THERMOMECHANICAL PROPERTIES OF MAGNESIUM ALLOY Mg0,7Zr. VŠB Technical university of Ostrava, 17. listopadu 15/2172, Ostrava, Czech Republic

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1 Acta Metallurgica Slovaca, 13, 27, 4 ( ) 654 THERMOMECHANICAL PROPERTIES OF MAGNESIUM ALLOY Hanus A. 1, Lichý P. 1, Kozelský P. 1, Čížek L. 2, Cagala M. 1 1 Foundry Department, 2 Department of Materials Engineering, Faculty of Metallurgy and Materials Engineering, VŠB Technical university of Ostrava, 17. listopadu 15/2172, Ostrava, Czech Republic TERMOMECHANICKÉ VLASTNOSTI HO Ř Č ÍKOVÉ SLITINY Hanus A. 1, Lichý P. 1, Kozelský P. 1, Čížek L. 2, Cagala M. 1 1) Katedra slévárenství 2) Katedra materiálového inženýrství, Fakulta metalurgie a materiálového inženýrství, VŠB Technická univerzita Ostrava, 17. listopadu 15/2172, Ostrava, Česká Republika Abstrakt Hoř č ík je nejlehč í ze všech kovů, které jsou užívány jako základní konstrukč ní slitiny. Tato vlastnost je využíváno v automobilovém prů myslu jako náhrada tě žších materiálů, nejen oceli, litiny a slitin mě di, ale také hliníkových slitin. Požadavek na snižení hmotnosti autodílů je dů sledkem zpř ísň ujících se emisních norem. Hoř č ík a hoř č íkové slitiny jsou primárně používány v leteckém a automobilovém prů myslu díky kombinaci jejich charakteristických mechanických a fyzikálních vlastností - pevnost v tahu ( MPa), modul pružnosti (45Gpa) a nízká hustota (174 kg/m 3 2/3 hustoty hliníku). Hoř č íkové slitiny mají vysoký pomě r hodnot pevnosti a hmotnosti porovnatelnými s ostatními konstrukč ními materiály. Znalosti relaxač ních vlastností nejen kovových konstrukč ních materiálů za vysokých teplot jsou nezbytné př i produkci odlitků pro ově ř ování jejich náchylnosti ke tvorbě defektů bě hem výroby a tím následně zamezení vzniku tě chto vad. Pomocí tahových zkoušek za vysokých teplot je možné zjistit teplotní rozsahy zkoušeného materiálu, ve kterých se generují nejvyšší hladiny pnutí. Vzniklá pnutí v odlitku jsou př íč inou tvorby a rozvoje defektů. Stále nároč ně jší požadavky na dopravní prostř edky (snižování spotř eby, vyšší mě rné výkony motorů, otázky bezpeč nosti a další) vedou k potř ebě používat materiály na odlitky s vyššími mechanickými, fyzikálními a technologickými vlastnostmi. Z výše uvedených dů vodů narů stá podíl odlitků z lehkých neželezných kovů a to i pro nejnároč ně jší použití na výrobu hlav válců a bloků př eplň ovaných motorů. Z tě chto materiálů dominují hliníkové slitiny, zvyšuje se však i celkový podíl strojních souč ástí slitin hoř č íku [1-4]. Slitiny hoř č íku př edstavují v souč asné době skupinu kovových materiálů s nejrychlejším nárů stem objemu výroby - bě hem posledních deseti let je roč ní nárů st výroby 7 procent. (Fig.1) Abstract Magnesium is the lightest of all metals used as the basis for constructional alloys. It is this property which entices automobile manufacturers to replace denser materials, not only steels, cast irons and copper base alloys but even aluminium alloys by magnesium based alloys. The requirement to reduce the weight of car components as a result in part of the introduction of legislation limiting emission has triggered renewed interest in magnesium. The growth rate over the next 1 years has been forecast to be 7% per annum.(fig. 1) A wider use of magnesium base alloys necessitates several parallel programs. These can be classified as alloy development,

2 Acta Metallurgica Slovaca, 13, 27, 4 ( ) 655 process development:improvement and design considerations. These will be discussed briefly and followed by some examples of the increasing uses of magnesium and future trends. Magnesium and magnesium alloys are primarily used in aeronautical and automobile industry in wide variety of structural characteristics because of their favorable combination of tensile strength (16 to 365 MPa), elastic modulus (45 GPa), and low density (1 74 kg/m 3 ), which is two-thirds that of aluminum). Magnesium alloys have high strength-to-weight ratio (tensile strength/density), comparable to those of other structural metals [1-4] Knowledge of the relaxation properties of metal materials at elevated temperatures is necessary for the verification of susceptibility of castings to the creation of defects during the production and forming processes. Temperature limits of materials where highest tension values are generated may be detected with tensile tests under high temperatures. Key words: relaxation properties at high temperatures, Mg alloy Fig.1 Increase in pressure die-cast components in USA and Europe 1. Introduction In our research, we have used magnesium alloy. Zirconium is very important additive as grain refining agent. The maximum solid solubility of Zr in Mg is 1,42 at.% Zr. (fig. 2) Can be used with alloys containing zinc, rare earths, thorium, ytrrium or a combination thereof but not in alloys containing aluminium or manganese as these form stable compounds with zirconium. It also forms stable compounds with iron, silicon, carbon, nitrogen, oxygen and hydrogen in the melt. Only dissolved zirconium is effective as grain refiner. [3] 2. Experimental construction During the projects research awarded by the Czech Republic Grant Agency, a station for measurement of solid and plastic qualities of ferrous materials, including measurement of temperatures of irons, casted irons and alloys made of non-ferrous metals. The scheme of the test machine is drawn in the Fig. 3. In the research we have used a machine intended for measurement of mechanical qualities of metals and alloys delivered by the company INOVA Praha, with the possibility to measure a load upto 2kN. The machine was accompanied by a heating furnace of a unique design and with heat control. The measurement was controlled by a PC which enables exact setting of the heating effect and loading capacity, and measurement of force, location and deformation. The testing programs run in WINDOWS and the data was processed by MS EXCEL. The measurement machine complies with no. 1 degree of accuracy given by the international standards.

3 Acta Metallurgica Slovaca, 13, 27, 4 ( ) 656 Fig.2 Binary system Mg-Zr Fig.3 Experimental set for measurement of solid and plastic qualities of ferrous materials treated in high temperatures 3. Description of methodology The measurement of solid and plastic qualities was performed with use of test rods which were mechanically treated to the final dimensions (see Fig. 5,6). The tested samples of magnesium alloy were obtained from the semi-products which had been produced by means of gravity casting. Fig.5 Drawing of the test rod used in rupture tests

4 Acta Metallurgica Slovaca, 13, 27, 4 ( ) 657 Heating of the test rods was performed inside a vertical electricity-run resistance furnace with a pipe-shaped graphite heating body. The heating procedure is performed in two steps during the experiment. Firstly, the sample reaches the temperature intended for the measurement in speed of 1 C/min and when the demanded temperature is reached, a three minutes long isothermal time follows, in which the sample is applied load of 6 mm/min. Finally, the sample is placed into the furnace which maintains a controlled atmosphere so that oxidation of fracture areas is limited, intended for further metallographic and fracture-graphic analysis. Fig.6 The test rod used in rupture tests 4. Achieved results The relation between solid-state and elasticity-plastic qualities and temperature (from 2 C to 35 C) is drawn in the Fig 7, 1. The final values represent the average values of three measurements. We may see that thermomechanical values rapidly decrease in relation to the increasing temperature. 5. Conclusion The achieved results proved the data which was previously studied in the literature which dealt with the possibilities to use the molds, made of these types of materials, under more demanding conditions. The lower measured values correspond to the means of samples preparation. The values were obtained from the research of foundry semi-products during their gravity casting. The experiments still continue and new samples, purposed for heat treatment, have been prepared Tensile stress [MPa] Fig.7 Record of tensile stress curve

5 Acta Metallurgica Slovaca, 13, 27, 4 ( ) Contraction [%] Fig.8 Record of contraction curve 3 Deformation to fracture [mm] Fig.9 Record of deformation to fracture curve Fracture work [J] Fig.1 Record of fracture work curve

6 Acta Metallurgica Slovaca, 13, 27, 4 ( ) 659 Acknowledgements The paper has been co-financed by project MSM Preparation and qualities of highly-purified and structurally-defined special materials, INTERREG IIIA No. CZ /2.3.CZ.1/32 and developed in the framework of solution of the projects and MSM Literature [1] ASM specialty Handbook- Magnesium and Magnesium Alloys, ed. Avedesian, M.M., Baker, H., ASM International, USA, 1999, s [2] Ptáč ek L.: Slitiny hoř č íku- souč asný stav vývoje a použití, In Sborník konference METAL 21, Ostrava, Tanger, 21, s [3] Friedrich H. E., Mordike B. L.: Magnesium technology, Metallurgy, Design Data, Applications, ISBN , Springer Berlin Heidelberg New York 26. [4] Rouč ka J.: Metalurgie neželezných slitin, skriptum VUT Brno, Brno 24.