- Email: [email protected]
A novel Al–Ti–B alloy for grain refining Al–Si foundry alloys
Journal of Alloys and Compounds 486 (2009) 219–222
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
A novel Al–Ti–B alloy for grain refining Al–Si foundry alloys Yucel Birol ∗ Materials Institute, Marmara Research Center, Gebze, 41470 Kocaeli, Turkey
a r t i c l e
i n f o
Article history: Received 23 January 2009 Received in revised form 2 July 2009 Accepted 2 July 2009 Available online 18 July 2009 Keywords: Metals Casting Al–Ti–B master alloy Powder metallurgy
a b s t r a c t Al–Ti–B refiners with excess-Ti (Ti:B > 2.2) perform adequately for wrought aluminium alloys but they are not as efficient in the case of foundry alloys. Silicon, which is abundant in the latter, forms silicides with Ti and severely impairs the potency of TiB2 and Al3 Ti particles. Hence, Al–Ti–B alloys with excess-B (Ti:B < 2.2) and binary Al–B alloys are favored to grain refine hypoeutectic Al–Si alloys. These grain refiners rely on the insoluble (Al,Ti)B2 or AlB2 particles for grain refinement, and thus do not enjoy the growth restriction provided by solute Ti. It would be very attractive to produce excess-B Al–Ti–B alloys which additionally contain Al3 Ti particles to maximize their grain refining efficiency for aluminium foundry alloys. A powder metallurgy process was employed to produce an experimental Al–3Ti–3B grain refiner which contains both the insoluble AlB2 and the soluble Al3 Ti particles. Inoculation of a hypoeutectic Al–Si foundry alloy with this grain refiner has produced a fine equiaxed grain structure across the entire section of the test sample which was more or less retained for holding times up to 15 min. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Grain refinement has become a standard melt treatment practice in aluminium foundries world-wide with well documented technical and economical advantages [1,2]. Commercial refiners are manufactured from the Al–Ti–B ternary system, often with more Ti than that required to form TiB2 [2–16]. Hence, the microstructure of these alloys typically comprises, in addition to the insoluble TiB2 , the soluble Al3 Ti particles dispersed in an aluminium matrix. The former act as heterogeneous nucleation sites while Al3 Ti particles readily dissolve in the melt and provide solute Ti, the partioning of which between the solid and liquid phases during solidification, slows down the growth process [17,18]. The very popular excess-Ti Al–Ti–B refiners are known to perform adequately for wrought aluminium alloys except when the alloy to be inoculated contains one or more of the elements whose borides are more stable than TiB2 [19,20]. However, they fail to perform as efficiently in the case of foundry alloys with adverse effects on the as-cast structure and inferior properties in cast parts [21–24]. The high content of Si, which forms silicides with Ti and thus severely impairs the potency of TiB2 and Al3 Ti particles, is claimed to be responsible for the poor response of foundry alloys to grain refinement with Al–Ti–B master alloys [25–29]. AlB2 particles, on the other hand, take advantage of high Si levels which enhance their nucleation potential. The superior performance of Al-borides,
∗ Tel.: +90 262 6412300; fax: +90 262 6412309. E-mail address: [email protected]. 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.07.064
which are not as efficient in the absence of Si, may be attributed to the dissolved Si in the foundry alloys [25]. While there are a number of Al–Ti–B alloys with excess-B compositions and binary Al–B alloys in the market for foundry alloys, these alloys rely on the insoluble (Al,Ti)B2 or AlB2 particles for grain refinement, and thus do not enjoy the growth restriction provided by solute Ti. The mutual presence of Al3 Ti and AlB2 particles in Al–Ti–B alloys, on the other hand, could offer to maximize the grain refining efficiency for aluminium foundry alloys. However, the manufacture of such grain refiners is not straightforward as the conventional practices provide Al–Ti–B alloys with either Al3 Ti and TiB2 particles in excess-Ti composition range or merely (Al,Ti)B2 particles in the case of excess-B alloys. The present work was undertaken to synthesize an Al–Ti–B alloy which contains both the insoluble AlB2 and the soluble Al3 Ti particles and to explore its potential as a grain refiner for hypoeutectic Al–Si foundry alloys. 2. Experimental Al–3B alloy powder and K2 TiF6 salt were thoroughly mixed to obtain a blended mixture. Al–3B master alloy was produced in the laboratory, with the conventional halide salt practice, i.e. by reacting KBF4 salt with molten aluminium at 800 ◦ C. The ratio of individual components in the mixture were adjusted so as to obtain 3 wt%Ti and 3 wt%B in the final alloy. Sample taken from the powder mixture was heated in the differential scanning calorimetry (DSC) cell until 750 ◦ C to identify the reaction sequence during thermal exposure. The DSC experiment was performed under flowing argon at a heating rate of 10 ◦ C/min. Larger volume of Al–3B/K2 TiF6 mixture was heated in a tube furnace in the same manner as that employed in the DSC experiment, to various temperatures where it was held for ½ h and then quenched to room temperature to retain the microstructural features thus introduced. The heat treated samples were subsequently analyzed with X-ray diffraction (XRD) and metallographic techniques, in order to identify the reactions responsible for the DSC
220
Y. Birol / Journal of Alloys and Compounds 486 (2009) 219–222
signals. XRD was conducted with CuK␣ radiation at a scan rate of 0.5◦ /min in order to improve the counting frequency. Samples for metallographic analysis were prepared using conventional practices and were examined with an optical microscope. Scanning electron microscopy (SEM) with energy dispersive X-ray analysis (EDS) was also used to identify the particles in powder grains. The experimental grain refiner was assessed for its grain refining performance. Al wt%7Si casting alloy was melted in a medium frequency induction furnace and the temperature of the melt was fixed at 700 ◦ C. A reference sample was taken from the melt in order to identify the grain size before inoculation. The heat treated Al–3B/K2 TiF6 mixture was dosed into 1.65 g samples which were subsequently compacted into pellets with a hydraulic press. This is the exact amount of an Al–3Ti–3B master alloy needed to bring the B concentration of 1000 g aluminium melt to 0.005 wt%. The melt was stirred with a graphite rod for 30 s right after inoculation. Samples were taken from the melt starting 2 min after the addition and were solidified in small copper molds. As the inoculated grain sizes obtained with the novel Al–3Ti–3B and Al–3B master alloys were nearly comparable after 30 min of holding, the grain refining performance tests were terminated after 30 min of holding time. Standard metallographic procedures were employed to prepare these sections for grain size measurements. They were etched with Poulton’s reagent and were then examined under a light microscope.
3. Results and discussion The Al–3B/K2 TiF6 powder blend is a homogeneous mixture of ␣-Al grains with a dispersion of AlB2 particles (Fig. 1) and halide salt grains. Unlike those in the commercially available Al–B master alloys, AlB2 particles in the laboratory processed Al–3B alloy are not clustered and are relatively smaller. The XRD spectrum of this mixture shows reflections of the AlB2 phase and K2 TiF6 salt, in addition to those of ␣-Al, as expected (Fig. 2a). Several unindexed reflections, too weak to be identified, are believed to belong to the impurities in the halide salt. The response of the Al–3B/K2 TiF6 mixture to thermal exposure was investigated in detail in order to identify the heat treatment conditions that would facilitate the formation of Al3 Ti phase without risking the AlB2 particles. The DSC scan of the Al–3B/K2 TiF6 mixture reveals a small endothermic signal at approximately 370 ◦ C (signal 1) and two much larger neighbouring signals between 613 ◦ C and 642 ◦ C and 653 ◦ C and 688 ◦ C, respectively (signals 2 and 3 in Fig. 3). The former is inherited from the DSC spectrum of K2 TiF6 [30]. The first of the latter two signals is exothermic while the second one is an endothermic doublet. Hence, melting of a species other than aluminium must be taking place between 653 ◦ C and 688 ◦ C. With these features the DSC scan of the Al–3B/K2 TiF6 mixture is almost identical to that of the Al/K2 TiF6 mixture [30]. AlB2 particles in the former have apparently no impact on the response to thermal exposure of the Al/K2 TiF6 mixture. One can see from the XRD spectrum of the Al–3B/K2 TiF6 mixture heat treated at 500 ◦ C, before signal 2 (Fig. 2b), that both AlB2 and
Fig. 1. Microstructure of the Al–3B alloy used to prepare Al–3B/K2 TiF6 powder blend.
Fig. 2. XRD patterns of Al–3B/K2 TiF6 powder blends (a) in the as-mixed state, after heating to (b) 500 ◦ C (below signal 2) and (c) 650 ◦ C (above signal 2) and held at these temperatures for 1/2 h (: ␣-Al, : AlB2 , ♦: K2 TiF6 , 䊉: Al3 Ti, : KAlF4 , : K3 AlF6 ).
K2 TiF6 compounds have prevailed in this temperature range with no evidence of a measurable reaction with each other nor with the aluminium matrix. The XRD spectrum of the Al–3B/K2 TiF6 mixture heated at 650 ◦ C, right after signal 2, is markedly different. Al3 Ti, KAlF4 and K3 AlF6 reflections have fully replaced K2 TiF6 lines in this sample (Fig. 2c). K2 TiF6 salt was apparently reduced by aluminium shortly before the melting of aluminium producing Al3 Ti and the inorganic salts, KAlF4 and K3 AlF6 . Signal 2 is then perfectly accounted for by the following reaction: 3K2 TiF6 + 13Al → 3Al3 Ti + 3KAlF4 + K3 AlF6
(1)
The formation of Al3 Ti is evidenced also by the metallographic analysis of samples heat treated in this temperature range. One can see many small blocky Al3 Ti particles dispersed inside aluminium
Fig. 3. DSC spectrum of the Al–3B/K2 TiF6 mixture.
Y. Birol / Journal of Alloys and Compounds 486 (2009) 219–222
221
Fig. 5. Change in free energy of formation with temperature for Al3 Ti, AlB2 and TiB2 . Fig. 4. Optical micrographs of the Al–3B/K2 TiF6 mixture heat treated at 650 ◦ C (above signal 2).
grains (Fig. 4). K2 TiF6 and Al can apparently react in the solid state contrasting the results of Prasad et al. [31] who claim that K2 TiF6 reacts only with molten aluminium. It is plausible to associate the initial trough of the endothermic signal 3 with the melting of the inorganic salts, KAlF4 and K3 AlF6 , produced by reaction (1). It is clear from the foregoing that a process which relies on a solid-state reaction between aluminium and K2 TiF6 can be used to generate Al3 Ti particles in a mixture which already has preformed AlB2 particles. The conventional route, i.e. adding to molten aluminium KBF4 and K2 TiF6 salts, inevitably favors the more stable of the two potential borides, TiB2 . Even when the halide salts are added sequentially so as to form AlB2 first, one would expect AlB2 to transform to TiB2 as soon as K2 TiF6 is added in the melt, according to, 3K2 TiF6 + 3AlB2 + Al → 3TiB2 + 3KAlF4 + K3 AlF6
(2)
since TiB2 is more stable than AlB2 (Fig. 5). The solid-state process not only avoids the AlB2 → TiB2 transformation, but also offers exceptional microstructural features. Al3 Ti particles generated by a reaction between K2 TiF6 and aluminium in the solid state were shown to be much smaller than those available in commercial Al–Ti master alloys yielding a superior grain refining performance [30]. Simply mixing powders of Al–B and Al–Ti master alloys already
available in the market to provide AlB2 and Al3 Ti particles together would not be as efficient. The Al–3Ti–3B pellet produced so as to contain both Al3 Ti and AlB2 particles was found to be a fast acting effective grain refiner for the Al–7 wt%Si alloy (Fig. 6). Inoculation with the present alloy has produced a fine equiaxed grain structure across the entire section of the test sample which was more or less retained for 15 min after inoculation. The reduction in grain size is remarkable in spite of the fact that the addition level was a modest 0.005 wt%B. Besides, the dendritic as-cast structure was improved into a more homogeneous one, dominated by equiaxed ␣-Al rosettes (Fig. 7). The performance of the present grain refiner is clearly superior than that of the binary Al–3B alloy confirming the favorable contribution of Al3 Ti on grain refinement of hypoeutectic Al–Si foundry alloys (Fig. 6). Fading of the grain refinement effect was noted, however, at holding times longer than 15 min. The present grain refiner is apparently not as resistant to fade as the popular Al–5Ti–1B alloy. Similar observations were made by other investigators for B-excess grain refiner alloys [32]. The settlement of the AlB2 particles in the melt may be partially responsible for fading. Loss of the grain refinement effect also raises questions regarding the stability of AlB2 particles in aluminium melts at temperatures in the neighbourhood of 700 ◦ C. Given enough time, transformation of the AlB2 particles to TiB2 variety at these temperatures is not at all unlikely. The latter is known to be a relatively less efficient nucleating agent in hypoeutectic Al–Si alloys and could be responsible for the reduced
Fig. 6. Performance tests of the Al–3Ti–3B and Al–3B grain refiners used with the Al–7Si alloy at an addition level of 0.005 wt%B.
222
Y. Birol / Journal of Alloys and Compounds 486 (2009) 219–222
a fine equiaxed grain structure across the entire section of the grain refining test sample which was more or less retained for holding times up to 15 min. Acknowledgements It is a pleasure to thank Osman C¸akır and Fahri Alageyik for their help in the experimental part of the work. References
Fig. 7. Microstructure of the Al–7Si alloy inoculated with the experimental Al–3Ti–3B grain refiner at a B addition level of 0.005 wt.
efficiency. The present alloy can nevertheless be used effectively when and where the grain refiner additions are made shortly before casting. 4. Conclusions The process employed in the present work relies on a solid-state reaction between aluminium and K2 TiF6 to generate Al3 Ti particles in a mixture which already has preformed AlB2 particles. This process not only avoids the AlB2 → TiB2 transformation, but also offers Al3 Ti particles much smaller than those available in commercial grain refiners yielding a superior grain refining performance. The experimental Al–3Ti–3B pellet thus produced contained both the insoluble AlB2 and the soluble Al3 Ti particles and was found to effectively grain refine a hypoeutectic Al–Si foundry alloy. Inoculating an Al–7 wt%Si alloy with the present grain refiner has produced
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32]
D.G. McCartney, Int. Mater. Rev. 34 (1989) 247. B.S. Murty, S.A. Kori, M. Chakraborty, Int. Mater. Rev. 47 (2002) 3. L. Arnberg, L. Backerud, H. Klang, Met. Technol. 9 (1982) 14. M.S. Lee, B.S. Terry, P. Grieveson, Metall. Trans. 24B (1993) 955. D. Apelian, G.K. Sigworth, K.R. Whaler, AFS Trans. (1984) 297. A. Cibula, J. Inst. Metal. 76 (1949) 321. Y. Birol, J. Alloys Compd. 420 (2006) 71. C.D. Mayes, D.G. McCartney, G.J. Tatlock, Mater. Sci. Technol. 9 (1993) 97. A. Hardman, F.H. Hayes, Mater. Sci. Forum. 217–222 (1996) 247. G.P. Jones, J. Pearson, Metall. Trans. 7B (1976) 223. M.S. Lee, B.S. Terry, Mater. Sci. Technol. 7 (1991) 609. Y. Birol, J. Alloy Compd. 420 (2006) 207. Y. Birol, J. Alloy Compd. 427 (2007) 142. Y. Birol, J. Alloy Compd. 440 (2007) 108. Y. Birol, J. Alloy Compd. 443 (2007) 94. Y. Birol, J. Alloy Compd. 458 (2008) 271. M.A. Easton, D.H. StJohn, Metall. Mater. Trans. A 30A (1999) 1613. P. Schumacher, A.L. Greer, J. Worth, P.V. Evans, M.A. Kearns, P. Fisher, A.H. Green, Mater. Sci. Technol. 14 (1998) 394. Y. Birol, J. Alloy Compd. 422 (2006) 128. Y. Birol, J. Alloy Compd. 430 (2007) 179. M.A. Easton, D.H. StJohn, Metall. Mater. Trans. A 36A (2005) 1911. D. Qiu, J.A. Taylor, M.-X. Zhang, P.M. Kelly, Acta Mater. 55 (2007) 1447. Y.C. Lee, A.K. Dahle, D.H. StJohn, J.E.C. Hutt, Mater. Sci. Eng. A259 (1999) 43. S.A. Kori, B.S. Murty, M. Chakraborty, Mater. Sci. Eng. A283 (2000) 94. G.K. Sigworth, M.M. Guzowski, AFS. Trans. 93 (1985) 907. J.A. Spittle, S. Sadli, Mater. Sci. Technol. 11 (1995) 533. T. Sritharan, H. Li, J. Mater. Process Technol. 63 (1997) 585. P.S. Mohanty, J.E. Gruzleski, Acta Mater. 44 (1996) 3749. P.S. Mohanty, F.H. Samuel, G.E. Gruzleski, Metall. Trans. B 26 (1995) 103. Y. Birol, J. Alloys Compd. 478 (2009) 265. K.V.S. Prasad, B.S. Murty, P. Pramanik, M. Chakraborty, Mater. Sci. Technol. 12 (1996) 766. J.A. Spittle, Int. J. Cast Metal Res. 19 (2006) 210.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
A novel Al–Ti–B alloy for grain refining Al–Si foundry alloys Yucel Birol ∗ Materials Institute, Marmara Research Center, Gebze, 41470 Kocaeli, Turkey
a r t i c l e
i n f o
Article history: Received 23 January 2009 Received in revised form 2 July 2009 Accepted 2 July 2009 Available online 18 July 2009 Keywords: Metals Casting Al–Ti–B master alloy Powder metallurgy
a b s t r a c t Al–Ti–B refiners with excess-Ti (Ti:B > 2.2) perform adequately for wrought aluminium alloys but they are not as efficient in the case of foundry alloys. Silicon, which is abundant in the latter, forms silicides with Ti and severely impairs the potency of TiB2 and Al3 Ti particles. Hence, Al–Ti–B alloys with excess-B (Ti:B < 2.2) and binary Al–B alloys are favored to grain refine hypoeutectic Al–Si alloys. These grain refiners rely on the insoluble (Al,Ti)B2 or AlB2 particles for grain refinement, and thus do not enjoy the growth restriction provided by solute Ti. It would be very attractive to produce excess-B Al–Ti–B alloys which additionally contain Al3 Ti particles to maximize their grain refining efficiency for aluminium foundry alloys. A powder metallurgy process was employed to produce an experimental Al–3Ti–3B grain refiner which contains both the insoluble AlB2 and the soluble Al3 Ti particles. Inoculation of a hypoeutectic Al–Si foundry alloy with this grain refiner has produced a fine equiaxed grain structure across the entire section of the test sample which was more or less retained for holding times up to 15 min. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Grain refinement has become a standard melt treatment practice in aluminium foundries world-wide with well documented technical and economical advantages [1,2]. Commercial refiners are manufactured from the Al–Ti–B ternary system, often with more Ti than that required to form TiB2 [2–16]. Hence, the microstructure of these alloys typically comprises, in addition to the insoluble TiB2 , the soluble Al3 Ti particles dispersed in an aluminium matrix. The former act as heterogeneous nucleation sites while Al3 Ti particles readily dissolve in the melt and provide solute Ti, the partioning of which between the solid and liquid phases during solidification, slows down the growth process [17,18]. The very popular excess-Ti Al–Ti–B refiners are known to perform adequately for wrought aluminium alloys except when the alloy to be inoculated contains one or more of the elements whose borides are more stable than TiB2 [19,20]. However, they fail to perform as efficiently in the case of foundry alloys with adverse effects on the as-cast structure and inferior properties in cast parts [21–24]. The high content of Si, which forms silicides with Ti and thus severely impairs the potency of TiB2 and Al3 Ti particles, is claimed to be responsible for the poor response of foundry alloys to grain refinement with Al–Ti–B master alloys [25–29]. AlB2 particles, on the other hand, take advantage of high Si levels which enhance their nucleation potential. The superior performance of Al-borides,
∗ Tel.: +90 262 6412300; fax: +90 262 6412309. E-mail address: [email protected]. 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.07.064
which are not as efficient in the absence of Si, may be attributed to the dissolved Si in the foundry alloys [25]. While there are a number of Al–Ti–B alloys with excess-B compositions and binary Al–B alloys in the market for foundry alloys, these alloys rely on the insoluble (Al,Ti)B2 or AlB2 particles for grain refinement, and thus do not enjoy the growth restriction provided by solute Ti. The mutual presence of Al3 Ti and AlB2 particles in Al–Ti–B alloys, on the other hand, could offer to maximize the grain refining efficiency for aluminium foundry alloys. However, the manufacture of such grain refiners is not straightforward as the conventional practices provide Al–Ti–B alloys with either Al3 Ti and TiB2 particles in excess-Ti composition range or merely (Al,Ti)B2 particles in the case of excess-B alloys. The present work was undertaken to synthesize an Al–Ti–B alloy which contains both the insoluble AlB2 and the soluble Al3 Ti particles and to explore its potential as a grain refiner for hypoeutectic Al–Si foundry alloys. 2. Experimental Al–3B alloy powder and K2 TiF6 salt were thoroughly mixed to obtain a blended mixture. Al–3B master alloy was produced in the laboratory, with the conventional halide salt practice, i.e. by reacting KBF4 salt with molten aluminium at 800 ◦ C. The ratio of individual components in the mixture were adjusted so as to obtain 3 wt%Ti and 3 wt%B in the final alloy. Sample taken from the powder mixture was heated in the differential scanning calorimetry (DSC) cell until 750 ◦ C to identify the reaction sequence during thermal exposure. The DSC experiment was performed under flowing argon at a heating rate of 10 ◦ C/min. Larger volume of Al–3B/K2 TiF6 mixture was heated in a tube furnace in the same manner as that employed in the DSC experiment, to various temperatures where it was held for ½ h and then quenched to room temperature to retain the microstructural features thus introduced. The heat treated samples were subsequently analyzed with X-ray diffraction (XRD) and metallographic techniques, in order to identify the reactions responsible for the DSC
220
Y. Birol / Journal of Alloys and Compounds 486 (2009) 219–222
signals. XRD was conducted with CuK␣ radiation at a scan rate of 0.5◦ /min in order to improve the counting frequency. Samples for metallographic analysis were prepared using conventional practices and were examined with an optical microscope. Scanning electron microscopy (SEM) with energy dispersive X-ray analysis (EDS) was also used to identify the particles in powder grains. The experimental grain refiner was assessed for its grain refining performance. Al wt%7Si casting alloy was melted in a medium frequency induction furnace and the temperature of the melt was fixed at 700 ◦ C. A reference sample was taken from the melt in order to identify the grain size before inoculation. The heat treated Al–3B/K2 TiF6 mixture was dosed into 1.65 g samples which were subsequently compacted into pellets with a hydraulic press. This is the exact amount of an Al–3Ti–3B master alloy needed to bring the B concentration of 1000 g aluminium melt to 0.005 wt%. The melt was stirred with a graphite rod for 30 s right after inoculation. Samples were taken from the melt starting 2 min after the addition and were solidified in small copper molds. As the inoculated grain sizes obtained with the novel Al–3Ti–3B and Al–3B master alloys were nearly comparable after 30 min of holding, the grain refining performance tests were terminated after 30 min of holding time. Standard metallographic procedures were employed to prepare these sections for grain size measurements. They were etched with Poulton’s reagent and were then examined under a light microscope.
3. Results and discussion The Al–3B/K2 TiF6 powder blend is a homogeneous mixture of ␣-Al grains with a dispersion of AlB2 particles (Fig. 1) and halide salt grains. Unlike those in the commercially available Al–B master alloys, AlB2 particles in the laboratory processed Al–3B alloy are not clustered and are relatively smaller. The XRD spectrum of this mixture shows reflections of the AlB2 phase and K2 TiF6 salt, in addition to those of ␣-Al, as expected (Fig. 2a). Several unindexed reflections, too weak to be identified, are believed to belong to the impurities in the halide salt. The response of the Al–3B/K2 TiF6 mixture to thermal exposure was investigated in detail in order to identify the heat treatment conditions that would facilitate the formation of Al3 Ti phase without risking the AlB2 particles. The DSC scan of the Al–3B/K2 TiF6 mixture reveals a small endothermic signal at approximately 370 ◦ C (signal 1) and two much larger neighbouring signals between 613 ◦ C and 642 ◦ C and 653 ◦ C and 688 ◦ C, respectively (signals 2 and 3 in Fig. 3). The former is inherited from the DSC spectrum of K2 TiF6 [30]. The first of the latter two signals is exothermic while the second one is an endothermic doublet. Hence, melting of a species other than aluminium must be taking place between 653 ◦ C and 688 ◦ C. With these features the DSC scan of the Al–3B/K2 TiF6 mixture is almost identical to that of the Al/K2 TiF6 mixture [30]. AlB2 particles in the former have apparently no impact on the response to thermal exposure of the Al/K2 TiF6 mixture. One can see from the XRD spectrum of the Al–3B/K2 TiF6 mixture heat treated at 500 ◦ C, before signal 2 (Fig. 2b), that both AlB2 and
Fig. 1. Microstructure of the Al–3B alloy used to prepare Al–3B/K2 TiF6 powder blend.
Fig. 2. XRD patterns of Al–3B/K2 TiF6 powder blends (a) in the as-mixed state, after heating to (b) 500 ◦ C (below signal 2) and (c) 650 ◦ C (above signal 2) and held at these temperatures for 1/2 h (: ␣-Al, : AlB2 , ♦: K2 TiF6 , 䊉: Al3 Ti, : KAlF4 , : K3 AlF6 ).
K2 TiF6 compounds have prevailed in this temperature range with no evidence of a measurable reaction with each other nor with the aluminium matrix. The XRD spectrum of the Al–3B/K2 TiF6 mixture heated at 650 ◦ C, right after signal 2, is markedly different. Al3 Ti, KAlF4 and K3 AlF6 reflections have fully replaced K2 TiF6 lines in this sample (Fig. 2c). K2 TiF6 salt was apparently reduced by aluminium shortly before the melting of aluminium producing Al3 Ti and the inorganic salts, KAlF4 and K3 AlF6 . Signal 2 is then perfectly accounted for by the following reaction: 3K2 TiF6 + 13Al → 3Al3 Ti + 3KAlF4 + K3 AlF6
(1)
The formation of Al3 Ti is evidenced also by the metallographic analysis of samples heat treated in this temperature range. One can see many small blocky Al3 Ti particles dispersed inside aluminium
Fig. 3. DSC spectrum of the Al–3B/K2 TiF6 mixture.
Y. Birol / Journal of Alloys and Compounds 486 (2009) 219–222
221
Fig. 5. Change in free energy of formation with temperature for Al3 Ti, AlB2 and TiB2 . Fig. 4. Optical micrographs of the Al–3B/K2 TiF6 mixture heat treated at 650 ◦ C (above signal 2).
grains (Fig. 4). K2 TiF6 and Al can apparently react in the solid state contrasting the results of Prasad et al. [31] who claim that K2 TiF6 reacts only with molten aluminium. It is plausible to associate the initial trough of the endothermic signal 3 with the melting of the inorganic salts, KAlF4 and K3 AlF6 , produced by reaction (1). It is clear from the foregoing that a process which relies on a solid-state reaction between aluminium and K2 TiF6 can be used to generate Al3 Ti particles in a mixture which already has preformed AlB2 particles. The conventional route, i.e. adding to molten aluminium KBF4 and K2 TiF6 salts, inevitably favors the more stable of the two potential borides, TiB2 . Even when the halide salts are added sequentially so as to form AlB2 first, one would expect AlB2 to transform to TiB2 as soon as K2 TiF6 is added in the melt, according to, 3K2 TiF6 + 3AlB2 + Al → 3TiB2 + 3KAlF4 + K3 AlF6
(2)
since TiB2 is more stable than AlB2 (Fig. 5). The solid-state process not only avoids the AlB2 → TiB2 transformation, but also offers exceptional microstructural features. Al3 Ti particles generated by a reaction between K2 TiF6 and aluminium in the solid state were shown to be much smaller than those available in commercial Al–Ti master alloys yielding a superior grain refining performance [30]. Simply mixing powders of Al–B and Al–Ti master alloys already
available in the market to provide AlB2 and Al3 Ti particles together would not be as efficient. The Al–3Ti–3B pellet produced so as to contain both Al3 Ti and AlB2 particles was found to be a fast acting effective grain refiner for the Al–7 wt%Si alloy (Fig. 6). Inoculation with the present alloy has produced a fine equiaxed grain structure across the entire section of the test sample which was more or less retained for 15 min after inoculation. The reduction in grain size is remarkable in spite of the fact that the addition level was a modest 0.005 wt%B. Besides, the dendritic as-cast structure was improved into a more homogeneous one, dominated by equiaxed ␣-Al rosettes (Fig. 7). The performance of the present grain refiner is clearly superior than that of the binary Al–3B alloy confirming the favorable contribution of Al3 Ti on grain refinement of hypoeutectic Al–Si foundry alloys (Fig. 6). Fading of the grain refinement effect was noted, however, at holding times longer than 15 min. The present grain refiner is apparently not as resistant to fade as the popular Al–5Ti–1B alloy. Similar observations were made by other investigators for B-excess grain refiner alloys [32]. The settlement of the AlB2 particles in the melt may be partially responsible for fading. Loss of the grain refinement effect also raises questions regarding the stability of AlB2 particles in aluminium melts at temperatures in the neighbourhood of 700 ◦ C. Given enough time, transformation of the AlB2 particles to TiB2 variety at these temperatures is not at all unlikely. The latter is known to be a relatively less efficient nucleating agent in hypoeutectic Al–Si alloys and could be responsible for the reduced
Fig. 6. Performance tests of the Al–3Ti–3B and Al–3B grain refiners used with the Al–7Si alloy at an addition level of 0.005 wt%B.
222
Y. Birol / Journal of Alloys and Compounds 486 (2009) 219–222
a fine equiaxed grain structure across the entire section of the grain refining test sample which was more or less retained for holding times up to 15 min. Acknowledgements It is a pleasure to thank Osman C¸akır and Fahri Alageyik for their help in the experimental part of the work. References
Fig. 7. Microstructure of the Al–7Si alloy inoculated with the experimental Al–3Ti–3B grain refiner at a B addition level of 0.005 wt.
efficiency. The present alloy can nevertheless be used effectively when and where the grain refiner additions are made shortly before casting. 4. Conclusions The process employed in the present work relies on a solid-state reaction between aluminium and K2 TiF6 to generate Al3 Ti particles in a mixture which already has preformed AlB2 particles. This process not only avoids the AlB2 → TiB2 transformation, but also offers Al3 Ti particles much smaller than those available in commercial grain refiners yielding a superior grain refining performance. The experimental Al–3Ti–3B pellet thus produced contained both the insoluble AlB2 and the soluble Al3 Ti particles and was found to effectively grain refine a hypoeutectic Al–Si foundry alloy. Inoculating an Al–7 wt%Si alloy with the present grain refiner has produced
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32]
D.G. McCartney, Int. Mater. Rev. 34 (1989) 247. B.S. Murty, S.A. Kori, M. Chakraborty, Int. Mater. Rev. 47 (2002) 3. L. Arnberg, L. Backerud, H. Klang, Met. Technol. 9 (1982) 14. M.S. Lee, B.S. Terry, P. Grieveson, Metall. Trans. 24B (1993) 955. D. Apelian, G.K. Sigworth, K.R. Whaler, AFS Trans. (1984) 297. A. Cibula, J. Inst. Metal. 76 (1949) 321. Y. Birol, J. Alloys Compd. 420 (2006) 71. C.D. Mayes, D.G. McCartney, G.J. Tatlock, Mater. Sci. Technol. 9 (1993) 97. A. Hardman, F.H. Hayes, Mater. Sci. Forum. 217–222 (1996) 247. G.P. Jones, J. Pearson, Metall. Trans. 7B (1976) 223. M.S. Lee, B.S. Terry, Mater. Sci. Technol. 7 (1991) 609. Y. Birol, J. Alloy Compd. 420 (2006) 207. Y. Birol, J. Alloy Compd. 427 (2007) 142. Y. Birol, J. Alloy Compd. 440 (2007) 108. Y. Birol, J. Alloy Compd. 443 (2007) 94. Y. Birol, J. Alloy Compd. 458 (2008) 271. M.A. Easton, D.H. StJohn, Metall. Mater. Trans. A 30A (1999) 1613. P. Schumacher, A.L. Greer, J. Worth, P.V. Evans, M.A. Kearns, P. Fisher, A.H. Green, Mater. Sci. Technol. 14 (1998) 394. Y. Birol, J. Alloy Compd. 422 (2006) 128. Y. Birol, J. Alloy Compd. 430 (2007) 179. M.A. Easton, D.H. StJohn, Metall. Mater. Trans. A 36A (2005) 1911. D. Qiu, J.A. Taylor, M.-X. Zhang, P.M. Kelly, Acta Mater. 55 (2007) 1447. Y.C. Lee, A.K. Dahle, D.H. StJohn, J.E.C. Hutt, Mater. Sci. Eng. A259 (1999) 43. S.A. Kori, B.S. Murty, M. Chakraborty, Mater. Sci. Eng. A283 (2000) 94. G.K. Sigworth, M.M. Guzowski, AFS. Trans. 93 (1985) 907. J.A. Spittle, S. Sadli, Mater. Sci. Technol. 11 (1995) 533. T. Sritharan, H. Li, J. Mater. Process Technol. 63 (1997) 585. P.S. Mohanty, J.E. Gruzleski, Acta Mater. 44 (1996) 3749. P.S. Mohanty, F.H. Samuel, G.E. Gruzleski, Metall. Trans. B 26 (1995) 103. Y. Birol, J. Alloys Compd. 478 (2009) 265. K.V.S. Prasad, B.S. Murty, P. Pramanik, M. Chakraborty, Mater. Sci. Technol. 12 (1996) 766. J.A. Spittle, Int. J. Cast Metal Res. 19 (2006) 210.