Key Engineering Materials Vols. 611-612

Abstract: In the paper the concept of electrochemically assisted microturning process was presented. This is a hybrid machining process in which the microcutting directly removes the material while the electrochemical passivation process is changing the conditions of cutting. The experimental part consist of description of passive layer formation process on 304 steel samples and results discussion of passive layer thickness and mechanical properties measurement with ellipsometry and nanoindentation technique.

Abstract: The main requirements of carbide metal working are higher precision and high quality surface which can be fulfilled by electrical discharge machining. This procedure is accompanied with formation of heat affected zones (white layers) during the discharge process negatively. Therefore, the essential post-processing reduces the efficiency of this process and shows the importance of process energy sources (PES) with ultra short discharge in favor of a clearly differentiated cutting volume. By means of simulations of crater geometry and channel expansion the influence of discharge rise time is defined as determining factor for the cut volume and formation of white layers. The technological section presents two different approaches of realizing ultra-short pulses.

Abstract: In aeronautics, economic and environmental aspects become increasingly important. As those are very much influenced by the frictional drag of the airplane, a reduction of skin friction which causes a major portion of total aerodynamic drag is desirable. One possible approach for passive drag reduction is the application of riblets small longitudinal grooves orientated in flow direction. Through an adapted rolling process, riblets can be brought into metal sheets on a large scale. For this process a thin high-strength steel wire is wound around a work roll to structure it with the negative riblet imprint. In a subsequent step the riblet profile is rolled into the sheet material. Different parameters can influence the process and the quality of the resulting riblet structure. Those parameters that depend on the sheets sheet thickness, material strength, and composition of the sheet are discussed in this paper. Form filling is used as an indicator for riblet quality. It is found that decreasing sheet thickness is beneficial for form filling, but a process dependent minimum sheet thickness exists for which this effect will reverse. Material strength is found to have a much smaller influence on form filling. Nevertheless, harder alloys seem to need a slightly smaller thickness reduction, but higher rolling forces and pressures to achieve desired form filling. Using clad instead of bare materials has a positive influence on form filling and riblet structuring. Furthermore, riblet rolling does not reduce the fatigue strength of the clad material.

Abstract: Electromagnetic Forming (EMF) is a high-speed forming process that can be applied for shaping, joining and cutting of workpieces made of electrically conductive material eg. aluminium. This paper proposes a dual electromagnetic forming method. Energy efficiency of the dual electromagnetic forming is compared with the single sided electromagnetic forming using FEM simulations. In uniform pressure rectangular coil, the top layer of the coil assists in the deformation of the workpiece while the bottom layer hinders the workpiece deformation.To make the bottom layer of the coil also to assist in the deformation of the workpiece, a uniform pressure rectangular coil is designed and placed between the two sheet metal workpiece. The efficiency of the two processes are compared by determing the maximum deformation obtained for each case.

Abstract: Laser forming ofopen-cell aluminum foams can be modeled by means of 3D thermo-mechanical models but the correct evaluation of the alloy material properties is a key-factor for obtaining good predictions. In order to increase the model predictability from a quantitative point of view, further information about the material behavior under laser exposure is necessary. In this study the effect of the temperature on the mechanical properties of a commercial aluminum sponge has been evaluated in terms of yielding stress and tangent modulus. Experimental tests have been performed by compression and used to infer mechanical properties by means of a 3D FE model. The same approach has been used also to evaluate the effect of a heat treatment of the sponge on the material behavior during forming. In conclusion numerical simulation of laser heating has been used to show the effect of the laser-material interaction on the final homogeneity of processed foams.

Abstract: Electrochemical machining (ECM) is an appropriate technology for structuring a wide range of metals and alloys independent of their mechanical properties. Further advantages the occurence of no tool wear and almost no change in the material properties of the work piece material due to the machining. As different studies showed it is possible to manufacture structures in the µm-range and also in the sub µm-range. The resulting geometry and feature size depend on the electrolyte, the working gap, the electric field, the flow conditions within the working gap and the material structure. The working gap is the most important parameter regarding the critical dimensions of ECM. Actual high resolution processes are limited by the process time due low current densities, while faster processes are limited by the resolution due to the dimensions of the double layer and the need for purging.\\Regarding these limits an enhanced understanding of the electrochemical processes within the working gap is mandatory for the optimization of the ECM. Therefore a measurement device is realised utilising electrochemical impedance spectroscopy (EIS), which is one technique to obtain a wide set of thermodynamic and kinetic parameters related to the electrochemical processes like reactions, adsorption, desorption and diffusion in the gap. There the impedance, which is the complex ratio between the voltage and the current in an AC circuit, is measured as a function of the frequency and it is compared to an equivalent circuit, composed out of basic elements representing the chemical sub processes. The spectrometer is realized by combining a commercial potentiostat with computer based signal processing realized in MATLAB\copyright. The spectrometer implements time-domain measurement techniques (Fourier transformation) for a strong reduction of measurement time. This time reduction allows in situ measurements.\\The presented spectrometer enables characterization of standard material systems like cooper, as well as the identification of the time-critical chemical processes and steps in more complex systems.

Abstract: The presented work is an investigation of fluid-particle separation phenomena and compression stress resistance performance of magnetorheological (MR) fluids under squeeze mode. The squeeze mode is very significant to MR machining application. Material used in this study was silicone oil based MR fluid with 20% volume fraction of carbonyl iron particle. Compression test was performed by integrating the developed squeeze mode testing rig with a 50 kN Universal Testing Machine (UTM). The tests were conducted at constant speed and current. Each test was conducted at an initial gap of 2 mm and was stopped at a final gap of 0.5 mm. Force-displacement data was recorded and was analysed using TestExpert® II software. Full factorials with 27 experiments were designed using Design Expert 7 software. Three factors investigated in the design of experiments were carrier fluid viscosity, supplied current, and compression speed. Responses measured were strain energy and compression stress at maximum strain. Macro images of the phenomenon were recorded and evaluated qualitatively. From the compression stress-strain results, carrier fluid viscosity was significant to vary the MR fluid properties. The observed phenomenon shows that fluid-particle separation occurred in the low viscosity carrier fluid, low compression speed and high applied current. The parameters effect on strain energy and compression stress suggests that the fluid-particle separation is significant to the squeeze mode MR fluid performance. The relationship between stress resistance performance and fluid-particle separation phenomena were significant in designing innovative MR fluid devices.

Abstract: In projection stereolithography, the cure depth and thereby the layer thickness in the manufacturing process may be varied by changing the operation wavelength. The change can be made even in the middle of the manufacturing process, which opens up possibilities for additional flexibility and speed-up of the process.In this paper, we investigate how the penetration depth of the curing light in the resin depends on the wavelength. The measurements were performed for wavelengths between 400 nm and 550 nm using a standard commercial resin. The penetration depth was found to almost double when moving from the short wavelengths to the long ones.

Abstract: Friction stir welding assisted by electrical joule effect is a development of conventional friction stir welding with the ability to eliminate or minimize the root defects by lack of penetration. The lack of penetration, mainly relevant in the welding of aluminium alloys, still constitute one major constrain to a wider dissemination of friction stir welding into industrial applications. The concept is based on assisting friction stir welding by an external electrical heat source integrated in the FSW tool. The innovative tool feature enables to increase the temperature in the weld root by Joule effect and improve material viscoplasticity in this region. A new tool was designed, manufactured and implemented. The resulting welds produced were analyzed via metallography and Electrical conductivity measurements that have proven to be a valuable technique to identify the different zones of solid-state welded joints with a good correlation with the microstructure and hardness. The potential of this variant was shown reducing the thickness of weld root defect, even for significant levels of lack of penetration, although affecting the grain size of the HAZ in the vicinity of the root surface.

Abstract: Magnesium is one of the most promising materials for the application as degradable biomaterial for load bearing implants due to the initial stability and extraordinary biocompatibility. But up until now magnesium degrades too fast before the ingrowing bone can support itself. One possible approach is the application of an interconnecting channel structure. This design increases the surface area to promote bone regeneration and gives the bone the possibility to build a supporting structure throughout the implant. In combination with a surface modification which increases the corrosion resistance the bone can regain enough stability to support itself before the implant has dissolved noticeably. Due to the low process forces EDM is very suited for the machining of the necessary filigree structures. But up until now there are no standard technologies for the machining of this material available. Therefore a fundamental analysis of the material specific influence of magnesium on the EDM process is necessary. In this paper the machining of three different magnesium alloys are compared to the machining of a tool steel. Due to the numerous influencing factors and their interdependencies it is normally not possible to analyze the impact of each input parameter of a typical EDM technology on the continuous process separately making the correlation of input and output parameters very difficult. Therefore the electrical signals are recorded during the machining process to identify and monitor the resulting discharge conditions and frequency. These results are compared to the fundamental influence of the material derived from single discharge experiments in which the boundary conditions are constant to a large extent. By this method the material removal rate as well as the resulting surface roughness of the continuous process can be linked to the material dependent process conditions. As a result technologies for the machining of new materials can be developed and optimized based on fundamental knowledge.