Note: Descriptions are shown in the official language in which they were submitted.
1~1'3~
~ his i~vention relates to the production of metallic
articles.
It i8 known that within limited temperature ranges and
at limited strain rates certain alloys may be processed to
give a very fine grain structure and thereafter be capable
of deforming superplastically. Providing that the
processed structure is sufficiently fine these alloys then
exhibit abnormally high plasticity under relatively low
loads when compared with the same alloy~ that do not posses~
extremely fine grain sizes. It is also known that the
phenomenon of superplastic deformation ma~ be employed to
enable the relatively cheap manufacture of articles from metal
blanks which have been processed to have extremel~ fine grain
sizes.
It is an object of this i~vention to provide a means of
forming metallie articles from certain metallic blanks which
have not been processed to possess extremely fine grain sizes.
According to one aspect of the present invention there
is provided a method of prod~cing simultaneously a fine
recrystallised grain ætructure in a metallic alloy having a
composition suitable for superplastic deformation but having
a grain structure which precludes such deformation and of
forming an axticle from said allo~ by s~perplastic
deformation comprising raising a blank of the alloy to a
forming temperature, applying a force to the blank at said
temperature to deform the blank non-superplastically and
induce dynamic strain recrystallisation and continuing the
application of said force 80 that said fine recrystallised
grain structure is progressivel~ developed and the partly
formed blank iB superplastically deformed to form the article.
So far aB predominantly aluminium alloys are concerned,
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1 as exemplified for example by those disclosed in our earlier
Canadian Patent 1,006,014 and Canadian application 190,403,
it had been believed that the basic alloy as cast and subse-
quently mechanically worked would need additional heat treatment
to form a sufficiently fine grain structure to achieve super-
plasticity. However, it has now been found that metallic blanks
rolled from suitable aluminium alloys may be formed into compo-
nents without the necessity for a blank conditioning stage.
In this specification all percentages are by weight.
According therefore to another aspect of the present
invention, there is provided a method of producing simultaneously
a fine recrystallised grain structure in an aluminium alloy and
of forming an article from said alloy by superplastic deformation
comprising raising a blank of the alloy to a forming temperature,
applying a force to the blank at said temperature to deform the
blank non-superplastically and induce dynamic strain recrystalli-
sation and continuing the application of said force so that said
fine recrystallised grain structure is progressively developed
and the partly formed blank is superplastically deformed to form
the article, said alloy being predominantly aluminium of a
substantially single phase solid solution and which includes
one or more elements selected from one or more of the
: following Cu, Zn, Mg, Mn, Si, Li and Fe to encourage recry-
stallisation and at least one of the elements Zr, Nb, Ta and
Ni in an amount of at least 0.25% substantially all of
which is present in solid solution to inhibit grain
coursening, the total amount of the latter elements not
exceeding 1~. The forming temperature is preferably in the
range 380C to 580C.
It has previously been believed that, because the
fi. - 3 -
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,. ,-~
1 stacking fault energy of aluminium is high, it would not be
possible to obtain dynamic recrystallisation (i.e. recrystalli-
sation simultaneously with hot deformation) in aluminium and
its alloys. We have found that the addition of elements, such
as copper or zinc or zinc and magnesium does enable dynamic
recrystallisation to occur. Additionally, by casting the alloy
in such a way that the cast ingot is supersaturated with not
less than 0.25% Zr ~or Nb, Ni or Ta) substantially the whole
of which is in solid solution it is possible to produce during
subsequent processing a dispersion of very fine particles of
ZrA13 which restrict the growth of newly formed grains. When
a heavily cold worked sheet of an Al-lO~Zn-0.5%Zr alloy is raised
to the superplastic deformation temperature and held at that
temperature without deformation it will eventually recrystallise
to a coarse non-uniform grain size. However, if an identical
alloy sheet is raised to the same temperature and subjected to a
mechanical force to deform the sheet non-superplastically a
fine recrystallised grain structure will progressivley develop
over about the first 200% strain so that superplastic deformation
then occurs. During the commercial manufacture of, for example,
the alloys described in our Canadian Patent 1,006,014 and
Canadian application 190,403 the semi-finished product would
generally be rolled sheet the structure of which consists
of a heavily cold worked matrix containing a dispersion
of very fine particles of ZrA13 derived from the zirconium
supersaturation of the cast ingot during subsequent
processing. Some other precipitates may also be present.
We have discovered that when the sheet is heated to the
superplastic forming temperature some recovery and recry-
stallisation occurs but it is only during the application
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1 of a mechanical strain that dynamic recrystallisation to a
fine grain size takes place and this enables superplastic
deformation to occur.
In our Canadian Patent 1,006,014 and Canadian application
190,403 we have disclosed particularly suitable alloys which
in their broadest form are:-
1. A superplastically deformable aluminium-base alloy consisting
of an aluminium-base alloy selected from nonheat treatable
alluminium-base alloys containing at least 5% Mg or at least
. 10 1~ Zn and heat-treatable aluminium-base alloys containing
one or more of the elements Cu, Mg, Zn, Si, Li and Mn in
known combinations and quantities, and at least one of
the elements Zr, Nb, Ta and Ni in a total amount of at least
0.30% substantially all of which is present in solid solution,
said total amount not exceeding 0.80%, the remainder being
normal impurities and incidental elements known to be incor-
porated in the said aluminium-base alloys.
2. A superplastically deformable aluminium base alloy con-
sisting of a non-heat treatable base material selected
from the group consisting of:
1. Aluminium of normal commercial purity;
: 2. Aluminium of 0.75 to 2.5% manganese;
3. Aluminium and 0.25 to 0.75% manganese; and
4. Aluminium and 1 to 4% magnesium;
together with dynamic recrystallisation modifying additives
for these materials to achieve fine structure respectively
consisting of:
1. 0.4% to 2% iron and 0.4% to 2% silicon;
2. 0.4% to 1% iron
3. nil;
4. 0.25% to 0.75% manganese;
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and at least one of the elements Zr, Nb, Ta and Ni in an
amount of at least 0.3% substantially all of which i8
present in solid solution, the total amount of said
elements not exceeding 1% and the remainder being
normal impurities and known incidental elements.
3. We have al~o found that it is possible to obtain good
results with alloys containing only 0.25% Zr, provided
the zirconium is virtually all in solid solution in the
cast block, as may be ensured by cooling the liquid metal
quickly from the alloying temperature to the freezing
point and solidifying it rapidly.
The invention also extends to articles produced by the
above methods.
Preferably for aluminium-copper-zirconium alloys and
for aluminium-copper-magnesium-zirconium alloy~ the
temperature range should be 430C-50QC. ~or alloys of
aluminium with zinc magnesium and zirconium the forming
temperature should be in the range 470C-580C whereas for
alloys of aluminium, zinc, magnesium, copper a~d zirconium
the preferred forming temperature range is 430 -500C. The
elem-ent-s Nb,~~a~or ~ m-ay be added-in place of Zr ln the-above
alloys.
When the rate of forming is too fast dynamic
recrystallisation does not occur and the blank will fail after
relatively low strains. ~hu~ when an Al-10/OZn-0.5%Zr alloy
was deformed at a stra~h rate of 3.4 x 10~2sec 1 at 580C an
elongation of only 16CPh was obtained and the structure was
largely unrecrystallised. ~he same alloy recrystallised
simultaneously with deformation gave an elongation of 690% at
580C when deformed at a strain rate of 4.2 x 10 3aec 1.
Alternativel~ at very low strain rates greater
1~3 ~
deformatio~ is possible without failure but the forming method
may then be too slow to be feasible commercially. Preferably
the strain rate is not greater than 5 x 10 2sec 1 and with
advantage not greater than 5 x lO~~sec~l. ~he table
illustrates the influence of strain rate on ductilit~ for an
Al-6/~u-0.5%Zr alloy. ~he ductility results are from uniaxial
tensile te~ts performed with a constant cross head velocity at
a temperature of 450C.
Cross headI Corresponding initial
velocity strain rate Elongation
, - _
0.1 in/min.3.4 x 10~3sec~l 985%
0.2 in/min.6.7 x 10~3sec~l 6~5%
0.5 in/min.1.7 x 10~2sec~l 413%
1.0 in/min.3,4 x 1o~2seC~1 273%
When the strain rate remains constant but the forming
temperature is increased the elongation in a tensile test
(which is equivalent to forming capacity in a component
manufacturing operation) increases to a maximum value and
then decreases from that value. At the lower temperatures
complete d~namic recrystallisation does not occur, while at
the optimum temperature the specimens r0crystallise
dynamically to a fine grain size. At temperatures above the
optimum temperature elongation decreases again because some
grain coarsening occurs at the higher temperature. ~his
effect is illustrated for the Al-6/~u-0.5%Zr alloy in the
following table.
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Deformation Elongation (%) at
~emperature constant cross head
(C) velocity of 0.1 in/min.
440 300
460 1100
480 107~
500 650
. ., . __ . ...
Increasing the rate of deformation will increase the
stress necessary to cause deformation so that greater
pressures will be necessary to form a co~ponent more rapidly.
Alternatively, the temperature of deformation may be
increased in order to reduce forming times or pressure when
- forming shallow components but the ductility may then be
reduced. ~hus shallow articles may be formed from the
Al-6/~u-0.5%Zr alloy at about 500C while deeper articles may
be formed at lower temperature~ of the order of 450C-480C.
Forming pressures for sheet 0.060in. thick would generally be
less than 60 p.æ.i. although to reproduce fine detail in a
reasonable time the preæsure may be increased up to 120 p.8~i.
~he following table illustrates the increase in flow stre~s
accompan~ing increase in strain rate for the Al-6%Cu-0.5%Zr
alloy at temperatures of 460C and 500C.
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~est Initial strain Strain rate Flow stress
~&m~. rate E (per sec.)sensitivityo~ MN/metres2
. .
460 5 x 10-4 0.36 5.20
1 x 10-3 0.42 7.40
2 x 10-3 0.45 11~00
5 x 10-3 0.40 18.00
1 x 1o~2 0.32 25.00
500 5x 10-4 0.44 3.30
` 1 x 10-3 0.49 5,00
2 x 10-3 0.50 8.20
5 x 10-3 0.42 14.00
1 x 10-2 0.33 20.00
The initial grain size in the startin~ blank may be as
coarse as 300~u although this size varies according to the
production histo~y of the blank. During deformation this
grain structure is trans~brmed by dynamic recr~stallisation
and will generally be less than about 15Ju when recrystall-
isation i8 completed. In the Al-6/dCu-0.5%Zr alIoy the
cr~stallised grain size ma~ be les~ than 5JU.
~his invention would apply to the forming of an article
by causing the blank to flow into a female mould by the
application of pressure or equally to the production of an
article b~ the application of pressure to make the blank form
o~er a male mould.
In one example a cup-like article having a diameter of
5~n~ and a depth of 2~" was formed from Al-6y~u-0.5%Zr sheet
of starting thickness 0.98 mms. ~he article had a final
thicknes~ of about 0.33 mmæ and was formed from a circular
blank of 10" diameter by blowi~g into a female mould with a
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pressure of 20 p.s.i. The average start rate was about
2 x 10 3sec 1 with a startin~ grain size in the blank of
350JU and a final grain size in the article of about 3~u.
The total moulding time was approximatel~ four minutes.
It will be understood that depending upon the thicknes~
and composition of the alloy sheet and the size and shape of
the article to be moulded, the moulding time will vary
considerabl~. It may, for example, be as low as 30 seconds
up to 10 minutes.
With aluminium allo~s containing less than 0.30%Zr it is
desirable that in the original casting operation the liquid
metal should be cooled quickly from the alloying temperature
employed to the freezing point of the allo~ to achie~e rapid
solidification. ~or example, with an aluminium alloy
containing 0.26yOZr, 0.03yOFe < 0.01~Si and 6.0~u, a total
residence time in the liquid metal sump during the casting
operation of about 0.7 minutes provides an alloy capable Or
superplastic elon~ation of 93C~/o. ~his residence time of less
than 1 minute compares with a time of about 2 minutes for the
alloys previousl~ discussed.
Although predominantly aluminium alloys have been
discussed above, it is also believed that superplastic
properties may be exhibited by alloys which are predominantl~
Or copper, nickel, zinc and magnesium with generally similar
alloying constituents, such constituents being so selected as
to promote the occurrence of dynamic strain recrystallisation
when subjected to hot deformation at strain rates appropriate
to superplastic forming operations.
While this description has mainly considered the for-
3D mation of articles from a semi-finished sheet product the
invention would al80 apply to the manufacture of an article
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by a slow forging operation starting from a rolled or
extruded bar or even cast metal.
