INVESTIGATIONS OF MECHANICAL PROPERTIES OF ULTRA-FINE GRAINED COMMERCIAL PURE ALUMINUM

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1 , Brno, Czech Republic, EU INVESTIGATIONS OF MECHANICAL PROPERTIES OF ULTRA-FINE GRAINED COMMERCIAL PURE ALUMINUM Rossen RADEV a, Valentin GAGOV a, Emil YANKOV a, Danail GOSPODINOV a a University of Ruse, Department of Materials Science & Technology, 8 Studentska St., 717 Ruse, Bulgaria, EU, rradev@uni-ruse.bg Abstract The ultra fine-grained (UFG) metallic materials are widely investigated during the past years with a view to the opportunities for various industrial applications of their improved mechanical and technological properties. Most of these materials are produced by severe plastic deformation, which allows large bulk samples to be obtained. This paper presents some preliminary results for commercial pure aluminum processed by equal channel angular pressing. Special laboratory equipment has been designed on the purpose of these experiments and some important changes in the structure and the mechanical properties after deformation are described. Keywords: 1. INTRODUCTION Metallic materials with grains size smaller than 1 µm have received much attention in the past decade. These materials have been classified as ultrafine-grain (UFG) materials (grain sizes in the range of 1 to 1 nm) and nanomaterials (grain size <1 nm) depending on the grain size. The growing interest is defined because of the their special properties, superior to those of conventional, coarse-grained materials. Hence designing potentially cost efficient and environmentally friendly products with better performance is possible. Among the different procedures in use for the production of nanostructured materials, only severe plastic deformation (SPD) processes exhibit a potential for producing relatively large samples suitable for industrial applications. Severe plastic deformation is metal forming processes in which an ultra-large plastic strain is introduced into a bulk metal in order to create nano- or ultrafine- grained structure at low homologous temperatures (typically below.3 of the melting temperature). In order to obtain smallest microstructural sizes plastic strains more than 6 to 8% are necessary. Such highest degrees of plastic deformation are possible because one sample can be subjected several times to SPD in order to accumulate the total amount of plastic strain. 2. FABRICATING OF ULTRA-FINE GRAINED ALUMINUM SAMPLES Recently, at least the following basic SPD techniques are available: Equal Channel Angular Pressing (ECAP) [1] (Fig. 1), High Pressure Torsion (HPT) [2], Multiaxial forging (MF) [3], Repetitive Corrugation and Straightening (RCS) [4], Cyclic Extrusion Compression (CEC) [5], Twist extrusion (TE) [6], Accumulative Roll-Bonding (ARB) [7]. The most promising of these processes for producing large size stock that is suitable for industrial application is the equal channel angular pressing (ECAP) process. This process involves introducing large shear strain in the workpiece by pushing it through a die that consists of two channels with the same crosssectional shape that meet at an angle to each other. Since the cross-sections of the two channels are the same, the extruded product can be re-inserted into the entrance channel and pushed again through the die.

2 , Brno, Czech Republic, EU Repeated extrusion through the ECAP die accumulates sufficient strain to breakdown the microstructure and produce ultra fine grain size. Fig. 1. Equal Channel Angular Pressing (ECAP). In this study, commercial pure aluminum was chosen for ECAP processing. The chemical composition of deformed material is shown on Table 1. Chemical Element Content [%] Table 1. Chemical composition of investigated commercial pure aluminum Si Fe Mg Cu Al,3242,2173,829,1725 Balance A particular ECAP tool was designed and realized. The tool, together with channel dimensions, deformation sequence and sliding planes at the time of processing are shown on Fig. 2. The initial billet was with dimensions 7 х 9,9 х 9,9 mm and was passed six times by the tool. After every pass the billet was rotated before the next pass according to Fig.2c. For six passes the deformed material accumulated strain more then 1%. c) a) b) d) Fig. 2. ECAP Tool а) 3D drawing, b) channel dimensions, c) deformation sequence, d) sliding planes. 3. MICROSTRUCTURE AND MECHANICAL PROPERTIES OF UFG ALUMINUM In order to research the microstructure changes of the billet material, microscopic pictures have been prepared (Fig. 3). The metallographic grains of the aluminum before deformation are visible with optical microscope. For deformed by ECAP material it is not possible. This fact calls additional investigation in order to obtain information about grains sizes after ECAP processing. The deformed samples were processed by focused ion beam (FIB) and then were observed by scanning electron microscope (SEM). The obtained image is shown on Fig. 4. It is evident the dimensions of metallographic grains after deformation are in the scale of ultra fine-grained materials.

3 , Brno, Czech Republic, EU Fig. 3. Microstructure of initial (left) and six times deformed (right) commercial pure aluminum (magnification X 6). Data about some mechanical properties of studied material also have been obtained. Tensile and compression tests have been carried out. The tensile specimen from UFG aluminum is shown on Fig. 5. The results of mechanical testing are shown on Fig. 6. Analyzing the received data, it can be seen that the tensile strength of ECAP deformed aluminum is more 2 times higher comparing with initial material. This is valid for tensile yield and for compressive yield, too. Fig. 4. Microstructure of commercial pure aluminum after deformation (magnification X 16). Fig. 5. Tensile specimen from UFG aluminum. In Table 2 are present data for mechanical properties, obtained by tensile mechanical tests and by hardness measurement of aluminum specimens before and after ECAP. Table 2. Mechanical properties of aluminum specimens Tensile testing Hardness State of materials Rm, MPa σb, MPa Ag, % A5, % HBS 15,6/2,5/3 HMV,5 Before deformation 94,3 37,9 35,1 96, After ECAP deformation 198,5 81,7 12,7 54,

4 , Brno, Czech Republic, EU 3 6 Load (N) 2 1 Load (N) Extension (mm) Extension (mm) a ) b) Compressive load (kn) Compressive extension (mm) Compressive load (kn) Compressive extension (mm) c ) d) Fig. 6. F L curves: a) tensile test before ECAP; b) tensile test after ECAP; c) compressive test before ECAP; d) compressive test after ECAP. a b As a part of the entire investigation, the compressed specimens have been observed after testing. Their geometrical forms in plane after the end of compressive tests are shown on Fig. 7. It can be supposed that the form and dimensions depends on the grain sizes obtained during the previous ECAP process. The more dispersive structure ensures the more uniform deformation due to the larger number of grains and appropriate orientated crystallographic plains. In this regard, it can be expected the ECAP-ed commercial pure aluminum samples have better plastic behavior. 4. CONCLUSIONS Fig. 7. Specimens after compressive tests a) ECAP-ed; b) before ECAP. Processing by ECAP is very effective in producing bulk aluminum samples with ultra fine grain sizes. After processing the investigated

5 , Brno, Czech Republic, EU material manifest significant changes of microstructure and mechanical properties. It is necessary additional researches in order to optimize the process of fabricating of UFG metal materials and to obtain data for correct simulation of their mechanical behavior and for their successful implementation in industry. ACKNOWLEDGEMENT This study is part of the project D2-158/28 Metal Forming Simulation of Ultra Fine-grained Aluminum Alloys", funded by the National Science Fund of Bulgaria. LITERATURE [1.] СЕГАЛ, В.М., РЕЗНИКОВ, В.И., ДРОБЫШЕВСКИЙ, А.Е., КОПЫЛОВ, В.И. Пластическая обработка металлов простым сдвигом, Изв. АН СССР. Металлы, 1981, c [2.] VALIEV, R., KORZNIKOV, A., MULYUKOV, R., Structure and Properties of Ultrafine-Grained Materials Produced by Severe Plastic Deformation, Mater. Sci. Eng. A, 168, 1993, p [3.] GHOSH, A., HUANG, W., Severe deformation based process for grain subdivision and resulting microstructures, Investigations and applications of Severe Plastic Deformation, (T. Lowe & R. Valiev, Ed.) 2, p [4.] ZHU, Y., HUANG, J., JIANG, H., LOWE, T., Processing of Bulk Nanostructured Copper by Repetitive Corrugation and Straightening (RCS), Innovation in Processing and Manufacturing of Sheet Metals, (M. Demeri, Ed.) 21, p [5.] KORBEL, A., RICHERT, M., RICHERT, J., The Effect of Very High Cumulative Deformation on Structure and Mechanical Properties of Aluminum, Proc. 2nd Riso Int. Symp. on Metall. and. Mat. Sci., 1981, [6.] BEYGELZIMER, Y., ORLOV, D., VARYUKHIN, V., A new severe plastic deformation method: Twist Extrusion, Ultrafine Grained Materials II. (Y. Zhu, T. Langdon, R. Mishra, S. Semiatin, M. Saran, T. Lowe, Ed.) 22, p [7.] SAITO, Y., TSUJI, N., UTSUNOMIYA, H., SAKAI, T., HONG, R., Ultra-Fine Grained Bulk Aluminum Produced by Accumulative Roll-Bonding (ARB) Process, Scripta Mater , p