Effects of Process Parameters on the morphology of TiAl 3 particle during the production of Al-Ti-B Master Alloy by Flux Reaction

Transcription

1 Effects of Process Parameters on the morphology of TiAl 3 particle during the production of Al-Ti-B Master Alloy by Flux Reaction Min Ryou*, Sang-Ho Choi** and Myung-Ho Kim *. * School of Materials Science and Engineering, Inha University #253 Yonghyeon-dong, Nam-gu, Incheon, , South Korea ** Dongyang Junior Technical College Abstract The effect of process parameters, such as reaction temperature and holding time, on the microstructure of Al-Ti-B master alloy had been studied by using SEM/EDS and EPMA. The flux reactions between K 2 TiF 6, KBF 4 and molten aluminum were carried out at various temperatures and holding times. The results indicated that the morphology of TiAl 3 particles in master alloy was strongly influenced by reaction temperature and holding time. The TiAl 3 particles with blocky type which was known as an effective refiner were observed at lower reaction temperature and in shorter holding time, whereas finer TiB 2 particles were found to be distributed uniformly independent of the reaction temperatures. The size of TiAl 3 particles in the master alloy was significantly increased with increasing of the reaction temperature, and the increased temperature resulted in the change of the morphology of TiAl 3 particles from blocky to plate-like type. The TiAl 3 particles with morphology of blocky type shows longer fading time compare to the plate-like type. Key words Grain refiner, Al-5Ti-1B alloy, TiAl 3 particle, Flux reaction 106/1

2 Introduction In aluminum industry, Al-Ti-B Master alloy has been widely used for grain refinement that is an essential process to decrease casting defects, to improve castability, workability, mechanical properties, and so on[1][2]. These effects of grain refinement have been known to be affected by the type and size of the existing phases in the master alloy, such as TiAl 3 and TiB 2. The type and size of these particles were mainly affected by the temperature of molten metal and holding time during the production of Al- Ti-B alloy by flux reaction. Studies on the grain refining ability of Al-5wt.%Ti-1wt.%B master alloy, and the effects of process variables during the flux reaction on the microstructure of the master alloy have been widely studied so far, and especially the effect of TiAl 3 phase on grain refinement behavior has been examined by many investigators[3-6]. However, as the reaction mechanism during the manufacture of Al-Ti-B grain refiner by flux reaction is so complex, significant variations in product performance are reported to be existed. And the influence of boron on the formation of TiAl 3 intermetallic phases during manufacture of the master alloy, and additional effects of the boride phase on the grain refinement behavior of the TiAl 3 intermetallic phases are not clearly understood[7].. As a first step to investigate the influence of duplex intermetallic phases, TiAl 3 intermetallic phases covered with TiB 2 boride, on the grain refinment of aluminum, the effect of process variables during the flux reaction, such as reaction temperature and holding time, on the microstructure of Al- 5wt.%Ti-1wt.%B master alloy, particularly for the morphological change of the TiAl 3 and TiB 2, and also the grain refining ability of the TiAl 3 with different morphology and its effect on the fading time were investigated by using SEM and image analysis. Experimental Procedure In this experiment, two kinds of flux, K 2 TiF 6 and KBF 4, were selected, and were responded to 700, 750, 800, 850, and 900 with reference to Fig.1. To produce Al-5wt.%Ti-1wt.%B master alloy, the amount of pure aluminum and added for each runs was K 2 TiF g, and KBF g, aluminum g, corresponding to the stoichiometric ratio. To observe the morphological change of the TiAl 3 and TiB 2 particles with holding times, the mixture were induction heated at each temperature and held for 10, 20, 30, 60, 120, 240, and 480 min. respectively. The master alloy samples were cast in the graphite mold with temperature of 200, and the microstructure was observed by using Scanning Electron Microscope (SEM), electron probe micro-analyzers (EPMA) after grinding up to #2000 and electropolishing by 5% Percloric Acid etchant. To investigate the effect of TiAl 3 particles with different morphology on the grain refinement of commercial purity aluminum, wt.% master alloy was added to the molten metal at 800, and then hold for 10, 20, 30, 60, 106/2

3 120, 240, and 480 min., and then cooled at room atomospher, respectively. Results Fig.2 shows the typical microstructure of the master alloy specimen, and Fig.3 shows the SEM/EDS analysis of the microstructure. According to these figures, the particle A is considered to be salt inclusions. In Al-5Ti- 1B master alloy, generally two kinds of intermetallics, TiAl 3 and TiB 2 particles, were reported to be observed[2,4], However, the size of TiB 2 particles is too small to analyze with SEM/EDS, these boride particles were observed with EPMA, as shown in Fig.4. From Fig.3 and Fig.4, it could be conformed that both the particle B and particles C are TiAl 3. Fig.5 shows the microstructural change of the master alloy specimen with reaction temperatures and holding times, in general. This figure indicates that the higher the reaction temperature, the bigger the size of TiAl 3 particles, and the amount of blocky type particles decreased but the amount of plate-like type particles increased. It is also indicated that increase in holding time resulted in increase in TiAl 3 particle size. However, the size and type of TiB 2 was not significantly influenced by the reaction temperature and holding time. According to Arnberg[6], the master alloy with a lots of regular size TiAl 3 and TiB 2 particles exhibit very effective grain refining ability. In this experiment, it was found that the blocky type TiAl 3 particles with regular size, which is known as a very effective grain refiner could be produced at the reaction temperature of 750, 800 and holding time of 10 min. 15 min., which was considered to be the optimum conditions for manufacturing of the master alloy. Fig.6(a) exhibits the variation of the TiAl 3 particle size with holding time and reaction temperature. According to this figure, the size of particles tends to be grown with the holding time and the growth rate became accelerated with increase of temperature. Fig.6(b) shows the effect of reaction temperature and holding time on the number of TiAl 3 particles. As you see, the number of particles tends to decrease with the holding time and reaction temperatures. These are considered to be due to the development of TiAl 3 cluster at higher temperature and longer holding time. Fig. 7 shows the changes in aspect ratio of TiAl 3 particles with holding times and reaction temperatures. At low reaction temperatures, the values of aspect ratio is about 1.5 to 2, and it s doesn t change significantly with holding times. This means that the TiAl 3 particles formed at low reaction temperature are blocky type, and the shape doesn t change with holding time. However, in case of the reaction temperature of 900, the aspect ratio of TiAl 3 particles became bigger, and this means the morphology of 106/3

4 TiAl 3 particles was converted from blocky type to plate-like type at the higher temperature. Fig. 8 shows the grain refining ability of the TiAl 3 with different morphology of the blocky and plate-like type and its effect on the fading time. From the figure, it could be understood that the TiAl 3 with morphology of blocky type exhibits better grain refining ability compare to the particles with plate-like type, as reported before[6]. In addition, interestingly, the TiAl 3 particles with morphology of blocky type shows longer fading time compare to the particles with plate-like type. It seems to be due to the change in morphology of TiAl 3 particles from blocky to plate-like type with increasing of holding time [7]. Conclusions From the above study, the following conclusions could be drawn: 1. By reaction of the fluxes K 2 TiF 6 and KBF 4 with aluminum Al-5%Ti- 1%B master alloy with TiAl 3 and TiB 2 particles of very effective grain refiner can be produced. 2. The size of TiAl 3 particles tends to decrease at low reaction temperature and shorter holding times, and increase in holding time resulted in increase in the TiAl 3 particle size. However the size and shape of TiB 2 was not significantly influenced by the reaction temperatures and the holding times. 3. The TiAl 3 particles formed at low reaction temperature are blocky type, and the type doesn t change with holding times. However, the morphology of TiAl 3 particles was converted from blocky type to plate-like type at higher reaction temperatures. 4. The TiAl 3 particles with morphology of blocky type exhibits better grain refining ability compare to the plate-like type, and the TiAl 3 particles with morphology of blocky type shows longer fading time compare to the particles with plate-like type. References 1. A. Cibula : J. Inst. Metals. 76 ( ) D.G. MCCARTNEY: Int. Mater. Rev. 34 (1989) M.M. Guzowaski, G.K. Sigworth. And D.A. Sentner, Metall. Trans. A.18A (1987) D.G. MCCARTNEY, Metall. Trans. A., 19A (1988) C.D. Mayes, D.G. McCartney, G.J. Tatlock, : Mater. Sci. Technol., 9 (1993) L. Arnberg, L. Backerud, and H. Klang, Metals. Techno. 9 (1982) John E. Gruzleski, Bernard M. Closset, The treatment of liquid Al-Si Alloys. (1990) 135 Figures 1. Phase diagram of the binary system K 2 TiF 6 and KBF /4

5 2. Typical microstructure of Al-5Ti-1B alloy by flux reaction at 850 for 15min.. 3. SEM/EDS results of A, B and C phases exhibited in Fig EPMA results of TiAl 3 and TiB 2 phases in the master alloy. 5. Microstructures of Al-5Ti-1B master alloy at the various reaction temperatures and holding times. 6. (a)size and (b)number of TiAl 3 particles as a function of holding time at the various reaction temperatures. 7. Aspect ratio of TiAl 3 particles in the master alloy as a function of holding time at the various reaction temperatures. 8. Grain refining ability of blocky type TiAl 3 and plate-like type TiAl 3 with time. Fig. 1 Phase diagram of the binary system K 2 TiF 6 and KBF /5

6 Fig. 2. Typical microstructure of Al-5Ti-1B alloy by flux reaction at 850 deg C for 15min.. Fig. 3. SEM/EDS results of A, B and C phases exhibited in Fig /6

7 Fig. 4. EPMA results of TiAl 3 and TiB 2 phases in the master alloy. 106/7

8 Fig. 5. Microstructures of Al-5Ti-1B master alloy at the various reaction temperatures and holding times. 106/8

9 Fig. 6. (a)size and (b)number of TiAl 3 particles as a function of holding time at the various reaction temperatures. 106/9

10 Fig. 7. Aspect ratio of TiAl 3 particles in the master alloy as a function of holding time at the various reaction temperatures. Fig. 8. Grain refining ability of blocky type TiAl 3 and plate-like type TiAl 3 with time. 106/10