Deoxidation of Titanium alloy using hydrogen

international journal of hydrogen energy 34 (2009) 8958–8963

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Deoxidation of Titanium alloy using hydrogen Yanqing Su*, Liang Wang, Liangshun Luo, Xiaohong Jiang, Jingjie Guo, Hengzhi Fu School of Materials Science and Engineering, Harbin Institute of Technology, 92 West Dazhi Street, Harbin, Hei LongJiang 150001, PR China

article info

abstract

Article history:

In this paper we present a simple and effective method to reduce the oxygen content of

Received 7 July 2009

titanium alloys by using the mixture of hydrogen (H2)/Ar gases as the reactive atmosphere

Received in revised form

during the remelting process of titanium alloys. The experimental results show that the

12 August 2009

decrease of oxygen content of Ti64 alloy is related to the hydrogen fraction of the mixture

Accepted 14 August 2009

gas and the melting time. When the hydrogen fraction is 10%, the best deoxidation takes

Available online 24 September 2009

place. The oxygen contents, in the titanium alloy, can be effectively reduced, leading to the microstructure of titanium alloy and the micro-hardness can be refined and decreased,

Keywords:

respectively. Additionally, hydrogen absorbed in our process can be easily removed by

Hydrogen

vacuum heat treatment.

Titanium alloys

ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.

Oxygen Microstructures Micro-hardness

1.

Introduction

Titanium and its alloys have been widely used in modern industry for their attractive properties such as high strengthto-weight ratio, good corrosion resistance, and good biocompatibility, etc [1,2]. However, the high affinity for oxygen, based on titanium alloys, limits their practical uses at elevated temperatures [2]. When exposing to oxidizing environment at relatively high temperatures [3–5], titanium alloys will suffer from the extensive scaling. As the titanium and its alloys have been exposed to any oxygen containing atmosphere at high temperatures, an oxide layer will form on the surface and an oxygen diffusion zone beneath it [2] owing to the TiO2 layer cannot hinder the inter-diffusion of oxygen into the matrix and form the solid solution [6] which can reduce the tensile ductility and fatigue resistance [7–10]. On the other hand, the lower materials usage ratio leads to a lot of titanium

wastes generate because of poor forming ability. Unfortunately, the global production volume of titanium is substantially low [11] compared to Fe and Al metal, so the recycling of titanium wastes is a very significant work. Usually, the oxygen content, which cannot be easily reduced, of the titanium wastes is relatively high because of hot processing and cutting. Up to now, little effective methods on removing the oxygen in titanium alloy have been reported. So it’s foreseen that finding a simple and effective method to remove the oxygen from the titanium alloys is very useful for the future technology. It’s well known that titanium and conventional titanium alloys also have high affinities for hydrogen and hydrogen can improve the hot workability, machinability and refine the casting microstructure of titanium alloys [12–16]. Additionally, it has been proven that hydrogen can reduce oxygen contents in steel chromium [17].

* Corresponding author. Tel.: þ86 045186417395. E-mail addresses: [email protected] (YQ. Su), [email protected] (L. Wang), [email protected] (LS. Luo), [email protected] (XH. Jiang), [email protected] (JJ. Guo), [email protected] (HZ. Fu). 0360-3199/$ – see front matter ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2009.08.053

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international journal of hydrogen energy 34 (2009) 8958–8963

Based on the above background, the aim of the present paper is to study the effect of hydrogen on the oxygen content of Ti–6Al–4V (Ti64) alloy through the remelting route in electric arc furnace.

Table 1 – The variation of oxygen content in sponge titanium alloy. Sponge titanium As received alloy Remelted in H2(10%)/Ar gaseous mixture

2.

Oxygen fraction (wt.%) 0.04 0.016

Experimental

Ti64 alloy has been chosen for this study, which covers about 50% of total titanium production [2,18,19]. The composition of the as received Ti64 bar in weight percentage is 6.27%Al, 4.26%V, 0.12%O, 0.098%Fe, 0.04%Si, 0.02%N, 0.01%C, 0.001%H and the balance is Ti. Sponge titanium is also used in this experiment and chemical composition in weight percentage is 0.06%Fe, 0.02%Si, 0.06%Cl, 0.02%C, 0.02%N, 0.04%O and the balance is Ti. In the deoxidation process, Ti64 alloy were melted in H2(10%)/Ar gaseous mixture in the electric arc furnace. The hydrogen percentage in the gaseous mixture can be measured by JF-2000 analysis system. The schematic diagram of the experimental equipments is shown in Fig. 1. This process includes 4 steps: (1) the specimen preparation and evacuation of the melting chamber; (2) filling the melting chamber with argon and hydrogen gas; (3) melting the specimen; (4) solidification in the water cooling copper crucible. The oxygen content in as received alloy is relatively low. For further study on the deoxidation capability of hydrogen, TiO2 powder was added into titanium alloy to replace the waste titanium. In a typical oxidation process, Ti64 alloy and TiO2 powder were melted in argon gas in the electric arc furnace. The Sartorius electronic analytic balance was used to measure the mass of the specimens and TiO2 power. Microstructures of the Ti64 alloy were observed by optical microscopy, scanning electron microscope (SEM). Phase analysis was performed using X-ray diffraction (XRD). Moreover, hardness measurements

were carried out on the cross-sections of the polished samples with a HuaYin HVS-50 Vickers micro-hardness tester. When the oxygen content is relatively low (less than 0.2 wt.%), it was measured by chemical analysis in Harbin welding Institute. The oxygen content higher than 0.2 wt.% in Ti64 alloy was determined by the Inductively Coupled Plasma (ICP) spectrometer in Tsinghua University.

3.

Results and discussion

3.1. Affections of the H2 contents on the decrease of the oxygen contents within Ti64 alloy Most of titanium alloys are prepared from sponge titanium. During the remelting process to prepare titanium ingot, it was found the oxygen content in sponge titanium decreased about 60% as shown in Table 1 when 10% H2, added into the gaseous mixture. To clearly see the decreases of the oxygen contents by varying the H2 contents, the affections of the H2 contents on the decrease of the oxygen contents within Ti64 alloy have been illustrated in Table 2. The oxygen content in the as received Ti64 alloy is 0.12 wt.%. When the H2 content is 1% in the gaseous mixture, the oxygen content has been decreased to 0.099 wt.% and the decrease of oxygen content is 0.021 wt.%. When the H2 content is 10%, the biggest decrease among the specimens melted can be obtained, the oxygen

Fig. 1 – The schematic diagram of the experimental equipments.

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Table 2 – The variation of oxygen content in the as received Ti64 alloy. Ti–6Al–4V alloy As received alloy Remelted in H2(1%)/Ar gaseous mixture Remelted in H2(10%)/Ar gaseous mixture Remelted in H2(20%)/Ar gaseous mixture Remelted in H2(30%)/Ar gaseous mixture

Oxygen fraction (wt.%) 0.12 0.099 0.028 0.045 0.043

content has been decreased to 0.028 wt.% and the decrease of oxygen content is 0.092 wt.%. Increasing the H2 contents to 20% and 30% H2, the decreases of oxygen contents are 0.075 wt.% and 0.077 wt.%, respectively. From those results mentioned above, it can be found that the gaseous mixture with 10% H2 has the best deoxidation effect on titanium alloys, which will be further studied in the paper. As mention above, hydrogen can reduce the oxygen content within the as received Ti64 alloy (0.12 wt.%). The deoxidation of titanium wastes with a high oxygen content is also need to study. According to actual condition, the waste titanium was replaced by the titanium alloy, in which TiO2 was added to. With this method, Ti64 alloy with different oxygen content was prepared. As shown in Fig. 2, TiO2 phase was identified when the oxygen content is relatively high. The oxygen content of this kind alloy variations after deoxidation by hydrogen were studied. Ti64 alloy containing different oxygen contents were remelted in the gaseous mixture with 10% H2 for about 3 min. As shown in Fig. 3, the oxygen contents decreased remarkably. It can be seen that the specimen with a higher oxygen content has a bigger oxygen content decrease. The remelting time also has an affect on the deoxidation of Ti64 alloy. With the remelting time increasing, the oxygen content will further decrease. When the remelting time increase to 20 min, the oxygen content even can decrease about 84% as shown in Fig. 4. Based on the experiment and the previous studies [17], it is found that the deoxidation mechanism is: H2 þ O ¼ H2O. Fig. 5 presents the schematic diagram of the deoxidation mechanism by H2. There are 3 steps during the remelting process: firstly,

Fig. 2 – XRD results of Ti64 alloy containing with 1.62 wt. % oxygen.

Fig. 3 – Decreasing of the oxygen contents after melted in gaseous mixture with 10% H2.

oxygen in titanium alloy moves to the gas–liquid boundary; then water molecules generates because of reaction of oxygen in the gas–liquid boundary with H2; finally, water in the form of water molecules flows into the gaseous mixture from the reaction surface. It is found that there are two advantages deoxidation with H2: one is that the deoxidation product is gas; another one is that hydrogen can be removed easily by vacuum heat treatment, so the hydrogen existence in titanium alloy is not discussed.

3.2.

Microstructural examinations

Fig. 6 presents the affection of oxygen on SEM microstructure of Ti64 alloy. It is found that oxygen coarsens the microstructure of Ti64 alloy. The Widmansta¨tten-like microstructure is a typical microstructure of as-cast Ti64 alloy [20] as shown in Fig. 6(a). When the oxygen content is 0.6 wt.%, the martensite lamellar with average thickness 2 mm is obtained. With the oxygen content further increase to 0.9 wt.%, the martensite lamellar thickness increased to

Fig. 4 – The variation of oxygen content with melting time.

international journal of hydrogen energy 34 (2009) 8958–8963

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Fig. 5 – The schematic diagram of the deoxidation mechanism by H2. about 6mm(Fig. 6(c)). Some blocks are found in Ti64 alloy with 0.9 wt.% oxygen, especially as shown in Fig. 6(d), because the oxygen content is higher in this area. But this kind blocks are not found in Ti64 alloy with 0.6 wt.% oxygen. To clearly see the affection of deoxidation on the microstructure of Ti64 alloy, the alloy with 0.9 wt.% was remelted in

gaseous mixture with 10% H2 for about 15 min. Fig. 7(b) presents the microstructure of Ti64 alloy after deoxidation. Compared with Fig. 6(c) and (d), it can be found that most of the thick martensite lamellar and blocks disappeared. It means that the deoxidation by hydrogen has a noticeable effect on the microstructure of Ti64 alloy. Fig. 7(a) is the

Fig. 6 – SEM micrographs of Ti64 alloy (a) as received alloy; (b) 0.6 wt.% oxygen; (c) and (d) 0.9 wt.% oxygen.

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Fig. 7 – SEM micrographs of Ti64 alloy melted in gaseous mixture with 10% H2. (a) as received alloy; (b) 0.9 wt.% oxygen.

micrograph of Ti64 alloy (as received alloy), which is also melted in gaseous mixture with 10% H2. The basketweave-like microstructure, which is very similar to Fig. 7(b), is obtained. It indicates that hydrogen can not only reduce oxygen in Ti64 alloy, but also refine the microstructure. Fig. 8 presents the affection of melting time on optical microstructure of Ti64 alloy with 1.62 wt.% oxygen. Fig. 8(b) is the micrograph of Ti64 alloy with 1.62 wt.% oxygen, which was melted in gaseous mixture with 10% H2 for about 4 min. Compared with Fig. 8(a), it is found that this microstructure is refined a little, because of the refinement of the a-plate and the gain boundary. The variation of the oxygen content in Fig. 4 can explain the reason of refinement phenomenon. When Ti64 alloy (1.62 wt.% O) is deoxidized for about 16 min in the same condition, it is found that the microstructure is further refined. Nearly 50% oxygen in Ti64 alloy is removed for the first 4 min remelting, but the alloy still has a high oxygen content about 0.8 wt.%. The oxygen content in the alloy decreases to about

0.31 wt.% after remelted for about 16 min. As the remelting time increase to about 20 min, the microstructure as shown in Fig. 8(d) is obtained. It indicates that the microstructure is not further refined compared with Fig. 8(c). This microstructure is very similar to the fine lamellar structure obtained by THP [13], which means that hydrogen has a bigger affection than oxygen at this time.

3.3.

Hardness variation after deoxidation

The micro-hardness variation of Ti64 alloy with oxygen is shown in Fig. 9. It indicates that the micro-hardness of the alloy increases with oxygen content resulted from the phase transformation and the coarse microstructure. The increase extent is about 75%(225HV) when 1.5 wt.% oxygen adding into as received alloy. It is found that the micro-hardness of Ti64 alloy decreases about 10% after deoxidation.

Fig. 8 – Microstructures of Ti64 alloy (1.62 wt.% oxygen) melted in gaseous mixture with 10% H2 (a) no melted; (b) melted for 4 min; (c) melted for 16 min; (d) melted for 20 min.

international journal of hydrogen energy 34 (2009) 8958–8963

Fig. 9 – The hardness variation of Ti64 alloy with different oxygen content.

4.

Conclusion

(1) A deoxidation method for titanium alloys using hydrogen as the reactive component in the atmosphere is proved. As a positive factor, hydrogen, which can be removed easily by vacuum heat treatment, can reduce most of oxygen in titanium alloys. (2) After deoxidation by hydrogen, the microstructure is refined remarkably when the oxygen content of titanium alloy is relatively high. (3) The micro-hardness of Ti64 alloy decreased about 10% after deoxidation.

Acknowledgements The authors would like to acknowledge the financial supported by NSFC(50975060), and the foundation of National Key Laboratory of Advanced Welding Production Technology. The services of the Materials Analysis Department of Harbin Institute of Technology are greatly appreciated.

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