Influence of aluminium content on properties of magnesium cast alloys.

Transcription

1 Influence of aluminium content on properties of magnesium cast alloys. L.A. Dobrzański 1, T. Tański 1, L.Čížek 2 1 L. A. Dobrzański, Prof, T. Tański, MsC. :Silesian University of Technology, Institute of Engineering Materials and Biomaterials, Konarskiego St. 18a, Gliwice, Poland, Corresponding Autor Leszek.Dobrzanski@polsl.pl 2 L. Čížek, Prof.: VSB- Technical University of Ostrava, Faculty of Metallurgy and Materials EngineeringTř.17 listopadu 15, Ostrava. Czech Republic, Corresponding Autor e- mail: lubomir.cizek@vsb.cz Abstract In this paper there is presented the structure and proprieties of the modeling cast magnesium alloys as cast state and after heat treatment. The presented results concern X-ray qualitative and quantitative microanalysis, tensile tests and hardness measurements. Castings in the form of Ø42xØ56xh120 cones and 200x100x15 mm plates were melted in a resistance furnace using a protective salt bad Flux 12 equipped with two ceramic filters by the applied melting temperature of 750±30 C (according to the manufactured alloy). Caused trough the metallurgical casting quality efforts of the manufactured alloy a refining with a neutral gas wit the industry name Emgesalem Flux 12 was carried on. To improve the surface quality a protective layer Alkon M62 was applied. Castings were made in dies with betonite binder caused trough its excellent sorption properties. The caste material was heated in an electrical resistance furnace in protective argon atmosphere. The heat treatment involve the solution heat treatment (T4 - warming material in temperature 375 C by 3 hour, it elevation temperature to 430 C, warming by 10 hours ) and cooling in different cooling mediums as well water, air and furnace. The improvement of the manufacturing technique and chemical composition as well as of heat treatment and cooling methods leads to the development of a material designing process for the optimal physical and mechanical properties of a new developed alloy. In the analysed alloys a structure of α solid solution and fragile phase β (Mg 17 Al 12 ) occurred mainly on grain borders as well as eutectic and AlMnFe, Mg 2 Si phase. The investigation is carried out to testy the influence of the chemical composition and precipitation processes on the structure and mechanical properties of the magnesium cast alloys with different chemical composition in its as cast alloys and after heat treatment. 1. Introduction At the contemporary stage of the development of the engineering thought, and the product technology itself, material engineering has entered the period of new possibilities of designing and manufacturing of elements, introducing new methods of melting, casting, forming, and heat treatment of the casting materials, finding wider and wider applications in many industry branches. Engineers whose employment calls for significant expenditure of labour and costs strive to reduce material consumption. Therefore the development of engineering aims at designs optimizing, reducing dimensions, weight, and extending the life of devices as well as improving their reliability [1, 2, 11]. The material selection is preceded by the analysis of many factors like: mechanical, design, environmental, urbanization, recycling, cost, availability, and weight related issues, which may change the existing conditions and emerge the needs resulting from the supplier-

2 customer relation [5]. The strive to decrease the weight of products becomes an important challenge for designers and process engineers. The excessive weight verifies a significant extent the possibilities of employing particular material groups. Contemporary materials should possess high mechanical properties, physical and chemical, as well as technological ones, to ensure long and reliable use. The above mentioned requirements and expectations regarding the contemporary materials are met by the nonferrous metals alloys used nowadays, including the magnesium alloys. Magnesium alloys and their derivatives, alike materials from the lightweight and ultra-lightweight family, characterize of low density ( g/cm 3 ) and high strength in relation to their weight [1-4]. Moreover, the magnesium alloys demonstrate good corrosion resistance, no aggressiveness towards the mould material and low heat of fusion, which make it possible to use pressure die casting that ensures good shape reproducibility. The designers closer and closer cooperate with magnesium alloy manufacturers, which is a good example of the fact that currently about 70% of the magnesium alloy castings are made for the automotive industry. Lowering car weight by 100 kg makes it possible to save 0.5 l of petrol per 100 km. It is anticipated that in the following years the mass of castings from magnesium alloys in an average car will rise to 40 kg, internal combustion engines will be made mostly from the magnesium alloys and car weight will decrease from 1200 kg to 900 kg [6, 9, 10]. The concrete examples of the employment of castings from magnesium alloys in batch production in the automotive industry are elements of the suspension of the front and rear axes of cars, propeller shaft tunnel, pedals, dashboards, elements of seats, steering wheels, elements of timer-distributors, air filters, wheel bands, oil sumps, elements and housings of the gearbox, framing of doors and sunroofs, and others (Table 1) [7, 8]. Table 1. The examples of uses magnesium alloys in industry car [2] Company Part Model Clutch housing, oil pan, steering column Ranger Ford Four-wheel-drive transfer case housing Aerostar 1994 Manual transmission case housing Bronco Valve cover, air cleaner, clutch Corvette housing(manual) General Motors Induction cover North Star V Clutch pedal, brake pedal, steering column brackets W Oldsmobile, Pontiac, Buick Drive brackets, oil pan Jeep 1993 Chrysler Steering column brackets LH midsize 1993 Drive brackets, oil pan Viper Daimler-Benz Seat frames 500 SL Alfa-Romeo Miscellaneous components (45Kg) GTV Porsche AG Miscellaneous components (53 kg) 911 Wheels (7.44 kg each) 944 Turbo

3 A good capability of damping vibrations and low inertia connected with a relatively low weight of elements have predominantly contributed to the employment of magnesium alloys for the fast moving elements and in locations where rapid velocity changes occur; some good examples may be car wheels, combustion engine pistons, high-speed machine tools, aircraft equipment elements, etc. [9, 10]. The increasing use of magnesium alloys is caused by the progress in the manufacturing of new reliable alloys with the addition of Zr, Ce i Cd and very light alloys are made from Li (used for constructions in the air-industry and for space vehicles) [1, 2]. Because of the growing requirements for materials made from light alloys revealing mechanical properties, corrosion resistance, manufacturing costs and the influence on the environment, these efforts can be considered as very up-to-date from the scientific point of view and very attractive for further investigation. For the reason of growing requirements for materials made from light alloys concerning mechanical properties, corrosion resistance, manufacturing costs and the influence no the environment this efforts can be consider as very up-to-date from the scientistic view and very attractive for investigation. The goal of this paper is to present of the investigation results of the casting magnesium alloy in its as-cast state and after heat treatment. 2. Experimental procedure The investigations were carried out on test pieces from the casting magnesium alloy made by ČKD Motory a.s. Hradec Králové in the as-cast state and after heat treatment (Table 1) with the chemical composition given in Table 2. Castings in the form of Ø42xØ56xh120 cones and 200x100x15 mm plates were melted in a resistance furnace using a protective salt bad Flux 12 equipped with two ceramic filters by the applied melting temperature of 750±10 C (according to the manufactured alloy). Caused trough the metallurgical casting quality efforts of the manufactured alloy a refining with a neutral gas wit the industry name Emgesalem Flux 12 was carried on. To improve the surface quality a protective layer Alkon M62 was applied. Castings were made in dies with betonite binder caused trough its excellent sorption properties. The caste material was heated in an electrical resistance furnace in protective argon atmosphere. The heat treatment involve the solution heat treatment (T4) and cooling in different cooling mediums as well water, air and furnace. Fractures of the investigated materials were examined using the Philips Xl-30 scanning microscope. The X-ray quantitative and qualitative analyses of the investigated alloy were carried out on the transverse microsections on the Philips scanning microscope with the EDX energy dispersive radiation spectrometer at the accelerating voltage of 20 kv. Hardness tests were made using Zwick ZHR 4150 TK hardness tester in the HRF scale. Ten measurements were made for each test piece and the average value and standard deviation was calculated. Tensile strength tests were made using Zwick Z100 testing machine. Ten tests were made for each test piece and the average value and standard deviation was calculated. Table 1. Parameters of heat treatment of investigation alloy Sing the state of heat Conditions of solution heat treatment treatment Temperature, C Time of warming, h Way coolings 0 As-cast Water Air In furnace

4 Table 2. Chemical composition of investigation alloy The mass concentration of main elements, % Al Zn Mn Si Fe Mg Rest 9,09 0,77 0,21 0,037 0,011 89,7905 0, Discussion of experimental results Examinations of the chemical composition of the casting magnesium alloys using the EDX spectrometer confirmed the presence of the main alloying elements: magnesium, aluminium, manganese, and zinc. It was found out that the cast magnesium alloys were characteristic of the α solid solution microstructure featuring the alloy matrix and the β Mg 17 Al 12 intermetallic phase was located mostly at grain boundaries (Fig. 1). Moreover, in the vicinity of the β intermetallic phase precipitations the presence of the (α+β) eutectics was revealed (Fig. 1, 4). One can clearly observe in the structure of the casting magnesium alloys, not only the Mg 17 Al 12 phase precipitations, the distinct aluminium, manganese and silicon concentrations, which indicate the presence of the AlMnFe and Mg 2 Si type precipitations in the alloy structure (Fig. 5) µm Fig. 1. Microstructure alloy with 9 % Al without heat treatment as-cast Fig. 2. Microstructure alloy with 9 % Al after cooling in the water 20 µm Fig. 3. Microstructure alloy with 9 % Al after cooling in the air Fig. 4. Microstructure alloy with 9 % Al after cooling in the furnace

5 A 50 µm Mg Al Mn Si Fe Fig. 5. The area analysis of chemical elements alloys with 9 % Al, after cooling in the air Phases containing Mg and Si are colored violet and have sharp edges in the shape of hexahedrons (Fig. 2, 3, 5). Phases with high Mn concentration (are colored red) are irregular with a non plain surface, they often occur in the form of blocks or needles (Fig. 5). After the air-cooling of the alloy the remainder amounts of the β(mg 17 Al 12 ) and AlMnFe, Mg 2 Si phases were identified in the alloy structure in the α solid solution. After water cooling, the alloy precipitations in the α phase structure were revealed (Mg 17 Al 12, AlMnFe and Mg 2 Si). After cooling the alloy in the furnace the α structure was revealed and, moreover, locations of the α + β eutectic occurrences, precipitations of the β(mg 17 Al 12 ) phases were located at the grain boundaries and AlMnFe, Mg 2 Si precipitation. Structure of this alloy is similar to the as-cast alloy structure.

6 Observations of fractures after the static tensile strength test of the analysed alloy made it possible to determine the effect of heat treatment on the nature of the investigated fractures (Figs 6-13) µm Fig. 6. The brittle fracture surface of tensile test without heat treatment as-cast Fig. 7. The brittle fracture surface of tensile test without heat treatment as-cast 50 µm Fig. 8. The ductile fracture surface of tensile test after cooling in the water Fig. 9. The ductile fracture surface of tensile test after cooling in the water µm Fig. 10. The ductile fracture surface of tensile test after cooling in the air Fig. 11. The ductile fracture surface of tensile test after cooling in the air

7 50 µm 20 µm Fig. 12. The ductile fracture surface of tensile test after cooling in the furnace Fig. 13. The ductile fracture surface of tensile test after cooling in the furnace The as-cast alloy, without the heat treatment and after the type 3 treatment demonstrates a brittle fracture. The casting magnesium alloy after the heat treatment of type 1 and 2 is characteristic of a ductile fracture. The alloy has acquired the highest hardness after cooling in the furnace (treatment 3). In case of the alloy after treatment 1 and 2 its hardness decreases insignificantly compared to the as-cast state 0 (Table 3). The results of the static tensile strength test make it possible to determine and compare the mechanical and plastic properties of the alloy without the eat treatment A and after the heat treatment (1, 2, 3) (Table 3. It was found out in the strength tests that subjecting the alloy to heat treatment improves significantly its mechanical properties. Heat treatment contributes to improvement of mechanical properties, yield point and hardness with the slight reduction of the elongation. Furnace-cooled test pieces demonstrate the highest strength. Alloys after treatment 3 are distinguished by a significant increase of the R 0.2 yield point, and alloys subjected to the treatments 1 and 2 increases slightly compared to the as-cast alloy. Elongation, however, nearly twice for the case of 1 and 2 in comparison with the as-cast state. Table 3. Mechanical properties analysis magnesium alloys Kind of cast Yield point Rp0,2, [MPa] Tensile strength RM, [Mpa] Percentage elongation A, [%] Hardness HRF 0 121,20 182,44 5,34 65, ,10 241,65 12, ,07 247,76 10,15 60, ,93 266,67 2,868 71,20 4. Summary The results of the EDX chemical composition analysis confirm the presence of magnesium, aluminum, manganese, and zinc, constituting the structure of α solid solution with the Mg 17 Al 12 placed mainly on the grain order in the form of plates (Fig. 3-9).

8 Also the phase AlMnFe with irregular shape, occurred often in the shape of blocks or needles (Fig.5.) and the Laves phase Mg 2 Si. The phases containing Mg and Si coloured violet are characterized with sharp outlines with plain edges in form of particles. The different heat treatment kinds employed contributed to the improvement of mechanical properties of the alloy at the slight reduction of its plastic properties. References 1. Magnesium Alloys and their Application, ed. Kainer, K.U., Sb. Int. Congress Magnesium Alloys and their Application, Mnichov, 2000, p.3-8, J.F.Grandfield, J.A.Taylor : Tensile cohereny in semi-solid AZ91, Held during the TMS Annual Meting in Seattle, Washington, USA, ASM specialty Handbook- Magnesium and Magnesium Alloys, ed. Avedesian, M.M., Baker, H., ASM International, USA, 1999, p J.M. Tartaglia, J.C.Grebetz: Observations of Intermetallic Particle and Inclusion Distributions in Magnesium Alloys., The Minerals, Metals&Materials Society, Mordike B.L., Ebert T.: Magnesium. Properties-applications-potential, Materials Science and Enrineering nr A302, Journal of Materials Processing Technology, 117 (2001), p Čížek L., Greger M., Pawlica L., Dobrzański L.A., Tański T.: Study of selected properties of magnesium alloy AZ91 after heat treatment and forming, Journal of Materials Processing Technology, (2004), p L.A.Dobrzański, T.Tański, W.Sitek, L. Čížek: Modeling of mechanical properties magnesium alloy, 12 th Scientific International Conference on Achievements in Mechanical and Materials Engineering AMME 2003, Gliwice Zakopane, 2003, p Dobrzański L.A., Tański T., Čížek L: Influence of modification with chemical elements on structure of magnesium casting alloys, 13 th Scientific International Conferences Achievements in Mechanical and Materials Engineering AMME, 2005, Gliwice Wisła 2005, p A. Fajkiel, P. Dudek: Foundry engineering - Science and Practice, Publishers Institute of Foundry engineering, Cracow, 2004, p A. Fajkiel, P. Dudek, G. Sęk-Sas: Foundry engineering XXI c. Directions of metallurgy development and Ligot alloys casting, Publishers Institute of Foundry engineering, Cracow, Friedrich H., Schumann S.: Research for a new age of magnesium in the automotive industry, Journal of Materials Processing Technology, 117 (2001), pp