Patent Application
20120152415
1. Field of the Invention
The invention relates to aluminum-copper-lithium alloy products, and more particularly such products, their manufacturing processes and use, designed in particular for aeronautical and aerospace engineering.
2. Description of Related Art
Hat-rolled products made of aluminum alloy are developed to produce parts of high strength designed in particular for the aircraft and aerospace industry.
Aluminum alloys containing lithium (AlLi) are of great interest in this respect, because lithium can reduce the density of aluminum by 3% and increase the modulus of elasticity by 6% for each percent of added lithium weight. For these alloys to be selected for aircraft, their performance as compared to the other usual properties must attain that of alloys in regular use, in particular in terms of the compromise between static mechanical resistance properties (tensile and compression yield stress, ultimate tensile strength) and damage tolerance properties (fracture toughness, resistance to fatigue crack propagation), these properties being in general contradictory. For certain parts such as the upper surfaces of wing skins the compression yield stress is an essential property. These mechanical properties must moreover be preferably stable over time and have good thermal stability, i.e. not be significantly modified by thermal exposure at operating temperature.
These alloys must also have sufficient corrosion resistance, be capable of being formed according to usual processes and have low residual stresses in order to be able to be integrally machined.
U.S. Pat. No. 5,032,359 describes a vast family of aluminum-copper-lithium alloys in which the addition of magnesium and silver, in particular between 0.3 and 0.5 percent by weight, makes it possible to increase the mechanical resistance.
U.S. Pat. No. 5,455,003 describes a manufacturing process for Al—Cu—Li alloys which have improved mechanical resistance and fracture toughness at cryogenic temperature, in particular owing to appropriate working and aging. This patent particularly recommends the composition, expressed as a percentage by weight, Cu=3.0-4.5, Li=0.7-1.1. Ag=0-0.6, Mg=0.3-0.6 and Zn=0-0.75.
U.S. Pat. No. 7,438,772 describes alloys including, expressed as a percentage by weight, Cu: 3-5, Mg: 0.5-2, Li: 0.01-0.9 and discourages the use of higher lithium content because of a reduction in the balance between fracture toughness and mechanical resistance.
U.S. Pat. No. 7,229,509 describes an alloy including (% by weight): (2.5-5.5) Cu, (0.1-2.5) Li, (0.2-1.0) Mg, (0.2-0.8) Ag, (0.2-0.8) Mn, 0.4 max Zr or other grain-refining agents such as Cr, Ti, Hf, Sc, V.
US patent application 2009/142222 A1 describes alloys including (percentage by weight), 3.4 to 4.2% Cu, 0.9 to 1.4% Li, 0.3 to 0.7% Ag, 0.1 to 0.6% Mg, 0.2 to 0.8% Zn, 0.1 to 0.6% Mn and 0.01 to 0.6% of at least one element for controlling the granular structure. This request also describes a manufacturing process for extruded products.
There exists a need for flat-rolled products made of aluminum-copper-lithium alloy having improved properties as compared to those of known products, in particular in terms of compromise between static mechanical resistance properties, in particular tensile yield stress and compression, and damage tolerance properties, in particular fracture toughness, thermal stability, corrosion resistance and machinability, while having a low density.
In addition there exists a need for a reliable and economic manufacturing process for these products.
A first subject of the invention is a manufacturing process for a flat-rolled product made of an aluminum alloy in which the following operations are performed in succession:
A second subject of the invention is a flat-rolled product of thickness between 8 and 50 mm and of substantially unrecrystallized granular structure obtainable by the process according to the invention having at mid-thickness at least one of the following combinations of characteristics:
Another subject of the invention is a structural element for an airplane, preferably an upper wing skin, including a product according to the invention.
Still another subject of the invention is the use of a product according to the invention or a structural element according to the invention for aeronautical engineering.
Unless otherwise stated, all the indications concerning the chemical composition of the alloys are expressed as a percentage by weight based on the total weight of the alloy. The expression 1.4 Cu means that the copper content expressed as a percentage by weight is multiplied by 1.4. Alloys are designated in conformity with the rules of The Aluminium Association, known to those skilled in the art. The density depends on the composition and is determined by calculation rather than by a method of weight measurement. The values are calculated in compliance with the procedure of The Aluminium Association, which is described on pages 2-12 and 2-13 of “Aluminum Standards and Data”. The definitions of the metallurgical states are indicated in European standard EN 515.
The tensile static mechanical characteristics, in other words the ultimate tensile strength Rm, the conventional yield stress at 0.2% of elongation Rp0.2 and elongation at break A %, are determined by a tensile test according to standard EN ISO 6892-1, sampling and test direction being defined by standard EN 485-1.
The compression yield stress was measured at 0.2% of compression as per standard ASTM E9.
The stress intensity factor (KQ) is given according to standard ASTM F 399. Standard ASTM E 399 gives the criteria which make it possible to determine whether KQ is a valid value of K1C. For a given test specimen geometry, the values of KQ obtained for various materials are comparable with each other insofar as the yield stresses of the material are of the same order of magnitude.
A plot of the stress intensity versus crack extension, known as the R curve, is determined according to ASTM standard E561. The critical stress intensity factor KC, in other words the intensity factor that makes the crack unstable, is calculated starting from the R curve. The stress intensity factor KCO is also calculated by assigning the initial crack length to the critical load, at the beginning of the monotonous load. These two values are calculated for a test piece of the required shape. Kapp denotes the KCO factor corresponding to the test piece that was used to make the R curve test.
Unless otherwise specified, the definitions of standard EN 12258 apply.
“Structural element” of a mechanical construction here refers to a mechanical part for which the static and/or dynamic mechanical properties are particularly important for the performance of the structure, and for which a structural analysis is usually prescribed or performed. These are typically elements the failure of which is likely to endanger the safety of said construction, its users or others. For an aircraft, these structural elements include the parts which make up the fuselage (such as the fuselage skin, stringers, bulkheads, circumferential frames), the wings (such as the upper or lower wing skin, stringers or stiffeners, ribs and spars) and the tail unit, made up of horizontal and vertical stabilizers, as well as floor beams, seat tracks and doors.
According to the present invention, a selected class of aluminum alloys which contain specific and critical quantities of lithium, copper, magnesium, silver and zirconium makes it possible to prepare, in certain transformation conditions, flat-rolled products having an improved compromise between fracture toughness, tensile yield stress and compression yield stress.
The present inventors noted that, surprisingly, it is possible to improve the compression yield stress for these alloys by choosing specific transformation process parameters, in particular during hot working and stress relieving by controlled stretching.
The copper content of the products according to the invention lies between 4.2 and 4.6% by weight. In an advantageous embodiment of the invention, the copper content is at least 4.3% by weight. A maximum copper content of 4.4% by weight is preferred.
The lithium content of the products according to the invention lies between 0.8% or 0.80% and 1.30% by weight and preferably 1.15% by weight. Advantageously, the lithium content is at least 0.85% by weight. A maximum lithium content of 0.95% by weight is preferred.
The increase in the copper content and, to a lesser extent, the lithium content contributes to improving static mechanical resistance; however, as copper has a detrimental effect in particular on density, it is preferable to limit the copper content to the preferred maximum value. The increase in the lithium content has a favorable effect on density; however the present inventors noted that for alloys according to the invention, the preferred lithium content ranging between 0.85% and 0.95% by weight in an embodiment makes for an improved compromise between mechanical resistance (tensile yield stress and compression) and fracture toughness and, in addition, the fracture toughness attained for aging at the peak or close to the peak is higher. In another embodiment wherein compression yield stress and low density are favored for a lower toughness, the preferred lithium content ranges between 1.10% and 1.20% by weight, with preferably a magnesium content ranging between 0.50% or preferably 0.53% and 0.70% or preferably 0.65% by weight.
The magnesium content of the products according to the invention lies between 0.3% or 0.30% and 0.8 or 0.80% by weight. Preferably, the magnesium content is at least 0.40% or even 0.45% by weight, which simultaneously improves static mechanical resistance and fracture toughness. The present inventors noted that the combination of a magnesium content ranging between 0.50% or preferably 0.53% and 0.70% or preferably 0.65% by weight and a lithium content ranging between 0.85% and 1.15% by weight and preferably between 0.85% and 0.95% by weight led to a compromise between mechanical resistance (tensile and compression yield stress) and particularly advantageous fracture toughness, while keeping an acceptable failure rate during the transformation, and thus satisfactory reliability of the manufacturing process.
The zirconium content lies between 0.05 and 0.18% by weight and preferably between 0.08 and 0.14% by weight. In an advantageous embodiment of the invention, the zirconium content is at least 0.11% by weight.
The manganese content lies between 0.0 and 0.5% by weight. In an advantageous embodiment of the invention, the manganese content is between 0.2 and 0.4% by weight. In another embodiment of the invention, the manganese content is lower than 0.1% by weight and preferably lower than 0.05% by weight, which makes it possible, for the products obtained by the process according to the invention, to decrease the quantity of insoluble metal phases and to improve damage tolerance still further.
The silver content lies between 0.05% and 0.5% by weight. In an advantageous embodiment of the invention, the silver content is between 0.10 and 0.40% by weight. The addition of silver helps to improve the compromise of the mechanical properties of the products obtained by the process according to the invention.
The sum of the iron content and the silicon content is at the most 0.20% by weight. Preferably, the iron and silicon contents are each at the most 0.08% by weight. In an advantageous embodiment of the invention the iron and silicon contents are at the most 0.06% and 0.04% by weight respectively. A controlled and limited iron and silicon content helps to improve the compromise between mechanical resistance and damage tolerance.
The alloy also contains at least one element which may contribute to the control of grain size chosen from Cr, Sc, Hf and Ti, the quantity of the element, if it is chosen, being from 0.05 to 0.3% by weight for Cr and Sc, 0.05 to 0.5% by weight for Hf and from 0.01 to 0.15% by weight for Ti. Preferably, it is chosen to add between 0.01 and 0.10% by weight of titanium and to limit the Cr, Sc and Hf content to 0.05% by weight maximum, as these elements can have a detrimental effect, in particular on density and are added only to further help obtain a primarily unrecrystallized structure if necessary.
Zinc is an undesirable impurity, in particular because of its contribution to the density of the alloy. The zinc content is lower than 0.20% by weight, preferably Zn≦0.15% by weight and preferably still Zn≦0.05% by weight. The zinc content is advantageously lower than 0.04% by weight.
It is possible to select the content of the alloy elements so as to minimize density. Preferably, the elements added that contribute to increasing density such as Cu, Zn, Mn and Ag are minimized and the elements that contribute to decreasing the density such as Li and Mg are maximized in order to reach a density of less than 2.73 g/cm3 and preferably less than 2.70 g/cm3.
The manufacturing process for the products according to the invention includes the steps of production, casting, homogenization, rolling at a temperature higher than 400° C., solution hardening, quenching, stretching between 2 and 3.5% and aging.
In the first step, a molten metal bath is produced in order to obtain an aluminum alloy composed according to the invention.
The molten metal bath is then cast in the form of rolling slab.
The rolling slab is then homogenized in order to reach a temperature ranging between 450° C. and 550° and preferably between 480 and 530° C. for a length of time ranging between 5 and 60 hours. The homogenization treatment can be carried out in one or more steps.
After homogenization, the rolling slab is in general cooled down to room temperature before being preheated ready for hot rolling. The aim of the pre-heating is to reach a temperature making it possible to maintain a temperature of at least 400° C. and preferably of at least 420° C. during hot rolling. Intermediate reheating is carried out if during hot rolling the temperature decreases excessively. Hot rolling is carried out down to a thickness ranging preferably between 8 and 50 mm and preferably between 12 and 40 mm.
No significant cold working is performed, in particular by cold rolling, between hot rolling and the solution heat treatment. Such a cold rolling step would be likely to lead to a recrystallized structure which is undesirable within the framework of the invention. Significant cold working is typically a deformation of at least approximately 5% or 10%.
The product so obtained is then solution heat treated by thermal treatment making it possible to reach a temperature ranging between 490 and 530° C. for 15 min to 8 hours, then quenched typically with water at room temperature or preferably with cold water.
The combination of the chosen composition, in particular the zirconium content, and the transformation range, in particular the hot working temperature and the absence of cold working before solution heat treatment, make it possible to obtain a primarily unrecrystallized granular structure. “Primarily unrecrystallized granular structure” is taken to mean an unrecrystallized structure granular content at mid-thickness greater 70% and preferably greater than 85%.
The product then undergoes controlled stretching with a permanent set of 2 to 3.5% and preferably 2.0% to 3.0%. Controlled stretching with a maximum permanent set of approximately 2.5% is preferred. The present inventors noted that, surprisingly, the compression yield stress decreases with the increasing permanent sets during controlled stretching while the yield stress under traction increases in these conditions. There is therefore an optimal permanent set by controlled stretching making it possible to obtain a high compression yield stress while maintaining a sufficient tensile yield stress. Advantageously, the permanent set by controlled stretching is selected so as to obtain a compression yield stress at least equal to the tensile yield stress. The present inventors additionally noted that, surprisingly, the effect of the rate of permanent set on the compression yield stress is specific to flat-rolled products; tests on extruded products showed that such an effect is not observed in this case.
Known steps such as rolling, flattening, straightening or shaping may optionally be performed after solution heat treatment and quenching and before or after controlled stretching. In an embodiment of the invention a cold rolling step of at least 7% and preferably at least 9% and at the most 15% is carried out after solution heat treatment and quenching and before controlled stretching. But especially given the cost of the additional cold rolling step, it is advantageous in another embodiment to realize directly controlled stretching after solution treatment and quenching.
Aging is performed in which the product reaches a temperature ranging between 130 and 170° C. and preferably between 150 and 160° C. for 5 to 100 hours and preferably from 10 to 70 hours. Aging may be performed in one or more steps.
It is known that for age-hardening alloys such as Al—Cu—Li alloys the yield stress increases with the duration of aging at a given temperature up to a maximum value known as the hardening peak or “peak”, then decreases with aging time. Within the framework of this invention, the aging curve is the change in yield stress according to the equivalent duration of aging at 155° C. An example of an aging curve is given in
To obtain an approximate value for PN, for a point N of the curve in an under-aged state, one can determine the slope of the right-hand side passing through point N and through the previous point N−1 obtained for time tN-1<tN and having a yield stress Rp0.2 (N-1); this gives PN≈(Rp0.2(N)−Rp0.2 (N-1))/(tN−tN-1). In theory, the exact value of PN is obtained when tN-1 tends towards tN. However, if the difference tN−tN-1 is low, the variation in yield stress is likely to be insignificant and the value inaccurate. The present inventors noted that a satisfactory approximation to PN is in general obtained when the difference tN−tN-1 lies between 2 and 20 hours and is preferably about 3 hours.
Equivalent time ti at 155° C., is defined by the formula:
where T (in Kelvin) is the instantaneous treatment temperature of the metal, which changes with time t (in hours), and Tref is a reference temperature fixed at 428 K. ti is expressed in hours. The constant Q/R=16,400 K is derived from the enablement energy of the diffusion of Cu for which the value Q=136,100 J/mol was used.
The tensile or compression yield stress can be used to determine whether aging makes it possible to reach a state close to the peak; the results are, however, not necessarily identical. Within the framework of the invention, it is preferred to use the values of compression yield stress to optimize aging.
In general, for alloys of the Al—Cu—Li type, the clearly under-aged states correspond to compromises between the static mechanical resistance (Rp0.2 Rm) and damage tolerance (fracture toughness, resistance to spreading of fatigue cracks) of more interest than at the peak and, a fortiori, beyond the peak. However, the present inventors noted that a state close to the peak both provides a good compromise between static mechanical resistance and damage tolerance and makes it possible to improve performance in terms of corrosion resistance and thermal stability.
In addition, the use of a state close to the peak makes it possible to improve the robustness of the industrial process: a variation in the conditions of aging leads to a slight variation in the properties obtained.
So it is advantageous to carry out a temper essentially under-aged close to the peak of the compression yield, i.e. a temper essentially under-aged with the conditions of time and temperature equivalent to those of a point N of the aging curve under compression at 155° C. such that the tangent to the aging curve at this point has a slope PN, expressed in MPa/h, such that −1<PN≦3 and preferably −0.5<PN≦2.3.
The flat-rolled products obtained by the process according to the invention have, for a thickness ranging between 8 and 50 mm, at mid-thickness at least one of the following combinations of characteristics:
Airplane structural elements according to the invention include products according to the invention. A preferred airplane structural element is an upper wing skin.
The use of a structural element incorporating at least one product according to the invention or manufactured from such a product is advantageous, in particular for aeronautical engineering. The products according to the invention are particularly advantageous for the production of airplane upper wing skins.
These aspects, as well as others of the invention are explained in greater detail using the following illustrative and non-restrictive examples.
In this example, a slab of section 406×1520 mm made of an alloy from the process according to the invention, the composition of which is given in table 1, was cast.
The slab was homogenized at about 500° C. for about 20 hours. The slab was hot rolled at a temperature greater than 445° C. to obtain plates of thickness 25 mm. The plates were solution heat treated at approximately 510° C., for 5 hours and quenched with water at 20° C. The plates were then stretched with a permanent elongation ranging between 2% and 6%.
The plates underwent single-step aging of 40 hours at 155° C. for 2 and 3% stretching, 30 hours for 4% and 20 hours for 6%, this aging making it possible to attain a tensile yield stress and compression at the peak or close to the peak. Samples were taken at mid-thickness to measure the static mechanical characteristics under stretching and compression, together with fracture toughness KQ. The test specimens used for fracture toughness measurement were of width W=40 mm and thickness B=20 mm. The measurements made were valid according to standard ASTM E399. The results are given in Table 2.
The structure of the plates obtained was primarily unrecrystallized. The unrecrystallized granular structure content at mid-thickness was 90%.
In this example, several slabs of section 120×80 mm, the composition of which is given in table 3, were cast.
The slabs were homogenized by a two-step treatment of 8 hours at 500° C. followed by 12 hours at 510° C., then surface-machined. After homogenization, the slabs were hot rolled to obtain plates with a thickness of 9.4 mm, with intermediate reheating if the temperature decreased to less than 400° C. The plates were solution heat treated for 5 hours at approximately 510° C., quenched with cold water and stretched with a permanent elongation of 3%.
The structure of the plates obtained was primarily unrecrystallized. The uncrystallized granular structure content at mid-thickness was 90%.
The plates underwent aging ranging between 15 and 50 hours at 155° C. Samples were taken at mid-thickness to measure the static mechanical characteristics under stretching, under compression, and fracture toughness KQ. The test specimens used for fracture toughness measurement were of width W=25 mm and thickness B=8 mm. The validity criteria of K1C were met for certain samples. Fracture toughness measurements were also obtained on CCT samples of width 300 mm and thickness 6.35 mm. The results obtained are given in table 4.
The combination of the preferred composition (Alloy N° 3) with the process according to the invention gives, in particular for a 50-hour aging at 155° C., the most favorable aging from the point of view of thermal stability, a particularly favorable compromise between compression yield stress, tensile yield stress and fracture toughness.
In this example, a slab of section 406×1525 mm made of an alloy from the process according to the invention, the composition of which is given in table 1, was cast.
The slab was homogenized at about 500° C. for about 30 hours. The slab was hot rolled at a temperature greater than 400° C. to obtain plates of thickness 25 mm. The plates were solution heat treated at approximately 510° C. for 5 hours and quenched with water at 20° C. The plates were then stretched with a permanent elongation of 2% or 3%.
The plates underwent single-step aging of 10 h to 30 h at 155° C. Samples were taken at mid-thickness to measure the static mechanical characteristics under stretching and compression, together with fracture toughness KQ. The test specimens used for fracture toughness measurement were of width W=40 mm and thickness B=20 mm. The measurements made were valid according to standard ASTM E399. The results are given in Table 6.
The structure of the plates obtained was primarily unrecrystallized. The unrecrystallized granular structure content at mid-thickness was higher than 90%.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1004962 | Dec 2010 | FR | national |
This application claims priority to FR1004962 filed Dec. 20, 2010 and to U.S. Provisional application No. 61/424,970, filed Dec. 20, 2010, the content of which is incorporated herein by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 61424970 | Dec 2010 | US |
