Field of the Invention
[0001] 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
[0002] 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.
[0003] 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.
[0004] 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.
[0005] 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 G47, where specimens
are tested using alternate immersion and drying cycles with NaCl solution (ASTM G44)
and typically using lower stress.
[0006] 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 µm), on a mesoscopic scale (100-1000 µm) 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.
[0007] 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).
[0008] The patent
US 8,323,426 proposes a solution to improve crack branching for some Al-Cu-Li alloys.
[0009] However crack deviation improvement is often related to higher fatigue crack growth
rate in the original cracking plane, before deviation.
[0010] 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.
[0011] 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.
[0012] US Patent application N°
US20050167016A1 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.
[0013] 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.
[0014] 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.
[0015] PCT Patent application No WO2004090183 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.
[0016] 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 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.
[0017] 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 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.
[0018] 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.
[0019] 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.
[0020] 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.
[0021] 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,
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.
SUMMARY OF THE INVENTION
[0022] 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.
[0023] 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.
[0024] 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
30 to 75 % of grains are recrystallized and non-recrystallized grains have an aspect
ratio in a L/ST cross section less than 3.
[0025] 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-%)
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;
- b) homogenizing the ingot;
- c) hot rolling said homogenized ingot to a rolled product with a final thickness of
at 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 L/ST cross section less than 3.
BRIEF DESCRIPTION OF THE DRAWINGS
[0026]
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.
DETAILED DESCRIPTION
[0027] 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).
[0028] 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).
[0029] Unless otherwise specified, the definitions of standard EN 12258 apply.
[0030] The symbol * is used for "multiplied by".
[0031] The fracture toughness K
1C is determined according to ASTM standard E399 (2012).
[0032] 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.
[0033] 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 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.
[0034] 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.
[0035] 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.
[0036] 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.
[0037] 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.
[0038] 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.
[0039] 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.
[0040] 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 L/ST cross section less than
3.
[0041] 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.%.
[0042] 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.%.
[0043] 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.%.
[0044] 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.
[0045] 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) 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 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 permanent set of preferably less than 5% and preferentially from 1 to 4%, and, (f)
conducting an artificial aging treatment.
[0046] 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 L/ST cross section less than 3. Advantageously the
hot rolling starting temperature is at least 145*Zr
-0.313 - 20 and preferably at least 145*Zr
-0.313 - 10. Preferably the hot rolling starting temperature is at most 145*Zr
-0.313 + 20 and preferably at least 145 *Zr
-0.313 + 10. Zr is the weight percent concentration of Zirconium in the alloy.
[0047] 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.
[0048] 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.
[0049] The equivalent time t(eq) at 155°C being defined by the formula :
where T is the instantaneous temperature in °K during annealing and T
ref is a reference temperature selected at 155 °C (428 °K). t(eq) is expressed in hours.
[0050] 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 ΔK = 15
MPa√m is less than 2.0 10
-4 mm/cycle, preferably less than 1.0 10
-4 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.
[0051] 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.
[0052] 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 MPa (t being the thickness of the product in mm),
- c) a K1C toughness in the L-T direction measured at quarter thickness of at least 32 - 0.1*t
MPa√m and preferably 34 - 0.1*t MPa√m and even more preferably 36 - 0.1*t MPa√m (t
being the thickness of the product in mm).
[0053] Rolled products according to the present invention are advantageously used as or
incorporated in structural members for the construction of aircraft.
[0054] 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.
[0055] 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
[0056] 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 |
B |
0.05 |
0.09 |
1.64 |
2.25 |
6.10 |
0.02 |
0.11 |
[0057] 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 %.
[0058] 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.
[0059] 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.
[0060] 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 (MPa) |
TYS (MPa) |
E (%) |
UTS (MPa) |
TYS (MPa) |
E (%) |
UTS (MPa) |
TYS (MPa) |
E (%) |
A |
548 |
518 |
8,4 |
550 |
502 |
6,5 |
525 |
473 |
4,8 |
B |
502 |
448 |
12,1 |
514 |
443 |
7,5 |
495 |
428 |
5,8 |
[0061] Results of the fracture toughness testing are provided in Table 3.
Table 3: Fracture toughness properties of the samples
Alloy |
K1C |
L-T (MPa√m) |
T-L (MPa√m) |
S-L (MPa√m) |
A |
26.9 |
25.1 |
28.6 |
B |
35.1 |
29.5 |
29.3 |
[0062] 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 (MPa) |
Test Method |
Number of Days to Failure |
|
|
|
|
Sample 1 |
Sample 2 |
Sample 3 |
A |
473 |
402 |
Constant Strain |
30 |
43 |
60 |
[0063] The plate made of alloy A resisted in average 33 days under a stress of 350 MPa for
SCC testing under ASTM G47.
[0064] 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 load of 4 KN. The results are presented in Table 5.
Table 5. Results of the L-S fatigue crack growth rate test (da/dN at ΔK = 15 MPa√m)
|
Alloy A |
Alloy B |
|
1/2 thickness |
1/4 thickness |
1/2 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 |
[0065] 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.
[0066] 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°.
[0067] All documents referred to herein are specifically incorporated herein by reference
in their entireties.
[0068] As used herein and in the following claims, articles such as "the", "a" and "an"
can connote the singular or plural.
[0069] 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.
1. A rolled aluminum-based alloy 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
30 to 75 % of grains are recrystallized and non-recrystallized grains have an aspect
ratio in a L/ST cross section less than 3.
2. A product according to claim 1 wherein Cu 1.95 - 2.25 and preferably Cu : 2.00 -2.20.
3. A product according to claim 1 or claim 2 wherein during a fatigue crack growth rate
test according to standard ASTM E647 the crack remains within a cone of ±20° 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 ΔK = 15 MPa√m is less than 2.0
10-4 mm/cycle, preferably less than 1.0 10-4 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 .
4. A product according to any of claims 1 to 3, wherein said product has at least one
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 30 days and preferably of at least 40 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 MPa, t being the thickness of the product in mm,
c) a K1C toughness in the L-T direction measured at quarter thickness of at least 32 - 0.1*t
MPa√m and preferably 34 - 0.1*t MPa√m and even more preferably 36 - 0.1*t MPa√m, t
being the thickness of the product in mm.
5. A product according to any of claims 1 to 4 wherein the thickness thereof is from
80 to 200 mm, or advantageously from 100 to 180 mm.
6. A structural member suitable for aircraft construction wherein said structural member
is used in wing ribs, spars and frames, comprising a product according to any of claims
1 to 5.
7. A process for the manufacture of a rolled aluminum-based alloy product comprising
the steps of:
a) casting an ingot 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;
b) homogenizing the ingot;
c) hot rolling said homogenized ingot to a rolled product with a final thickness of
at least 80 mm ;
d) solution heat treating and quenching the product;
e) stress-relieving the solution heat treated and quenched 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 L/ST cross section less than 3.
8. A process according to claim 7 wherein the hot rolling starting temperature is at
least 145*Zr-0.313 - 20 and preferably at least 145*Zr-0.313 - 10.
9. A process according to claim 7 or claim 8 wherein the hot rolling starting temperature
is at most 145*Zr-0.313 + 20 and preferably at least 145*Zr-0.313 + 10.
10. A process according to anyone of claims 7 to 9 wherein the equivalent aging time t(eq)
is comprised between 8 and 30 hours and preferentially between 12 and 25 hours, the
equivalent time t(eq) at 155°C being defined by the formula :
where T is the instantaneous temperature in °K during annealing and T
ref is a reference temperature selected at 155 °C (428 °K). t(eq) is expressed in hours.
11. A process according anyone of claims 7 to 10 wherein the solution heat treatment temperature
is from 460 to about 510 °C or preferentially from about 470 to about 500 °C.