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PROCESSING AND PERFORMANCE OF Al-Li-Cu-X EXTRUSIONS
M. Skillingberg, R. Ashton
To cite this version:
M. Skillingberg, R. Ashton. PROCESSING AND PERFORMANCE OF Al-Li-Cu-X EXTRUSIONS.
Journal de Physique Colloques, 1987, 48 (C3), pp.C3-179-C3-186. �10.1051/jphyscol:1987321�. �jpa- 00226551�
JOURNAL DE PHYSIQUE
Colloque C3, supplement au n09, Tome 48, septembre 1987
PROCESSING AND PERFORMANCE OF A1-Li-Cu-X EXTRUSIONS
M.H. .SKILLINGBERG and R.F. ASHTON
Reynolds Metals Company, Metallurgy Laboratory, ~ichaond, V A 23219, U.S.A.
ABSTRACT
The extrusion chmckristics of 2090 (Al-Cu-Li) and 8090 and 8091 (Al-Li-Cu-Mg) alloys from the standpoints of ease of extrusion and ampatibility with plant operations were investigated. Ekprhtation
was condlucted to determine the effects of extrusion temperature and extmsion rate on extrudability and prcduct p e r f o ~ c e . Structure studies were carried out to explain variations in-cal properties.
E x p r h t a l extrusion limit diagrams for Al-Li-Cu and Al-Li-Cu-Mg alloys showed that the addition of magnesium lowered the working range by increasing the flow stress and enhancing the onset of incipient melting ard surface tearing, particularly at the higher alloying levels.
Plant trials were conducted with 2090 alloy. The extrudability of this allby was mre similar to 6061 alloy £ram the shxipints of press effort and run out speed than to the conventional 2)(XX and 7 W aircraft alloys.
The mechanical properties of the extrusions, were a function of micros-e and test direction. High longitudmal ultimate tensile and yield strength (>600 MPa) and gocd toughness were obtained in the pfz+ strength condition when the extrusions had fibrous, elongated grams. Transverse strengths and toughness were substantially lower.
Material with a mre remy.stal.l,ized, quiaxed grain structure showed less variation with test dneclxon, but significantly lower strengths were obtained.
It was concluded that 2090, 8090 and 8091 alloys can be extruded using conventional equipment am3 methods. Alloy 2090 is easier to extrude than the Mg-hearhg ccnapositions. T h e mechamcal properties of these alloys are very structure sensitive, w h i c h can be controlled to scam extent by the extrusion paramters.
There has been a virtual explosion of published infomation concerning Al-Li alloys since 1980 when the First International Alminutn-Lithium Conference was held (1-3). However, the bulk of information has been related to the physical metallurgy and properties of the alloys. In order to gain confidence in Al-Li alloys, the effects of variations in processing m t be better ~erstood an3 the information disseminated. Earlier papers covered the effects of processing variations on the properties of Al-Li-Cu and Al-Li-Cu-Mg alloy plate (4,5). The present paper discusses the extrudability of Al-Li-Cu-(Mg) alloys. The rela- tionships between processing conditions and extrusion properties were examined along with the cmnpatibility of A1-Li alloy extrusion with current plant P
- .
EWERlMENTAZlPROCEIXlRES
The bulk of the material used in the laboratory trials was DC cast as 20 x 40
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987321
C3-180 JOURNAL DE PHYSIQUE
Cm rectangular ingots from which 7.5 cm diameter by 23 a long extrusion b i l l e t s were machined. A limited rnrmber of b i l l e t s were machined f m 10 x 20 cm rectangular permanent mold ingots. Alloys 2024 and 6061, which, were extruded for mnpmson, were DC cast as 7.5 an &ameter logs. The 2090 b l l l e t s extruded m plant trials w e r e IX: cast either a t Reynolds Metals as 35 cm diameter b i l l e t o r a t Kaiser Almbmm as 40 cm diameter billet. The alloys were homcgenized based on practices previously developed l# Ashton e t al. (4,5) for rolling i n g ~ t s ; ~ t h e s e practices were 24 hours a t 541 C for 2090 and 8090 and 24 hours a t 532 C for alloys 2091 and 8091.
The laboratory extrusions were produced using a 640 ton direct press w i t h a 8.25 an diameter container. The b i l l e t s w q preheated i n a circulating a i r furnace a t 524-537Oc and extruded a t 385-524 C with the container t a @ m t u r e typically 42-56O~ belai the b i l l e t t a p r a t u r e . Extmsion ratios ranged between
1 O : l an3 6 4 : l and ram speeds were varied f m 15-56 cm per minute. All the extrusions were fan cooled. FXcept for a limited number of rectangular sections extruded for cconparison with 2024 and 6061 alloys, all of the laboratory extrusions were p r d u c d as round rod.
The solution heat treatment practices used were 30 minutes a t 541°c for 2090 and 8090 and 30 minutes a t 532Pc for 8091. A l l the extrusions were cold water quenched and stretched 5%. Aguq practices used t o develop W g e d and near peak aged properties are given i n Table 1 for the various alloys.
LDngitudinal tension tests were performe3 on laboratory extruded m a t e r i a l using standard and subsize Xm B 557 threaded-erd round specimens. The tension specimen size depended upon the original rcd diameter, with the largest compatible specimen used i n each instance. A limited number of tension tests were also performed on f u l l SBction rcd samples. Tuu#mess testing was performed on the 1.75 an and 2.5 an rcd using subsize threaded-erd 1.27 cm sharpnotch tensile specimens (RS?M E 602).
Tension tests on the plant produced extrusions were performed i n both the longitudinal and transverse directions us* either recbng+ar o r mud specimens. Toughness testing was pesformed in the LrT and T-L -ions using sharpnotch (50.001 inch radius) Cbarpy impact spechem (Modified ASlM E 23) .
RESULTS AND DISCUSSION LAEORATORY 'l?BTmG
Al-Li-cu Alloy
IXlring initial laboratory trials a series of 2090 b i l l e t s was extruded under a range of processing conditions which included variations in extrusion ratio, ram
speed and b i l l e t taperature. A series of curves of peak extrusion pressure versus b i l l e t tempsrature was developed and are Shawn i n Figures 1 and 2. Feak pressure varied directly w i t h extrusion r a t i o and ram speed and inversely with b i l l e t tenperature, which is consistent with data reported by others for both conventional and Al-u-X alloys (6,7) .
The range of conditions under w h i c h an allay can be extruded is governed by the available pressure of the press and by the susceptibility of the alloy t o incipient melting and surface tearing. Extrusion limit diagrams can be developed, us* these two limiting factors, which idicate the potential range of processing condrtions. This T emay be further limited by the need or desire for specific structures, mechamcal properties or additional processing limitations. Laue and S'cenger (8) have sham a g e d i z e d example of these limit diagram, and experimental limit diagrams for A1-Li-Mg and Al-Li-Mg-Cu alloys have been developed by Parson and Sheppard (9).
A similar l i m i t diagram was developed for 2090 alloy as sham i n Figure 3.
Increasing either the extrusion ratio o r the ram speed reduces the acceptable ranqe of extrusion t a p m t u r e s by increasing the pressure or increasing the
+ . Acceptable Unacceptable = 22-/min.
Surhce
A Unacceptable Limited
Table 1. Aging Practices
9 5 -
Alloy Under Age Near Peak Age
Temp. C Hrs. Temp. C Hrs. =
2090 1 2 1 36 149 36 'z 4 - * \ \ *. '\ A
8090 135 34 1 6 1 38 Y \
1 6 1 48 \
8091 1 2 1 36 II
1 3 -
- \ \
Billet Temperature -- Degrees F (Degrees C)
Figure 1. Effect o f Extrusion Ratio on the Peak Pressure of 2090 Alloy Extrusions.
Billet Temperature -- Degrees F (Degrees C) Figurc 3. Experimental Extrusion Limit
Extrusion Ram Speed Ratlo V l m i n . 22"Imin.
1000 psi
700 (400) ' 'O0 (d;O) '
(5001 Billet Temperature -- Degrees F
ID... CI
Extrusion Ratio / MPa
. - . - . . . - ,
Figure 2. Effect of Ram Speed on Peak Pressure Figure 4. Typical Longitudinal Grain Structure Near of 2090 Alloy Extrusions. Trailing End o f 13 m m 2090 Rod Extrusion
A. Surface B. Center Diagram -- 2090 Billet
JOURNAL DE PHYSIQUE
arcmunt of heat generated (or reducing the heat dissipated) at the die bearing Surface w h i c h pmmtes the onset of surface tearing or incipient melting.
An examination of the extrusion microstrum after solution-heat treatment revealed a duplex structure consisting of a recrystallized layer at the surface
a fibrous, mreaystallized core structure, as shuwn in Figure 4. This. duplex structure was most promuncd near the tail-end of the extrusion where the recrystdllized zone was thickest. At the leading of the extrusions the depth and degree of m l i z a t i o n was mch less than near the trailing end. A
slight trerx3 of uLcreaslng recrystallization w i t h increased extrusion temperature
was chccved and is Shawn in Figure 5 . This is apposite of the trend reported by Parson and Sheppazd (9) for Z+-Li;Mg allays. Imreasd extrusion ratio also resulted in increased remystalllzatlon as was seen by Parson and She@.
An examination of as-extxuded m--i mealed a duplex structure similar to that seen after heat treatment but with same differences. The zone near the surface of the as-extruded rods appeared to consist of very fine
I I ~ubgrains~~ with similar crystallcgrapkic orientations as wideme3 by a lack of
contrast when examined after a Barker's electrolytic etch. The subgrain size was also a finer size (-1-3ym) than the recrystallized grain size (-5-lop) after solution heat trea-. The depth of the fine subgrain region in the as-extruded rcd was very similar to that of the fine recrystallized grain region in solution treated rods. T h i s wcRild indicate that wkile same or all rexystallization may have ocmrred during s u b s q m k heat-treaatmentS, the final microstructure is
largely determined by what cccurs during the extrusion process. Additional testing, including TEN examination of the micrpstrudures, is needed to detexmine the exact nature and relati-p between the as-extruded subgrain structure and the recrystallized structure after heat treatment.
The results of tension tests with reduced section round specimens showed only a slight tmn3 of decreased s t r a q t h with increased extrusion tenperature, *as
&awn in Figure 6. A mre significant relationship between strength and extrusion tmperature had been e x p c t d based on the variation in rexystallization &served but was probably lessened due to the 1- of the near-surface s- during machining of the specimens. To deterrmne whether recrystallization reduced
tension stmyth, full section rod specinens were tested frcnu the head (largely unrecrystdllized) and tail (substantial recrystallization) regions of additional
rcd extrusions. Figure 7 clearly shaws 1- strength in the mre recrystallized tail regions as had been ezrpected.
?he effect of B%J on extmdability was examined by a limited rnrmber of 2091, 8090 +. 8091 billets using a 22:l extmsion ra= shmin Figure 8, the m-cmkmmg alloys cited higher peak pressures than 2090 alloy at a given extrusion knpemture. The lcw Cu content of 8090 offset most of the effect of Mg and resulted in only slightly higher peak pressure as cmpred to 2090. The higher total alloying levels of 8091 and 2091 resulted in prcgressively increasing peak pressures. An estimated extrusion limit diagram for the Mg containing alloys based on the results of the single extrusion ratio examined is ,given in Figure 9
where 8091 and 2091 have been ca&ined for sinplicity. The -P
reqxiqd to tha m-containing allays -the =range of extrusion corditions as shown on the left (1- temperature limit) side of the figure. In addition, the higher total alloying content of 2091 and 8091 substantially reduces the upper tmpzature limit due to the onset of incipient melting and surface tearing. T h i s was particularly evident when using the h i e ram speed wfiere no acceptable surfaces were obtained even at the lcrwest (-400 C) extntsion knpemture evaluated. It appeared that for 8090 alloy the surface limited conditions would be similar to those for 2090 and that overall the two allays would extrude soaaewhat similarly. However, 2091 and 8091 allays appear to have a ma& more limited range of acceptable extrusion conditions.
75 - 62: 0 0 . 4000 - -7 + 8090 8091
.. 10 A A - 25
- -
: so - - 20
Y
2000 -
I I
700 1000 - I I
(400) (450) (sO0) 'OoO 700 (400) 800 (450) (500) loo0
Billet Tempcrrture -- D~~~~~~ F Billet Temperature -- Degree* F
(Degrees C) (Degrees C)
F t u r e 5. Effect of Extrusion Conditions o n Recrystallization Figure 8. Effect of Alloy o n Peak Extrusion Pressure.
of 2090 Rods A f t e r Solution Heat Treatment.
ksi MPa
Extrusion Ratio = 22
800
I Peak Aged
MPa
f ; Ram Speed W i t h / = Tail
6"/min. 2T'lmin. Without = Head
I
;e 2090
= 22 =A= 2091 Ratio 6"lmin. 22"lmin.
70 Under Aged
I , I I I
700
(400) '0° SO)(: (500) looo
Billct Temperature -- Degrees F (Degrees C)
Figure 6. Effect o f Processing Conditions o n the Yield Strength of 2090 Extrusions. (Reduced Section Specimens)
psi
ksi
Peak Aped
I I I I I
700 (4d0) 800 (PA) (5b0) looo
Billet Temprraturr -- Degrrrr F
( D c ~ t e * C )
Figure 7. Effect of Processing Conditions and Specimen Location o n the Yield Strength o f 2090 Extrusions (Full Section Tension Specimens)
Billet Temperature -- Degrees F (Degrees C)
Figure 9. Experimental Extrusion Limit Diagram
With I = Tail Without = Head
I I , I I
700
( 4 k ) 'O0 900 1000
(450) WlO)
bi MPa
Billet Temperature -- Degrees F
(Degrees C)
Figure 10. Effect o f Processing Conditions o n the Yield Strength of 8090 and 8091 Extrusions.
100 Extrusion Ratio = 22 Ram Speed
- 6"Imin. 22"lmin. 700
8090 0
8091
JOURNAL DE PHYSIQUE
The 8090 and 8091 extrusion micmstructures follawed the same trends as did the 2090 extrusions with a duplex grain s+axc%wx and a slight increase in recrystallization with increasing extrusion . Differences were observed in that the 8090 and 8091 had a 1- --ot anmunt of recrystallization
and that both the recrystallized and, particularly, the unreaystallized core grains were smaller than those of the 2090 extrusions.
T h e 8090 and 8091 extrusions also follaJed the same trend of decreasing strength with increasing extrusion tenperature as did 2090 (see Figure 10). A limited of notch tensile tests were conduckl with 2090, 8090 and 8091. The results (see Figure 11) shaw that the 8 m extrusions have a higher notch strength/yield strength ratio than the 2090 extrusions at a given yield strength.
A strong inverse relationship between yield strength and the &-yield ratio was seen for both groups. No correlation of notch-yield ratio with extrusion ratio, extrusion temperature, aging practice or specimen location was obsaxed, other than the effect these variables had on yield strength.
A canparison was made between the extrudability of 2090 and 8090 alloys and that of conventional extrusion alloys 2024 and 6061. An extrusion ratio of 15 to 1 and a ram speed of 15-30 an per minute were used to prcduce a 6 m flat bar.
The d t s , given in Figure 12, show 2090 and 8090 to extrude more easily than 2024 and at pressures and t e n p r a t u r e s which approach those used for 6061.
Bar stock and channel extrusions have been produoed f m 35 an diame*
billet and an I-beam type shape (see Figure 13) f m 40 a billet. No particular difficulties were exprienced in extruding these sections, and the extrudability was between that of 6061 and 2024, as had been indicated by the laboratory testing.
4- curves developed using material f m the heavy section of the I-beam shape are given in Figure 14 and shw that high longitudhal yield strengths (>500
MPa) can be achieved with a range of aging conditions. When d i n e d with CS.larpy impact energy, as Shawn in Figure 15, it can be seen that various strerqth-toughness cabhations can be obtained depe.m%ng upon the processing conditions selected.
High and law strength practices were select& for £urther characterization of the extrusions with the work being concentrated on the I-beam section due to its canbination of heavy and thin cross-Wonal areas. G o d uniformity was obsaved the 1- of the extrusions, but large variations in strength and toucjiness were seen due to test direction and section thickness. As shown in Figure 16, the highest ambination of tension strength ard CCnarpy impact energy was in the longitudinal (W) direction of the heavy section. Substantially lmer strength and impact energies were obtained when the heavy sections were tested in the tral~~erse (T-L) direction. The lighter section (web) exhibited little variation due to +st direction and had yield strength-charpy impact energy canbinations in-ate to those of the thicker sections. Similar variations, although of a lesser magnitude, have been reported by Reynolds et al. (10) for Al-Li-Cu-!Q
alloys. In addition to test direction and section thidmess, test location affected properties which were particvlarly notable in the heavier sections.
Differences of -35-85 MPa were noted between the edge and center of various sections, but unlike other reports (lo), the lower strength area was not always at the standard test location.
Examination of the -m-i of the various extrusion test locations indicated that grain stru- and aspect ratio along with texture variations account for most of the property variations. In the case of the lfiI-beamll section, the heavier sections had primarily an elongated fib- grain structure with only a mall amount of recrystallization. The thinner section had a mre I1pancake1I shaped g + n s t r w t u ~ and a higher percMtage of recrystallization. The interrelatlonskip between grain structure and aging practices and their effect on
8090/8091
7
" t . 0
With 1 = Tail .
Without = Head
Yield Strength -- bi (MP.1
Figure 11, Notch Tensile Strength-Yield Strength Ratio Vr.
Yield Strength. Billet Temperature -- Ovgre~s F
(Degree. C) Figure 12. Comparison of Peak Extrusion Pressures
of At-Li Alloys w i t h Conventional Extrusion Alloys.
rA 275 deg. F (135 deg 250 deg F (121 deg T7 C) C) TOO
Asins Time -- Hours
Figure 13. Cross-Sectional View of 2090 'I-Beam"
Extrusion. Figure 14. Aging Curves for 2090 Extrusion.
(Section Thickness -1.0")
Longitudinal Yield Strength -- ksi IMP=) Figure 15. Effect af Aging Practice on the
Strength and Charpy Inlpact Energy of 2090 Extrusion.
Yield Strength -- krf (MP.1
Figure 16. Effect of Test Direction and Section Thickness on the Yield Strength and Charpy Impact Energy of a 2090 -1-Beam*
Extrusion.
JOURNAL DE PHYSIQUE
fracture paths and stzrem$h--s cambinations appeared to be typical of that reported elsewfiere (4,5,11-13) and will not be further discussed here.
1. Exp=rimental extrusion limit diagrams for Al-Li-Cu and Al-Li-Cu-I@ alloys indicate that the addition of I@ lcrwers the working range by both increasing the flaw stress and enhancing the onset of incipient melting ard surface tearing particularly at the higher alloying levels.
2. The extrusion grain structure is influenced by the extrusion ratio and billet -ture w i t h increased teqerat;w" and ratios in the range examined bxduq to cause mm recrystallization.
3. Mechanical properties are strongly influenced by recrysfallization and gfzlin shape. Elongated, umsrystallized grains resulted m high lorgitudmal s t m q t b and t o u ~ e s s but law tmnsverse prcrperties. Recrystallization and lpmxlce*l shaped grains resulted in less arusotropy but lmer lcagitulhal strength and toughness.
4. Alloy 2090 can readily be exkuded in a praductibn e n v h r r m ~ t . The extrudability of 2090 is closer to that of 6061 from the stan3point of press effort and extrusion tenperatures as canpar& to current high strength 2XXX and 'IXXX alloys.
1. AlminimLithium Allow, E. A. Starke, Jr. an3 T. H. Sanders, Jr., editors.
' I M S - m , Warrenddle, PA, 1981.
2. Alminium-r;ithim Allow 11, E. A. Starke, Jr. and T. H. Sandess, Jr., editors, 'IMS-AIME, Warrenddle, PA, 1984.
3. A l m m d t h i u m Allow 111, C. Bdker, P. J. G r q a n , S. J. Harris, an3 C.
J. Peel, editors, The Institute of Metdls, Lcmlon, 1986.
4. R. F. Ashton, et al., Ibid, pq. 66-77.
5. R. F. Ashton, et dl., Alununium Allovs - Their Fhvsical and Mechanicdl hraDertiw-Vol. 1, EMAS, 1986, pp. 403-417.
6. N. C. J?arson and T. Sheppard, Aluminium-Lithium All- 11, I M S - A m , 1984,
m. 53-64.
7. K sheppxd, S. J. Paterson, M. G. Micxcstructural Control in Aluminium Allws: Deformation, Recovery and -- Recmstallization, W - A I M E , 1985. pp. 123-154.
8. K. Iaue and H. Stenger, IWrusion: hrocesses. Machbhf. ?bolbq, X N , M e w s Park, OH, 1981, pp. 34-37.
9. N. C. Parson and T. sheppd, Aluminium-Lithium Allavs 111, Ihe Institute of Metals. Um3on, 1986. m. 222-232.
lo. M. A. hqm1ds; et ai.;- mid, pp. 57-65.
11. W. S. Miller etal., Ibid, pp. 584-594.
12. K. V. Jatd and E. A. Starke, Jr., Ibid, pp. 247-255.
13. S. Srrresh and A. K. ,-V Ibid, pp. 595-601.
