345. Surface Tension and Viscosity of Industrial Alloys From Parabolic Flight Experiments - Results of the ThermoLab Project

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SurfaceTensionandViscosityofIndustrialAlloysfromParabolicFlightExperiments-ResultsoftheThermoLabProject

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INTRODUCTION

The numerical simulation of casting and microstructure forma-tion is used increasingly by industry [1] for the optimization ofcasting and the improvement of product quality. Modelsdescribing fluid and heat flow are combined with thermody-namic models of nucleation and growth kinetics, phase selec-tion, and phase stability in the solid phase [2]. The increasingsophistication of numerical simulation is, however, met by apronounced lack of thermophysical property data which is owedto the high chemical reactivity of many metallic alloys in theliquid phase. As a consequence, for many high-temperaturealloys conventional thermoanalytical techniques where thespecimen is in contact with a container are difficult to apply andfraught with error. In order to overcome these limitations, con-tainerless methods for thermophysical property measurementshave been developed and applied under reduced-gravity condi-tions [3, 4].

Knowledge of the surface tension is relevant for the predic-tion of defects such as gas porosity, the contribution ofMarangoni convection to fluid transport, and for an estimate ofthe effect of liquid - mould interaction. The viscosity is relevantfor the calculation of the convective contribution to heat trans-fer, for simulations of mould filling and, in general, for allprocesses involving moving fluid. The experiments to bedescribed were conducted within the framework of theThermoLab project which was initiated to provide thermophys-ical property values of industrial alloys by the application ofcontainerless processing under reduced gravity conditions andin ground-based laboratory including also conventional tech-niques.

EXPERIMENTAL PROGRAMME

The alloy selection was mainly driven by the industrial usergroup of the ThermoLab project. While for the Ni-based alloysmeasurements of the surface tension and of the viscosity could also be performed by the sessile drop and the oscillating cupmethod in ground-based laboratory, this was not possible for the

© Z-Tec Publishing, Bremen Microgravity sci. technol. XVI

R. Wunderlich, R. Aune, L. Battezati, R. Brooks et al: Surface Tension and Viscosity of Industrial Alloy - the ThermoLab Project

11

Ragnhild Aune1, Livio Battezzati2, Rob Brooks3, Ivan Egry4, Hans-Jörg Fecht5, Jean-PaulGarandet6, Ken C. Mills7, Alberto Passerone8, Peter N. Quested3, Enrica Ricci8, StephanSchneider4, Seshadri Seetharaman1, Rainer K. Wunderlich5, Bernard Vinet6

Surface Tension and Viscosity of IndustrialAlloys from Parabolic Flight Experiments –Results of the ThermoLab Project

Authors:

1 Department of Materials Science and Engineering, Royal Institute of Technology, Stockholm, Sweden

2 Università degli Studi di Torino, Torino, Italy3 Materials Centre, National Physical Laboratory,

Teddington, United Kingdom4 ZEUS (Zentrum für Erstarrung Unterkühlter Schmelzen)

at DLR, Cologne, Germany5 Materials Division, University of Ulm, Ulm, Germany6 Department of Materials, Imperial College STM, London, United Kingdom7 IENI-CNR Ge, Genoa, Italy8 Département des Technologies pour les Engergies Nouvelles,

CEA/DTEN, Grenoble, France

Mail Address:

Rainer K. Wunderlich, Materials Division, University of Ulm, Albert-Einstein-Allee 47, 89081 Ulm, Germany

The surface tension and the viscosity of a series of industrialalloys have been measured by the oscillating drop techniquewith an electromagnetic levitation device under reduced gravityconditions in several parabolic flights. It was demonstrated thatthe 20 seconds of reduced gravity available in a parabola weresufficient for melting, heating into the liquid phase, and coolingto solidification of typically 7 mm diameter metallic specimen.The surface tension and the viscosity were obtained from the fre-quency and the damping time constant of the oscillation whichwere evaluated from the temperature signal of a highresolutionpyrometer. Alloys processed included steels, Ni-based superal-loys, and Ti-alloys which were supplied by industrial partners tothe project. Three to four parabolas were sufficient to obtain thesurface tension and the viscosity over a large range in temper-ature.

Ti-alloys because of their high melting temperatures and highchemical reactivity in the liquid phase. In Table 1, an overviewof the specimen processed on the parabolic flights is shown. TheTi6242 alloy is a variant of the Ti6Al4V alloy with an additionof Zr and W in the few per cent range. In order to investigate theinfluence of the oxygen concentration, a Ni-based superalloyand a Ti-alloy with different oxygen concentrations wereprocessed.

EXPERIMENTAL SET-UP AND SAMPLE PREPARATION

The parabolic flight experiments were performed on board anAirbus A320. The experimental set-up on the parabolic flightswas similar to that of the containerless electromagnetic pro-cessing facility TEMPUS which has been used in the IML-2 andMSL-1 Spacelab experiments for the measurement of thermo-physical properties of liquid metals and alloys [3,4]. The speci-men were contained in an ultra-high vacuum compatible vessel.Positioning was by a radio-frequency quadrupole field and heat-ing by a dipole field of frequencies 240 kHz and 340 kHz,

respectively. The temperature was measured by an opticalpyrometer in the top position which was calibrated at the liq-uidus temperature obtained from high temperature calorimetry,for the Ti 6-4 and Ti 6-242 alloys values provided by the indus-trial partner have been used. The pyrometer had a sampling rateof 100 Hz, and a temperature resolution of 0.02 K. The speci-men shape was recorded by two 100 Hz video cameras in theside and the top view position. The experimental chamber wasoperated under flowing or static argon and a He-Ar mixture.The 7 mm diameter specimen were contained either in an opencage structure, or in a cup with observation slits from the sideon a SiN pedestal.

Specimen were polished and stored in a glove box with high-purity argon. The oxygen concentration of representative speci-men and of the processed samples was analyzed with the LECOhot gas extraction method. The influence of oxygen on the sur-face tension and the viscosity was investigated for a CMSX-4Ni-based superalloy and a Ti6Al4V specimen. Specimen withdifferent oxygen concentrations were prepared by re-melting inan arc melter either with or without using additional Ti-getter.Typical oxygen concentrations of the low and high oxygen con-centration specimen were 50 ppm and 350 ppm, and 1450 ppmand 1720 ppm for the CMSX-4 and Ti6Al4V specimen, respec-tively.

PROCESSING

The parabolic flights provided approx. 20 seconds of reducedgravity (µ-g). Specimen were preheated to approx. 1000°Cunder an atmosphere of 20 mbar argon. At the onset of the µ-gphase when the specimen was freely suspended, the heatingfield was turned on. After having reached maximum tempera-ture, the chamber was flooded with about 300 mbar of highpurity helium to increase the cooling rate which was in therange of 60Ks-1 to 25Ks-1. A typical processing sequence of a Ti-6-4 alloy is shown in Figure 1 with melting (1), heating to max-imum temperature (2), free cooling with application of surfaceexcitation pulses (3), and undercooling and recalescence (4). Inthis way, the surface tension could be obtained over a largerange in temperature from a single parabola. Three to fourparabolas were sufficient to obtain the surface tension and vis-cosity values. All Ti-alloys processed on the parabolic flightsexhibited an undercooling of about 300K with the Ti6Al4Valloys undercooling close to the hypercooling limit. For the Ti-alloys, the undercooling was independent of the oxygen con-tent. The Ni-based superalloys undercooled typically by 90-100K.

DATA EVALUATION

For a force free, spherical droplet of mass Μ the surface tension,σ, is obtained from the frequency, ν, of the l=2 mode of the sur-face oscillations by the Raleigh formula:

σ = 3/8 π ν 2 Μ

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Alloy T1 / °C Flight

Ti6Al4V 1655 Oct 2001

Ti 6242 1705 Oct. 2002

γ-TiAl 1566 June 2003

CMSX-4 1382 Oct. 2002

IN738LC 1345 June 2003

MM247LC 1368 June 2003

C263 1355 June 2003

low alloyed steel 1480 June 2003

cast iron 1367 June 2003

Table 1. Overview of the alloys processed on parabolic flights for themeasurement of the surface tension and the viscosity.

Fig.1. Temperature-time profile of a Ti6Al4V specimen shown on theleft hand ordinate, heater current shown on the right hand ordinate.

Higher oscillation modes are usually not observed because oftheir much stronger damping. The viscosity, η, is obtained fromthe damping time constant, τ, of the surface oscillations accord-ing to:

η = 3/(20π) (Μ/R) τ −1

Two methods are available for the evaluation of the oscillationfrequency and damping time constant. In digital image analysis,the video recordings of the specimen shape are analyzed. Thismethod requires a rather high software and digital processingeffort [5]. Alternatively, the surface oscillations can be evaluat-ed from the high-frequency modulation of the temperature sig-nal [6] originating from the variation of the surface normal withrespect to the optical axis of the pyrometer and from the varia-tion of the surface position with respect to focal plane of thepyrometer. This approach has the advantage of being fast andeasy to apply. It is, however, more susceptible to sample move-ments which result in an additional amplitude modulation of thepyrometer signal appearing as sidebands in the Fourier spec-trum and beat oscillations which can impede the evaluation ofthe damping time constant. The results presented here havebeen obtained with the temperature analysis method. In Figure2a,b the high frequency oscillations of the temperature signalextracted by Fourier filtering and the corresponding Fourierspectrum are shown. The sidebands are symmetric, and split bythe frequency of the sample movements. A further splitting inthe main peak is not resolved because of the small samplingtime interval.

RESULTS

Ni-based superalloysAs a typical result for the Ni-based superalloys, the surface ten-sion as a function of temperature of the alloy MM247LC isshown in Figure 5. The data exhibit low scatter and can be verywell fit by a linear regression. The results were compiled from4 parabolas. In Table 2, a compilation of the surface tension ofthe Ni-based superalloys obtained on the parabolic flights isshown. For CMSX-4, very good agreement of σ(Tl) and dσ/dT

is observed with results obtained in ground-based laboratorieswith the sessile drop method. In Figure 4, Arrhenius plots of theviscosity of the high and low oxygen CMSX-4 alloys obtainedfrom the parabolic flight are shown. A significant increase in theviscosity of the high-oxygen specimen is apparent. No compa-rable data have been found in the literature. For applications incasting simulations, the lower viscosity data are recommendedbecause this oxygen concentration is closer to that of the indus-trial alloys. The results show that the oxygen concentration is acritical parameter in viscosity measurements for these alloys.

R. Wunderlich, R. Aune, L. Battezati, R. Brooks et al: Surface Tension and Viscosity of Industrial Alloy - the ThermoLab Project

Alloy σ(T1)/ Nm-1 dσ/dT Source

IN738LC 1.85 -1.48 10-3 PF June 2003

MM247LC 1.86 -1.36 10-3 PF June 2003

C263 1.74 -0.69 10-3 PF June 2003

CMSX-4 low ox. 1.78 -1.28 10-3 PF Oct. 2002

CMSX-4 high ox. 1.75 -1.80 10-3 PF Oct. 2002

CMSX-4 1.97 -1.38 10-3 sessile drop 1-g

CMSX-4 1.83 -1.45 10-3 sessile drop 1-g

Table 2. Surface tension at Tl and temperature coefficient of Ni-based superalloys. The two bottom rows show values obtained in theground-based programme.

Fig.2a. High frequency component of the temperature signal shown on the left hand side; 2b. Fourier spectrum of a 0.64 s time slice

Fig.3. Surface tension as a function of temperature of the Ni-basedalloy MM247LC results from three parabolas indicated by , and .

Microgravity sci. technol. XVI-1 (2005) 13

Ti-alloysAs an example of the quality of the data obtained in the para-bolic flights for the Ti-alloys the surface tension as a function oftemperature of Ti6Al4V is shown in Figure 5. Very good agree-ment with a value obtained by the pendant drop technique at theliquidus temperature, i.e., σ(Tl) = 1.52 Nm-1, was obtained giv-ing support to the results of the parabolic flights where no com-parison values over a large range in temperature are availablefrom other techniques. Moreover, a reproducibility of the sur-face tension at Tl of better 1 % was obtained in data obtainedfrom two different parabolic flights and with specimen of dif-ferent diameter as shown in Table 3. Viscosity values for the Ti-Al-V alloys obtained on the parabolic flights are given in Table4.

CONCLUSIONSParabolic flights were proved as a valuable tool for the meas-urement of the surface tension and the viscosity of high meltingpoint industrial alloys. Measurements are carried out almostroutinely and a large amount of property values has beenacquired. A very good reproducibility of surface tension valuesfor Ti-alloys were obtained on different parabolic flights. Itcould be shown that electromagnetic processing conditions hadno effect on the surface tension values. Further improvementsmay be expected from an optimization of the temperature-timeprofiles for viscosity measurements which will, however,require a larger number of parabolas to cover the same temper-ature range as for the surface tension measurements.

ACKNOWLEDGEMENTSThis work was supported by the European Space Agency (ESA)Microgravity Applications Support Programme (MAP) undercontract number AO-99-022 (14306/01/NL/SH) and by theGerman Aerospace Center (DLR) under contract number 50WM 0041. The parabolic flight opportunities were provided byDLR and ESA. Support from the facility developer EADSSpace Systems and the Microgravity User Support Center atDLR Cologne is gratefully acknowledged.

REFERENCES[1] Dantzig, J. A.: Solidification Modeling: Status and Outlook, JOM, vol. 52,

no. 12, p. 18 (2000).[2] Saunders, N., in: Superalloys 1996. Kissenger, R. D. et al. (Eds.), TMS,

Warrendale, Pennsylvania, p. 101 (1996).[3] Egry, I., Lohöfer, G., Jacobs, G.: Surface Tension of Liquid Metals:

Results from Measurements on Ground and in Space, Phys. Rev. Letters, vol. 75, p. 4043 (1995).

[4] Wunderlich, R. K., Fecht, H.-J.: Thermophysical Properties of Bulk Metallic Glass Forming Alloys in the Stable and Undercooled Liquid – A Microgravity Investigation, J. Mat. Trans. JIM, vol. 42, no. 4, p. 565 (2001).

[5] Egry, I., Jacobs, G., Schwartz, E., Szekely, J.: Surface Tension Measurements of Metallic Melts under Micro-Gravity, Int. J.Thermophys., vol. 17, p. 1181 (1996).

[6] Rösner-Kuhn, M., Hofmeister, W. H., Kuppermann, G., Morton, C. W., Bayuzick, R. J., Frohberg, M. G., in: Solidification 1999. Hofmeister, W. H., Rogers, J. R., Marsh, S., Singh, N. B., Vorhees, P. W. (Eds.), TMS, Warrendale, Pennsylvania, p. 33 (1999).

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R. Wunderlich, R. Aune, L. Battezati, R. Brooks et al: Surface Tension and Viscosity of Industrial Alloy - the ThermoLab Project

Fig.5. Ti6Al4V, surface tension as a function of temperature.

Alloy T / °C h / mPa.s

Ti64 1824 +/- 100 3.92

Ti64 1670 +/- 80 4.82

Ti6242 1740 +/- 100 3.70

Ti6242 1626 +/- 80 4.76

Table 4. Viscosity of TiAlV alloys.

Alloy σ(T1)/ Nm-1 dσ/dT Source

Ti6Al4V 1.52 -5.52 10-4 PF Nov. 2001

Ti6Al4V 1.49 -4.10 10-4 PF Oct. 2002

Ti6Al4V 1.52 -4.50 10-3 PF Oct. 2002

Ti-6-242 1.51 -8.73 10-3 PF Oct. 2002

Ti6Al4V 1.52 n.a pendant drop 1-g

Table 3. Surface tension at Tl and temperature coefficient of Ni- asedsuperalloys. The two bottom rows show values obtained in theground-based programme.

Fig.4. CMSX-4; Arrhenius plot of the viscosity of the low ( ) and high( ) oxygen alloy.