THE EFFECT OF THERMOMECHANICAL PROCESSING VARIABLES ON ANISOTROPY IN MECHANICAL PROPERTIES OF Al-Li ALLOYS

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THE EFFECT OF THERMOMECHANICAL PROCESSING VARIABLES ON ANISOTROPY IN

MECHANICAL PROPERTIES OF Al-Li ALLOYS

K. Takahashi, K. Minakawa, C. Ouchi

To cite this version:

K. Takahashi, K. Minakawa, C. Ouchi. THE EFFECT OF THERMOMECHANICAL PROCESSING

VARIABLES ON ANISOTROPY IN MECHANICAL PROPERTIES OF Al-Li ALLOYS. Journal de

Physique Colloques, 1987, 48 (C3), pp.C3-163-C3-169. �10.1051/jphyscol:1987319�. �jpa-00226549�

JOURNAL DE PHYSIQUE

Colloque C3, suppl6ment au n"9, Tome 48, septembre 1987

THE EFFECT OF THERMOMECHANICAL PROCESSING VARIABLES ON ANISOTROPY IN MECHANICAL PROPERTIES OF A1-Li ALLOYS

K. TAKAHASHI, K. MINAKAWA and C. OUCH1

Advanced Technology Research Center, Nippon Kokan K.K. 1-1 Minamiwatarida. Kawasaki-ku, Kawasaki, 210, Japan

ABSTRACT

The effect of thermanechanical processing on rnicrostn,*ure and mechanical properties of AL-Li-a-Zr alloy was studied. Particular attention was given to anisotropy in mchanical properties. It is found that the low slab reheating temperature (400'~) with the low finish-rolling temperature (300°c) resulted in the highest strength as well as the best strength-ductility balance. It is also found that anisotropy in rechanical properties is considerably inproved in the plate reheated at low t~ature(400°C) with the low finish-rolling tei1perature(300~C). The same hot working condition inproves exfoliation resistance of the alloy. The cbtained results are discussed in terms of grain mrphology, grain boundary precipitation, grain size and recrystallized structure.

INTRC%)UcrION

In recent years Al-Li alloys have received considerable attention frcm both scientific and industrial camunities because of their excellent low density- high modulus property. However, poor ductility of the alloys is an intrinsic problem, and this has to be inproved before the alloys are extensively used in service. Previous studies revealed that the ductility problem observed in A1-Li alloys is mainly due to strain looalization caused by 6' precipitates and 6 precipitation along the grain boundaries. (1) As in other hi~h strength Al alloys (2)(3), A1-Li alloys generally show considerable anisotropy in mechanical properties.(l) Microstructural control through themawchanical.

processing is -ted to inq?rwe the poor ductility and mechanical property anisotropy in A1-Li alloys. In the present study, the effects of

themxxwchanical processing on microstructure, mhanical behavior and exfoliation resistance are investigated for the Al-Li-Ol-Zr alloy.

EXPEEDENTAL PRCXEDURS

The alloy examined is a vacuum induction rnelted Al-Li-Cu-Zr alloy and chemical capmition of the dlloy is listed in Table I. Ingots were huncgenized at 540°c for 24 hours and were slabbed at 550'~. Both A and B slabs were hot rolled to 27mn and 52mn thick plates under several different rolling

conditions shown in Table 11. After solution treating at 550°C for 2 hours,each plate received a aging treatment at lQO°C for 24 hours to --age strength. In order to study the effect of thmncxmchanical processing an microstructure and mechanical behavior, tensile properties and microstnictures in the

longitudinal(l), long-transverse(I5) and short-transverse(ST1 directions were examined for both the 27m and 52rrPn thick plates. Fracture toughness and exfoliation resistance were also examined for 5 2 m thick plates. Fracture toughness tests using 3/4 inch thick -act type specimens were carried out according to ASlM E399. For exfoliation tests, coupon specimens(8m in thickness, 40mn in width, 80mn in length) were prepared frcm the 52wn plates and the tests were carried out acording to ASlM G34,

RESULTS

Fig.1 shows the microstructures of heat treated 2 7 m thick plates. The kigh slab reheating tanperature(550°c) with the high finish-rolling temperature

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987319

C3-164 JOURNAL DE PHYSIQUE

(450'~) results in an elongated unrecrystallized microstructure. As the finish- rolling temperature decreases at the same reheating temperature, the recrystallization fraction increases and fine equiaxed recrystallized grains are observed along the large elongated unrecrystallized grain boundaries. The low reheating temperature (400'~) results in a greater velum fraction of equiaxed recrystallized grain as q a r e d with the high reheating temperature (550'~). As shown in Fig.2, the lowest finish-rolling temperature (300'~) develops the greatest v o l ~ fraction of fine recrystallized microstruduxe in the plate. With respect to the effect of hot-working condition on

microstructure, a similar trend was observed in the 52nrm thick plates as sham in Fig.3. The results of tensile tests for the 2 7 m thick plates are shown in Fig.4. Also shown in Fig.5 are the tensile test results plotted as a function of recrystallized fraction.

For all hot working conditions examined, the differences in strength and ductility between the L and LT directions was found to be d l , but the levels of tensile strength, yeild strength and ductility in the ST direction were lower than those in the L and LT directions. It is noted that the levels of tensile strength and yeild strength inprove as the recrystallization fraction increases and that the low slab reheating tqerature (400'~) with the low finish-rolling temperature (300'~) results in the highest strength level as well as the best strength-ductility balance in all directions. It is interesting to ention that the recrystallized microstructure improves ductility in the ST direction, but not in the L and LT directions. Fracture appearances of the tensile specimens are shown in Fig.6. Intergranular failure occurs at the elongated unrecrystallized grain boundaries for the plates reheated at 550'~. Hawever, for the plates reheated at 400'~ which consist of a considerable amount of fine recrystallized structure, intergranular failure occurs at the fine recrystallized grain boundaries and little intergranular cracking at the large elongated unrecrystallized grain boundaries is observed.

The results of fracture toughness tests and exfoliation tests are given in Table I11 and Table TV ,respectively. Valid Kzcvalues were obtained in only the LT direction. Fracture toughness values of unrecrystallized structure are slightly higher than those of the partially recrystallized structure in all directions. Fig.7 shows surface appearances of the exfoliation test specimens.

The specimen taken from the plate reheated at 550°C shows extensive

exfoliation. Hwever, for the plate reheated.at 400°c, pitting corrosion is a dainant mode. A mtallographic study on the longitudinally sectioned specimen surface revealed that the observed difference in corrosion behavior in due rminly to the difference in grain mrphology as shown in Fig.8.

DISCUSSION

The present study clearly indicated that mchanical properties and exfoliation resistance of the Al-Li alloy are strongly influenecd by hot working conditions. W n g the microstructures examined, the wecrystallized structure shows the lowest strength levels and the greatest mechanical

property anisotropy. The coarse elongated grains of unrecrystallized structure appear to be responsible for the low strength levels. As shown in Fig.5, the lowest level of recrystallization results in the poorest Ciuctility in the ST direction. The reason for this is also the coarse elongated grains, since the elongated grain boundaries provide an easy crack path when a specimen is loaded in the ST direction. (4) It should be noted t k t

6

precipitates on the boundaries of elongated grains are also contributed to the low ductility.(l)

The partially recrystallized structure considerably imprwes the strength level of Al-Li alloy, although the level of elongation in the L and LT directions slightly decreases with increasing the strength level. In the ST direction, however, the recrystallized structure inprwes both the strength and ductility. This indicates that the fine equiaxed recrystallized structure is effective in inproving the strength-ductility balance in the ST direction, which in turn improves anisotropy in mechanical property.

Table I Chemical wmposition (Wt. %)

Table I1 Hot working conditions.

Fig. 1 Microstructures of 2 M plates. t

80 -

6 0 -

*>

_{Heating temp.-400}

C

40

- \

2 0

-

O'

o V A t e m p . - 5 5 0 C Ingot

A

B

300 350 400 450 500 Finish. rolling temp. [OC)

Slab reheat. temp.

PC) 550 400 550 400

Fig.2 Recrystallization fraction of hot- rolled plate cmder various rolling wndi tions

.

Finish. rolling temp.

ec) 450,350,300 370.350.300

450 300 Ingot size

(kg) 18

42

F$t:&~ess

(mrn) 120

-

²⁷

180--52

JOURNAL D E PHYSIQUE

Fig.3 Microstructures of 5 d thick plates.

Fig.4 Tensile test results for 27mn thick plate.

I

ST

0-0-

0--- 0--C ---,-

* w e

0---- _-0-- ' 0 TS

--- -- - - -

^-0

0-

0--- *-. _o---,- 0.2% PS

---_

^-0

I I I

300 350 400 450 10

- r

^-

ci

0 500-

.;; 450-

(L

E

400 t- d a E 350 N.

"

300

Finish-rolling t e m p e r a t u r e ("C)

I I

-

L -.

---0

---

⁰

o--

. / C

I

:----,:=*

--..

TS

- . .. _..

-0

0-

*-• 0.2% PS 0---

- O---

---_

-

- 1

-

300 350 400 450

I I 1 I

LT ..

---

-0--*---O

*-*/

:-- ^*-

o--t_

-.

TS

..

--

- ^.

-0 --

0-

*-•

0.2% PS

-- ^0--- _0--- --. _---o

I I

300 350 400 450

.-• L A-A L T

4-4 ST

A

0'

Volume fraction o f

recrystallized s t r u c t u r e (%) Fig.5 Tensile test results a s a function

of recrystallization fraction.

Table I11 Mechanical properties of 52 mn thick plate.

Fig.6 Fracture appearanQs of tensile specimm in the ST direction ( a l h e a t h g temp. : 400° C, finish-rolling temp. : 300° C ( b ) h e a t h g temp. : 550° C, finish-rolling temp. : 450° C

Slab reheot. temp.

('C)

550

400

Finish. rolling temp.

('C)

450

300

Direction

L L T ST L L T ST

TS (MPa)

452 442 396 510 497 453

KQ', Klc

( M P O ~ ) 31.4' 21.7 30.4' 28.9' 17.9 27.5' O.Z%PS

>a) 379 371 317 424 417 355

El (%) 8.2 8.6 2.0 5.5 5.9 3.0

JOURNAL DE PHYSIQUE

T a b l e IV E x f o l i a t i o n test results.

Specimen Unrecrystallized

Exfoliation r o t i n g E B structure(')

P a r t i a l l y r e c r y s t a l l i z e d

P i t t i n g corrosion structurec2)

Reagents: NoCl (4.OM) (1) Slab reheat. temp. : 55O0C KN03 (0.5M) Finish. rolling temp. : 4 5 0 9 C HN03 (0.1M) (2) Slab reheat. temp. : 400'C Condition : 25OCx48h Finish. rolling temp. : 300°C

F i g . 7 Apparanes of e x f o l i a t i o n test spcimms.

( a l r e h e a t temp. : 400° C, finish-rolling temp. : 300° C (b) reheat temp. : 550° C, f i n i s h - r o l l i n g temp. : 450° C

Fig. 8 m a n e s of l m g i t u d i n a l l y s e c t i c n e d specimens shm in F i g . 7.

( a ) reheat temp. : 400° C, finish-rolling temp. : 300° C

@ ) r e h e a t temp. : 550° C , finish-rolling tenp. : 450° C

The mchanical properties of the alloy in the ST direction always lower than those in the L and LT directions. This may be an indication that texture is an additional factor for the anisotropy in mechanical property, as suggested in the previous study.(5)(6)

The mrphology and size of microstructure also plays an important role in exfoliation resistance. As shown in Fig.7, the coarse elongated

unrecrystallized grains together with the grain boundary

6

precipitates provide preferable sites for exfoliation attack. In contrast with the uncrystallized structure, the partially recrystallized structure shows the localized corrosion behavior in along the fine exiaxed grain boundaries, although the initial process of corrosion appears to be the s a m as that of unrecrystallized structure.

The present study revealed that the low slab reheating temperature with the low finish-rolling temperature results in a great extent of recrystallized structure which significantly alters the matrerial's behavior. At a temperature below 450°C, undissolved

6

phase particles are present. The 6 particles (7) and Al3Zr particles (8)(9) are considered to be strain accumuration sites and pramote recrystallization during solution heat

treatment. The importance of the undissolved 6 particles for recrystallization is easily seen in Fig 2. This figure indicates that at the reheating

temperature of 40U°C which allows the presence of undissolved 6 particles, the v o l m fraction of recrystallized structure reaches dlmost 75 % with decreasing the finish rolling temperature, but without the 6 particles( reheating

temperature at 550°C 1, 80 8 of the microstructure remains as a coarse unrecrystallized structure.

a3NCLUSIONS

(1)Microstructural morphology and size of the Al-Li alloy are strongly influenced by thenncmechachanical processing. The low slab reheating

temperature cambined with low temperature finish-rolling results in a greater v o l m of fine recrystallized structure.

( 2 ) Undissolved 6 phase particles form at reheating temperature lower than

450 C. These 6 phase particles are considered to be strain accumulation sites which p r m t e recrystallization.

(3)The low slab reheating ternperatwe (400°c) with the low finish-rolling temperature (300°C) results in the highest strength and the best

strength-ductility balance. This thermomechamical processing reduces anisotropy in tensile properties. The abtained improvement in property is considered to be due to the refinement of microstructure through recrystallization.

(4) Corrosion behavior of the Al-Li-Ch-Zr alloy depends upon grain mrphology and grain boundary precipitates. The elongated unrecrystallized grain boundaries together with grain boundary precipitates provide a preferable crack path, resulting in extensive exfoliation. The microstructure containing partially recrystallized grains improves exfoliation resistance.

FBFERENCES

(l)T.H.Sanders,Jr., Mat. Sci. and Eng.,43,(1980),p247 (2)E.D.Russo et al., Mat. Sci. and Eng.,l4,(1974),p23 (3)J.Waldmn et al., Net. Trans.A,5A,(1974),p573

(4)P.J.hrbar et al., "Aluminium-Lithium Alloys 111" p.496,(1986),The Institute of Metals

(5)I.G.Palmer et al., ibid.,p.56

(6)P. J.Gregsson and H. J. Flower, Acta. Met., 33, (1985) ,p527

( 7)M. Niikura et al

. ,

"Aluminium-Lithium Alloys 111" p.213, (1986 ,The Institute of Metals

(8)E.A.Starke,Jr.et al., J. of Metals,August,(l981) p.24

( 9 )G.Channani et dl., "Aluminum-Lithium Alloys" p.369 ,AIMF, Warrendale ,PA.