Patent Application
20080145266
This invention pertains to aluminum aerospace alloys. More particularly, this invention pertains to aluminum alloys of the AA6xxx-series (or AA6000-series) that are suitable for welding, yet have improved performance properties, particularly corrosion resistance and damage tolerance properties.
As will be appreciated herein below, except as otherwise indicated, alloy designations and temper designations refer to the Aluminum Association designations in Aluminum Standards and Data and the Registration Records, as published by the Aluminum Association.
For any description of alloy composition or preferred alloy compositions, all references to percentages are by weight percent unless otherwise indicated.
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. Aluminum alloys 6061 and 6063 are well known heat treatable aluminum alloys. These alloys have useful strength and toughness properties in both T4 and T6 tempers. As is known, the T4 condition refers to a solution heat treated and quenched condition naturally aged to a substantially stable property level, whereas T6 tempers refer to a stronger condition produced by artificially ageing. These known alloys lack, however, sufficient strength for most structural aerospace applications. Several other Aluminum Association (“AA”) 6000 series alloys are generally unsuitable for the design of commercial aircraft which require different sets of properties for different types of structures. Depending on the design criteria for a particular aircraft component, improvements in strength, fracture toughness and fatigue resistance result in weight savings, which translate to fuel economy over the lifetime of the aircraft, and/or a greater level of safety. To meet these demands several 6000 series alloys have been developed.
U.S. Pat. No. 4,589,932 (Alcoa) discloses an aluminum alloy for automobile, rail, naval or aeronautical construction, with the following composition, in weight percent:
This aluminum alloy was subsequently registered in March 1983 by the Aluminum Association under the designation AA6013. The registered compositional ranges for AA6013 are, in weight percent:
The AA6013 alloy has attractive mechanical properties for use amongst others as a fuselage skin and furthermore this alloy is also weldable. However, there are at least two factors limiting the application of this AA6013 alloy. The first one is that the AA6013 alloy is susceptible to intergranular corrosion (IGC) attack, which can increase local stress concentrations when an alloy product is subjected to stress conditions such as repeated pressurization and depressurization of an aircraft fuselage in use, see for example the paper of T. D. Burleigh, “Microscopic Investigation of the Intergranular Corrosion of Alloy 6013-T6”, ICAA3, Trondheim, 1992, p. 435. And the second drawback is that the AA6013 alloy has significant lower damage tolerant properties compared to its AA2x24 counterpart.
Another AA6xxx-series alloy suitable for aerospace application is the AA6056-series alloy. The registered compositional ranges for AA6056 are, in weight percent:
However, it has been reported that also this AA6056 alloy is susceptible to intergranular corrosion. The resistance to intergranular corrosion of the AA6056 has been improved by overageing, (i.e. artificially ageing by a practice that causes the metal to go past peak strength to a lower strength condition). In order to obtain the improved corrosion resistance it is also essential for the disclosed overageing process that in the aluminum alloy the Mg/Si ratio is less than 1. This specific overaging practice has been disclosed in U.S. Pat. No. 5,858,134, but has amongst others the drawback of a significant decrease in strength compared to peak aged tempers.
Another method of controlling the resistance to intergranular corrosion of the AA6056 alloy is by providing it with a dilute AA7072 cladding having 0.25-0.7 wt. % Zn as disclosed in EP-1170118.
Yet another method to improve the properties of the AA6056 alloy for its application as aircraft structural component is disclosed in US-2002/0014290-A1. This document discloses an ageing practice to improve the static mechanical characteristics and the tolerance to damage.
In order to take full advantage of the potential cost savings offered by fuselage skin panel welding as a low cost alternative to fastening them with rivets, therefore, it would be desirable to develop a weldable aluminum alloy suitable for aerospace application having sufficient strength combined with improved damage tolerance properties and improved resistance to intergranular corrosion.
A principal objective of the present invention is to provide an improved AA6xxx series alloy that is weldable, yet exhibits improved corrosion resistance properties.
An object of the present invention is to provide a weldable AA6xxx-type series alloy product having improved resistance to intergranular corrosion compared to its AA6013 counterpart.
A further object is to provide a weldable AA6xxx-type series alloy product having improved damage tolerance properties compared to its AA6013 counterpart.
Another object is to provide a weldable AA6xxx-type series alloy product having an improved balance of intergranular corrosion resistance and damage tolerance properties compared to its AA6013 counterpart.
These and other objects and further advantages are met or exceeded by the present invention concerning an aluminum alloy wrought product consisting essentially of, in weight percent:
In the peak aged condition (i.e. T6 type condition), the aluminum alloy of this invention offers greater resistance to intergranular corrosion resistance compared to its AA6013 aluminum alloy counterpart. Furthermore, in the peak aged condition, the aluminum alloy of this invention offers an improved ratio of UPE (Unit Propagation Energy) versus tensile strength.
By an AA6013 counterpart it is meant an aluminum alloy wrought product having a composition as defined above for AA6013 and processed and heat treated and having the same dimensions of length, width and thickness as the wrought product of the present invention to which it is compared.
The present invention provides a weldable aluminum alloy wrought product having high strength and improved resistance to intergranular corrosion, the alloy consisting essentially of, in weight percent:
In referring to an element, “up to” includes zero except that, when an element is stated to be present, such excludes zero since the element is stated to be present.
In the alloy product according to the invention it is preferred to control the Mg and Si contents such that Mg+1.1Si<2.0, and more preferably Mg+1.1Si<1.85. This is to facilitate achieving the required strength levels by ensuring that second phase particles of Mg2Si can be fully dissolved during the homogenization and/or preheat prior to the hot working operation. It has been found that such a compositional control can increase both the Unit Propagation Energy (“UPE”) and the ratio of TS/Rp (“Tear Strength/Yield Strength”) significantly.
Apart from the Si, Mg, Cu and Mn in the defined ranges, and preferred narrower ranges, an important alloying element in the alloy according to this invention is titanium. The addition of Ti at levels of more than 0.06% to the alloy according to this invention has the effect of increasing the corrosion resistance, and the resistance against intergranular corrosion in particular. Ti levels at significant lower levels (e.g. at about 0.03% or less) can be found also in aluminum alloys, but at such low levels it is commonly purposively added to obtain a grain refining effect during industrial scale casting of ingots or stock for rolling, extrusion or forging. No effect on the corrosion resistance could be found at such low levels.
A similar effect on the corrosion resistance has been found for the purposive addition of Cr up to about 0.25%.
However, an even significant further improvement in the corrosion resistance, and intergranular corrosion in particular, has been found according to this invention in the case of the combined addition of Ti with Cr, and optionally with a further addition of Zr. To achieve the best improvements the Ti content is in the range of about 0.06 to 0.19%, and preferably about 0.09 to 0.19%. Preferably the Cr content should be in the range of up to about 0.25%, and preferably about 0.05 to 0.25%, more preferably about 0.08 to 0.19%. The combined addition of Ti plus Cr should be in the range of about 0.12 to 0.3%, and preferably about 0.15 to 0.28%. The combined addition of Ti and Cr has also a very favorable effect of the strength levels and the Unit Propagation Energy (“UPE”) making the alloy product a very attractive candidate for aerospace applications.
The ranges for the Ti and Cr contents are very critical. For example it has been found that the addition of more than 0.2% Ti may result in the formation of large primary phases which significantly reduce amongst others the tear strength (“TS”) and the UPE.
Zr may be added to the aluminum alloy according to this invention up to 0.2%. If purposively added to the alloy it is preferably in the range of about 0.06 to 0.18%. Adding Zr to the alloy has the effect of maintaining favorable UPE levels while offering an increased yield strength. The intergranular corrosion resistance is slightly decreased compared to the alloy variant with solely the combined addition of Ti plus Cr. However, the overall balance of strength, damage tolerance and corrosion resistance is still favorable compared to its AA6013 counterpart.
In another embodiment the Zr content is less than 0.05%, and more preferably the aluminum alloy is substantially free from Zr to obtain a fully recrystallised microstructure.
In an embodiment of the aluminum alloy according to this invention there is no purposive addition of Zn, but it may be tolerated as an impurity. In this embodiment the Zn content is in a range of less than about 0.25%, preferably less than about 0.05%, and more preferably less than about 0.02%.
In another embodiment of the aluminum alloy according to this invention there is a purposive addition of Zn to further improve the strength, wherein the Zn is preferably present in a range of about 0.5 to 0.9%, and preferably in a range of about 0.6 to 0.85%. A too high Zn content may have an adverse effect on the intergranular corrosion performance.
In a preferred embodiment the aluminum alloy according to the invention is substantially free from each of V, Sr, and Be.
For this invention, with “substantially free” and “essentially free” we mean that no purposeful addition of this alloying element was made to the composition, but that due to impurities and/or leaching from contact with manufacturing equipment, trace quantities of this element may, nevertheless, find their way into the final alloy product.
The best results are achieved when the alloy rolled products have a recrystallised microstructure, meaning that 80% or more, and preferably 90% or more of the grains in a T4 or artificially aged condition are recrystallised.
Increased intergranular corrosion resistance is particularly useful for applications that expose the metal to corrosive environments, such as the lower portion of an aircraft fuselage. Moisture and corrosive chemical species tend to accumulate in these areas of an aircraft as solutions drain to the bottom of the fuselage compartment. In a preferred embodiment the alloy product according to this invention has in a T6 temper an intergranular corrosion depth of attack of less than 100 micron when measured according to the MIL-H-6088 test, and preferably less than 90 micron, and in the best examples less than 50 micron.
The aluminum alloy wrought product according to this invention is preferably provided as a rolled product such as a sheet or plate. However, the advantages in improved corrosion resistance and damage tolerance properties can be obtained also when the wrought product is in the form of an extruded product, and less preferentially in the form of forged product shapes using regular product manufacturing processes.
It is important to note that the alloy composition of this invention works well at resisting intergranular corrosion in both its clad and unclad varieties. For some clad versions, the alloy layer applied overtop the invention is a AA7xxx-series alloy cladding, more preferably an AA7072 series alloy or the AlZn-cladding as disclosed in EP-1170118 (incorporated herein by reference), or the more commonly known cladding of the AA1xxx-series, such as the AA1145 aluminum.
In another embodiment the alloy product according to the invention is being provided with a cladding thereon on one side of the AA1000-series and on the other side thereon of the AA4000-series. In this embodiment corrosion protection and welding capability are being combined. In this embodiment the product may be used successfully for example for pre-curved panels. In case the rolling practice of an asymmetric sandwich product (1000-series alloy+core+4000-series alloy) causes some problems such as banaring, there is also the possibility of first rolling a symmetrical sandwich product having the following subsequent layers 1000-series alloy+4000-series alloy+core alloy according to this invention+4000-series alloy+1000-series alloy, where after one or more of the outer layer(s) are being removed, for example by means of chemical milling.
Aerospace applications of this invention may combine numerous alloy product forms, including, but not limited to, TIG welding, laser and/or mechanically welding (i.e. friction stir welding): sheet to a sheet or plate base product; plate to a sheet or plate base product; or one or more extrusions to such sheet or plate base products. One particular embodiment envisions replacing the manufacture of today's airplane fuselage parts from large sections of material from which significant portions are machined away. Using the alloy composition set forth above, panels can be machined or chemically milled to remove metal and reduce thickness at selective strip areas to leave upstanding ribs between the machined or chemically milled areas. These upstanding ribs provide good sites for welding stringers thereto for reinforcement purposes. Such stringers can be made of the same or similar composition, or of another AA6xxx-series alloy composition, so long as the combined components still exhibit good resistance to intergranular corrosion attack.
In a further aspect of the invention there is provided a method of manufacturing the alloy product according to this invention, the method comprising the steps of:
The aluminum alloy as described herein can be provided in process step (a) as an ingot or slab for fabrication into a suitable wrought product by casting techniques currently employed in the art for cast products, e.g. DC-casting, EMC-casting, EMS-casting. Slabs resulting from continuous casting, e.g. belt casters or roll caster, may be used also.
Typically, prior to hot rolling the rolling faces of both the clad and the non-clad products are scalped in order to remove segregation zones near the cast surface of the ingot.
The cast ingot or slab may be homogenised prior to hot working, preferably by means of rolling and/or it may be preheated followed directly by hot working. The homogenisation and/or preheating of the alloy prior to hot working should be carried out at a temperature in the range of 490 to 580° C. in single or in multiple steps. In either case, the segregation of alloying elements in the material as-cast is reduced and soluble elements are dissolved. If the treatment is carried out below 490° C., the resultant homogenisation effect is inadequate. If the temperature is above 580° C., eutectic melting might occur resulting in undesirable pore formation. The preferred time of the above heat treatment is between 2 and 30 hours. Longer times are not normally detrimental. Homogenisation is usually performed at a temperature above 540° C. A typical preheat temperature is in the range of 540 to 570° C. with a soaking time in a range of 4 to 16 hours.
After the alloy product is cold worked, preferably after being cold rolled, or if the product is not cold worked then after hot working, the alloy product is solution heat treated at a temperature in the range of 480 to 590° C., preferably 530 to 570° C., for a time sufficient for solution effects to approach equilibrium, with typical soaking times in the rang of 10 sec. to 120 minutes. With clad products, care should be taken against too long soaking times to prevent diffusion of alloying element from the core into the cladding detrimentally affecting the corrosion protection afforded by the cladding.
After solution heat treatment, it is important that the alloy product be cooled to a temperature of 175° C. or lower, preferably to room temperature, to prevent or minimise the uncontrolled precipitation of secondary phases, e.g. Mg2Si. On the other hand cooling rates should not be too high in order to allow for a sufficient flatness and low level of residual stresses in the alloy product. Suitable cooling rates can be achieved with the use of water, e.g. water immersion or water jets.
While the invention is particularly suited to fuselage skins, it also may find other applications such as automotive sheet, railroad car sheet, and other uses.
The invention will now be illustrated with reference to non-limiting embodiments according to the invention.
Five different alloys have been DC-cast into ingots, then subsequently scalped, pre-heated for about 6 hours at 560° C. (heating-up speed about 30° C./h), hot rolled to a gauge of 8 mm whereby the hot-mill entry temperature was about 480° C., cold rolled to a final gauge of 2 mm, solution heat treated for 10 min. at 560° C., water quenched, 2% stretch, aged to a T6-temper by holding for 4 hours at 190° C. and followed by air cooling to room temperature. Table 1 gives the chemical composition of the five alloys cast. The alloy composition of Alloy No. A is a regular 6013 alloy for reference purposes. Alloy No. B is the 6013 alloy with increased Cr content, and is also a reference alloy. Alloys Nos. C to E are according to this invention.
The tensile testing has been carried out on the bare sheet material in the T6-temper and having a fully recystallised microstructure. For the tensile testing in the L-T direction small euro-norm specimens were used, average results of 3 specimens are given, and “Rp” stands for yield strength, “Rm” for ultimate tensile strength, and El for elongation (A50). The results of the tensile tests have been listed in Table 2. The UPE versus the yield strength is also schematically shown in
From the results of Table 2 and
From the results of Table 3 and
The combined addition of Ti, Cr and Zr results still in improved IGC performance compared to the 6013 counterpart, while balancing this with a small further increase in strength. As can be seen from
In a further experiment the effect of homogenisation and pre-heat temperature prior to hot working has been investigated on the AA6013 alloy of Table 1 above. After casting the ingots were scalped, homogenised for about 6 hours at different temperatures, hot rolled to a gauge of 8 mm whereby the hot-mill entry temperature was about 480° C., cold rolled to a final gauge of 2 mm, solution heat treated for 15 min. at 565° C., water quenched, 2% stretch, aged to a T6-temper by holding for 4 hours at 190° C. and followed by air cooling to room temperature. The results on the mechanical properties are listed in Table 4.
From the results of Table 4 it can be seen that the UPE and Tear Strength systematically increase with increasing homogenization temperature whereas the yield strength does not change. Although illustrated on the AA6013 alloy, the same trend can be found on the alloy product according to this invention. Furthermore, contrary to prior research (see for example the paper by V. G. Davydov et al., “Influence of SSTT, Ageing Regime and Stretching on IGC, Complex of Properties and Precipitation Behavior of 6013 alloy.”, Materials Science Forum Vols. 331-337, (2000), pp. 1315-1320) increasing the preheat or homogenization temperature does not have an adverse effect on the IGC resistance when producing the alloy product according to this invention.
Having described the presently preferred embodiments, it is to be understood that the invention may be otherwise embodied within the scope of the appended claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 06012379 | Jun 2006 | EP | regional |
This claims the benefit of United States provisional patent application No. 60/814,068 filed Jun. 16, 2006, and claims priority of European patent application number 06012379 filed Jun. 16, 2006, both incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 60814068 | Jun 2006 | US |
