Article

Using severe plastic deformation to prepare of ultra fine - grained materials by ECAP method

Archives of Materials Science and Engineering 01/2007;
Source: DOAJ

ABSTRACT Purpose: The purpose of the paper is to design a tool and propose a procedure for verification of development of structure at equal channel angular pressing. The goal is to obtain after extrusion the semi-products of AlCuMg alloys a fine-grain structure which one hand increases strength properties and plasticity, and on the other hand is possible to use it at selected cases for subsequent deformations under conditions of „super-plastic state“.Design/methodology/approach: The experiments were aimed the verification of functionality of the proposed equipment, determination of deformation resistance, deformability and change of structure at extrusion of the alloy AlCu4Mg2. Deformation forces were measured at extrusion. The average grain size in cross direction was determined by quantitative metallographic methods. TEM analysis of the structure of AlCu4Mg2 were also made.Findings: The structural analysis of AlCu4Mg2 alloy made by TEM has demonstrated a perfect suitability of the ECAP die design. The process results in a very fine grain structure (100-200 nm) throughout the sample overall volume, at which the starting average grain size was 150 !m.Practical implications: Aluminium alloys of super fine granularity structure are basic intermediate products realised by ECAP technologies. The state of super fine granularity facilitates forming of material in the so-called ‘superplastic state’. The achievement of the desired structure depends primarily on the tool geometry, number of passages through the die, magnitude and speed of deformation, process temperature, and lubrication mode.Originality/value: It has been demonstrated that the extrusion technology is suitable for attaining of grain nano-structure in the material investigated in order to determine the number of extrusion cycles needed and the appropriate canal angle with corresponding internal and external bend radii. The obtained results make for success of further investigative efforts in the area.

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    ABSTRACT: Purpose: of this paper was to investigate the effect of grain refinement in AZ31 magnesium alloy processed by ECAP and to study microstructure evolution and hardness response of AZ31 alloy associated with ECAP processing.Design/methodology/approach: The microstructure of AZ31 magnesium alloy after two passes of equal channel angular pressing at 150°C, 180°C and 250°C was studied by means of metallographic and transmission electron microscopy. The hardness changes after ECAP processing were determined by Vickers hardness.Findings: The grain refinement in AZ31 magnesium alloy was successfully carried out using ECAP processing at 150, 180 and 250°C. The grain size decreases nearly 10 times after 2 passes of ECAP at 150 and 180°C, but microstructure is characterized by bimodal grains structure. The rather homogenous grains were achieved after ECAP processing at 250°C. Processes of dynamic recrystallization during ECAP were observed. The hardness increase related to grain refinement proceeded by ECAP is in accordance with Hall-Petch relationship.Research limitations/implications: The ECAP processing were carried out only after 2 passes, therefore in order to describe in detail the microstructural changes connected with grain refinement, the analysis of more passes of ECAP processing should be done.Practical implications: The development of highly ductile magnesium alloys allows to apply these materials as structural materials. The grain refinement of materials leads to the significant improvement of the mechanical properties and plasticity. The present results extend the knowledge about grain refinement in AZ31 alloy proceeded by ECAP.Originality/value: The microstructural studies of AZ31 alloy after grain refining by ECAP processing performed by transmission electron microscopy were presented.
    Archives of Materials Science and Engineering. 01/2009;
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    ABSTRACT: Purpose: Purpose of this paper is to determine the effect of Equal Channel Angular Pressing (ECAP) processing on the microstructure and hardness of α+β brasses. The effect of deformation temperature and number of passes was investigated particularly on the shape and size of grains of both phases.Design/methodology/approach: The specially constructed channel with 90° pressing angle, allowing heating of the tool with the sample was used for ECAP processing. The grain size was investigated using optical and transmission electron microscopy. The hardness and measurements microhardness were used to determine the effect of ECAP on the hardness of both phases.Findings: Significant grain refinement down to 300 nm from the initial 20 mm was observed after ECAP processing at 300°C. At 400°C grain refinement occurred down to 1-3 mm. Frequent microtwins were observed within a phase. The microhardness of the b phase was higher than that of a phase, 235 HV and 173 HV respectively.Research limitations/implications: The limitation is a size of the sample which makes difficult future applications. Another one is elevated temperature (minimum 300°C) otherwise the samples forms crack. This limits also the grain refinement which is above the range of nanomaterials.Practical implications: Significant grain refinement allows to increase the hardness and strength of the sample preserving a good plasticity. The limitation is the size of the channel what limits the application. The material could be used in such cases when high strength of brasses is needed with sufficient plasticity and good conductivity.Originality/value: In this paper detailed TEM studies were performed for α+β brasses showing high density of microtwins and higher density of dislocations within α phase, than in the b phase. Higher hardness of the b phase results from the ordering, which hinder deformation of this phase.
    Archives of Materials Science and Engineering. 01/2009;

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