Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C21/00—Alloys based on aluminium
  • C22C21/10—Alloys based on aluminium with zinc as the next major constituent
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22F—CHANGING THE PHYSICAL STRUCTURE OF NON-FERROUS METALS AND NON-FERROUS ALLOYS
  • C22F1/00—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working
  • C22F1/04—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working of aluminium or alloys based thereon
  • C22F1/053—Changing 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%.

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