is made possible. To this end a model has been developed, which simulates the motion of the platelets during the deformation process based on a finite-element computed velocity profile development of the extrusion flow. The interaction between the reinforcing particles is considered by using an interaction coefficient. In detail the orientation-behaviour of the embedded particles is analyzed as a function of their geometry and volume ratio in models with variable boundary conditions. Subsequently the theoretical results, obtained with these calculations, are set against the experimental results, which were determined on extruded composites with the aid of an automatic image-analyzing system. This comparison documents the suitability of the presented simulation-model.     

Mikroanalytische untersuchungen Faserverstärkter keramischer Werkstoffe

Microanalytical investigations have been made on samples of ceramic fibers (SiC-fibres (Nicalon), C-fibre coated with TiN) and fibre-reinforced ceramics (SiC- and glass-matrices). High resolution Auger electron spectroscopy (HRAES), electron probe microanalysis (EPMA) and scanning electron microscopy were employed for these examinations. Analysis was best performed with HRAES on account of its lateral and depth resolution. AES depth profiles of ceramic fibres are reported and compared with the surface analysis of fibers in the composites after being broken in situ.     

Berechnung der Wärmevorgänge beim Abbrennstumpfschweißen

Calculation of Thermal Processes in Flash Butt Welding For the calculation of the thermal processes in flash butt welding the Fourier equation for heat conduction is used paying regard to resistance heating. With a model of calculation the assumptions and simplifications made can be explained. Cooling of the clamps, losses by flashing and heat transfer are taken into account. The input of heat at the gap can be treated as a flat heating source. Particularities with respect to preheating, flashing and cooling may be introduced into the calculation by corresponding inital and boundary conditions. As a result we get the dependence of temperature on the welding parameters. Experiment and calculation are in good agreement.     

Untersuchungen über das Strangpressen kubisch-flächenzentrierter Metalle

Investigation of the extrudability of face-centered cubic metals Part I represents investigations of the behaviour of the interface during the extrusion of lead and lead alloys. The alloying tendency (welding) and the coefficient of friction μ do not show any relationship to the power requirements during the process of extrusion. Due to the high pressure inside the extrusion presses an interfacial movement (friction) can take place only if the coefficient of friction is in the range of μ ≦ 0,001. By knowing the normal pressure and the shear strength of the material the coefficient of friction for slip-stick conditions can be determined from a chart. The dynamic friction is presented as the shearing off of the interfacial contact points and the stationary friction as the creep of these contact points. Attempts to calculate the power requirements of industrial presses on the basis of the shear strength of the material and shear area were successful.     

Aufbau und Eigenschaften glasfaserverstärkter Eisenverbundwerkstoffe

Properties and Microstructure of Iron-Glassfiber Composites It is possible to produce powdermetallurgically glasfibre reinforced iron composites after high deformation by extrusion. These materials represent some interesting technological aspects because an anomalous increasing in tensile strength and other mechanical properties can be found, although the Young's-Modulus of the additive is lower than of the matrix. On the other hand electrical conductivity and Young's Modulus of the composites show a linear decreasing with the volume fraction of the second phase. This behaviour is reasoned in the fibre structure of the composites. Other physical properties such as thermal conductivity or the internal friction show an anomalous behaviour because of an excursive phononscattering at the interphases, which the curled microstructure possess in multiple forms.     

Zyklische Beanspruchung drahtverstärkter Verbundstrangpressprofile mit Aluminiummatrix

Cyclic loading of wire‐reinforced aluminium matrix composite extrusions Aluminium matrix composite extrusions reinforced with wires featuring high strength and stiffness represent an innovative materials concept for lightweight structures. The use of reinforcing elements should improve the mechanical properties and hence enhance the performance of the lightweight structures. Composite extrusions made from the aluminium alloy EN AW‐6060 reinforced with reinforcing elements made from the spring steel 1.4310 and the cobalt‐base alloy Haynes 25 were examined under cyclic loadings which are of vital importance for the desired applications. Initially, load controlled multiple step tests at a load ration of R = ‐1 allowed for the determination of the cyclic stress‐strain‐curve. Afterwards, lifetime predictions were determined from these results by using mechanical models proposed by Morrow and Basquin, which were reviewed Woehler tests without mean stress. Furthermore, light and electron microscopy served for the clarification of damage and failure mechanisms. The investigations have been carried out with varying materials, configurations and surface treatments of the reinforcing elements. The investigations strived for the identification of the parameters’ influence on the lifetime behaviour to optimize the materials systems regarding the fatigue behaviour.     

Mikromechanisches stochastisches Versagensmodell uniaxial faserverstärkter Verbundwerkstoffe

Micromechanical stochastic failure model of uniaxial fibre-reinforced composites A theoretical model of stress transfer between a transversal isotropic fibre and the surrounding matrix material in a uniaxially fibre-reinforced composite near a single matrix flaw is discussed including friction controlled fibre-matrix interface debonding. The rise of fracture toughness due to frictional fibre sliding is studied accounting for Weibull strength distribution of fibres. The total dissipative work may be used as figure of merit regarding the damage tolerance. A critical evaluation is presented concerning some previous models of local failure probabilities. Numerical results are demonstrated. Conditions for an optimized C/Al-composite are presented.     

Zur Viskosität von Glasschmelzen und porösen Sintergläsern

Qn the viscosity of glass melts and porous sintered glasses Suspensions of solid phase particles in molten glasses modify their flow behavior and therefore their viscosity. This work deals with the dependence of the effective viscosity (η_eff) of isotropic suspensions on the concentration of the solid phase particles. Independently of the shape of the inclusions the values of the effective viscosities lie between upper [η_eff = η_L (1 ? C_D)^?14 ]and lower bounds [η_eff = η_L (1 ? C_D)^?3], where η_L is the viscosity of the molten glass and C_D is the volume fraction of solid inclusions. The lower bound is also valid for the effective viscosity of suspensions containing spherical inclusions. Pores present in a glass matrix affect its flow behavior and consequently its creep and sintering behavior. The effective viscosity of porous glasses (η_P) as a function of the volume fraction of pores, or porosity (P), also varies between and upper [η_P = η_M (1 ? P)^1.04] and a lower bound [η_P = η_M (1 ? P)¹⁴], where η_M is the viscosity of the nonporous glass matrix. For spherical porosity the equation is: Measured values for the viscosity of different suspensions as reported in the literature as well as own experimental data on two glass-solid systems are compared with the theoretical values predicted by these equations. In a similar way literature data for the viscosity of porous sintered glasses are compared with the respective equations. In all cases a fair agreement between experiment and theory was found in some cases the agreement was excellent. Therefore the presented equations constitute a reliable approach for the prediction of the viscosity of suspensions and porous sintered glasses and since they do not contain fitting parameters, they are of substantial practical relevance (for a comprehensive english treatment of the matter compare Glastechnische Berichte, Proc. Otto-Schott-Colloquium 1994).     

Numerische Untersuchung zum Verhalten von Rißwiderstandskurven: Simulation mit dem Kohäsivmodell

Characteristics of Crack Resistance: Simulation with a new Cohesive Model Cracks in structures induce high stress and strain concentration and reduce significantly their load capacity. The resistance of the material during stable crack extention is determined using global quantities measured on specific crack specimens. However, such determined resistance of a material is dependent on several parameters, even under small scale yielding as shown here. The behaviour of the resistance curves, determined by Finite Element simulations using a new GKSS specific cohesive model, will be presented. New aspects also arise for the choise, design, and assessment of materials.     

Strangpressen von verschleißbeständigen Fe‐Basis MMC

Hot extrusion of wear resistant Fe‐base metal matrix composites (MMC) Increasing demands on technical surfaces, i.e. thermal load, corrosion or wear, often prompt the development of tailored materials or coatings. In highly abrasive environments the progress in powder metallurgy has lead to the production of highly wear‐resistant materials based on metal‐matrix composites (MMC). Such materials are produced from a metal matrix (MM) based on Fe, Ni or Co and additional hard phases (HP), such as carbides, nitrides, borides or oxides. Moreover, powder metallurgical techniques can be used to adapt the particle size, the distribution and the content of the hard phases to the wear system on a large scale. HIP cladding is an established method of producing such MMC, but due to its near net shape capsule technique it is quite expensive. Because of this reason hot direct extrusion of capsules filled with powder blends was researched in a DFG‐Project as a method of producing long cylindrical products. Aiming at a high abrasive wear resistance, powder blends of hardenable steels with additions of fused tungsten carbide (WSC) or titanium carbides (TiC) were used. The extruded MMC were investigated with respect to their densification and microstructure, their bending strength and their wear resistance.