Key Engineering Materials Vols. 611-612

Abstract: The evolution of the mold temperature during squeeze casting of EN-AB46000 aluminum alloy has been correlated with the final mechanical performances of cast ingots. Starting from a material model which expresses hardness and yielding stress of cast aluminum alloys as a function of the cooling rate during the melt solidification, an experimental approach has been used to provide a useful tool for process monitoring. As a result, the mold temperature increase during the melt squeezing phase is directly correlated with the main mechanical and microstructural parameters. Experiments were made by squeeze cast small cylinders (14 mm in diameter and 18 mm height) at different values of squeezing pressure, mold pre-heating temperature, and melt temperature. Microscopic observations of the sample sections were made as well as hardness measurements and indentation tests. In conclusion, because of the material solidification, a temperature gradient has been observed in the sample which can be directly related with the aluminum alloy dendrite size and, in turn, with microhardness and yielding stress.

Abstract: EDM of electrically conductive oxide ceramics with addition of titanium carbide have been successfully applied as wear resistant tool inserts in ceramic injection molding or extrusion. In recent years especially alumina based ceramic composites toughened by zirconia have shown their potential in the field of ED machinable ceramics however revealing some drawbacks resulting from their moderate fracture toughness. This study focuses on the zirconia based ceramics with addition of different amounts of titanium carbide as electrically conductive phase (26-36 vol.-%) in order to improve the toughness of ED machinable ceramics. Additionally the influence of the titanium on removing mechanisms during machining as well as the hardness and strength of the material was investigated. It was found that the use of zirconia as matrix material does improve the toughness and strength compared to alumina based composites whereas the drawback of zirconia based materials concerning machinability and lower hardness can be only partially compensated by adjusting the titanium carbide content.

Abstract: The design of processes like magnetic pulse forming and electrohydraulic forming involves multiphysical couplings that require numerical simulation, and knowledge on dynamic behaviour of metals. The forming process is completed in about 100 μs, so that the workpiece material deforms at strain-rates between 100 and 10 000 s^-1. In this range, the mechanical behaviour can be significantly different than that in quasi-static conditions. It is often noticed that the strength and the formability are higher. The main goal of this study is to use an electromagnetically driven test on tubes or sheets to identify the constitutive behaviour of the workpiece material. In the case of tube, an industrial helix coil is used as inductor. Simulations with the code LS-Dyna^® permit to find a configuration where the tube deforms homogeneously enough to allow axisymmetric modelling of the setup. The coil current is measured and used as an input for the simulations. The radial expansion velocity is measured with a Photon Doppler Velocimeter. The parameter identification is lead with the optimization software LS-Opt^®. LS-Dyna axisymmetric simulations are launched which different set of parameters for the constitutive behaviour, until the computed expansion velocity fits the experimental velocity. The optimization algorithm couples a gradient method and a global method to avoid local minima. Numerical studies show that for the Johnson-Cook constitutive model, two or three experiments at different energies are required to identify the expected parameters. The method is applied to Al1050 tubes, as received and annealed. The parameters for the Johnson-Cook and Zerilli-Armstrong models are identified. The dynamic constitutive behaviour is compared to that measured on quasi-static tensile tests, and exhibits a strong sensitivity to strain-rate. The final strains are also significantly higher at high velocity, which is one of the major advantages of this kind of processes.

Abstract: Electrical discharge machining uses the pulse electrical discharges generated between the closest asperities existing on the workpiece surface and the active surface of the tool electrode in dielectric fluid. Essentially, some distinct electrical discharge machining schemas could be used in order to obtain cylindrical external surfaces; within this research, one preferred a machining schema based on the use of a cooper plate in which there were small diameter holes, by taking into consideration the existence of a ram electrical discharge machine. The results of the machining process analysis were presented. A thin copper was considered to be used as tool electrode, in order to diminish the spurious electrical discharges, able to generate shape errors of the machined surface. Some experimental researches were developed by changing the sizes of the process input parameters. As output factors, the test piece and tool electrode masses decreases were considered. Power type empirical mathematical models were determined, in order to highlight the influence exerted by the pulse on time, off time and machining process duration on the output parameters values.

Abstract: In this study, solid state foaming was used to produce epoxy foam samples with shape memory properties. Foams were indented at room and high temperature by using flat pins with diameter from 1 to 5 mm. Micro-indentations were performed as well only at room temperature. The indentation marks were measured before and after thermal recovery to evaluate the ability of the material to reach the initial shape. For a better understanding of the overall process, a study was made also to predict the initial precursor density as a function of the compaction parameters. This way, it was also evaluated that the effect of the compaction process is covered by the effect of the foaming step.

Abstract: For the EDM, it is important that the surface topology can be determined for given process parameters in advance. Simulations with constant plasma channel radius are very inaccurate and only give a limit of thermally-influenced zone to. Many functional relationships of the temporal change of the plasma channel are only for small time periods and do not reflect properties of the dielectric. In the paper is shown how various properties of the dielectric, contribute to the change of the plasma channel and the plasma channel as different behaviors can be treated with one simulation program. The maximum thermal load may be calculated by selective application of simulation, but also the topologies through moving or split plasma channels. This extended simulation is important because in the micro-EDM process instabilities leading to the splitting of the plasma channels. The simulations are reviewed on the basis of individual discharges and corrected corresponding weighting factors.

Abstract: Recent studies showed that electrical conductivity is a valuable technique to identify the different zones of solid-state welded joints with a good correlation with the microstructure and hardness. This is a relevant result since this technique is expedite and, in some cases, non destructive. The concept was applied to other welding processes as the ones involving fusion and to a wide range of materials. For this, a comprehensive study was performed using friction stir welding, tungsten inert gas (TIG) and gas metal arc (MAG) welding processes in either bead on plate or butt joints in: carbon steel, magnesium and titanium. Eddy current non-destructive testing (NDT) was used to measure the electrical conductivity at different depths in transverse sections of the processed materials. The obtained profiles were compared to the hardness profiles in the same sections. As a result, a good correlation was observed in most materials welded by solid state and by fusion processes. The variation of the electrical conductivity closely follows the one detected in the hardness. Another interesting conclusion is that, even for fusion welding of carbon steels, the technique has potential to complement the hardness measurements and microstructural observations, allowing to identify the distinct zones of welds in materials commonly used in industry.

Abstract: Silicon Carbide is a ceramic material with extraordinary properties. Not only does it excel in mechanical properties, such as very high hardness and flexural strength, but also shows excellent thermal properties with a low thermal expansion and operating temperatures above 1000°C. These properties predestine silicon carbide for applications in harsh environments. Structuring silicon carbide in the sintered state with conventional methods is not feasible due to its hardness. A non-conventional process to structure materials independent of their mechanical properties is electrical discharge machining (EDM). A certain conductivity is however required for this process. To fulfill this requirement, the method of an assisting electrode (AE) is used. During the process, an intrinsic AE is generated from the cracked dielectric oil and deposited on the ceramic surface. The process can therefore continue even after the applied AE has been penetrated. For a deeper understanding of the present removal mechanism EDM of non-conducting ceramics, especially in the area of micro EDM, an investigation of the influence of conductivity is necessary. Therefore three silicon carbide ceramics with different electrical conductivity (S-SiC: 1 10^-7 S/cm; LR-SiC: 10 S/cm; HO-SiC: 5 10^-9 S/cm) have been microstructured and analysed. It is found that the conductivity of the silicon carbide materials has no influence on the machinability, all samples can be microstructured. The microsections of the machined samples show that the near-surface structure of the SiC materials is not negatively influenced by the EDM process. The analysis of the surface revealed indications that for S-SiC and for HO-SiC, thermal spalling is the present removal mechanism. The LR-SiC surface shows melting structures. The material removal rate of LR-SiC is 8 × 10^-3 mm3/min, whereas the material removal rate of the S-SiC and the HO-SiC ranges at 3 × 10^-3 mm3/min. The high MRR of the LR-SiC indicates a removal mechanism analog to Silicon infiltrated Silicon carbide (SiSiC), with removal of a conductive phase.

Abstract: . Direct molding is a simple molding process to produce small parts at low costs. A single pellet or few pellets are directly molded into a transparent mound by means of an IR lamp. A new micro-molding machine has been developed for this study and used to mould polymers for biomedical applications (PCL and PHBV). Due to the small amount of molded material, the proposed process is particular suitable for biomedical applications which are characterized by small numbers of parts and high costs of the raw materials. Moreover, direct molding is able to reduce material degradation and frozen stresses. The new machine has been used to mold small disks from single pellets. This way, the effect of the main process parameters on the molded polymer density has been evaluated. Moreover, DSC analyses have shown the effect of the molding process on the polymer properties. In conclusion, the micro-molding system has been used to produce small components which were tested in terms of dimensional stability and mechanical performances.

Abstract: Characterized by excellent material properties such has high mechanical, thermal and chemical stability technical ceramics such as ZrO₂, SiC, Si₃N₄ and AlN are increasingly being used for various applications. Traditional means of machining sintered ceramics are expensive and limited by geometry. Electrical discharge machining (EDM) is an electro-thermal machining process used to structure conductive materials. By applying a conductive layer (denoted as assisting electrode) on top of the non-conductive material, the EDM process can also be used to structure insulating ceramics. This paper presents a comparative study on the major machining parameters affecting the µEDM process of non-conductive SiC, ZrO₂, Si₃N₄ and AlN ceramics. The influence of five major machining parameters (current, open-circuit voltage, gap voltage, duty-cycle and servo) over two responses (material removal rate (MRR) and tool wear rate) is investigated for each ceramics material. The underlying reason for the variation in the MRR among the different ceramics is examined by comparing the material properties. Melting point of the ceramics material has an effect on the MRR for the µEDM of different ceramics. The bulk resistance value of the ceramic material does not have an influence on the MRR for the µEDM of different ceramics. Scanning electron microscope (SEM) images of the cross section of the unprocessed and µEDM processed surface of these ceramics have been analyzed. The SEM micrographs show that the µEDM process does not affect the ceramics bulk. It also confirmed spalling as one of the dominant material removal mechanism for ZrO₂ ceramics.