U.S. patent application number 10/334388 was filed with the patent office on 2007-01-04 for aluminum sheet products having improved fatigue crack growth resistance and methods of making same.
Invention is credited to Dhruba J. Chakrabarti, Diana K. Denzer, Paul E. Magnusen, Anthony Morales, Roberto J. Rioja, Anne E. Roberts, Gregory B. Venema, Robert W. Westerlund.
Application Number | 20070000583 10/334388 |
Document ID | / |
Family ID | 24368434 |
Filed Date | 2007-01-04 |
United States Patent
Application |
20070000583 |
Kind Code |
A1 |
Rioja; Roberto J. ; et
al. |
January 4, 2007 |
Aluminum sheet products having improved fatigue crack growth
resistance and methods of making same
Abstract
Aluminum sheet products having highly anisotropic grain
microstructures and highly textured crystallographic
microstructures are disclosed. The products exhibit improved
strength and improved resistance to fatigue crack growth, as well
as other advantageous properties such as improved combinations of
strength and fracture toughness. The sheet products are useful for
aerospace and other applications, particularly aircraft
fuselages.
Inventors: |
Rioja; Roberto J.; (Alcoa
Center, PA) ; Westerlund; Robert W.; (Alcoa Center,
PA) ; Roberts; Anne E.; (Alcoa Center, PA) ;
Chakrabarti; Dhruba J.; (Alcoa Center, PA) ; Denzer;
Diana K.; (Alcoa Center, PA) ; Morales; Anthony;
(Alcoa Center, PA) ; Magnusen; Paul E.; (Alcoa
Center, PA) ; Venema; Gregory B.; (Alcoa Center,
PA) |
Correspondence
Address: |
INTELLECTUAL PROPERTY
ALCOA TECHNICAL CENTER, BUILDING C
100 TECHNICAL DRIVE
ALCOA CENTER
PA
15069-0001
US
|
Family ID: |
24368434 |
Appl. No.: |
10/334388 |
Filed: |
December 31, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09591904 |
Jun 12, 2000 |
6562154 |
|
|
10334388 |
Dec 31, 2002 |
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Current U.S.
Class: |
148/694 |
Current CPC
Class: |
C22F 1/05 20130101; C22F
1/04 20130101; C22C 21/08 20130101; C22F 1/053 20130101; C22C 21/10
20130101; C22C 21/16 20130101; C22C 21/02 20130101; C22F 1/057
20130101 |
Class at
Publication: |
148/694 |
International
Class: |
C22F 1/04 20060101
C22F001/04 |
Claims
1-100. (canceled)
101. a method of making an aluminum allov sheet product, the
method: comprising: providing an aluminum alloy; hot rolling the
aluminum alloy to form a sheet; recovery annealing the hot rolled
sheet; solution heat treating the recovery annealed sheet; and
recovering a sheet product comprising an anisotropic microstructure
defined by grains having an average length to width aspect ratio of
greater than about 4 to 1.
102. The method of claim 101, wherein the recovery anneal is
performed at a temperature of from about 300 to about 1,000.degree.
F. for a time of from about 0.5 to about 96 hours.
103. The method of claim 101, wherein the recovery anneal is
performed at a temperature of from about 400 to about 700.degree.
F.
104. The method of claim 103, wherein the sheet product is
unrecrystallized.
105. The method of claim 101, wherein the recovery anneal is
performed at a temperature of from about 600 to about 1,000.degree.
F.
106. The method of claim 105, wherein the sheet product is
recrystallized.
107. The method of claim 101, further comprising intermediate
annealing the sheet prior to the recovery anneal.
108. The method of claim 107, wherein the intermediate anneal is
performed during the hot rolling.
109. The method of claim 107, wherein the intermediate anneal is
performed at a temperature of from about 400 to about 1,000.degree.
F.
110. The method of claim 101, further comprising cold rolling the
sheet after the hot rolling.
111. The method of claim 110, further comprising intermediate
annealing the, sheet prior to the recovery anneal.
112. The method of claim 111, wherein the intermediate anneal is
performed during the cold rolling.
113. The method of claim 112, further comprising intermediate
annealing during the hot rolling.
114. The method of claim 110, wherein the intermediate anneal is
performed at a temperature of from about 400 to about 1,000.degree.
F.
115. The method of claim 101. wherein the hot rolling step includes
multiple hot rolling operations.
116. The method of claim 115, wherein the hot rolling operations
include finish hot rolling prior to the recovery anneal.
117. The method of claim 116, further comprising intermediate
annealing the sheet during the finish hot rolling.
118. The method of claim 101, further comprising cold working the
solution heat treated sheet.
119. The method of claim 101, wherein the aluminum alloy is an
Al--Cu alloy comprising aluminum, from about 1 to about 5 weight
percent Cu, up to about 6 weight percent Mg, up to about 1 weight
percent Mn, and up to about 0.5 weight percent Zr.
120. The method of claim 101, wherein the Al--Cu base alloy
comprises at least about 3 weight percent Cu.
121. The method of claim 101, wherein the Al--Cu base alloy
includes from about 3.5 to about 4.5 weight percent Cu, from about
0.6 to about 1.6 weight percent Mg, from about 0.3 to about 0.7
weight percent Mn, and from about 0.08 to about 0.13 weight percent
Zr.
122. The method of claim 101, wherein the Al--Cu base alloy
includes from about 3.8 to about 4.4 weight percent Cu, from about
0.3 to about 0.7 weight percent Mn, from about 1.0 to about 1.6
weight percent Mg, and from about 0.09 to about 0.12 weight percent
Zr.
123. The method of claim 101, wherein the Al--Cu base alloy
includes from about 3.4 to about 4.0 weight percent Cu, from 0 to
about 0.4 weight percent Mn, from about 1.0 to about 1.6 weight
percent Mg, and from about 0.09 to about 0.12 weight percent
Zr.
124. The method of claim 101, wherein the Al--Cu base alloy
includes from about 3.2 to about 3.8 weight percent Cu, from about
0.3 to about 0.7 weight percent Mn, from about 1.0 to about 1.6
weight percent Mg, from about 0.09 to about 0.12 weight percent Zr,
and from about 0.25 to about 0.75 weight percent Li.
125. The method of claim 101, wherein the aluminum alloy is an
Al--Mg base alloy comprising aluminum, from about 0.2 to about 7
weight percent Mg, from 0 to about 1 weight percent Mn, from 0 to
about 1.5 weight percent Cu, from 0 to about 3 weight percent Zn,
and from 0 to about 0.5 weight percent Si.
126. The method of claim 101, wherein the aluminum alloy is an
Al--Mg--Si base alloy comprising aluminum, from about 0.1 to about
2.5 weight percent Mg, from about 0.1 to about 2.5 weight percent
Si, from 0 to about 2 weight percent Cu, from 0 to about 3 weight
percent Zn, and from 0 to about 1 weight percent Li.
127. The method of claim 101, wherein the aluminum alloy is an
Al--Zn base alloy comprising aluminum, from about 1 to about 10
weight percent Zn, from about 0.1 to about 3 weight percent Cu,
from about 0.1 to about 3 weight percent Mg, from 0 to about 2
weight percent Li, and from 0 to about 2 weight percent Ag.
128. The method of claim 101, wherein the sheet product has a
thickness of up to about 0.35 inch.
129. The method of claim 101, wherein the sheet product is
unrecrystallized.
130. The method of claim 129, wherein the unrecrystallized sheet
product has a Brass texture of greater than 20.
131. The method of claim 129, wherein the unrecrystallized sheet
product has a Brass texture of greater than 30.
132. The method of claim 129, wherein the unrecrystallized sheet
product has a Brass texture of greater than 40.
133. The method of claim 101, wherein the sheet product is
recrystallized.
134. The method of claim 133, wherein the recrystallized sheet
product has a Goss texture of greater than 20.
135. The method of claim 133, wherein the recrystallized sheet
product has a Goss texture of greater than 30.
136. The method of claim 133, wherein the recrystallized sheet
product has a Goss texture of greater than 40.
137. A method of making an aluminum alloy sheet product, the method
comprising: providing an aluminum alloy; hot rolling the aluminum
alloy to form a sheet; intermediate annealing the hot rolled sheet;
solution heat treating the intermediate annealed sheet; recovering
a sheet product comprising an anisotropic microstructure defined by
grains having an average length to width aspect ratio of greater
than about 4 to 1.
138. The method of claim 137, wherein the intermediate anneal is
performed at a temperature of from about 400 to about 1,000.degree.
F.
139. The method of claim 137, wherein the intermediate anneal is
performed during the hot rolling.
140. The method of claim 137, further comprising recovery annealing
the sheet after the intermediate anneal and prior to the solution
heat treatment.
141. The method of claim 137, further comprising cold rolling the
sheet after the hot rolling.
142. The method of claim 141, wherein the intermediate anneal is
performed during the cold rolling.
143. The method of claim 142, further comprising recovery annealing
the sheet after the cold rolling.
144. The method of claim 142, further comprising performing another
intermediate annealing during the hot rolling.
145. The method of claim 144, further comprising recovery annealing
thesheet after the cold rolling.
146. The method of claim 137, further comprising cold working the
solution heat treated sheet.
147. The method of claim 137, wherein the aluminum alloy is an
Al--Cu alloy comprising aluminum, from about 1 to about 5 weight
percent Cu, up to about 6 weight percent Mg, up to about 1 weight
percent Mn, and up to about 0.5 weight percent Zr.
148. The method of claim 147, wherein the Al--Cu base alloy
comprises at least about 3 weight percent Cu.
149. The method of claim 147, wherein the Al--Cu base alloy
includes from about 3.5 to about 4.5 weight percent Cu, from about
0.6 to about 1.6 weight percent Mg, from about 0.3 to about 0.7
weight percent Mn, and from about 0.08 to about 0. 13 weight
percent Zr.
150. The method of claim 147, wherein the Al--Cu base alloy
includes from about 3.8 to about 4.4 weight percent Cu, from about
0.3 to about 0.7 weight percent Mn, from about 1.0 to about 1.6
weight percent Mg, and from about 0.09 to about 0.12 weight percent
Zr.
151. The method of claim 147, wherein the Al--Cu base alloy
includes from about 3.4 to about 4.0 weight percent Cu, from 0 to
about 0.4 weight percent Mn, from about 1.0 to about 1.6 weight
percent Mg, and from about 0.09 to about 0.12 weight percent
Zr.
152. The method of claim 147, wherein the Al--Cu base alloy
includes from about 3.2 to about 3.8 weight percent Cu, from about
0.3 to about 0.7 weight percent Mn, from about 1.0 to about 1.6
weight percent Mg, from about 0.09 to about 0.12 weight percent Zr,
and from about 0.25 to about 0.75 weight percent Li.
153. The method of claim 137, wherein the aluminum alloy is an
Al--Mg base alloy comprising aluminum, from about 0.2 to about 7
weight percent Mg, from 0 to about 1 weight percent Mn, from 0 to
about 1.5 weight percent Cu, from 0 to about 3 weight percent Zn,
and from 0 to about 0.5 weight percent Si.
154. The method of claim 137, wherein the aluminum alloy is an
Al--Mg--Si base alloy comprising aluminum, from about 0.1 to about
2.5 weight percent Mg, from about 0.1 to about 2.5 weight percent
Si, from 0 to about 2 weight percent Cu, from 0 to about 3 weight
percent Zn, and from 0 to about 1 weight percent Li.
155. The method of claim 137, wherein the aluminum alloy is an
Al--Zn base alloy comprising aluminum, from about 1 to about 10
weight percent Zn, from about 0.1 to about 3 weight percent Cu,
from about 0.1 to about 3 weight percent Mg, from 0 to about 2
weight percent Li, and from 0 to about 2 weight percent Ag.
156. The method of claim 137, wherein the sheet product has a
thickness of up to about 0.35 inch.
157. The method of claim 137, wherein the sheet product is
unrecrystallized.
158. The method of claim 157, wherein the unrecrystallized sheet
product has a Brass texture of greater than 20.
159. The method of claim 157, wherein the unrecrystallized sheet
product has a Brass texture of greater than 30.
160. The method of claim 157, wherein the unrecrvstallized sheet
product has a Brass texture of greater than 40.
161. The method of claim 137, wherein the sheet product is
recrystallized.
162. The method of claim 161, wherein the recrystallized sheet
product has a Goss texture of greater than 20.
163. The method of claim 161, wherein the recrystallized sheet
product has a Goss texture of greater than 30.
164. The method of claim 161, wherein the recrystallized sheet
product has a Goss texture of greater than 40.
165. The method of claim 101, wherein the sheet product comprises
unrecrystallized grains having a Brass texture of greater than 20
and/or recrystallized grains having a Goss texture of greater than
20.
166. The method of claim 165, wherein the aluminum alloy is
substantially free of Li, and comprises a maximum of 0.7 weight
percent Mn and a maximum of 0.2 weight percent Si.
167. The method of claim 165, wherein the aluminum alloy comprises
from about 1 to about 5 weight percent Cu, up to about 6 weight
percent Mg, up to about 0.7 weight percent Mn, up to about 0.5
weight percent Zr, up to about 0.2 weight percent Si, and is
substantially free of Li.
168. The method of claim 165, wherein the aluminum alloy is
substantially free of Li, and comprises a maximum of 4.0 weight
percent Cu and a maximum of 0.2 weight percent Si.
169. The method of claim 165, wherein the aluminum alloy is
substantially free of Li, and comprises a maximum of 4.0 weight
percent Cu, a maximum of 0.7 weight percent Mn and a maximum of 0.2
weight percent Si.
170. The method of claim 137, wherein the sheet product comprises
unrecrystallized grains having a Brass texture of greater than 20
and/or recrystallized grains having a Goss texture of greater than
20.
171. The method of claim 170, wherein the aluminum alloy is
substantially free of Li, and comprises a maximum of 0.7 weight
percent Mn and a maximum of 0.2 weight percent Si.
172. The method of claim 170, wherein the aluminum alloy comprises
from about 1 to about 5 weight percent Cu, up to about 6 weight
percent Mg, up to about 0.7 weight percent Mn, up to about 0.5
weight percent Zr, up to about 0.2 weight percent Si, and is
substantially free of Li.
173. The method of claim 170, wherein the aluminum alloy is
substantially free of Li, and comprises a maximum of 4.0 weight
percent Cu and a maximum of 0.2 weight percent Si.
174. The method of claim 170, wherein the aluminum alloy is
substantially free of Li, and comprises a maximum of 4.0 weight
percent Cu, a maximum of 0.7 weight percent Mn and a maximum of 0.2
weight percent Si.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to the production of rolled
aluminum products having improved properties. More particularly,
the invention relates to the manufacture of aluminum sheet products
having controlled microstructures, which exhibit improved strength
and fatigue crack growth resistance. The sheet products are useful
for aerospace applications such as aircraft fuselages, as well as
other applications.
BACKGROUND INFORMATION
[0002] Aircraft components such as fuselages are typically
fabricated from aluminum sheet products. Resistance to the growth
of fatigue cracks in such aerospace products is very important.
Better fatigue crack growth resistance means that cracks will grow
slower, thus making aircraft safer because small cracks can be more
readily detected before they achieve a critical size which could
lead to a catastrophic failure. In addition, slow crack growth can
have an economic benefit because longer inspection intervals may be
used. U.S. Pat. No. 5,213,639 to Colvin et al. discloses aluminum
alloy products useful for aircraft applications.
[0003] The present invention provides rolled aluminum sheet
products having improved resistance to fatigue crack growth, as
well as other advantageous properties including improved
combinations of strength and fracture toughness.
SUMMARY OF THE INVENTION
[0004] Aluminum sheet products fabricated in accordance with the
present invention exhibit improved resistance to the propagation of
cracks. Aluminum alloy compositions and processing parameters are
controlled in order to increase fatigue crack growth resistance.
This resistance is a result of a highly anisotropic grain
microstructure which forces cracks to experience a transgranular or
an intergranular tortuous propagation path. The number of cycles
required to propagate these tortuous cracks to a critical crack
length is significantly greater than the number of cycles required
to propagate a crack that follows a smooth intergranular or
non-tortuous path.
[0005] In an embodiment of the invention, alloy compositions,
thermo-mechanical and thermal practices are controlled in order to
develop an unrecrystallized microstructure or a desired amount of
recrystallization. The microstructures are controlled with the help
of dispersoids or precipitates which are formed at intermediate
processing steps, or precipitation treatments to yield obstacles
for dislocation and grain boundary motion. The sheet products
comprise elongated grains, which form a highly anisotropic
microstructure.
[0006] In accordance with one embodiment, the anisotropic
microstructure may be developed as a result of hot rolling and
additional thermal practices. The hot rolling temperature is
controlled in order to facilitate the desired type, volume fraction
and distribution of crystallographic texture. In one embodiment, a
recovery anneal after hot rolling yields the desired anisotropic
microstructure after final solution heat treating and optional
stretching and tempering operations. Additional intermediate
anneals may be used to control the driving force for
recrystallization.
[0007] The compositions of the aluminum products are preferably
selected in order to provide dispersoid forming alloying elements,
which control recrystallization and recovery processes during
production. In one embodiment, mixtures of alloying elements that
form the coherent Cu.sub.3Au prototype structure (L12 in the
structurebereight nomenclature) are preferred. Such elements
include Zr, Hf and Sc. In addition, alloying elements that form
incoherent dispersoids such as Cr, V, Mn, Ni and Fe may also be
utilized. Combinations of such alloying elements may be used.
[0008] An aspect of the present invention is to provide a rolled
aluminum alloy sheet product having high levels of crystallographic
anisotropy.
[0009] Another aspect of the present invention is to provide an
Al--Cu base alloy sheet product having high levels of
crystallographic anisotropy.
[0010] A further aspect of the present invention is to provide an
aircraft fuselage sheet comprising a rolled aluminum alloy sheet
product having an anisotropic microstructure.
[0011] Another aspect of the present invention is to provide a
method of making an aluminum alloy sheet product having a highly
anisotropic grain microstructure. The method includes the steps of
providing an aluminum alloy, hot rolling the aluminum alloy to form
a sheet, recovery/recrystallize annealing the hot rolled sheet,
solution heat treating the annealed sheet, and recovering a sheet
product having an anisotropic microstructure.
[0012] These and other aspects of the present invention will be
more apparent from the following description.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1 is a partially schematic drawing of an airplane
including an aluminum alloy fuselage sheet, indicating the
orientation of typical fatigue cracks which tend to develop in the
fuselage sheet.
[0014] FIG. 2 is a fabrication map for an aluminum sheet product
having an anisotropic microstructure produced in accordance with an
embodiment of the present invention.
[0015] FIG. 3 is a fabrication map for an aluminum sheet product
having an anisotropic microstructure produced in accordance with
another embodiment of the present invention.
[0016] FIGS. 4a and 4b are photomicrographs illustrating the
substantially "equiaxed" grains of Aluminum Association alloy 2024
and 2524 sheet products which are conventionally used as fuselage
sheet.
[0017] FIGS. 5a and 5b are photomicrographs illustrating the
anisotropic microstructure of an aluminum sheet product produced in
accordance with an embodiment of the present invention.
[0018] FIGS. 6a and 6b are photomicrographs illustrating the
anisotropic microstructure of another aluminum sheet product
produced in accordance with an embodiment of the present
invention.
[0019] FIGS. 7a and 7b are photomicrographs illustrating the
anisotropic microstructure of a further aluminum sheet product
produced in accordance with an embodiment of the present
invention.
[0020] FIGS. 8a and 8b are photomicrographs illustrating the
anisotropic microstructure of another aluminum sheet product
produced in accordance with an embodiment of the present
invention.
[0021] FIGS. 9a and 9b are photomicrographs illustrating the
anisotropic microstructure of a further aluminum sheet product
produced in accordance with an embodiment of the present
invention.
[0022] FIGS. 10a and 10b are photomicrographs illustrating the
anisotropic microstructure of another aluminum sheet product
produced in accordance with an embodiment of the present
invention.
[0023] FIG. 11 illustrates the layout of specimens taken from sheet
samples for testing.
[0024] FIG. 12 is a graph illustrating tensile yield strength
values for sheet samples of the present invention in different
orientations.
[0025] FIGS. 13 and 14 are graphs illustrating crack growth
resistance curves for sheet samples of the present invention.
[0026] FIG. 15 is a graph illustrating fracture toughness and
tensile yield strength for sheet samples of the present
invention.
[0027] FIG. 16 is a graph illustrating fatigue test results for two
of the present alloys exhibiting unrecrystallized
microstructures.
[0028] FIG. 17 is a graph illustrating tensile yield strengths for
sheet samples of the present invention in different
orientations.
[0029] FIG. 18 is a photomicrograph illustrating the anisotropic
microstructure of an aluminum sheet product produced in accordance
with an embodiment of the present invention.
[0030] FIG. 19 is a photomicrograph illustrating the anisotropic
microstructure of another aluminum sheet product produced in
accordance with an embodiment of the present invention.
[0031] FIG. 20 is a photomicrograph illustrating the anisotropic
microstructure of a farther aluminum sheet product used in
accordance with an embodiment of the present invention.
[0032] FIG. 21 is a photomicrograph illustrating the anisotropic
microstructure of another aluminum sheet product produced in
accordance with an embodiment of the present invention.
[0033] FIG. 22 is a graph illustrating tensile yield strength
values for sheet products of the present invention in different
orientations.
[0034] FIGS. 23-26 are graphs illustrating fracture toughness and
tensile yield strength values for sheet products produced in
accordance with embodiments of the present invention.
[0035] FIG. 27 is a graph illustrating duplicate fatigue test
results for two alclad alloys exhibiting elongated recrystallized
grains.
[0036] FIG. 28 is a graph illustrating results from S/N fatigue
testing for two alclad alloys exhibiting elongated recrystallized
grains.
DETAILED DESCRIPTION
[0037] In accordance with the present invention, a rolled aluminum
alloy sheet product is provided which comprises a highly
anisotropic microstructure. As used herein, the term "anisotropic
microstructure" means a grain microstructure where the grains are
elongated unrecrystallized grains or elongated recrystallized
grains with an average aspect ratio of length to thickness of
greater than about 4 to 1. The average grain aspect ratio is
preferably greater than about 6 to 1, more preferably greater than
about 8 to 1. In a particularly preferred embodiment, the
anisotropic microstructure has an average grain aspect ratio of
greater than about 10 to 1. In both instances of recrystallized or
unrecrystallized grains, the common feature among recrystallized
and unrecrystallized grain microstructures is that the grains are
elongated. Observation of these grains may be done, for example, by
optical microscopy at 50.times. to 100.times. in properly polished
and etched samples observed through the thickness in the
longitudinal orientation. For recrystallized products, the
anisotropic microstructures achieved in accordance to the present
invention preferably exhibit a Goss texture, as determined by
standard methods, of greater than 20, more preferably greater than
30 or 40. For unrecrystallized products, the anisotropic
microstructures preferably exhibit a Brass texture, as determined
by standard methods, of greater than 20, more preferably greater
than 30 or 40.
[0038] As used herein, the term "sheet" includes rolled aluminum
products having thicknesses of from about 0.01 to about 0.35 inch.
The thickness of the sheet is preferably from about 0.025 to about
0.325 inch, more preferably from about 0.05 to about 0.3 inch. For
many applications such as some aircraft fuselages, the sheet is
preferably from about 0.05 to about 0.25 inch thick, more
preferably from about 0.05 to about 0.2 inch. The sheet may be
unclad or clad, with preferred cladding layer thicknesses of from
about 1 to about 5 percent of the thickness of the sheet.
[0039] As used herein, the term "unrecrystallized" means a sheet
product that exhibits grains that relate to the original grains
present in the ingot or intermediate slab. The original grains have
only been physically deformed. As a result, the unrecrystallized
grain microstructures also exhibit a strong hot rolling
crystallographic texture. The term "recrystallized" as used herein
means grains that have formed from the original deformed grains.
This occurs typically during hot rolling, during solution heat
treating or during anneals, these anneals can be intermediate
between hot rolling and/or prior to solution heat treating.
[0040] In one embodiment of the invention, the sheet products are
useful as aircraft fuselage sheet. FIG. 1 schematically illustrates
an airplane 10 including a fuselage 12 which may be made of the
present wrought aluminum alloy sheet. The aluminum alloy sheet may
be provided with at least one aluminum cladding layer by methods
known in the art. The clad or unclad sheet of the present invention
may be assembled as an aircraft fuselage in a conventional manner
known in the art. The arrows A and B in FIG. 1 indicate the
orientations and propagation paths of fatigue cracks, which tend to
develop in airplane fuselage sheet. In accordance with an
embodiment, the anisotropic microstructure of the present sheet
product is oriented on the fuselage such that the lengths of the
high aspect ratio grains are substantially perpendicular to the
likely fatigue crack propagation paths through the fuselage sheet.
For example, either the longitudinal and/or long transverse
orientations of the sheet may be positioned substantially
perpendicular to the directions A or B shown in FIG. 1.
[0041] In accordance with the present invention, aluminum alloy
compositions are controlled in order to increase fatigue crack
growth resistance. Some suitable alloy compositions may include
Aluminum Association 2xxx, 5xxx, 6xxx and 7xxx alloys, and variants
thereof. For example, suitable aluminum alloy compositions for use
in accordance with the present invention include Al--Cu base
alloys, such as 2xxx alloys. A preferred Al--Cu base alloy
comprises from about 1 to about 5 weight percent Cu, more
preferably at least about 3 weight percent Cu, and from about 0.1
to about 6 weight percent Mg.
[0042] An example of a particularly preferred Al--Cu base alloy
comprises from about 3.5 to about 4.5 weight percent Cu, from about
0.6 to about 1.6 weight percent Mg, from about 0.3 to about 0.7
weight percent Mn, and from about 0.08 to about 0.13 weight percent
Zr. In accordance with another preferred embodiment, the rolled
aluminum alloy sheet product has a composition of from about 3.8 to
about 4.4 weight percent Cu, from about 0.3 to about 0.7 weight
percent Mn, from about 1.0 to about 1.6 weight percent Mg, and from
about 0.09 to about 0.12 weight percent Zr. In accordance with a
further preferred embodiment, the rolled aluminum sheet product has
a composition of from about 3.4 to about 4.0 weight percent Cu,
from 0 to about 0.4 weight percent Mn, from about 1.0 to about 1.6
weight percent Mg, and from about 0.09 to about 0.12 weight percent
Zr. In accordance with another preferred embodiment, the rolled
aluminum alloy sheet product has a composition of from about 3.2 to
about 3.8 weight percent Cu, from about 0.3 to about 0.7 weight
percent Mn, from about 1.0 to about 1.6 weight percent Mg, from
about 0.09 to about 0.12 weight percent Zr and from about 0.25 to
about 0.75 weight percent Li.
[0043] The Al--Cu base alloys produced in accordance with the
present invention may comprise up to about 1 weight percent of at
least one additional alloying element selected from Zn, Ag, Li and
Si. These elements, when properly heat treated, may give rise to
the formation of strengthening precipitates. Such precipitates form
during natural aging at room temperature or during artificial
aging, e.g., up to temperatures of 350.degree. F.
[0044] The Al--Cu base alloys may further comprise up to about 1
weight percent of at least one additional alloying element selected
from Hf, Sc, Zr and Li. These elements, when properly heat treated,
may give rise to the formation or enhancement of coherent
dispersoids. Such dispersoids may enhance the ability of the
microstructure to be produced with elongated recrystallized or
unrecrystallized grains.
[0045] The Al--Cu base alloys may further comprise up to about 1
weight percent of at least one additional alloying element selected
from Cr, V, Mn, Ni and Fe. These elements, when properly heat
treated, may give rise to the formation of incoherent dispersoids.
Such dispersoids may help to control recrystallization and grain
growth.
[0046] In addition to Al--Cu base alloys, Al--Mg base alloys,
Al--Si base alloys, Al--Mg--Si base alloys and Al--Zn base alloys
may be produced as sheet products having anisotropic
microstructures in accordance with the present invention. For
example, Aluminum Association 5xxx, 6xxx and 7xxx alloys, or
modifications thereof, may be fabricated into sheet products having
anisotropic microstructures.
[0047] Suitable Al--Mg base alloys have compositions of from about
0.2 to about 7.0 weight percent Mg, from 0 to about 1 weight
percent Mn, from 0 to about 1.5 weight percent Cu, from 0 to about
3 weight percent Zn, and from 0 to about 0.5 weight percent Si. In
addition, Al--Mg base alloys may optionally include further
alloying additions of up to about 1 weight percent strengthening
additions selected from Li, Ag, Cd and lanthanides, and/or up to
about 1 weight percent dispersoid formers such as Cr, Fe, Ni, Sc,
Hf, Ti, V and Zr.
[0048] Suitable Al--Mg--Si base alloys have compositions of from
about 0.1 to about 2.5 weight percent Mg, from about 0.1 to about
2.5 weight percent Si, from 0 to about 2 weight percent Cu, from 0
to about 3 weight percent Zn, and from 0 to about 1 weight percent
Li. In addition, Al--Mg--Si base alloys may optionally include
further alloying additions of up to about 1 weight percent
strengthening additions selected from Ag, Cd and lanthanides,
and/or up to about 1 weight percent dispersoid formers such as Mn,
Cr, Ni, Fe, Sc, Hf, Ti, V and Zr.
[0049] Suitable Al--Zn base alloys have compositions of from about
1 to about 10 weight percent Zn, from about 0.1 to about 3 weight
percent Cu, from about 0.1 to about 3 weight percent Mg, from 0 to
about 2 weight percent Li, and from 0 to about 2 weight percent Ag.
In addition, Al--Zn base alloys may optionally include further
alloying additions of up to about 1 weight percent strengthening
additions selected from Cd and lanthanides, and/or up to about 1
weight percent dispersoid formers such as Mn, Cr, Ni, Fe, Sc, Hf,
Ti, V and Zr.
[0050] In accordance with the present invention, processing
parameters are controlled in order to increase fatigue crack growth
resistance of the rolled aluminum alloy sheet products. A preferred
process includes the steps of casting, scalping, preheating,
initial hot rolling, reheating, finish hot rolling, optional cold
rolling, optional intermediate anneals during hot rolling and/or
cold rolling, annealing for the control of anisotropic grain
microstructures, solution heat treating, flattening and stretching
and/or cold rolling. An example of a fabrication map is shown in
FIG. 2. Another example of a fabrication may is shown in FIG.
3.
[0051] As illustrated in FIG. 2, a recovery anneal step is
preferably utilized in the production of sheet products in
accordance with the present invention. As illustrated in FIG. 3,
intermediate anneals during hot rolling and/or cold rolling may be
used in addition to, or in place of, the recovery anneal. It should
be noted that the anneals can be provided by controlled heating or
by single or multiple holding times at one or several
temperatures.
[0052] Depending on the particular alloy composition, the
preheating step is preferably carried out at a temperature of
between 800 and 1,050.degree. F. for 2 to 50 hours. The initial hot
rolling is preferably performed at a temperature of from 750 to
1,020.degree. F. with a reduction in thickness of from 0.1 to 3
inch percent per pass. Reheating is preferably carried out at a
temperature of from 700 to 1,050.degree. F. for 2 to 40 hours. The
finish hot rolling step is preferably performed at a temperature of
from 680 to 1,050.degree. F. with a reduction in thickness of from
0.1 to 3 inch per pass.
[0053] The optional intermediate anneals during hot rolling or cold
rolling, e.g., as illustrated in FIG. 3, are preferably carried out
at a temperature of between about 400 and about 1,000.degree. F.
for 0.5 to 24 hours.
[0054] The cold rolling step is preferably carried out at room
temperature with a reduction in thickness of from 5 percent to 50
percent per pass.
[0055] The recovery/elongated grain recrystallization anneals,
e.g., as illustrated in FIG. 2, are preferably carried out at a
temperature of between about 300 and about 1,000.degree. F. for 0.5
to 96 hours. Unrecrystallized anisotropic microstructures typically
require anneals at relatively low temperatures, for example, from
about 400 to about 700.degree. F. Recrystallized anisotropic
microstructures typically require anneals at relatively high
temperatures, for example, from about 600 to about 1,000.degree.
F.
[0056] Solution heat treatment is preferably carried out at a
temperature of from about 850 to about 1,060.degree. F. for a time
of from about 1 to 2 minutes to about 1 hour.
[0057] The quenching step is preferably carried out by rapid
cooling using immersion into a suitable cooling fluid or by
spraying a suitable cooling fluid.
[0058] The flattening and stretching steps are preferably carried
out to provide no more than 6 percent of total cold
deformation.
[0059] After solution heat treatment, cold working may optionally
be performed, preferably by stretching or cold rolling. The cold
working process preferably imparts a maximum of 15 percent cold
deformation to the sheet product, more preferably a maximum of
about 8 percent.
[0060] The sheet products fabricated in accordance with the present
invention exhibit substantially increased strength and/or
resistance to the growth of fatigue cracks as a result of their
anisotropic microstructures. In a preferred embodiment, the rolled
sheet products exhibit longitudinal (L) tensile yield strengths
(TYS) greater than 45 ksi, more preferably greater than 48 ksi. The
rolled sheet products preferably exhibit long transverse (LT)
tensile yield strengths greater than 40 ksi, more preferably
greater than 43 ksi. In the long transverse (T-L) orientation, the
rolled sheet in the T3 temper preferably exhibits a fatigue crack
growth rate da/dN of less than about 5.times.10.sup.-6 inch/cycle
at a .DELTA.K of 10 ksi inch, more preferably less than about
4.times.10.sup.-6 or 3.times.10.sup.-6 inch/cycle. In the T36
temper, the rolled sheet exhibits a T-L orientation fatigue crack
growth rate da/dN of less than about 4.times.10.sup.-6 inch/cycle
at a .DELTA.K of 10 ksi inch, more preferably less than
3.times.10.sup.-6 or 2.times.10.sup.-6 inch/cycle.
[0061] Furthermore, the present wrought aluminum alloy sheet
products exhibit improved fracture toughness values, e.g., as
tested with 16 by 44 inch center notch fracture toughness specimens
in accordance with ASTM E561 and B646 standards. For example, sheet
products produced in accordance with the present invention
preferably exhibit longitudinal (L-T) or long transverse (T-L)
K.sub.c fracture toughness values of greater than 130 or 140 ksi
inch. The sheet products also preferably possess L-T or T-L
K.sub.app fracture toughness values of greater than 85 or 90 ksi
inch.
[0062] Thus, in addition to improved fatigue crack growth
resistance, the present sheet products exhibit improved
combinations of strength and fracture toughness.
[0063] FIGS. 4a and 4b are photomicrographs illustrating the
substantially equiaxed grains of conventional alloy 2024 and 2524
sheet products which are used as fuselage sheet. Unlike
conventional fuselage sheet such as shown in FIGS. 4a and 4b, the
anisotropic microstructure of the present sheet products enables
aircraft manufacturers to orient the sheet in directions which take
advantage of the increased mechanical properties of the sheet, such
as improved longitudinal and/or long transverse fatigue crack
growth resistance, fracture toughness and/or strength.
[0064] Table 1 below lists compositions of some sheet products,
which may be processed to provide anisotropic microstructures in
accordance with embodiments of the present invention.
TABLE-US-00001 TABLE 1 Sheet Product Alloy Compositions (Weight
Percent) Alloy Sample No. Cu Mn Mg Zr Sc Li Fe Si Al 770-308 (Zr
alloy) 3.74 0 1.36 0.12 0 0 0.03 0.04 balance 770-311 (Zr + Li
alloy) 3.19 0 1.22 0.10 0 0.31 0.03 0.04 balance 770-309 (Mn + Zr
alloy) 4.26 0.57 1.4 0.10 0 0 0.07 0.04 balance 770-310 (Zr + Sc
alloy) 3.7 0 1.36 0.10 0.06 0 0.04 0.03 balance 770-312 (Zr + Sc +
Li alloy) 3.56 0 1.36 0.10 0.06 0.31 0.04 0.03 balance 596-367 (Mn
+ Zr + Li alloy) 3.37 0.58 1.21 0.12 0 0.76 0.04 0.02 balance
[0065] The sheet products having compositions listed in Table 1
were made as follows. Ingots measuring 6 inches.times.16
inches.times.60 inches were cast using direct chill (DC) molds. The
compositions reported in Table 1 were measured from metal samples
obtained from the molten metal bath. Ingots were first stress
relieved by heating to 750.degree. F. for 6 hours. The ingots were
then scalped to remove 0.25 inch surface layer from both rolling
surfaces and side sawed to 14 inch width. For preheating, ingots
were heated to 850.degree. F., soaked for 2 hours, then heated to
875.degree. F. and soaked an additional 2 hours. Ingots taken from
the preheating furnace were cross rolled 22 percent to a 4.5 inch
gauge followed by lengthening to a 2 inch gauge. Metal temperature
was maintained above 750.degree. F. with reheats to 850.degree. F.
for 15 minutes. The 2 inch slab was sheared in half and reheated to
915.degree. F. for 8 hours, table cooled to 900.degree. F. and hot
rolled to 0.25 inch gauge. Suitable reheats were provided during
hot rolling to 915.degree. F. for 15 minutes. Metal temperature was
kept above 750.degree. F. After hot rolling, sheet product 0.150
inch gauge was fabricated. Recovery anneals prior to solution heat
treatment of from 8 to 24 hours at temperatures from 400.degree. F.
to 550.degree. F. yielded unrecrystallized microstructures after
solution heat treatment.
[0066] After rolling, solution heat treating and quenching, all
pieces of sheet were ultrasonically inspected to Class B and they
all passed. Microstructural analyses revealed that all samples
exhibited unrecrystallized microstructures in the final temper.
FIGS. 5a to 10b are photomicrographs illustrating the anisotropic
microstructures of the sheet products listed in Table 1. In each
case, the sheet possesses high levels of crystallographic
anisotropy and exhibits elongated grains. The grain anisotropy is
most pronounced in the longitudinal direction (L) of each sheet,
but is also present in the long transverse direction of each
sheet.
[0067] Fabricated samples in accordance with the present invention
were tested for mechanical properties. The diagram in FIG. 11 shows
the locations and orientations of samples taken for the different
tests.
[0068] Results from tensile testing in the L, LT and 45 directions
are shown in FIG. 12. Alloy 367 listed in Table 1 showed the
highest strength in all three directions. However, the other alloys
listed in Table 1 also exhibited favorable strength levels.
[0069] Fracture toughness tests were conducted from 16 by 44 inch
center notch specimens with 4 inch initial center cracks. FIGS. 13
and 14 illustrate R-curves from fracture toughness testing, showing
that the test specimens of the present sheet products possess
favorable fracture toughness values comparable to alclad 2524 T3
sheet. The R curves are comparable for all of the alloys
tested.
[0070] The improved strength/toughness combinations attained are
shown in FIG. 15. FIG. 15 also shows an average value from 2524-T3
plant fabricated alclad sheet for comparison purposes. The minimum
values shown in FIG. 15 correspond to a minus 3 times the standard
deviation extrapolated value.
[0071] Fatigue testing under constant amplitude is shown in FIG.
16. These tests were conducted in samples that appeared to be most
promising from the strength and toughness tests. These results
revealed that the products made according to the present invention
exhibit substantially lower rates of crack growth, i.e., improved
resistance to fatigue crack growth.
[0072] Samples in the T36 temper exhibited the properties shown in
FIG. 17. In FIG. 17, the T36 temper was attained by providing 5
percent cold deformation either via cold rolling stretching. The
strengths of the cold rolled samples are slightly higher.
[0073] The results from the foregoing tests revealed that the
strength and the resistance to fatigue crack growth were
substantially improved in accordance with the present invention. By
hot rolling at relatively high temperatures using recovery anneals,
and by adding Zr and/or Sc as dispersoid forming additions, it was
possible to fabricate unrecrystallized microstructures in sheet
gauges. The Li additions also appear to aid in the attainment of
the unrecrystallized microstructures for unknown reasons. In 2xxx
alloys, copper appears to have a substantial effect on
strengthening. Scandium additions help attain unrecrystallized
microstructures but may be detrimental for strengthening. Manganese
additions are beneficial for strength properties. Cold rolling,
e.g., 5 percent increases the strength significantly without a
reduction in fatigue or fracture toughness, this also was a
surprise. Alloys containing Li may exhibit larger improvements in
properties as a result of the cold deformation than alloys without
the Li addition.
[0074] A plant rolling trial was performed with the object of
producing an anisotropic grain microstructure in a sheet product to
exhibit higher strength and higher resistance to the propagation of
fatigue cracks. The alloys shown in Table 2 were cast as 15,000 lb
ingots and fabricated in accordance with the methods of the present
invention, using a fabrication route similar to that shown in FIG.
2. TABLE-US-00002 TABLE 2 Sheet Product Alloy Compositions (Weight
Percent) Alloy Sample No. Cu Mn Mg Zr Fe Si Al 354-371 4.08 0.29
1.36 0.12 0.02 0.01 balance (low Cu-low Mn) 354-381 4.33 0.30 1.38
0.10 0.01 0.00 balance (high Cu-low Mn) 354-391 4.09 0.58 1.35 0.11
0.02 0.01 balance (low Cu-high Mn) 354-401 4.22 0.60 1.32 0.10 0.01
0.01 balance (high Cu-high Mn)
[0075] The sheet products having compositions listed in Table 2
were made as follows. Ingots measuring 14 inches.times.74
inches.times.180 inches were cast using direct chill (DC) molds.
The compositions reported in Table 2 were measured from metal
samples obtained during casting. Ingots were first stress relieved
by heating to 750.degree. F. for 6 hours. The ingots were then
scalped to remove 0.50 inch surface layer from both rolling
surfaces. For preheating, ingots were heated to 850.degree. F.,
soaked for 2 hours, then heated to 875.degree. F. and soaked an
additional 2 hours. Ingots taken from the preheating furnace were
roll bonded to alcald 1100 plate and rolled to 6.24 inch gauge. The
alcald 6.24 inch slab was reheated to 915.degree. F. for 8 hours,
table cooled to 850.degree. F. and hot rolled to 0.180 inch gauge.
Metal temperature was kept above 600.degree. F. After hot rolling,
the sheet product was given a recrystallization anneal at
700.degree. F. for 8 hours prior to solution heat treatment. The
sheet product was batch solution heat treated at 925.degree. F. for
11 minutes and water quenched. Sheet was flattened with a gauge
reduction from 0.180 inch to 0.17746 inch. Then T3 and T36 tempers
were fabricated. The aluminum cladding had a thickness of 2.5
percent of the final thickness. The anisotropic microstructures
comprising elongated recrystallized grains attained in the final T3
temper are shown in FIGS. 18-21.
[0076] Results from tensile strength measurements are shown in FIG.
22. Measurements of tensile properties indicated that the high Mn
variants listed in Table 2 exhibited higher strengths than the low
Mn variants. The strengthening effect of Mn was surprisingly higher
than that of Cu.
[0077] Fracture toughness measurements were conducted using 16 inch
by 44 inch center notch toughness specimens. Results from strength
and toughness measurements are shown in FIGS. 23 to 26. These
figures also show an average value for 2524-T3 alclad sheet for
comparison purposes. The minimum values shown in these figures
correspond to a minus 3 times the standard deviation extrapolated
value. The strength and toughness combinations of the sheet
products with high Mn variants are better than those of 2524-T3.
Surprisingly, the low Cu-high Mn sample exhibits higher properties
than the high Cu-low Mn sample.
[0078] FIG. 27 shows the da/dN performance of the low Cu-high Mn
variant for the T3 and T36 tempers. The tests were conducted in
duplicate and resulted in good correlation from the duplicate
tests. Note that these results indicate that, at a delta K of 10,
the rate of growth of fatigue cracks is reduced for the T3 tempers
and reduced even more for the T36 tempers. These results indicate
that the products fabricated in accordance with the present
invention exhibit better FCG performance.
[0079] FIG. 28 shows results from the testing of S/N fatigue. Note
that for a given value of the number of cycles, the maximum stress
is higher for products fabricated in accordance with the present
invention. This means that components can be subjected to higher
stresses than conventional components to experience the same life.
The S/N fatigue performance of the products fabricated in
accordance with this invention is also better than that of alclad
2524-T3 sheet product.
[0080] Table 3 shows the results from compressive yield strength
tests, in which compressive strength properties in the longitudinal
(L) and long transverse (LT) orientations for alloy 2524 and one of
the alloys of the present invention (the low Cu-high Mn variant
354-391) are compared. A significant improvement in compressive
yield strength properties is achieved by the present sheet products
in comparison with the conventional 2524 sheet product.
TABLE-US-00003 TABLE 3 Measured Compressive Yield Strengths for
Alloy 2524 and 354-391 Low Cu-High Mn Gauge L (ksi) LT (ksi) Temper
2524-T3 Measurements 0.200 42.8 49.3 T3 0.200 43.0 48.4 T3 0.249
42.9 48.7 T3 0.249 42.2 47.3 T3 0.249 42.5 48.5 T3 0.249 43.7 49.2
T3 0.310 40.9 44.4 T3 MLHDBK5 39.0 43.0 T3 354-391 Measurements
0.177 51.5 54.8 T3 0.177 51.5 56.2 T3 0.177 54.1 60.5 T36 0.177
55.2 62.1 T36
[0081] The anisotropic microstructures of some recrystallized and
unrecrystallized sheet products of the present invention were
measured in comparison with conventional alloy 2024 and 2524 sheet
products. Table 4 lists the Brass and Goss texture components of
2024-T3 and 2524-T4 sheet products in 0.0125 inch gauges. These are
compared with the 770-309 and 770-311 unrecrystallized sheet
products of the present invention listed in Table 1, and the
354-391 and 354-401 recrystallized sheet products of the present
invention listed in Table 2. TABLE-US-00004 TABLE 4 Maximum
Intensity of Texture Components (X Times Random) Alloy
Microstructure Brass Goss 2024-T3 recrystallized equiaxed grains
1.0 12.0 2524-T4 recrystallized equiaxed grains 1.9 15.3 770-309
unrecrystallized elongated grains 36.1 0 770-311 unrecrystallized
elongated grains 34.9 0 354-391 recrystallized elongated grains 1.3
42.7 354-401 recrystallized elongated grains 8.6 56.7
[0082] As shown in Table 4, the unrecrystallized sheet samples
770-309 and 770-311 of the present invention possess Brass texture
components of greater than 30, indicating their highly anisotropic
microstructures. The recrystallized sheet samples 354-391 and
354-401 of the present invention possess Goss texture components of
greater than 40, well above the Goss texture components of the
conventional 2024-T3 and 2524-T4 recrystallized sheet products.
[0083] The products and methods of the present invention provide
several advantages over conventionally fabricated aluminum
products. In accordance with the present invention, aluminum sheet
products containing high anisotropy in grain microstructure are
provided which exhibit high fracture surface roughness and
secondary cracking and branching, making the products better suited
for applications requiring low fatigue crack growth. In addition,
the products exhibit favorable combinations of strength and
fracture toughness.
[0084] Whereas particular embodiments of this invention have been
described above for purposes of illustration, it will be evident to
those skilled in the art that numerous variations of the details of
the present invention may be made without departing from the
invention as defined in the appended claims.
* * * * *