Influence of aluminium content on properties of magnesium cast alloys.

L.A. Dobrzański1, T. Tański1, L.Čížek2

1L. A. Dobrzański, Prof, T. Tański, MsC. :Silesian University of Technology, Institute ofEngineering Materials and Biomaterials, Konarskiego St. 18a, 44-100 Gliwice, Poland,Corresponding Autor e-mail: Leszek.Dobrzanski@polsl.pl

2L. Čížek, Prof.: VSB- Technical University of Ostrava, Faculty of Metallurgy and MaterialsEngineeringTř.17 listopadu 15, 708 33 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 magnesiumalloys as cast state and after heat treatment. The presented results concern X-ray qualitativeand quantitative microanalysis, tensile tests and hardness measurements. Castings in the formof Ø42xØ56xh120 cones and 200x100x15 mm plates were melted in a resistance furnaceusing a protective salt bad Flux 12 equipped with two ceramic filters by the applied meltingtemperature of 750±30°C (according to the manufactured alloy). Caused trough themetallurgical casting quality efforts of the manufactured alloy a refining with a neutral gas witthe industry name Emgesalem Flux 12 was carried on. To improve the surface quality aprotective layer Alkon M62 was applied. Castings were made in dies with betonite bindercaused trough its excellent sorption properties. The caste material was heated in an electricalresistance furnace in protective argon atmosphere. The heat treatment involve the solutionheat treatment (T4 - warming material in temperature 375 ° C by 3 hour, it elevationtemperature to 430 ° C, warming by 10 hours ) and cooling in different cooling mediums aswell water, air and furnace. The improvement of the manufacturing technique and chemicalcomposition as well as of heat treatment and cooling methods leads to the development of amaterial designing process for the optimal physical and mechanical properties of a newdeveloped alloy. In the analysed alloys a structure of α solid solution and fragile phase β(Mg17Al12) occurred mainly on grain borders as well as eutectic and AlMnFe, Mg2Si phase.The investigation is carried out to testy the influence of the chemical composition andprecipitation processes on the structure and mechanical properties of the magnesium castalloys 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 producttechnology itself, material engineering has entered the period of new possibilities ofdesigning and manufacturing of elements, introducing new methods of melting, casting,forming, and heat treatment of the casting materials, finding wider and wider applications inmany industry branches. Engineers whose employment calls for significant expenditure oflabour and costs strive to reduce material consumption. Therefore the development ofengineering aims at designs optimizing, reducing dimensions, weight, and extending the lifeof 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, whichmay change the existing conditions and emerge the needs resulting from the supplier-

customer relation [5]. The strive to decrease the weight of products becomes an importantchallenge for designers and process engineers. The excessive weight verifies a significantextent 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 mentionedrequirements and expectations regarding the contemporary materials are met by the non-ferrous metals alloys used nowadays, including the magnesium alloys. Magnesium alloysand their derivatives, alike materials from the lightweight and ultra-lightweight family,characterize of low density (1.5-1.8 g/cm3) and high strength in relation to their weight[1-4]. Moreover, the magnesium alloys demonstrate good corrosion resistance, noaggressiveness towards the mould material and low heat of fusion, which make it possibleto use pressure die casting that ensures good shape reproducibility.The designers closer and closer cooperate with magnesium alloy manufacturers, which isa good example of the fact that currently about 70% of the magnesium alloy castings aremade for the automotive industry. Lowering car weight by 100 kg makes it possible tosave 0.5 l of petrol per 100 km. It is anticipated that in the following years the mass ofcastings from magnesium alloys in an average car will rise to 40 kg, internal combustionengines will be made mostly from the magnesium alloys and car weight will decreasefrom 1200 kg to 900 kg [6, 9, 10].The concrete examples of the employment of castings from magnesium alloys in batchproduction in the automotive industry are elements of the suspension of the front and rearaxes 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 ofthe 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

Four-wheel-drive transfer case housing Aerostar 1994Ford

Manual transmission case housing Bronco

Valve cover, air cleaner, clutchhousing(manual)

Corvette

Induction cover North Star V-8 1992General Motors

Clutch pedal, brake pedal, steering columnbrackets

„W“ Oldsmobile,Pontiac, Buick

Drive brackets, oil pan Jeep 1993

Steering column brackets LH midsize 1993Chrysler

Drive brackets, oil pan Viper

Daimler-Benz Seat frames 500 SL

Alfa-Romeo Miscellaneous components (45Kg) GTV

Miscellaneous components (53 kg) 911Porsche AG

Wheels (7.44 kg each) 944 Turbo

A good capability of damping vibrations and low inertia connected with a relatively lowweight of elements have predominantly contributed to the employment of magnesium alloysfor the fast moving elements and in locations where rapid velocity changes occur; somegood 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 ofnew 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 thegrowing requirements for materials made from light alloys revealing mechanical properties,corrosion resistance, manufacturing costs and the influence on the environment, these effortscan be considered as very up-to-date from the scientific point of view and very attractive forfurther investigation.For the reason of growing requirements for materials made from light alloys concerningmechanical properties, corrosion resistance, manufacturing costs and the influence no theenvironment this efforts can be consider as very up-to-date from the scientistic view and veryattractive for investigation.The goal of this paper is to present of the investigation results of the casting magnesium alloyin 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 byČKD Motory a.s. Hradec Králové in the as-cast state and after heat treatment (Table 1) withthe chemical composition given in Table 2.Castings in the form of Ø42xØ56xh120 cones and 200x100x15 mm plates were melted in aresistance furnace using a protective salt bad Flux 12 equipped with two ceramic filters by theapplied melting temperature of 750±10°C (according to the manufactured alloy). Causedtrough the metallurgical casting quality efforts of the manufactured alloy a refining with aneutral gas wit the industry name Emgesalem Flux 12 was carried on. To improve the surfacequality a protective layer Alkon M62 was applied. Castings were made in dies with betonitebinder caused trough its excellent sorption properties. The caste material was heated in anelectrical resistance furnace in protective argon atmosphere. The heat treatment involve thesolution heat treatment (T4) and cooling in different cooling mediums as well water, air andfurnace.Fractures of the investigated materials were examined using the Philips Xl-30 scanningmicroscope. The X-ray quantitative and qualitative analyses of the investigated alloy werecarried out on the transverse microsections on the Philips scanning microscope with the EDXenergy dispersive radiation spectrometer at the accelerating voltage of 20 kV. Hardness testswere made using Zwick ZHR 4150 TK hardness tester in the HRF scale. Ten measurementswere 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 foreach test piece and the average value and standard deviation was calculated.

Table 1. Parameters of heat treatment of investigation alloy

Conditions of solution heat treatmentSing the state of heattreatment Temperature , °C Time of warming, h Way coolings

0 As-cast1 430 10 Water2 430 10 Air3 430 10 In furnace

Table 2. Chemical composition of investigation alloy

The mass concentration of main elements, %Al Zn Mn Si Fe Mg Rest9,09 0,77 0,21 0,037 0,011 89,7905 0,0915

3. Discussion of experimental results

Examinations of the chemical composition of the casting magnesium alloys using the EDXspectrometer confirmed the presence of the main alloying elements: magnesium, aluminium,manganese, and zinc. It was found out that the cast magnesium alloys were characteristic ofthe α solid solution microstructure featuring the alloy matrix and the β Mg17Al12 intermetallicphase was located mostly at grain boundaries (Fig. 1). Moreover, in the vicinity of the βintermetallic 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 theMg17Al12 phase precipitations, the distinct aluminium, manganese and silicon concentrations,which indicate the presence of the AlMnFe and Mg2Si type precipitations in the alloystructure (Fig. 5).

Fig. 1. Microstructure alloy with 9 % Alwithout heat treatment – as-cast

Fig. 2. Microstructure alloy with 9 % Alafter cooling in the water

Fig. 3. Microstructure alloy with 9 % Alafter cooling in the air

Fig. 4. Microstructure alloy with 9 % Alafter cooling in the furnace

10 µm20 µm

20 µm20 µm

A Mg

Al Mn

Si FeFig. 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 ofhexahedrons (Fig. 2, 3, 5). Phases with high Mn concentration (are colored red) are irregularwith 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(Mg17Al12) and AlMnFe,Mg2Si phases were identified in the alloy structure in the α solid solution. After watercooling, the alloy precipitations in the α phase structure were revealed (Mg17Al12, AlMnFeand Mg2Si).After cooling the alloy in the furnace the α structure was revealed and, moreover, locations ofthe α + β eutectic occurrences, precipitations of the β(Mg17Al12) phases were located at thegrain boundaries and AlMnFe, Mg2Si precipitation. Structure of this alloy is similar to theas-cast alloy structure.

50 µm

Observations of fractures after the static tensile strength test of the analysed alloy made itpossible to determine the effect of heat treatment on the nature of the investigated fractures(Figs 6 - 13).

Fig. 6. The brittle fracture surface of tensiletest without heat treatment – as-cast

Fig. 7. The brittle fracture surface of tensiletest without heat treatment – as-cast

Fig. 8. The ductile fracture surface of tensiletest after cooling in the water

Fig. 9. The ductile fracture surface of tensiletest after cooling in the water

Fig. 10. The ductile fracture surface oftensile test after cooling in the air

Fig. 11. The ductile fracture surface oftensile test after cooling in the air

20 µm100 µm

50 µm50 µm

50 µm100 µm

Fig. 12. The ductile fracture surface oftensile test after cooling in the furnace

Fig. 13. The ductile fracture surface oftensile test after cooling in the furnace

The as-cast alloy, without the heat treatment and after the type 3 treatment demonstrates abrittle fracture. The casting magnesium alloy after the heat treatment of type 1 and 2 ischaracteristic of a ductile fracture. The alloy has acquired the highest hardness after cooling inthe furnace (treatment 3). In case of the alloy after treatment 1 and 2 its hardness decreasesinsignificantly compared to the as-cast state 0 (Table 3).The results of the static tensile strength test make it possible to determine and compare themechanical and plastic properties of the alloy without the eat treatment –A and after the heattreatment (1, 2, 3) (Table 3. It was found out in the strength tests that subjecting the alloy toheat treatment improves significantly its mechanical properties. Heat treatment contributes toimprovement of mechanical properties, yield point and hardness with the slight reduction ofthe elongation. Furnace-cooled test pieces demonstrate the highest strength. Alloys aftertreatment 3 are distinguished by a significant increase of the R0.2 yield point, and alloyssubjected 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 castYield pointRp0,2, [MPa]

Tensile strengthRM, [Mpa]

PercentageelongationA, [%]

HardnessHRF

0 121,20 182,44 5,34 65,681 126,10 241,65 12,10 632 132,07 247,76 10,15 60,703 158,93 266,67 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 theMg17Al12 placed mainly on the grain order in the form of plates (Fig. 3-9).

50 µm20 µm

Also the phase AlMnFe with irregular shape, occurred often in the shape of blocks or needles(Fig.5.) and the Laves phase Mg2Si. The phases containing Mg and Si coloured violet arecharacterized with sharp outlines with plain edges in form of particles.The different heat treatment kinds employed contributed to the improvement of mechanicalproperties of the alloy at the slight reduction of its plastic properties.

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