Patent Application
20080121317
The present invention relates to a high damage tolerant Al—Cu alloy product having a high toughness and an improved fatigue crack growth resistance while maintaining good strength levels, to a method for producing such a rolled high damage tolerant Al—Cu alloy product having a high toughness and an improved fatigue crack growth resistance and further to a rolled alloy sheet product for aeronautical applications. More specifically, the present invention relates to a high damage tolerant Al—Cu—Mg alloy designated by the Aluminum Association (“AA”)2xxx-series for structural aeronautical applications with improved properties such as fatigue crack growth resistance, strength and fracture toughness. The invention also relates to a rolled alloy product which is suitable used as fuselage skin or lower wing skin of an aircraft.
It is known in the art to use heat treatable aluminum alloys in a number of applications involving relatively high strength such as aircraft fuselages, vehicular members and other applications. The aluminum alloys 2024, 2324 and 2524 are well known heat treatable aluminum alloys which have useful strength and toughness properties in T3, T39 and T351 tempers.
The design of a commercial aircraft requires various properties for different types of structures on the aircraft. Especially for fuselage skin or lower wing skin it is necessary to have properties such as good resistance to crack propagation either in the form of fracture toughness or fatigue crack growth. At the same time the strength of the alloy should not be reduced. A rolled alloy product either used as a sheet or as a plate with an improved damage tolerance will improve the safety of the passengers, will reduce the weight of the aircraft and thereby improve the fuel economy which translates to a longer flight range, lower costs and less frequent maintenance intervals.
It is known in the art to have AA2×24 alloy compositions with the following broad compositional range, in weight percent:
U.S. Pat. No. 5,593,516 discloses a high damage tolerant Al—Cu alloy with a balanced chemistry comprising essentially the following composition (in weight %):
U.S. Pat. No. 5,897,720 discloses a high damage tolerant Al—Cu alloy with a “2024”-chemistry comprising essentially the following composition (in weight %):
the balance aluminum and unavoidable impurities wherein the alloy is annealed after hot rolling at a temperature at which the intermetallics do not substantially dissolve. The annealing temperature is between 398° C. and 455° C.
U.S. Pat. No. 5,938,867 discloses a high damage tolerant Al—Cu alloy with a “2024”-chemistry comprising essentially the following composition (in weight %):
EP-0473122, as well as U.S. Pat. No. 5,213,639, disclose an aluminum base alloy comprising essentially the following composition (in weight %):
the remainder aluminum, incidental elements and impurities, wherein such aluminum base is hot rolled, heated and again hot rolled, thereby obtaining good combinations of strength together with high fracture toughness and a low fatigue crack growth rate. More specifically, U.S. Pat. No. 5,213,639 discloses an inter-anneal treatment after hot rolling the cast ingot with a temperature between 479° C. and 524° C. and again hot rolling the inter-annealed alloy wherein the alloy contains one or more elements from the group consisting of Cr, V, Hf, Cr, Ag and Sc, each within defined ranges. Such alloy is reported to have a 5% improvement over the above mentioned conventional 2024-alloy in T-L fracture toughness and an improved fatigue crack growth resistance at certain ΔK-levels.
EP-1170394-A2 discloses an aluminum sheet product with improved fatigue crack growth resistance having an anisotropic microstructure defined by grains having an average length to width aspect ratio of greater than about 4 to 1 and comprising essentially the following composition, (in weight %):
Furthermore, it is described that low copper-high manganese samples exhibited higher properties than high copper-low manganese samples. Results from tensile strength measurements showed that high manganese variants exhibited higher strength values than the low manganese variants. The strengthening effect of manganese was reported to be surprisingly higher than that of copper.
It is a preferred object of the present invention to provide a high damage tolerant 2024-series type alloy rolled product having a high toughness and an improved fatigue crack growth resistance while maintaining good strength levels of conventional 2024, 2324 or 2524 alloys. It is another preferred object of the present invention to provide an aluminum alloy sheet product having an improved fracture toughness and resistance to fatigue crack growth for aircraft applications such as fuselage skin or lower-wing skin.
Yet a further preferred object of the present invention is to provide rolled aluminum alloy sheet products and a method for producing those products so as to provide structural members for aircrafts which have an increased resistance to fatigue crack growth and to provide an improved fracture toughness while still maintaining high levels of strength.
More specifically, there is a general requirement for rolled AA2000-series aluminum alloys within the range of 2024 and 2524 alloys when used for aeronautical applications that the fatigue crack growth rate (“FCGR”) should not be greater than a defined maximum. A FCGR which meets the requirements of high damage tolerance 2024-series alloy products is, e.g., FCGR below 0.001 mm/cycles at ΔK=20 MPa√m and 0.01 mm/cycles at ΔK=40 MPa√m.
The present invention preferably solves one or more of the above mentioned objects.
The foregoing and other features and advantages of the alloy according to the invention will become readily apparent from the following detailed description of preferred embodiments. Some of the enhanced high damage tolerant properties are shown in the appended drawings, in which:
In accordance with the invention there is disclosed a high damage tolerant Al—Cu alloy having a high toughness and an improved fatigue crack growth resistance by maintaining high levels of strength which comprises essentially the following composition (in weight %):
It has surprisingly been found that lower levels of manganese result in a high toughness and an improved fatigue crack growth resistance specifically in areas where the toughness and fatigue crack growth resistance under tensile load are critical. The alloy of the instant invention in a T3 temper has significant improved high damage tolerance properties by lowering the amount of manganese and by partially replacing manganese-containing dispersoids by zirconium containing dispersoids. At the same time it is important to carefully control the chemistry of the alloy.
The main improvement of the alloy according to the present invention is an improved fatigue crack growth resistance at the lower ΔK-values which leads to significant longer lifetimes. The balance of high damage tolerance properties and mechanical properties of the alloy of the present invention is better than the balance of conventional 2024 or 2524-T3 alloys. At the same time the toughness levels are equal or better to 2524 alloy levels. It has been found that the high damage tolerance properties such as fracture toughness or strength may be further improved by adding zirconium.
The amount (in weight %) of manganese is preferably in a range of 0.20 to 0.45%, most preferably in a range of 0.25 to 0.30%. Mn contributes to or aids in grain size control during operations. The preferred levels of manganese are lower than those conventionally used in conventional AA2×24 alloys while still resulting in sufficient strength and improved damage tolerance properties. In order to optimize the improved high damage tolerance properties the chemical composition of the alloy of the present invention preferably meets the proviso that Zr≧0.09 when Mn≦0.45 and Cu≧4.0.
The amount (in weight %) of copper is in a range of 4.0 to 4.4, preferably in a range of 4.1 to 4.3. Copper is an important element for adding strength to the alloy rolled product. It has been found that a copper content of 4.1 or 4.2 results in a good compromise in strength, toughness, formability and corrosion performance while still resulting in sufficient damage tolerance properties.
The preferred amount (in weight %) of magnesium is in a range of 1.0 to 1.4, most preferably in a range of 1.1 to 1.3. Magnesium provides also strength to the alloy rolled product.
The preferred amount (in weight %) of zirconium is in a range of 0.09 to 0.15 thereby partially replacing Mn-containing dispersoids. The balance of manganese and zirconium influences the recrystallisation behavior. Throughout the addition of zirconium more elongated grains may be obtained which also results in an improved fatigue crack growth resistance. Zirconium may also be at least partially replaced by chromium wherein [Zr] +[Cr]≦0.20. Preferred amounts (in weight %) of chromium and zirconium are in a range of 0.05 to 0.15, preferably in a range of 0.10 to 0.13. The balance of zirconium and chromium as well as the partial replacement of Mn-containing dispersoids and Zr-containing dispersoids result in an improved recrystallisation behavior and more elongated grains.
A preferred alloy composition of the present invention comprises the following composition (in weight %):
Another preferred alloy according to the present invention consists of the following composition (in weight %):
Even more preferred, an alloy according to the present invention consists of the following composition (in weight %):
The balance in the rolled alloy product according to the invention is aluminum and inevitable impurities and incidental elements. Typically, each impurity element is present at 0.05% maximum and the total of impurities is 0.20% maximum. Preferably the alloy product is substantially Ag-free. The best results are achieved when the alloy rolled products have a recrystallised microstructure meaning that 75% or more, and preferably more than 80% of the grains in a T3 temper, e.g. T39 or T351, are recrystallised. In a further aspect of the microstructure it has the grains have an average length to width aspect ratio of smaller than about 4 to 1, and typically smaller than about 3 to 1, and more preferably smaller than about 2 to 1. Observations of these grains may be done, for example, by optical microscopy at 50× to 100× in properly polished and etched samples observed through the thickness in the longitudinal orientation.
The alloy according to the present invention may further comprise one or more of the elements Zn, Hf, V, Sc, Ti or Li, the total amount less than 1.00 (in weight %). These additional elements may be added to further improve the balance of the chemistry and enhance the forming of dispersoids.
In another aspect the invention provides a method for producing a rolled high damage tolerant Al—Cu alloy product having a composition as set out above and having a high toughness and an improved fatigue crack growth resistance according to the invention comprises the steps of:
After hot rolling the ingot it is possible to anneal and/or re-heat the hot rolled ingot and again hot rolling the rolled ingot. It is believed that such re-heating or annealing enhances the fatigue crack growth resistance by producing elongated grains which—when recrystallized—maintain a high level of toughness and good strength. It is furthermore possible to conduct a surface heat treatment between hot rolling and cold rolling at the same temperatures and times as during homogenisation, e.g. 1 to 5 hours at 460° C. and about 24 hours at 490° C. The hot rolled ingot is preferably inter-annealed before and/or during cold rolling to further enhance the ordering of the grains. Such inter-annealing is preferably done at a gauge of about 4.0 mm for one hour at 350° C. Furthermore, it is advisable to stretch the rolled and heat-treated product in a range of 1 to 5%, preferably in a range of 1 to 3%, and then naturally aging the stretched product for more than 5 days, preferably about 10 to 20 days, and more preferably for 10 to 15 days, to provide a T3 temper condition, in particular a T351 temper condition.
The present invention provides a high damage tolerant rolled Al—Cu alloy sheet product which has high toughness and an improved fatigue crack growth resistance with the above described alloy composition which is preferably produced in accordance with the above described method. Such rolled alloy sheet product has preferably a gauge of around 2.0 mm to 12 mm for applications such as fuselage skin and about 25 mm to 50 mm for applications such as lower-wing skin. The present invention thereby provides an aircraft fuselage sheet or an aircraft lower-wing member sheet with improved high damage tolerance properties. In particular when used as aircraft fuselages, the sheet may be unclad or clad, with preferred cladding layer thickness of from about 1 to about 5 percent of the thickness of the sheet.
The foregoing and other features and advantages of the alloy according to the invention will become readily apparent from the following examples. Some of the enhanced high damage tolerant properties are shown in the appended drawings, in which:
On an industrial scale 7 different aluminum alloys have been cast into ingots having the following chemical composition as set out in Table 1.
The alloys have been processed to a 2.0 mm sheet in the T351 temper. The cast ingots were homogenized at about 490° C., and subsequently hot rolled at about 410° C. The plates were further cold rolled, surface heat treated and stretched by about 1%. All alloys have been tested after at least 10 days of natural aging.
Then the ultimate tensile strength properties and the unit propagation energy as well as the Kahn-tear has been measured in the L and T-L direction. The testing has been done in accordance with ASTM-B871 (1996) for the Kahn tear tests, and EN-10.002 for the tensile tests.
As identified in Table 2 and shown in
In order to identify the fatigue crack growth rate (“FCGR”) all alloys were tested according to ASTM E-647 on 80 mm wide M(T) panels at R=0.1 at constant load and a frequency of 8 Hz. The lifetime as shown in Table 3 is defined as the time (in number of cycles) that the crack grows from a length of 5 mm to 20 mm. The maximum stress was 54 MPa. The initial notch was 4.1 mm. Anti-buckling device are not used. The results are presented in Table 3 and
From the results of Table 3 and
From the results of Table 3 and according to
Having now fully described the invention, it will be apparent to one of ordinary skill in the art that many changes and modifications can be made without departing from the spirit or scope of the invention as hereon described.
| Number | Date | Country | Kind |
|---|---|---|---|
| 02078443.5 | Aug 2002 | EP | regional |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 10642507 | Aug 2003 | US |
| Child | 11948614 | US |
