PROPERTIES OF FORGINGS FORM MAGNESIUM ALLOYS AND TEHIR USE IN INDUSTRY

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1 PROPERTIES OF FORGINGS FORM MAGNESIUM ALLOYS AND TEHIR USE IN INDUSTRY Miroslav GREGER 1, Milena WIDOMSKÁ 1, Vlastimil KARAS 2, 1 VŠB Technická univerzita Ostrava, FMMI, 17. listopadu 15, , Ostrava-Poruba miroslav.greger@vsb.cz, milena.widomska@vsb.cz 2 KOVOLIT, a.s, Nádražní 344, Modřice, vlastimil.karas@kovolit.cz Abstract Advantage of magnesium alloys is in their low specific mass. This is the lowest of usual technical alloys. Specific strength (Rm/ ) is double in comparison with aluminium. These alloys are at forming characterised by low cold formability caused by hexagonal crystal lattice. The main alloying elements of magnesium alloys are aluminium, zinc and manganese, or possibly also Si, Zr, Th and elements of rare earth metals. In case of higher of Al, or Zn and Th it is possible to use hardening in order to increase the strength. Magnesium alloys are used in automotive industry for an extensive assortment of components, such as parts for chassis, sheets and wheels. Keywords: magnesium alloys, forgings, mechanical properties 1. INTRODUCTION Automotive industry, which is characterised by the biggest potential of development, belongs to important outlets for magnesium materials. Use of magnesium in vehicles was for decades limited to castings of complicated shapes for engines and wheels. Traditional die casting dominated for economic reasons. A possibility of use of magnesium materials components also for chassis and drives is now being considered. It turns out that it is suitable to replace the use parts made of steel and aluminium with magnesium alloys. Use of magnesium alloys for components of chassis puts high requirements to their strength, toughness and service life. Majority of these properties is achieved by forging. Importance of application of forgings from magnesium alloys in passenger vehicles in comparison with the currently used die cast castings is continuously increasing. Use of magnesium alloys in cars depends on price relation between aluminium and magnesium alloys (Tab. 1). The table compares the current economic possibilities of replacement of aluminium alloys with magnesium alloys, as well as price relations expected in years to come [1]. Table 1. Price relations between forgings made of aluminium and magnesium alloys Price relation Aluminium Magnesium current price Magnesium target price aluminium - magnesium EUR/kg EUR (dm 3 ) EUR/kg EUR (dm 3 ) EUR/kg EUR (dm 3 ) Basic metal Initial blank to to to to 3-7 Forging and 5 to to to to 36 5 to 10 9 to 18 finishing Total costs 8 to to to to to to 28 Comparison with Al alloys 100% 100% 210 to 280% 140 to 180% 120 to 160% 80 to 100% Density of magnesium alloys is by 25% lower than that of aluminium alloys. Density of magnesium alloys varies in dependence on the on the content of alloying elements from 1350 to 1830 kgm -3. Magnesium alloys have the biggest ratio between strength and density (Rm/ of all structural materials. They are characterised by good Machinability and majority of alloys also by good weldability at arc welding under protective atmosphere. Certain drawback of magnesium alloys consists in their lower resistance to corrosion.

2 Another disadvantage is their high reactivity (in some cases even at machining) and low strength at high temperatures, as well as low notch toughness. Mechanical properties of cast structure may be increased by forging, Fig. 1. Fig. 1. Mechanical properties of cast and forged magnesium alloys 2. MAGNESIUM ALLOYS Mechanical properties of Mg can be substantially increased by alloying by aluminium (up to 10%), zinc (up to 5 to 6%), manganese (up to 2.5%) and zirconium (up to 1.5%). Aluminium and zinc form a solid solution with magnesium. Inter-metallic phases of the type Mg 4 Al 3 and MgZn 2 are formed at its higher contents. In both cases the quantity of admixtures increases the basic mechanical properties. Manganese forms with magnesium a solid solution. Solubility of manganese in magnesium decreases with the decreasing temperature and phase precipitates from the solid solution. Addition of manganese does not influence the achieved strength characteristics, but it influences favourably resistance to corrosion. Increase of the level of resistance to corrosion can be explained by the fact that a thin layer of Mg - Mn oxides is formed on the surface. Addition of manganese decreases effect of iron in magnesium. Manganese and Fe form a compound of high density, which settles at melting at bottom of the bath. Apart from basic addition elements also addition of tin is used in magnesium alloys. Tin is soluble in magnesium at the temperature of 645 C up to the content of approx. 10%. Its solubility decreases with temperature with simultaneous precipitation of phase (Mg 2 Sn). Complex alloys Mg-Al-Mn alloyed additionally with 5% of Sn have good hot formability. Silicon is insoluble in magnesium. It forms with Mg an inter-metallic phase of the type Mg 2 Si, which strongly strengthens the basic matrix. Due to significant increase of brittleness the content of silicon in alloys is under 0.3%. Alloying of magnesium alloys with zirconium refines grain, the achieved level of mechanical properties increases and at the same time resistance to corrosion decreases. Elements of rare earth metals or thorium increase refractoriness of magnesium alloys. Beryllium in the amount from % decreases oxidation of alloys at melting, casting and heat treatment. Wide assortment of products made of the formed magnesium alloys covers numerous products from forgings to sheets. Forgings are made from the following alloys: AZ31B-F, AZ61A-F, AZ80A-T5, AZ80-T6, M1A-F, ZK31-T5, ZK60A-T5, ZK61-TS and ZM21-F (in the state F to T6). Tab.2 gives chemical composition of selected magnesium alloys.

3 Table 2. Chemical composition of magnesium alloys for forgings Alloy Content of alloying elements in wt.% Al Zn Mn Si Cu Fe Zr Th Ni AZ AZ AZ ZK M1A AZ HM PROPERTIES OF SELECTED MAGNESIUM ALLOYS Magnesium and majority of its alloys crystallises in hexagonal system. This system is characterised by reduced formability, which is caused by small number of slip mechanisms. Slip of dislocations takes place in selected crystallographic planes and directions, and it is controlled by three known laws. Up to the temperature of 220 C the only slip plane in magnesium is basal plane (0001) and directions [1120]. At higher temperatures the slip begins at the planes (1010) in direction [1120], and in the planes (1011) in direction [1120]. These are the planes and directions in HTU lattice, which are occupied by atoms the most densely. Formability increases significantly with an increase of slip systems. Values of critical slip stress ( kr ) are low for pure magnesium. Value of critical slip stress depends on purity of metal, structure and thermo-dynamic conditions of deformation. The higher purity of metal, the lower magnitude of critical slip stress. Impurities forming solid solutions with the basic metal increase kr more intensively than impurities that are insoluble in basic metal. If metal and admixture form a solid solution, then the value of critical stress increases in dependence on the on difference between magnitude of atoms of both metals, and on the difference of electro-chemical properties of both metals. Admixture elements in magnesium interact with dislocations and they increase critical slip stress. Influence of admixture elements on kr can be determined by the following equation: n kr c (1) where c is concentration of admixture atom, n is exponent (n ~ 0.5 to 0.66). Value of critical slip stress decreases in majority of metals with increasing temperature. Influence of tem is not unequivocal in case of magnesium and its alloys. Various slip planes can function at various temperatures. For example at room temperature Mg alloys have only one system of slip planes. Number of active slip planes increases with increase of temperature, which is manifested by rapid decrease of slip stress. Yield strength of magnesium alloys can be determined approximately from the equation: kr k (2) m where m is Schmid s factor (m max ~ 0.5). Table 3 presents the basic parameters of technical procedure of forging of magnesium alloys, as well as their mechanical and technological properties

4 Table 3. Forging temperatures, mechanical and technological properties of forgings from magnesium alloys Alloy Forging temperatures, o C Mechanical properties Technological properties for forgings for die forgings Re [MPa] Rm [MPa] A [%] Weldability Resistance to corrosion AZ o g AZ g g AZ91A g g ZK nr sa Note: o - outstanding, g - good, sa - satisfactory, nr not recommended Basic properties of magnesium alloys depend on the achieved structural state, which is function of chemical composition, applied deformation and heat treatment. Strengthening of matrix also depends on as well. In this relation it is advisable to pay attention to pars of processing of alloys and to their optimisation aimed at reliable achievement of required and reproducible properties. 4. HEAT TREATMENT OF MAGNESIUM ALLOYS Forgings are used in heat treated, as well as in non-treated state. Input blanks are before forming subjected homogenisation annealing at temperatures of C. Duration of annealing is h. The objective is to remove segregation heterogeneities of admixture elements. During homogenisation annealing the segregated phases on grain boundaries dissolve in basic matrix and chemical composition of the alloy is more homogenous, Fig. 2. This improves formability and enhances level of mechanical properties [2]. a) b) Fig. 2. Micro-structure of the alloy AZ91 in as-cast state (a) and after homogenisation annealing (b) [4] Re-crystallisation annealing is performed at the temperature around 350 C. Beginning of re-crystallisation of magnesium alloys strengthened by deformation lies in temperature interval of C. This temperature interval depends on the degree of strain hardening [3]. Majority of magnesium alloys alloyed by manganese or aluminium is used heat treated condition, i.e. after quenching and aging. Achieved higher strength is connected with changed solubility of admixture elements - Al, Zn, Zr in dependence on the on temperature. Heating before quenching is selected in such a way that segregated inter-metallic phases of the type MgZn 2, A1 3 Mg 4, Mg 3 A1 2 Zn 2 are dissolved in solid solution. A homogenous oversaturated solid solution is obtained

5 after quenching. During aging the strengthening phases precipitate. Characteristic property of magnesium alloys is small rate of diffusion processes, that s why the processes of phase transformation run very slowly. During heating before quenching the dwells of 4 to 24 hours are applied. Artificial aging runs in magnesium alloys within the interval of 16 to 24 hours. Selected magnesium alloys can be quenched also by cooling on air from the finish forging temperature. Consequential aging directly from the finish forging temperature is used, without inclusion of previous solution annealing and quenching. Temperatures of solution annealing of magnesium alloys vary around 380 to 420 C. Controlled aging is performed at temperatures of 200 to 300 C. This procedure of heat treatment is marked as T1 and T4. For achievement of maximal level of strengthening it is necessary to apply aging temperature of 175 to 200 C. Changes of properties achieved by aging are smaller in magnesium alloys in comparison with aluminium alloys. Increase of strength properties after aging is not higher than 20 to 35%. Plastic properties of alloys, however, decrease after aging. For these reasons the most frequently used heat treatment is homogenisation annealing. Mechanical properties are enhanced as a result of more homogeneous structure. Application of natural aging does not lead practically to more significant changes of strength properties. 5. FORGING OF MAGNESIUM ALLOYS Deformation behaviour and development of structure of six alloys and several shapes of products were verified experimentally. Initial samples had a shape of cylinder with diameter from 30 to 120 mm. Mass of initial blanks varied between 120g to 1500 g. The paper deals with three alloys only and one shape of product. Shape of forgings is shown in Fig. 3.. Fig. 3. Shape of forgings from the alloy AZ61 Structure of initial blanks for forging was in as-cast state. One half of the samples was in the state after homogeneous annealing, the second was without annealing. Secondary phases and zinc and aluminium based precipitates dissolved during heat treatment in basic matrix. Deformation behaviour and evolution of structure was verified by forging at the temperatures of 380 and 420 C. After forging a fine-grained structure was obtained, but with different grain size along the cross-section of forgings. Average grain size varied in dependence on the on temperatures around 50 to 60 m. It depended on chemical composition, forging temperature, magnitude of deformation and manner of cooling from finish forging temperatures [4]. Mechanical properties of forgings and their evolution in dependence on the on heat treatment were verified by tensile test and by hardness HB [5]. Results of hardness are given in Fig. 4.

6 Hardness HB , Brno, Czech Republic, EU AZ31 AZ91 AZ Initial state Formed Heat treated Fig. 4. Hardness of initial blanks and forgings made from magnesium alloys 6. CONCLUSIONS Magnesium alloys are very interesting for applications in automotive industry. They have the biggest ratio Rm/p of all structural materials, as well as high characteristics of vibration damping. At present numerous car components are made of various magnesium alloys. Wheels made of the alloy Electron belong to the most frequently used products, which are supplied in two executions cast or forged. Deformation behaviour of the alloys AZ31, AZ61, AZ91, ZK60, AZ63a M1A at die forging was verified experimentally. Influence of forging technology and homogeneous annealing on structure and properties of forgings was compared. Forging procedures differed by forging temperature. Influence of heat treatment and forming temperature on final structure and mechanical properties was evaluated. Results confirmed suitability of inclusion of heat treatment before the heating and forging. This procedure enables obtaining of forgings with more homogeneous structure. Within the investigated temperature interval no significant differences in structure of samples were obtained. The highest strength and hardness was obtained in the alloy AZ91. ACKNOWLEDGEMENT This paper was created within the project No. P106/09/1598 of the Czech Science Foundation and by research project No. CZ.1.05/2.1.00/ " Regional Materials Science and Technology Centre" within the frame of the operation programme "Research and Development for Innovations " financed by the Structural Funds and from the state budget of the Czech Republic. LITERATURE [1] DOEGE, E., HALLER, E., JANSEN,S. Precision forging of magnesium alloys. Wire 2002, 5, pp [2] BARTEČEK, R., GREGER, M. Light metals and their alloys. Kovárenství, 2004, vol. 14, No. 25, pp [3] GREGER, M., ČÍŽEK, L., WIDOMSKÁ, M. et al. Forming of magnesium alloys. In Nowe technologie i materialy w metalurgii i inženyrii materialowej. Katowice: Politechnika Slaska, 2004, pp [4] JÍLEK, L., GREGER, M., KARAS,V. et al. Forging of magnesium alloys. Kovárenství, 2008, 31, pp [5] GREGER, M. et al. Mechanical properties and microstructure of Mg-A1 alloys after forming. In CAM3S Zakopane: TU Gliwice, 2005, pp