Materials Science and Engineering A 413–414 (2005) 527–532
On the precipitation of TiB
2
in aluminum melts from
the reaction with KBF
4
and K
2
TiF
6
Jonas Fjellstedt
a
, Anders E.W. Jarfors
b,*
a
Outokumpu Copper R&D, Trefasgatan 1, Box 594, 721 10 V¨ aster˚ as, Sweden
b
Corrosion & Metals Research Institute, Drottning Kristinas v¨ ag 48,
114 28 Stockholm, Sweden
Received in revised form 30 June 2005
Abstract
The divergent reaction between Al and the two salts K
2
TiF
6
and KBF
4
are used in the manufacturing of Al–Ti–B master alloys for grain
refinement. It is also possible to use the same reaction to make aluminum based metal matrix composites. The formation sequence of TiB
2
from
the reaction with K
2
TiF
6
and KBF
4
is discussed in the view of the formation of a salt/metal emulsion and agglomeration. The highly exothermic
reaction with K
2
TiF
6
will aid the formation of an emulsion and Al
3
Ti form that in a subsequent step transforms into TiB
2
. It will also support high
element transfer efficiency. The reaction with KBF
4
causes less heat to evolve and the tendency towards formation of an emulsion and thus lower
transfer efficiency resulting in a sluggish formation of AlB
2
. Simultaneous additions of K
2
TiF
6
and KBF
4
increase the transfer efficiency but will
cause the formation of stringer defects in a layered structure.
© 2005 Elsevier B.V. All rights reserved.
Keywords: K
2
TiF
6
; KBF
4
; Al; Particulate composite; In situ reaction
1. Introduction
It is well known that addition of a master alloys containing
Ti and B refines the microstructure in Al alloy castings. These
master alloys are produced by letting Ti- and B-bearing fluoride
salts react with the molten aluminum, yielding a ternary Al–Ti–B
alloy [1]. This process includes the transfer of Ti and B from
the salts to molten Al, where the atoms dissolve in the melt
and precipitate as Al
3
Ti and TiB
2
. It is also suggested that in
situ metal matrix composite (MMC), can be made by a slightly
modified process route. The aim is to achieve a suitable fraction
of finely dispersed TiB
2
-particles of optimum size of 1–2 m
[2], by moving into the Liquid (L)-TiB
2
two-phase region in the
ternary phase diagram and thus avoid the formation of Al
3
Ti.
In this paper, the reactions between molten aluminum and Ti-
and B-bearing potassium fluoride salts, K
2
TiF
6
and KBF
4
, have
been studied to increase the understanding of the formation of
the different phases.
*
Corresponding author. Tel.: +46 8 4404866; fax: +46 8 4404535.
E-mail address: anders.jarfors@kimab.com (A.E.W. Jarfors).
1.1. The characteristics of K
2
TiF
6
and KBF
4
Prasad et al. [3] studied K
2
TiF
6
- and KBF
4
-salts, using differ-
ential thermal analysis (DTA), differential scanning calorimetry
(DSC), and thermogravimetry (TG) up to 1073 K.
It was found that bonded water was released from K
2
TiF
6
at
648 K. At 835 K, K
3
TiOF
5
formed due to reaction with moisture
in the surrounding atmosphere. TG examination showed that
the salt started to lose mass at 648 K and as the sample reached
1073 K the total mass-loss was about 11%. Dissociation into KF
and TiF
4
was not observed up to 1073 K.
Prasad et al. [3] found that at 562 K, KBF
4
showed a transition
from orthorombic to cubic structure, slightly higher than 556 K
found in JANAF tables [4]. Prasad et al. [3] could not identify
a melting temperature of KBF
4
, which contradicts the reported
melting temperature of 843 K [4]. The melting temperature of
K
2
TiF
6
is reported to 1172 K [5].
DTA performed on a mixture of the two different salts show
that the endothermic reaction at 835 K for K
2
TiF
6
disappeared,
suggesting that some reaction take place. X-ray diffraction
(XRD) of the salt mixture heat-treated at 873 K and 973 K that
showed some unidentified peaks confirmed this. Prasad et al. [3]
could not identify the nature of this reaction.
0921-5093/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.msea.2005.09.054