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Journal of Materials Processing Technology 210 (2010) 1089–1094

Contents lists available at ScienceDirect

Journal of Materials Processing Technology

journa l homepage: www.e lsev ier .com/ locate / jmatprotec

nvestigation of electron beam melting and refining of titanium andantalum scrap

. Vutova ∗, V. Vassileva, E. Koleva, E. Georgieva, G. Mladenov, D. Mollov, M. Kardjievnstitute of Electronics, Bulgarian Academy of Sciences, 72 Tzarigradsko shosse blvd., 1784 Sofia, Bulgaria

r t i c l e i n f o

rticle history:eceived 15 July 2009eceived in revised form 20 February 2010ccepted 24 February 2010

a b s t r a c t

In this paper obtained experimental and theoretical data for Ti and Ta electron beam melting regenera-tion from waste products are presented. Different technological regimes and methods are realized andthe obtained results are discussed. Element analyses of the impurities’ concentration of the materialsbefore electron beam melting and refining (EBMR) and of the ingots after EBMR are compared. Statis-tical approach is applied for optimization of process parameters for Ti. Material losses less than 1% and

eywords:lectron beam melting and refining (EBMR)efractory and reactive metalscrap regenerationitanium

oxygen concentration less than 400 ppm after EBMR of Ti scrap are achieved at 11.5–12 kW beam powerand 0.09–0.14 mm/s casting velocity. The optimal process conditions and purification data for Ti refiningby minimization of all impurities’ concentrations and the material losses at the same time are obtainedat 11.25 kW electron beam power and 0.0835 mm/s casting velocity. For the performed experiments thebest purification of Ta (99.985) is obtained at 24 kW beam power and 0.029 mm/s casting velocity, the

ont s

antalum residence times on the frrespectively.

. Introduction

For a couple of decades the electron beam technologies and inarticular the technologies for electron beam melting and refiningEBMR) of metals and alloys have been recognized as a compet-tive and even a single method for obtaining new materials usedn various fields – mechanical engineering, instrument engineer-ng, electrical engineering, electronics, transport, power industry,

edicine, etc. The process of scrap recycling of refractory elementse.g. tantalum), reactive metals elements (e.g. titanium) and alloyshrough electron beam method results not only in reducing thecrap quantity but also in reuse of these expensive metals and alloysith unique properties. The world’s recourses of these metals are

xceedingly limited. That is why they are an expensive and strate-ic resource. For countries (e.g. Bulgaria) that do not fabricate suchetals, it is very important to recover the wastes containing them

o the maximum extent.Titanium and tantalum have unique physicochemical and

hemical properties: they are corrosion resistive, wear-proof inhemically aggressive environments (at normal working temper-

tures), mechanically robust at high and low temperatures. Thats why these metals are of present interest to various branchesf science and engineering such as metallurgy, power industry,edicine, chemical industry, electronics, military industry, nuclear

∗ Corresponding author. Tel.: +359 2 9795922; fax: +359 2 9753201.E-mail addresses: vutova@ie.bas.bg, katia@van-computers.com (K. Vutova).

924-0136/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.jmatprotec.2010.02.020

ide of the feeding block and in the liquid metal pool are 2 min and 5 min,

© 2010 Elsevier B.V. All rights reserved.

power industry, etc. Even though the manufacturers are interestedin high purity metals and alloys, they lack the technologies neededand the research in thermal and refining processes of materials pro-cessing is still in an opening stage. In Mitchell and Wang (2000) itis concluded that the main place of purification is the molten poolin the upper part of the cast ingot. In Mladenov et al. (1996) it isshown that the purification for the gaseous contaminants in the rawmaterial takes place on the front of feeding remelting material. InAblitzer et al. (1992) it is shown that the complexity of EBMR makesprocess control particularly difficult.

The production of each one of these metals is unique. Selectiveremoval of impurities from the used metals is important because inprevious applications or processing the metals were enriched withimpurities. This imposes the need to search for specific technologi-cal schemes for electron beam refining for different initial resourcesand for each particular equipment.

Investigations dealing with the relations of process parametersand impurity composition at EBMR of Ti and Ta are carried out.The aim is improving the composition and quality of the producedmetals as well as choosing optimal process conditions. Experimentsfor regeneration of Ti and Ta wastes are presented and discussed.Optimization approaches for the process of EBMR in the case of Tiand Ta are applied.

2. EBMR of titanium

Electron beam melting and refining of Ti was performed using60 kW equipment (ELIT-60) with horizontal feeder and the drip

1090 K. Vutova et al. / Journal of Materials Processing Technology 210 (2010) 1089–1094

F–c

mw

ttcf2TptTppirtmdomb–w

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Fig. 2. Contour plot of the final oxygen concentration.

TE

ig. 1. Principal scheme of the electron beam melting and refining drip process: 1started metal rod, 2 – generated droplets, 3 – molten pool (in the water-cooled

rucible).

olten metal crystallized in a water-cooled copper crucible. Theorking vacuum pressure was 5–8 × 10−3 Pa.

In order to examine the refining kinetics and to investigatehe influence of the process conditions during EBMR, experimen-al investigation was performed using free of oil contamination Tiold pressed wastes. In agreement with Harker (1983) the Ti scrapalls into three categories regarding size: “bulk weldable” typically.27 kg and up; “feedstock” – 25.4–76.2.mm and machine chips.itanium chips are not as easily recycled. As feeding material coldressed waste rods with diameter ≈45 mm were used and the dis-ance between the rod and the metal pool surface was 50 mm.he investigated ranges of the process parameters (electron beamower P, casting velocity vC and refining time �) are: the beamower P is in the range of 11.25–18.75 kW, the casting velocity vC

s in the range of 0.05–0.15 mm/s, and the refining time � is in theange of 2.78–11.85 min, respectively. The values of the refiningime are estimated as the overall time during which the processed

aterial is in liquid state in the three reaction zones in the case ofrip melting (Fig. 1) where 1 zone – molten layer on the front sidef the melting block, 2 zone – the drops falling toward the liquidetal pool and 3 zone – the liquid metal pool on the top of the cast

lock in the crucible. The measured values of another parameterthe surface temperature T of the molten metal in the crucible –ere in the range of 2370–2670 K.

Statistical approach, described in Koleva et al. (2007), is appliedere for the estimation of kinetic dependencies of the impurities’oncentrations and the material losses based on experimentallybtained data (Table 1). The material losses are estimated usinghe weight of the initial and the obtained ingots. They are mainly

ue to evaporation but they also occur due to splashes during theefining process. The estimated experimental kinetic dependencesf the impurities’ concentrations Ci (ppm) at EBMR of Ti take intoccount the multiple correlations between the process parameters.he standard deviation of the oxygen content is 0.01% (100 ppm)

able 1xperimental data for EBMR of Ti.

No. Process parameters Ci (ppm)

P (kW) T (K) vC (mm/s) � (min) O Al

1 11.25 2370 0.05 11.85 520 4002 11.25 2370 0.10 5.68 320 4003 11.25 2370 0.15 4.95 480 4004 15.00 2460 0.05 10.45 660 4005 15.00 2460 0.10 4.54 660 4006 15.00 2460 0.15 2.78 430 5007 18.75 2670 0.05 9.98 410 5008 18.75 2670 0.10 4.50 310 4009 18.75 2670 0.15 3.32 420 500Concentrations before EBMR 1500 600

Fig. 3. Contour plot of the final Al concentration.

and the standard deviation of the chemical analysis concerning thecomponents is 10 ppm.

Figs. 2–9 present contour plots of the final impurities’ concen-trations (in ppm) and the material losses (in %) vs. the independentprocess parameters – the electron beam power P and the castingvelocity vC. The values of the other dependent process parameters Tand � can be calculated (using P and vC) by the following estimatedrelations:

T = 2820 − 88.0P + 4.26667P2; (1)

� = 36.477 − 1.8933P − 255.97vC + 0.05618P2 + 926.00vC2. (2)

Fig. 2 presents the contour plot of oxygen concentration depend-ing on the beam power and the casting velocity. The oxygen

V Fe Si Mn Ni Cr Ca Cu

300 1200 200 30 220 120 40 30300 1200 200 30 260 140 40 50300 900 200 30 230 150 30 20300 1000 200 30 220 120 40 30300 1000 200 30 250 100 30 20300 900 200 30 240 140 30 40300 1100 200 30 250 100 40 20300 1100 100 30 240 120 40 30300 1000 100 30 230 130 40 30300 1200 200 55 300 450 100 100

K. Vutova et al. / Journal of Materials Processing Technology 210 (2010) 1089–1094 1091

Fig. 4. Contour plot of the final Fe concentration.

rdtooaTKp

Fig. 7. Contour plot of the final Ca concentration.

Fig. 5. Contour plot of the final Ni concentration.

emoval from the molten metal pool is accomplished by: (i)egassing of oxygen that is in the form of a solid solution, (ii)he removal of the bound oxygen via the evaporation of metalxides, predominately sub-oxides of the base metal and volatile

r unstable (at melting temperatures) impurity oxides with higherffinity to the oxygen and (iii) direct electron beam irradiation.his new mechanism was shown in Vassileva et al. (2005) andoleva et al. (2007). Two areas with minimal concentration at EBower 11–13 kW and 17–19 kW are seen. The oxygen concentra-

Fig. 6. Contour plot of the final Cr concentration.

Fig. 8. Contour plot of the final Cu concentration.

tion decreases at higher casting velocities (0.13–0.15 mm/s). Thedecrease of the oxygen concentration at lower EB powers could beexplained by achieving a low melting rate on the front of the feedingmelting rod and considerable release of oxygen being in solid metalsolution there. At the same time a low temperature of the molten

pool surface together with a residence time of remelted titanium240–660 s lead to minimal oxygen concentration due to removalof oxides inclusions in the cast ingot that is in agreement with theresult in Vassileva et al. (2005). The second optimum of the oxygenrefining is connected to higher superheating of the molten metal

Fig. 9. Contour plot of the material losses.

1 rocessing Technology 210 (2010) 1089–1094

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nd better reduction of the oxygen content independently from theesidence time in the molten state of the refining Ti. This also is ingreement with the data for oxygen concentration dependence onhe refining time in Vassileva et al. (2005). Obviously, at the studiedegimes and EB melting plant the casting velocities 0.14–0.15 mm/sesemble a good value of that control parameter of EBMR. In thease of the intermediate EB powers (13.5–17 kW) the superheatingf the molten pool surface is not so high but optimal conditions forxygen reduction are achieved only at higher values of the castingelocity.

The behavior of Fe content (Fig. 4) could be explained by thebserved insensitivity to the residence time and superheatinginetic dependencies of Fe metal contamination during EBMR of Tibtained in Vassileva et al. (2005). In contrast to the oxygen removalase, the EB powers lower than 13 kW or higher than 17.5 kW leado lower purification of the cast ingot. By analogy with the casef the oxygen removal, velocity ranging between 0.14 mm/s and.15 mm/s is optimal for all studied EB powers in EBMR.

Similar behavior of low sensitivity to the casting velocity and theB power for the concentration variation of Ni is shown in Fig. 5.imilar situation is the one for V and Si concentrations that are nothown as contour plots here.

Different behaviors could be seen in Fig. 3 and in Figs. 6–8or refining of Al, Cr, Ca and Cu. The areas with minimal impu-ity concentrations and the corresponding ranges for the powernd the velocity can be seen. This could be connected to theulti-exponential kinetic dependencies of the contaminant con-

entrations on the residence time in the liquid state and on theuperheating of the melted material obtained in Mladenov et al.1996) and Vassileva et al. (2005).

The material losses (Fig. 9) are estimated on the base of weighteasurements of the initial remelting rod and the cast ingot

btained after EBMR. In the cases of lower as well as of higherB power the losses are due to evaporation of contaminants (atower power) and evaporation of the base metal as well as dueo the splashes (at higher powers) during the remelting of theaw material. The minimal material losses are achieved at theollowing optimal parameter values: 13–16 kW beam power and.1–0.14 mm/s casting velocity.

The optimal conditions for concentration minimization of eachne of the components are different. Therefore, if there are specialequirements for the content of more inclusions, an overall crite-ion for the parameter optimization should be formulated and aompromise optimal solution should be found. The oxygen removalogether with the minimization of the material losses is of majormportance during the refining of the processed Ti. The initialxygen concentration is 1500 ppm. Contour plots of the oxygenoncentration of 400 ppm and the material losses of 1% depend-ng on the independent process parameters – the EB power andhe casting velocity are shown in Fig. 10 (using Figs. 2 and 9). 1%

aterial losses are chosen as an acceptable level of the losses.The marked area (Fig. 10) presents the common area of these

ontours. Each point from this common area corresponds to valuesf the electron beam power and the casting velocity at which theaterial losses are less than 1% and the oxygen concentration is less

han 400 ppm after EBMR of Ti scrap. The decrease of the oxygenoncentration will lead to an increase of the material losses. In thatay a compromise solution satisfying both requirements can be

ound.In the case of application of analogous geometry optimization

or another contour plot of impurity concentration, the common

rea will be different. If the optimization task is formulated withespect to the concentrations of all the impurities and the materialosses, a compromise solution is obtained. This is applied for thease when the minimization of all impurities’ concentrations andf the material losses is equally important. This optimal solution is

Fig. 10. Graphical optimization – the material losses (<1%) and the oxygen concen-tration after EBMR (<400 ppm).

called Pareto-optimal solution and it is found by minimization ofall impurities’ concentrations and the material losses at the sametime – Table 2. A change in the obtained optimal regime condi-tions will worsen the values of one or more from the consideredcriteria. Table 2 also presents the individual minimal values of theconcentration of each impurity and the overall material losses.

For optimization (reduction) of the impurities’ concentration,besides the change of the process parameters, the intentionalchange of the initial concentration can be performed and investi-gated by additional experiments; the increase of the feeding blockdiameter will increase the reaction surface and will support therefinement of non-metallic elements, forming a solid solution inthe bulk metal and the refinement of the highly volatile impurities(1 reaction zone), etc.

3. EBMR of tantalum

Experiments for Ta regeneration from waste product were con-ducted using EBMR installation with power 60 kW (ELIT-60). Afteranalysis and evaluation of potential ways for preliminary treat-ment (oil removal, compacting, preliminary melting, etc.) of theinitial material (the waste) for regeneration, adequate preliminarytreatment of the particular Ta scrap was chosen. Used capacitorsfrom electronic circuits were the initial material for EBMR of Ta.They were preliminary separated, oil removed and compacted inthe form of disks with diameter 40 mm and height 15 mm. Thepurity of the obtained, in this way, initial material was Ta 98.9%.The obtained results showed that the product after EBMR processwas Ta 99.95%, using the following scheme:

• Preliminary treatment of the initial material for regeneration:o electronic components disjoining and separation;o chemical treatment of Ta waste in acid;o washing in H2O and alcohol and drying ando compacting in the form of disks.

• Electron beam melting and refining (ELIT-60).

For investigation of the refining processes in Ta many exper-iments (25) were performed using 60 kW EBMR plant (ELIT-60),equipped with one electron gun, feeding mechanism for horizontalinput of the raw material and water-cooled copper crucible witha moving bottom and pulling mechanism. Different technological

regimes (conditions) are realized for tantalum regeneration. Thebeam moves consecutively on the front side of the feeding block(1 zone) and in the liquid metal pool (3 zone) because the meltingtemperature of Ta is high. Attention is paid to the ratio betweenthe e-beam power of the 1 zone and 3 zone. The values of the ratio

K. Vutova et al. / Journal of Materials Processing Technology 210 (2010) 1089–1094 1093

Table 2Values of optimal process parameters and optimal inclusions’ concentrations.

Incl. P (kW) vC (mm/s) T (K) � (min) Ci,opt (ppm) C(O) (ppm) � (%) Pareto-optimal solution

O 18.75 0.1045 2670 4.09 300. 8 300. 8 2.1406 P = 11.2500 kWvC = 0.0835 mm/s� = 7.3698 minC(O),opt = 364.4570 ppmCAl,opt = 402.4978 ppmCFe,opt = 1199.8 ppmCNi,opt = 257.5228 ppmCCr,opt = 134.9620 ppmCCa,opt = 36.1959 ppmCCu,opt = 35.7617 ppm�opt = 1.2519%

Al 17.14 0.0710 2565 7.02 338.9 557.6 2.1173Fe 14.51 0.1500 2441.5 3.27 861.1 429.8 1.0638Ni 15.08 0.0500 2463 10.22 221.5 651.0 2.8258Cr 15.90 0.0845 2499.5 5.56 99.0 665.7 1.0103Ca 16.73 0.1255 2541.7 2.99 25.0 507.6 1.0068Cu 16.2 0.0775 2514.1 6.27 16.4 650.6 1.3977� (%) 14.29 0.1135 2433.7 3.77 – 582.1 0.2633

Table 3Impurities’ concentration at EBMR of Ta (sample no. 6).

Concentration Fe K Na Nb W As Al Mo Mn Cr Cu

Before EBMR, C0 (ppm) 300 100 300 200 100 200 10 40 30 20 30After single EBMR, C (ppm) 10 30 30 20 10 10 10 20 30 <10 10After double EBMR, C (ppm) 10 30 10 10 10 10 10 10 20 <10 10

Table 4Optimal inclusions’ concentration (minimal total inclusions) for EBMR of Ta (sample no. 11).

Fe K Na Nb W As Al Mo Mn Cr Cu

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C0 (ppm) 600 300 300 200C (ppm) 10 10 10 10(C0 − C)/C0 × 100% 98 97 97 95

1/�3, between the refining times in the 1 zone (on the front part ofhe feeding rod – Fig. 1) and in the 3 zone (the surface of the moltenool in the water-cooled copper crucible – Fig. 1), are in the rangef 1/5–2/3. The investigated ranges for the e-beam power densityare: 3.82–5.35 kW/cm2 for the 1 zone (p1) and 6.87–8.40 kW/cm2

or the 3 zone (p3). For the experiments the following scheme con-erning the technological parameters (regimes) were realized: (i)he value of the ratio p1/p3 is a constant and the value of the ratio1/�3 is also a constant; (ii) the value of the ratio p1/p3 is a constanthile the value of the ratio �1/�3 varies; (iii) the value of the ratio

1/p3 varies and the value of the ratio �1/�3 also varies.Data about chemical analysis of the impurities’ concentra-

ion of the starting materials (before EBMR) and of the ingotsfter EBMR of Ta are obtained and analyzed. In Table 3 data

bout inclusions’ concentration after single and double refin-ng are presented. The values of the process parameters are:1 = 4.04 kW/cm2, p3 = 6.87 kW/cm2 and �1/�3 = 2/5 for the singleBMR; p1 = 4.58 kW/cm2, p3 = 6.87 kW/cm2 and �1/�3 = 2/4 for theouble EBMR of Ta, respectively.

Fig. 11. Dependence of impurity concentration (purification) C/C0 × 100 on proc

0 200 30 20 10 10 200 40 10 10 10 10 200 80 67 50 0 0 0

These results and the kinetic dependencies for contaminants,given in Koleva et al. (2006), show that the concentrations ofsome impurities such as Mo, Fe, K, Na, As, Nb, Al and W aresignificantly less than the corresponding initial impurities’ concen-trations (more than 10 times). The refining process of Mn, Cr andCu is difficult due to thermodynamic limits at the refining condi-tions and their concentrations decrease slightly (up to 2–3 times).The oxygen concentration decreases two times from 800 ppm upto 400 ppm after EBMR.

The best purification of Ta (99.985) is obtained for thefollowing regime conditions: �1/�3 = 2/5, p1 = 4.48 kW/cm2 andp3 = 7.64 kW/cm2, that correspond to 24 kW e-beam power and0.029 mm/s casting velocity at our conditions. The results areshown in Table 4.

The removal of the impurities depends on the temperature upto which the molten metal is superheated and on the refining time.Fig. 11 presents compressed data for W and Nb concentrations afterEBMR of Ta for different regimes (conditions). These dependenciescould be used for appropriate regime parameter’s choice. The points

ess parameters p1/p3 and �1/�3 × 100 for: (a) W and (b) Nb in EBMR of Ta.

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094 K. Vutova et al. / Journal of Materials P

orresponding to the values of the p1/p3, �1/�3 × 100 and C/C0 × 100C is the impurity concentration of the ingot after EBMR of Ta and0 is the impurity concentration of the starting material) repre-ent vertices of inscribed triangles. We can approximately evaluatehe purification (impurity concentration after EBMR) for other val-es of the process parameters. For example, when p1/p3 = 0.65 and1/�3 × 100 = 50, the value of C/C0 × 100 (purification) is between.25 and 5 in the case of W impurity. The purification C/C0 × 100 ofb is in the range of 1.7–3.3 when p1/p3 = 0.68 and �1/�3 × 100 = 67.sing such type triangular diagrams the value of one of the param-ters can be approximately evaluated when the other two are fixed.

. Conclusion

Generally, almost all the refractory and reactive metals andlloys are obtained and processed by EBMR. Technology and equip-ent optimization are hindered and made more expensive due to

nsufficient knowledge of the complex physical and physicochem-cal processes of interaction between the intensive electron beamnd the material.In this paper experimental and theoretical inves-igations of the process parameters at EBMR of Ti and Ta scrapith the purpose of improving the composition of the performed

ngots are presented. Beam powers in the range of 14–17 kW lead toower oxygen removal, while higher purification of the cast ingots achieved at 13–17.5 kW power for refining of Fe, Ni, V and Si.learly defined areas with minimal impurity concentrations areeen for refining of Cr, Ca, Cu and Al, namely at: EB power 16 kWnd 0.75 mm/s casting velocity for Cr; 15–18 kW beam power andhe same casting velocity for Cu; power 16–17 kW and 0.12 mm/sasting velocity for Ca; 17–18 kW power and 0.05 mm/s castingelocity for Al. Minimal material losses are seen at 14 kW beam

ower and 0.11 mm/s casting velocity. Material losses less than% and oxygen concentration less than 400 ppm after EBMR of Ticrap are achieved at 11.5–12 kW power and 0.09–0.14 mm/s cast-ng velocity. Compromise optimal solution for the chosen criterionf minimal content of all inclusions and the material losses at the

ing Technology 210 (2010) 1089–1094

same time for Ti refining is found at 11.25 kW e-beam power and0.0835 mm/s casting velocity. In the case of EBMR of Ta, 25 experi-ments are performed and attention is paid to the ratio between thee-beam power on the front side of the feeding rod and the power inthe liquid metal pool. For these experiments the best purificationof Ta is obtained at 24 kW power and 0.029 mm/s casting velocityfor residence times 2 min and 5 min on the front side of the feedingblock and in the liquid pool, respectively.

Acknowledgements

This research was funded by the National Fund for ScientificResearch at the Ministry of Education, Youth and Science of Repub-lic of Bulgaria under contracts nos. TK01/0073 (DO 02-200/2008)and BIn-5/2009. The authors thank Eng. T. Nikolov, R. Nikolov andA. Stoilov for the technical assistance.

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