Journal of Materials Processing Technology 162–163 (2005) 116–120

NiTi SMA production using ceramic filter during pouring the melt夽 O.D. Rigo a , J. Otubo a,b , C. Moura Neto b , P.R. Mei a,∗ a

Universidade Estadual de Campinas, DEMA/FEM, 13083-970 Campinas, SP, Brazil b Instituto Tecnol´ ogico de Aeron´autica, 12228-900 S. J. Campos, SP, Brazil

Abstract The usual process to produce NiTi shape memory alloys (SMA) is by vacuum induction melting (VIM) using graphite crucibles which contaminate the bath with carbon. Carbon reacts with titanium precipitating TiC influencing the matrix composition which, in turn, affects the martensitic transformation temperatures. Furthermore, the presence of TiC makes the final product difficult for mechanically process using cold working steps. In this study, we present preliminary results of our attempt to remove the TiC from the melt using ceramic filters during pouring and analyzed the efficiency of the filtering process and some results will be presented. © 2005 Elsevier B.V. All rights reserved. Keywords: NiTi; Shape memory; VIM; Ceramic filter

1. Introduction Although the NiTi shape memory alloys (hereafter called NiTi SMA) are known worldwide since the 1970s [1], the literature related to processing are few in comparison to that concerning characterization and martensitic transformation phenomena mainly due to production difficulties (there are very few producers in the world). In Brazil, there are few research groups working on NiTi SMA and Koshimizu and Yamamoto [2,3] and Andrade [4,5] did the pioneering works. Our group has been working on NiTi SMA processing since 1997 [6–8] using two processes: electron beam melting (EBM) and vacuum induction melting (VIM) in an attempt to produce pilot scale ingots. The main problem in NiTi production is the contamination by carbon and oxygen making the final product brittle. Beside that, the contamination by carbon and oxygen causes deviation in the martensitic transformation temperatures to lower values. The main commercial process for producing NiTi SMA is by VIM using graphite crucibles and graphite ingot molds. The contamination by carbon comes from the graphite crucible, which reacts with both nickel and titanium. The use of other types of crucibles such as MgO and Al2 O3 contaminates the bath with oxygen 夽 FAPESP Grant #00/09730-1 ∗

Corresponding author. E-mail address: [email protected] (P.R. Mei).

0924-0136/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2005.02.177

[1]. The contamination by oxygen can also come from residual oxygen inside the melting chamber since the pressure is not so low (1–10 Pa). This work analyzes the efficiency of using ceramic filters to remove mainly TiC particles from the melt during the pouring process. The melting and casting were done in graphite crucibles and graphite molds producing 1.5 kg ingots. 2. Experimental procedure The ingots were melted in a VIM furnace with the melting power ranging from 15 to 17 kW and the melting chamber internal pressure from 1 to 10 Pa. Two distinct melting procedures were used: the first consisted of producing three ingots using extruded high porosity (low density) graphite crucibles and molds with no ceramic filter during pouring the melt. The charge consisted of grade 1 titanium plates intercalated with electrolytic nickel plates consolidated by TIG welding. This was then set over a NiTi block which was already inside the graphite crucible. The NiTi block served as a melting starter. It should be noted that the carbon solubility in NiTi compound is lower than for elemental nickel and titanium. The ingots produced in this step were used as a reference. Using the same charge mounting procedure as described above, the second set of experiments consisted of using low porosity (high density)

O.D. Rigo et al. / Journal of Materials Processing Technology 162–163 (2005) 116–120

117

and ZrO2 –PSZ (partially stabilized zirconia) with 25 ppi and maximum operating temperature of 1700 ◦ C.

3. Results and discussion

Fig. 1. Ceramic filter inside the graphite holder.

graphite crucibles and pouring the melt through a ceramic filter. The ceramic filter was mounted inside a graphite holder (Fig. 1) and the set mounted on the top of graphite mold (Fig. 2). The mold shown in Fig. 2 was also used in first stage with no ceramic filter. The final ingot dimension was 180 mm long by 19 mm in diameter weighing around 1.5 kg. The filters tested included SiC with 15 and 25 ppi (pores per inch) with maximum operating temperature of 1590 ◦ C

Fig. 2. Ceramic filter mounted on the top of graphite mold.

Fig. 3 shows the typical ingots produced in both the stages with no surface oxidation or cold junctions. Table 1 presents the chemical composition of the ingots produced with and without ceramic filters: ingots 1, 2 and 3 were cast without ceramic filter showing very high and increasing values of carbon content from 2440 to 4010 ppm; ingots 4–11 were produced using ceramic filters during pouring and contain much lower carbon contents ranging from 990 to 2430 ppm. The high carbon content in the ingots produced without filters is attributed to the use of extruded graphite crucible that presents high porosity, promoting large actual contact area between liquid bath and graphite crucible surface and this contact area increases (for the same liquid volume) as the number of melts increases (using the same crucible), increasing the carbon content as presented by ingots 2 and 3. The low carbon content presented by the second set of ingots (4–11) compared to ingots produced in first stage is due to the use of high density graphite crucible. The higher carbon content presented by ingots 8, 10 and 11 is due to the degradation of crucible inner surface, but still lower than the lowest values found in the first stage. Also from Table 1, it can be seen that the carbon content is practically the same before and after filtering, showing that the ceramic filter was inefficient in removing TiC precipitates although, as shown below by SEM analyses, the adhesion of TiC on the porous surface is quite visible. The oxygen content varied from 329 to a maximum of 636 ppm and nitrogen content from 4 to 19 ppm showing no difference between two processes. It should be emphasized that the oxygen and nitrogen contents in the final product are much more dependent on the initial raw material than the melting process itself due to operation in vacuum. Fig. 4 shows the top view of SEM images of ZrO2 –PSZ ceramic filter before filtering and Fig. 5 after filtering, showing the thickening of pores wall and also the closing of interconnection channels due to adhesion of TiC precipitates. Fig. 6 presents a magnified view of surface shown in Fig. 5 containing agglomerated TiC particles with maximum dimension of

Fig. 3. NiTi ingots with 180 mm long by 19 mm in diameter.

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O.D. Rigo et al. / Journal of Materials Processing Technology 162–163 (2005) 116–120

Table 1 Ingots chemical composition Ingot 1 2 3 4 5 6 7 8 9 10 11 a

Filter

Ni (wt%)

Ti (wt%)

Ca (ppm)

C (ppm)

O (ppm)

N (ppm)

– – – SiC SiC ZrO2 –PSZ ZrO2 –PSZ ZrO2 –PSZ ZrO2 –PSZ ZrO2 –PSZ ZrO2 –PSZ

55.31 54.60 54.89 55.56 55.20 55.26 55.21 55.35 55.45 54.63 54.10

Remainder Remainder Remainder Remainder Remainder Remainder Remainder Remainder Remainder Remainder Remainder

2440 2750 4010 1400 2430 1150 1020 1600 990 1800 2300

– – – 1200 – 1100 1000 1680 1000 1880 2400

358 – 632 – – 621 390 636 476 329 –

19 – 8 – – 3 9 13 12 4 –

Carbon content before filtering.

the order of 10 ␮m. Fig. 7 shows the inner part (cross section) of a ZrO2 –PSZ ceramic filter for ingot 7 that presented one of the lowest carbon contents of 1000 ppm. The homogeneous large area denoted by (A) is the NiTi alloy that solidified after stopping the pouring. The area denoted by (B) is the layer

Fig. 4. ZrO2 –PSZ ceramic filter before filtering.

Fig. 5. ZrO2 –PSZ ceramic filter after filtering.

of TiC particles trapped on the ceramic surface and, finally, the region denoted by (C) is the matrix of the ceramic filter. Fig. 8 shows the inner portion of the SiC ceramic filter for ingot 5 which had one of highest carbon contents of 2430 ppm. Here clearly three distinct regions can be seen: the dark area

Fig. 6. Magnified view of Fig. 4 showing agglomerated TiC particles trapped on the filter surface.

Fig. 7. Inner portion of ceramic filter: ZrO2 –PSZ, ingot 7 containing 1000 ppm of carbon.

O.D. Rigo et al. / Journal of Materials Processing Technology 162–163 (2005) 116–120

Fig. 8. Inner portion of ceramic filter: SiC, ingot 5 containing 2430 ppm of carbon.

119

Fig. 10. Orange colored TiC precipitates from VIM ingot 0.058 wt% C (200×).

denoted by (A) is the solidified NiTi alloy; (B) the trapped TiC particles and (C) the ceramic matrix. Both figures were taken at the same magnification. Fig. 9 presents the micrography structure of ingot 3 (first stage, as cast) with the highest carbon content (4010 ppm) showing large precipitates of TiC, orange colored, with irregular distribution and also tiny precipitates of Ti2 Ni (or Ti4 Ni2 O) phase. This picture is typical for samples with carbon contents above 1000 ppm. Fig. 10 shows the micrography of another VIM processed NiTi ingot which presents 580 ppm of carbon content, typical of commercial products. Note the smaller amount of TiC particles. For comparison, Fig. 11 shows the micrography of TiC precipitates free electron beam melted NiTi sample with very low carbon content of 130 ppm. See reference [8] for EBM processing details and results. The results presented above show that the change from the extruded high porosity graphite crucible to the low porosity one drastically reduced the carbon content mainly due to the reduction in real contact area between the crucible surface and

the liquid bath for the same liquid volume. Another aspect that should be pointed out, independent of graphite type, is the degradation of the crucible inner surface as a function of usage. The carbon content in the final ingot increases as the number of melting runs increase. The ideal condition should be reworking the crucible every two runs. Another aspect that should be analyzed is the dichotomy between the results presented in Table 1 that shows no difference in the carbon content before and after filtering, and the SEM micrographies in Figs. 5–8 that show clearly the adhesion of TiC particles on the surface of the ceramic filter. The explanation could be as follows: taking into account that the primary TiC particle dimensions are some orders of magnitude smaller than the pore dimensions, the basic retention mechanism of these particles during filtering is related to the occurrence of contact between those particles and the pores surface [9–11]. The probability of particle adhesion onto pores surface increases with the increasing contact area and longer liquid paths through the ceramic filter channel. The thickening of TiC particles layer or the agglomeration onto the pores surface occurs by the successive adhesion

Fig. 9. Micrograph of ingot 3, as cast, 4010 ppm carbon, magnification: 400×.

Fig. 11. TiC precipitates free EBM ingot, 0.013 wt% C (400×).

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O.D. Rigo et al. / Journal of Materials Processing Technology 162–163 (2005) 116–120

of those particles passing through the pores being higher in channels between adjacent pores as can be seen in Fig. 7. On the other hand, to keep enough fluidity during pouring, it is necessary to superheat the bath between 150 and 200 ◦ C. Since the melting point of NiTi SMA (for the compositions tested) is around 1310 ◦ C, the bath temperature should be kept between 1450 and 1500 ◦ C. This temperature is sufficient to keep carbon in solution or to dissolve TiC particles. At the end, the bath passing through the ceramic filter should acquire a stationary equilibrium condition of adhesion and dissolution justifying the results shown in Table 1. Finally, it should be emphasized that although apparently the SiC ceramic filter is more efficient to remove the TiC particles from the melt as shown in Fig. 8, its use was interrupted because this type of ceramic is chemically bonded, being its maximum operating temperature of 1550 ◦ C, which is too close to the bath temperature (approximately 1500 ◦ C). In this range of temperature, the SiC filter is easily damaged and its fragments are carried into the bath and, consequently, the final product contains higher carbon contents.

4. Conclusions The change from a high porosity to a low porosity graphite crucible resulted in a drastic reduction of carbon content due to reduction in real contact area between crucible surface and the liquid bath. Independent of graphite type, its inner surface degradation as a function of usage is very high requiring reworking the crucible. Although SEM micrographs have shown TiC particle adhesion onto the pores surface, the chemical analyses show that the use of ceramic filter of both types is not efficient to remove those particles. The resulting carbon content before and after filtering is the same. Since the TiC particle dimensions are some orders of magnitude smaller than the pore dimensions, the particle retention is related to the probability of contact between these particles and the pore surfaces being higher, the longer is the liquid path through the ceramic filter.

The superheat of 150–200 ◦ C, necessary to keep the liquid fluidity through the ceramic filter, keeps carbon in solution or may dissolve the TiC particles, generating a stationary condition of adhesion and re-dissolution. As can be seen from Fig. 11 and reference [8], it is possible to obtain very clean NiTi alloy by EBM, resulting probably in better corrosion resistance and important aspect as a biomaterial.

Acknowledgements To FAPESP, AEB, Villares Metals, UNICAMP, ITA/CTA, DEMA/UFSCar and IPT for supporting this project.

References [1] C.M. Jackson, H.J. Wagner, R.J. Wasilewski, NASA-SP 5110 (1972). [2] S. Koshimizu, S. Yamamoto; Proceedings of 10th CBECIMAT, 1992, pp. 449–451. [3] S. Koshimizu, C.S. Yamamoto, REM: Revista Escola de Minas, Ouro Preto 46 (1/3) (1993) 40–47. [4] M.S. Andrade, Final Project Report, Fundac¸a˜ o Centro Tecnol´ogico de Minas Gerais, B.H., 1985, p. 146. [5] M.S. Andrade, J.E. Silva, M.H.S. Lara, Anais do 8 CBECIMAT, UNICAMP, Campinas, S.P., 1988, pp. 256–259. [6] 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, 12–16 July 1998, Honolulu, Hawaii, USA, pp. 1063–1068, The Minerals, Metals and Materials Society, TMS. [7] J. Otubo, O.D. Rigo, C. Moura Neto, M.J. Kaufman, P.R. Mei, Scale up of NiTi shape memory alloy production by EBM, J. Physique, IV France 112 (2003) 873–876. [8] J. Otubo, O.D. Rigo, C. Moura Neto, M.J. Kaufman, P.R. Mei, Low carbon content shape memory alloy produced by electron beam melting, Mater. Res. 7 (2) (2004) 263–267. [9] W.H. Sutton, J.R. Morris, 31st Annual Meeting of the Investment Casting Industry, Dallas, TX, 3–5 October 1983, 9:01–9:21. [10] D. Apelian, W.H. Sutton, Fifth International Symposium on Superalloys, Seven Springs, PA, 7–11 October 1984, pp. 421–432. [11] S.C. Jones, Proceeding Conference Scaninject V Part 2, Lulea, Sweden, 6–8 June 1989, pp. 553–572.

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