Materials Science and Engineering A 438–440 (2006) 679–682

The effects of vacuum induction melting and electron beam melting techniques on the purity of NiTi shape memory alloys J. Otubo a,b,∗ , O.D. Rigo b , C. Moura Neto a , P.R. Mei b a

Instituto Tecnologico de Aeronautica (ITA), 12228-900 S.J. Campos, SP, Brazil b DEMA-FEM-UNICAMP, 13083-000 Campinas, SP, Brazil

Received 21 April 2005; received in revised form 16 January 2006; accepted 7 February 2006

Abstract The usual process to produce NiTi shape memory alloys is by vacuum induction melting (VIM) using graphite crucible that contaminates the bath with carbon. The contamination by oxygen comes from residual oxygen inside the melting chamber. A new alternative process to produce NiTi alloys is by electron beam melting (EBM) using water-cooled copper crucible that eliminates the carbon contamination and the oxygen contamination would be minimized due to operation in high vacuum. This work compares the two processes and shows that the carbon contamination is four to ten times lower for EBM compared to VIM products and that the final oxygen content is much more dependent on the starting raw materials. The purity of the final product should be very important mainly in terms of biomedical applications and the contaminations by carbon and oxygen affect the direct and reverse martensitic transformation temperatures. © 2006 Elsevier B.V. All rights reserved. Keywords: NiTi; Shape memory alloys; Vacuum induction melting; Electron beam melting

1. Introduction Although the NiTi shape memory alloys (hereafter called NiTi SMAs) are known worldwide since the 1970s [1], the literature relating its processing is limited owing to production difficulties (there are few producers in the world). Our group has been working on NiTi SMA processing since 1997 using two processes: electron beam melting (EBM) and vacuum induction melting (VIM) in order to obtain ingot in pilot scale [2–4]. The conventional process to produce NiTi alloy is by VIM that presents some advantages such as ease chemical composition control due to magnetic agitation, relatively ease operational control and possibility of degassing. One of the disadvantages is the contamination of the bath by carbon which comes from the use of graphite crucible and graphite mold. The use of other types of crucible such as MgO and Al2 O3 could contaminate the bath with oxygen [1]. An alternative to VIM process is the EBM process to produce NiTi SMA. This process is known since the 1950s for refining refractory and reactive metals. More recently this process has been used to produce alloys such as Ti6Al4V [5–8] and very

clean superalloys [9]. The use of EBM process to produce NiTi SMA is quite rare [10,11] and practically started with this group [2,3]. The ingot production by EBM completely eliminates the carbon contamination due to melting in a water-cooled copper crucible, and also the oxygen contamination is minimum due to operation in high vacuum (better than 10−2 Pa). Therefore, the carbon and oxygen contents in the final product depend only on their levels in the initial raw material. One of the disadvantages of working in high vacuum during melting and remelting is the difficulty of controlling the nominal chemical composition due to some component evaporation mainly nickel which has higher vapor pressure than Ti. Our recent work producing small NiTi SMA ingots using EBM has proved that the composition control is perfectly possible as far as certain care is taken during the feeding material preparation [3,4]. This work compares some results of producing NiTi SMA by EBM and VIM processes mainly in terms of final ingot purity and how those impurities affect the martensitic transformation temperature. 2. Experimental procedures 2.1. VIM process



Corresponding author. Tel.: +55 12 39475905; fax: +55 12 39475906. E-mail address: [email protected] (J. Otubo).

0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.02.171

By this process three experimental procedures were planned to check the influence of graphite crucible quality and its dimen-

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J. Otubo et al. / Materials Science and Engineering A 438–440 (2006) 679–682

sions on the final ingots contamination mainly by carbon: in the first stage, it was used extruded high porosity graphite crucible and in the second experiment, it was replaced by high density low porosity graphite crucible keeping its dimensions, therefore, the quality of crucible is being tested producing a 19 mm in diameter by 180 mm long ingot weighing around 1.5 kg. In the third stage, we kept the low porosity graphite for the crucible but its dimensions were scaled up such as to minimize the ratio between apparent contact area and liquid metal volume which were 0.68 for the initial two experiments and 0.43 for the last one. The ingots dimensions for the last stage were 55 mm in diameter by 260 mm long of a mass around 4.3 kg. Each charge consisted of grade 1 Ti plates intercalated with electrolytic Ni plates consolidated by Tungsten Inert Gas (TIG) welding, then set over a NiTi alloy block, previously prepared. The melting power ranged from 15 to 17 kW and the melting chamber internal pressure from 1 to 10 Pa. 2.2. EBM process For preparing the EBM ingots, we used a 80 kW EB furnace with EB power of up to 13.5 kW and melting chamber internal pressure up to 10−3 Pa. The raw materials were grade 1 Ti + electrolytic Ni consolidated according to melting procedure. Three processes were used as follows: A static process where the elements (Ni + Ti) are charged at once with desired nominal composition onto water-cooled copper crucible that serves also as the casting mold; a semi-dynamic process where the raw materials are continuously fed and cast into constant volume cylindrical water-cooled copper mold and the last one is a dynamic process with continuous feeding of raw materials and continuous ingot casting scaling up the size and weight of the ingots. The details of fabrication by EBM could be found in the following references [2–4]. 3. Results and discussions In terms of ingot surface quality, the VIM products cast in graphite mould showed better results with smooth surface

while EBM products presented some cold-junctions defects (less than 3 mm deep), that is typical of refrigerated copper mould casting. Table 1 shows the chemical composition of the VIM and EBM processed ingots. The influence of graphite crucible quality on the carbon contamination on the final ingot could be seen comparing results of ingot 1 (stage 1) and ingots 2, 3 and 4 (stage 2). The ingot produced in high porosity graphite crucible presented highest carbon contamination of 2440 ppm (ppm are in wt.%). Changing to low porosity crucible, the contamination reduced by a factor of two which is the average of ingots 2, 3 and 4. This decrease is due to decrease of actual contact area going from low porosity graphite crucible to high porosity crucible of same apparent contact area. The effects of scaling up the dimensions could be seen comparing ingots 5–9 to ingots 2–4 of the second stage. For those ingots, the average carbon content was around 600 ppm reducing by another factor of two. This reduction is promoted by the change in the ratio between apparent contact area to liquid volume which decreased from 0.68 to 0.43 representing a reduction of 37% while the ingot mass increased from 1.5 kg in the second stage to 4.3 kg in the third stage, that is, almost three times. Continuing on Table 1, the ingots 10 and 11 were produced by EBM static process whose limitation is the maximum weight of about 350 g. Larger weight requires higher EB power and promotes loss of homogeneity along the ingot. The ingot 12 was produced by a semi-dynamic process that also presents weight limitation of around 500 g and ingot length of 50 mm. The ingot 13 was produced by a dynamic process avoiding the above limitations scaling up weight and size. The nickel loss which was the initial concern in using EBM can be overcome controlling the EB power and initial composition of raw materials. The highlights of the EBM processed ingots are the very low contamination by carbon. Those ingots show the lowest carbon content ranging from 70 to 150 ppm. That is, the highest value of 150 ppm is 3.5 times lower than the lowest value of 520 ppm of VIM ingot. The oxygen content varied from 640 to 1055 ppm. Irrespective of processing technique, the value of oxygen content in the final product depends upon the purity of initial raw materials as far

Table 1 Chemical composition of VIM and EBM processed ingots (weight-ppm)a Stage

Ingot

Weight (g)

Ni (wt.%)

Ni (at.%)

C (ppm)

O (ppm)

MP (◦ C)

AP (◦ C)

MP (◦ C)

1

1

1500

55.31

50.21

2440

358

−18.0

4.1

−55.5

2

2 3 4

1500

55.26 55.21 54.63

50.17 50.13 49.60

1100 1000 1880

621 390 329

−67.5 −46.0 −26.5

−39.1 −21.9 −3.8

−107.5 −89.0 −85.5

3

5 6 7 8 9

4300

55.08 55.27 55.61 54.72 55.54

50.01 50.18 50.49 49.68 50.42

543 600 580 660 520

924 564 837 1113 570

22.0 21.2 −1.9 51.2 −4.9

52.7 47.2 17.8 81.9 17.0

−25.5 −18.5 −22.5 −6.5 −31.0

10 11 12 13

175 341 330 2200

55.78 54.70 54.90 54.70

50.58 49.50 49.74 49.66

150 70 140 130

1055 1050 640 640

−29.9 58.3 55.0 60.4

−10.8 91.0 86.3 93.7

−56.0 −1.2 −1.4 2.3

EBM

a

Grade 1 Ti, 99.80 wt.% purity (balance); electrolytic Ni, 99.95 wt.% purity.

J. Otubo et al. / Materials Science and Engineering A 438–440 (2006) 679–682

Fig. 1. Martensitic transformation temperature MP as a function of nickel content.

as both processes operate in a relatively high vacuum. Details on EBM processing can be found in [2–4]. The influence of carbon and oxygen content on direct (MP ) and reverse (AP ) martensitic transformation temperature peaks are also shown in Table 1 obtained from the DSC measurements. In the Fig. 1, only MP values are plotted as a function of nickel content. Beside the data from VIM (square dot) and EBM (round dot) processed samples, there are also data from small arc melted samples (triangle dot) specially prepared with carbon content below its solubility limit and the influence of oxygen above its solubility limit also was taken into account according to Ti4 Ni2 O formation. Therefore, in principle, the outer curve of Fig. 1 serve as a reference curve showing only the influence of nickel content on the direct martensitic transformation temperatures peaks that decrease very fast as the nickel content increases above the equiatomic composition. From Fig. 1, one can see that excepting ingot 13, all the samples presented negative MP temperatures deviation compared to reference curves which are shown in Table 1 as MP (=MP − MPREF ). Separating in groups according to degree of contamination, one can start analyzing samples from stages 1 and 2 that presented carbon content higher than 1000 ppm but with oxygen content below 400 ppm excepting ingot 2. Correspondingly, those samples presented highest negative MP deviation of almost −90 ◦ C. The set of samples from stage 3 (with carbon and oxygen content usually presented by commercial materials) presented lower negative MP deviation compared to first set of samples. The highest deviation was −31 ◦ C for ingot 9. Excepting ingot 11, the EBM ingots with carbon content lower than 150 ppm presented practically no deviation of MP compared to reference curve. The influence of oxygen itself can be analyzed in terms of ingots 10 and 11 which present carbon content below its limit of solubility in the alloy and oxygen content above 1000 ppm. The effect of oxygen is practically null when the nickel content is below equiatomic composition as for ingot 11 (49.50 at.% Ni) and this affirmation could be corroborated by ingot 8 with 49.68 at.% Ni and 1113 ppm of

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oxygen content with negative deviation of just −6.5 ◦ C. The effect of oxygen starts to appear when the nickel content is above equiatomic composition as for ingot 10 with 50.58 at.% Ni which presented negative MP deviation of −56 ◦ C. The presence of carbon and oxygen above their solubility limit precipitate TiC and Ti4 Ni2 O, respectively withdrawing titanium from the matrix. The result is the nickel enrichment in the matrix compared to initial nominal composition and as shown by reference curve, higher the nickel content, above equiatomic composition, lower is the martensitic transformation temperatures. Therefore to have final ingot close to original composition, the contaminants carbon and oxygen should be kept below its solubility limit. In terms of processing, for VIM the relevant aspects are the graphite quality and scale used for the crucible to minimize the carbon contamination of final ingot. It was shown that NiTi production by EBM is perfectly possible with final ingot with very low carbon content. The degree of purity of final product should be very important such as in biomedical application. 4. Conclusions In terms VIM processing the change from high porosity graphite crucible to one with low porosity keeping its dimensions reduced the final carbon content by a factor of two. Using low porosity graphite and scaling up the crucible dimensions as to have lower contact area, the carbon contamination was reduced by another factor of two. The average carbon content was 600 ppm usually found in commercial products. It was shown that EBM to produce NiTi SMA is a viable process resulting in ingots with carbon content as low as 70 ppm. For both processes, the final oxygen content depends upon the initial raw materials nickel and titanium. The martensitic transformation temperatures are decreased by the presence of carbon and oxygen and depending upon the degree of contamination, the deviation from the reference curve could be as high as 100 ◦ C. The influence of oxygen content in decreasing the martensitic transformation temperature is especially important for nickel content above equiatomic composition. Acknowledgements To FAPESP, grant 00/09730-1 and FINEP, grant CT-INFRA 03/2003/30, INFRAFICTA. References [1] C.M. Jackson, H.J. Wagner, R.J. Wasilewski, NASA-SP 5110 (1972). [2] J. Otubo, P.R. Mei, S. Koshimizu, L.G. Martinez, in: M.A. Imam, R. DeNale, S. Hanada, Z. Zhong, D.N. Lee (Eds.), The Third Pacific Rim International Conference on Advanced Materials and Processing—PRICM-3, vol. 1, Honolulu, Hawaii, USA, 12–16 July, The Minerals, Metals & Materials Society, TMS, 1998, pp. 1063–1068. [3] J. Otubo, O.D. Rigo, C. Moura Neto, M.J. Kaufman, P.R. Mei, J. Phys. IV Fr. 112 (2003) 873–876.

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[4] J. Otubo, O.D. Rigo, C. Moura Neto, M.J. Kaufman, P.R. Mei, Mater. Res. 7 (2) (2004) 263–267. [5] A. Choudhury, ISIJ Int. 32 (1992) 554–563. [6] T. Nakayama, A. Kuroda, K. Kurita, ISIJ Int. 32 (1992) 583–592. [7] S. Watanabe, K. Suzuki, K. Nishikawa, ISIJ Int. 32 (1992) 625– 629.

[8] A.N. Lalinyuk, N.P. Trigub, V.N. Zamkov, O.M. Ivasishin, P.E. Markovsky, R.V. Teliovich, S.L. Semiatin, Mater. Sci. Eng. A346 (2003) 178– 188. [9] A. Mitchell, ISIJ Int. 32 (1992) 557–562. [10] H. Matsumoto, J. Mater. Sci. Lett. 10 (1991) 417–419. [11] H. Matsumoto, J. Mater. Sci. Lett. 10 (1991) 596–597.

The effects of vacuum induction melting and electron ...

The effects of vacuum induction melting and electron beam melting techniques on the purity of NiTi shape memory alloys. J. Otuboa,b,∗. , O.D. Rigob, C. Moura ...

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