High strain rates have a similar influence to large deformations on the refinement of microstructure. In both cases, at large strains and high deformation rates, a strong tendency to form microbands is observed. It was found, that the width of the microbands is very sensitive to changes of the deformation parameters. It has been observed particularly, that in severely deformed materials, the width of the microbands is reduced to nanometric dimensions. Hydrostatic extrusion, which has been used in for the deformation of copper in the current work, strain rates exceeding 2 1 3.84 10 − ⋅ ε = × s were employed. In all the samples investigated, numerous microbands were found in the microstructure. The width of microbands varied from 20 to about 400 nm. Thus, the width of some of the microbands exhibited dimensions typical of nanometric materials. Additionally, a special feature was the appearance of large areas of subgrains with an average dimension of about 200 nm. These areas were identified as recrystallized dynamically, or post-dynamically. Large misorientations were found between the microbands and the surrounding “matrix’. Such misorientation facilitates the formation of high angle boundaries, which in turn contribute to the changes of microstructure and mechanical properties. The mechanism for the creation of high misorientation in the microband areas is probably different from that operating during the process of dynamic recrystalization. The results confirm the possibility of obtaining a nanometric structure at lower deformation, but at higher strain rates.

No abstract

Electrical Discharge Machining (EDM) is a non-conventional machining process in which the material removal occurs by utilizing thermoelectric energy. EDM’s inherent properties render it a feasible method in machining hard-to-cut materials, like tool steels. The current study presents an experimental study regarding the machining of Calmax, a chromium - molybdenum - vanadium alloyed steel, with EDM. The control parameters are the pulse-on current, the pulse-on time and the open-circuit voltage, while the machining performance was estimated in terms of the Material Removal Rate (MRR), the Tool Wear Ratio (TWR) and the Arithmetic Average Roughness (Ra). The experiments were carried out based on Taguchi DOE and an L16 orthogonal array design. Finally, for the aforementioned performance indexes, Analysis Of Variance (ANOVA) was performed to determine how the machining parameters impact the process’ results.