Key Engineering Materials Vols. 504-506

Abstract: Electrically insulating high performance ceramics like zirconia are increasingly used for high technological applications due to their extraordinary properties. Meanwhile even the spark erosion process can be used to structure non-conductive ceramics by using an assisting electrode (AE). This conductive AE is placed on top of the work piece to enable the process. The electrical contact is sustained during the process by carbonic residua of cracked oil used as a dielectric. For an enhanced understanding of the removal mechanisms during the spark erosion process of such non-conductive ceramics, we analysed the surface of ceramic samples during the entire spark erosion process chain. In our investigation a zirconium oxide with yttrium oxide stabilisation was chosen. For this purpose X-ray diffraction (XRD) and scanning electron microscopy (SEM) analysis was performed. The XRD results showed that the zirconium oxide reacts under the presence of carbonic residua during processing into zirconium carbide. This material supports sustaining the conductivity additionally, as it is conductive itself. In a subsequent thermal cleaning step with oxide atmosphere (T = 750°C), remaining carbonic residua are oxidised and thus removed. The XRD measurements after cleaning showed that the zirconium carbide was no longer detectable. Instead a monoclinic zirconium phase was found. This shows that a reverse reaction of zirconium carbide into zirconium oxide took place. To prevent the formation of a non-stabilised monoclinic phase, the oven process was adapted to higher temperatures of 900°C with higher heating and active cooling rates of up to 10 K/s. This adjustment shows that the monoclinic phase can be supressed and the reverse reaction leads to a tetragonal zirconium oxide like the bulk material.

Abstract: This paper deals with developments of Wire-EDM technologies for fir tree slot production. The aim of these developments is to substitute certain conventional processes within turbine manufacturing that have been identified as non-optimal like the critical broaching process. Thereby the negative characteristics like inflexible manufacturing processes, high machine tool investment costs and huge energy consumption can be abolished. The objective targets of the conducted research are to meet all safety requirements of the critical components and get an economic manufacturing process. In the first step a special brass wire technology for cutting Ni-based super alloys was developed. Main focus was to meet the requirements of fir tree production concerning aspects of surface integrity and geometry. To measure these aspects on the one hand non-destructive analyses have been performed to guarantee surface roughness and accuracy. On the other hand destructive analyses in terms of cross section polishing for showing thermal influenced rim zones have been prepared. A capable Wire-EDM process is presented.

Abstract: Investigations on the special flushing conditions with an electrical discharge drilling machine were done to analyze single discharges. While flushing speeds in electrical discharge drilling are quiet higher as in electrical discharge sinking, the craters are influenced by the strong flushing. The topography of the craters in terms of depth, length and volume were analyzed. Crater depth and crater length are increasing with higher flushing speeds. Under a dielectric flushing the removed material on the tool electrode is less than on the workpiece. If the volume flow is high enough, the whole plasma channel is pushed in the direction of the flow. A computational fluid dynamics analysis was done to simulate the flow speeds in the narrow gap. Flow speeds above 500 m/s were simulated in the gap.

Abstract: As consequence of the development of electrical discharge machining process, the electrode is affected by wear; knowing the evolution of the electrode wear, a better estimation of its service life is possible. It is expected that the electrode wear depends on the energy of the electrical discharges and the mass of the electrode. It is known also that the nature of the workpiece material exerts influence on the evolution of the electrode wearing process. In the paper, some theoretical considerations are used to highlight the above mentioned aspects. A set of experimental tests was designed and developed in order to highlight the influence exerted by the nature of the workpiece material and by the size of the cross section of the electrode, respectively, on the electrode wear. Empirical mathematical models corresponding to the evolution of the electrode wear were established.

Abstract: The Electro-Contact-Discharge Machining (ECoDM) is a hybrid process composed of Electrical Discharge Machining (EDM), Arc Discharge Machining (ADM), Contact Heating Proc-ess (CoHP), and Mechanical Machining (MM). The article presents a comparison of different ap-plications and the different influence of the machining parts for a necessary result. The application field ranges from ECoDM-Cutting to the cleaning of railway tracks. The analysis of the different processing parts is necessary for monitoring the process parameters (electrical and mechanical). An advantage of the ECoDM is the cutting of electric-conductive and partial electric-non-conductive materials. The application field is not high-precise machining but rather recycling and pre-working processing. The examples show that the focal point is not the process energy source, but the selec-tion of the processing parts through the analysis of the voltage curve and/or the current curve.

Abstract: Electromagnetic forming (EMF) is a typical high speed forming process using the energy density of a pulsed magnetic field to form work pieces made of metals with high electrical conductivity like aluminum. In view of new lightweight constructions, special forming processes like EMF gain importance for the associated materials. In this paper modeling of electromagnetic sheet metal forming process is carried out by using commercial finite element software LS-DYNA®. A fully coupled numerical simulation method has been incorporated to study the interaction of the electromagnetic field and the structural deformation via transient analysis. Studies on the effect of first current pulse in electromagnetic forming are reported in the paper.

Abstract: The laser beam machining uses the effects generated at the impact of the workpiece surface with a laser beam that has adequate energetically and spatial - temporal characteristics. It is known that, as result of the laser beam impact with the workpiece material, local chemical and physical effects are signalized. In order to study such effects and simultaneously the technological possibilities of laser beam equipment, some experimental researches were developed on an Ytterbium fiber laser equipment of 300 W. Movements of the laser spot along established trajectories were ensured, changing the sizes of the machining parameters. The influence exerted by the machining parameters and by the materials characteristics on the material removal from the test piece was highlighted. The surface layer modifications were also studied.

Abstract: Laser forming tests have been performed on open-cell aluminum alloy foams with different pore size. Laser power was fixed at 150 W, a total of 150 laser scans led to a bending angle up to 60°, depending on the laser scan rate. At the end of the laser bending, the foams were left to cool and samples were extracted for analysis by means of an optic microscope. The alloy microstructure was investigated in different points of the samples and correlated with the processing conditions. Image analysis was also carried out to extract the percentage of melted area due to laser heating.

Abstract: The numerical simulation of the compression behavior of open-cell aluminum foams is discussed as a way to extract material property information for laser forming simulation. A bilinear isotropic model was implemented for the alloy base material whereas a parametric approach was used to build the finite element model of the foam structure. Compression tests were performed on commercial foams with different pore size and density, and the results of lower density foam were used for the model validation. Numerical results show a good agreement with experimental data in terms of foam deformation under compression and required loads.

Abstract: One can state that special place in group of micromanufacturing methods are connected with application of unconventional methods, such as electrochemical (EC) and electrodischarge (ED) micromilling, because of their high efficiency in shaping 3D structures. In the paper result of cylindrical tools manufacturing for EC and ED-micro milling process will be discussed. The special attention will be paid for electrochemical etching, electrodischarge turning and some hybrid methods application for micro-tools manufacturing