Note: Descriptions are shown in the official language in which they were submitted.
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AL- ZN-CU-MG ALLOYS AND THEIR MANUFACTURING
PROCESS
10 Field of the Invention
The present invention relates generally to aluminum base alloys and more
particularly,
Al-Zn-Cu-Mg aluminum base alloys, in particular for aerospace applications.
Description of Related Art
Al-Zn-Cu-Mg aluminum base alloys have been used extensively in the aerospace
industry
for many years. With the evolution of airplane structures and efforts directed
towards the
goal of reducing both weight and cost, an optimum compromise between
properties such
as strength, toughness and corrosion resistance is continuously sought. Also,
process
improvement in casting, rolling and heat treatment can advantageously provide
further
control in the composition diagram of an alloy and property compromise.
Thick rolled, forged or extruded products made of Al-Zn-Cu-Mg aluminum base
alloys
are used in particular to produce integrally machined high strength structural
parts for
the aeronautic industry, for example wing elements such as wing ribs, spars,
frames and
the like, which are typically machined from thick wrought sections.
The performance values obtained for various properties such as static
mechanical
strength, fracture toughness, resistance to corrosion, quench sensitivity,
fatigue
resistance, and level of residual stress will determine the overall
performance of the
product, the ability for a structural designer to use it advantageously, as
well as the ease
it can be used in further processing steps such as, for example, machining.
Among the above listed properties some are often conflicting in nature and a
compromise generally has to be found. Conflicting properties are, for example,
static
mechanical strength versus toughness and strength versus resistance to
corrosion.
Among corrosion or environmentally assisted cracking (EAC) properties, a
distinction
can be made between EAC under conditions of high stress and humid environment
and
EAC under conditions of standard stress corrosion cracking (SCC) tests, such
as ASTM
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G47, where specimens are tested using alternate immersion and drying cycles
with NaCl
solution (ASTM G44) and typically using lower stress.
Crack deviation, crack turning or also crack branching are terms used to
express
propensity for crack propagation to deviate from the expected fracture plane
.. perpendicular to the loading direction during a fatigue or toughness test.
Crack deviation
happens on a microscopic scale (<100 [tm), on a mesoscopic scale (100-1000
[tm) or on
a macroscopic scale (>1 mm) but it is considered detrimental only if the crack
direction
remains stable after deviation (macroscopic scale). The phenomenon is a
particular
concern for fatigue trials in L-S direction. The term crack branching is used
herein for
macroscopic deviation of cracks in L-S fatigue or toughness tests from the S
direction
towards the L direction which occurs for rolled products with a thickness of
30 mm or
higher. Crack branching may occur in relation to the rolled product
composition and
microstructure and to the test conditions.
Crack deviation has been considered as a major problem by aircraft
manufacturers
because it is difficult to take into account to design parts, using
traditional design
methods. This is because crack deviation invalidates conventional, mode I
based,
materials testing procedure and design models. The crack deviation problem has
proven
difficult to solve. Recently it was considered that in the absence of solution
for avoiding
crack deviation, efforts should be directed to predicting crack deviation
behaviors. (M. J.
.. Crill, D. J. Chellman, E. S. Balmuth, M. Philbrook, K. P. Smith, A. Cho, M.
Niedzinski,
R. Muzzolini and J. Feiger, Evaluation of AA 2050-T87 Al¨Li Alloy Crack
Turning
Behavior, Materials Science Forum, Vol 519-521 (July 2006) pp 1323-1328).
The patent US 8,323,426 proposes a solution to improve crack branching for
some Al-
Cu-Li alloys.
However crack deviation improvement is often related to higher fatigue crack
growth
rate in the original cracking plane, before deviation.
US Patent 5,560,789 describes AA 7000 series alloys having high mechanical
strength
and a process for obtaining them. The alloys contain, by weight, 7 to 13.5%
Zn, 1 to
.. 3.8% Mg, 0.6 to 2.7% Cu, 0 to 0.5% Mn, 0 to 0.4% Cr, 0 to 0.2% Zr, others
up to
0.05% each and 0.15% total, and remainder Al, corrosion properties are however
not
mentioned.
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US Patent No 5,865,911 describes an aluminum alloy consisting essentially of
(in weight
%) about 5.9 to 6.7% zinc, 1.8 to 2.4% copper, 1.6 to 1.86% magnesium, 0.08 to
0.15%
zirconium balance aluminum and incidental elements and impurities. The '911
patent
particularly mentions the compromise between static mechanical strength and
toughness.
US Patent application N U520050167016A1 discloses in particular an Al¨Zn¨Cu¨
Mg product comprising (in weight %) : 5.8-6.8% Zn, 1.5-2.5% Cu, 1.5-2.5% Mg,
0.04-
0.09% Zr remainder aluminum and incidental impurities, wherein said product
possesses
a recrystallization rate greater than about 35% at a quarter thickness
location, with
improved fatigue crack growth resistance.
US Patent No 6,027,582 describes a rolled, forged or extruded Al-Zn-Mg-Cu
aluminum
base alloy products greater than 60 mm thick with a composition of (in weight
%), Zn :
5.7-8.7, Mg : 1.7-2.5, Cu : 1.2-2.2, Fe : 0.07-0.14, Zr : 0.05-0.15 with Cu +
Mg < 4.1
and Mg>Cu. The '582 patent also describes improvements in quench sensitivity.
US Patent No 6,972,110 teaches an alloy, which contains preferably (in weight
%) Zn :
7-9.5, Mg : 1.3-1.68 and Cu 1.3-1.9 and encourages keeping Mg +Cu 3.5. The
'110
patent discloses using a three step aging treatment in order to improve
resistance to
stress corrosion cracking. A three step aging is long and difficult to master
and it would
be desirable to obtain high corrosion resistance without necessarily requiring
such a
thermal treatment.
PCT Patent application No W02004090183 discloses an alloy comprising
essentially (in
weight percent): Zn: 6.0 - 9.5, Cu: 1.3 - 2.4, Mg: 1.5 - 2.6, Mn and Zr < 0.25
but
preferably in a range between 0.05 and 0.15 for higher Zn contents, other
elements each
less than 0.05 and less than 0.25 in total, balance aluminium, wherein (in
weight
percent): 0.1[Cu] + 1.3 < [Mg] < 0.2[Cu] + 2.15, preferably 0.2[Cu] + 1.3 <
[Mg] <
0.1[Cu] + 2.15.
US Patent application No 2005/006010 a method for producing a high strength Al-
Zn-
Cu-Mg alloy with an improved fatigue crack growth resistance and a high damage
tolerance, comprising the steps of casting an ingot with the following
composition (in
weight percent) Zn 5.5-9.5, Cu 1.5-3.5, Mg 1.5-3.5, Mn<0.25, Zr<0.25, Cr<0.10,
Fe<0.25, Si<0.25, Ti<0.10, Hf and/or V<0.25, other elements each less than
0.05 and
less than 0.15 in total, balance aluminum, homogenizing and/or pre-heating the
ingot
after casting, hot rolling the ingot and optionally cold rolling into a worked
product of
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more than 50 mm thickness, solution heat treating, quenching the heat treated
product,
and artificially ageing the worked and heat-treated product, wherein the
ageing step
comprises a first heat treatment at a temperature in a range of 105 C to 135
C for
more than 2 hours and less than 8 hours and a second heat treatment at a
higher
temperature than 135 C but below 170 C for more than 5 hours and less than
15
hours. Again, such three step aging is long and difficult to master.
EP Patent 1 544 315 discloses a product, especially rolled, extruded or
forged, made of
an AlZnCuMg alloy with constituents having the following percentage weights:
Zn 6.7 -
7.3; Cu 1.9 - 2.5; Mg 1.0 - 2.0; Zr 0.07 - 0.13; Fe less than 0.15; Si less
than 0.15; other
1() elements not more than 0.05 to at most 0.15 per cent in total; and
aluminum the
remainder. The product is preferably treated by solution heat treatment,
quenching, cold
rolling and artificial aging.
US Patent No 8,277,580 teaches a rolled or forged Al-Zn-Cu-Mg aluminum-based
alloy
wrought product having a thickness from 2 to 10 inches. The product has been
treated
by solution heat-treatment, quenching and aging, and the product comprises (in
weight-
%): Zn 6.2-7.2, Mg 1.5-2.4, Cu 1.7-2.1. Fe 0-0.13, Si 0-0.10, Ti 0-0.06, Zr
0.06-0.13,
Cr 0-0.04, Mn 0-0.04, impurities and other incidental elements <=0.05 each.
US Patent No 8,673,209 discloses aluminum alloy products about 4 inches thick
or less
that possesses the ability to achieve, when solution heat treated, quenched,
and
artificially aged, and in parts made from the products, an improved
combination of
strength, fracture toughness and corrosion resistance, the alloy consisting
essentially of:
about 6.8 to about 8.5 wt. % Zn, about 1.5 to about 2.00 wt. % Mg, about 1.75
to about
2.3 wt. % Cu; about 0.05 to about 0.3 wt. % Zr, less than about 0.1 wt. % Mn,
less than
about 0.05 wt. % Cr, the balance Al, incidental elements and impurities and a
method for
making same.
None of the documents, which describe high strength 7xxx alloy products,
describe alloy
products without a tendency to crack deviation and low fatigue crack growth
rate and
having simultaneously high strength, high toughness properties and high
corrosion
resistance.
A problem that the present invention addresses is to obtain thick rolled
products of the
7XXX alloy series with improved fatigue crack growth rate without increased
tendency
of crack deviation, while maintaining a good balance between mechanical
strength,
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fracture toughness, resistance to corrosion, quench sensitivity, fatigue
resistance, and
level of residual stress. By thick rolled products it is meant products with a
thickness of
at least 80 mm or even of at least 100 mm.
5
SUMMARY OF THE INVENTION
An object of the invention was to provide an Al-Zn-Cu-Mg alloy having a
specific
composition range and manufacturing process that enables, for thick rolled
products, an
improved fatigue crack growth rate without increased tendency of crack
deviation.
Another object of the invention was the provision of a manufacturing process
of
wrought aluminum products which enables an improved compromise improved
fatigue
crack growth rate without increased tendency of crack deviation.
To achieve these and other objects, the present invention is directed to a
rolled
product having a thickness of at least 80 mm comprising (in weight %) :
Zn 6.85 ¨ 7.25,
Mg 1.55 ¨ 1.95,
Cu 1.90 ¨ 2.30,
Zr 0.04 ¨ 0.10,
Ti 0¨ 0.15,
Fe 0 ¨ 0.15,
Si 0¨ 0.15,
other elements < 0.05 each and < 0.15 total, remainder Al,
wherein at mid-thickness more than 75 % of grains are recrystallized or at mid-
thickness
to 75 % of grains are recrystallized and non-recrystallized grains have an
aspect ratio
25 in a LIST cross section less than 3.
To achieve these and other objects, the present invention is directed the
present
invention is directed to a process for the manufacture of a rolled aluminum-
based alloy
product comprising the steps of:
a) casting an ingot comprising, (in weight-%)
30 Zn 6.85 ¨ 7.25,
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Mg 1.55 ¨ 1.95,
Cu 1.90 ¨ 2.30,
Zr 0.04 ¨ 0.10,
Ti 0¨ 0.15,
Fe 0 ¨ 0.15,
Si 0¨ 0.15,
other elements < 0.05 each and < 0.15 total, remainder Al;
b) homogenizing the ingot;
c) hot rolling said homogenized ingot to a rolled product with a final
thickness of at
1() least 80 mm;
d) solution heat treating and quenching the product;
e) stress-relieving the solution heat-treated an quenched the product;
f) artificially aging the stress-relieved product;
wherein the hot rolling starting temperature is controlled to obtain after
step f at mid-
thickness more than 75 % of recrystallized grains or at mid-thickness 30 to 75
% of
recrystallized grains and non-recrystallized grains with an aspect ratio in a
LIST cross
section less than 3.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 shows the C(T) specimen used for the Fatigue Crack Growth Rate
testing. A
cone of 20 which origin is at the intersection of a line passing through the
holes
centers and the specimen axis of symmetry used for the criteria of crack
deviation is
represented as a bold line.
Figure 2a is a schematic of the C(T) specimen showing before the fatigue test
and the for
the criteria of crack deviation. Figure 2b shows a cracked specimen without a
tendency
to crack deviation: the cracks remains within the cone. Figure 2c shows a
specimen with
a tendency of crack deviation.
Figure 3 shows specimen of alloy A after Fatigue Crack Growth Rate testing.
Figure 4 shows specimen of alloy B after Fatigue Crack Growth Rate testing.
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Figure 5 shows specimen of alloy C after Fatigue Crack Growth Rate testing.
DETAILED DESCRIPTION
Unless otherwise indicated, all the indications relating to the chemical
composition of the
alloys are expressed as a mass percentage by weight based on the total weight
of the
alloy. In the expression Cu/Mg, Cu means the Cu content in weight % and Mg
means the
Mg content in weight %. Alloy designation is in accordance with the
regulations of The
Aluminium Association, known to those skilled in the art. The definitions of
tempers are
laid down in EN 515 (1993).
Unless mentioned otherwise, static mechanical characteristics, i.e., the
ultimate tensile
strength UTS, the tensile yield stress TYS and the elongation at fracture E,
are
determined by a tensile test according to standard NF EN ISO 6892-1 (2016),
the
location at which the pieces are taken and their direction being defined in
standard EN
485 (2016).
Unless otherwise specified, the definitions of standard EN 12258 apply.
The symbol * is used for "multiplied by".
The fracture toughness K1c is determined according to ASTM standard E399
(2012).
Except if mentioned otherwise, EAC under conditions of high stress and humid
environment was tested under a constant strain on a tensile sample at mid-
thickness as
described in standard ASTM G47 and using a load of about 80% of ST direction
TYS,
under 85% relative humidity, and at a temperature of 70 C. The minimum life
without
failure after Environmentally Assisted Cracking (EAC) corresponds to the
minimum
number of days to failure from 3 specimens for each plate.
The tendency to crack deviation is observed using a L-S Compact Tension C(T)
fatigue
specimen as defined in ASTM E647. The term "deviation" in is not meant herein
as
described in ASTM E647-15 (which definition is focused on the precision of
measurement of fatigue crack growth rate), but is meant as the crack remaining
within a
cone of 20 and preferably of 15 , which origin is at the intersection of a
line passing
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through the holes centers and a specimen axis of symmetry, illustrated by the
line A-A in
Figure 1. The C(T) specimen has a width W = 40 mm and a thickness B = 10 mm. A
representation of the specimen used is shown in Figure 1 which also
illustrates with a
bold line the cone of 20 . For the test specimen used L = 48 mm, W = 40 mm, Z
= 50
mm, C = 22 mm, B = 10 mm. The method to evaluate crack deviation is
illustrated by
Figure 2. Figure 2a shows schematically the CT specimen before the fatigue
test. Figure
2b shows a cracked specimen without a tendency to crack deviation: the cracks
remains
with the cone illustrated by bolded lines. Figure 2c shows a specimen with a
tendency of
crack deviation.
The term "structural member" is a term well known in the art and refers to a
component
used in mechanical construction for which the static and/or dynamic mechanical
characteristics are of particular importance with respect to structure
performance, and
for which a structure calculation is usually prescribed or undertaken. These
are typically
components the rupture of which may seriously endanger the safety of the
mechanical
construction, its users or third parties. In the case of an aircraft,
structural members
comprise members of the fuselage (such as fuselage skin), stringers,
bulkheads,
circumferential frames, wing components (such as wing skin, stringers or
stiffeners, ribs,
spars), empennage (such as horizontal and vertical stabilizers), floor beams,
seat tracks,
and doors.
The alloy of the invention has a specific composition and microstructure which
makes
possible to obtain products which have a very low fatigue crack growth rate
and do not
have a tendency to crack deviation.
A minimum Zn content of 6.85 and preferably 6.90 or even 6.90 is needed to
obtain
sufficient strength. However the Zn content should not exceed 7.25 and
preferably 7.20
or even 7.15 to obtain the sought balance of properties, in particular
toughness and
elongation.
A minimum Mg content of 1.55 and preferably 1.60 or even 1.65 is needed to
obtain
sufficient strength. However, the Mg content should not exceed 1.95 and
preferably 1.90
or even 1.85 to obtain the sought balance of properties in particular
toughness and
elongation and avoid quench sensitivity.
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A minimum Cu content of 1.90 and preferably 1.95 or 2.00, or even 2.05 is
needed to
obtain sufficient strength and also to obtain sufficient EAC performance.
However the
Cu content should not exceed 2.30 and preferably 2.25 in particular to avoid
quench
sensitivity. In an embodiment the Cu maximum content is 2.20.
In order to obtain products with low sensitivity to EAC under conditions of
high stress
and humid environment and avoid quench sensitivity, the sum Cu + Mg is
preferably
controlled between 3.8 and 4.2.
The alloys of the present invention further contains 0.04 to 0.10 wt.%
zirconium,
which is typically used for grain size control. The control of the zirconium
content in
combination with the hot rolling conditions is important to obtain the desired
microstructural properties of the invention which are at mid-thickness more
than 75 % of
recrystallized grains or at mid-thickness 30 to 75 % of recrystallized grains
and non-
recrystallized grains with an aspect ratio in a LIST cross section less than
3.
The Zr content should preferably comprise at least about 0.05 wt. %, but
should
advantageously remain below about 0.08 or even 0.07 wt.%.
Titanium, associated with incidental elements such as boron or carbon can
usually be
added if desired during casting in order to limit the as-cast grain size. The
present
invention may typically accommodate up to about 0.15 wt. % and preferably up
to about
0.06 wt.% Ti. In a preferred embodiment of the invention, the Ti content is
about 0.02
wt.% to about 0.06 wt.% and preferentially about 0.03 wt.% to about 0.05 wt.%.
The present alloy can further contain other elements to a lesser extent and in
some
embodiments, on a less preferred basis. Iron and silicon typically affect
fracture
toughness properties. Iron and silicon content should generally be kept low,
with a
content of at most 0.15 wt.%, and preferably not exceeding about 0.13 wt.% or
preferentially about 0.10 wt.% for iron and preferably not exceeding about
0.10 wt.% or
preferentially about 0.08 wt.% for silicon. In one embodiment of the present
invention,
iron and silicon content are < 0.07 wt.%.
Other elements are impurities or incidental elements which should have a
maximum
content of 0.05 wt.% each and < 0.15 wt.% total, preferably a maximum content
of 0.03
wt.% each and <0.10 wt. total.
A suitable process for producing rolled products according to the present
invention
comprises: (a) casting an ingot made in an alloy according to the invention,
(b)
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conducting an homogenization of the ingot preferably with at least one step at
a
temperature from about 460 to about 510 C or preferentially from about 470 to
about
500 C typically for 5 to 30 hours, (c) conducting hot rolling of said
homogenized ingot
in one or more stages by rolling, with an entry temperature preferably
comprised from
5 about 280 to about 420 C, to a rolled product with a final thickness of
at least 80 mm,
(d) conducting a solution heat treatment preferably at a temperature from 460
to about
510 C or preferentially from about 470 to about 500 C typically for 1 to 10
hours
depending on thickness and conducting a quench, preferentially with room
temperature
water, (e) conducting stress relieving by controlled stretching or compression
with a
10 .. permanent set of preferably less than 5% and preferentially from 1 to
4%, and, (f)
conducting an artificial aging treatment.
The hot rolling entry temperature is controlled in order to obtain the desired
microstructural properties of the invention which are at mid-thickness more
than 75 % of
recrystallized grains or at mid-thickness 30 to 75 % of recrystallized grains
and non-
recrystallized grains with an aspect ratio in a LIST cross section less than
3.
Advantageously the hot rolling starting temperature is at least 145*Zr-4'313 -
20 and
preferably at least 145*Zr-4'313 ¨ 10. Preferably the hot rolling starting
temperature is at
most 145*Zr-0 313 20 and preferably at least 145 *Zr- 313 10. Zr is the
weight percent
concentration of Zirconium in the alloy.
The present invention finds particular utility in thick gauges of greater than
about 80
mm. In a preferred embodiment, a rolled product of the present invention is a
plate
having a thickness from 80 to 200 mm, or advantageously from 100 to 180 mm
comprising an alloy according to the present invention. "Over-aged" tempers
("T7 type")
are advantageously used in order to improve corrosion behavior in the present
invention.
Tempers that can suitably be used for the products according to the invention,
include,
for example T6, T651, T73, T74, T76, T77, T7351, T7451, T7452, T7651, T7652 or
T7751, the tempers T7351, T7451 and T7651 being preferred. Aging treatment is
advantageously carried out in two steps, with a first step at a temperature
comprised
between 110 and 130 C for 3 to 20 hours and preferably for 4 or 5 to 12 hours
and a
second step at a temperature comprised between 140 and 170 C and preferably
between
150 and 165 C for 5 to 30 hours.
In an advantageous embodiment, the equivalent aging time t(eq) at 155 C is
comprised between 8 and 35 or 30 hours and preferentially between 12 and 25
hours.
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The equivalent time t(eq) at 155 C being defined by the formula:
exp(-16000 / T) dt
t(eq) =
exp(-16000 / Tref)
where T is the instantaneous temperature in K during annealing and Tõf is a
reference
temperature selected at 155 C (428 K). t(eq) is expressed in hours.
With the narrow composition range of the invention it is possible to obtain a
product
with a low tendency to crack deviation and with a very low fatigue crack
growth rate.
Thus, for the product of the invention, during a fatigue crack growth rate
test according
to standard ASTM E647, the crack remains within a cone of 20 , as illustrated
by
Figure 2b, and preferably of 15 , which origin is at intersection of a line
passing
through specimen holes centers and a specimen axis of symmetry and da/dN at AK
= 15
1VIPa-Vm is less than 2.0 10' mm/cycle, preferably less than 1.0 10' mm/cycle
and more
preferably less than 0.9 10-5 mm/cycle, on a L-S C(T) fatigue specimen at mid-
thickness
with W = 40 mm and B = 10 mm.
The narrow composition range of the alloy from the invention, selected mainly
for a
strength versus toughness compromise provided rolled products with
unexpectedly high
EAC performance under conditions of high stress and humid environment.
A product according to the invention also preferably has preferably one, more
preferably
two and most preferably three of the following properties:
a) a minimum
life without failure after Environmentally Assisted Cracking
(EAC) under conditions of high stress, at a short transverse (ST) stress level
of
80% of the product tensile yield strength in ST direction, and humid
environment
with 85% relative humidity at a temperature of 70 C, of at least 20 days and
preferably of at least 30 days,
b) a conventional tensile yield strength measured in the L direction at
quarter
thickness of at least 515¨ 0.279 * t MPa and preferably of 525¨ 0.279 * t MPa
and even more preferably of 535 ¨ 0.279 * t 1VIPa (t being the thickness of
the
product in mm),
c) a
Kic toughness in the L-T direction measured at quarter thickness of at
least 32 - 0.1*t 1VIPaAlm and preferably 34 ¨ 0.1*t 1VIPaAlm and even more
preferably 36 ¨ 0.1*t 1VIPaAlm (t being the thickness of the product in mm).
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Rolled products according to the present invention are advantageously used as
or
incorporated in structural members for the construction of aircraft.
In an advantageous embodiment, the products according to the invention are
used in
wing ribs, spars and frames. In embodiments of the invention, the rolled
products
according to the present invention are welded with other rolled products to
form wing
ribs, spars and frames.
These, as well as other aspects of the present invention, are explained in
more detail with
regard to the following illustrative and non-limiting examples.
EXAMPLE
Example 1
Two ingots were cast, one of a product according to the invention (A), and one
reference example (B) with the following composition (Table 1) :
Table 1 : composition (wt. %) of a cast according to the invention and a
reference cast.
Alloy Si Fe Cu Mg Zn Ti Zr
A 0.03 0.04 2.13 1.75 7.05
0.04 0.06
0.05 0.09 1.64 2.25 6.10 0.02 0.11
The ingots were then scalped and homogenized at about 475 C. The ingots were
hot
rolled to a plate of thickness of 102 mm (alloy A) or 110 mm (alloys B). Hot
rolling
entry temperature was 350 C for alloy A and 440 C for alloy B. The plates
were
solution heat treated with a soak temperature of about 475 C. The plates were
quenched and stretched with a permanent elongation comprised between 2.0 and
2.5 %.
The reference plates were submitted to a two-step aging of 4 hours at 120 C
followed
by approximately 15 hours at 155 C for a total equivalent time at 155 C of 17
hours, to
obtain a T7651 temper.
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The plates made of alloy A had at mid-thickness more than 75 % of
recrystallized grains
and the plates of alloy B were substantially unrecrystallized, with a volume
fraction of
recrystallized grains lower than 35% at mid-thickness.
The samples were mechanically tested, at quarter-thickness for L and LT
directions and
at mid-thickness for ST direction to determine their static mechanical
properties as well
as their fracture toughness. Tensile yield strength, ultimate strength and
elongation at
fracture are provided in Table 2.
Table 2: Static mechanical properties of the samples
Alloy L Direction LT Direction ST Direction
UTS TYS UTS TYS UTS TYS
E (0/0) E (0/0) E (0/0)
(MPa) (MPa) (MPa) (MPa) (MPa) (MPa)
A 548 518 8,4 550 502 6,5 525 473 4,8
502 448 12,1 514 443 7,5 495 428 5,8
Results of the fracture toughness testing are provided in Table 3.
Table 3: Fracture toughness properties of the samples
Alloy Kic
L-T T-L S-L
(MPa-Vm) (MPa-Vm) (MPa-Vm)
A 26.9 25.1 28.6
35.1 29.5 29.3
EAC under conditions of high stress and humid environment was measured with ST
direction tensile specimens which are described in ASTM G47. Testing stress
and
environment were different from ASTM G47 and used a load of about 80% of ST
direction TYS at t/2, under 85% relative humidity, and at a temperature of 70
C. The
number of days to failure is provided for 3 specimens for each plate,
The results are provided in Table 4
Table 4 Results of EAC under conditions of high stress and humid environment
Alloy ST TYS t/2 (MPa) EAC Stress Test Method Number of
Days to Failure
(MPa)
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Sample 1 Sample 2
Sample 3
A 473 402 Constant 30 43 .. 60
Strain
The plate made of alloy A resisted in average 33 days under a stress of
3501VIPa for
SCC testing under ASTM G47.
The L-S fatigue crack growth rate was measured according to standard ASTM E647
at
mid-thickness and quarter thickness in the L-S direction on CT specimen
(CT10W40
thickness 10 mm, width 40 mm) under a maximum load of 4 KN and R=0.1. The
results
are presented in Table 5.
Table 5. Results of the L-S fatigue crack growth rate test (da/dN at AK = 15
MPaVm)
Alloy A Alloy B
1/2
1/2 thickness 1/4 thickness thickness 1/4
thickness
da/dN
(mm/cycle) 7.9E-05 6.8E-05 8.2E-05 8.5E-05 2.5E-04 2.0E-04 2.1E-04
The L-S fatigue crack growth rate is reduced up to a factor at least 3 on CT
specimens
for the invention alloy A vs alloy B.
Images of the cracked specimen of alloy A are shown in Figure 3. None of the
cracked
specimen exhibited a tendency to crack deviation, and the cracks remained
within a cone
of 15 . The cracked specimen of alloy B are shown in Figure 4 and the cracks
remained
within a cone of 20 but not however within a cone of 15 .
Example 2
One additional ingot was cast, with a composition according to the invention
(C), (Table
6) :
Table 6 : composition (wt. %) of cast C.
CA 03121837 2021-06-02
WO 2020/127592 PCT/EP2019/086106
Alloy Si Fe Cu Mg Zn Ti Zr
0.03 0.04 2.15 1.65 7.11 0.03 0.10
The ingot was then scalped and homogenized at 475 C. The ingot was hot rolled
to a
plate of thickness of 152 mm. Hot rolling entry temperature was 420 C. The
plate was
5 solution heat treated with a soak temperature of 475 C. The plate was
quenched and
stretched with a permanent elongation comprised between 2.0 and 2.5 %.
Due to the high hot rolling temperature, the plate microstructure was not
according to
the invention, the plate made had at mid-thickness less than 20% of
recrystallized grains.
The L-S fatigue crack growth rate was measured according to standard ASTM E647
at
10 mid-thickness and quarter thickness in the L-S direction on CT specimen
(CT10W40
thickness 10 mm, width 40 mm) under a maximum load of 4 KN and R=0.1. The
results
are presented in Table 7.
Table 7. Results of the L-S fatigue crack growth rate test (da/dN at AK = 15
MPaVm)
Alloy C
1/2 thickness
da/dN
(mm/cycle) 1.4E-04 1.2E-04
Images of the cracked specimen of plate C are shown in Figure 4. The cracked
specimen
exhibited a tendency to crack deviation, and the cracks did not remain within
a cone of
.
20 All documents referred to herein are specifically incorporated herein by
reference in their
entireties.
As used herein and in the following claims, articles such as "the", "a" and
"an" can
connote the singular or plural.
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In the present description and in the following claims, to the extent a
numerical value is
enumerated, such value is intended to refer to the exact value and values
close to that
value that would amount to an insubstantial change from the listed value.
