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STATUS REPORT ON THE DEVELOPMENT OF ALUMINIUM-LITHIUM AT PECHINEY
G. Le Roy, R. Mace, D. Marchive, P. Meyer, R. Nossent, F. Schlecht
To cite this version:
G. Le Roy, R. Mace, D. Marchive, P. Meyer, R. Nossent, et al.. STATUS REPORT ON THE DE- VELOPMENT OF ALUMINIUM-LITHIUM AT PECHINEY. Journal de Physique Colloques, 1987, 48 (C3), pp.C3-33-C3-39. �10.1051/jphyscol:1987304�. �jpa-00226527�
JOURNAL DE PHYSIQUE
Colloque C3, suppl6ment au n 0 9 , Tome 48, septembre 1987
STATUS REPORT ON THE DEVELOPMENT OF ALUMINIUM-LITHIUM AT PECHINEY
G. LE ROY, R. MACE, D. MARCHIVE* , P. MEYER* * , R. NOSSENT* * * and F. SCHLECHT+
CBgedur-PBchiney, B.P. 4 2 , F-63502 Issoire Cedex, France
"FORGEAL, Le Piat, F-63501 Issoire Cedex, France
* * Cegedur-Pechiney, B.P. 27, F-38340 Voreppe, France viatube tube, B.P. 35, F-44471 Carquefou Cedex, France +CBg&dur-Pechiney, 4 , Avenue Victor Hugo,
F-49460 Montreuil-Juigne, France
ABSTRACT :
This paper p r e s e n t s t h e s t a t e of development a t PECHINEY in t e r m s of p r o p e r t y goals (replacement of existing alloys) and p r o d u c t forms. T h e m a i n a r e a s of r e s e a r c h in progress, a c h i e v e m e n t s a n d prospects regarding c o m m e r c i a l i z a t i o n a r e given. Typical values measured on sheets, extrusions, forgings and t u b e s a r e given, t o g e t h e r with engineering properties.
Most results r e l a t e t o d a m a g e t o l e r a n c e and medium s t r e n g t h t y p e s of r e p l a c e m e n t . They a r e all based on industrial production. Progress regarding t h e o t h e r p r o d u c t forms, mainly t h e t h i c k e r ones, and regarding higher s t r e n g t h alloys will a l s o b e outlined.
INTRODUCTION :
Aluminium-Lithium raises a d e e p i n t e r e s t a m o n g a i r c r a f t m a n u f a c t u r e r s a s t h i s n e w f a m i l y of alloys has proven potentially very a t t r a c t i v e . In particular, i t s lower density combined with a higher s t i f f n e s s c a n lead t o weight reductions of up t o 15 % in c o m m e r c i a l airliners.
As a major aluminium producer, PECHINEY is strongly c o m m i t t e d t o t h e d e v e l o p m e n t of AI-Li alloys, with a c h i e v e m e n t s described in t h e r e f e r e n c e p a p e r s listed in t h e end.
A f t e r e x t e n s i v e sampling t o European and American c u s t o m e r s and o t h e r s , precious feed-back w a s obtained, which helped p r e s e n t t h e overall s t a t u s of our AI-Li alloys in t h i s paper.
P R O P E R T Y TARGETS A N D CORRESPONDING AL-LI ALLOYS :
The g a g e t o g a g e r e p l a c e m e n t of c o m p o n e n t s without redesign, i.e., t h e substitution of conventional alloys by t h e lighter and m o r e rigid AI-Li, r e q u i r e s a t f i r s t sight t h a t a l l o t h e r properties b e e q u a l or b e t t e r . This r e q u i r e m e n t is e s s e n t i a l for t h o s e p r o p e r t i e s upon which t h e p a r t s w e r e designed, such a s :
2024-T3 : d a m a g e tolerance, including f a t i g u e 2214-T6 : medium s t r e n g t h a n d f a t i g u e r e s i s t a n c e 7075-T73 : stress-corrosion r e s i s t a n c e (ST direction) 7xxx-T6 : high s t r e n g t h
For p r a c t i c a l reasons, t h e r e p l a c e m e n t of t h e alloys (mostly 2024 a n d 7075) g u i d e t h e work o n AI-Li a l l o y d e s i g n , a l t h o u g h i t a p p e a r s t h a t i n d i v i d u a l "AI-Lit' a l l o y s ( w i t h i n t h e Al-Li-Cu-Mg-Zr family) a c h i e v e a t t r a c t i v e property c o m b i n a t i o n s of t h e i r own.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987304
C3-34 J O U R N A L D E PHYSIQUE
Four main AI-Li alloys h a v e been developed by PECHINEY, t o cover t h e r a n g e of s t r e n g t h of conventional alloys with a density reduction of 8 t o 12 %. Their main associated property is :
i n t e r n a l n b AA Nb d E
medium s t r e n g t h + toughness medium s t r e n g t h + low density
high s t r e n g t h very low density
OVERALL STATUS : T A R G E T VS P R O D U C T FORMS :
The overall s t a t u s is given in t a b l e 1. The thicker l e t t e r s correspond t o t h e product f o r m s which a r e c l o s e s t t o being qualified by customers, t h e n commercialized. The ** means t h a t g u a r a n t e e d values h a v e been established. The r e s t (thin l e t t e r s ) is s t i l l a t t h e development stage, though also produced on s c a l e 1.
t a b l e 1 : correspondance b e t w e e n al-li a n d conventional alloys.
* : strength-wise i.e., excluding s c c resistance.
thickness (mm) 376 12,7 10 10
CASTING EXPERIENCE : PRODUCT
GOAL
2024 - T 3
2214 - T6
*
7075 - T73
7075 - T6
- T76
With a n e x p e r i e n c e of s e v e r a l hundreds of c a s t i n g s i n b o t h slab and billet forms, and on a v a r i e t y of alloys and ingot cross-sections, t h e casting technology of AI-Li i s well m a s t e r e d o n t h e t w o c a s t i n g u n i t s l o c a t e d in t h e VOREPPE Research C e n t r e .
The ingot weight is 150 kg on t h e s m a l l e r one, 2 t (3 t by end of 1987) on t h e bigger one.
Regardless of t h e C P 27X alloy used, c u r r e n t ingots exhibit a good s u r f a c e condition, leading t o t h e s a m e scalping a s o n conventional alloys. Their processing i s performed solely in plant, on s c a l e 1, under fully industrial conditions. The good short-transverse properties on thick products (see l a t e r ) i l l u s t r a t e t h e good quality of t h e melt.
The successive scale-ups (150 K g t o 1.5 t and 2 t ) show t h a t t h e subsequent c o m m e r c i a l s c a l e up t o 8 t ingots should b e r e a d i l y made, a t t h e ISSOIRE plant. In f a c t , t h e f u t u r e (commercial) ingot s i z e s which will b e produced a t ISSOIRE a s of 1989 a r e a l r e a d y c a s t , with reduced length, o n t h e Voreppe 2 t c a s t i n g unit :
slab cross-section (mm) : 1100 x 300 billet d i a m e t e r : 450 m m 1300 x 446
s h e e t s
2091-T8X
xx
2091-T8
t h i n extrusions
209 1 -T8 1 8090-T6
8090-T8
C P 2 7 6 -T8 interm.
2091-T8X
X Y
2091-T8 8090-T8
C P 2 7 6 -T8
p l a t e s
2091-T8X
**
2091-T8 8090-T8
CP276 -T8
thick extrusions
209 1 -T8 1
8090-T8
CP276 -T8
thin forgings
2091-T6 2091-T7X
8090-T6
thick forgings
2091 -T8X
2091-T8
CP276 -T8
DAMAGE TOLERANCE : 2024 - T3X REPLACEMENT :
Compared with 2024, 2091 rolled products and extrusions exhibit t h e following s e t of properties :
- density reduction : - 8 % and modulus increase : + 7 % - comparable strength level
- good ductility and toughness, including in the ST direction
- good fatigue behaviour (close t o t h a t of the reference) and improved crack propagation resistance
- comparable corrosion resistance (slightly improved)
- a d e q u a t e s t a b i l i t y of p r o p e r t i e s i n t i m e ( s i m u l a t i o n of 20 y e a r s of s e r v i c e l i f e ) - s a m e possibility of cladding for sheets.
In particular, tensile properties on sheets and plates (1.2 to 40 mm in thickness) compare well with the target, in both average values and scatter. This leads t o minimum guaranteed values which, based on our experience so far, comply with existing 2 0 2 4 4 3 specifications :
table 2 : minimum guaranteed properties on sheets and plates optimized for damage tolerance. The reference corresponds t o European specs, e.g t h e AIR 9048 specification.
Rolled products
thickness (mm) direction YS (MPa) UTS (MPa)
El (%I
Note t h a t : - thin sheets ( t 5 3.5 mm) a r e recrystallized t o ensure a good isotropy, and under aged (T8X temper) for damage tolerance. Sheets clad with either 7072 or lxxx alloys yield adequate properties : the associated drop in strength i s identical t o t h a t of conventional alloys : around 6 % for s h e e t s clad 5 % on each side.
- a special version (called CPH) of the damage tolerant 2091-T8X sheet has been designed t o match a 200 MPa threshold in t h e a l t e r n a t e immersion-emersion tensile t e s t performed in 3.5 % Nacl solution (duration of t e s t : 30 days).
The first results obtained on industrial production a r e given in table 3. The fatigue resistance remains a t t h e same high level a s t h e standard version described so far. The guaranteed properties of the CPH material a r e being established t o prepare for the certification of stress-corrosion critical parts.
AI-Li : 2091-TSX(51) [reference :2024-T3(51)]
- thicker sheets and plates ( t > 3.5 rnm) a r e fibered with a low o r acceptable anisotropy and an excellent stress-corrosion resistance in t h e rolling plane ( b 250 MPa). Note t h e good short transverse properties of the underaged 2091-T8X51 plate (2024 replacement), confirmed by measurements on thick forgings (around 90 mm in thickness) :
1.2 t o 2.5 L
310 420 I 4
40
209 1 -T6X 2024-T452
ST 270 [270]
380 [380) 3C31 LT
310 [290]
420 t4451 14 1141
L 310 [310]
430 [430]
6[10]
YS (MPa) ST 335 310
6Oo/L 270 400 20
LT 300 [300]
430 [430]
6[101
UTS ( ~ P a ) l El (%) ST
405 430
KIc ( M P a G ) ST
7 9
L-T 40 41
T-L 35 35
S-L 32 30
C3-36 JOURNAL DE PHYSIQUE
Table 3 : First results obtained on 2091 sheets optimized for damage tolerance and stress- corrosion resistance.
In t h e case of extrusions, both 2091 and 8090 alloys yield a t t r a c t i v e property combinations under industrial processing conditions (table 41, s o t h a t t h e choice is l e f t to the customers :
Table 4 : typical properties measured on 2091-T851 and 8090-T851 extrusions compared with those of 2024-T351 ; thickness range : 10 to 30 mm.
One particular case of interest is t h a t of cold-drawn tubes used for control and structural rods. 8090 alloy is prefered here, t o maximize t h e specific modulus, which is critical for buckling.
It has been checked t h a t t h e fatigue life of 8090-T6 tubes matched t h a t of 2024-T4 tubes ( > 2.5 million cycles a t a load = 35% UTS) while t h e compression load t o failure (buckling) was significantly increased, leading t o a potential 22 % weight saving over conventional 2024 structural rods.
ALLOY and TEMPER DENSITY/MODULUS
.YS (MPa) L :
UTS (MPa) L :
El (%) L :
toughness L-T :
K q ( M P a f i ) T-L : EXCO rating
intercrystalline (pm) corrosion (core) (%) stress-corrosion LT :
Based on t h e experience conducted by customers and on t h e results of in-house programs, t h e engineering properties of 2091 appear high :
- machining c a n be performed on existing tools - chemical milling leads t o satisfactory surface aspects
- forming ability i s b e t t e r than t h a t of 2024, as checked on a c t u a l severe parts, without t h e need for cold chambers
- spot welding is easier and leads t o stronger structures
- surface protection by chromic anodizing is a t l e a s t a s efficient.
2091-T851 2.57 / 78 GPa
465-505 520-580 7 - 11 35-43 24-31 EA t o EC 150 t o 250 40 t o 100
8090-T85 1 2.53 / 8 I G P a
460-535 520-580 5 - 9 30-40 20-30 N t o EA 100 t o 120
pitting 380 MPa
CONCLUSION : 2091 is t h e recommended alloy for t h e replacement of 2024 in t h e different parts designed for damage tolerance, a p a r t from cases where stiffness is t h e key parameter : 8090 then appears t o be a b e t t e r choice.
2024-T35 1 2.78 / 73 GPa
400 530 1 3 39 36 N t o ED 100 t o 150
MEDIUM STRENGTH (2214-T6-7075-T73 STRENGTH LEVEL) :
2091 and 8090 alloys, both aged t o peak strength (or close enough) a r e candidates for this application : table I.
When comparing t h e particular combinations of properties, i t appears t h a t their winning cards are, respectively :
- density, modulus and exfoliation resistance for 8090,
- t o u g h n e s s a n d d u c t i l i t y ( e s p e c i a l l y in t h e s h o r t - t r a n s v e r s e d i r e c t i o n ) f o r 2091.
Results have been obtained on all possible product forms, e.g on plates :
Table 5 : typical properties on plates h e a t t r e a t e d to medium strength level.
Also note t h e short- transverse properties on thick products :
Work is g o i n g o n t o i n c r e a s e t h e SL t o u g h n e s s , w h i c h is s t i l l below t h e t a r g e t .
In the case of precision and thin-walled die-forgings, t h e reference alloy is 7175-T73.
Optimization work bears on both strength level and corrosion resistance, with still room for improvement : table 6.
2091-T8(51 o r 52) 2214-T6(51or52)
FORG1NGS:t : up t o 150 mm
Table 6 : typical properties on precision forgings.
YS (MPa) 420 430 PLATE :t=38.5 mm
Engineering properties have been tested in depth on both alloys : - machining i s performed successfully on existing tools
- chemical milling is made using existing bathes, leading to a smooth milled surface
- forming operations a r e satisfactorily performed in t h e as-quenched condition - riveting 8090 results in a n increased fatigue life (better than on 2024)
- spot vfelding is easier than on conventional alloys
- surface protection by chromic anodizing is more efficient.
UTS ( M P ~ ] EL (%) 450 1 3.5
470 1 6
YS (MPa) 385 410
7175 T73 420 500 14 EA-EB UTS ( ~ ~ a ) l El(%)
465 1 3
460 1 5.5
8090 T6 440 510 7 EA-EB ALLOY
TEMPER
YS(MPa) L :
UTS (MPa) L : E l ( M P a ) L : EXCO rating
209 1 T6 440 520 10 EC-ED
2091 T7X 370 450 9 EB
C3-38 JOURNAL DE PHYSIQUE
CONCLUSION : - for thinner products, especially plates, extrusions and thin-walled forgings, 8090 is a n a t t r a c t i v e candidate for the replacement of existing medium strength alloys, offering in particular excellent fatigue properties and a good resistance t o exfoliation.
- for thicker products, 2091 approaches t h e t a r g e t with acceptable strength and ductily. An optimized corrosion-resistant version i s being also developed on thin-walled -
forgings.
HIGH STRENGTH : 7xxx-T6/T76 REPLACEMENT :
C P 276 has been designed t o reach t h e optimum combination of properties in t h e high strength range, within t h e Al-Li-Cu-Mg-Zr family. Combined with i t s increased modulus, this alloy m e e t s t h e target, mainly for p a r t s subjected t o compressive loading.
Experience on C P 276 bears on plates, thick forgings and extrusions processed from 2.0 tons slabs and billets. Compared with 7xxx-T6fT76, C P 276 gives t h e following properties (table 7)
- density reduction : - 8 % and modulus increase : + 11 %
- comparable strength and elongation on plates and extrusions
- lower elongation on thick forgings
- good toughness (usually superior t o t h a t of 7xxx)
- good resistance t o exfoliation corrosion
- improved crack-propagation resistance (better than t h a t of 2024 !).
Table 7 : typical properties obtained on various product forms.
t = 10 t o 30 mm L - T :
The engineering properties have been evaluated a s well :
- TIG welding has been proved feasible, but a complete solution heat t r e a t m e n t is needed to obtain a n adequate ductility on welded specimens.
- Hard a n o d i z i n g h a s b e e n p e r f o r m e d w i t h e x c e l l e n t r e s u l t s ( t h i c k o x y d e l a y e r ) . EXCO rating
intercrystalline (core)
CONCLUSION : C P 276 m e e t s t h e strength levels requested for replacement of conventional high strength alloys. This hardening potential associated with a high modulus, makes i t t h e candidate alloy for compressive structures.
EA-EB EB
150 pm 60 ~ r n (locally)
S C R A P ISSUE :
The need t o s e g r e g a t e AL-Li s c r a p f r o m conventional s c r a p h a s been announced a n d explained by a l l producers. Industrial solutions h a v e been studied, with t h r e e basic c o n c e r n s :
- a minimum risk of c o n t a m i n a t i o n of conventional alloys by unwanted lithium, within t h e a c t u a l s c r a p processing loops,
- a minimum (or a c c e p t a b l e ) c h a n g e for t h e a i r c r a f t maker,
- m e a n s of recycling AI-Li scrap, both technically and economically.
A1-Li solid scrap : a s e v e r e segregation i s requested (e-g., p u t a l l cuttings, discarded parts, in a s e p a r a t e box). Technically speaking, t h i s solid s c r a p c a n b e r e c y c l e d within t h e AI-Li foundry loop. When solid s c r a p i s bought b a c k f r o m c u s t o m e r s , i t s e x p e c t e d value is higher than t h e a v e r a g e value of a e r o s p a c e scrap, d u e t o t h e lithium content.
AI-Li chips : ideally, t h i n s c r a p (swarf, t h i n cuttings, turnings) should also b e s e v e r e l y segregated, but f o r t h e industrial feasibility i n workshops, s o m e mixing h a s t o b e permitted.
As this ,swarf is improper for recycling within t h e AI-Li loop (for q u a l i t y considerations), o t h e r recycling r o u t e s h a v e been studied, such a s t h e r e c l a i m of t h e lithium i t contains.
For both t e c h n i c a l and e c o n o m i c a l reasons, only 5 % mixing of conventional within AI-Li swarf is allowed (up t o 10% exceptionally). As work i s going on, t h e v a l u e of thin s c r a p i s still t o b e d e t e r m i n e d .
Conventional scrap : again, producers m u s t insist o n t h e n e c e s s i t y of avoiding t h e mixing of unwanted lithium within conventional (non AI-Li) scrap, t h i c k o r thin.
The maximum a c c e p t a b l e lithium c o n t e n t depends o n both : 1) t h e alloy, and 2) t h e final application. T h e study b e a r s on wrought and c a s t i n g alloys, with r e s u l t s e x p e c t e d in 1988.
CONCLUSIONS :
The i n t e n s e development a c t i v i t y during four y e a r s now has l e d t o r e m a r k a b l e a c h i e v e m e n t s :
- 2091 a p p e a r s c a p a b l e of replacing 2024, t h e damage-tolerant alloy in u s e on major portions of a i r c r a f t s : s h e e t s and thin extrusions (fuselage), p l a t e s a n d t h i c k e x t r u s i o n s (lower wing).
I t c a n also b e brought t o medium s t r e n g t h (2214-T6 level), which widens i t s p o t e n t i a l use.
- 8090 g i v e s a d e q u a t e p r o p e r t i e s on s o m e product forms, with numerous possible applications (medium t y p e of s t r e n g t h ) : stringers, tubes, t h i n forgings.
- CP 276 is a promising c a n d i d a t e for high s t r e n g t h applications : forgings, extrusions, p l a t e s (upper wing, s t r u c t u r a l p a r t s ) : 7xxx-T6 replacement.
- only t h e stress-corrosion resistant t a r g e t on thick p r o d u c t s (short-transverse direction:
7075-T73 r e p l a c e m e n t ) still r e m a i n s a t t h e research stage.
- t h e physical a n d engineering p r o p e r t i e s remain similar t o t h o s e of conventional aluminium a l l o y s , s o t h a t t h e m a n u f a c t u r i n g t e c h n i q u e s a r e u n c h a n g e d f o r t h e a i r c r a f t m a k e r . The basic p r o p e r t y combinations being established, c u s t o m e r s c a n now launch e x t e n s i v e programs f o r t h e evaluation of s e r v i c e properties on a l a r g e scale, and f o r t h e qualification of t h e m o s t a d v a n c e d alloys.
REFERENCES : P. Meyer and B. Bubost, AI-Li 111 Conf., Oxford, July 1985
G. L e Roy and P. Meyer , AI-Li Symposium, Los Angeles, March 1985, 1986 a n d 19S7.
