Sintering of Titanium Hydride Powder Compaction - ScienceDirect

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ScienceDirect Procedia Manufacturing 2 (2015) 550 – 557

2nd International Materials, Industrial, and Manufacturing Engineering Conference, MIMEC2015, 4-6 February 2015, Bali Indonesia

Sintering of Titanium Hydride Powder Compaction

Dong-Won Lee1,a, Hak-Sung Lee1,b, Ji-Hwan Park2,c, Shun-Myung Shin3,d, and Jei-Pil Wang4*,e * 1T itanium G roup, K orea Institute of M aterials S cience (K IMS ), C hangwon 642-831, K orea 2R esearch C enter, MT IG L td., S eoul 135-833, K orea 3E xtractive M etallurgy G roup, K orea Institute of G eoscience and Mineral R esources, D aejeon 305-350, K orea 4D epartment of M etallurgical E ngineering, P ukyong National U niversity, B usan 608-739, K orea aldw1623@ kims.re.kr, byjy1706@ kims.re.kr, chslee@ kims.re.kr, dshin1016@ kigam.re.kr, ejpwang@ pknu.ac.kr *C orresponding author: jpwang@ pknu.ac.kr

Abstract Abstract. Sintering by the TiH2 and Ti powder compaction was performed at 1423~1623K for 2 hours at 1.33x10-3 Pa. The sinter-ability of TiH2 powder was higher than that of pure titanium powder, leading to near 99% of relative density, which was the competing level with that by HIP process. In the direct sintering of TiH2 powder compaction, the interstitial hydrogen atoms was released before sintering through the grain boundaries and meet the titanium oxide phase existing in the grain boundaries as the film forms. We found thermodynamically there that the hydrogen atoms could effectively reduce the titanium oxide existing on the powder surfaces or grain boundaries with a considerable driving force of about -250 kJ/mole. Such a self-reduction process by hydrogen atoms makes the grain boundaries very clean with oxide-free condition and helpful for sintering. Moreover, the de-hydrogenated titanium regions near grain boundaries could become unstable due to the formation of many vacancy defects by the release of hydrogen and also evoke the sintering ability. © 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license © 2015 The Authors. Published by Elsevier B.V. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and Peer-review Peer-reviewunder underresponsibility responsibility Scientific Committee of MIMEC2015. Selection and ofof thethe Scientific Committee of MIMEC2015 Keywords:Titanium; Hydride; Sintering; Self-reduction

* Corresponding author. Tel.: +0-000-000-0000 ; fax: +0-000-000-0000 . E-mail address:[email protected]

2351-9789 © 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and Peer-review under responsibility of the Scientific Committee of MIMEC2015 doi:10.1016/j.promfg.2015.07.095

Dong-Won Lee et al. / Procedia Manufacturing 2 (2015) 550 – 557

1. Introduction Many researches have recently concentrated on developing components by titanium powder metallurgy(P/M) technology for the medical components, leisure products, interior decorating materials, accessories, etc. [1,2]. Powder metallurgy is used to manufacture the metal products by compacting and sintering of powder. With respect to the advantages of power metallurgy, there is no need to increase the temperature to the melting point in the process of manufacture, and it is possible to manufacture the various complicated structures of parts In this study, titanium P/M process was performed with titanium and titanium hydride (TiH2) powders as raw materials. The main reason to employ TiH2 powder as raw materials is that manufacturing process of titanium powder also includes the dehydrogenation process of TiH2 powder in a vacuum condition. That is, the it can be accomplished the unified process during a vacuum sintering process instead of two steps, dehydrogenation and sintering and also gives a cost down by simple process. Even if some research work related with TiH2 compaction and sintering was found [3,4], the detail investigation to compare a sintering behavior in particular with thermodynamical study was not reported, yet. Therefore, we have tried to produce TiH2 powder by hydrogenation and milling with sponge titanium and compared the sintering behaviors of both compactions by Ti and TiH2 powders as well as microstructures, densities, hardness and purity of sintered parts. 2. Experimental procedure The sponge titanium with purity, 99.8% was hydrogenated at 923K for 5 hours to prepare TiH2 powders by ball milling process in cylindrical container using zirconia ball (ZrO2, dia:10 mm). The milled powder was sieved by 325 mesh screen to prepare under 45 μm in size. Dehydrogenated pure titanium powder was also produced in vacuum treatment of hydride powders at 1.33x10-3 Pa and 973K for 5 hours, and compaction and sintering were carried out with both powder. They were compacted to a cylindrical shape having 22 mm, diameter and 5~6 mm, height by the pressure of 7.8 MPa. The sintering temperature and time were 1423~1623 K for 2 hours, respectively and the vacuum level was maintained at less than 10-6 kPa during sintering. The microstructure, density, hardeness and chemical compositions were studied by scanning electron microscope (Zeiss SUPRA 55 VP-25-78), N/O determinator (ELTRA ON900) and etc. 3. Results and discussion The sintered microstructures by Ti and TiH2 compactions were examined on the effect of sintering temperature as shown in Fig. 1. Titanium was shown to be normal sintering behavior of densification by decrease of total porosity amount with increase sintering temperature. On the other hand, the structure by titanium hydride (TiH2) has shown that the amount of large pores is considerably small in even low-temperature showing denser than that by titanium powder. But it should be noticed that very small size of porosities located along the grain boundaries and it was imagined that such small pores were performed before starting sintering by phase transformation from TiH2 to Ti during dehydrogenation behavior, which normally occurred at below 823~923K [5]. In the sintering structure at higher 1523K of TiH2, the previously formed small pores were fully eliminated showing similar structure with that by Ti sample. We could suggest hence that TiH2 powder can give more effective sinter-ability particularly at low sintering temperature.

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a)

b)

c)

d)

100 μm Fig. 1. Microstructure after sintered at different temperatures. a) Ti, 1423K b) Ti, 1523K c) TiH2, 1423K d) TiH2, 1523K Fig. 2 shows sintered density (a) and hardness (b) measure in samples by of Ti and TiH2 powders. In particlar, it was showed that the sintered density of TiH2 reached to about 98% at 1527K. In case of pressure-less sintering, this level of sintered is considered to a very good property. Hardness also showed a behavior similar to the increase tendency of sintered density. The reason why the sintered density is high when TiH2 is used must be analyzed from the perspective of the material’s characteristics and it was found to be closely related to the dissociation behavior of TiH2. In general, the oxide layer on the surface of titanium powder is not reduced and eliminated during the sintering in even high-degree vacuum, which inhibits the sintering behavior. Thus, it can be imagined that better sinter-ability shown in TiH2 materials is obviously related with a modification of grain boundary mature in particular during dehydrogenation stage.


Fig. 2. Sintered density and hardness of Ti and TiH 2.

In Fig. 3, (b) showing the free energy change needed for hydrogen gas and the hydrogen atom to reduce the oxide layer, we should know here that the reaction with hydrogen atoms have a large free energy decrease of -200 kJ/mole. In other words, the hydrogen atoms formed may act as a catalyst that stimulates reduction effectively. Because the diffused and released hydrogen inside TiH2 material in vacuum condition before approaching sintering temperature exist in not molecular but atom state, it can reduce effectively the surface oxide leading to the formation of oxide-free boundaries, which can stimulate the initiation of sintering.

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Fig. 3. Standard free energy changes in the reaction for hydrogen reduction with different reductants. Fig. 4 describes the schematic behaviors of surface reduction by releasing hydrogen, which can evoke a sinter-ability by the effect of oxide-free clean surface. It was considered here another effect on sinter-ability in TiH2 material. It means that the peripheral region of the crystal grains of TiH2 become an unstable Ti-phase by phase transformation formed at relatively low temperature and this un-stability also can resulted in high sinter-ability when approached to sinterting temperature. It was considered that such un-stability can be explained with a large amount of vacancy effect formed in transformation and oxide reduction by released hydrogen atoms. And the micro- or nano- pores formed at dehydrogenation stage can be coarsened when the temperature is more increased. Fig. 5 shows the microstructure sintered at 1423K with the relatively low temperature for TiH2 powder, and the fine pores-crowded areas distributed along the grain boundaries. It is likely that these fine cores were formed by above mentioned effects.


Fig. 4. Comparison of sintering behaviors in use of TiH2 and Ti powder compacts.

Fig. 5. The microstructure of TiH2 sintered at 1150°C. Table 1 shows oxygen and hydrogen contents analyzed in initial Ti and TiH2 powders and samples sintered at 1,423K and 1,523K, respectively. In case of Ti powder, hydrogen was

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reduced to 0.07 wt.% by dehydrogenation. And the oxygen content, 0.35wt.% of the powder increased to 0.47wt.% after sintering. The increase of oxygen content can be explained by the existence possibility of oxygen partial pressure in spite of high vacuum condition. TiH2 case shows that hydrogen is almost 4 wt.% and oxygen is as low as 0.19wt.%, because oxygen is replaced by hydrogen. After sintering, the hydrogen is removed completely and oxygen concentration is kept as low as at a level of 0.22~0.23wt.%, which is lower levels compared with those by pure titanium and it might be also resulted from reduction effect. The limit of the elongation property restricts the application of sintered components using titanium powder to the industries. In general, 10% elongation needs to be ensured, but the results of many studies show that it is difficult to secure the elongation above 10%. There are many reasons for the decrease of elongation of titanium sintered components. The most critical reason is oxygen concentration[6]. It is known that to ensure excellent elongation, the oxygen concentration should be maintained at below 0.3wt.%. Thus the oxygen concentration of 0.23% in the sintered compounds obtained in this study using TiH2 probably can be satisfied for industrial application and such studies for mechanical properties will be continued. Table 1. Oxygen and hydrogen contents analyzed in produced titanium and titanium hydride powders and samples sintered at 1,423 K and 1,523 K, respectively. Materials Ti TiH2

wt.%

powder

Oxygen Hydrogen Oxygen Hydrogen

0.353 0.071 0.190 3.980

As sintered 1,423 K 0.486 < 0.001 0.219 < 0.001

1,523 K 0.491 < 0.001 0.231 < 0.001

4. Conclusion Sinter-ability was more improved when the sintering was conducted using TiH2 powder than using pure Ti power. The sintered density up to 98% was achieved in employing TiH2 powders. The reason for sinter-ability improvement by TiH2 powder was studied in view of the effect of ‘voluntary reduction’ behavior inducing reduction effectively by reacting with TiO2 existing along grain boundary and the hydrogen atoms released in dehydrogenation stage. As a result, the grain boundary is made of clean titanium metal and thus is accompanied by grain growth, which in turn increases the sintered density. Acknowledgements This study was co-supported by the NST (National Research Council of Science and Technology), R&D Center for Valuable Recycling (Global-Top Environmental Technology Development Program) funded by the Ministry of Environment (Project No.:GT-11-C-01-060-0) and by a grant from the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Trade, Industry & Energy, Republic of Korea.


References [1] F. H. Froes Sam, 8-Powder metallurgy of titanium alloy, Adv. Powd. Metall. (2013) 202-240. [2] R. R. Boyer, An overview on the use of titanium in the aerospace industry, Mater. Sci. Eng. A A213 (1996) 103-114. [3] I. M. Robertson, G. B. Schaffer, Comparison of sintering of titanium and titanium hidried powder, Powd. Metall. 53(1) (2010) 12-19. [4] Hongtao Wang, Michael Lefler, Z. Zak Fang, Ting Lei, Shuming Fang, Jiamin Zhang, Qun Zhao, Titaium and titanium alloy via sintering of TiH2, Key Eng. Mater. 436 (2010) 157-163. [5] V. Bhosle, E. G. Baburaj, M. Miranova, K. Salama, Dehydrogenation of TiH2, Mater. and Eng. A356 (2003) 190-199. [6] E. Baril, L. P. Lefebvre,Y. Thomas, Interstitial elements in titanium powder metallurgy: sources and control, Powd. Metall. 54(3) (2011) 183-187.

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