Production of Al–Ti–B grain refining master alloys from B2O3 and K2TiF6

Production of Al–Ti–B grain refining master alloys from B2O3 and K2TiF6

Journal of Alloys and Compounds 443 (2007) 94–98 Production of Al–Ti–B grain refining master alloys from B2O3 and K2TiF6 Y¨ucel Birol ∗ Materials Ins...

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Journal of Alloys and Compounds 443 (2007) 94–98

Production of Al–Ti–B grain refining master alloys from B2O3 and K2TiF6 Y¨ucel Birol ∗ Materials Institute, Marmara Research Center, TUBITAK, Gebze, Kocaeli, Turkey Received 14 September 2006; received in revised form 3 October 2006; accepted 4 October 2006 Available online 9 November 2006

Abstract It is very desirable to replace the KBF4 salt in the popular “halide salt” process to reduce the volume of fluoride-bearing particulate material to be added to molten aluminium. Several practices all relying on B2 O3 to supply B, were employed in the present work to produce Al–5Ti–1B grain refiners. Replacing the KBF4 salt in the halide salt process entirely with B2 O3 , has not only impaired the grain refining performance, but, with the dross generated, has also hurt the fluidity of the melt making pouring difficult. A significant improvement was noted both in the microstructural features and in the grain refining efficiency when B2 O3 was pre-mixed with a drossing flux to promote the seperation of oxides from the molten alloy. However, this practice has almost completely negated the particulate material savings offered by B2 O3 . The Ti recovery and the grain refining efficiency achieved when B2 O3 and KBF4 were used together to supply B, was nearly as good as when the latter was used alone. It is thus concluded that KBF4 can be replaced with B2 O3 , only partially, to take advantage of lower fluoride emissions and particulate material savings without a penalty in grain refining efficiency. © 2006 Elsevier B.V. All rights reserved. Keywords: Metals; Casting; Al–Ti–B master alloy; Grain refinement

1. Introduction The aluminium industry relies largely on grain refiner master alloys from the Al–Ti–B ternary system to control the cast grain size [1,2]. A typical practice involves the addition of Al–Ti–B grain refiner rods into molten aluminium on its way to the casting station. The uniform, fine, equiaxed grain structure thus obtained not only yields superior mechanical properties and surface quality but also provides alloy homogeneity and allows for good castability yielding a high casting output and reduced down stream processing costs. While numerous methods have been offered for the production of Al–Ti–B grain refiner alloys [3–17], that which involves adding a mixture of KBF4 –K2 TiF6 salts to molten aluminium, is the most popular [3]. The KBF4 and K2 TiF6 salts used in the conventional “halide salt” process are lean in B and Ti, respectively. A large volume of salt must thus be added to molten aluminium (nearly 370 kg KBF4 –K2 TiF6 mixture to produce one ton Al–5Ti–1B



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grain refiner alloy). While these halide salts, KBF4 in particular, are expensive compounds which add greatly to the raw material costs, a substantial portion of the charge is of no use, giving rise to intense fluoride emissions and troublesome dross. Besides, the cooling of the melt by a large volume of particulate material is hardly compensated for by the exothermic reaction it kicks off. Hence, it is very desirable to replace these halide salts with other precursor compounds richer in B and Ti. Several alternative B and Ti sources were recently offered to replace KBF4 and K2 TiF6 salts [18–25]. The present work was undertaken to replace KBF4 with B2 O3 in the production of Al–5Ti–1B grain refiners. B2 O3 is nearly four times richer in B with respect to KBF4 and is less expensive. Several different methods all involving B2 O3 were investigated to identify the best practice. 2. Experimental The process described in Ref. [15] was employed to produce, on a 1 kg batch scale, four experimental Al–5Ti–1B grain refiner alloys (Table 1). The first experiment involved the addition into molten aluminium of pre-mixed KBF4 and K2 TiF6 salts of commercial purity, to obtain a standard grain refiner (alloy 1). In the following experiment, KBF4 was replaced entirely with B2 O3 , to produce alloy 2. This practice has led to a savings of 84 g of particulate charge per

Y. Birol / Journal of Alloys and Compounds 443 (2007) 94–98 Table 1 Particulate material additions for the production of 1000 g of Al–5Ti–1B grain refiner alloy (g) Alloy 1 2 3 4

K2 TiF6

KBF4

B2 O3

Flux

Total

250.7

116.4 – – 58.2

– 32.2 32.2 16.1

– – 70 –

367.1 282.9 352.9 325.0

kilogram of grain refiner alloy. B2 O3 was once again the precursor B compound but was added into the melt pre-mixed with a drossing flux in the third experiment (alloy 3). In the final experiment to produce alloy 4, only half of the B content of the Al–5Ti–1B alloy was sourced from B2 O3 and the rest was supplied in the form of KBF4 . The alloy melt thus obtained was held at 800 ◦ C for 30 min in an electric resistance furnace in order to avoid the mixing of the spent salt and of the oxides in the melt. The molten K–Al–F salt was finally decanted, the dross was skimmed off and the melt was stirred thoroughly to rejuvenate the settled particles before it was cast into a permanent mold. The experimental master alloys thus produced were assessed for their grain refining performances using the test described in Ref. [15]. Standard metallographic procedures were employed to prepare the inoculated samples which were etched with 0.5% HF reagent and then photographed for visual assesment. The insoluble boride particles in the aluminium matrix were identified with X-ray diffraction (XRD) analysis.

3. Results and discussion Ti recovery in alloy 1, produced with the recently optimized “halide salt” process [15,16], is exceptional at 99.4%. The high Ti recovery is evidenced by a large number of soluble aluminide and insoluble boride particles dispersed in an ␣-Al matrix. The former are blocky and invariably smaller than 20 ␮m while the boride particles are numerous, often in clusters and are smaller

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than 1 ␮m (Fig. 1a). The majority of the borides were identified by XRD to be of the TiB2 variety (Fig. 2a). The grain refining efficiency of this alloy is remarkable with an average grain size of 100 ␮m 2 min after inoculation (Fig. 3a). The adequately refined grain structure and the absence of fading for contact times upto 60 min is attributed to the predominance of sub-micron TiB2 particles and to the blocky morphology and wide size range of Al3 Ti particles. Owing to very favorable microstructural features and an exceptional grain refining efficiency, alloy 1 sets the standard for the alternative grain refiner alloys investigated in the present work. Ti recovery in alloy 2, produced in exactly the same manner as alloy 1, but with B2 O3 as the precursor B compound, dropped to 70%. The lower Ti recovery of this alloy is manifested also in the metallographic studies. The aluminide particles are fewer but exhibit features similar to those in alloy 1 (Fig. 1b). Petallike aluminide particles are occasionally observed (Fig. 4). Such a change in Al3 Ti particle morphology would originate from a subtantial increase in the melt temperature [26]. It takes temperatures above 900 ◦ C to solutionize the blocky aluminides which crystallize in non-equiaxed shapes when rapidly solidified. However, the temperature increase in the melt upon the addition of the B2 O3 –K2 TiF6 mixture was less than that encountered with KBF4 –K2 TiF6 , the reaction of which with molten aluminium is highly exothermic. So, the petal-like aluminide particles in alloy 2 cannot be accounted for by high melt temperatures. The boride particles, on the other hand, are of the typical boride morphology and are in numbers comparable to alloy 1. A slight increase is noted, however, in the fraction of the AlB2 variety (Fig. 2b). Replacing the precursor B compound is expected to have no effect on the reaction sequence leading to the formation of Al3 Ti

Fig. 1. The microstructures of alloys: (a) 1, (b) 2, (c) 3 and (d) 4.

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or may occur through direct reduction of B2 O3 by solute Ti: 2B2 O3 + 5Ti → 2TiB2 + 3TiO2

Fig. 2. The XRD spectra of alloys: (a) 1, (b) 2, (c) 3 and (d) 4 (♦: ␣-Al, : AlB2 , : TiB2 and : Al3 Ti).

particles in the “halide salt” route: 3K2 TiF6 + 4Al → 3Ti + 3KAlF4 + K3 AlF6

(1)

3Ti + Al → Al3 Ti

(2)

K2 TiF6 is reduced by molten aluminium, releasing Ti which dissolves in the melt before precipitating out as Al3 Ti particles as soon as its solubility limit is exceeded. Formation of TiB2 particles, on the other hand, may involve the reduction of B2 O3 by molten aluminium and the subsequent reaction of AlB2 with the Al3 Ti phase: B2 O3 + 3Al → AlB2 + Al2 O3

(3)

AlB2 + Al3 Ti → TiB2 + 4Al

(4)

(5)

Reaction (4) of the former route was shown to be extremely sluggish even when both AlB2 and Al3 Ti particles were abundant in the melt [25]. Reaction (5), on the other hand, enjoys a much larger driving force [27] and provides, implying oxidation of Ti, a plausible account of the lower Ti recovery in alloy 2. Evidence, in the XRD spectrum, of a considerable amount of AlB2 seems to suggest that reaction (3) is also taking place. Alloy 2, which relied entirely on B2 O3 for B supply, is, but a poor grain refiner (Fig. 3b). Lower Ti recovery, petal-like aluminides and a relatively higher fraction of AlB2 particles are believed to be responsible for the inferior performance of this alloy. The petal-like variety of the aluminides and the AlB2 type of the borides are known to be less efficient in terms of grain refinement with respect to the blocky aluminides and TiB2 particles, respectively [28,29]. Replacing KBF4 with B2 O3 not only impaired the grain refining performance but also generated a considerable amount of dross (Table 2), partly suspended in the melt, hurting the fluidity of the molten alloy and making pouring rather difficult. This is not surprising since the oxidation of molten aluminium by B2 O3 is inevitable at these temperatures. The dross skimmed from the melt was shown by XRD to predominantly contain Al2 O3 and K–Al fluorides while there was hardly any evidence for TiO2 . Al3 Ti particles, however, were entrapped inside the dross in appreciable numbers (Fig. 5). The low Ti recovery in alloy 2 may thus be accounted for by the failure to skim from the melt a dry dross rather than by the loss of Ti through oxidation. Even if reaction (5) were involved in the formation of TiB2 , TiO2 , by-product of this reaction, must have transformed to Al2 O3 , the more stable of the two oxides. The last two experiments were designed to overcome the adversities encountered when producing alloy 2. The practice employed to produce alloy 3 involved the use of a commercial drossing flux with the B2 O3 –K2 TiF6 mixture to facilitate the seperation of oxides and to improve the fluidity of the melt. The drossing flux has helped to improve the Ti recovery while reducing the weight fraction of the dross by nearly 25% (Table 2). The increase in Ti recovery is evidenced by the metallographic analysis of alloy 3. The aluminides and borides are present in numbers and with features resembling alloy 1 (Figs. 1c and 2c). Hence, the grain refining efficiency of alloy 3 is clearly superior with respect to that of alloy 2 (Fig. 3c). Slight fading in the grain refinement performance was noted, however, 30 min after inoculation. While employing a drossing flux is judged to Table 2 Fraction of the different constituents produced in the grain refiner production experiments and the Ti recovery (wt.%) Constituents

Alloy 1

Alloy 2

Alloy 3

Alloy 4

Grain refiner alloy (wt.%) Spent salt (wt.%) Dross (wt.%) Ti recovery (%)

64 21 15 99.4

56 4 40 70.0

60 11 29 80.0

65 16 19 92.0

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Fig. 3. The grain refining performance test results of alloys: (a) 1, (b) 2, (c) 3 and (d) 4.

be effective in recovering the aluminides entrapped in the dross and in helping to make alloy 3 a better grain refiner, there is still room for improvement to match the performance of alloy 1. Besides, employing a drossing flux has almost completely erased the particulate material savings offered by B2 O3 (Table 2), a major objective in the present work. Only half of the B content was sourced from B2 O3 and the rest was supplied in the form of KBF4 when producing alloy 4. The purpose in doing so, was to reduce the amount of the oxidizing agent, i.e. B2 O3 , while promoting the generation of KAlF4 salt by using some KBF4 , instead of a separate drossing flux, to improve wetting of the oxides. Alloy 4 eventually enjoyed even a higher Ti recovery (92 wt.%) than alloy 3. The amount of the dross generated was the least among the three alternative grain refiner alloys investigated in the present work. Owing to the number density, the size, the morphology and type of Al3 Ti and TiB2 particles, which were found to be quite typical of alloy 1

Fig. 5. Al3 Ti particles entrapped inside the dross generated during production of alloy 2.

(Figs. 1 and 2), alloy 4 performed nearly as good as alloy 1 with no evidence of fading until 60 min after inoculation (Fig. 3). 4. Summary

Fig. 4. Petal-like Al3 Ti particles in alloy 2.

The grain refiner alloy which relied on B2 O3 for the B supply suffered a lower Ti recovery, revealed some petal-like aluminides and a relatively higher fraction of AlB2 particles and was thus a poor grain refiner. Replacing the KBF4 salt in the halide salt process with B2 O3 , has not only impaired the grain refining capacity but, having generated a considerable amount of dross, has also hurt the fluidity of the melt making pouring difficult. A significant improvement was noted in the Ti recovery and in the microstructural features when B2 O3 was added with a drossing flux to promote the seperation of oxides from the molten alloy. However, this practice has almost completely negated the particulate material savings offered by B2 O3 . The Ti recov-

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ery, microstructural features and the grain refining efficiency achieved when B2 O3 and KBF4 were used together to supply B, was nearly as good as when the latter was used alone. It is thus concluded that KBF4 can be replaced with B2 O3 only partially, to take advantage of lower fluoride emissions and particulate material savings without sacrificing an acceptable grain refining efficiency. Acknowledgements O. C ¸ akır, F. Alageyik, E. Ekiz and E. Karabeyoglu are thanked for their help in the experimental part of this work.

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