Patent Grant
7252723
1. Field of the Invention
The present invention relates generally to damage tolerant aluminum alloys, and in particular, to such alloys useful in the aerospace industry suitable for use in lower wing skin applications and as fuselage skin.
2. Description of Related Art
Materials particularly adapted for use in lower wing skin applications including 2×24 alloys are generally known, as described, for example, in U.S. Pat. Nos. 5,213,639 and 6,444,058 as well as in the PCT application WO 99/31287, the content of which is incorporated herein by reference in their entireties. Damage tolerance of 2×24 alloys is of particular importance and materials that have excellent properties in this regard are highly desirable. These 2×24 alloys, derived from the chemical composition of the 2024 alloy, usually contain manganese in a concentration of at least 0.15 to 0.20%, and up to 0.8 or 0.9%. This is the case of the 2×24 alloys which have been standardized by The Aluminum Association (AA): 2024, 2024A, 2124, 2224, 2224A, 2324, 2424, 2524.
European Patent Application EP 1 170 394 A discloses methods for manufacturing damage tolerant AlCuMg sheet. These methods involve unusual (hot cross rolling) or otherwise expensive manufacturing steps (repeated intermediate heat treatment) in order to obtain a precisely controlled microstructure.
According to the present invention, there is provided a substantially manganese-free aluminum alloy rolled product consisting essentially of (in percent by weight):
This product, as plate or sheet, presents a good compromise between fracture toughness and mechanical strength. It can be provided as plate or sheet, and is suitable for use in applications that require high damage tolerance, such as in lower wing skins or fuselage skin.
As used herein, the term “sheet” includes flat rolled aluminum products having a thickness form about 0.2 mm to about 12 mm, whereas the term “plate” is limited to products thicker than 12 mm. This definition is different from the one used in European Standard EN 12258-1.
Specifically, substantially Mn-free AlCuMg alloys for applications such as in lower wing skins are believed to be novel and to provide unexpectedly superior properties. As used herein, “substantially Mn-free” means up to 0.05% Mn. These alloys were compared against high damage tolerant material 2024 (Reference DT) according to prior art. According to embodiments of the present invention, manganese has been totally replaced by zirconium or by zirconium+300 μg/g of scandium.
Sheet or plate according to the present invention may have one or more of the following combinations of properties:
Plate according to the present invention may have one or more of the following combinations of properties:
Another object of the present invention involves providing methods for manufacturing sheet products and plate products in said substantially manganese-free alloys. These methods are particularly simple, especially for production of sheet.
Additional objects, features and advantages of the invention will be set forth in the description which follows, and in part, will be obvious from the description, or may be learned by practice of the invention. The objects, features and advantages of the invention may be realized and obtained by means of the instrumentalities and combination particularly pointed out in the appended claims.
The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate a presently preferred embodiment of the invention, and, together with the general description given above and the detailed description of the preferred embodiment given below, serve to explain the principles of the invention.
In accordance with the present invention, an attempt has been made to improve the damage tolerance of 2×24 alloys suitable for lower wing skin applications (in the form of plate of thickness typically of the order of 12 to 25 mm) and fuselage skin applications (in the form of sheet of thickness typically of the order of 3 to 9 mm). Some applications of 2×24 alloys include, for example, lower wing skin structural members and wing spar members.
Several alloys were tested:
A high damage tolerant 2024 with no addition of Scandium and Zirconium (internal designation DT, composition in agreement with AA2024A) is taken as the reference material.
Specifically, Mn-free 2×24 alloys for applications such as in lower wing skins are found to provide unexpectedly superior properties. As used herein, “Mn-free” means up to 0.05% Mn. Although a loss of strength is expected in some cases in the T351 temper, better damage tolerance can be achieved, owing to a lower volume fraction of AlFeMn-type coarse intermetallics.
In a preferred embodiment, the Scandium content was chosen at a level of 300 ppm in order to substantially avoid the precipitation of coarse (Al,Cu,Sc) primary phases while keeping a strong anti-recrystallization influence. However, different amounts of scandium might be possible as well without departing from the scope of the present invention.
According to preferred embodiments of the present invention, there is provided an Al alloy sheet or plate product comprising: 3.6-4.5% Cu, 1.0-1.6% Mg, 0.08-0.20% Zr (preferred 0.08-0.14% Zr), 0.0-0.06% Sc (preferred 0.02-0.05% Sc).
Al alloy sheet or plate products of the present invention preferably have a recrystallized volume fraction of 5% maximum according to some embodiments. In particularly advantageous embodiments there is provided an aluminum alloy sheet or plate product comprising 3.7-4.2% Cu (preferred 3.8-4.2%), 1.1-1.5% Mg (preferred 1.2-1.5%), 0.10-0.14% Zr, and 0-0.05% Sc (preferred 0.02-0.05% Sc). In one embodiment, there is provided an aluminum alloy sheet or plate product that is substantially Mn-free, which means here having less than 0.05% Mn. In further embodiments, said sheet or plate product contains up to 0.01% Mn. Scandium, if included, is preferably included in an amount from 0.02-0.05%; a Scandium content of 300 ppm (0.03%) by mass has been used in a preferred embodiment.
The products according to the present invention can be subjected to naturally aged tempers with various degrees of post-quench cold-working (T351, T37, T39 . . . ) and artificially aged tempers with various degrees of post-quench cold-working (T851, T87, T89 . . . ).
A preferred method for obtaining plate products according to the present invention comprises:
A preferred method for obtaining sheet products according to the present invention comprises:
This preferred method for obtaining sheet is very simple and does not involve reheating between hot-rolling steps, or recrystallization treatment.
The product according to the present invention is particularly suitable for use as a lower wing skin structural member. Another advantageous use is the use as fuselage skin sheet. Both sheet and plate can be clad.
A preferred sheet or thin plate with a thickness below about 12 mm in T351 temper has a da/dn in T-L direction which fulfills at least one, and preferably two or more, and even more preferably all of the following conditions:
A preferred plate in T351 temper has a da/dn in T-L direction which fulfills at least one, and preferably two or more, and even more preferably all of the following conditions:
Products according to the present invention exhibit in a corrosion test according to ASTM G 110 a maximum intergranular corrosion attack of less than 80 μm in T39 temper, and/or less than 200 μm in T851 temper, and/or less than 250 μm in T89 temper, and/or less than 300 μm in T351 temper. In a preferred embodiment, they have a maximum intergranular attack of less than 70 μm in T39 temper, and/or less than 180 μm in T851 temper, and/or less than 220 μm in T89 temper, and/or less than 270 μm in T351 temper.
It should be noted that according to some embodiments of the present invention, scandium, although preferred, can optionally be replaced by one or more of the following chemical elements: Hf, La, Ti, Ce, Nd, Eu, Gd, Tb, Dy, Ho, Er, Y, Yb, Cr. Typically, the concentration of each of these elements should not exceed about 0.1%, and the total of said elements should not exceed about 0.3%.
a) Manufacturing of Alloys/Tempers
Casting of several ingots was conducted at a laboratory scale cast house, on (320 mm×120 mm) slabs (2t casting unit). The compositions in weight % are given in Table 1.
Table 1 also gives the alloy designations that will be used hereinbelow:
The detailed conditions of the transformation of the slabs are provided below:
The details regarding the actual manufacturing parameters are given in Table 2.
b) Microstructural Characterization
The microstructural characterization program of these alloys was only conducted in the basic T351 temper. It consisted of Differential Scanning Calorimetry (DSC) and Optical micrography.
Table 3 below gives the main microstructural characteristics of the alloys in the T351 temper. According to the DSC results, all these alloys seem to be well solutionized. Detailed micrographs of some of the alloys are provided in
The alloys manufactured in Example 1 in the various T3X tempers were characterized as follows:
The static tensile properties in the T3X tempers are summarized in Table 4 and
The following effects are demonstrated:
8% + 2.5%
8% + 2.5%
Fracture toughness was evaluated by Kahn tear tests (see Table 5) and Kapp R-curve evaluation (see Table 6).
Kahn tear maximum stress Re of initiation energy Einit (energy spent until the maximum stress is reached) are indicative of the plane stress fracture toughness performance (the specimen thickness is about 5 mm).
The Kapp evaluation is conducted on thin (6.35 mm-0.25″) CT specimens (width 40 mm-1.6″) and corresponds to testing conditions close to the R-curve.
As for T3X fracture toughness results (
8% + 2.5%
8% + 2.5%
As regards the crack propagation performance of the alloys in T3X tempers, the following points can be stated (Table 6 and
The exfoliation corrosion ratings after the EXCO test (ASTM G34) are given in Table 7. The alloys containing no manganese seem to be slightly more sensitive (espically the DT+Zr+Sc variant which shows a very oriented grain structure).
The alloys manufactured in Example 1 (various T3X tempers) were artificially aged to T8X tempers as explained in Example 1.
The high manganese variant named 24HiMn was not selected for the T78X evaluation, due to its relatively poor toughness.
Prior to the artificial aging treatment, aging kinetics (using Vickers hardness as a strength indicator) have been conducted on the various alloys in different T3X conditions. The results are provided in
On some of the cases (apparently independent of alloy chemistry and T3X temper), an initial decrease of hardness is observed for low ageing times; this is probably due to retrogression phenomena. Then, hardness increases, owing to precipitation hardening. A peak in hardness is generally observed, before hardness slowly decreases by over-ageing.
Table 8 below gives the aging treatment duration chosen for the complete characterization program in the T8X tempers.
The static tensile properties in the T8X tempers are summarized in Table 9 and
Regarding the T8X fracture toughness results (Table 10 and
As regards the crack propagation performance (FCGR=Fatigue Crack Growth Rate) of the alloys in T8X tempers (Table 10 and
Table 11 below summarizes the EXCO results obtained on the T8X tempers for the different alloys. The results obtained on the T351 tempers are recalled. In the T8X tempers, it is noticed that the corrosion susceptibility decreases from T851 to T89 tempers, provided that the ageing treatment is the same (20 h at 173° C.). This is probably due to a more extensive intragranular precipitation in the case of strongly cold-worked tempers. When such a strong cold-work is followed by a shorter ageing treatment, the intragranular precipitation is probably not very different (in terms of solute content decrease) from that of the T351 temper, and corrosion susceptibility is similar.
Two alloys N and M with a chemical composition according to the invention were elaborated. The liquid metal was treated firstly in the holding furnace by injecting gas using a type of rotor known under the trade mark IRMA, and then in a type of ladle known under the trade mark Alpur. Refining was done with AT5B wire (0.7 kg/ton). 3.2 m-long ingots were cast, with a section of 320 mm×120 mm. They were relaxed for 10 h at 350° C.
The ingots were then homogenized at 500° C. for 12 hours and then hot rolled to a thickness of 6 mm. The exit temperature from the hot rolling mill was between 230° C. and 255° C. From ingot N, four sheets labeled N1, N2, N3 and N4 were obtained in this way. They were all solution heat treated in a salt bath furnace for 1 hour at 500° C., and then water quenched. Up to this point, the five sheets M, N1, N2, N3 and N4 were elaborated by the same process.
An alloy E according to prior art was elaborated using the same casting and hot rolling process as for alloy N. Solution heat treatment was done in a salt bath furnace for 1 hour at 500° C. on test coupons of size 600 mm×200 mm, followed by quenching in water (about 20° C.) and stretching to a permanent set of 2% (temper T351).
The chemical compositions of the alloys N and E alloys measured on a spectrometry slug taken from the launder, are given in Table 12:
No zinc and chromium were detected.
The ultimate tensile strength (UTS) Rm (in MPa), the tensile yield stress (TYS) at 0.2% elongation Rp0.2 (in MPa) and the elongation at failure A (in %) were measured by a tensile test according to EN 10002-1.
Table 13 contains the results of measurements of static mechanical characteristics:
The UTS and TYS of sheets M and N1, according to the invention, are almost comparable to those of sheet E, according to prior art, but their elongation is significantly higher. Sheet N2 (T39 temper), N3 (T851 temper) and especially N4 (T89 temper) exhibit improved mechanical properties compared to sheets M, N1 and E, as well as elongation values which are deemed sufficient for the application as fuselage skin sheet.
Damage tolerance was characterized in the T-L direction using the maximum stress Re (in MPa) and the creep energy Eec as derived from the Kahn test. The Kahn stress is equal to the ratio of the maximum load Fmax that the test piece can resist on the cross section of the test piece (product of the thickness B and the width W). The creep energy is determined as the area under the Force-Displacement curve as far as the maximum force Fmax resisted by the test piece. The Kahn test, well known to one skilled in the art, is described in the article “Kahn-Type Tear Test and Crack Toughness of Aluminum Alloy Sheet” published in the Materials Research & Standards Journal, April 1964, p. 151-155. The content of said article is incorporated herein by reference in its entirety. The test piece used for the Kahn toughness test is described in the “Metals Handbook”, 8th Edition, vol. 1, American Society for Metals, pp. 241-242. The results are given in table 14:
The maximum stress to which sheet N1 is capable of resisting is higher that that of sheet E, for a higher creep energy.
Fracture toughness was also determined for sheets N1, N2, N3, N4 and E by a measurement of the plane stress fracture toughness Kapp according to ASTM E 561 in the T-L direction using C(T) test pieces with W=127 mm. Results are given in table 15.
The sheet according to the present invention, and especially in T851 temper (sheets N3), show significantly improved Kapp values.
Fatigue resistance was determined according to ASTM E 647, by measuring the fatigue crack growth rate using C(T) test pieces with W=75 mm. The fatigue crack growth rate da/dN (in mm/cycle) for different levels of ΔK (expressed in MPa√m) was determined. Results are displayed in table 16.
All sheets according to the invention have a fatigue crack growth rate at least as good as sheet E according to prior art, most are significantly better, and especially sheets M and N1.
Corrosion resistance was evaluated according ASTM G 110. After etching and polishing, the maximum depth of corrosion attack was evaluated. All samples exhibited intergranular corrosion attack, but the maximum depth of corrosion was only 40 μm for N2, 165 μm for N3, 180 μm for N4 and 225 μm for N1, whereas sample E according to prior art exhibited a maximum depth of 350 μm. Sample N2 also showed pitting, but at maximum depth not exceeding 60 μm.
Additional advantages, features and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details, and representative devices, shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.
As used herein and in the following claims, articles such as “the”, “a” and “an” can connote the singular or plural.
All documents referred to herein are specifically incorporated herein by reference in their entireties.
This application claims priority from Provisional Application Ser. No. 60/394,234, filed Jul. 9, 2002, the content of which is incorporated herein by reference in its entirety.
| Number | Name | Date | Kind |
|---|---|---|---|
| 5213639 | Colvin et al. | May 1993 | A |
| 5593516 | Cassada, III | Jan 1997 | A |
| 6444058 | Liu et al. | Sep 2002 | B1 |
| 6562154 | Rioja et al. | May 2003 | B1 |
| 20040099353 | Benedictus et al. | May 2004 | A1 |
| 20040112480 | Benedictus et al. | Jun 2004 | A1 |
| Number | Date | Country |
|---|---|---|
| 1 170 394 | Jan 2002 | EP |
| 2717827 | Sep 1995 | FR |
| WO 9931287 | Jun 1999 | WO |
| Number | Date | Country | |
|---|---|---|---|
| 20040079455 A1 | Apr 2004 | US |
| Number | Date | Country | |
|---|---|---|---|
| 60394234 | Jul 2002 | US |
