PROGRESS TO ALUMINIUM-LITHIUM SEMI-FABRICATED PRODUCTS

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PROGRESS TO ALUMINIUM-LITHIUM SEMI-FABRICATED PRODUCTS

R. Grimes, T. Davis, H. Saxty, J. Fearon

To cite this version:

R. Grimes, T. Davis, H. Saxty, J. Fearon. PROGRESS TO ALUMINIUM-LITHIUM SEMI- FABRICATED PRODUCTS. Journal de Physique Colloques, 1987, 48 (C3), pp.C3-11-C3-24.

�10.1051/jphyscol:1987302�. �jpa-00226525�

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Colloque C3, supplement au n09, Tome 48, septembre 1987

PROGRESS TO ALUMINIUM-LITHIUM SEMI-FABRICATED PRODUCTS

R. GRIMES, T

.

.

British Alcan Aluminium plc, c/o Alcan International Limited, Southam Road, Banbury, GB-Oxon OX16 7SP, Great-Britain

* ~ l c a n Plate Limited, P.O. BOX 383, Kitts Green, GB-Birmingham B33 9QR, Great-Britain

* * ~ r i t i s h Alcan Sheet Limited, David's Loan.

GB-Falkirk F K 2 7 X T , Scotland, Great-Britain

* * *

Alcan High Duty Extrusions Limited, Lillyhall, GB-Workington CAI4 4JY, Cumbria, Great-Britain

Abstract

British Alcan Aluminium plc installed a small scale melting and casting facility for aluminium-lithium based alloys at the Alcan Plate factory, Rirmingham, England that was completed in early 1985. Since May 1985 the Lital alloys have been ir. regular production in the form of sheet, plate, extrusion, tube and forging stock. During the two years of production, attention has been paid to optimisation of all aspects of the manufacture of the semi-fabricated products and considerable improvements in properties have been achieved. This paper outlines the progress and compares current typical properties with guaranteed minima and the properties of the established aerospace alloys.

Good damage tolerant properties have been achieved with Lital C (8090) in a lightly aged condition. In sheet form toughness is slightly better than that of 2024 T3 at the same strength level and the corrosion/stress corrosion behaviour of the Lital alloy is superior to that of 2024. In plate and extruded forms strength properties match those for 2024 while toughness in the rolling plane is superior. Though thickness toughness matches that of 2024 while fatigue crack growth rate is less than for 2024.

In the medium strength Lital A (8090) condition constderably improved strengths and stren,qth/toughness relationships have been achieved by optimisation of stretching and final ageing practices. Strengths attainable are close to the targets originally set for the high strength Lital B (8091).

Plate up to 100 mm thick has been produced exhibiting good ductility and fracture toughness in the short transverse direction. In sheet form the medium strength alloy is unrecrystallised and, therefore, exhibits some anisotropy of properties but strength targets are being achieved.

Demand for the 8090 alloy has reduced the development time available for the high strength Lital B (8091) alloy. So far only limited production of Lital B has occurred at Kitts Green but very good strength properties have been achieved. Using optimised stretching and ageing practices, it appears that very high strength levels can be achieved in thin 8091 plate, although this is by sacrifice of the through thickness properties.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987302

C3-12 JOURNAL DE PHYSIQUE

Introduction

Development of the current generation of aluminium-lithium based alloys began in the United Kingdom under UK Ministry of Defence sponsorship in about 1970. Alcan's involvement began in 1977. By 1984 the aluminium-lithium based alloys, now registered as 8090 and 8091, had been developed in the laboratory and sufficient confidence had been gained in their properties to justify the factory installation of an aluminium-lithium melting and casting facility at the Alcan Plate factory in Birmingham. This unit has been in operation now for rather more than two years, initially working on a single shift basis but,

~ i n c e Aprtl 1 9 8 6 , operating on two shjfts. Despite this iccreose in oroduction activity it is not, currently, proving possible to keep up with demand for the product.

The alloys development targets, in the early stages, were to produce three materials that should match the properties of established damage tolerant, medium strength and high strength aluminium alloys while being 10%

lighter and 10% stiffer. Each of the new materials was to be available in all the usual semi-fabicated product forms. Reasonable success in reaching these initial targets has been achieved, so that the alloys have now been used in a number of aircraft and space programmes. However, during the course of the development the targets have, inevitably, changed and become more demanding.

These changing targets have provided the impetus to extract improved properties from the new alloys by optimisation of all aspects of semi-fabrication, so that it has now been demonstrated that material can be produced with properties considerably superior to the initial targets. Additionally, sufficient data have now been generated on products manufactured entirely within the commercial production system of British Alcan to define, with a very few exceptions, guaranteed property minima against which Alcan is prepared to supply the products.

In this paper, the properties that are currently being achieved in production material are summarised and compared with the guaranteed property minima. Scientific explanations for the results/improvements are not included as they are given in the papers by Miller et a1 ( 1 , 2 ) , Reynolds et a1 ( 3 ) , White et a1 (4) and Gray (5). Some description of the superplastic versions of

the alloys is given in the paper by Grimes et a1 (6).

Early Production Material

Figure 1 shows the typical properties obtained from 8090 plate produced during the first 12-18 months of the production development. The properties are very similar to those produced in plate rolled from laboratory cast ingot and, in strength properties and toughness in the rolling plane, gave no cause for concern. However, the fracture toughness in the short transverse direction was less than many customers were seeking and this led to a major programme of optimisation of all aspects of thermal/mechanical processing. In turn this has led to the introduction of the series of new tempers indtcated in Table 1. The temper designations are tentative at present and may well be changed in the light of discussions with the specifying authorities. In the meantime, however, the high strength plate variants designated T 8771 should be noted.

These tempera employ the well established technique (7) of introducing larger cold strains between solution treating and ageing to improve the strength/toughness relationship. Ageing conditions are also modified. The following sections briefly indicate the property situation for each of the product forms.

Table 1 : Tentative temper designations for the Lital alloys in various product forms

Plate

Figures 2(a) and 2(b) plot, respectively, the guaranteed minimum values for 0.2% PS and TS against plate thickness for the aluminium-lithium alloys and the baseline alloys. It can be seen that the high strength versions of the 8090 and 8091 alloys are very close to their targets, although the 8090 T 8771 falls slightly short of 8091 T 651. In the damage tolerant condition the 8090 is stronger than the baseline alloy but here, of course, it is toughness and fatigue performance that is most important. Guaranteed minima minimum T-L fracture toughness values are compared with the baseline alloys in figure 3.

Damage tolerant Medium strength 2XXX

replacement High strength 7XXX

replacement

In reality it is much more important to compare guaranteed minima with typical values. Table 2 makes the comparison for 8090 T 8151 damage tolerant plate and shows that all the typical values comfortably exceed the guaranteed minlma. The toughness values for the 8090 T 8151 plate are compared with those for 2024 in Figure 4. The 8090 is marginally better than the 2024 in the ST-L orientation and considerably superior in the rolling plane.

Plate 8090-T8151 8090-T8251 8090-T8771 8091-T851 8091-T8771

Table 2 : Typical tensile and fracture toughness values, with guaranteed minima for 8090-T8151 damage tolerant plate, 38-65mm thick.

Extrusion9 1 Tubes 8090-T81551 8090-T82551

8091-T8551 Sheet

8090-T81 8@90-T6/8

8091 -T6/8

It should be noted that the above values were all obtained on plate given a typical stretch of 2-3% whereas, in the higher strength tempers, the henefits of larger stretches as part of a route to improve the strengthftoughness relationship has been clearly demonstrated. Although the results from the 8090 T 8151 plate achieve all the target requirements, the question is inevitably raised as to whether further improvement could be obtained by the use of larger stretches in plate aged to a damage tolerant condition. While there was little doubt that a better strength/toughness relationship could be achieved from higher stretch material there were some concerns about the influence on fatigue

Forgings 8090-T6152 8090-T652

8091-2'652

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crack growth rate and Bauschinger effects. At present only very limited data are available, but assessment is continuing. In the initial test a 50mm plate was stretched by 2-3% and sampled before further stretching to 7% and sampltng again. The two samples were then given ageing treatments designed to produce the same damage tolerant strength before conducting fatigue crack growth rate tests. Figure 5 shows that the normally stretched material had a superior fatigue crack growth performance compared with 2024 but that the 7% stretched material was, in turn, superior to the lower stretch material. Table 3 demonstrates that Bauschinger effects, also, were not a problem with the higher stretch material.

Table 3 : Influence of prior stretching on the tensile and compressive proof stress of 8090 damage tolerant plate

The above results were achieved very recently and corresponding toughness values have not yet been determined. However, it is known that, at the same strength level, the higher stretch material is likely to be somewhat tougher and there is, therefore, a high probability that plate given a larger stretch will possess an overall combination of superior damage tolerant properties.

In earlier phases of the current development, doubts have been expressed about the ability to achieve acceptable levels of ductility and toughness through the thickness of thicker section products. Figure 6 shows averaged properties from three lOOmm thick 8090 T 8251 plates. Stretching procedures for these plates were the same as for the earlier 8090 T 651 temper but ageing practices were modified. Not only were reasonable strength levels achieved but the good short transverse ductility and fracture toughness values dispel earlier concerns about "inherent" short transverse property limitations.

The typical properties now being achieved in the higher strength T 8771 version of 8090 are compared with guaranteed mimima in Table 4. Clearly, if further experience confirms the very high values for toughness in the rolling plane some upward revlsion of the guaranteed minima will be required. Figure 7 demonstrates that in this temper the properties of the 8090 can exceed the minima for 7010 T 7651 although here, of course, the typical values for the lithium containing alloy are being compared with minimum values for the 7010.

Table 5, comparing the tensfle and compressive proof stresses for the higher strength verions of 8090 demonstrates that, as with the damage tolerant plate, high stretches should not lead to problems from the Bauschinger effect.

Table 4 : Typical tensile and fracture toughness values, with guaranteed minima (G.min.) for 8090-T8771 medium strength plate, 38-65mm thick

Table 5 : Influence of prior stretching on the tensile and compressive proof stress of 8090 in the near to peak aged temper

8091 T 8771 provides the highest strength capability of the Alcan aluminium-lithium based alloys. Production experience with this alloy is much more limited than for the 8090 so very few data for 8091 are included in this paper. Figure 8, however, demonstrates that when stretch.ing and ageing practices are optimised, properties that approach those for 7150 can be achieved. It must be appreciated that through thickness properties would be sacrificed at this very high level of strength so, in this condition, only relatively thin plate would be considered viable.

Sheet

-

Aluminium-lithium based alloys containing zirconium do not, in general, recrystallise readily. Perhaps for this reason, it was not immediately realised that for optimum toughness in the sheet product a fine recrystallised structure is required. Development has also been involved in defining a manufacturing route that consistently and readily produces the required grain structure. However, having defined the manufacturing route, the damage tolerant properties are very good. Figure 9 shows that, in the damage tolerant temper and with the fine recrystallised grain structure, the strength properties are remarkably isotropic. The strength in the direction of stretching is marginally higher than in the other directions but, otherwise, there is little variation relative to the rolling direction in tensile strength or 0.2% proof stress. Figure 10, comparing the fracture toughness of 8090 with non-recrystalltsed and recrystallised grain structures with 2024 and 2091 demonstrates, again, the fallacy of the claim for brittleness in the aluminium- lithium based alloys. Figure 11 demonstrates that in terms of fatigue crack growth rate, also, the 8090 T 81 is superior to 2024 T3. Indeed, it can be argued that 8090 T81 has superior damage tolerant properties to 2024 on an

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absolute basis, before taking any account of the fact that the 8090 is 10% less dense.

While use of a fully recrystallised grain structure gives a product with very good toughness it also reduces the strength potential of the material.

Thus for the higher strength tempers (8090 T6 and T8), the retention of a non- recrystallised grain structure is necessary and more anisotropy is then inevitable. Consistent properties are now being achieved and these are compared with guaranteed minima in Table 6. Of course, because of the lazk of stretch, the T6-T8 differences persist. Fatigue crack growth performance of this is somewhat better than that of 2014 T6 other than at high values of AK (Figure 12).

Table 6 : Typical tensfle properties wfth guaranteed minima for 8090-T6/8 medium strength sheet

Extrusion

Unlike plate, larger stretches are not being employed for the extruded product because it is felt that this would upset the balance of properties between the L and LT directions and, in any case, high strength properties can' be achieved relatively easily in this product form.

Table 7 compares the typical strength and toughness properties for 8090 extrusions in both the damage tolerant (T 81551) and medium strength (T 82551) tempers.

Table 7 : Typical tensfle property and fracture toughness values for 25mm x lOOmm extruded sections compared with guaranteed minima

(Damage tolerant)

(Medlum strength)

Forgings

Alcan does not manufacture forgings, so that the properties shown in Table 8 as guaranteed minima are not correctly described. However, the properties are those that, on the basis of a fairly large number of forgings produced from Alcan billet it is believed can be achieved in 8090 hand forgings that are cold compressed between solution treatment and ageing. With die forgings, of course, cold compression is more difficult and sufficient experience does not yet exist to indicate typical properties or guarantee minima.

Table 8 : Typical tensile and fracture toughness properties of cold compressed hand forgings for< lOOmm section thickness 8090-T652 medium strength alloy with guaranteed mimima

Production S t r a t e a Test

Direction L

L-T S-T

As indicated at the beginning of this paper it is not currently proving possible to keep up with the demand for aluminium-lithium alloys, despite working the Kitts Green unit on a two shift basis. When the unit was first installed it was estimated that it would have the capability of approximately 1000 cast tonnes per annum on a single shift basis and 4000 cast tonnes operated continuously. In practice, two years of operational experience has led to the belief that these estimates were slightly optimistic as it has not proved possible to produce 1000 tonnes on a single shift basis.

To improve the situation a second melting furnace is to be installed during 1987 (coming into operation at the end of the year) and three shift operation is also planned for introduction during 1987. The new melting furnace will not have a large impact upon output because it is largely intended to ensure that a melting furnace is always available. This will allow consumption of significant quantities of "in house" scrap without jeopardising output because of more frequent furnace re-lining. When both these steps to improve output are operational it is predicted that the unit will have a capacity of 3000 cast tonnes per annum.

Typical G. min.

Typical G. min.

Typical G. min.

In the above sections very little has been said about the high strength alloy 8091. Although limited quantities of full size 8091 rolling ingots have been produced at Kitts Green, the success rate was unacceptably low and quality was suspect. Development of improved and more consistent procedures for the high strength alloy are under intensive investigation in the laboratory and, it is planned to use the newly established techniques in a factory trial towards the end of the year. In the meantime semi-fabrication development and property development continues using rolling ingot cast in the laboratory programme.

0.2% P.S.

(MPa) 3 89 350 352 335 335 310

T.S.

(MPa) 4 90 430 4 82 4 20 4 73 4 00

El.

(I) 7.5 4 .O 8 .O 4 .O 4.2 2 -0

KQ (MPaJm).

2 9 23 2 0 19 15 14

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Conclusions

Alcan is now confident of producing the products described with guaranteed minimum properties. Damage tolerant and medium strength tempers are available in plate, sheet and extruded forms. Typical properties on plate, currently up to lOOmm thick, demonstrate that good through thickness strength and toughness can be achieved in thick products and tests on plate up to 150mm thick are continuing.

Development is continuing with the high strength alloy 8091.

Acknowledgements

While this paper has reported results on production material this would not have been possible without the development contributions from numerous colleagues within Alcan International and the Materials and Structures Department of the Royal Aircraft Establishment, Farnborough.

References

1. Miller W.S., White J., Reynolds M.A., McDarmaid D.S. and Starr G.M. Tbis volume

2. Miller W.S., White J. and Lloyd D.J. Tbis volume 3. Reynolds M.A. and Creed E. This volume

4. White J. and Miller W.S. This volume 5. Gray A. This volume

6. Grirres R., Miller W.S. and Butler R.G. This volume 7. Harris S.J., Noble B. and Dinsdale K. in

Aluminium ~ i t h i u m Alloys 11, Sanders T.H. and Starke E.A. (eds), Metallurgical Society of AIME, 1983

KQ ST-L ( M p a f i ) KQ T-L ( ~ P a h ) K Q L-T I M P a f i ) ELONGATION (%I

0

0

Figure 1 : Typical strength and toughness values for early production 8090 T651 plate

LONG TRANS. S.T. L-T T-L ST-L

om

z

p 475.. 8090-Tam

Figure 2 : Minimum Lital plate

8090-T8251 strength properties

compared with

2014-T651

80%-T8151 baseline alloys over

2024T351 the range of plate

3 7 5 7 : ; : : : ; : . . . thicknesses

0 10 20 30 40 50 60 70 80 90 100 110 lk Thickness (rnrn)

5w- 540.- -7l50-T651 520 ..

-

m..

-

^{- i}

2

8090-T877l

380- ^2014-T651 8090-T8251

O 10 20 30 40 50 60 70 80 90 100 110 120 Plate thtdoress(mm)

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V HIGH HIGH MEDIUM DAMAGE STRENGTH STRENGTH STRENGTH TOLERANT

Figure 3 :

Guaranteed minimum T-L f r a c t u r e toughness for 25mm Lital plate i n a range o f tempers and compared with baseline alloys

Figure 4 : Fracture

toughness of damage tolerant Lital 8090 T8151 plate compared with 2024 T351. Values for 2024 from Mil Handbook V

L " " I

1 E-6

w

W C 4 a

Y 8090 stretched 7%

U

4 12h. 150°C

5

5 6 7 8 9 1 0 20 30

RANGE IN STRESS INTENSITY FACTOR AK (~k6i;l)

Figure 5: Fatigue crack g r o w t h rates for 50mm Lital 8090 damage tolerant plate stretched by 2.5% a n d 7% compared with 2024 T351

Mechanical Properly

F i g u r e 6: Average mechanical properties from t h r e e lOOmm Lital 8090 T8251 plates

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6mK Q ELONGATION

0

0.2% e s .

Long TmnsS-T L-T T-L ST-L Long Tmns.S-T L-T T-L ST-L

Figure 7: Comparison of strength a n d toughness o f Lital 8090 T8771 plate with specification values for 7010 T7651 plate

ESl ^{K Q}

ELONGATION

0

0.2%P.S.

0

LONG TRANS. T-L LONG TRANS. T-L

Figure 8: Comparison o f t h e s t r e n g t h and toughness o f t h i n Lital 8091 T8771 with specification values for 7150 T651

T.S.(MPa)

v A

1 -

j ELONG. (%)

I

" LONG 10 20 30 40 5 0 60 7 0 8 0 TRANS.

TEST DIRECTION

Figure 9: Variation in tensile properties with testing direction for Lital 8090 T81 damage tolerant sheet

Figure 10: Comparison of the fracture toughness of damage tolerant sheet alloys

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Figure

5 10 20

RANGE IN STRESS INTENSITY FACTOR AK ( t l ~ a f i

1 1 : Fatigue crack growth rate for Lital 8090 tolerant sheet compared with ²⁰²⁴ T 3

10-61 I I I I I I I I I I I I

5 10 2 0

RANGE IN STRESS INTENSITY FACTOR AK (MPafi

damage

Figure 12: Fatigue crack growth rate for Lital 8090 T 8 medium strength sheet compared with 20111 T6