Superplastic Flow of Metals Extruded by KoBo Method

Abstract

The extrusion of metals by the KoBo method, which is a new technological solution, allows to perform the process at low temperature with severe plastic deformation (SPD) under cyclic change of deformation path conditions. Apart from the localized plastic flow in shear bands intensive generation of point defects (self interstitials) takes place. It is important that excessive concentration of interstitial atoms is preserved in extruded products and often takes the shape of nanoclusters. Their presence causes that the effect of superplasticity (e.g. about 800% elongation of aluminum alloy 7075) does not require nanometric sizes of the grains and appears in metals extruded by the KoBo method with grains of over dozen or even several dozen micrometers. The results suggest the necessity to verify the mechanisms currently considered as decisive of the superplastic flow of metals.

Lüders deformation and superplastic flow of metals extruded by KOBO method

“…For a comparison, however, some billets were extruded also at 350°C. Press container had an internal diameter of 40 mm, while the manufactured products had form of wires with a diameter of 4 mm (extrusion ratio λ = 100) or of flats with dimensions of 3 Â 11 mm (λ = 38) in case of direct extrusion ( Figure 1) or of tapes with dimensions of 1.5 Â 30 mm (λ ≈ 30) when extruded with lateral outflow ( Figure 2) [38,39]. It is necessary to emphasize that the lateral outflow of the material during the KOBO extrusion did not require an increase in the load as in conventional process, and it was possible due to highly viscous nature of the deformation mechanism to induct it under the scheme of straining used.…”

“…Korbel and W. Bochniak zinc subjected to tension at the rate of 2.7 Â 10 À4 s À1 at 180°C showed elongation higher than 600% ( Figure 25). [39,49]. Samples cut from flat extrudate of 7075 alloy were subjected to the tensile test at 450°C.…”

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

“…It was considered that both objectives can be achieved by comparing the mechanical properties of KOBO extruded products with direct and lateral (Fig. 2) [9,10] outflow, performed under constant force of extrusion maintained by continuous adjustment of the frequency of oscillating die (spindle) [11].…”

Constitutive modeling of cyclic deformation of metals under strain controlled axial extension and cyclic torsion

“…Such a mode of deformation was recently implemented in technological processes such as extrusion, forging and rolling, cf. Bochniak and Korbel (Eng Trans 47:351-367, 1999, J Mater Process Technol 134:120-134, 2003, Philos Mag 93:1883-1913, 2013, Mater Sci Technol 16:664-674, 2000. The constitutive model accounting for combined hardening (isotropic-kinematic) with both hardening and recovery effects is presented and calibrated for several materials: pure copper, aluminum alloy (2024), and austenitic steel.…”

“…In fact, in recent years, a growing interest is observed in metal forming processes assisted by cyclic loading. The so-called KOBO-type forming proposed by Korbel and Bochniak [9][10][11][12]47] and applied to extrusion of tubes and wires has demonstrated essential advantages with respect to classical forming processes. The significant reduction of required load, growth of ductility, fine grain structure are main beneficial factors, cf.…”