WO2006037648A1 - HIGH STRENGTH, HIGH TOUGHNESS Al-Zn ALLOY PRODUCT AND METHOD FOR PRODUCING SUCH PRODUCT - Google Patents

HIGH STRENGTH, HIGH TOUGHNESS Al-Zn ALLOY PRODUCT AND METHOD FOR PRODUCING SUCH PRODUCT Download PDF

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Publication number
WO2006037648A1
WO2006037648A1 PCT/EP2005/010809 EP2005010809W WO2006037648A1 WO 2006037648 A1 WO2006037648 A1 WO 2006037648A1 EP 2005010809 W EP2005010809 W EP 2005010809W WO 2006037648 A1 WO2006037648 A1 WO 2006037648A1
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product
range
alloy
temper
ageing
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PCT/EP2005/010809
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French (fr)
Inventor
Rinze Benedictus
Christian Joachim Keidel
Alfred Ludwig Heinz
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Aleris Aluminum Koblenz Gmbh
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Application filed by Aleris Aluminum Koblenz Gmbh filed Critical Aleris Aluminum Koblenz Gmbh
Priority to CA2592132A priority Critical patent/CA2592132C/en
Priority to JP2007533966A priority patent/JP5068654B2/en
Priority to EP05802352.4A priority patent/EP1831415B2/en
Priority to DE602005013429T priority patent/DE602005013429D1/en
Priority to CN2005800339556A priority patent/CN101068943B/en
Priority to BRPI0517538-0A priority patent/BRPI0517538B1/en
Publication of WO2006037648A1 publication Critical patent/WO2006037648A1/en

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    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C21/00Alloys based on aluminium
    • C22C21/10Alloys based on aluminium with zinc as the next major constituent
    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22FCHANGING THE PHYSICAL STRUCTURE OF NON-FERROUS METALS AND NON-FERROUS ALLOYS
    • C22F1/00Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working
    • C22F1/04Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working of aluminium or alloys based thereon
    • C22F1/053Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working of aluminium or alloys based thereon of alloys with zinc as the next major constituent

Definitions

  • 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.
  • Aluminium alloys AA7050 and AA7150 exhibit high strength in T6-type tempers. Also precipitation-hardened AA7x75, AA7x55 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 AA7x50, AA7x75 and AA7x55 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.
  • 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.
  • T74 temper 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.
  • T74 temper is performed by over-ageing the aluminium alloy product at temperatures of 121 0 C for 6 to 24 hours and followed by 171 0 C for about 14 hours.
  • each of EP-0377779, US-5,221 ,377 and US-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 0 C to 141 0 C for two hours or more and heating the product to one or more temperatures from 148 0 C to 174°C.
  • the present invention meets one or more of these objects by the characterizing features of the independent claims. Further preferred embodiments are described and specified within the dependent claims.
  • 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.
  • alloy product has a substantially fully unrecrystallized microstructure at the position T/10 of the finished product.
  • 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 AA7x50 or AA7x55 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.
  • 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 sunstantially unrecrystallized.
  • 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 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.
  • the improved corrosion resistance of the alloy according to the invention has exfoliation properties ("EXCO") of EB or better, preferably EA or better.
  • EXCO exfoliation properties
  • 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%.
  • a Sc-containing alloy is an excellent candidate for obtaining high strength versus high notch toughness levels.
  • Sc is 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%.
  • a preferred method for producing a high strength, high toughness Al-Zn alloy product with good corrosion resistance comprises the steps of a. casting an ingot with the following composition (in weight percent):
  • Hf and/or V ⁇ 0.25, optionally Sc and/or Ce 0.05 to 0.25, and optionally Mn 0.05 to 0.12, and inevitable impurities and balance aluminium, preferably other elements each less than 0.05 and less than 0.50 in total, b. homogenising and/or pre-heating the ingot after casting, c. hot working the ingot into a pre-worked product, d. reheating the pre-worked product, and either d1. hot rolling the reheated product to the final gauge, or d2 hot rolling and cold rolling the reheated product to the final gauge, e. solution heat treating and quenching the solution heat treated product, f.
  • 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.
  • the present invention it is advantageous to hot roll the re-heated product at low temperatures in the range of 300 0 C to 420 0 C so that the alloy does not recrystallise.
  • 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 0 C to 135 0 C, preferably around 120 0 C for 2 to 20 hours, preferably around 8 hours and a second heat treatment at a higher temperature then 135 0 C but below 210 0 C, preferably around 155 0 C for 4 to 12 hours, preferably 8 to 10 hours.
  • the alloys of Table 1 were processed using three processing variants (see step 5):
  • Homogenisation was performed by heating at a temperature rate of 40°C/h to a temperature of 460 0 C, then soaking for 12 hours at 460°C and another increase with 25°C/h to a temperature of 475 0 C with another soaking for 24 hours at 475 0 C, and air cooling to room temperature.
  • the lab scale ingots were hot rolled from 80 to 25 mm, thereby reducing the gauge by about 6 to 8 mm per pass.
  • Variant 2 The reheated product was hot rolled to 8.0 mm and thereafter cold rolled to 4.0 mm.
  • Variant 3 The reheated product was hot rolled to 5.0 mm and then cold rolled to 4.0 mm. 6. Solution heat treatment was done for 1 hour at 475°C, thereafter water quenched.
  • Stretching was done by 1.5 to 2.0% within about 1 hour after quenching.
  • the stretched products were aged in accordance with a T76 ageing procedure, thereby raising the temperature to 12O 0 C at a rate of 30°C/h and maintaining the temperature at 12O 0 C for 5 hours, raising the temperature at a rate of 15°C/h to a temperature of 160 0 C and soaking for 6 hours, and air cooling the aged product to room temperature.
  • Table 2a Strength and toughness properties of the alloys as shown in Table 1 in MPa and notch toughness (TYR) in accordance with Variant 1.
  • Table 2b Strength and toughness properties of the alloys as shown in Table 1 in MPa and notch toughness (TYR) in accordance with Variant 2.
  • Table 2c Strength and toughness properties of the alloys as shown in Table 1 in MPa and notch toughness (TYR) in accordance with Variant 3.
  • 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.
  • 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.
  • magnesium levels are of less than 2.4% with an optimum of about 1.7%.
  • magnesium levels are at about 1.7%, excellent toughness properties are obtained but the strength levels decrease.
  • magnesium levels of about 2.1 % the best strength levels are obtained.
  • magnesium is best in between 1.7 and 2.1%.

Abstract

The invention relates to a Al-Zn alloy wrought product, and to a method of manufacturing such a product, with an improved combination of high toughness and high strength by maintaining good corrosion resistance, said 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, Ti < 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.

Description

High strength, high toughness Al-Zn alloy product and method for producing such product
FIELD OF THE INVENTION 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. BACKGROUND OF THE INVENTION 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 AA7x75, AA7x55 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 AA7x50, AA7x75 and AA7x55 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 1210C for 6 to 24 hours and followed by 1710C 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, US-5,221 ,377 and US-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 790C to 1410C for two hours or more and heating the product to one or more temperatures from 1480C 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 7x50 counter- parts in the T76-temper condition. They still have at least about 5% higher strength than their similar- sized 7x50-T77 counterpart (7150-T77 will be used herein below as a reference alloy). SUMMARY OF THE INVENTION
It is an 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 meets one or more of these objects by the characterizing features of the independent claims. Further preferred embodiments are described and specified within the dependent claims.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS As will be appreciated hereinbelow, except 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):
Zn 6.0 to 11.0 Cu 1.4 to 2.2 Mg 1.4 to 2.4
Zr 0.05 to 0.15 Ti < 0.05, Hf and/or V < 0.25, optionally Sc and/or Ce 0.05 to 0.25, and optionally Mn 0.05 to 0.12, and inevitable impurities and balance aluminium, preferably other elements each less than 0.05 and less than 0.50 in total, and wherein the alloy product has a substantially fully unrecrystallized microstructure at the position T/10 of the finished product.
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 AA7x50 or AA7x55 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 sunstantially 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 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):
Zn 6.0 to 11.0 Cu 1.4 to 2.2
Mg 1.4 to 2.4
Zr 0.05 to 0.15
Ti < 0.05,
Hf and/or V < 0.25, optionally Sc and/or Ce 0.05 to 0.25, and optionally Mn 0.05 to 0.12, and inevitable impurities and balance aluminium, preferably other elements each less than 0.05 and less than 0.50 in total, b. homogenising and/or pre-heating the ingot after casting, c. hot working the ingot into a pre-worked product, d. reheating the pre-worked product, and either d1. hot rolling the reheated product to the final gauge, or d2 hot rolling and cold rolling the reheated product to the final gauge, e. solution heat treating and quenching the solution heat treated product, f. optionally stretching or compressing of the quenched alloy product or otherwise cold worked to relieve stresses, and g. optionally ageing the quenched and optionally stretched or compressed product to achieve a desired temper, and wherein the alloy product has a substanlially fully unrecrystallized microstructure at the position T/10 of the finished product.
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 3000C to 4200C 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 1050C to 1350C, preferably around 1200C for 2 to 20 hours, preferably around 8 hours and a second heat treatment at a higher temperature then 1350C but below 2100C, preferably around 1550C 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.
Example 1
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.
Table 1. Chemical compositions of alloys in thin plate form, in weight%, balance aluminium and inevitable impurities, Fe 0.06, Si 0.05, Ti 0.04 and Zr 0.12.
Figure imgf000008_0001
The alloys of Table 1 were processed using three processing variants (see step 5):
1. Homogenisation was performed by heating at a temperature rate of 40°C/h to a temperature of 4600C, then soaking for 12 hours at 460°C and another increase with 25°C/h to a temperature of 4750C with another soaking for 24 hours at 4750C, and air cooling to room temperature.
2. Pre-heating was done at 42O0C for 6 hours with a heating rate of 40°C/h.
3. The lab scale ingots were hot rolled from 80 to 25 mm, thereby reducing the gauge by about 6 to 8 mm per pass.
4. The 25 mm thick products were reheated to 420°C for about 30 min. 5. Variant 1 : The reheated product was hot rolled to 4.0 mm.
Variant 2: The reheated product was hot rolled to 8.0 mm and thereafter cold rolled to 4.0 mm. Variant 3: The reheated product was hot rolled to 5.0 mm and then cold rolled to 4.0 mm. 6. Solution heat treatment was done for 1 hour at 475°C, thereafter water quenched.
7. Stretching was done by 1.5 to 2.0% within about 1 hour after quenching.
8. Thereafter, the stretched products were aged in accordance with a T76 ageing procedure, thereby raising the temperature to 12O0C at a rate of 30°C/h and maintaining the temperature at 12O0C for 5 hours, raising the temperature at a rate of 15°C/h to a temperature of 1600C and soaking for 6 hours, and air cooling the aged product to room temperature.
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.
Table 2a. Strength and toughness properties of the alloys as shown in Table 1 in MPa and notch toughness (TYR) in accordance with Variant 1.
Figure imgf000010_0001
Table 2b. Strength and toughness properties of the alloys as shown in Table 1 in MPa and notch toughness (TYR) in accordance with Variant 2.
Figure imgf000010_0002
Table 2c. Strength and toughness properties of the alloys as shown in Table 1 in MPa and notch toughness (TYR) in accordance with Variant 3.
Figure imgf000011_0001
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.
Example 2
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. Table 3. Chemical compositions of thin plate alloys, in weight%, for all alloys balance aluminium and inevitable impurities, Fe 0.06, Si 0.05.
Figure imgf000012_0001
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.
Table 4. Strength and toughness properties of the alloys as shown in Table 3 in MPa and notch toughness (TS/Rp) in accordance with Variant 3.
Figure imgf000012_0002
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.
Example 3
Further tests were done with zinc levels of 8.6 and 8.7 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.
Table 5. Chemical compositions of thin plate alloys, in weight%, for all alloys balance aluminium and inevitable impurities, Fe 0.06, Si 0.05.
Figure imgf000013_0001
Table 6. Strength and toughness properties of the alloys as shown in Table 5 in MPa and notch toughness (TS/Rp) in accordance with Variant 3.
Figure imgf000014_0001
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 alloy 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.
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 scope or spirit of the invention as herein described.

Claims

1. Method for producing a high strength, high toughness Al-Zn alloy product with good corrosion resistance, comprising the steps of: a.) casting an ingot with the following composition, in weight percent:
Zn 6.0 to 11.0%
Cu 1.4 to 2.2%
Mg 1.4 to 2.4%
Zr 0.05 to 0.15% Ti < 0.05%
Hf and/or V < 0.25%, optionally Sc and/or Ce 0.05 to 0.25%, optionally Mn 0.05 to 0.12%, and inevitable impurities and balance aluminium, b.) homogenising and/or pre-heating the ingot after casting, c.) hot working the ingot into a pre-worked product, d.) reheating the pre-worked product and either d1.) hot rolling the reheated product to the final gauge, or d2.) hot rolling and cold rolling the reheated product to the final gauge, e.) solution heat treating and quenching the solution heat treated product, f.) optionally stretching or compressing of the quenched alloy product, and g.) optionally ageing the quenched and optionally stretched or compressed product to achieve a desired temper, and wherein the product in its final temper has a substantially fully unrecrystallized microstructure at least at the position T/10 of the finished product.
2. Method according to claim 1 , wherein hot rolling the reheated product to about 150 to 250 (in final-gauge%) and then cold rolling the hot rolled product to the final gauge.
3. Method according to claim 1 or 2, wherein hot rolling the reheated product to about 105 to 140 (in final-gauge%) and then cold rolling the hot rolled product to the final gauge.
4. Method according to any one of claims 1 to 3, comprising hot rolling the reheated product at low temperatures in the range of 3000C to 420°C to prevent the alloy product from recrystallising.
5. Method according to any one of claims 1 to 4, wherein the artificial ageing during step g.) is to temper selected from the group consisting of T79, and T76, and preferably by means of a two step ageing treatment.
6. Method according to any one of claims 1 to 5, wherein the artificial ageing during step g.) consists of a first ageing step at a temperature in a range of 105 to 1350C for
2 to 20 hours and a second ageing step at a temperature higher than 1350C but less than 2100C for 4 to 12 hours to a temper selected from T79 and T76 temper.
7. Method according to claim 6, wherein the artificial ageing during step g.) consists of a first ageing step at a temperature around 12O0C for 2 to 20 hours and a second ageing step at a temperature higher than 1350C but less than 21O0C for 4 to 12 hours to a temper selected from T79 and T76 temper.
8. Method according to claim 6 or 7, wherein the artificial ageing during step g.) consists of a first ageing step at a temperature around 12O0C for 2 to 20 hours and a second ageing step at a temperature around 1550C for 4 to 12 hours to a temper selected from T79 and T76 temper.
9. Method according to any one of claims 1 to 8, wherein the amount of Zn is in a range of 7.4 to 9.6 wt.%.
10. Method according to claim 9, wherein the amount of Zn is in a range of 8.0 to 9.6 wt.%, and preferably in a range of 8.4 to 8.9 wt.%.
11. Method according to any one of claims 1 to 10, wherein the amount of Cu is in a range of 1.7 to 2.2 wt.%, and preferably in a range of 1.8 to 2.1 wt.%.
12. Method according to claim 11 , wherein the amount of Mg is in a range of 1.7 to 2.2 wt.%, and preferably in a range of 1.7 to 2.1 wt.%.
13. Method according to any one of claims 1 to 12, wherein the amount of Sc is in a range of [Zr]+1.5 [Sc]<0.15 wt.%.
14. Method according to any one of claims 1 to 13, wherein the amount of Sc is in a range of 0.03 to 0.06%, and wherein the amount of Ce is in a range of 0.03 to 0.06%.
15. Method according to any one of claims 1 to 14, wherein the amount of inevitable impurities are < 0.05 wt.% each, and total < 0.5 wt.%.
16. Method according to any one of claims 1 to 15, wherein of the finished rolled product more than 80%, and preferably more than 90%, of the gauge has a substantially unrecrystallized microstructure.
17. Method according to any one of claims 1 to 16, wherein the Al-Zn product is a thin plate having a gauge in the range of 20 to 60 mm, and preferably 30 to 50 mm.
18. Method according to any one of claims 1 to 17, wherein the Al-Zn product is a product selected from the group consisting of thin aircraft member, an upper-wing member, a thin skin member of an upper-wing, or a stringer of an aircraft.
19. An Al-Zn alloy wrought product with an improved combination of high toughness and high strength by maintaining good corrosion resistance, said alloy consisting of (in weight percent):
Zn 6.0 to 11.0
Cu 1.4 to 2.2
Mg 1.4 to 2.4
Zr 0.05 to 0.15 Ti < 0.05
Hf and/or V < 0.25, and optionally Sc and/or Ce 0.05 to 0.25, and optionally Mn 0.05 to 0.12, other elements each less than 0.05 and less than 0.50 in total, balance aluminium, wherein the Al-Zn alloy wrought product has a substantially fully unrecrystallized microstructure at least at the position T/10 of the finished product.
PCT/EP2005/010809 2004-10-05 2005-10-04 HIGH STRENGTH, HIGH TOUGHNESS Al-Zn ALLOY PRODUCT AND METHOD FOR PRODUCING SUCH PRODUCT WO2006037648A1 (en)

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