Selective evaporation of some titanium alloys components in the electron beam furnace

Materials Chemistry 2 (1977) 51 - 67 © CENFOR S.R.L. - Printed in Italy

SELECTIVE EVAPORATION

OF SOME TITANIUM ALLOYS

COMPONENTS IN THE ELECTRON BEAM FURNACE* P.M. S T R O C C H I * a n d M. S O C C I * *

*

Laboratorio Tecnologia Materiali Metallici non Tradizionali o f the National Research Council, CINISELLO B., (Milano).

**

Metallurgica del Tirso S.p.A., B O L O T A N A , (Nuoro).

Received 24 December 1977 - The use of an electron beam furnace for casting parts in titanium alloys assures high purity products, avoids any undesirable contamination, permits a high overheating of the bath and a direct recycling of scraps. On the other hand there are some disadvantages, and specificaUy the loss of the most volatile alloy elements due both to the high overheating and the high vacuum degree. Therefore the manufacture of certain critical castings such as some aircraft cast structures, whose composition has to face very strict tolerances, becomes a delicate problem. The purpose of this work was to study the properties of melting, overheating and casting in the electron beam furnace of two types of titanium alloys: Ti-5A1-2.SSn and Ti-6A1-4V; as a reference the behaviour of commercially pure titanium was also investigated. The evaporation tests were carried out on cylindric specimens in a laboratory type electron beam furnace, working under controlled conditions which may be easily reproduced. After the treatment each test piece was cut into sections, metallographycally examined and analyzed by an electron microprobe, in order to obtain the composition profiles on several planes normal to the base. In this way evaporation rates of the various alloy components as well as vapour phase compositions were determined; by employing a sufficiently large number of samples it was, therefore, possible to set out the evaporation curve of each Summary

* Paper presented to the Third International Conference on Titanium, Moscow, May 18-25, 1976.

52

alloy component. The obtained results, which have been also theoretically discussed, enable useful suggestionsfor the proper use of an electron beam furnace for industrial production.

INTRODUCTION For applications such as the fabrication of complicated configuration the casting process would be the most economical, if not the only practical, means of producing the parts. Although, volume-wise, titanium castings represent only a small portion of titanium market, the much progress has been made recently in improving the quality of products and applications have broadened from strictly chemical equipment elements of titanium to components of various alloys to be employed in mechanical and aeronautical constructions. The procedures and characteristics of melting and casting of titanium in the electron beam furnace lead to some real advantages like the utilization of scraps, high degree of superheating, the possibility of carrying out continuous processes, the achievement of an optimum of economy and product quality. On the other hand melting and casting under a high vacuum such as that of the electron beam furnaces may cause the loss of volatile alloy components like aluminum and tin and therefore the electron beam melting and casting of certain alloys such as the Ti-6A1-4V and Ti-5A1-2.5Sn, may be difficult to be controlled as far as the final analysis of products is concerned. This paper describes a first step of a study aimed to determine the behavior_of two titanium alloys melted in an electron beam furnace as a function of time at a constant energy input level. Rates of evaporation for aluminum, tin and vanadium were determined by weight loss and chemical analysis. A theoretical treatment of the results leads to the evaluation of the "evaporation coefficient" whose knowledge allows to predict the behavior of an alloy during electron beam melting and casting.

53 THEORETICAL CONSIDERATIONS Vacuum processes are industrially performed in a relatively cold chamber far from equilibrium conditions which could be only reached if all the metal in the crucible were transferred by evaporation to the infinite sink provided by the cold chamber walls. Vacuum melting of alloys involves the simultaneous evaporation of basis metal and metallic alloying elements, i.e. a vacuum distillation; therefore, liquid and vapor compositions generally change continuously with the time of treatment. Only in the case of pure substances or azeotropic mixtures both liquid and vapor show obviously the same constant composition. Some authors 1-2-3-4-5-6 have developed various formalisms based on the Herz-Knudsen-Langmuir equation, that give a relationship between concentration of volatile substances and the evolution of melt composition. According to the Olette treatment 1,6 developed for binary alloys, an "evaporation coefficient" can be defined as follows: (1)

~ x - -"Yx "Ya

V ~ Ma - Pox Poa

where % Po and M are, respectively, activity coefficients, vapor pressures and atomic weights; a denotes the solvent (for dilute solutions 7a = 1) and x the solute, i.e. an alloying element or impurity. It has been shown that the evaporation coefficient at a given temperature is the proportionality factor between the ratio of weight evaporation rates of solute and solvent d m x / d m a and the weight ratio of constituents • mx for a dilute binary solution containing m a parts by ma weight of solvent and m x of alloying element 7 ,8 . Thus:

(2)

dm X

mX

dm a

c¢x ma

By integration of (2) the weight fractions x and A (as per cent)

54

which have evaporated may be evaluated by: (3)

X = 1 0 0 - 100 (1 - ~ )ax 100

Knowledge of olx is of direct interest to predict the possibility of selective evaporation of the element involved when the alloy is vacuum treated: according to whether 0~x is greater or smaller than unity, the melt will impoverish or enrich in x; in practice, however, a x must be higher than 2 in order to observe any valuable effect. A high ax value is a favourable characteristic for an impurity which can be easily removed by electron-beam refining; on the contrary, a low value is wanted for alloying elements for avoiding them to be selectively evaporated by vacuum meking. Since the value of a x depends on the thermodynamic properties of the melt, it is possible to use evaporation experiments to determine some of the specific data of the system. On the other hand, when ax can be calculated by thermodynamic data, it is possible to predict the evolution of the liquid system consequent to the selective evaporation of its components.

EXPERIMENTAL Melting procedure and materials The melting experiments were conducted in a modified 9 mod. ES 1/2/20 Leybold-Heraeus electron beam furnace which consists of a water-cooled melting chamber, a system of pumps and valves for evacuating the system up to values of 10 -6 torr, and an electrical power supply. The melting chamber houses a 20 kV, 25 kW electron-gun assembly and a water-cooled copper unit carrying five 25 mm diameter button molds. Samples for electron beam melting were prepared from 25 mm bars of the following materials: a) grade 2 commercial titanium; b) Ti6A1-4V and c) Ti-5A1-2.5Sn alloy, whose analysis is given in Table 1.

55 With a lathe small ¢ 23 mm cylindrical samples were cut off; 7, 10 and 13 mm high for titanium; 13 mm high for the alloys. Five samples were placed into the molds and then the system was evacuated to 4.10 -6 torr or better. Then the first sample of the series Table 1 - Analysis of experimental materials used.

~ e r i a l Element

Gr. 2 comm. Ti

Ti-6AI-4V

Ti-5AI-2.5Sn

C

0.018

0.015

0.022

O

0.140

0.165

0.157

N

0.016

0.012

0.014

H

0.004

0.002

0.010

Al

--

6.20

5.10

V

-

3.9

Sn

-

--

-

2.6

Fe

0.06

0.14

0.32

Ti

balance

balance

balance

was melted for the established time at a power level of 9.7 kW; transient from zero to maximum power was kept between 5" (short duration experiments) and 15" (long duration). By the same procedure the remaining four samples were melted. After the system had been completely cooled, the furnace was blanked of from the pumps, argon admitted into the melting chamber and the amount of evaporation determined for each b u t t o n by weight loss. During the experiments beam power and oscillation, vacuum, water tlow-rate into the mold-piece and its inlet and exit temperatures were carefully controlled in order to keep these parameters as constant as possible. In such a way 75 melting tests were carried out.

56 Results

Weight losses The weight change which occurred on each sample of grade 2 titanium are shown in Fig. 1. It can be observed that samples of various sizes lost weight early in the melting process at a rate which appears to be almost constant. Then, after a certain time (about 8 and 16 minutes for the 13 and 19 g samples respectively) the slope of the curves decreases. No change of slope is shown by the 24 g samples up to 24 minutes of treatment. The deviation from linearity shown by 13 and 19 g titanium samples within the treating period experimented, which may be interpreted in terms of a surface temperature reduction due to the influence of the mold-piece walls, suggested the 24 g size samples to be the most suitable for performing the test on alloys. Fig. 2 shows the'results of melting experiments on 24 g size samples of Ti-6A1-4V and Ti-5A1-2.5Sn alloys compared with those obtained with 24 g size of titanium. After the very early treatment time, the evaporation of the two alloys can be approximated also by a straight line up to weight losses of about 7 g (30% of the total weight of the sample). Within the above-mentioned range the weight losses (WL: g) of titanium, and Ti-6A1-4V, Ti-5A1-2.5Sn alloys may be expressed by WLTi = 0.33 t WL64= 0.48 + 0 . 5 0 t WL52= 0.32 + 0 . 7 2 t where t is measured in minutes.

Pool depth studies. Melt composition The depth and shape of the liquid layer was determined for each

57

8

I

l

01 m.

0

m

/ //

1"

o~6

/

i m

[]

4

0

fI01

2

Z~

I

8

o 13g samples 19g

,,

[] 2 4 g

,,

I

1

~

16

I

24 time • min

F@ 1 - Weight losses vs. time o f grade-2 titanium specimens; sizes: 24, 19 a~d Id .4, electron beam melted. Beam power: 9.7 kW; pressure: 1 . 3 - 3 . 5 x 10 .6 tort; temperature (calculated): 1880°C (Morrison), 1945°C (Dushman, Honig).

58

8

I

'

I

ml

0 J

0

~6 i m

0

2

~/

' 7

oTi-6AI-4V-alloy ~ Ti- 5AI - 2.5Sn -alloy 4

8

12 16 time .min

Fig. 2 - Weight losses vs. time o f grade-2 titanium, li-6A1-4V and Ti-5A1-2.5Sn, 24 g specimens, electron beam melted. Beam power 9.7 kW; pressure 1 . 3 - 3 . 5 x lO'6toi'r; temperature (evaluated) 1880°C (Morrison), 1945°C (Dushman, Ho-

,ug).

59 b u t t o n to guide the removal of representative samples for chemical analysis and to establish a basis for the mass balance between liquid and vapor which characterizes the evaporation process. Each b u t t o n was cut diametrally and a cross-section surface polished up to 600 mesh paper, cerium bioxide high polished and then macrographically etched. Fig. 3 and 4 show typical buttons produced with the Ti-6A1-4V and Ti-5A1-2.5Sn alloys, respectively. Samples were taken from each button and analyzed; fine milled chips were removed from the top of each b u t t o n and care was taken to be sure the samples were entirely from the melted area. Aluminum, vanadium and tin were determined by atomic absorption. Distribution of significative elements in both melted and unmelted skull section, determined by electron-microprobe, evidenced composition gradient in both sections showing that mixing in the liquid state is so fast that its composition is kept uniform despite removal of alloying elements from the surface and the skull maintains about its initial compositions. In Table 2 the most significant data are reported.

DISCUSSION Approximate surface temperature At least in the range of vapor pressures below about 1 torr, the rate of evaporation of a substance makes it possible to calculate the corresponding value of the temperature 1° b y the relation: (4)

logW = C -

0.5 logT

B T

where W = rate of evaporation in grams per square centimeter per second. Values of the constant B and C for titanium are given b y Dushman and Honig 11 as: B = 23230, C = 9.11. A determination of the vapor pressure of titanium by Morrison, gives values of the temperature considerably lower: the constant based on such data are: B = 24659 and C = 10.0811.

60

,

4~ i

6'

8'

12'

Fig. 3 - Typical buttons cross-section of Ti-6Al-4V alloy for pool depth studies. Melting times in minutes are #Den.

61

2'

4 y

,

8'

12'

Fig. 4 Typical buttons cross-section o f Ti-5A l-2.5Sn alloy f o r pool depth stu, dies. Melting times in minutes are given.

62

~q

c~j L~

c~

E~

c~

r~

C~ 0 C~

A C~

~q~m

63 The experimental rate of evaporation for the linear extent of the titanium evaporation curves, employing an estimated value of the bath surface of about 6 cm z, results to be 0.00091 g. cm -2" sec "1 . A numerical solution of the equation (4) yields sur(ace temperature values of 1880 and 1945°C according to whether Morrison or Dushman constants are used. It appears to us reasonable, due to the strict control of experimental conditions employed, to infer that the surface temperature of alloy melts would have about the same values of titanium ones. Selective evaporation of alloying elements In order to interprete the behavior of the two alloys studied when they are electron beam melted, the theoretical consideration on selective evaporation of alloying elements developed in this paper are now applied to our experimental data. The following simplifying assumptions are adopted: a) The skull material, whose weight is arbitrarly considered to remain constant during the whole experiment, do not take part in the evaporation process which involves the liquid and vapor phases only. b) In each experiment the initial "active" weight of the alloy is calculated as the sum of the liquid final weight and the total weight loss of the sample (evaporated). According to this assumption, knowledge of the initial (constant) and final analysis of the alloy makes it possible to calculate the mass balance for each evaporation process. c) The concept of evaporation coefficient, which has been originally defined for a binary alloy, has been extended to multicomponent alloys such as those under study: titanium acts as the solvent; aluminum and vanadium in the Ti-6A1-4V and aluminum and tin in the Ti-5A1-2.5Sn, behave independently as dilute solutes. From the several sets of experimental data concerning each alloy, the relative percentages (by weight) of titanium (At) and alloying elements (Xmt) have been calculated. Thus: Ti evaporated (wgt) A t = 100 Ti initial in liquid (wgt)

64

~ 80

~ 60 E

0 0 >,

40

0 m

V ~=0.4 20

20 4O titanium evaporated ."wgt. % ,

I

I

I

I

I

2

4

6

8

12

time.min

Fig. 5 - Ti-6AI-4V- Relationship between the proportion o r A l and V evaporated and the loss of titanium. Experimental results: dots; predicted by equation 0 ) : solid curves. Evaluated temperature: 1880°C (Morrison), 1945°C (Dushman, Ho~).

65

,

~ ~

°

' ~

~ 80 ~ 60

~ '9'°

Sn c~4

,

~~o 20

/

-j

l

I

I

20 40 titanium evaporated -wgt.% I

I

I

I

2

4

6

8

time : min

Fig. 6 -- Ti-5Al-2.5Sn - Relationship between the proportion o] Al and Sn evaporated and the loss o f titanium. Experimental results: dots; predicted by equation (3): solid curves. Evaluated temperature: 1880°C (Morrison), 1945°C (Dushman, Honig).

66 Alloy component evaporated (wgt) Xmt = 100 Alloy component initial in liquid (wgt) The values of A t and Xmt obtained were put into the equation (3) which was numerically solved for a. The resulting a values calculated in this way appear to be reasonably constant for each component of a given alloy; those values are, as an average, at the operating temperature, as follows: Ti-6A1-4V

:

O~A1 = 15;

a V = 0.4

Ti-5A1-2.5Sn

:

O~Al = 13;

OtSn = 4

The relationship between the proportion o f the alloy elements evaporated and the loss of titanium for the two alloys experimented is represented in Fig. 5 and 6; the dots indicate experimental values; the solid curves show the theoretically computed prediction. The results obtained by this laboratory-scale experiments, which allow us to determine the evaporation coefficients, can be used to predict the behavior of a titanium alloy to be melted in a commercial size electron bram furnace, also in order of programming a continuous casting cycle. On this basis some approximate evaluations have been already performed on an industrial precision casting plant: first results obtained appear to be quite encouraging.

Acknowledgements The experimental part of this work was performed at and supported by the Laboratorio per la Tecnologia dei Materiali Metallici non Tradizionali of the National Research Council. The authors are grateful to Messrs. Turisini for their help in experiments, Caciorgna and Cucchiaro for metallography, Garulli and Dr. Rondelli for chemical analysis.

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