CA1039158A - Production of metallic articles - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
Alloys having a composition suitable for super-plastic deformation ussually require heat treatment after casting and mechanical working in order to produce in the alloy the necessary fineness of grain structure to permit such deformation to occur. It has now been found that some such alloys including in particular ranged of aluminium alloys containing zirconium (or Nb, Ta or Ni) may be heated to a superplastic forming temperature and non-superylastically deformed at that temperature to induce dynamic recrystal-lisation and simultaneously produce a fine recrystallised grain structure and superplastic deformation.

Description

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 -,- -., ,. ,-~

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 1~3~
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.

Deformation Elongation (%) at ~emperature constant cross head (C) velocity of 0.1 in/min.

480 107~

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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 _ g _ 3~
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 1 ~3 ~

by a slow forging operation starting from a rolled or extruded bar or even cast metal.

Claims (18)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method of producing simultaneously a fine recrystallised grain structure in a metallic alloy having a composition suitable for superplastic deformation but having a grain structure which precludes such deformation 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 recrystallisation and continuing the application of said force so that said fine recrystal-lised grain structure is progressively developed and the partly formed blank is superplastically deformed to form the article.

2. A method of producing simultaneously a fine recry-stallised grain structure in an aluminium alloy and of forming an article from said alloy by superplastic defor-mation 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 so that said fine recrystallised grain structure is progressivley 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 Cu, Zn, Mg, Mn, Si, Li and Fe to encourage recrystallisation 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 coarsening, the total amount of the latter elements not exceeding 1%.

3. A method according to claim 2, in which the forming temperature is in the range 380°C to 580°C.

4. A method according to claim 2 in which the blank is of an aluminium-base alloy selected from non-heat treatable aluminium-base alloys containing at least 5% Mg or at least 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 alloy.

5. A method according to claim 2 in which the blank is of a non-heat treatable base material selected from the group consisting of:
a. Aluminium of normal commercial purity;
b. Aluminium of 0.75 to 2.5% manganese;
c. Aluminium and 0.25 to 0.75% manganese; and d. Aluminium and 1 to 4% manganese; 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;
and at least one of the elements Zr, Nb, Ta and Ni in an amount of at least 0.3% substantially all of which is present in solid solution, the total amount of said elements not exceeding 1% and the remainder being normal impurities and known incidental elements.

6. A method according to claim 2 in which the blank contains less than 0.30%Zr and in which the casting from which the blank is formed has been cooled quickly from the alloying temperature to freezing point and solidified rapidly.

7. A method according to claim 6 in which the cooling time is less than one minute.

8. A method according to claim 7 in which the cooling time is no greater than 0.7 minutes.

9. A method according to claim 2 in which for blanks of alloys of aluminium, copper and one of the elements selected from Zr, Nb, Ta or Ni and for such alloys addition-ally inlcuding magnesium the forming temperature range is 430°C to 500°C.

10. A method according to claim 2 in which for blanks of alloys of aluminium, zinc, magnesium and one of the elements selected from Zr, Nb, Ta or Ni the forming tem-perature range is 472°C and 580°C.

11. A method according to claim 2 in which for blanks of alloys of aluminium, zinc, magnesium, copper and one of the elements selected from Zr, Nb, Ta or Ni the forming temperature range is 430°C to 500°C.

12. A method according to claim 2 in which the initial strain rate of deformation is between 5 X 10-2sec-1 and 5 X 10-4sec-1.

13. A method according to claim 12 in which the initial strain rate is not greater than 5 X 10-2sec-1.

14. A method according to claim 12 in which the initial strain rate is not greater than 5 X 10-3sec-1.

15. A method according to claim 2 in which the grain size of the formed article is less than 15µ.

16. A method according to claim 15 in which the grain size of the formed article is less than 5 µ.

17. A method according to claim 15 in which the grain size of the blank is at least 300 µ.

18. A method according to claim 2 in which the pressure applied to the blank is within the range 20 p.s.i. to 120 p.s.i.