Papers by Author: Detlef Löhe

Abstract: For an increase in safety against crack initiation and growth in metallic structures of airplanes different concepts were developed in the past. In the focus of this work are profiles made of continuously reinforced extruded aluminum. The production and the used die set of these profiles is presented as well as problems occurring in terms of geometrical inaccuracies of the embedded high strength wires. In addition, this paper attends to the problem of lateral seam weld formation. The interface between the AA-2099 as well as AA-6056 aluminum alloy and the high strength wires Nivaflex and Nanoflex were characterized by metallurgic investigations and push-out tests. As a result it can be stated that a sufficient geometrical accuracy could be achieved and a high interface strength can be accomplished even if a slight gap is still present in the interface layer between matrix and reinforcing element.

Abstract: Electromagnetic compression of tubular profiles with high electrical conductivity is an innovative joining process for lightweight structures. The components are joint using pulsed magnetic fields which apply radial pressures of up to 200 MPa to tubular work pieces causing a symmetric reduction of the diameter with typical strain rates of about 104 sec-1. Since there is no contact between the components to be joined and the joining machine, any damage of component’s surface can be avoided. Friction stir welding (FSW) is a relatively new solid state joining technique and has been extensively developed for aluminum, magnesium, copper and titanium alloys as well as steels. The primal advantages of the process in comparison to conventional fusion welding are better mechanical properties, low residual stresses and distortion, and reduced occurrence of defects. In the present article, the influence of process and material parameters on the joint’s characteristics, material’s microstructure and the mechanical properties of electromagnetic compressed joints and friction stir welds using reinforced aluminum profiles is analyzed. The strength of the joint is determined by tensile tests. Finally, possible improvements of both techniques are outlined.

Abstract: A possibility to increase both stiffness and strength of aluminium-based structures for the application in lightweight profiles for vehicle space frames is the use of composite extrusions in which high-strength metallic reinforcements are incorporated. Within the scope of the present investigations, composite-extruded profiles with wire-reinforcements made of austenitic spring steel 1.4310 (X10CrNi18-8), in an aluminium matrix AA6060 (AlMgSi0.5), which were exposed to different corrosive media for different times, were characterised in terms of the debonding shear strength using the push-out-technique. The formation of a galvanic couple could be conceived mathematically in regard of terms describing the formation of a shear-impeding layer and the corrosive attack. Thereby the parameters for the different media could be determined.

Abstract: Powder-metallurgical (P/M) produced components may be used as mass parts in a very large quantity. Due to the multistage manufacturing process which consists of the pressing of the prepared powder and the sintering of the green bodies also complex shaped components can be produced very economically and precisely. They can be utilised without any further post-processing if the whole production process is optimised. However, it is still difficult and only possible with considerable technical and financial effort to produce highly stressable components profitably using the sinter technique. Therefore, the examinations on hand had the intention to create the basis for the use of the sinter technology also for the production of highly stressable case-hardened components. To this, at first bending specimens were fabricated with modern pressing procedures and sinter facilities and first characterized in the as sintered state. Then the conditions for the case-hardening was analysed and the parameters for an optimised case-hardening procedure fixed. With these parameters specimens were case-hardened and their lifetime behaviour estimated under different bending loading conditions. Finally it should be checked whether the knowledge gained from the specimens could be applied to complex components. To this, gear wheels were produced using powder-metallurgy. The cyclical tooth foot strength of this gear wheels were analysed in the only sintered as well as in the case-hardened state. It could be demonstrated that the improvement of the fatigue strength of the bending specimens by case-hardening also appears at the tooth foot strength of the gear wheels.

Abstract: Electromagnetic compression of tubular profiles with high electrical conductivity is an innovative joining process for lightweight structures. The components are joint using pulsed magnetic fields which apply radial pressures of up to 200 MPa to tubular work pieces causing a symmetric reduction of the diameter with typical strain rates of about 104 sec-1. Since there is no contact between components to be joined and the joining machine, any damage of component’s surface can be avoided. The load, which the joint can transmit, is strongly dependent on the residual stresses in the region of the joint. In the present article, the influence of charging energy, gap width before joining and diameter on the development of the residual stresses is analysed using geometry changes during splitting of the components. Besides, the contact zone between the components is analysed and the pullout force of the joints is determined by tensile tests. This allows the evaluation of correlations between joining process, state after joining and mechanical properties.

Abstract: In a three-dimensional Finite-Element-Simulation of shot peening, a combined isotropickinematic viscoplastic material description was introduced in order to describe the cyclic softening effects during peening. After verifying the model in the simulation of push-pull tests at different strain amplitudes it could be used for the shot peening simulation. The simulated residual stress profile is compared with experimental results determined by X-ray diffraction and with simulated results of a simpler isotropic viscoplastic material model.

Abstract: Compound-extruded unidirectionally reinforced lightweight profiles are a novel class of materials for the realisation of load-bearing structures. They may be fabricated in a flexible and rapid near-net-shape process. The authors present investigations of the reinforcing effect of wires in compound-extruded aluminum profiles under quasi-static tension and compression. In particular, the compounds were characterized by metallographic examinations focusing on the fracture morphology. Furthermore, specimens subject to compression tests were examined using micro computer tomography (µ-CT) and light microscopy (LM). It is shown, that the mechanical properties of wire-reinforced profiles are improved under both positive and negative quasi-static loads in comparison to non-reinforced profiles.

Abstract: Electromagnetic Compression of tubular profiles with high electrical conductivity is an innovative joining process for light weight structures. The components are joined using pulsed magnetic fields which apply radial pressures of up to 200 MPa to tubular work pieces causing a symmetric reduction of the diameter with typical strain rates of about 104 sec-1. This process avoids any surface’s damage of the workpiece because there is no contact between the components and the forming tool. The load, which the joints can transmit, strongly depends on the process parameters. Of them, the charging energy and initial gap between components are the most important. In the present article, the influence of these two parameters on the joint’s characteristics, material’s microstructure and the mechanical properties is analyzed. The strength of the joint is determined by tensile tests and by measurements of the residual stresses. Finally, conclusions for the joint design are given.

Abstract: Analysis of the residual stress state, the microstructure and surface topography of ceramic microcomponents made of Y2O3-stabilized ZrO2 produced by micro powder injection moulding were performed. During the production of the microspecimens sintering conditions were varied. The measurement of residual stresses has been carried out using the MAXIM (MAterials XRay IMaging) diffractometer at HASYLAB beamline G3 at DESY in Hamburg (Germany). The microstructure and surface topography of differently moulded specimens were examined using a scanning electron microscope (SEM) and a confocal white light microscope type NANOFOCUS. The findings are used to establish correlations between process parameters and characteristics of the microcomponents. These will allow to improve the production process with respect to the mechanical properties of the microcomponents.

Abstract: Brazing of cemented carbides to steel bodies gives rise to the development of complex stress states and distortions which influence the fatigue behaviour of the parts. It is quite important to estimate the residual stresses with numerical methods whose agreement is to be guaranteed with experimental characterization of the brazed parts. In this work FEM simulations and X-ray as well as neutron stress analysis were used to examine the residual stresses of brazed samples. Joints with different geometries and dimensions made of cemented carbide and different steels showing different phase transformation behaviours were investigated.