In the paper, results of the study on temperature effects on the rate of aluminium evaporation from the Ti-6Al-4V alloy during smelting in a vacuum induction furnace are presented. During smelting at 1 973 – 2 023 K, 10 Pa and 100 Pa, up to 26 % reduction in the aluminium content in the alloy compared to the initial value is observed. The determined values of overall mass transport coefficient are 1,48∙10-5 m∙s sup>-1 – 1,95∙10 sup>-5m∙s sup>-1

A. Szmal

D. Jama

G. Siwiec

J. Mizera

R. Burdzik

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The effects of temperature on the kinetics of aluminium evaporation from the Ti-6Al-4V alloy

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225METALURGIJA 53 (2014) 2, 225-227G. SIWIEC, J. MIZERA, D. JAMA, A. SZMAL, R. BURDZIKTHE EFFECTS OF TEMPERATURE ON THE KINETICS OF ALUMINIUM EVAPORATION FROM THE Ti-6Al-4V ALLOYReceived – Prispjelo: 2013-04-25Accepted – Prihvaćeno: 2013-09-15Preliminary Note – Prethodno priopćenjeISSN 0543-5846METABK 53(2) 225-227 (2014)UDC – UDK 669.295:621.365.5:669.041=111G. Siwiec - Silesian University of Technology, Faculty of Materials En-gineering and Metallurgy, Katowice, PolandJ. Mizera - Warsaw University of Technology, Faculty of Materials En-gineering Warszawa, PolandD. Jama - Silesian University of Technology, Faculty of Applied Math-ematics, Gliwice, PolandA. Szmal - Silesian University of Technology, Faculty of Organization and Management, Zabrze, PolandR. B urdzik - S ilesian Un iversity o f Technology, F aculty, F aculty o f Transport, Katowice, PolandIn the paper, results of the study on t emperature eff ects on the rat e of aluminium evaporation fr om the Ti-6Al-4V alloy during smelting in a vacuum induc tion furnace are presented. During smelting at 1 973 – 2 023 K , 10 Pa and 100 Pa, up to 26 % r eduction in the aluminium c ontent in the allo y compared to the initial value is obser ved. The determined values of overall mass transport coeffi  cient are 1,48∙10-5 m∙s-1 – 1,95∙10-5m∙s-1.Key words: Ti-6Al-4V alloy, vacuum induction furnace, smelting, overall mass transport coeffi  cientINTRODUCTIONTitanium alloys belong to the class of metallic mate-rials which recently have been of increasing interest in many industries, mainly due to their specifi c properties, such as small densities, high tensile strength and good corrosion resistance in many chemically aggressive en-vironments. These properties ensure the use of alloys for production of e.g. implants, endoprostheses and components of systems used in cardiovascular sur gery, such as cardiac pacemakers and valves. The Ti-6Al-4V is an example of increasing applications of titanium al-loys. One of its production methods is smelting in a vacuum induction furnace (a conventional technology) or with the use of ‘skull melting’ technology. The proc-ess of smelting is very often associated with two disad-vantageous ef fects. The fi rst ef fect is metal bath con-tamination due to a reaction of the matrix component, i.e. titanium, with the ceramic materials of the melting pot. The other ef fect is a potential for signifi  cant alu-minium elimination due to its evaporation which is caused by a large difference in vapour pressures of alu-minium and titanium. Aluminium loss is also induced by low pressures in the smelting system [1 - 5].In the paper, results of the study on the temperature effects on the rate of aluminium evaporation from theTi-6Al-4V alloy during smelting in the vacuum induc-tion furnace are presented.EXPERIMENTAL PARTFor the experiments, a Ti-6Al-4V alloy containing Al 5,5 %mass was used. The smelting system was a VIM-20 - 50 vacuum induction furnace manufactured by Seco -  Warwick. It is a modern device which ensures smelting at a broad range of operating pressures due to a system of  vacuum pumps consisting of e.g. a diffusion pump and a Roots pump. The time of liquid alloy holding at a given  temperature and pressure was 10 minutes. During each  experiment, alloy samples were collected and subjected  to a chemical analysis to determine titanium, aluminium  and vanadium contents. The experiments were performed at 1 972 – 2 023 K, 10 and 100 Pa.Changes in the aluminium content in a liquid bath  were determined with the use of the following equation: tACCAlkAl ⋅=− 0log   (1)where: C kAl and CAl0  - initial and fi nal aluminium concen-trations in the alloy, respectivelyt – time,A – constant.Examples of changes in aluminium concentration in the alloy are presented in Figures 1 and 2.DISCUSSION OF RESULTSWhile analysing the process of aluminium evapora-tion, it was assumed to be determined by the fi rst-order rate equation [6 - 8]. Thus, in order to determine the overall mass transfer coeffi cient, the following equation was used: 2,303 log tVFkCCAloAlkAl ⋅−=   (2)where:kAl – overall mass transfer coeffi cient,226  METALURGIJA 53 (2014) 2, 225-227G. SIWIEC et al.: THE EFFECTS OF TEMPERATURE ON THE KINETICS OF ALUMINIUM EVAPORATION FROM...In Table 1, the estimated values of apparent rate con-stant ′eAlk  and average fl ux of Al loss are presented.The determined kAl, and ′eAlk  values allowed for defi n-ing the contribution of mass transfer resistance related to the evaporation process to the overall resistance of the process using the following relation: 10011⋅=′AleAlekkU  % (5)The results have shown that for both pressures, the contribution of resistance related to the evaporation process, Ue, to the overall process resistance exceeds50 %, which means that the investigated process is ki-netically controlled (Figure 3).In order to confi rm that the investigated process is kinetically controlled, the value of its activation energy was determined with the use of Arrhenius equation: TREk AAl ⋅−=ln  (6)where:EA – activation energy of the processIn Figure 4, Arrhenius relationships and changes in the overall mass transfer coeffi  cient (kAl) value versus temperature are presented in the coordinate system.For both operating pressures, the estimated activa-tion energies are far above 100 kJ/mol, which is typical of kinetically controlled processes. For instance, the ac-tivation energy values were 200 kJ/mol and 161 kJ/mol for 100 Pa and 10 Pa, respectively. In the case of diffu-sion control in the liquid phase, these values are far smaller and comparable to the diffusion activation ener-gies in liquids. For example, the dif fusion activation energy value for cobalt and iron in liquid copper is11 kJ/mol, while for chromium, nickel, manganese and vanadium in liquid iron, it is 15 to 25 kJ/mol.F –  evaporation area assumed as the inner surface area of the melting pot,V – bath volume.In order to determine the kAl value from equation (2), the value of ‘A’ slope in equation (1) was used. In Table 1, the overall mass transfer coeffi  cient values, deter -mined from the experimental results, are presented.In order to determine the effects of liquid phase-gas-eous phase physical change rate (on the interface) on the overall process rate, the values of apparent evapora-tion rate constant were estimated with the use of the following equation: '5,0)2(eAlAlkRTM=πα ϕ  (3)where:  TiTiAloAl Mpργϕ⋅⋅=  (4)where:γAl – coeffi cient of Al activity in liquid alloy,oAlp  - vapour pressure of Al over pure bath,MTi – titanium molar mass,MAl  - aluminium molar mass,ρTi – liquid titanium density,α=1 – constant [9].Figure 1  Changes in aluminium concentration during Ti-6Al-4V melting at 100 Pa, 1 973 K and 2 023 KFigure 2  Changes in aluminium concentration during Ti-6Al-4V melting at 10 Pa, 1 973 K and 2 023 KTable 1  The values of pressure, temperature, average fl ux of Al loss, overall mass transfer coeffi  cien t, apparent evaporation rate constant for aluminum evaporation process from Ti-6Al-4V alloyPressure/PaTemp/KAverage fl ux of Al loss/g∙cm-2∙s-1Overall mass transfer co-effi  cient kAl/m∙s-1Apparent evaporation rate constant ke’Al/m∙s-1100 1 973 6,30·10-5 1,48∙10-5 1,90∙10-5100 1 973 6,31·10-5 1,48∙10-5 1,90∙10-5100 1 998 6,26·10-5 1,57∙10-5 2,37∙10-5100 1 998 6,24·10-5 1,57∙10-5 2,37∙10-5100 2 023 6,22·10-5 1,64∙10-5 2,97∙10-5100 2 023 6,21·10-5 1,62∙10-5 2,97∙10-510 1 973 6,08·10-5 1,52∙10-5 1,90∙10-510 1 973 6,01·10-5 1,52∙10-5 1,90∙10-510 1 998 6,00·10-5 1,86∙10-5 2,37∙10-510 1 998 5,92·10-5 1,95∙10-5 2,37∙10-510 2 023 5,92·10-5 1,94∙10-5 2,97∙10-510 2 023 5,91·10-5 1,93∙10-5 2,97∙10-5227METALURGIJA 53 (2014) 2, 225-227G. SIWIEC et al.: THE EFFECTS OF TEMPERATURE ON THE KINETICS OF ALUMINIUM EVAPORATION FROM...CONCLUSIONSBased on the experimental results of Ti-6Al-4V smelting in the vacuum induction furnace at 1 973 – 2 023 K, 10 and 100 Pa as well as the kinetic analysis fi ndings, it was demonstrated that:•  Temperature rise from 1 973 K to 2 023 K at 100 Pa is accompanied by aluminium loss from the al-loy: 20 % to 22 % compared to the initial concen-tration (5,5 % mass).Simultaneously, the value of overall mass transfer coeffi cient, kAl, increases from 1,48·10 -5 m·s -1 to 1,64·10-5 m·s-1.•  The temperature rise from 1 973 K to 2 023 K at 10 Pa is accompanied by aluminium loss from the alloy: 23 % to 26 % compared to the initial con-centration. Simultaneously , the value of overall mass transfer coeffi  cient, kAl, increases from 1,52·10-5 m·s-1 to 1,95·10-5 m·s-1.•  Kinetically, it has been shown that for the whole pressure range, the contribution of resistance re-lated to the evaporation process to the overall process resistance exceeds 50 %, which means that the process is kinetically controlled. It is confi rmed by determined values of the activation ener gy of the process which are over 160 kJ/mol.AcknowledgementsThe study was conducted under the Research Project No. N N508 589439, fi nanced by the Ministry of Sci-ence and Higher Education – Poland.REFERENCES[1]  G. Siwiec: Archives of Metallurgy and Materials Materials 57 (2012), 4, 951-956.[2]  L Blacha, J. Łabaj: Metalurgija 51 (2012), 4, 529-533.[3] L. Blacha, R. Burdzik, A. Smalcerz, T. Matuła: Archives of Metallurgy and Materials 58 (2013), 1, 197-201.[4] L. Blacha, B. Oleksiak, A. Smalcerz, T. Matuła: Archives of Materials Science and Engineering 58 (2012), 1, 28-32.[5]  L. Blacha, G. Siwiec, B. Oleksiak: Metalurgija 52 (2013), 3, 301-304.[6]  R. Harris: Metallurgical Transactions B 15 (1984), 2, 251-257.[7]  L. Blacha: Archives of Metallurgy 48 (2003), 1, 105-127.[8]  R. Ohno: Metallur gical Transactions B 7 (1976), 4, 647-653.[9]  B. Bakish, O. Winkler: Vacuum metallurgy, Elsever, Am-sterdam, 1972.Note: Nowak P. is responsible for English language, Katowice, Poland0204060801001 970 1 980 1 990 2 000 2 010 2 020 2 030Temperature / K(1/ke'Al/1/kAl).100 %10 Pa 100 Pa10,810,910,911,011,011,111,111,211,24,92E-04 4,96E-04 5,00E-04 5,04E-04 5,08E-041/T / 1/K-lnk Al100Pa 10PaFigure 3  Changes in the contribution of resistance related to evaporation to the overall process resistance depending on the pressureFigure 4  The overall mass transport coeffi  cient, kAl, versus temperature

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The effects of temperature on the kinetics of aluminium evaporation from the Ti-6Al-4V alloy

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