STUDY OF PLASTIC DEFORMATION OF MAGNESIUM ALLOYS WITH GRADUATE ALUMINIUM CONTENT

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1 Acta Metallurgica Slovaca, 12, 2006, 4 ( ) 490 STUDY OF PLASTIC DEFORMATION OF MAGNESIUM ALLOYS WITH GRADUATE ALUMINIUM CONTENT Čížek L. 1, Kocich R. 1, Greger M. 1, Praźmowski M. 2, Tański T. 3 1 VŠB Technical University of Ostrava, 17. listopadu 15, Ostrava, Czech Republic, lubomir.cizek@vsb.cz, radim.kocich@vsb.cz, miroslav.greger@vsb.cz 2 Technical University of Opole, S. Mikolajczyka 5, Opole, Poland, marpraz@po.opole.pl 3 Silesian University of Technology, Konarskiego St. 18a, Gliwice, Poland, tomasz.tanski@polsl.pl STUDIUM PLASTICKÉ DEFORMACE HOŘČÍKOVÝCH SLITIN S ROSTOUCÍM OBSAHEM HLINÍKU Čížek L. 1, Kocich R. 1, Greger M. 1, Praźmowski M. 2, Tański T. 3 1 VŠB Technická Univerzita Ostrava, 17. listopadu 15, Ostrava, Česká republika lubomir.cizek@vsb.cz, radim.kocich@vsb.cz, miroslav.greger@vsb.cz 2 Politechnika Opolska, S. Mikolajczyka 5, Opole, Polsko, marpraz@po.opole.pl 3 Politechnika Slaska, Konarskiego St. 18a, Gliwice, Polsko tomasz.tanski@polsl.pl Abstrakt Přestože slitiny typu AZ91 patří do skupiny k nejdéle využívaných hořčíkových slévárenských slitin ke konstrukčním účelům, stále se objevují nové možnosti dalšího využití spojené s neustálým zkvalitňováním výrobních procesů, vedoucích ke zvyšování jejich užitných vlastností. S tímto rozvojem souvisí i zájem o detailnější poznání fyzikálně-metalurgických procesů probíhajících při jejich výrobě a zpracování. Použití hořčíku a jeho slitin k výrobě tvarově velmi rozmanitých odlitků má svou dlouhodobou tradici a souvisí především s rozvojem leteckého průmyslu. Jsou známy dlouhodobé snahy vyrábět a používat kovové materiály s vysokými hodnotami pevnostních vlastností, zajištujících dlouhodobou životnost mechanicky, fyzikálně nebo chemicky či kombinovaně namáhaných strojních součástí při dosažení minimálních hodnot jejich specifické pevnosti. Snižování hmotnosti řady polotovarů i finálních výrobků při zachování jejich tvarové tuhosti a životnosti je celkovým trendem v konstrukci strojů a zařízení. Rozsah použití slévárenských slitin hořčíku se neustále rozšiřuje a je nutno se velmi aktivně zabývat zlepšováním vlastností jednotlivých slitin, optimalizací jejich chemického složení, studiem otázek jejich metalurgické přípravy, experimentálním prověřováním jejich slévárenských vlastností i podmínkami úspěšného odlévání odlitků jednotlivými metodami, včetně jejich tepelného zpracování Znalost plastické je velmi důležitou veličinou pro komplexní posuzování vlastností při komerčním využití hořčíkových slitin. V předloženém příspěvku je hodnocen vliv proměnných podmínek při prováděných testech na plasticitu hořčíkových slitin s rostoucím obsahem hliníku ve výchozím litém stavu a po tepelném zpracování. V práci byly použity slitiny AZ91, AZ61 and AZ31 (podle ASTM). Cílem práce bylo stanovení mechanických vlastností použitých slitin v závislosti na jejich zpracování, včetně metalografického rozboru mikrostruktury a analýzy lomových ploch pomocí SEM. Dále byl sledován vliv předchozí

2 Acta Metallurgica Slovaca, 12, 2006, 4 ( ) 491 deformace metodou ECAP na plasticitu vybrané hořčíkové slitiny za zvýšené teploty a malé rychlosti deformace při které byl pozorován jev superplasticity. Abstract Magnesium alloys has been used for a wide variety of applications, namely from the reason of their low density and high strength to weight ratio. Their material selection is preceded by analysis of many factors, including considerations concerning mechanical properties, design, environmental, urbanization, recycling, cost, availability etc. Low inertia, which results from its low density, is advantageous in rapidly moving parts, for example automobile wheels and other automobile parts. The similar situation is in the aeronautical market and air frame application. Scope of utilisation of foundry magnesium alloys is continuously being extended, so if we want to operate as competitive producers, it is necessary to investigate very actively properties of individual alloys, optimise their chemical composition, study issues of their metallurgical preparation, verify experimentally their casting properties and conditions of successful casting of castings by individual methods, including heat treatment. Recently, however, increases also utilisation of formed magnesium alloys. Knowledge of plastic properties is very important for complex evaluation of magnesium alloys for commercial using. The paper presents the influence of changing testing conditions on plasticity of magnesium alloys with graduated aluminium content at cast state and after heat treatment. Alloys AZ91, AZ61 and AZ31 (after ASTM Standard) were used. Objective of the work was determination mechanical properties of magnesium alloys in dependence on method of its processing, including investigation of structure and fracture characteristics with use light metallographic analyse and SEM. Impact of previous deformation after equal channel angular pressing by ECAP method on mechanical properties of selected alloys was considered as another important factor. At special conditions an effect of superplasticity was observed. Keywords: magnesium alloys, mechanical properties, plasticity, heat treatment, metallographic and fracture analysis 1. Introduction Material selection is preceded by analysis of many factors, including considerations concerning mechanical properties, design, environmental, urbanization, recycling, cost, availability e.t.c. [2]. Magnesium alloys has been used for a wide variety of applications, namely from the reason of their low density and high strength to weight ratio. Low inertia, which results from its low density, is advantageous in rapidly moving parts, for example automobile wheels and other automobile parts and others products. See Fig.1. For practical design are usually used two categories of magnesium alloys: First category (I.) represents magnesium alloys with content of 2-10 % of Al, pertinently with minor content of Zn and Mn. These alloys are produced with low (relatively low) costs and their mechanical quality rapidly falls down at higher temperatures. Second category (II.) represents magnesium alloys with wide variety of chemical elements (for ex. Zn, Th, Ag and Si) instead of Al, but always with effectively low content of Zr

3 Acta Metallurgica Slovaca, 12, 2006, 4 ( ) 492 which means close-grained structure and higher mechanical quality. These alloys have better characteristics at higher temperatures but more expensive elements together with special production technology means higher production costs. Fig.1 Sony Ericsson K750i, model W800. From other point of view magnesium alloys are divided on Casting alloys (Gravity and Low-pressure casting, Die-casting) Wrought alloys Composites Powder materials Scope of utilisation of foundry magnesium alloys is continuously being extended, so if we want to operate as competitive producers, it is necessary to investigate very actively properties of individual alloys, optimise their chemical composition, study issues of their metallurgical preparation, verify experimentally their casting properties and conditions of successful casting of castings by individual methods, including heat treatment [1,2,3]. Magnesium alloys are subjected to heat treatment mostly for the purpose of improvement of their mechanical properties or as an intermediary operation, to prepare the alloy to other specific treatment processes. The type of heat treatment depends namely on the chemical composition of the alloy. A change of the heat treatment basic parameters has an influence on a change of the properties. Annealing significantly decreases the mechanical properties and causes improvement of plastic properties, thus facilitating further treatment. 2. Used material and experimental methods Experimental investigation was made with use of cast plates (size 10x20x150 mm) of magnesium alloy AZ91 - Samples A, AZ61 - Samples B and AZ31 - Samples C (after ASTM Standard) in initial state as cast. Chemical composition of alloys is given in Table 1. Table 1 Chemical composition of used alloys (in weight %) Alloy Al Zn Mn Si Fe Zr Sn Ni Pb Ce A-AZ91 8,95 0,76 0,21 0,041 0,008 0,003 0,01 0,003 0,059 0,01 B-AZ61 5,92 0,49 0,15 0,037 0,007 0,003 0,01 0,003 0,034 0,01 C-AZ31 2,96 0,23 0,09 0,029 0,006 0,003 0,01 0,002 0,013 0,01

4 Acta Metallurgica Slovaca, 12, 2006, 4 ( ) 493 Samples A were heat treated (T4-signed after ASTM Standard): pre-heating 375 o C/3h heating 415 o C/18h, cooling in air (A1), water (A2) and furnace (A3). Testing of mechanical properties was made on tensile testing machine INOVA- TSM 50. Samples for tensile test in cast state had a form of bar with length 115 mm, diameter 6 mm, in central part the diameter was reduced to 4 mm in the length of 30 mm. Samples for ECAP method after application of heat treatment T4 (after ASTM) and rolling were used for next investigation [4]. Size of sample for ECAP method was 8 x 8 x 50 mm. Samples for tensile test after ECAP had a form of plate and measured size of sample was 3 x 12 x 1,5 mm. Total length of sample was 40 mm. Tensile test of samples after ECAP method was made at 250 o C with strain rate s -1. This temperature on the base of previous measurement was selected from the reason of best plasticity. The results of this testing were following: Tensile strength R m = 35 MPa and percentage elongation was 250%. Percentage reduction from the reason impossibility of correct fracture area measurement was approximately determined 400%. In order to complete the obtained results of mechanical properties and to clarify measurement dependencies an evaluation of microstructure and character of fracture were performed in the relevant samples. 3. Results of tests and discussion Results of tensile tests of investigated alloys (samples A, B, C, A1, A2, A3) are summarised in Table 2. Table 2 Results of tensile tests of used alloys Alloy R p 0,2 [MPa] R m MPa] A [%] A B C A A A As it is seen from Table 2 at ambient temperature Proof stress R p 0,2 falls with increasing content of aluminium in alloys while Tensile strength R m and Percentage elongation A are increased. As it is seen from this table the significant modifications of mechanical properties, namely increasing of plastic deformation was occured after heat treatment alloy AZ91 (signed alloy A). Microstructure of used magnesium alloys as cast state are showed in Fig.2. Microstructure in initial as cast state of sample A is formed by crystals of matrix on the basis of magnesium, surrounded by minority massive phase of the type Mg 17 Al 12 in almost continuous formations in interdendritic areas along grain boundary, which represent places of initiation and propagation of failure at tensile test (Fig.2a). Occurrence of minority massive phase of the type

5 Acta Metallurgica Slovaca, 12, 2006, 4 ( ) 494 Mg 17 Al 12 in the case of alloys B is reduce and (Fig.2b) in the case of alloys C this phase is not presented, only changing of etching in grain boundary regions is observed (Fig.2c) a) b) c) Fig.2 Microstructure of the as-cast magnesium alloys: a) Alloy A, b) Alloy B, c) Alloy C Microstructure magnesium alloy AZ91 (sample A) after heat treatment T4 is shown in Fig.3. Occurrence of minority massive phase of the type Mg 17 Al 12 is strongly reduced after cooling on air and water from the reason of its dissolving (Fig.3.a,b), while in case of cooling in furnace this phase under eutectic temperature if again appear at form fine precipitate nearly in all grain areas (Fig.3c) [5,6] a) b) c) Fig.3 Microstructure magnesium alloys after heat treatment T4: a) cooling on air, b) cooling in water, c) cooling in furnace a) b) c) Fig. 4 Analysis of fracture areas with use of SEM (electron microscope JEOL 50A) samples in initial state: a) alloy A, b) alloy B, c) alloy C.

6 Acta Metallurgica Slovaca, 12, 2006, 4 ( ) 495 The metallographic investigation was completed by fracture analysis of fracture analysis (Fig.4). Fracture surface of sample alloy A as cast state is showed in Fig.4.a. There occurs interdendritic character of failure. This process is accompanied by forming of micropores in interdendritic areas contributing also to initiation of crack propagation along the phase boundary. Fracture surface of sample alloy B as cast state is showed in Fig.4.b. Fracture surface of sample alloy A as cast state is showed in Fig.4.c. As it is seen character of fracture area becomes more ductility. The microstructure of sample after rolling before ECAP method application is shown in Fig.5a and microstructure of samples after ECAP method application is shown in Fig.5b. As shown this figure fine grain microstructure vas occurred. a) b) Fig.5 Microstructure of samples used for ECAP method: a) after rolling, b) after ECAP method application 4. Conclusions The following conclusions can be drawn from results of evaluation of mechanical properties, structural and fracture characteristics of the magnesium alloys with graduated aluminium content at cast state and after heat treatment: Microstructure of the alloy in initial state is formed by solid solution and by minority phases Mg 17 (Al,Zn) 12 in massive and dispersion form. Microstructure has dendritic character, minority phases are comparatively continuously distributed in interdendritic areas, which represent suitable places for initiation and propagation of cracks under load. Results of tensile test of alloys AZ91, AZ61 and AZ31 at ambient temperature show that proof strength is falls with icreasing content of aluminium in alloys while Tensile strength and Percentage elongation are increased. Possibility of superplasticity for magnesium alloys AZ91 after heat treatment and application ECAP method was confirmed. Acknowledgements This paper was developed in the framework of solution of the project MSM (Ministry of Education, Youth and Sports of the Czech Republic) and cofinanced by Grant Agency CR (project 106/04/1346) and project INTERREG IIIA. Literature [ 1] Aghion E., Bronfin B.: Magnesium Alloys Development towards the 21th Century, In Proceedings of Conference: Materials Science Forum, Switzerland: Trans. Tech. Publication, 2000, p

7 Acta Metallurgica Slovaca, 12, 2006, 4 ( ) 496 [2] ASM specialty Handbook- Magnesium and Magnesium Alloys, ed. Avedesian, M.M., Baker, H., ASM International, USA, 1999, p [3] Fajkiel A., Dudek P.: Foudry Engineering-Science and practice, Publ.Inst. of Foundry Engineering, Cracow, 2004 [4] Greger M.. Čížek L., Widomska M.: Structural characteristics Magnesium Alloys along of the Equal Channel Angular Pressing. In: Advanced Metallic Materials and their joining, Bratislava 2004, p [5] Dobrzański L.A., Tański T., Čížek.: Influence of heat treatment on structure and mechanical property of the casting magnesium alloys. Proceedings of International Scientific Conference Contemporary Achievements in Mechanics, Manufacturing and Materials Science CAM3S 2005, Gliwice-Zakopane, 2005, 6 ps. (elektronic medium CD). [6] Čížek L., Greger M., Pavlica L., Dobrzański L.A., Tański T.: Study of selected properties of magnesium alloy AZ91 after heat treatment and forming. In: Journal of Materials Processing Technology, , 2004, p