Andrzej Korbel

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.

Formation of shear bands during cyclic deformation of aluminium

Abstract A study of shear band formation in aluminium during cyclic extrusion-compression at high strain amplitude was performed. The techniques employed enabled the metal to be deformed far beyond the strains available by conventional monotonic deformation methods. The measurements of microhardness, along with optical and TEM observations, have shown that at strains on the order of 6.0 aluminium enters the saturation stage. Prior to the saturation, formation and rapid multiplication of shear bands was observed. It has been found that formation of shear bands is independent of the mode of deformation (monotonic or cyclic). The structural features of shear bands and their effect on the mechanical performance of aluminium has been revealed. The origin of shear bands is discussed in terms of geometrical vs structural softening models.

Strain hardening of aluminium at high strains

Abstract The paper brings a critical analysis of the shape of the strain hardening curve of aluminium within the range of high strains. Basing on a statistical analysis it has been shown that the hardening curve has a series of plateaus. The structural investigations indicate that the plateaus on the hardening curve correspond to heterogeneous plastic deformation in the form of shear band the result of which is a form of cyclic heterogeneity in the structure and the properties of the material during deformation.

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.

The effect of strain localization on mechanical properties of A199,992 in the range of large deformations

The work is an attempt to answer the question about the cause of the plateau on the strain hardening curve of aluminium in the range of very large deformations, greatly exceeding the conventional range of strains. It has been found that the strain localization in the long-range shear bands crossing the whole volume of the samples is the main deformation mechanism in the range of plateau strains. As a result of mutually crossing of shear bands and microbands the almost equiaxial subgrains are formed, inducing homogenization of the structure in the range of plateau strains.

Mode of deformation and the rate of energy storage during uniaxial tensile deformation of austenitic steel

The mechanism of slip and its consequence in the process of energy storage during uniaxial tension of austenitic steel was studied. The interpretation of the energy storage process in terms of the slip development and microscopic shear band formation is presented.

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.