Fluidity of Magnesium alloys, an Experimental & Numerical approach

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1 Fluidity of Magnesium alloys, an Experimental & Numerical approach Shehzad Saleem Khan 1, Norbert Hort 1*, Emir Subasic 2, and Siegfried Schmauder 3 1 GKSS Forschungszentrum GmbH, Max Planck Strasse 1, D Geesthacht 2 Gießerei-Institut, RWTH Aachen, Intzestraße 5, D Aachen, Germany 3 Institut für Materialprüfung, Werkstoffkunde und Festigkeitslehre (IMWF), Universität Stuttgart, Pfaffenwaldring 32, D Stuttgart The interest in metal fluidity has been concentrated mainly on aluminium alloys and on ferrous metals. Fluidity tests have been developed and are used commercially as quality checks to determine the flowing qualities of molten metal. It is well known that fluidity relates to the mold and metal parameters, as well as to casting conditions imposed during mold feeding. Among these some of the most important factors usually considered are solidification mechanism, superheat, viscosity and surface tension. In this investigation, the mold filling ability characteristics have been evaluated for binary Mg-Al alloys. The goal of this work is to increase the general knowledge base of magnesium fluidity behaviour with different mold geometries and optimization of the parameters to study their influence and a detailed discussion about the design and the constraints evolved in the fabrication of an optimized mold system. The later section considers the collection of the pre-requisite thermo physical properties for simulation of the whole casting process using finite difference based simulation software. The last section focuses on the comparison of the simulated and the experimental results and provides validation of the results.keywords: Fluidity simulations, mold optimization, castability, magnesium alloys, permanent mold casting. Introduction Research and development of magnesium alloys depend largely on the metallurgist s understanding and ability to control the microstructure of the as-cast part. Currently, few sources of magnesium solidification information and as-cast microstructures exist. So far, there has not been any subsequent progress in the field of casting simulations for magnesium fluidity. This research work *Corresponding author s addressnorbert.hort@gkss.de comprises the determination of input experimental parameters to run a successful and vast informative simulation using state of the art softwares. Therefore, the goal of this paper is to increase the general knowledge base of magnesium fluidity behavior with different mold geometries and to simulate the resultant microstructures. Various thermodynamic equipments have been used to study the thermophysical properties of different magnesium alloys with inclusion of the Calculation of Phase Diagram analysis. These analyses (CALPHAD) have been performed on 13 binary magnesium aluminum alloys, and the resultant microstructures have been simulated and then have been compared with the experimental output. This paper presents an overview of a range of ideas that have been undertaken to improve the understanding on the gravity diecasting behavior & solidification characteristics of Mg-Al alloys. It follows the solidification process of binary Mg-Al alloys, beginning with the nucleation. Next, the design and the constraints evolved in the fabrication of an optimized mould system are discussed in detail. Also the collection of the pre-requisite thermo physical properties for simulation of the whole casting process using Finite difference based Magmasoft and the simulated microstructure obtained using MICRESS (Micro Structure Evolution Simulation Software) are presented. The last section advises on the comparison of the simulated and the experimental results and provides validation of the results. In this paper the development of physical data concerning the dissipation of heat during filling and solidification is also being discussed. Main objective of these simulations is to optimize the conditions for the fluidity of the particular alloy. The thesis bifurcates into two main portions, namely fluidity simulation and microstructure simulations. MAGMASOFT operates on a Finite Difference Algorithm and has the ability to simulate the fluidity of 329

2 particular alloy provided input thermodynamic and physical data. Fluidity of Magnesium alloys The term fluidity has come to a meaning quite different to the foundry engineer than to the physicist. To the physicist, it is the reciprocal of viscosity; and to the foundry man, it is an empirical measure of a processing characteristic 1. Fluidity, in the casting sense, refers to the property of a metal which allows it to flows when it is poured into a standard fluidity test channel. Fluidity is therefore a length, usually in millimeters or meters. It is an important property for casting alloys and determines the ability of the liquid metal to fill the mold cavity. As mentioned before it is quantified by the distance the metal flows before it is stopped by solidification through different kinds of fluidity tests 2,3. This channel may be straight or it may be in the form of a spiral. Figure 1: The mold made of steel, maintained at constant 250 C for all casting experiments. The cross-section may be round, half round, trapezoidal or rectangular. In GKSS a familiar fluidity spiral have been used (Fig. 10). The channel was wound into a spiral, thereby simplifying handling and levelling problems. However, the fluidity of magnesium alloys and the influence of alloying elements have not yet been well studied. The purpose of this test rig is to develop a type test, which would be simple and easy to use and would also: Afford precise control over the metallostatic head and permit this pressure head to be reached before any metal entered the fluidity spiral by carefully controlling the process. Provide control over metal turbulence as the metal enters the spiral. Filter any dross or other foreign materials from the metal before the metal entered the fluidity spiral without altering the metal head. Be of flat cross-section to simulate problems encountered in pouring sand castings of thin section in the foundry. Experimental study It is well known, that fluidity relates to the mold and metal parameters, as well as to casting conditions imposed during mold feeding. Among these, the most important factors usually considered are solidification mechanism, superheat, viscosity and surface tension etc. of the cast alloy. It is thus possible to quantatively determine the values of mold filling ability at various pressure heads. In the present investigation, the mold filing ability characteristics have been evaluated for binary Mg-Al alloys. The evaluation has been carried out in three phases given below: Evaluation (experimental and simulated) of the mold filling ability values of all binary Mg-Al alloys containing up to 12% of aluminium by weight at various pressure heads and superheats. Study the influence of process parameter on mold filling ability of these alloys. Evaluation of alloy and optimization of the mold geometry and mold parameters to maximize mold filling ability values.although, magnesium can be fabricated by virtually all manufacturing techniques 4, this research focuses on casting and the alloys specifically designed for casting. Thermo physical properties of the cast alloy The exact knowledge of thermo physical properties of molten phases is essential in modern metallurgy. It leads to optimized process windows including better metal/slag separation, suitable slag selection or reduced slag/refractory wetting. The most important properties are melting characteristics (solidus and liquidus), thermal conductivity, melting and transition enthalpies, wetting angle, density, viscosity and surface tension. The material properties of the cast alloy are most important for simulation also. Depending on the simulation different material properties are needed. The more details needed to receive from simulation results, the bigger are the demands on quality and range of the data needed for the simulation. The differential thermal analyzer (DTA) is a device, which is designed to measure the difference in temperature between two samples which are subjected to the same heating/cooling regimen 5. One of the samples is a reference, commonly an inert material over the range of temperature being investigated. The sample material and reference materials are not required to have any similarities, although it can be advantageous to select reference materials with thermal similarities, such as thermal conductivity and heat capacity 5,6. 330

3 Table 1: The conditions for the DTA experimental set-up. Property Equipment Heating rate Cooling rate Temperature range Atmosphere Weight of sample Crucible Manufacturer Conditions DTA(Differential Thermal Analysis) 10 C / min 10 C / min C Flowing Argon 390~410 mg Steel X5 CrNi (1.4301) Mettler Toledo GmbH Experimentally it is possible to determine specific heat using the DSC equipment (Fig.2). Problems are encountered if the latent heat of fusion is not subtracted from the dissipated heat. This is why baseline method was applied (a technique commonly used, it involves the pre-determined heat during the exothermic or endothermic reaction of a reference material, in this case Sapphire). Table 2: The conditions for the DSC experimental set-up. Property Equipment Heating rate Cooling rate Temperature range Atmosphere Weight of sample Crucible Manufacturer Conditions DSC(Differential Calorimetry) 5 C / min 5 C / min C Scanning Flowing Argon 40~60 mg Al 2 O 3 NETZSCH Gerätebau GmbH Figure 3: The coefficient of thermal expansion of all the 13 binary Mg/Al alloys. For the determination of the coefficient of thermal expansion (CTE), NETZSCH Dilatometer 402 C device was used (Fig.3). The dilatometer was operated over a temperature range from room temperature to 450 C, at heating rates of 5 K/min, with programmed heating, cooling and isothermal sections, under either air or a protective atmosphere. Samples up to 25 mm long and with a diameter of up to 12 mm, with a maximum shrinkage/expansion of 5 mm, were measured up to an accuracy of 1.25 nm. Figure 4: Heat transfer coefficient for the mold. Figure 2: The specific heat of all the binary Mg/Al alloys. While mentioning the properties of the cast alloys, the heat transfer coefficient of the mold cannot be neglected. The mold is made of Chromium steel (X38CrV5_1) and as it is seen around 250 C it transmits more heat then it does at C (Fig. 4). In all the fluidity experiments the mold temp is kept constant. One of the salient cause for retardation in fluidity is the high α values of the mold. 331

4 Latent heat of fusion Latent heat of fusion can also be calculated using the DSC equipment. Before initializing the experimental calibration it is necessary to derive the sensivity of the specimen carrier denoted by H (Table 3). Latent heat was determined using the DSC equipment. Table 3: The experimental results for the heat of fusion. wt.% Al Sens. H(μVs/mg) T ( C) Latent heat (J/g) Feeding effectivity Prior to simulations, the calculations of feeding effectivety is a must, provision of a value at this point that defines the range of feeding is vital. This value describes the solidified fraction of the melt up to which macroscopic feeding can occur. The solidified fraction is expressed in percent and is strongly dependent on the solidification morphology. The feeding effectivity is taken from the database provided for the according material. MAGMAsoft automatically calculates the effective feeding temperature onces not being provided the value. The empirical formula it used is: Upon solidification the dendrites arms starting growing in the reverse direction as that of the heat flow. Melting range of a particular alloy might be the difference in solidus and liquidus temperatures but the fluid range is different from that. Feeding effectivety refers to the temperature after which the fluid ceases to follow through the mold cross-section even there is a good amount of liquid still present in the semi-solid mushy region. Mould optimization As an established method, a mold featuring a coil shaped cavity has been used for the quantitative description of castability in gravity casting. The geometric shape of the coil was designed following a VDG standard (Verein Deutscher Giessereifachleute e.v.). This standard originally refers to mold material, made of steel but because of the lengthy labor involved in the fabricating the die, breaking it after every test and expandability concerns. A permanent steel mold concept was developed. There are several reasons for the permanent mold: Less work necessary compared to a spiral made of sand. Another problem with sand is the additional ingredients needed to stabilize the mold porosity levels, humidity, reactions with SiO 2 and degassing etc. To find out the rise in temperature caused by the hot melt 28 holes were drilled and Cr-Ni-Cr (capable of withstanding high temperatures) thermocouples were placed to all these holes. The mold stays at a constant temperature of 250 ºC for all the alloys and experiments and the castings vary from 750 ~ 650 C, depending on the alloy s liquidous temperature. The position of the thermocouples are shown in Fig. 6. Figure 5: The position of the thermocouple for the determination of temperature distribution in the mold. Observe the importance of the feeding system as revealed in Fig. 7. The mold is confined in a furnace to attain desired temperature, which in this case was 250 C. Nowadays, molds are available with an integrated heating system; this is very beneficial for visibility but lacks range of operation (temperature). This mold furnace setup can be used till 700 C. One practical aspect of this setup is the leveling of the mold as it can be seen in the Fig. 7. The mold is kept on two blocks to ensure the proper perpendicular position with respect to the flow of the cast metal. 332

5 being considered, Mg-1%Al freezes at (solidus point) 913K where as Mg- 12%Al freezes around 854K. The feeding temperature kept at a constant 100K above the liquidus point. Heat of fusion Another major cause for the rise in fluidity is the latent heat of fusion (Tab. 3). It reduces as the content of Al is increased that essentially means at high Al concentration less heat content is being released. The alloy subsequently attains a higher temperature making it to be more fluidable. Figure 6: The schematic diagram of the die, mold in the furnace. Figure 8: Grain size and the fluidity values (experimentally) vs. content of Al. Figure 7: DSC curves for Mg-5Al till Mg- 12Al. There is no thermodynamic reaction occurring prior to solidus temperature. The DSC signal is almost constant till the solidus, beyond that an endothermic reaction occurs and the melting starts. The onset at this point is referred to solidus and the peak is the liquidous. The peak before the solidus temperature represents the endothermic reaction caused by the γ-phase, the concentration of the particular phase then increases with the increase in Al content. Discussion The fluidity value of the binary Mg-Al alloys increases if the Al content increases. This is because of some factors mentioned below: Long freezing range The freezing range widens with the increase in Al content and this is major cause of high fluidity. If the values are Figure 9: Casting parameters vs. Al content. To keep all the alloys apparent to symmetric thermal conditions, fluidity of all the alloys was determined with input temp of 100 C above liquidous. As can be seen in Fig. 10, the difference between the feeding temperature and the liquidous temp line is constant. There is a gap between the solidus line and the feeding threshold (refers to the feeding effectivety). 333

6 The results with the pressure head of 25m bar and the superheat of 100 C over the liquidous temperature. This is due to the fact that the alloy is in the liquid state until the pouring is completed unlike the case of lower superheats where partial solidification occurs during the pouring itself Fabrication of the new geometry The experimental and the simulated results revealed one observation: The old geometry fabricated for the fluidity tests (Fig. 1) had huge chunk of mass around the spiral cavity which serves as a heat sink and the cast alloy s heat content is being cooled down relatively quicker.the reservoir feeding system was effective in controlling the turbulence because of its ability to allow fixed quantity of mass to the mold. Figure 10: The pre-solidification of the cast alloy in the old feeding system. Figure 13: The optimized geometry with the new feeding system. Conclusions Start to Continuous with the vertical alignment to the top Fig.12 shows the effect of metallostatic pressure head and degree of superheat on the simulated fluidity. 1. Simulations with 25mbar pressure head and 100 C superheat (100 C above liquidus). 2. Simulations varying the pressure head only and keeping the superheat const. at 100 C. 3. Simulations with varying superheat (150 C) and keeping the pressure constant at 25mbar. The simulations drew the following trends in the fluidity of the alloys: Rise in superheat temperature by 50 C (from C) increases the fluidity up by almost 15%.Elevated pressure head of 15mbar (from 25-40mbar) increases the fluidity by 10%. For lower concentration of Al (wt %) in Mg, the superheat and pressure have no significant change as the freezing range is very short. References 1 C.J.Cooksey, V. a. (Sep 1959). The Casting Fluidity of Some Foundry Alloys. In The British Foundryman, vol 52 (p. 31). 2 B. Wunderlich (1990). Thermal Analysis. New York: Marcel Dekker. Figure 112: Fluidity comparison of different metallostatic pressure head and degree of superheat vs. Al content. At the same time a portion of the cast alloy got solidified in the feeding system itself, thus, hindering the path of the casting into the cavity. Fig.12 shows a new type of feeding system which serves both purposes turbulences and solidification. 3 M.M. Avedesian, H. B. (1999). In ASM Speciality Handbook Magnesium and Magnesium Alloys (pp. 4-6). Materials Park, Ohio: ASM International. 4 F.Speyer, F.Roberts (1994). Thermal Analysis of Materials. NewYork [u.a]. 5 C. Flemings, E.F. Niiyama, H.F. Taylor (1961). AFS Trans. 69, Fluidity of Aluminum: An experimental and quantitative evaluation C. Flemings, E.F. Niiyama, H.F. Taylor (1962). AFS Trans. 70, Fluidity of Aluminum: An.metal in thin section