Note: Descriptions are shown in the official language in which they were submitted.
CA 02361484 2001-11-08
09/710175
CHO 00-1319
PROnUCTlQN OF ULTRA..~'INC GltAUV ST1ZUCTURC !N
AS-CAST ALtJMXNUM Al.t,OYS
1. I~ield of the Inyention
This invention relates to production of as-cast almninum alloys with
ulri~a-fine grain stxuctuz-e. More particularly, it relates to methods of
adding a gain
refiner to a coolten aluminum alloy at levels which reduce the ~-aui size to
less
than about 200 nnicrons.
2, rior Art
The size and shape of grains in as-cast alun'tzt~.um alloy impacts
properties of wrought or cast products. Casting with large grains,
particularly
dendritic graitxs, is highly prone to cracking during casting and reduce
ductility,
fracture toughness and fatigue properties. Reduction of the size as well as
the
form of the grains may be accomplished by mechanical or electromagnetic
stirring
to break up the grains. Grain size may also be controlled metallurgically by
adding a grain refiner to the aluminum alloy melt. A typical grain refiner
used for
aluminum alloys is either an Al-Ti-13 alloy or an Al-~'i-C alloy in the form
of a rod
or waffle. A grain refiner consists of nwnerous fine boridc or carbide
patrticles in
an aluminum matrix. When these grain refiners are added to tle aluminum alloy
melt, the boride of carbide particles are dispersed into the melt and serve as
nucleating sites for grains during solidification. Commercially available
grain
refiners include alloys containing about 3-5 wt, % Ti and about 0.15-1 wt. % B
or
CA 02361484 2001-11-08
C and the balance AI. According; to ibis practice, a residual amount of Ti is
present in or is added to the alutninutn melt (e.g. less than O.OlSwt. %), and
a
e~ntrolled amount of the grain refiner is added thereto which increases the
total 'Ci
concentration in the ftnal melt by about 0.001-0.003 wt. %. In this manner,
the
amount of B or C added to the melt via the grai,z~. refiner is about 0,0001-
0.001
wt. %. The addition of gram refiners at these conventional levels can control
the
size of dendtitic grains to be about 250-1000 microns, ror certain cast or
wrought
aluminum products, such a grain structure is suffteaently fine and cracking or
other
mechanical problems are not experienced.
Howcvct, a need remains for as-cast alumhaum alloys with an ultra-
fine grain structure, i.c. about 200 microns in size or less. It has been
found that
ultra-fine grain size tnay be achieved by supersaturating a molten alloy with
dispetsoid-forming elements such as Zr, Mn, Cr, V, Ti, Sc and T~f as disclosed
iii
If.S. fatertt No. 6,004,506. That process requires the addition of a
specialized,
pre-alloyed ribbon of material containing the dispersoid-fornctiztg elements
into a
pool of molten metal formed dutixig ingot castinb. Accordingly, a nerd remains
for a method of producing aluminum alloys with ultra-f;.ne grain structure
using
readily available additives.
This need is met by the method ofthe prESCnt invention for
producing fine grain aluminum and the as-cast aluminum alloy prepared thereby.
The inventive method includes the steps of a) providing a iuolton aluminum
alloy
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CA 02361484 2001-11-08
including an alloying element selected from the group consisting of Ti, Sc,
fir, V,
11F, Nb and Y; b) adding a grain refiner to the molten aluminum alloy to form
a
melt, wherein the grain refiner comprises (i) Ti and (ii) B or C, such that
the
concentration in the melt of B or C after addition of grai~~ refiner is about
0.003-
0.010 w~. %; and c) solidifying the melt to foam an ingot. Preferably, the
alloying
clement is Ti at a concentl-ation of about 0.015-0.030 wt. % in the molten
almninum alloy or Sc at a concentration of about 0.030-0.10 wt. % in the
molten
aluminum alloy. The alloy may be a wrought alloy of the 1XXX, 2XXX, 3XX?C,
SXXX, 6XXX, 7~:XX or 8XXX Aluminum Associafiion (AA) series, preferably
an alloy of the ZXXX or 7XXX AA series or a casting alloy such as a 2XX, 3XX,
d.XX, SXX, 7XX or 8XX series alloy. Particularly prefeiTed wrought alloys u-e
7055 and 7050 alloys. The grains in the ingot formed according to the present
invention are sized about Z00 microns or less, preferably 100 microns or less.
Other features of the present invention will be further described in
tl~e following related description of the preferred embodiment which is to be
considered together with the accompanying drawings wherein:
Fig. 1 is a photomicrograph of 7055 alloy produced according to the
present invention; and
Fig. 2 is photonuerograph of 7055 alloy produced according to the
prior art.
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For purposes of the description hereinafter, it is to be understood that
the invention may assume a number of alternative variations and step
sequences,
except where expressly specifted to the contrary. (t is also to be understood
that
the specific devices and processes illustrated in the attached drawings, and
described in the following specification, axe simply exemplary embodiments of
the
invention. IIence, specific dimensions aTad other physical eh~.racteristics
related to
the embodiments disclosed herein are not to be considered as limitinb.
The present invention includes a method of controlling the grain sine
in cast aluminum alloys to about 200 microns or less. According to the
inventive
mcfihod, an alloying clement is added to a molten aluminum alloy. Prefen-ed
residual alloying elements arc Ti, Sc, Zr, V, Hf, Nb and Y, more preferably Ti
and
Sc. A prefezTed conccnixation of the alloying element Ti in the molten
aluminuzxz
alloy is about 0.015-0.030 wt. %. When the alloying element is Sc, a preferred
concenlrt~tion of residual Sc in the molten aluminum alloy is about 0.030-0.1
wt.
%. 'W'hen~the alloying element is Zr, V, Hf, Nb or Y, the preferred
concentration
thereof is on the order of the preferred co~acentrations of Ti and Sc.
A grain refiner is added to the molten aluminum alloy containing the
residual alloying element to form a melt. Commercially available grain,
refiners
are alloys consisting ofTi and 13 or C with the balance Zluminutn. Typical
concentrations in the brain refiner are about 3-5 wt. % Ti and about 0.15-I
wt.
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B or C. The final concentration of Q or C in the melt from the grain refiner
is
about 0.003-0.010 wt. %.
Suitable comruercially available grain refiners have compositions
such as 3 wt. %'1'i, 1 wt. % B and balance A1 (referred to as A1~3%'ri-1%B),
A1-
5%Ti-1%B, A1 ~%Ti-0.2%8, Al-5%Ti-0.2%B, AI-3%Ti-0.15%C, or AI-3%Ti-
0.3%C. 'These gcaila refiners typically are provided in the fonn of a rod or
waffle.
The ratio of 13 oa C to Ti in the grain refiner is aomnally f xed; hence, the
amount
of grain refiner added to the melt controls the final amount of Ti present in
the
melt. For example, in order to aclueve a concentration of B of about 0.003-
0.010
wt. % in 100 pounds of a melt contai~x~ing 0.02 wt. % Ti, about 0.003-0.010
pounds
of 13 are needed nn the melt. When a lain refiner consisting of Al-3%Ti-1%B is
used, about 0.3-1 pound of grain refiner arc added to the melt. 'this results
in an
additional about 0.009-0.030 wt. % Ti added to the melt from the grain refiner
fox-
a total concentzation of Ti in the melt of about 0.029-0.050 wt. %.
Conventional gxaan refining practice dictates using less than 0.001
wt. % 13 in tl~c melt which is 3 to 10 times less grain refiner than is added
according to the present invention. Likewise, the incremental concentration of
Ti
from the grain refiner for conventional practice is 3 to 10 times less than
tl~e
amotmt of Ti added to the melt by the grain refiner according to the present
invcotion.
The present invention may be used to control grain size in wrought
and cast alloys. Suitable alloys include Aluminum Association (AA) wrought
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alloys of Ihc 1 XXX, 2XXX, 3XXX, SXXX, 6XXX, 7XXX and 8XXX series attd
cast alloys of the AA ZXX, 3 XX, 4XX, SXX, 7XX aad 8XX series, Alloys of the
AA 2XXX and 7x;XX series are patrticulzrly suited to treatment according to
the
present invention.
As-cast aluminum alloy produced according to the present invention
has globular grains which are about 200 microns or less in size, typically
about 80
microns in size. 1n contt~ast, conventional ,grain refining practice of adding
a grain
refiner of Al-3%Ti-1%B such that the concentration of B in the melt is 0.001
wt.
produces dendritic grains sized about 1000 microns. These large dendritic
grains interlock with each other and render the cast alloy a-igid and prone to
cracking, whereas the small, globular grains formed by the method of the
present
ilivention reduce crack irlitxation during casting and improve formability
during
deform ation.
Although the invention has been described generally above, the
particular example gives additional illustration o.f the product and process
steps
typical of the present invention.
Irxam,~le
50 Pounds (lbs) of aluminum alloy 7055 was melted, and 0.0154 lb
of 97% Ti powder compact was added to the molten alloy to achieve a
concentration of about 0.03 wt. % Ti. A grain refiner of Al-3%Ti-1%B (0.335
lbs)
was added to the melt at 1300° F. The melt was stirred for 1 minute
after the
addition of the grain refiner. The final concentration of B in the melt was
about
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0.0066 wt. %, and the find concentration of 1'i in the ztnelt was O,OS wt. %
(0.03
wt. % residual Ti, plus 0.02 wt, % Ti added from the grain refiner.) The mclt
was
cast into an ingot. 'fhe microshucture of the ingot is shown in rig. 1. The
bright
areas of globular grains are cleanly seen and are less than about 100 microns
in
size.
SO founds of aluminuzn alloy 7055 was melted. I~ grain refiner of
A1,3%Ti-1%!i was added to the melt at 1300° F according to
conventional
commercial practice. The melt was stirred for 1 minute after the addition of
the
grain refiner. The melt was cast into an ingot. The microstzucture of the
ingot is
Shown fn Fig. 2. The bright areas of dendritic grains are clearly seen and are
up to
about 1000 microns in sine.
It will be readily appreciated by those skilled in the art that
modifications may be made to the invention without depai*ing from the concepts
disclosed iit the foregoing description. Such ttaodiCcations are to be
considered as
included within the following claims unless the claims, by their language,
expressly state otherwise. Accordingly, the pa~*icular embodiments described
in .
detail herein are illustrative only and we not limiting to the scope of the
invention
which is to be given the full breadth of the appended clairu.s and any and all
equivalents thereof.
