U.S. patent number 3,785,807 [Application Number 05/137,986] was granted by the patent office on 1974-01-15 for method for producing a master alloy for use in aluminum casting processes.
This patent grant is currently assigned to Granges Aluminium AB. Invention is credited to Stig Lennart Backerud.
United States Patent |
3,785,807 |
Backerud |
January 15, 1974 |
METHOD FOR PRODUCING A MASTER ALLOY FOR USE IN ALUMINUM CASTING
PROCESSES
Abstract
A method for producing a master alloy for use in aluminum
casting processes in which an aluminum melt containing 0.02-6
percent by weight titanium and 0.01-2 percent by weight boron is
produced under conditions under which the boron is bound to
titanium in the form of titanium diboride, whereafter the melt
containing titanium diboride is held under agitation at a
temperature ranging from the melting point of the material to
900.degree. C for a period of at least 15 minutes and at most 9
hours.
Inventors: |
Backerud; Stig Lennart
(Akersberga, SW) |
Assignee: |
Granges Aluminium AB
(Kubikenborg, Fack, Sundsvall, SW)
|
Family
ID: |
20267440 |
Appl.
No.: |
05/137,986 |
Filed: |
April 27, 1971 |
Foreign Application Priority Data
|
|
|
|
|
Apr 28, 1970 [SW] |
|
|
5881/70 |
|
Current U.S.
Class: |
420/552;
75/684 |
Current CPC
Class: |
C22C
1/03 (20130101); C22C 21/00 (20130101) |
Current International
Class: |
C22C
1/03 (20060101); C22C 21/00 (20060101); C22c
001/02 () |
Field of
Search: |
;75/138,135,68R |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Dean; Richard O.
Attorney, Agent or Firm: Waters, Roditi, Schwartz &
Nissen
Claims
What I claim is:
1. A method for preparing a master alloy intended to be added to an
aluminum melt for refining the grains of the aluminum during
solidification thereof comprising the steps of
a. preparing an alloy melt consisting essentially of 0.02-6 percent
by weight of titanium and 0.01-2 percent by weight of boron, the
balance being aluminum and wherein the boron is bound to titanium
in the form of titanium diboride, and
b. maintaining the alloy melt at a temperature between the melting
point of the alloy and 900.degree. C. for a period of time of at
least 15 minutes and at most 9 hours with agitation until the
grains of titanium diboride in the alloy become embraced by a
crystal shell comprising substantially Al.sub.3 Ti.
2. A method according to claim 1 wherein the cooling step comprises
cooling the alloy melt to a temperature below the melting point of
the alloy and then re-heating the cooled alloy to a temperature
between the melting point of said alloy and 900.degree. C.
3. A method according to claim 2 wherein the step of cooling the
alloy melt to a temperature below the melting point of the alloy
comprises casting the alloy melt into cooled moulds.
4. A method according to claim 1, wherein the titanium is present
in an amount of 0.02-2 percent by weight, based on the total weight
of the alloy.
5. A method according to claim 1, wherein the boron is present in
an amount of 0.01-1 percent by weight, based on the total weight of
the alloy.
6. A method according to claim 1, wherein the alloy melt is
maintained at a temperature of 680.degree.-720.degree. C.
7. A method according to claim 6, wherein the alloy melt is
maintained at 680.degree.-720.degree. C for from 45 minutes to 2.5
hours.
8. A method according to claim 1 wherein the step of preparing the
alloy melt comprises dispersing titanium diboride in molten
aluminum.
9. A method according to claim 1 wherein the step of preparing the
alloy melt comprises adding titanium and boron to molten aluminum
at a temperature above about 1,200.degree. C.
Description
The present invention relates to a method for producing a master
alloy which can be added to an aluminum melt before the melt
solidifies, thereby to obtain finer grain size of the cast aluminum
product with subsequent increase in the quality of the same.
It is known that in order to obtain a satisfactory product from an
aluminum casting process, for example, it is necessary to add a
substance which facilitates the formation of crystals during the
period of solidification, the substance preventing the aluminum
melt from solidifying to a coarse-crystal product. To this end,
different grain refining substances are generally incorporated in
the aluminum as master alloys, which are added to the aluminum melt
in solid form, for example in the form of small ingots or a wire
which is continuously fed into the melt. The master alloy may also
be added in a molten state.
The master alloys previously used have comprised mainly titanium,
boron and a combination of titanium and boron. Typical master
alloys contain 2-10 percent by weight titanium in aluminum, 0.3-5
percent by weight boron in aluminum and 2-10 percent by weight
titanium together with 0.3-5 percent by weight boron in aluminum. A
usual composition is one containing 5 percent by weight titanium
and 1 percent by weight boron in aluminum. Master alloys of this
type are available commercially.
It has now been surprisingly found that by producing a
titanium-boron-aluminum master alloy in a special manner it is
possible to improve considerably the grain refinement while using
considerably lower total quantities of titanium and boron than has
been possible with the hitherto commercially available master
alloys.
Master alloys containing titanium and boron are normally produced
by dissolving the required quantities of titanium and boron in an
aluminum melt at temperatures in excess of approximately
1,200.degree. C. When practicing this method, it is first necessary
to dissolve a specific quantity of titanium before adding the
boron. The boron is added in the form of a boron salt, normally
potassium borofluoride (KBF.sub.4). The boron salt is dissociated
in the melt and the liberated boron then rapidly combines with the
titanium present in the melt. It is also possible to disperse
fine-grain titanium diboride in the melt. In accordance with the
present invention there is produced a master alloy which is
intended to be added to an aluminum melt to afford a grain refining
effect during the solidification period, an aluminum melt
containing 0.02-6 percent by weight titanium and 0.01-2 percent by
weight boron being produced, in which the boron is bound to the
titanium in the form of titanium diboride, by either first
dissolving titanium at a temperature such that the quantity added
passes into solution, and then adding boron, or by dispersing
titanium boride in an aluminum melt, and the method is
characterized by the step of maintaining the melt containing
titanium diboride at a temperature between the melting point of the
mixture and 900.degree. C while stirring the melt and for a period
of time of at least 15 minutes and at most 9 hours.
If large quantities of titanium, for example of the order of 10
percent by weight, are to be dissolved, it is necessary, for
thermodynamic reasons, that the temperature during the dissolution
phase reaches at least 1,200.degree. C. Consequently, it is
naturally necessary to cool the aluminum melt rapidly down to a
temperature below 900.degree. C, in order to prevent the occurrence
of undesirable reactions. Since it is difficult to rapidly cool the
alloy to a temperature immediately above the melting point a
particularly suitable method for carrying out such a cooling
process is one in which alloy is cast in small, water-cooled
moulds, whereafter the metal is re-melted at a temperature below
900.degree. C.
The titanium content of the master alloy is preferably 0.2-2
percent by weight and the boron content is preferably 0.1-1 percent
by weight and the temperature used during the dissolution phase is
from 1,200.degree. to 1,500.degree. C. The alloy is then cooled to
the holding temperature between the melting point and 900.degree.
C. A preferred holding temperature is 680.degree.-720.degree. C and
a preferred holding time is from 45 minutes to 2.5 hours.
Subsequent to the heat treatment process, the pre-alloy can be used
directly or subsequent to solidifying, although it is normal
practice to decant the molten master alloy to prevent the formation
of large agglomerates of titanium diboride and other impurities
from accompanying the master alloy.
The conditions prevailing in the aluminum-titanium system are
evident from available constitutional diagrams, from which it can
be seen that pure aluminum solidifies at approximately 660.degree.
C and that a peritectic solidification line exists from a titanium
content of roughly 0.5 percent by weight at 665.degree. C to the
stoichiometric composition for Al.sub.3 Ti at roughly 37.5 percent
by weight Ti. In order that Al.sub.3 Ti can be formed, the content
of Ti at 665.degree. C must thus be at least 0.15 percent by
weight. At 900.degree. C the liquid solubility for titanium equals
1 percent.
When titanium and boron are dissolved in aluminum, a compound
between titanium and boron, TiB.sub.2, is rapidly formed, Al.sub.3
Ti crystallizing out at reduced temperature during the holding
period to embrace the compound. The formation of Al.sub.3 Ti
presumes that the concentration of Ti in the system exceeds the
content necessary for forming Al.sub.3 Ti at the temperatures in
question. In this particular instance, a titanium concentration
gradient is obtained around the TiB.sub.2 grains. This
concentration gradient is obtained as a result of the fact that
titanium is disassociated from titanium diboride and is replaced
therein with aluminum. This enables the titanium diboride and
aluminum diboride to have the same crystal structure and to replace
each other in the crystal lattice.
It is thus necessary to exceed the solubility limit or
liquiduscurve in the constitutional diagram for Al.sub.3 Ti, which
can be effected by raising the concentration of titanium or by
changing the position of the solubility curve by means of
appropriate additives. In this way, Al.sub.3 Ti will crystallize
around the TiB.sub.2 grains and form small crystals, which
constitute the actual crystallization nuclei. The formation of
Al.sub.3 Ti takes place during the holding time at the
aforementioned temperature interval of the invention. If the
titanium content of the master alloy is of such magnitude that
Al.sub.3 Ti can be formed in the whole melt, large quantities of
Al.sub.3 Ti crystals will be formed, which when the master alloy is
used will dissolve and give high titanium contents to the final
product, but will of course also act as crystallization nuclei to a
lesser extent, owing to the fact that these crystals will become
considerably larger and fewer than those which are formed around
the TiB.sub.2 -grains.
The initally irregular grains of TiB.sub.2 will, after
approximately 1 hour, have been embraced by a more regularly shaped
crystal shell comprising substantially Al.sub.3 Ti. The formed
crystals added to the aluminum melt are able to refine the grains
rapidly and effectively. If the master alloy is not subjected to
the crystallization of Al.sub.3 Ti around the TiB.sub.2 -particles
during the holding period and during simultaneous agitation of the
system, TiB.sub.2 will form aggregates which will be practically
totally precipitated out by gravitational separation, and will
either not be included during the casting process or will be
entrained with the casting material, thereby rendering its use
impossible for, for example, foil rolling, where the agglomerated
TiB.sub.2 -particles cause the foil to be torn during the rolling
operation. For the same reason, a large quantity of TiB.sub.2 will
fall to the bottom of the furnace without fulfilling its function
as a grain refining agent, whereupon it becomes necessary to add
the TiB.sub.2 in excessive quantities, which considerably impairs
the economy of aluminum casting processes using this agent. A large
addition of grain refining agent also causes a large quantity of
titanium to be dissolved in the melt. An increase in the titanium
content of aluminum gives rise to several undesirable effects, such
as the formation of feathery grains and changes in the conductivity
of the final product.
The new master alloy of the present invention can be used in
considerably small quantities or with a lower content of titanium
and boron, since it is possible to utilize actively all the
titanium and boron present therein.
It is obvious that the final, desired aluminum melt can be
considered totally as a master alloy and that the melt can be
treated in a manner whereby titanium and boron are first dissolved
at higher temperatures and the whole melt than maintained at a
temperature of approximately 700.degree. C for a period of 1 hour
under agitation. In this way grain refinement would be equivalent
to that obtained with the master alloy of the present invention.
However, such treatment of an aluminum melt is expensive, extremely
difficult to carry out technically and gives an undesirable content
of titanium in the product. It is, instead, particularly desirable
to produce a master alloy which can be used in continuous casting
processes externally of the furnace in a special container or in
the actual pouring stream. The master alloy of the present
invention is particularly suited for this purpose, since it can be
passed to the melt just before the melt is to be transferred to the
mould and intimately blended with the melt. In this way, the grain
refining agent is able to exert its influence immediately and a
superior product is obtained with a considerably smaller total
quantity of titanium and boron in the finished product.
* * * * *