Papers by Author: Georgy I. Raab

Abstract: In this paper, we investigated the process of rotary forging of commercially pure copper grade M2 ​​using standard and special-shaped anvils and presented the results of studies obtained by the method of numerical and physical modeling. It is established that the use of anvils with special geometric shapes provides a higher level of accumulated strain and the formation of more dispersed structural states with the same elongation ratio under conditions of multi-cycle processing [1]. The formation of a finer structure in its turn increases the hardness and strength of the material. In addition, the special shape of the anvils provides a positive field of values ​​of the Lode-Nadai coefficient in the cross section of the samples, predominantly in a range of 0.3-0.7 and, correspondingly, a more "comfortable" stress state close to non-uniform all-round compression, which contributes to increasing technological plasticity.

Abstract: Physical simulation of the stress-strain state and microstructure evolution, which are similar to that occurring during asymmetric rolling with a large strain, is very important for design of technologies of producing ultra fine grained metallic materials. This paper presents the results of optimization of specimen geometry and a special multi-cycle shear-compression technique for the physical simulation of asymmetric rolling with a large strain up to e ~ 4. The specimen consisted of a parallelepiped having an inclined gauge section created by two diametrically opposed semi-circular slots which were machined at 45°. The specimen was compressed between two flat dies during shear-compression testing in accordance to the special multi-cycle scheme. Each cycle of the shear-compression testing consisted of two steps. The first step included height reduction of specimen, after that specimen was rotated by 90º. The second step included length reduction of the specimen for getting the quasi original shape of a parallelepiped. The specimen provided simultaneous pure and simple shear in an inclined gauge-section. The level of effective strain was controlled through adjustment of the specimen geometry, height reduction, load application direction and number of cycles of shear-compression. Gauge thickness, width and radius of the specimen were optimized by FEM with using of software DEFORM 3D. Numerical simulation and comparison of the stress-strain state during shear-compression testing and asymmetric rolling of low-carbon steel AISI 1010 were performed. Results of FEM analysis of the applicability of the multi-cycle shear-compression testing to the modeling of asymmetric rolling were discussed.

Abstract: Grain refinement by severe plastic deformation can make conventional metallic materials several times stronger, but it leads to dramatic loss of their ductility. Gradient structure through the thickness of processed material represents a new strategy for producing a superior combination of high strength and good ductility. In gradient metallic materials the grain size increases gradually from nanoscale at the surface to coarse-grained in the core. Strain gradient can be considered as a mechanism of creating of such microstructures. Providing of predetermined strain gradient in the metallic materials can be achieved by asymmetric rolling (AR), when circumferential speeds of the top and bottom work rolls are different. Since the AR is a continuous process, it has great potential for industrial production of large-scaled sheets. Searching the optimal process parameters which can provide special strain gradients through sheet thickness is very important. This paper presents the distributions of the effective strain through sheet thickness of low-carbon steel AISI 1015 processed by a single-pass AR. Influence of process parameters was investigated by the finite element method with using software DEFORM 2D. Extremely high strain gradient e ≈ 4...8 through sheet thickness during a single-pass AR was found. FE analysis of the deformation characteristics, presented in this study, can be used for optimization of the AR process as a method of fabrication of metallic materials with gradient microstructures.

Abstract: Various contributions to the overall strength of the Cu-1Cu-0.7Al-0.2Zr alloy after the combined severe plastic deformation treatment have been calculated and compared with those after the standard industrial processing. Contrary to the common viewpoint, the SPD increases the strength not only due to the structure refinement, but also because of greater contribution of the dispersion strengthening. It is argued that this effect is linked to the deformation-induced phase transitions upon the SPD.

Abstract: The paper considers the features of the manifestation of dynamic strain aging (DSA) effect during severe plastic deformation processing via equal-channel angular pressing of low-carbon steel 10 and during the deformation processing via rolling of steel 20Kh. The deformation mechanisms under different regimes of deformation processing are analyzed. The temperature ranges for the manifestation of the DSA effect during the deformation by rolling of steel 20Kh and by equal-channel angular pressing of steel 10 are established. It is demonstrated that the deformation of low-carbon steels in the temperature range of DSA leads to further structure refinement and, as a consequence, to the enhancement in strength properties.

Abstract: The paper analyzes the regularities of structure formation in low-alloyed carbon steels. During the investigation of ferritic-pearlitic steel samples it has been found that the structure formation in pearlite essentially lags behind structural changes in ferrite grains, and this delay is observed at all stages of deformation. An important feature of structure formation in pearlite is crack nucleation in cementite, accompanied by dislocation pile-up in the ferrite interlayers of pearlite. Using the method of dislocation dynamics, the relationship between structural transformations and the parameters of strain hardening is analyzed. It is demonstrated that the proposed method of computer analysis reflects well the processes taking place in a material during plastic deformation. The character of the theoretical curve of strain hardening is determined by the dislocation structure that forms in a material at various stages of deformation.

Abstract: This paper presents the results of a computer and experimental study of a promising severe plastic deformation (SPD) technique, called Multi-ECAP-Conform (M-ECAP-C), for the fabrication of long-length nanostructured billets (wire rods) with an enhanced strength and electrical conductivity from the aluminum alloy EN-AW 6101 per one processing cycle. On the basis of the obtained results, a new rational geometry of the pressing channel for the M-ECAP-C process has been developed. The strained state of pilot samples of wire rods have been studied. Using the method of dividing grids shows the character of accumulation and the achieved level of deformation shear during the experimental treatment, and the average value of the shear deformation of the central region of the workpiece is 3.015. It was found that the error between the obtained values of the experimental method of grids, computer modeling and calculation is, respectively, 10 % and 17 %. The error between the experimental and computer modeling is not so important, so application of the computer modeling to estimate the strain state of the method of the Multi-ECAP-Conform is completely adequate.

Abstract: Severe plastic deformation is one of promising techniques for omproving metal properties. In this work a new SPD technique – Multi-ECAP-Conform – is studied. The technique ensures an accumulation of a true strain degree to е>2.5 during one processing cycle. Models for force characteristics calculation have been developed on the basis of theoretical procedures of metal forming. The derived equations determine force parameters of the Multi-ECAP-Conform process.

Abstract: The present work deals with the evolution of mechanical properties and structure of low-carbon Fe-1,12Mn-0,08V-0,07Ti-0,1C (wt.%) steel after severe plastic deformation (SPD) and high-temperature annealing. Steel in initial ferritic-pearlitic state was deformed by equal channel angular pressing (ECAP) at T=200°C and high pressure torsion (HPT) at room temperature. The evolution of ultrafine grained structure and its thermal stability were investigated after annealing at 400-700°C for 1 hour. The results shown that SPD leads to formation of structure with an average size of (sub-) grain of 260 nm after ECAP and 90 nm after HPT. Ultrafine grained structures produced by SPD reveal a high thermal stability up to 500°C after ECAP and 400°C after HPT. At higher annealing temperatures a growth of structural elements and a decrease in microhardness were observed.

Abstract: This work reports on the results of investigation of microstructure change of commercially pure titanium Grade 4 with the increase of the number of ECAP-Conform passes. There has been investigated influence of continuous equal-channel angular pressing by the scheme “Conform” (ECAP-C) on the structure and properties of commercially pure titanium Grade 4. It has been demonstrated that as a result of first two ECAP-C cycles titanium structure is strongly fragmented and deformation bands are formed. With the further increase of ECAP-C passes to 6 the band structure is transformed into ultrafine-grained (UFG) structure with the grain size of about 250 nm. The strength of titanium regularly grows with the increase of the number of ECAP-C passes, while ductility, which settles after first cycle on the level of 12%, is almost not changed with the further strain degree increase. As a result of the subsequent drawing of titanium after ECAP-C its strength additionally increases to 1300 MPa, with retention of ductility about 11%.