Sintering of HDH Ti powder

Transcription

1 Sintering of HDH Ti powder Kováčik Jaroslav 1, Emmer Štefan Keywords: powder metallurgy, sintering, titanium, HDH powder Abstract Titanium powders prepared by hydro-dehydration process (HDH powder) were pressure less sintered in vacuum oven at different temperatures, time and green density. The sintering properties of powders of two particle sizes - 30 and 150 microns were investigated. The usual powder metallurgical (PM) results were observed, i.e., decreasing final porosity with increasing sintering temperature and time at constant heating rate. Higher green density leading to higher final density for both powder sizes was also observed. The obtained results will be used as comparative material for future sintering experiments of Ti based composites. 1 INTRODUCTION Titanium is widely used in different applications from airplanes, cars, chemical reactors or biomaterials. Ti has been for a long time rare and expensive material. The reason is that conventional metallurgical methods are ineffective in the case of Ti due to its tendency to react at elevated temperatures with broad range of elements. At present, Ti is produced by Kroll process [1] which consists of four basic operations: extraction of Ti from ore by chlorination (TiO 2, TiC, TiN), production of TiCl 4 in the oven at 800 C, production of Ti sponge by reducing of TiCl 4 with Mg or Na in protective atmosphere. The final step is re-melting of obtained Ti sponge. Ti and its alloys are usually prepared in vacuum furnaces by induction re-melting and as well as by vacuum casting. These methods of preparation are very expensive. Price of Ti limits significantly its applicability, e.g., in 2009 it was produced only 115 kt of Ti sponge [2]. Due to the problematic casting the final goods are made from titanium ingots by machining. Therefore there is a great loss of expensive material. Currently the aerospace industry buys about eight times more Ti as it finally uses in the aircrafts finished parts [3]. Machine ability of Ti is worse than other metals because its surface becomes brittle due to oxygen and nitrogen [4]. The low thermal conductivity also causes the sticking of Ti on the edge of cutting tool and decreases its sharpness. For these reasons there is growing interest in the world in Ti powder metallurgy (PM) as a cost-effective way of direct production of complex parts from Ti and its alloys [5]. PM approach brings additional benefits in significantly improved chemical and microstructural homogeneity of components. Ti parts may be produced by a wide range of PM techniques. These, depending on the complexity and size of the final components include standard 1 1 Institute of Materials and Machine Mechanics, Slovak Academy of Sciences, Račianska 75, Bratislava, Slovak Republic, ummsjk@savba.sk 2 Institute of Technologies and Materials, Faculty of Mechanical Engineering, IVMA STU, Slovak University of Technology, Pionierska 15, Bratislava, Slovak Republic, stefan.emmer@stuba.sk

2 compaction procedure followed by sintering (the press-and-sinter), sintering followed by compacting using hot plastic deformation, by direct rolling and extruding of loose powders, method of hot isostatic pressing (HIP) and MIM (metal injection moulding). Press-and-sinter approach is technically most simple and economically most attractive near-to-shape approach of parts production. Limiting fact is that the sintering ought to be done in vacuum that restricts reproducibility and residual porosity leads to applications under non fatigue conditions [6]. Starting material is Ti powder, which, depending on production method is of different size, morphology, purity and price [7]. Due to high ductility Ti sponge cannot be directly grind to powder. From an economic point of view it seems to be the most effective to use Ti powders prepared by hydro-dehydration process (HDH powder), which allows preparation of a wide range of particle sizes with relatively good purity. Hydrated Ti sponge is brittle TiH 2 that is easy to grind. Then grinded TiH 2 powder is dehydrated by annealing in a vacuum. Therefore the aim of this paper is to investigate the sintering properties of HDH Ti powder of two different particle sizes. 2 EXPERIMENTAL Two various HDH pure titanium powders of particle sizes of 30 and 150 microns (Kimet, China) were investigated. The powders are of typical fragmented shape due to the HDH preparation method (see Fig. 1). Figure 1: Characteristic fragmented shape of used HDH titanium powders (30 micron). The powder size distribution was determined using Fritch Analysette 22 laboratory equipment. For finer 30 micron powder d 50 = 25 m and d 90 = 46 m (see Table 1) were determined. On the contrary Table 2 indicates that for coarser 150 micron powder d 50 = 29 m and d 90 = 97 m. The results indicate that the producer makes finer powder from coarser one simply by sieving. The powders were compacted by uniaxial pressing at 278 MPa (maximal load of the equipment). For comparison of the densification methods the CIPing was also used for compaction

3 (maximal possible pressure of 200 MPa) and the samples were machined to the geometry of uniaxially pressed samples. The typical geometry of samples before sintering was as follows: diameter of 15 mm, height of 2.7 mm. Then the samples were weighted and the green density was calculated. The sintering experiments were made in vacuum furnace at constant heating rate of 50 K/minute for different temperatures, time and green densities. The final density was again determined from geometry and the weight of the samples after sintering. Table 1: Particle distribution of 30 micron HDH powder. There Q3(x) is cumulative percentile for 10, 50 and 90% of powder size; CV is standard deviation of mean value. Q3(x) [%] Mean [µm] CV [%] Table 2: Particle distribution of 150 micron HDH powder. There Q3(x) is cumulative percentile for 10, 50 and 90% of powder size; CV is standard deviation of mean value. Q3(x) [%] Mean [µm] CV [%] Electron microscope JEOL 7600F, equipped with a Schottky thermal-emission cathode (thermal FEG - W-coated ZrO2) as well as energy-and wavelength spectrometers from Oxford Instruments were used for microstructure characterisation. 3 RESULTS AND DISCUSSION The experiments confirmed that HDH titanium powder can be successfully pressure less sintered in vacuum. As indicates Fig. 2 even at such low temperature as 1100 C (0.66 T m ) after 30 minutes of sintering material with porosity less than 20% can be obtained when fine 30 micron HDH powder is used. The effect of compaction method for both powders is demonstrated on Fig. 3. Both powders compacted by uniaxial pressing at 278 MPa possess higher green density than after compaction via CIPing at 200 MPa. The higher green density leads also to higher final density after sintering. It is evident that fine 30 micron powder can achieve final density above 90% of theoretical density after sintering at 1300 C (0.78 T m ) for 30 minutes (see Fig. 4). At these conditions coarse 150 micron powder is achieving scarcely 85% of theoretical density. Fig. 4 also illustrates that with increasing sintering temperature at constant sintering time (30 minutes in this case) the final porosity in sintered specimen significantly decrease. This observed decrease is basically nonlinear at given heating rate of 50 K/minute. Moreover the

4 sintering behaviour is similar for fine and coarse powder in the investigated sintering temperature range. Figure 2: The microstructure of sintered Ti specimens: sintering temperature 1100 C, sintering time 30 minutes left 30 micron powder - final porosity 16.1%, right 150 micron powder - final porosity 26.3%. Figure 3: Sintered density of titanium specimens as function of its green density for different powder size and pressing conditions: lower green density values for each powder size are after CIPing (200 MPa), upper green density values after uniaxial pressing (278 MPa): sintering temperature 1300 C, sintering time 30 minutes.

5 Another possibility how to decrease final porosity is to increase sintering time (see Fig. 5). Only 1 sample prepared from coarse 150 micron HFH powder was sintered for longer time - 60 minutes at 1200 C. What is not good is the fact that 30 minutes more of sintering led to only 5% decrease of porosity. Figure 4: Final porosity of titanium specimens as function of its powder size and sintering temperature (sintering time 30 minutes). Figure 5: Final porosity of titanium specimens as function of its powder size and sintering time (sintering temperature 1200 C).

6 4 CONCLUSIONS The experiments confirmed that both HDH titanium powders (30 and 150 micron) can be successfully pressure less sintered in vacuum. It was further shown that final density depends on green density, sintering temperature and time. It was observed that the higher final densities were observed for fine 30 micron powder. The influence of preparation technology was studied also. It was observed that the consolidation by various methods (uniaxial pressing, CIPing) leads to different green density which is ruled by limited load of used equipment: 278 MPa for uniaxial pressing and 200 MPa for CIPing. Moreover, higher green density led to higher final density for both powders after sintering. It must be concluded that the usual PM results were observed: Decreasing final porosity with increasing sintering temperature or time at constant heating rate. The obtained results will be used as comparative material for future sintering experiments of Ti based composites. ACKNOWLEDGEMENT Authors thank for the financial support to APVV agency under the project VMSP II and VEGA agency under the projects 1/0189/12 and 2/0158/13. REFERENCES [1] W. Kroll, Z. Metallkunde 29 (1937) [2] M. Qian, Powder Metal. 46 (2010) [3] J.E. Barnes, W. Peter, C.A. Blue, Mater. Sci. Forum (2009) [4] Wikipedia.org [5] Powder processing, Consolidation and Metallurgy of Titanium Ti (PM Titanium 2011), Conerence Proceedings, Brisbane, December 2011 [6] P.J. Anderson, V.M. Svoyatkovsky, F.H. Froes, Y. Mahajan, D. Eylon, Modern Developments in Powder Metallurgy, Metal Powder Industries Federation, Princeton NJ (1981) [7] C.G. McCracken, Ch. Motchenbacher, D.P. Barbis, Powder Metal. 46 (2010)