Analysis of the Thermodynamic Conditions ofRefining during Electron Beam Melting ofRefractory MetalsAnalyse der thermodynamischen Zustande des Verfeinerns wahrend desElektronenstrahl-Schmelzens von hochschmelzenden Metallen

V. Vassileva, K. Vutova, G. Mladenov

The electron beam melting and refining (EBMR) decreases theconcentration of an impurity element through evaporation, removalof dissolved gases, lessening the size and density of the particulateimpurities complexes by either flotation, either sedimentation (dueto the difference in component densities) as well as by its dissolu-tion in the liquid metal. The paper is focused on the analysis of thethermodynamic conditions for EB refining of wastes of the titani-um, zirconium and hafnium with high oxygen contamination andother refractory metals using 60kW and 250kW EBMR plants.

Keywords: electron beam melting; refractory metals; vacuummetallurgy; refining thermodynamic conditions.

Das Elektronenstrahl-Schmelzen und Verfeinern (EBMR) ver-ringert die Konzentration der verunreinigenden Elemente durch

Verdampfung, Entfernung der aufgelosten Gase, Verminderungder Große und der Menge der Anteile der bestehenden verunreini-genden Komplexe durch deren Flotation, Sedimentation (infolgeder unterschiedlichen Komponentendichte) sowie durch deren Auf-losung im flussigen Metall. Der Bericht ist konzentriert auf dieAnalyse der thermodynamischen Bedingungen bei EB Raffinierungder Abfalle von Titan, Zirkonium und Hafnium mit hohem Sauer-stoffgehalt sowie von anderen hochschmelzendenMetallen bei Ver-wendung von 60kW und 250kW EBMR-Anlagen.

Schlusselworte: Elektronenstrahlschmelzen; hochschmelzendeMetalle; Vakuummetallurgie; verfeinernde thermodynamische Be-dingungen

1 Introduction

Refractory and reactive metals and their alloys are used atmastering cosmic apparatus, electron tubes, plasma and mi-crowave devices, chemical reactors, power generators andothers. This is because of their ability to protect surfaces ofcorrosion, to emit electrons (in heated state in vacuum orrare gases), to work as getter, improving vacuum in electrondevices as well as to possess mechanical stability at very highand low temperatures.

The use of electron beam melting and refining (EBMR) ofrefractory and reactive metals is increasingly broadening thetraditional frames of their current application. New grades ofpurity are dictated by new application fields such as the struc-tural elements and steel additive alloying constituents in nu-clear reactors, aerospace devices, chemical industry, electro-nics etc. From economical point of view, the major problem isto regenerate wastes of these expensive materials as well as toprocess intermediate products obtained at earlier stages ofconventional metallurgy and mechanical treatment. A suc-cessful combination of (i) prevention of contact of liquid me-tal with ceramic pots and slag, (ii) superheating of the meltedmetal in vacuum and (iii) various liquid state dwell times isEBMR. This method permits selective refining of processingmetal or alloy from various undesired metallic componentsand non-metallic impurities. As a result of these advantagesthe method is suitable for the recycling of wastes of refractoryand the reactive metals.

The pure refractory and reactive metal scrap has compara-tively lower content of metallic components. Due to the high

chemical activity of these components at high temperatures,these wastes can contain a big quantity of gases picked up dur-ing treatment or work (for example oxygen). From economic-al point of view an addition of refractory or reactive metalsponge is effective to be used at that regeneration. The extrac-tion of non-metallic and undesirable impurities during EB re-fining of metals and alloys is a complex process, which is ac-companied by various phenomena, such as heat exchange,mass transport and chemical interactions among the impuri-ties and the metal being refined.

To provide an efficient application of every concrete refin-ing process it is important to know:l the concentration of the investigated component at thermo-

dynamic equilibrium at concrete conditions (pressure andtemperature) giving the principle possibility of refining;

l the rates and limits of the processes, before achieving thethermodynamic equilibrium, as well as the parameters thatinfluence these rates or limits.The present work is focused on the first part of the deter-

mined task, namely on the analysis of the thermodynamic con-ditions for EB refining of wastes of the titanium, zirconiumand hafnium with high oxygen contamination as a base ofEBMR optimization. The accumulated research experienceand some theoretical and experimental data, concerning thedrip EBMR of highly oxidized titanium, hafnium andzirconium (as well as of sponge of these metals as additionto the re-melting scrap) are presented. Oxygen removal pos-sibilities and some typical metal-impurities purification of thestudied metals of the IV group of the Periodic system arediscussed.

F 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 613

Mat.-wiss. u. Werkstofftech. 2006, 37, No. 7 DOI: 10.1002/mawe.200500014

2 Results and Discussion

2.1 Analysis of thermodynamic conditions ofdegassing during electron beam re-melting ofrefractory metals (titanium, zirconium andhafnium)

The refining processes at EB melting and refining takeplace mainly on the reaction surfaces of the liquid metal(its interface with vacuum) on three reaction zones: the frontpart of the re-melting block, the flying droplets and the moltenpool in the water cooled crucible. At the condition of longdwell time in liquid state one could not be accented to the im-purity transport to the interface molten pool/vacuum and payattention to possibility of refining (or opposite-enrichment) ofchosen base metal component.

Depending on the thermodynamic melting conditions andon the removed component, the EBMR could be realizedthrough one of the following two purification pyro-vacuummethods:l degassing – removal of components (metallic or nonmetal-

lic) with vapor pressure (pi),which is higher than the vaporpressure of re-melted base metal (pR), i.e. (pi)> (pR). Thefinal state of the removed product is in gaseous state.

l distillation – evaporation of more volatile compounds of themetallic components. The final state of the removed pro-duct is a solid condense state.Through thermal degassing the gases, presented in the re-

fining metal, are removed. The condition necessary for therealization of that kind of refinement is that the partial pres-sure of the removed gas (pg) has to be higher than the equili-brium pressure of the metal vapors (pR) at the treatment tem-perature - namely (pg> pR). In the case of (pg)>10-3 Torr thedegassing will be fast enough.

The oxygen occurs in the metals as dissolved in solid phaseor as oxides of the base metal and of its metallic components.

The solubility of the oxygen could be evaluated by Sivert’slaw:

½%O� ¼ K½O�ffiffiffiffiffiffiffi

p½O�p ð1Þ

where [%O] is the equilibrium concentration of the oxygendissolved in the metal; p[O] is the partial pressure of the oxy-gen; K[O] is the coefficient of the equilibrium concentration ofthe oxygen dissolved in the metal at atmospheric pressure.The solubility of oxygen depends on the pressure and onthe temperature of treated metal.

The calculated by eq.(1) values of equilibrium concentra-tions of oxygen, dissolved in zirconium and titanium at 1273K at atmospheric pressure and in vacuum are shown in Table1. It can be shown that during heating of the refractory metalsin vacuum the equilibrium concentration of the oxygen, dis-

solved in zirconium and titanium, sharply decrease when thetechnology chamber pressure decreases [1]. Therefore oxygendegassing of these metals can occur at EB melting.

The temperature dependence of solubility of oxygen in ti-tanium, hafnium and zirconium for studied temperature range2000-2800 K are shown in Fig.1. The data are calculated atworking pressure in the vacuum chamber 10–3 Pa. Duringheating of the metals in vacuum the oxygen solubility de-creases. This becomes a stronger trend during heating of zir-conium and slightly decreases for hafnium and titanium.Ther-efore during heating of the molten metal in vacuum, the oxy-gen is removed from the surface of the superheating liquidmetal through the thermal degassing.

At studying the conditions of oxygen removal of this part ofoxygen, which is bounded as oxides, involving base metal orits components, the ratio between the partial pressures of thebase metal (pR) and the metal components (pMe) to the oxides-respectively (pRO) and (pMeO) is important from the thermo-dynamic point of view. From that ratio it is obvious thatthe value depends on whether the oxide molecules will be va-porized from the surface or not. When (pMeO) > (pMe) the re-moving of the metallic component is possible through eva-poration of its oxide from the interface between the moltenpool and the vacuum (namely through distillation if the im-purity oxide is stable or will take place evaporation, proceed-ing after a thermal dissociation of the oxide).

Fig. 1. Equilibrium thermodynamic constant of the oxygen solubi-lity in titanium, hafnium and zirconium.

Abb. 1. Thermodynamische Gleichgewichtskonstante fur die Sau-erstoffloslichkeit im Ti, Hf und Zr.

Table 1. Equilibrium concentration of oxygen, dissolved in refractory metals at atmospheric pressure and in vacuum.

Tabelle 1. Gleichgewichtskonzentration des Sauerstoffes, aufgelost in den hochschmelzenden Metallen bei atmospharischem Druck undim Vakuum.

Me T,K 105,Pa

2.1x10-1,Pa

2.1x10-5,Pa

Titanium 1273 11.26% 1.87.10-2% 1.87.10-4%

Zirconium 1273 5.08% 8.45.10-3% 8.45.10-5%

614 V. Vassileva, K. Vutova and G. Mladenov Mat.-wiss. u. Werkstofftech. 2006, 37, No. 7

In the case of (pRO)>(pR), a part of the oxygen leaves thereaction interface between the molten metal and vacuum inthe form of oxide of the refined base metal. Loss of this metalcould be observed. For obtaining high efficiency of the refin-ing the inequality must be satisfied:

ðpMeOÞ > ðpMeÞ > ðpROÞ > ðpRÞ ð2Þ

In the case when the condition (2) is unsatisfied for eachcomponent i.e. (pRO)> (pMeO); (pRO)>(pMe) or (pRO)<(pR),the losses of the base metal are higher and the refining isnot efficient. Note, that purification through a volatile sub-oxide of the base metal (sacrificial de-oxidation) in case ofIV group reactive metals is negligible [2].

In Fig. 2 the values of the partial equilibrium pressures oftitanium, hafnium and zirconium, as well as some typical me-tal components, at EBMR conditions (vacuum 10–3Pa andtemperature region 1500-3000 K) are shown. For the studiedtemperature range the conditions (2) are satisfied for siliconand iron, being components of the titanium as well as for allstudied components for hafnium and zirconium. Exceptions inthe last case are the molybdenum impurities. For the alumi-nium, calcium, nickel and copper, which could be obtained asrefractory metals’ components, the partial pressure of thesecomponents in the temperature range under study is greaterwith two-three orders than partial pressure of respective re-fractory base metal. At conditions of the EBMR this compo-nent refining is realized through their evaporation from thereaction surface. In this case the refining is an efficient processhere and refining base metal losses are minimal.

For the impurities, for which inequality (2) is not satisfied,as for vanadium, chromium and manganese in titanium basemetal and molybdenum in all studied refractory metals, theEBMR can take place using another mechanism namely dis-tillation of a more easily evaporating compound under theconditions of the refining.

From the partial pressures of titanium, zirconium and haf-nium, shown in Fig. 2 one can see that zirconium, being a

component of hafnium, as well as a component of zirconiumand hafnium in base titanium could not be removed by thermaldegassing. To decrease its concentration in base refractorymetal conditions of distillation of their more volatile com-pounds must be found.

In Fig.3 the partial pressures of titanium, zirconium andhafnium, are compared with the partial pressures of their oxi-des and nitrides at heating the metals in the temperature range1800-3000K and working pressure in the vacuum chamber of10–1–10-3Pa.

The vapour pressures of the studied reactive refractory me-tals are higher than the pressures of their oxides. Therefore thethermodynamic conditions of EB refining of their wastes arenot suitable for conductance of distillation of these oxides andremoval of oxygen using this mechanism is not realistic.

The metal oxides usually have higher melting temperatureand lower density compared to these of respective metal (Ta-ble 2) [3,4]. Therefore during EB interaction the metal oxidespresented in the molten pool are not in liquid phase but in theform of solid particles float to the molten pool surface due tothe lower density. Being in the surface layer, which is super-heated and has a higher temperature [5] their dissociation tothe free oxygen and the respective metal can take place. Ad-ditional reason for this is the direct irradiation of the floatingparticles located on the surface.

The interaction of the refining metal, as well as the metalcomponent with the oxygen can be presented using the follow-ing equations:

TiO2 ¼ Tiþ O2 ð3Þ

ZrO2 ¼ Zr þ O2 ð4Þ

NiO ¼ Niþ 1

2O2 ð5Þ

Fig. 2. Partial pressure of studied metals in the temperature range1500 – 3000 K.

Abb. 2. Partialdruck der untersuchten Metalle im Temperaturbe-reich 1500-3000 K.

Fig. 3. Partial pressures of zirconium, hafnium, titanium and theiroxides and nitrides.

Abb. 3. Partialdrucke von Zr, HF, Ti und deren Oxide und Nitride.

Mat.-wiss. u. Werkstofftech. 2006, 37, No. 7 Analysis of the Thermodynamic Conditions 615

Cr2O3 ¼ Cr þ 3

2O2 ð6Þ

MoO2 ¼ Moþ O2 ð7Þ

FeO ¼ Feþ 1

2O2 ð8Þ

The conditions for the reactions’ execution and for the finalstate of the system coming in the thermodynamic equilibriumare defined by the chemical thermodynamics laws [3].

The equilibrium constant keq for reactions of the kind(3)–(8) is calculated using the Vant Hoff’s equation which de-scribes the relationship between the variation of free energyDFO for a particular chemical reaction and its equilibrium va-lue keq (9):

DF0 ¼ �RT ln keq ð9Þ

Equation (9) gives opportunity to calculate the equilibriumconstants of interaction of the oxygen and each of the metalliccomponents at given pressure and temperature. According tothe thermodynamics’ laws all reactions (3)-(8), can be exe-cuted in the same time if in the EBMR condition (high tem-perature and vacuum) the requirement DF<0 is executed. Thetotal process can be presented in the form (10):

RO2 þMe ¼ MeOþ Rþ DF; ð10Þ

where R u RO2 are the corresponding refined refractory me-tal and its oxide and Me = MeO are: the metal component andthe stable oxide.

The interaction (10) is going to reach the thermodynamicequilibrium in the system at free energyDF=0.When the pres-sure decreases the thermodynamic equilibrium for the pro-cesses, which are fulfilled with pick out of one gas phase,is shifted to the performed gas. So, the chemical interactionsgiven by the equations (3)-(8) will be executed fully in thedirection of the dissociation of the oxides. The values of iso-bar-isothermal potential (free energy) DFO at atmosphericpressure and DF at p= 10-3Pa [4] are shown in Table 3.

For all studied metals the dissociation of the respective oxi-des can be further realized in the vacuum conditions (the va-lues of DF increase sharply when the pressure decreases). Theoxidization in vacuum is not possible for the chromium andiron (DF>0). The oxides of the rest metal component, as wellas the base refractory metal oxides, reach the reaction surfaceand can be removed through distillation of the oxide mole-cules. Those of the oxides, which are unstable at high tem-peratures of the superheated metal of the surface layer, aredissociated and oxygen molecules are removed from liquidmetal. In that way refining is executed by thermal dissocia-tion. The equilibrium dissociate pressure that characterizesthe dissociate processes performed on the interface liquid me-tal/vacuum as a function of surface superheated layer tem-perature [4] is shown in Fig.4.

Table 2. Melting temperature and density of refractory metals, as well as their oxides and the oxides of metal components into these basemetals.

Tabelle 2. Schmelztemperatur und Dichte der schwer schmelzbaren Metalle, sowie derer Oxide und der Oxide von anwesenden metal-lischen Beimischungen.

metal/oxyde Ti TiO2 Zr ZrO2 Hf HfO2

Tmelt, K 1941 2123 2125 2923 2500 3083

q, g/cm3 4.5 4.26 6.5 5.68 13.31 9.68

oxyde FeO Cu2O MgO MnO CaO NiO Cr2O 3 Al 2O3

Tmelt, K 1693 1508 3073 2058 2853 2263 2373 2323

q, g/cm3 5.7 6.0 3.58 5.45 3.35 4.85 5.21 3.7

Table 3. Isobar-isothermal potential of the investigation oxygen-metal.

Tabelle 3. Isobar-isothermisches Potential der Wechselwirkung Sauerstoff-Metall.

T,K DF,kJ/kg DF, kJ/kg

TiO2 ¼ Tiþ O2 1273 – 14312 – 6186

ZrO2 ¼ Zr þ O2 1273 – 9378 – 3076

NiO ¼ Niþ 12O2 1473 – 3590 – 219

Cr2O3 ¼ Cr þ 32O2 1473 –14 509 366

FeO ¼ Feþ 12O2 1473 – 3032 1251

MoO2 ¼ Moþ O2 1873 – 2 964 – 22

616 V. Vassileva, K. Vutova and G. Mladenov Mat.-wiss. u. Werkstofftech. 2006, 37, No. 7

2.2 Experiment

For testing the given thermodynamic analysis of the condi-tions and the possibilities for EBMR of the refractory metals,experimental investigations in EBMR plants of power 60kWand of 250kWwas executed. The plants are equipped with oneor respectively six electron guns with feeding mechanisms forhorizontal input of the raw material, water cooled hearth in the250 kW plant, copper-water-cooled-crucible with a movingbottom and connected with it pushing mechanism for drawingthe cast ingot.

As re-melting raw material at EBMR of titanium stronglyoxidized titanium wastes from surgery implants prepared pre-viously by hot pressing (at atmospheric ambience) is used.This scrap was pressed using discs with diameter 45mm

and height 15mm. The same preliminary preparation wasdone for starting raw material (sponge) from zirconium.

When hafnium is a raw starting material, work-out smalldimension scrap particles that were preliminary heated in va-cuum till purification of easy evaporating components, wereused. Then this material was pressed in the form of discs ofdiameter 60mm and height 10mm and fed in horizontal direc-tion.

The experiments at electron beam power P0 ranging from 7kW to 20kW and melting speed vmon 2.2 kg/h, 6.4 kg/h and12.7 kg/h were performed in the case of titanium and zirco-nium. In the case of hafnium the EBMR trials were producedfor the melting rates ranging from 2 to 4 kg/h, at electron beampower of 25 kW, 30 kW, 35 kWand 40 kWand circle oscilla-tions. In Table 4 is presented the chemical composition of thestarting metal and refined through EBMR process for some ofinvestigated regimes.

The data shown in Table 4 are connected both with the ther-modynamic trends, discussed above, and with the kinetics ofmass-transfer processes in the liquid metal (see [7,8] for cop-per, titanium and cobalt base alloys).

During the EBMR of oxidized wastes of IV group reactiverefractory metals (which are chemically active at high tem-peratures – titanium, hafnium and zirconium) the oxygen in-clusions removal could be presented in the following way. Theoxygen that is in the solid raw material as small particles ofoxides during the heating and melting of the base metal can bedissolved or moved to the molten pool surface. Some smallinclusions are incorporated through their upwards trip. Dueto their lower density the bigger oxide particles float onthe interface liquid metal/vacuum and dissociate to gaseousproducts. On the molten pool surface liquid metal is heatedto higher temperatures and due to the electron bombardmentthis process takes place for more stable oxides too.

From the data shown in Table 4 one can conclude, that theEBMR is suitable method for performing pure IV group re-active metals from raw wastes, including highly oxidized dur-ing previous treatment or use. After presented experiments of

Table 4. Example of pure refractory metals (titanium, hafnium and zirconium) without oxygen, EB re-melted.

Tabelle 4. Beispiel von reinen schwer schmelzbaren Metallen (Ti, HF und Zr) mit niedrigem Sauerstoffgehalt, umgeschmolzen durchElektronenstrahl.

Ti Pb Ingot D Concentration, (ppm)

kW mm Ti, % (O) Mn Ca Ni Al Fe Cu

Before EBMR 99.86 1500 40 40 300 800 100 100

After BMR 15 60 99.97 150 30 30 10 30 30 10

Hf Pb, Ingot D Concentration, (ppm)

kW mm Hf, % (O) Cr Ca Ni Al Fe Mg

Before EBMR 99.82 1000 40 30 200 200 200 100

After BMR 25 60 99.97 240 10 10 20 10 10 2

Zr Pb, Ingot D Concentration, (ppm)

kW mm Zr, % (O) Cr Ca Ni Al Fe Cu

Before EBMR 99.73 1000 40 40 300 800 200 300

After BMR 15 60 99.95 144 10 30 200 10 100 30

Fig. 4. Equilibrium dissociate pressure of metal oxides.

Abb. 4. Gleichgewichts-Dissoziationsdruck der Metalloxide.

Mat.-wiss. u. Werkstofftech. 2006, 37, No. 7 Analysis of the Thermodynamic Conditions 617

one way EB refinement the oxygen content in hafnium wasdecreased 4 times, in zirconium -7 times and in titanium10 times. It is observed also decrease (of order of 30) ofthe content of easy evaporating components (as aluminium,calcium, nickel and copper). As a result from the investiga-tions carried out (see also [6]) the metal impurities can be di-vided into three groups:l impurities which do not change their concentrations (sili-

con, vanadium, iron in titanium; copper, aluminum, molyb-denum in hafnium) during EBMR process,

l impurities whose evaporation depends only on the durationof the electron beam influence (aluminum, nickel, molyb-denum in titanium; chromium in hafnium),

l impurities whose evaporation depends both on the durationof the electron beam influence and the temperature changesof the molten pool surface (calcium, chromium in titanium;iron, calcium, zirconium, titanium in hafnium).

3 Conclusion

At EBMR of studied reactive refractory metals the oxygenremoval is realized using two mechanisms:l Due to the decrease of oxygen solubility in the liquid metal

at high temperatures and low pressure, the dissolved oxy-gen is eliminated through thermal degassing. For the stu-died three metals- titanium, zirconium and hafnium thismechanism has different relative weight in the total refin-ing. In the case of zirconium it is predominant. In the case ofhafnium it is less important and in the case of EBMR oftitanium the role of this mechanism is the smallest.

l In the case of EBMR of titanium more important is the oxy-gen reduction through oxides removal. In the studied groupof reactive metals these are not the base metal oxides, butoxides of the metallic components presented in the base me-tal.

l The thermodynamic analysis shows that impurities as alu-minum, nickel, molybdenum, calcium and copper, pick theoxygen from the base metal oxide. The impurity mediatelow-density oxides transport the oxygen to the moltenpool surface. There thermodynamic equilibrium of reaction(10) shifts to the full execution of the dissociation.

l The results and accumulated experience show that duringregeneration and purification of strongly oxidized scrap

of IV group reactive refractory metals, chemical composi-tion of which has a low metallic component content, addi-tion of a metal sponge is not only economically required,but also technologically needed (for achievement of highefficiency of the oxygen refining).

Acknowledgements

This research was funded from the National Council forScientific Research at Ministry of Education and Scienceof Republic of Bulgaria under contracts BIn-2/04 and BIn-3/04.

References

1. A. N. Krestovnikov, A.V. Krupin, V. N. Chernishev, B. L. Line-ckiy, V. S. Bogoliubov, presented at Simp.41 of Moscow Institutefor steel and alloys, Moscow, Russia, 1966, 189–198.

2. I. G. Sharma, N. Krishnamurthy, A. K. Suri, presented at theIndo-Bulgarian Workshop Electron Beam Technologies and Ap-plications, Mumbai, India, November 19–21, 2004, 42–50.

3. B. V. Linchevskiy, Thermodynamics and kinetics of the interac-tion of gases with liquid metals, Publ. House Metallurgy, Mos-cow, 1986. (in Russian).

4. G. S. Burhanova, Y. V. Efimova, Refractory metals and alloys,Publ. House Metallurgy, Moscow, 1986. (in Russian).

5. K. Vutova, V. Vassileva, G. Mladenov, Vacuum, 1997, 48, 143.6. G. Georgiev, V. Vassileva, T. Nikolov, N. Dimitrov, G. Mlade-

nov, Vacuum, 1990, 41, 2161.7. V. Vassileva, K. Vutova, L. Georgiev, T. Nikolov, G. Mladenov,

presented at the Conf. EBMR, Reno, Nevada, USA, October,1992, 233–242.

8. G. Mladenov, V. Vassileva, K. Vutova, T. Nikolov, Vacuum,1990, 47, 825.

Corresponding author: Assoc. Prof. Dr. Katia Vutova, Institute ofElectronics, Bulgarian Academy of Sciences, 72, boul.Tzarigrads-ko shosse, 1784 Sofia, Bulgaria, Fax +359 2 9753201,E-mail: vutova@ie.bas.bg

Received in final form: December 6, 2005 [T 14]

618 V. Vassileva, K. Vutova and G. Mladenov Mat.-wiss. u. Werkstofftech. 2006, 37, No. 7