Włodzimierz Bochniak

KOBO Type Forming: forging of metals under complex conditions of the process

Abstract Copper workpieces were subjected to forging in open and closed dies by the conventional method and by a complex one utilizing additional reversible rotary motion of the punch. The experimental data obtained in this way became a basis for comparative analysis. It has been found that the complex process of forging enables plastic forming of metal applying small forging force, and the forgings are characterized by homogeneous structure and good geometry while retaining low strength properties. The most effective application of the new method of plastic forming of metals (KOBO Type Forming) presented in this study, has been combined with the production in a single operation of forging preforms with the geometry close to that of a final product.

The structure based design of metal forming operations

Abstract The work summarizes the recent experimental studies and theoretical considerations on the nature of plastic deformation in metallic materials. Specifically, it concentrates upon the rules which the heterogeneous deformation obeys and which differ radically from those of the homogeneous deformation. Formulation of such rules is based upon the experiments which provide the very fundamental information about the criterion of slip in crystalline materials, choice and sequence of operation of slip systems and conditions under which “crystallographically determined” slip may be replaced by transgranular shear. The correlation of this form of slip with metal structure and global mechanical properties is given. The results of the studies allow to predict the onset of heterogeneous mode of deformation and what is even more important, they show the means due to which the mode of deformation may have taken under control. The very practical aspect of the work consists itself in the concept “of the structure based design” of metal forming operation. It is shown, that the scheme and sequence of straining in a particular forming operation appears extremely important from the point of view of the energy consumption of the process and the final structure of the metal. The examples of “energy saving” metal forming operations, designed according to the concept, are presented.

Energy balance and macroscopic strain localization during plastic deformation of polycrystalline metals

Abstract The energy balance during uniaxial test of the annealed FeSi sheet and the rolled one has been studied. The method of estimation of the energy storage rate in the stage of macroscopic strain localization has been presented. Heterogeneous temperature distribution on the surface of the loaded sample as an experimental indicator of the macroscopic localization of strain was used. The experimental evidence for the existence of recovery process during a development of the localized plastic deformation has been provided.

Lüders deformation and superplastic flow of metals extruded by KOBO method

The work brings the results of the study on mechanical properties of some metallic materials subjected to very large plastic deformation by KOBO extrusion. The unexpected features of the KOBO products like Lüders deformation in pure metals and superplastic flow in coarse grain materials are discussed in terms of micro- and nano-scale elements of their structure. The choice to the experiment materials having different crystallographic and phase structure (commercial purity aluminium, multiphase aluminium 7075 alloy, pure zinc and multiphase magnesium AZ91 alloy) and different history (extrusion, casting) allowed to identify the common nano-size elements of the structure generated during the KOBO deformation which seems to be responsible for the mechanical behaviour of these materials. In particular, clusters of point defects (self-interstitials) formed under the KOBO extrusion conditions (cyclic change in the deformation path, high hydrostatic pressure) were found in these materials regardless of grain size and material early history. They correlate with appearance of unstable Lüders-like or even Portevin–LeChatelier deformation at ambient and superplastic flow at elevated temperatures.

The `natural' metal–matrix composite formation by thermo-mechanical treatments

Abstract The progress in understanding the nature of strain localization in metallic materials and in particular understanding the role of external (deformation conditions) and internal (material substructure) factors in the development of the tendency toward concentration of plastic flow within transgranular shear bands is the background of the concept given in the title. Following this idea, the practical possibility of replacing the homogeneous in a micro-scale mode of deformation by transgrasnular shear bands gives a chance to change the arrangement of the dislocation substructure in the material from that typical for homogeneous deformation into long wavelength (transgranular), pseudo-periodic dislocation walls. The presence or formation of such a substructure in a thermodynamically unstable matrix may result in the formation of the products of phase transformation along shear bands, giving rise to a composite-like structure. This work is aimed at providing a coherent justification to the concept of a natural metal–matrix composite and summarizes the results of experimental work in this area.

Microstructure and texture of Mg-based AZ alloys after heavy deformation under cyclic strain path change conditions

The effects associated with the change of the deformation path - such as the replacement of homogeneous multi-slip by heterogeneous deformation and a decrease of global strain hardening - have been utilised in the metal forming operation termed KOBO technology. In the case of extrusion it consists in reversible, cyclic twisting of a billet under the extrusion force. The technology enables extrusion of metals with very large deformation in one operation at low temperature. A complex scheme of straining, large cumulated deformation and low temperature of the process results in a fine grained microstructure of the extruded material (product). The new technology requires detailed studies of the mechanism of the plastic deformation with the specific geometry of the zone of metal flow during extrusion. Essential in these studies is the information on the texture and microstructure in the deformation zone. The aim of this work is therefore to disclose the deformation mechanisms on the basis of the observations of microstructure and texture evolution in the zone of plastic flow of the extrudate. Coarse grained polycrystalline billets of magnesium alloys AZ31 were extruded by KOBO at room temperature and also by a conventional method at about 400°C. Methods of texture topography as well as optical observations reveal the specific microstructure and texture in mezzo and micro scale of heavily deformed material after extrusion. It is worth mentioning that the KOBO process leads to compact and rather homogeneous extrudates even in the case of AZ alloys. These hexagonal metals cannot be cold-formed to a high reduction with conventional techniques.

A new constitutive approach to large strain plastic deformation

Abstract This paper presents the results of studies regarding the effect of nonconventional plastic deformation on the mechanism of plastic flow of metals. The applied method, called KOBO extrusion, in which monotonic large deformation is accompanied by cyclic reversible plastic twisting, falls into the category of severe plastic deformation. The cyclic reversible plastic twisting, which proved to be redundant in terms of changes in the materials geometry, appears to be a particularly efficient source of point defects. The experimentally proven Newtonian viscosity of metals at low temperatures under KOBO extrusion conditions implies that it is necessary to conduct a study on the crystal plasticity from the point of view of the constitutive law in which a crystals viscosity totally controls the kinetics of the process. The well known dependency of the viscosity parameter on the diffusion coefficient, which, in turn, is a function of point defect concentration, dictates the link between the frequency of reversible plastic twist and the crystals viscosity and, consequently, determines the mode of plastic flow. The results of this work provide experimental verification which justifies this new approach to crystal plasticity.

Strain localization and superplastic-like flow in copper at high temperature

Abstract The localization of plastic deformation at high temperature and its influence on the mechanical performance of polycrystalline copper were studied. The results showed that the instability of the load during high-temperature deformation is caused by the localization of plastic flow within shear bands. This in turn leads to heterogeneity of the dislocation structure and becomes a motive for selective static recrystallization along shear bands. Under appropriate conditions the balance between the formation of new shear bands and recrystallization (or recovery) within already inactive shear bands leads on a global scale to superplastic-like behaviour of the metal.

Mechanical properties of aluminum extruded via the KOBO method with direct and lateral outflow

Abstract Aluminum was extruded via the KOBO method with direct and lateral outflow at ambient temperature with an extrusion ratio of λ = 100, keeping constant extrusion force all the time. Despite the far different metal flow geometries, in both cases products were obtained with very good quality (die shape reflection and surface state) and a slight variation in mechanical properties. It was also found that higher torsion angle of extruded metal lowers their levels. The results show both the real possibilities of conducting the extrusion process with lateral outflow, as well as a successive argument supporting the concept of superplastic flow of metal in the KOBO process.

Nano Grained Structure in KOBO Extruded Bulk Products

There are several known severe plastic deformation (SPD) methods, which allow receiving nano-grained structure of materials. One of the methods is called the KOBO method (the name is composed of the first two letters of the surnames of the inventors: KOrbel and BOchniak). The method was used for extrusion of copper alloys at room temperature to manufacture thin-wall tubes. During the KOBO extrusion, reversible cyclic torsion of the material results in multiple changing the deformation path. Hence, a heavy localized plastic flow leads to shear bands development that reduces the total strain hardening effect and results in effective structure refinement. In particular, thin-wall tubes produced from single-phase copper alloys, intensively cooled close to the die outlet, had homogeneous structure consisting of fine grains/subgrains of a few hundreds nanometers in size. The final structure of the material is practically independent on the initial structure of processed billets. If the accelerated cooling of extruded material was omitted, grain coarsening to the size of dozens of micrometers was observed. In multi-phase copper alloys, the grain refinement was limited to about 1 μm, both for intensively cooled and air-cooled products.