Owing to the low formability of single-crystal nickel-based materials, single-crystal components are typically cast. Subsequently, a multi-stage heat treatment is carried out in order to partially compensate the dendrite segregation and to stabilize the precipitated γ'-phase. Such components possess a high resistance to creep at elevated temperatures. Since it is known that electrical impulses can be used to increase the formability of various materials, the potential of an electrical high current-density impulse treatment was evaluated for forming of the high strength nickel-based superalloy CMSX-4. Heat treated and pre-deformed specimens were loaded in compression and subjected to short pulses with a high-current density of 2.3 kA/mm 2. Depending on the microstructural state, the material demonstrated work hardening or softening as a consequence of the impulse treatment. In addition, experiments were carried out on crept specimens to test whether the current-impulse treatment can be used to reverse creep-related segregation of alloying elements (raft formation). It was possible to observe a change in the concentration of the elements in the γ/γ'-phase transition following the currentimpulse treatment. In particular, a local increase in the γ-forming elements Cr, Co and a decrease in the γ'-forming elements Ta, Ti was observed after the impulse treatment.
Magnesium alloys are important engineering materials due to their good combination of strength and very low densities. However, the low ductility imposed by the hcp-lattice has thus far limited the application of magnesium alloys as sheet material. The use of the electroplastic effect offers a route to increase formability of magnesium alloys while being more energy efficient than conventional hot forming. The underlying mechanism (s) of this effect have not yet been fully understood. This study investigates the impact of high current density electrical pulses on magnesium alloys. Special consideration was given to the effect of the orientation of the applied electric current relative to the mechanical loading of the specimens. The results show that the mechanical properties of coarse-grained materials are more strongly affected by the current pulses than finer grained material. Applying the current parallel to the compressive load shows a more pronounced softening of the material than pulses applied perpendicular to the mechanical stress. Microstructure investigations revealed the formation of twinning solely in the interior of grains even at stresses below the yield point for both configurations.
In order to reduce the number of process steps of steel forging processes. and thus also tool wear and process costs, tailored preform geometries can be produced by casting. By only one subsequent forming operation it is possible to improve the casting microstructure, eliminate possible defects and achieve the required mechanical properties. To evaluate the properties of the cast microstructure and the closure of possible casting defects during warm/hot forming, cylindrical steel billets (C45/1.0503) were produced by sand casting and then upset in a hydraulic press. Global plastic strain φ was varied between 0.3 and 0.7 while forging temperatures of 600 °C and 1200 °C were applied to detect possible temperature effects. Conventional rolled bar material formed under the same conditions was used as a reference. After forming, the specimens were tempered and the mechanical properties were determined by tensile tests (ISO 6892-1) and notch impact tests (similar to ISO 148-1). The microstructures were examined by metallographic analysis while defects were characterised using an optical wide-area 3D measurement system and digital image processing. It could be observed that the ultimate tensile strength of the cast-forged specimens depends on the forming temperature and is about 15 % lower in comparison to the reference material at a forming temperature of 600 °C and 5 % lower at 1200 °C, respectively. The impact energies show a strong dependence on plastic strain for both, the castforged and reference specimens. The values of the mechanical properties of the reference specimens were higher than those of the cast-forged specimens. These results allow a deeper understanding of the cast-forging of steel and will contribute to the cast-forging design of more complex steel parts.
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