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SUPERPLASTICITY OF 2090 SPF SHEET AT HOT ROLLED GAUGE
C. Cho, B. Cheney, D. Lege, J. Petit
To cite this version:
C. Cho, B. Cheney, D. Lege, J. Petit. SUPERPLASTICITY OF 2090 SPF SHEET AT HOT ROLLED GAUGE. Journal de Physique Colloques, 1987, 48 (C3), pp.C3-277-C3-283.
�10.1051/jphyscol:1987331�. �jpa-00226562�
JOURNAL DE PHYSIQUE
Colloque C3, suppl6ment au n09, Tome 48, septembre 1987
SUPERPLASTICITY OF 2090 SPF SHEET AT HOT ROLLED GAUGE
C.W. CHO, B.A. CHENEY, D.J. LEGE and J.I. PETIT
Aluminum Company of America, Alcoa Laboratories, Alcoa Center.
PA 15069, U.S.A.
ABSTRACT
The s u p e r p l a s t i c p r o p e r t i e s o f r e l a t i v e l y t h i c k , 0.44 cm, gauge s u p e r p l a s t i c 2090 sheet produced by a s p e c i a l thermomechanical h o t r o l l i n g process a r e examined. S u p e r p l a s t i c t e n s i l e elongations are h i g h e s t i n t h e l o n g i t u d i n a l o r i e n t a t i o n and lowest i n t h e long t r a n s v e r s e o r i e n t a t i o n w i t h a l l values i n excess o f 600%. The p o s t - s u p e r p l a s t i c a l l y formed s t r e n g t h l e v e l s are lower than those o f nondeformed 2090. The l o s s o f s t r e n g t h i s a t t r i b u t e d t o dynamic
r e c r y s t a l l i z a t i o n and c a v i t a t i o n d u r i n g forming.
INTRODUCTION
S u p e r p l a s t i c a l l y formable (SPF) h i g h s t r e n g t h aluminum sheet i s o f i n t e r e s t f o r use i n a i r c r a f t s t r u c t u r e s because i t a7lows f o r s i g n i f i c a n t cost and design b e n e f i t s over c o n v e n t i o n a l l y formed p a r t s . A1-Li SPF sheet i s e s p e c i a l l y a t t r a c t i v e due t o i t s low d e n s i t y and h i g h s t i f f n e s s . Generally, h i g h s t r e n g t h a l l o y s such as 7475 have r e q u i r e d h i g h amounts o f c o l d work and r a p i d h e a t i n g r a t e s t o r e c r y s t a l l i z a t i o n t o achieve t h e very f i n e g r a i n s t r u c t u r e s r e q u i r e d f o r optimum s u p . e r p l a s t i c i t y (1). I n c o n t r a s t , t h i s paper presents r e s u l t s obtained f o r as-hot r o l l e d 2090 sheet i n an i n i t i a l l y u n r e c r y s t a l l i z e d c o n d i t i o n . As designers begin t o t a k e advantage o f h i g h l e v e l s o f SPF, t h i c k e r s t a r t i n g gauges w i l l be r e q u i r e d so t h a t s u f f i c i e n t sheet thickness w i l l remain i n t h e f i n a l formed p a r t . The a b i l i t y t o o b t a i n SPF c a p a b i l i t i e s i n as-hot r o l l e d sheet w i l l f a c i l i t a t e t h e p r o d u c t i o n o f t h i c k e r sheet gauges.
Conventional 2090 sheet re1 ie s on a very f i n e d i s t r i b u t i o n o f strengthening p r e c i p i t a t e s , promoted by c o l d working a f t e r s o l u t i o n heat treatment and p r i o r t o age (2), and on an u n r e c r y s t a l l i z e d m i c r o s t r u c t u r e t o o b t a i n maximum strength. However, i n a SPF 2090 p a r t , c o l d working between s o l u t i o n heat treatment and aging i s n o t p r a c t i c a l s i n c e t h e p a r t i s i n i t s f i n a l shape p r i o r t o heat treatment. Also, as wi 11 b.z discussed i n t h i s paper, t h e 2090 undergoes m i c r o s t r u c t u r a l changes d u r i n g s u p e r p l a s t i c deformation which i n f l u e n c e i t s mechanical p r o p e r t i e s . Therefore, t h e s t r e n g t h p o t e n t i a l o f 2090 SPF sheet a f t e r SPF forming can be q u i t e d i f f e r e n t than conventional 2090 sheet.
To u t i l i z e SPF 2090 sheet f o r a i r c r a f t s t r u c t u r a l components, i t i s e s s e n t i a l t o understand t h e e f f e c t o f s u p e r p l a s t i c forming p r a c t i c e on t h e post-SPF p r o p e r t i e s . Since SPF p a r t forming s t r a i n s t h e m a t e r i a l i n m u l t i p l e o r i e n t a t i o n s and since A1-Li a l l o y s a r e known t o be h i g h l y a n i s o t r o p i c , th:!
impact o f SPF deformation on p r o p e r t i e s i n various o r i e n t a t i o n s i s o f importance.
T h i s study i n v e s t i g a t e s t h e e f f e c t o f v a r i o u s l e v e l s o f SPF deformation i n various o r i e n t a t i o n s r e l a t i v e t o t h e r o l l in g d i r e c t i o n on t h e microstructure, c a v i t a t i o n and s t r e n g t h o f 0.437 cm gauge SPF 2090 sheet.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987331
JOURNAL DE PHYSIQUE
MATERIALS
A r e c t a n g u l a r 2090 i n g o t , 4 1 cm t h i c k X 127 cm wide, was h o t r o l l e d v i a a s p e c i a l thermomechanical process t o 0.437 cm gauge sheet. To t h e best o f our knowledge, t h i s i s t h e f i r s t l o t o f s u p e r p l a s t i c A1-Li based a l l o y product which was s p e c i a l l y f a b r i c a t e d f o r SPF i n a f u l l s c a l e p l a n t operation. The f i n a l gauge sfieet was c o i l e d a f t e r h o t r o l l i n g on a continuous r o l l i n g m i l l l o c a t e d i n Alcoa's Davenport, Iowa f a c i l i t y . The chemical composition, measured a t t h e f i n a l sheet gauge, i s as follows:
EXPERIMENTAL PROCEDURES
Two types o f s u p e r p l a s t i c t e n s i l e t e s t s were c a r r i e d o u t t o c h a r a c t e r i z e t h e SPF c h a r a c t e r i s t i c s and post-SPF p r o p e r t i e s .
To evaluate t h e e l o n g a t i o n t o f a i l u r e , small specimens w i t h 0.477 cm wide X 0.953 cm l o n g gauge s e c t i o n were t e s t e d a t t h e temperature o f 527OC and a t a constant t r u e s t r a i n r a t e o f 0.0004/sec. Constant t r u e s t r a i n r a t e t e s t i n g was achieved by a p r o g r a m a b l e servo-hydraul i c t e s t i n g system which c o n t i n u o u s l y adjusted cross head speed t o m a i n t a i n t h e constant t r u e s t r a i n r a t e . Such a s h o r t
specimen gauge l e n g t h was necessary t o accommodate up t o 900% e l o n g a t i o n w i t h i n a c o n t r o l l e d temperature zone i n t h e furnace. No superimposed h y d r o s t a t i c pressure was used i n t h i s t e s t system. Load and s t r o k e were monitored d u r i n g t h e t e s t s , and t r u e s t r e s s - t r u e s t r a i n data were c a l c u l a t e d from these data assuming u n i f o r m deformation w i t h i n t h e 0.953 cm e f f e c t i v e gage l e n g t h and constant volume. True f r a c t u r e s t r a i n s i n t h e specimen w i d t h and t h i c k n e s s were c a l c u l a t e d from dimensions measured w i t h a micrometer. The t r u e l e n g t h f r a c t u r e s t r a i n was c a l c u l a t e d from t h e o t h e r two s t r a i n measurements assuming constant volume.
For post-SPF mi c r o s t r u c t u r a l and mechanical p r o p e r t y evaluations, s p e c i a l l y designed 3.81 cm wide specimens were used w i t h various i n i t i a l gauge l e n g t h s so t h a t t h e f i n a l gauge l e n g t h a f t e r predetermined s u p e r p l a s t i c deformation would be 10.16 cm long. Such a l a r g e specimen design enabled t h e machining o u t o f a t e n s i l e t e s t specimen o r a c r y s t a l l i t e o r i e n t a t i o n f u n c t i o n (CODF) sample from each s u p e r p l a s t i c a l l y deformed sample. The SPF deformation was conducted a t a constant temperature o f 527OC w i t h a constant s t r a i n r a t e o f 0.0004/sec. For t h e l a r g e specimen t e s t i n g , t h e constant t r u e s t r a i n r a t e was approximated by
a d j u s t i n g crosshead speed a f t e r every 35% elongation.
Both t h e small and t h e l a r g e SPF t e n s i l e deformation specimens were deformed i n l o n g i t u d i n a l (L), l o n g transverse (LT) and 45O o r i e n t a t i o n s w i t h respect t o t h e o r i g i n a l r o l l e d d i r e c t i o n t o i n v e s t i g a t e t h e e f f e c t o f s u p e r p l a s t i c forming d i r e c t i o n s on t h e e l o n g a t i o n t o f a i l u r e o r t h e e v o l u t i o n o f m i c r o s t r u c t u r e s and post-SPF p r o p e r t i e s .
RESULTS AND DISCUSSION SPF T e n s i l e Tests
D u p l i c a t e u n i a x i a l SPF t e n s i l e t e s t s t o determine SPF e l o n g a t i o n t o f a i l u r e were conducted f o r t h e 2090 w i t h L, LT and 45' specimens. S u p e r p l a s t i c d u c t i l i t i e s are shown i n Table 1.
The r e s u l t s i n d i c a t e t h a t on average t h e 2090 sheet tends t o have maximum
d u c t i l i t y i n t h e L d i r e c t i o n and minimum d u c t i l i t y i n t h e LT d i r e c t i o n . The
v a r i a b i l i t y o f e l o n g a t i o n r e s u l t s f o r any given t e s t o r i e n t a t i o n may be as
s i g n i f i c a n t as t h e d i f f e r e n c e s between d i r e c t i o n s , however. The d i r e c t i o n a l i t y
trends e x h i b i t e d by t h e t r u e f r a c t u r e s t r a i n measurements tend t o be more
consi s t e n l .
Representative t r u e s t r e s s - t r u e s t r a i n curves determined from t h e l o a d - s t r o k e measurements a r e shown i n F i g u r e 1. A l l t e s t s e x h i b i t a s t r e s s maximum t h a t occurs a t a t r u e s t r a i n between 0.33 and 0.55 (elongations o f between 40% and 75%). This maximum t r u e s t r e s s may correspond t o t h e onset o f dynamic
r e c r y s t a l l i z a t i o n . Specimens o r i e n t e d i n t h e 45' d i r e c t i o n t e n d t o reach maximum s t r e s s e a r l i e s t , w h i l e specimens o r i e n t e d a t 90° t o t h e r o l l i n g
d i r e c t i o n reach i t a t t h e h i g h e s t s t r a i n s . The d u p l i c a t e t e s t s conducted f o r t h e 45' specimens r e s u l t e d i n t h e h i g h e s t maximum stresses (approximately
4.8 MPa). The lowest maximum s t r e s s was determined f o r a t r a n s v e r s e specimen
(approximately 4.25 MPa).
=*-$5 I I I I I
Table 1
TOTAL ELONGATION
DIRECTION SPECIMEN
., - - -
897%
0-degrees -
839%
0 Average
90-degrees 746% 744%
0 0.5 1.0 1.5 2.0 2.590-degrees 622% 622%
True strain90 Average 684%
Typical True Stress and True Strain Curves for
Post-SPF P r o p e r t i e s Superplastic Tensile Tests in Three Orientations
Figure 1
Evaluations o f t h e post-forming mechanical p r o p e r t i e s of 2090 sheet were made u s i n g t h e l a r g e SPF samples described above. T e n s i l e specimens were machined from t h e gauge 'section o f s u p e r p l a s t i c a l l y formed t e n s i l e samples, s o l u t i o n heat t r e a t e d , aged t o peak s t r e n g t h and t e s t e d a t room temperature. SPF c o n d i t i o n s evaluated i n c l u d e d nominal s t r a i n s ranging from 50 t o 500%
e l o n g a t i o n i n t h e L d i r e c t i o n and 200% e l o n g a t i o n i n t h e LT and 45' d i r e c t i o n s . Data from these e v a l u a t i o n s are p l o t t e d i n Figures 2 and 3. F i g u r e 2
i l l u s t r a t e s t h e observed decrease i n y i e l d s t r e n g t h w i t h i n c r e a s i n g superpl a s t i c e l o n g a t i o n f o r samples s u p e r p l a s t i c a l l y deformed i n t h e l o n g i t u d i n a l d i r e c t i o n . It i s o f i n t e r e s t t o note t h a t t h e sample-to-sample v a r i a t i o n i n y i e l d s t r e n g t h increases d r a m a t i c a l l y a t t h e h i g h e s t l e v e l o f SPF s t r a i n . Note, too, t h a t t h e y i e l d s t r e n g t h dropped by approximately 20% over t h e range examined. A s i m i l a r t r e n d of s t r e n g t h l o s s a f t e r s u p e r p l a s t i c deformation was r e p o r t e d from Supral 100, Supral 150 and Supral 220 a l l o y s which a l s o dynamically r e c r y s t a l l i z e d d u r i n g s u p e r p l a s t i c forming ( 3 ) , w h i l e no s i g n i f i c a n t s t r e n g t h l o s s was observed from SPF 7475 ( 4 ) . T h i s behavior, t h e r e f o r e , can be a t t r i b u t e d t o : a ) t h e decreasing degree o f c r y s t a l l o g r a p h i c t e x t u r e accompanying t h e dynamic r e c r y s t a l 1 i z a t i on; and b ) t h e development of c a v i t a t i o n , e s p e c i a l l y a t t h e h i g h e r s u p e r p l a s t i c s t r a i n s .
F i g u r e 3 i s a comparison o f t h e t e n s i l e y i e l d s t r e n g t h s between samples t e s t e d i n t h e L, LT and 45' d i r e c t i o n s a f t e r SPF s t r a i n s o f approximately 200%. From t h i s f i g u r e , i t i s apparent t h a t t h e s t r e n g t h i n t h e L d i r e c t i o n i s s l i g h t l y h i g h e r than those observed i n t h e o t h e r d i r e c t i o n s , and t h a t t h e LT and 45"
s t r e n g t h l e v e l s a r e e s s e n t i a l l y i d e n t i c a l . This behavior i s q u i t e a departure from t h a t t y p i c a l l y observed f o r conventional 2090, wherein t h e 45' y i e l d s t r e n g t h i s t y p i c a l l y much l o w e r than t h a t i n e i t h e r t h e -L o r -LT d i r e c t i o n . T h i s observation i s , again, a t t r i b u t a b l e t o t h e decreasing degree o f
c r y s t a l l o g r a p h i c t e x t u r e accompanying t h e dynamic r e c r y s t a l l i z a t i o n and g r a i n
growth process.
C3-280
Yield strength (MPa) Yield strength (ksl)
Superplastic tensile elongation (%) Orientation (nominal 200% elongation)
Variation in Room Temperature Yield Strength With Increasing Superplastic Deformation
Figure 2
Variation in Room Temperature Yield Strength With Orientation
Figure 3
Cavitation
Cavitation measurements were made adjacent to the subsize tensile specimens, approximately 0.3 cm from the centerline of the deformed sample. Measurement was accomplished via an SEM based image analysis system. Twelve fields were evaluated for each specimen, at a magnification of 270x. In each case, the area fraction of cavities and the distribution of cavity length were
determined.
Cavity length distribution data, normalized with respect to area examined, are presented in Figures 4 and 5. Figure 4 illustrates the increasing amount of cavitation observed as superplastic deformation increases. Note that the distribution widens to larger cavity sizes as superplastic strain increases, but that the origin remains essentially constant, indicating that existing cavities grow while new cavities are being nucleated. Also of interest in the apparent bimodal nature of the curves, possibly influenced by differences in cavity nucleation sites, i.e., grain boundary triple points vs. second phase particles.
Cavtties/mm2 in class Cavities/mm2 in class
Length of cavity (/jm)
Length of cavity (fjm)Cavity Size Distribution as a Function of SPF Elongation in Longitudinal Direction
Figure 4
Cavity Size Distribution as a Function of Superplastic Tensile Test Direction at Approximately 240% Elongation
Figure 5
Figure 5 is a comparison of the cavity distributions measured in specimens superplastically deformed in the -L, -LT and 45° directions to a strain level of approximately 200%. Fewer cavities were observed in the -LT and 45°
directions than in the -L direction, possibly due to the directionality of the microstructure; that is, the alignment of second phase particles and
boundaries in the rolling direction.
Area f r a c t i o n s o f c a v i t i e s f o r samples e v a l u a t e d i n t h i s i n v e s t i g a t i o n and f o r 7475-02 samples p r e v i o u s l y e v a l u a t e d i n a s i m i l a r manner u s i n g t h e s m a l l e r t e n s i l e specimens ( 5 ) a r e p r e s e n t e d i n F i g u r e 6. From F i g u r e 6 i t can be seen t h a t w h i l e c a v i t i e s n u c l e a t e a t l o w e r s t r a i n s i n 2090 t h a n i n 7475, the growth
r a t e s a r e comparable.
Mi c r o s t r u c t u r a l E v o l u t i o n
As has been discussed, t h e SPF 2090 e x h i b i t s s i g n i f i c a n t s t r e n g t h changes w i t h SPF d e f o r m a t i o n . Therefore, i t i s i m p o r t a n t t o understand how t h e
m i c r o s t r u c t u r e e v o l v e s w i t h s u p e r p l a s t i c deformation. As shown i n F i g u r e 7 ( a ) , undeformed SPF sheet has an u n r e c r y s t a l l i z e d s t r u c t u r e w i t h s u b g r a i n s i z e o f
2 mm t o 3 mm. W i t h SPF d e f o r m a t i o n , s i g n i f i c a n t m i c r o s t r u c t u r a l .changes t a k e place. As shown i n F i g u r e 7 ( b ) f o r a sample h a v i n g been deformed 250% by SPF, t h e g r a i n s t r u c t u r e appears t o r e c r y s t a l l i z e d u r i n g SPF d e f o r m a t i o n .
There a r e t w o t y p e s o f m i c r o s t r u c t u r e s o f A1 a l l o y sheet which e x h i b i t s u p e r p l a s t i c i t y as r e p o r t e d i n t h e l i t e r a t u r e . One s t r u c t u r e i s a s t a t i c a l l y r e c r y s t a l l i z e d f i n e g r a i n s t r u c t u r e such as t h a t o f SPF 7475 sheet (6). The second s t r u c t u r e i s an o r i g i n a l l y u n r e c r y s t a l l i z e d ( o r c o n t i n u o u s l y
r e c r y s t a l 1 i zed) s t r u c t u r e which undergoes dynamic r e c r y s t a l l i z a t i o n d u r i n g s u p e r p l a s t i c forming such as Supra1 100 ( 7 ) . SPF 2090 a l s o f a l l s i n t h i s l a t e r category. I n 2090 a1 l o y , t h e r e c r y s t a l l i z a t i o n d u r i n g SPF c o n t r i b u t e s t o t h e s t r e n g t h l o s s .
F i g u r e 7
( a ) O p t i c a l m i c r o g r a p h showing u n r e c r y s t a l l i z e d g r a i n s t r u c t u r e o f SPF 2090 sheet p r i o r t o s u p e r p l a s t i c d e f o r m a t i o n . ( b ) O p t i c a l m i c r o g r a p h showing d y n a m i c a l l y r e c r y s t a l l i z e d g r a i n s t r u c t u r e a f t e r 250 % s u p e r p l a s t i c e l o n g a t i o n , deformed i n l o n g i t u d i n a l d i r e c t i o n .
As a m a t e r i a l r e c r y s t a l l i z e s , t h e i n t e n s i t y o f a s - r o l l e d t e x t u r e would be expected t o decrease. T h e r e f o r e , t h e i n t e n s i t y o f c r y s t a l l o g r a p h i c t e x t u r i n g i n t h e s u p e r p l a s t i c a l l y deformed m a t e r i a l was measured a t v a r i o u s SPF
e l o n g a t i o n l e v e l s t o m o n i t o r t h e degree o f dynamic r e c r y s t a l l i z a t i o n . The
r e s u l t s a r e shown i n F i g u r e 9 where "ODF T e x t u r e I n d e x " v a l u e s , as a measure
o f degree o f t e x t u r i n g , a r e p l o t t e d a t v a r i o u s SPF e l o n g a t i o n s , p u l l e d i n
t h r e e d i r e c t i o n s . A l t h o u g h t h e s t r i c t r e l a t i o n s h i p between t h e decrease i n
C3-282 JOURNAL DE PHYSIQUE
" O M Texture Index" and t h e degree o f r e c r y s t a l l i z a t i o n i s s t i 11 speculative, t e n t a t i v e i n t e r p r e t a t i o n i s attempted here. An ODF study o f s u p e r p l a s t i c a l l y deformed 2090 sheet i s i n progress and w i l l be r e p o r t e d elsewhere.
a&-.
2.5 I I I I I
Longitudinal test direction Straln rate = 0.0004 s-1 2.0
-
7475-02
.
2090,;
1.5-
- -
>U
.g 1.0
-
0.5
-
0 100 200 300 400 500 600
% elongation
Comparison of Cavitation Behaviors of SPF
7475-02and
SPF 2090Figure 6
SPF deformation (%)
Evolution ot "ODF Texture Index" as a Function of Superplastic Elongation
Figure
