Short-Term Material Characterization by Electrohydraulic Incremental Extrusion through Micro Channels.

Conventional testing procedures for characterizing the mechanical behavior of materials require intense preparation in geometry and in the handling of the samples to apply specific stress conditions. Furthermore, these procedures are time consuming. In a novel method for high-throughput development of new material, spherical and cylindrical micro samples should also be tested within a short time. For mechanical testing, the samples need to be exposed to specific types of stress. As most conventional testing procedures are not applicable, new testing procedures are demanded. The incremental electrohydraulic extrusion of micro samples through micro channels is a new testing procedure that was introduced for short-term material characterization. Loading energy is used to cause shock waves that incrementally push the samples through the forming die. The resulting deformation progress is measured between the forming steps. In this research, process simulations are used for channel design and material flow analysis. The designed channels that cause specific stress in samples are realized by stacking elements radially or axially. The stacking enables sample access for measurement and unloading and ensures good machinability of the forming channels. New testing cases for short-term characterization of cylindrical as well as spherical micro samples by electrohydraulic extrusion are presented according to monotone tensile, compression, and torsion testing. Furthermore, production-related testing and cyclic load testing are introduced by incremental electrohydraulic extrusion. By measuring the deformation due to the dependence on supplied energy, flow curve equivalents are determined that correspond to values from conventional material testing procedures.

Material Flow in Infeed Rotary Swaging of Tubes.

Rotary swaging is an incremental metal forming process widely used to reduce the cross-section of parts. For tubular parts, the final wall thickness also changes during the process. The lubricant condition is a factor, which affects these geometry changes. Beneath the change of the geometry, the complex material flow during the process determines the final geometry and the mechanical properties. Therefore, with a thorough insight into the material flow, it could be understood how to control it in order to achieve desired properties. Producing tubes with uniform outer diameter and changing inner profiles is an application of this method. Furthermore, applying this method, different local cold hardening could be achieved by different total strain. In this study, the dependency of the material flow on the lubrication conditions was investigated. Simulations with combined hardening material models were verified by the change of the wall thickness of tubes. It was found that friction condition significantly influences the back shifting of the workpiece and the elongation caused by each stroke. Results from simulations and experiments showed that a certain lubricant condition leads to the highest axial elongation of the workpiece.

Diamond-Like-Carbon Coated Dies for Electromagnetic Embossing.

Electromagnetic forming is a high-speed process, which features contactless force transmission. Hence, punching operations can be realized with a one-sided die and without a mechanical punch. As the forces act as body forces in the part near the surface, the process is especially convenient for embossing microstructures on thin sheet metals. Nevertheless, the die design is critical concerning wear like adhesion. Several die materials were tested, like aluminum, copper as well as different steel types. For all die materials adhesion phenomena were observed. To prevent such adhesion an a-C:H-PVD (Physical Vapor Deposition)-coating was applied to steel dies (X153CrMoV12) and tested by embossing aluminum sheets (Al99.5). By this enhancement of the die adhesion was prevented. Furthermore, the die surface was structured with tribology-effective patterns that were generated by micro hard milling. The embossing quality was topographically analyzed with respect to different initial surface states of the sheets. It was identified that thicker sheets facilitate better embossing results. Moreover, the initial sheet surface has a decisive influence on the embossing quality, whereby the characteristic of the topography showed different susceptibility on the initial sheet surface state.

Influence of Process Fluctuations on Residual Stress Evolution in Rotary Swaging of Steel Tubes.

Infeed rotary swaging is a cold forming production technique to reduce the diameter of axisymmetric components. The forming is achieved discontinuously by a series of radial strokes that are spread over the shell of the part. Due to tolerances within the rotary swaging machine, these strokes perform individually and the resulting stroke pattern is not homogeneous with regards to circumferential and longitudinal distribution. Nevertheless, in combination with the high number of performed strokes and the large contact area between the dies and the part, the external part properties, such as diameter, roundness and surface roughness, show even values along the finished part. In contrast, strength-defining internal part properties, like microstructure and residual stress components, are more sensitive to the actual pattern and temporal sequence of the individual strokes, which is investigated in this study. The impact of process fluctuations during the conventional process, which are induced by the tolerances of machine tool components, was verified by numerical simulations, physical tests and measurements of residual stress distributions at the surface and at depth. Furthermore, a method is introduced to maintain the stroke following angle ∆φ at zero by flat dies, and thus the actual pattern and temporal sequence of the strokes was homogenized. The results show that the residual stress fluctuations at the surface could be controlled and reduced. Furthermore, it is demonstrated that the depth profile of the residual stresses at a distance of 300 µm from the surface developed independently from the process fluctuations.