Patent Grant
7883591
The present invention relates to a high-strength high-toughness Al—Zn alloy wrought product with elevated amounts of Zn for maintaining good corrosion resistance, and to a method for producing such a high-strength high-toughness Al—Zn alloy product and to a plate product of such alloy. More specifically, the present invention relates to a high strength, high toughness Al—Zn alloy designated by the AA7000-series of the international nomenclature of the Aluminum Association for structural aeronautical applications. Even more specifically, the present invention relates to a new chemistry window for an Al—Zn alloy having improved combinations of strength and toughness by maintaining good corrosion resistance, which does not need specific ageing or temper treatments.
It is known in the art to use heat treatable aluminium alloys in a number of applications involving relatively high strength, high toughness and corrosion resistance such as aircraft fuselages, vehicular members and other applications. Aluminium alloys AA7050 and AA7150 exhibit high strength in T6-type tempers. Also precipitation-hardened AA7×75, AA7×55 alloy products exhibit high strength values in the T6 temper. The T6 temper is known to enhance the strength of the alloy, wherein the aforementioned AA7×50, AA7×75 and AA7×55 alloy products which contain high amounts of zinc, copper and magnesium are known for their high strength-to-weight ratios and, therefore, find application in particular in the aerospace industry. However, these applications result in exposure to a wide variety of climatic conditions necessitating careful control of working and ageing conditions to provide adequate strength and resistance to corrosion, including both stress corrosion and exfoliation.
In order to enhance resistance against stress corrosion and exfoliation as well as fracture toughness it is known to artificially over-age these AA7000-series alloys. When artificially aged to a T79, T76, T74 or T73-type temper their resistance to stress corrosion, exfoliation corrosion and fracture toughness improve in the order stated (T73 being best and T79 being close to T6) but at the cost of strength compared to the T6 temper condition. A more acceptable temper condition is the T74-type temper which is a limited over-aged condition, between T73 and T76, in order to obtain an acceptable level of tensile strength, stress corrosion resistance, exfoliation corrosion resistance and fracture toughness. Such T74 temper is performed by over-ageing the aluminium alloy product at temperatures of 121° C. for 6 to 24 hours and followed by 171° C. for about 14 hours.
Depending on the design criteria for a particular aircraft component even small improvements in strength, toughness or corrosion resistance result in weight savings, which translate amongst others to fuel economy over the life time of the aircraft. To meet these demands several other 7000-series alloys have been developed.
For example each of EP-0377779, U.S. Pat. No. 5,221,377 and U.S. Pat. No. 5,496,426 disclose alloy products and an improved process for producing an 7055 alloy for sheet or thin plate applications in the field of aerospace such as upper-wing members with high toughness and good corrosion properties which comprises the steps of working a body having a composition consisting of, about in wt.%: Zn 7.6 to 8.4, Cu 2.2 to 2.6, Mg 1.8 to 2.1 or 2.2, and one or more elements selected from Zr, Mn V and Hf, the total of the elements not exceeding 0.6 wt. %, the balance aluminium plus incidental impurities, solution heat treating and quenching the product and artificially ageing the product by either heating the product three times in a row to one or more temperatures from 79° C. to 163° C. or heating such product first to one or more temperatures from 79° C. to 141° C. for two hours or more and heating the product to one or more temperatures from 148° C. to 174° C. These products are reported to have an improved exfoliation corrosion resistance of “EB” or better with about 15% greater yield strength than similar sized 7×50 counter-parts in the T76-temper condition. They still have at least about 5% higher strength than their similar-sized 7×50-T77 counterpart (7150-T77 will be used herein below as a reference alloy).
It is a preferred object of the present invention to provide an improved Al—Zn alloy preferably for plate products with high (compressive) strength and high toughness. Corrosion resistance should not deteriorate.
More specifically, it is an object of the present invention to provide an alloy product which can be used for upper wing applications in aerospace with an improved compression yield strength and a high unit propagation energy with properties which are better than the properties of a conventional AA7055-alloy in the T77 temper.
It is another object of the invention to obtain an AA7000-series aluminium alloy which exhibits strength in the range of T6-type tempers and toughness and corrosion resistance properties in the range of T73-type tempers.
It is another object of the invention to provide a method of manufacturing the aluminium alloy product according to this invention.
The present invention relates to a Al—Zn alloy wrought product, and a to method of manufacturing such a product, with an improved combination of high toughness and high strength by maintaining good corrosion resistance, the alloy including (in weight percent): Zn 6.0-11.0, Cu 1.4-2.2, Mg 1.4-2.4, Zr 0.05-0.15, T <0.05, Hf and/or V <0.25, and optionally Sc and/or Ce 0.05-0.25, and Mn 0.05-0.12, other elements each less than 0.05 and less than 0.50 in total, balance aluminium, wherein such alloy has an essentially fully unrecrystallized microstructure at least at the position T/10 of the finished product.
As will be appreciated hereinbelow, except as otherwise indicated, alloy designations and temper designations refer to the Aluminum Association designations in Aluminum Standards and Data and the Registration Records, all published by the US Aluminum Association.
One or more of the above mentioned objects of the invention are achieved by using an Al—Zn alloy product with an improved combination of high toughness and high strength by maintaining good corrosion resistance, said alloy comprising, and preferably consisting of, (in weight percent):
Such chemistry window for an AA7000-series alloy exhibits excellent properties when produced to relatively thin plate products, and which is preferably useable in aerospace upper-wing applications having gauges in the range of 20 mm to 60 mm.
The above defined chemistry has properties which are comparable or better than existing alloys of the AA7×50 or AA7×55 series in the T77-temper, without using the above described cumbersome and complicated T77 three-step ageing cycles. The chemistry leads to an aluminium product which is more cost effective and is also simpler to produce since less processing steps are necessary. Additionally, the chemistry allows new manufacturing techniques like age forming or age creep forming which is not feasible when a T77-temper alloy is applied. Even better, the chemistry as defined above can also be aged to the T77-temper whereby the corrosion resistance further improves.
According to the invention it has been found that a selected range of elements, using a higher amount of Zn and a specific combination of a particular range of Mg and Cu, exhibit substantially better combinations of strength and toughness and maintaining a good corrosion performance such as exfoliation corrosion resistance and stress corrosion cracking resistance.
The present invention uses the chemistry also in combination with a method to produce a rolled product from such chemistry, as explained herein below, to obtain a substantially fully unrecrystallized microstructure at least at the position T/10 of the finished product. More preferably the product is unrecystallized across the whole thickness. With unrecystallized we mean that more than 80%, preferably more than 90% of the gauge of the finished rolled product is substantially unrecrystallized. Hence, the present invention is disclosing an alloy product which is in particular suitable for upper wing skin applications for aircrafts and having a thickness in the range of 20 to 60 mm, preferably 30 to 50 mm.
It has been found that it is not necessary to slowly quench the rolled product or to increase the gauge of the rolled product to obtain superior compression yield strength and toughness properties.
Copper and magnesium are important elements for adding strength to the alloy. Too low amounts of magnesium and copper result in a decrease of strength while too high amounts of magnesium and copper result in a lower corrosion performance and problems with the weldability of the alloy product. Prior art techniques used special ageing procedures to ameliorate the strength while low amounts of magnesium and copper are used in order to achieve a good corrosion performance. In order to achieve a compromise in strength, toughness and corrosion performance copper and magnesium amounts (in wt. %) of between 1.7 and 2.2%, preferably between 1.7 and 2.1% for Mg and 1.8 and 2.1% for Cu have been found to give a good balance for thin plate products. Throughout the claimed chemistry of the present invention it is now possible to achieve strength levels in the region of a T6-temper alloy while maintaining corrosion performance characteristics similar to those of T74-temper alloys.
Apart from the amounts of magnesium and copper the invention discloses a balance of magnesium and copper amounts to zinc, especially the balance of magnesium to zinc, which gives the alloy these performance characteristics. The improved corrosion resistance of the alloy according to the invention has exfoliation properties (“EXCO”) of EB or better, preferably EA or better.
The amount (in weight %) of zinc is preferably in a range of 7.4 to 9.6%, more preferably in a range of 8.0 to 9.6%, most preferably in a range of 8.4 to 8.9%. Testing has found an optimum zinc level of about 8.6%. Further details are given in the examples as described in more details hereinbelow.
It has furthermore been shown that, according to a preferred embodiment of the present invention, a Sc-containing alloy is an excellent candidate for obtaining high strength versus high notch toughness levels. By adding Sc to an alloy comprising copper, magnesium, zinc, zirconium and titanium it has been found that the microstructure remains unrecrystallized, thereby showing superior properties with regard to strength and toughness. Hence, preferred amounts of Sc (in weight %) are in a range of [Zr]+1.5 [Sc]<0.15%. Preferred amounts (in weight %) of Sc or Ce are in a range of 0.03 to 0.06% when the amount of Zn is about 8.70% and Mg and Cu are about 2.10%. The levels of the unit propagation energy are considerably good for an alloy with additional Sc, Ce or Mn alloying elements.
A preferred method for producing a high strength, high toughness Al—Zn alloy product with good corrosion resistance according to the present invention comprises the steps of
a. casting an ingot with the following composition (in weight percent):
It has been found that the microstructure of the alloy product remains substantially fully unrecrystallized underneath its surface when the inventive method step of pre-working the product and hot rolling and/or or cold rolling the pre-worked product are applied.
In accordance with an embodiment of the present invention the method includes a first hot rolling of the ingot which has been homogenised into a pre-worked product, hot rolling the re-heated product to about 150 to 250 (in final-gauge %) and then cold rolling the hot rolled product to the final gauge or hot rolling the re-heated product to about 105 to 140 (in final-gauge %) and then cold rolling the hot rolled product to the final gauge. “Final-gauge %” means a percentage in thickness compared to the thickness of the final product. 200 final-gauge % means a thickness which is twice as much as the thickness of the finally worked product. That means that it has been found that it is advantageous to first hot roll the pre-heated product to a thickness which is about twice as high as the thickness of the final product and then cold rolling the hot rolled product to the final thickness or to hot roll the pre-heated product to a thickness which is about 20% higher than the thickness of the final product and then cold rolling the product, thereby obtaining another about 20% reduction of the gauge of the hot rolled product.
According to another embodiment of the present invention it is advantageous to hot roll the re-heated product at low temperatures in the range of 300° C. to 420° C. so that the alloy does not recrystallise. Optionally, it is possible to artificially ageing the worked and heat-treated product with a two-step T79 or T76 temper or to use a T77-three step temper if SCC performance shall be improved.
The present invention is useful for hot-working the ingot after casting and optionally cold-working into a worked product with a gauge in the range of 20 to 60 mm.
The present invention also concerns a plate product of high strength, high toughness Al—Zn alloy of the aforementioned composition which plate product is preferably a thin aircraft member, even more preferably an elongate structural shape member such as an upper-wing member, a thin skin member of an upper-wing or of a stringer of an aircraft.
The properties of the claimed alloy may further be enhanced by an artificial ageing step comprising a first heat treatment at a temperature in a range of 105° C. to 135° C., preferably around 120° C. for 2 to 20 hours, preferably around 8 hours and a second heat treatment at a higher temperature then 135° C. but below 210° C., preferably around 155° C. for 4 to 12 hours, preferably 8 to 10 hours.
The foregoing and other features and advantages of the alloys according to the present invention will become readily apparent from the following detailed description of preferred embodiments.
On a laboratory scale 14 different aluminium alloys have been cast into ingots, homogenised, pre-heated for more than 6 hours at about 410° C. and hot rolled to 4 mm plates. Solution heat treatment was done at 475° C. and thereafter water quenched. Thereafter, the quenched product was aged by a two-step T76 ageing procedure. The chemical compositions are set out in Table 1.
The alloys of Table 1 were processed using three processing variants (see step 5):
Strength was measured using small Euronorm and toughness were measured in accordance with ASTM B-871(1996). The results of the three above-mentioned variants are shown in Table 2a to 2c.
From the results presented in Tables 2a to 2c it is clear that a minor degree (10 to 20%) of cold rolling is beneficial for an optimum toughness versus strength balance. The purely hot rolled material in accordance with Variant 1 (Table 2a) is close to the optimum but in general the Variant 3-alloys are better.
Furthermore, it can be seen that Sc-containing alloy 14 is advantageous if high strength versus high notch toughness is needed. Small amounts of manganese do increase the strength but at the cost of some toughness.
Additional chemistries have been processed in accordance with the above-mentioned processing steps 1 to 8, thereby using the variant 3 of step 5 of example 1 above and a T76 ageing.
The properties of the alloys mentioned in Table 3 have been tested in the L-direction for the strength and in the L-T-direction for the toughness.
The toughness versus tensile yield strength (Rp) shown in Table 4 clearly shows that the best toughness versus tensile yield strength value is obtained for alloys having around 8.6 to 8.7 weight % zinc. Alloys with lower levels of zinc will show similar toughness values but the tensile strength is—generally speaking—lower whereas high levels of zinc result in higher strength levels but lower toughness levels. Small amounts of manganese do increase the strength at the cost of toughness.
Further tests were done with zinc levels of 8.6 and 8.7 wt. % thereby varying copper and magnesium levels. It can be shown that toughness levels can be elevated at the same strength levels. Some additional alloys were processed similar as to the ones in Example 2, thereby using the processing steps 1 to 8 as described above and Variant 3 of step 5 of Example 1.
As shown in Table 6 it is advantageous to have magnesium levels of less than 2.4% with an optimum of about 1.7%. When magnesium levels are at about 1.7%, excellent toughness properties are obtained but the strength levels decrease. With magnesium levels of about 2.1% the best strength levels are obtained. Hence, magnesium is best in between 1.7 and 2.1%.
All above mentioned alloys have been tested on exfoliation corrosion according to ASTM G-34. They all showed a performance of EB or better.
Furthermore, it has been shown that the addition of Ce or Sc enhances the microstructure of the alloy thereby reducing recovery processes. Since the recovery within the allow material is low, nearly no recrystallization takes place even though a solution heat treatment is used in accordance with the standard route. Sc represses recrystallization so that usually more than 90% of the thickness of the thin plate products remains unrecrystallized.
According to another embodiment of the present invention it is advantageous to hot roll the re-heated product at low temperatures in the range of 300° C. to 420° C. so that the alloy does not recrystallise. Optionally, it is possible to artificially age the worked and heat-treated product with a two-step T79 or T76 temper or to use a T77-three step temper if SCC performance shall be improved.
| Number | Date | Country | Kind |
|---|---|---|---|
| 04077721 | Oct 2004 | EP | regional |
This claims priority from U.S. provisional patent application Ser. No. 60/616,227 filed Oct. 7, 2004 and European patent application no. 04077721.1 filed Oct. 5, 2004, both incorporated herein by reference in their entirety.
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