Classifications

• • 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
• • 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
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
  • Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
  • Y10T428/00—Stock material or miscellaneous articles
  • Y10T428/12—All metal or with adjacent metals
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
  • Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
  • Y10T428/00—Stock material or miscellaneous articles
  • Y10T428/12—All metal or with adjacent metals
  • Y10T428/12389—All metal or with adjacent metals having variation in thickness

Definitions

• the invention relates to a wrought Al-Zn-Mg-Cu aluminium type (or 7000- or
• the present invention is related to an age-hardenable, high strength, high fracture toughness and highly corrosion resistant aluminium alloy and products made of that alloy. Products made from this alloy are very suitable for aerospace applications, but not limited to that.
• the alloy can be processed to various product forms, e.g. sheet, thin plate, thick plate, extruded or forged products.
• alloy members and temper designations used herein are in accordance with the well-known aluminium alloy product standards of the Aluminum Association. All percentages are in weight percents, unless otherwise indicated.
• FCGR fatigue crack growth rate
• plane stress fracture toughness a combination of fatigue crack growth rate (FCGR)
• FCGR fatigue crack growth rate
• high damage tolerant AA2x24-T351 see e.g. US-5,213,639 or EP- 1026270-A1
• Cu containing AA6xxx-T6 see e.g. US-4,589,932, US-5,888,320, US- 2002/0039664-A1 or EP-1143027-A1
• For lower wing skin a similar property balance is desired, but some toughness is allowably sacrificed for higher tensile strength.
• AA2x24 in the T39 or a T8x temper are considered to be logical choices (see e.g. US-5,865,914, US- 5,593,516 or EP-1114877-A1), although AA7x75 in the same temper is sometimes also applied.
• the compressive strength, fatigue (SN-fatigue or life-time) and fracture toughness are the most critical properties.
• the preferred choice would be AA7150, AA7055, AA7449 or AA7x75 (see e.g. US-5,221 ,377, US-5,865,911 , US- 5,560,789 or US-5,312,498).
• These alloys have high compressive yield strength with at the moment acceptable corrosion resistance and fracture toughness, although aircraft designers would welcome improvements on these property combinations.
• the present invention is directed to an AA7xxx-series aluminium alloy having the capability of achieving a property balance in any relevant product that is better than property balance of the variety of commercial aluminium alloys (AA2xxx, AA6xxx, AA7xxx) nowadays used for those products.
• a preferred composition of the alloy of the present invention comprises or consists essentially of, in weight %, about 6.5 to 9.5 zinc (Zn), about 1.2 to 2.2 % magnesium (Mg), about 1.0 to1.9% copper (Cu), about 0 to 0.5% zirconium (Zr), about 0 to 0.7% scandium (Sc), about 0 to 0.4% chromium (Cr), about 0 to 0.3% hafnium (Hf), about 0 to 0.4% titanium (Ti), about 0 to 0.8% manganese (Mn), the balance being aluminium (Al) and other incidental elements.
• Zn zinc
• Mg magnesium
• Cu copper
• Zr zirconium
• Sc scandium
• Cr chromium
• Hf hafnium
• Ti titanium
• Mn manganese
• a more preferred alloy composition according to the invention consist essentially of, in weight %, about 6.5 to 7.9% Zn, about 1.4 to 2.10% Mg, about 1.2 to 1.80% Cu, and preferably wherein (0.9Mg-0.5) ⁇ Cu ⁇ 0.9Mg, about 0 to 0.5% Zr, about 0 to 0.7% Sc, about 0 to 0.4% Cr, about 0 to 0.3% Hf, about 0 to 0.4% Ti, about 0 to 0.8% Mn, the balance being Al and other incidental elements.
• a more preferred alloy composition according to the invention consist essentially of, in weight %, about 6.5 to 7.9% Zn, about 1.4 to 1.95% Mg, about 1.2 to 1.75% Cu, and preferably wherein (0.9Mg-0.5) ⁇ Cu ⁇ (0.9Mg-0.1), about 0 to 0.5% Zr, about 0 to 0.7% Sc, about 0 to 0.4% Cr, about 0 to 0.3% Hf, about 0 to 0.4% Ti, about 0 to 0.8%
• the lower limit for the Zn-content is 6.7%, and more preferably 6.9%.
• the lower limit for the Mg-content of 1.90%, and more preferably 1.92%.
• This lower-limit for the Mg-content is in particular preferred when the alloy product is being used for sheet product, e.g. fuselage sheet, and when used for sections made from thick plate.
• the above mentioned aluminium alloys may contain impurities or incidental or intentionally additions, such as for example up to 0.3% Fe, preferably up to 0.14% Fe, up to 0.2% silicon (Si), and preferably up to 0.12% Si, up to 1 % silver (Ag), up to 1 % germanium (Ge), up to 0.4% vanadium (V).
• the other additions are generally governed by the 0.05-0.15 weight % ranges as defined in the Aluminium Association, thus each unavoidable impurity in a range of ⁇ 0.05%, and the total of impurities ⁇ 0. 5%.
• the iron and silicon contents should be kept significantly low, for example not exceeding about 0.08% Fe and about 0.07% Si or less. In any event, it is conceivable that still slightly higher levels of both impurities, up to about 0.14% Fe and up to about 0.12% Si may be tolerated, though on a less preferred basis herein. In particular for the mould plates or tooling plates embodiments hereof, even higher levels of up to 0.3% Fe and up to 0.2% Si or less, are tolerable.
• the dispersoid forming elements like for example Zr, Sc, Hf, Cr and Mn are added to control the grain structure and the quench sensitivity.
• the optimum levels of dispersoid formers do depend on the processing, but when one single chemistry of main elements (Zn, Cu and Mg) is chosen within the preferred window and that chemistry will be used for all relevant products forms, then Zr levels are preferably less than 0.11%.
• a preferred maximum for the Zr level is a maximum of 0.15%.
• a suitable range of the Zr level is a range of 0.04 to 0.15%.
• a more preferred upper-limit for the Zr addition is 0.13%, and even more preferably not more than 0.11 %.
• the addition of Sc is preferably not more than 0.3%, and preferably not more than 0.18%.
• the sum of Sc+Zr should be less then 0.3%, preferably less than 0.2%, and more preferably at a maximum of 0.17%, in particular where the ratio of Zr and Sc is between 0.7 and 1.4.
• Another dispersoid former that can be added, alone or with other dispersoid formers is Cr.
• Cr levels should be preferable below 0.3%, and more preferably at a maximum of 0.20%, and even more preferably 0.15%.
• the sum of Zr + Cr should not be above 0.20%, and preferably not more than 0.17%.
• the preferred sum of Sc+Zr+Cr should not be above 0.4%, and more preferably not more than 0.27%.
• Mn can be added alone or in combination with one of the other dispersoid formers.
• a preferred maximum for the Mn addition is 0.4%.
• a suitable range for the Mn addition is in the range of 0.05 to 0.40%, and preferably in the range of 0.05 to 0.30%, and even more preferably 0.12 to 0.30%.
• a preferred lower limit for the Mn addition is 0.12%, and more preferably 0.15%.
• the sum of Mn + Zr should be less then 0.4%), preferably less than 0.32%, and a suitable minimum is 0.14%.
• the alloy is free of Mn, in practical terms this would mean that the Mn-content is ⁇ 0.02%, and preferably ⁇ 0.01 %, and more preferably the alloy is essentially free or substantially free from Mn.
• 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 alloy consists essentially of, in weight percent:
• Mn optionally in a range of 0.05 to 0.19, and preferably 0.09 to 0.19, or in an alternative embodiment ⁇ 0.02, preferably ⁇ 0.01 Si ⁇ 0.07, and typically about 0.04
• the alloy consists essentially of, in weight percent:
• Mg 1.90 to 1.97, preferably 1.92 to 1.97, and typically about 1.94
• the alloy product according to the invention can be prepared by conventional melting and may be (direct chill, D.C.) cast into ingot form. Grain refiners such as titanium boride or titanium carbide may also be used. After scalping and possible homogenisation, the ingots are further processed by, for example extrusion or forging or hot rolling in one or more stages. This processing may be interrupted for an inter- anneal. Further processing may be cold working, which may be cold rolling or stretching. The product is solution heat treated and quenched by immersion in or spraying with cold water or fast cooling to a temperature lower than 95°C.
• the product can be further processed, for example by rolling or stretching, for example up to 8%, or may be stress relieved by stretching or compression up to about 8%, for example, from about 1 to 3%, and/or aged to a final or intermediate temper.
• the product may be shaped or machined to the final or intermediate structure, before or after the final ageing or even before solution heat treatment.
• FCGR damage tolerant properties under tensile loads
• the important material properties for an upper wing skin product are the properties under compressive loads, i.e. compressive yield strength, fatigue life and corrosion resistance.
• the important material properties for machined parts from thick plate depend on the machined part. But, in general, the gradient in material properties through thickness must be very small and the material properties like strength, fracture toughness, fatigue and corrosion resistance must be a high level.
• the present invention is directed at an alloy composition when processed to a variety of products, such as, but not limited to, sheet, plate, thick plate etc, will meet or exceed the desired material properties.
• the property balance of the product will outperform the property balance of the product made from nowadays commercially used alloys. It has been found very surprisingly a chemistry window within the AA7000 window, unexplored before, that does fulfil this unique capability.
• the present invention resulted from an investigation on the effect of Cu, Mg and Zn levels, combined with various levels and types of dispersoid former (e.g. Zr, Cr, Sc, Mn) on the phases formed during processing.
• dispersoid former e.g. Zr, Cr, Sc, Mn
• Some of these alloys were processed to sheet and plate and tested on tensile, Kahn-tear toughness and corrosion resistance.
• Interpretations of these results lead to the surprising insight that an aluminium alloy with a chemical composition within a certain window, will exhibit excellent properties as well as for sheet as for plate as for thick plate as for extrusions as for forgings.
• a method of manufacturing the aluminium alloy product according to the invention comprising the processing steps of: a) casting an ingot having a composition as set out in the present description; b) homogenising and/or pre-heating the ingot after casting; c) hot working the ingot into a pre-worked product by one or more methods selected from the group consisting of: rolling, extruding and forging; d) optional reheating the pre-worked product and either, e) hot working and/or cold working to a desired work piece form; f) solution heat treating (SHT) the formed work piece at a temperature and time sufficient to place into solid solution essentially all soluble constituents in the alloy; g) quenching the solution heat treated work piece by one of spray quenching or immersion quenching in water or other quenching media; h) optionally stretching or compress
• the alloy products of the present invention are conventionally prepared by melting and may be direct chill (D.C.) cast into ingots or other suitable casting techniques. Homogenisation treatment is typically carried out in one or multi steps, each step having a temperature preferably in the range of 460 to 490°C.
• the pre-heat temperature involves heating the rolling ingot to the hot-mill entry temperature, which is typically in a temperature range of 400 to 460°C.
• Hot working the alloy product can be done by one or more methods selected from the group consisting of rolling, extruding and forging. For the present alloy hot rolling is being preferred.
• Solution heat treatment is typically carried out in the same temperature range as used for homogenisation, although the soaking times can be chosen somewhat shorter.
• the artificial ageing step i.) comprises a first ageing step at a temperature in a range of 105°C to 135°C preferably for 2 to 20 hours, and a second ageing step at a temperature in a range of 135°C to 210°C preferably for 4 to 20 hours.
• a third ageing step may be applied at a temperature in a range of 105°C to 135°C and preferably for 20 to 30 hours.
• a surprisingly excellent property balance is being obtained in whatever thickness is produced.
• the properties will be excellent for fuselage sheet, and preferably the thickness is up to 1 inch.
• the thin plate thickness range of 0.7 to 3 inch the properties will be excellent for wing plate, e.g. lower wing plate.
• the thin plate thickness range can be used also for stringers or to form an integral wing panel and stringer for use in an aircraft wing structure. More peak-aged material will give an excellent upper wing plate, whereas slightly more over- ageing will give excellent properties for lower wing plate.
• thicker gauge products When processed to thicker gauges of more than 2.5 inch up to about 11 inch or more excellent properties will be obtained for integral parts machined from plates, or to form an integral spar for use in an aircraft wing structure, or in the form of a rib for use in an aircraft wing structure.
• the thicker gauge products can be used also as tooling plate or mould plate, e.g. moulds for manufacturing formed plastic products, for example via die-casting or injection moulding.
• thickness ranges are given hereinabove, it will be immediately apparent to the skilled person that this is the thickness of the thickest cross sectional point in the alloy product made from such a sheet, thin plate or thick plate.
• the alloy products according to the invention can also be provided in the form of a stepped extrusion or extruded spar for use in an aircraft structure, or in the form of a forged spar for use in an aircraft wing structure. Surprisingly, all these products with excellent properties can be obtained from one alloy with one single chemistry.
• the component increased elongation compared to its AA7050 aluminium alloy counterpart.
• the elongation (or A50) in the ST testing direction is 5% or more, and in the best results 5.5% or more.
• the component has a fracture toughness Kapp in the L-T testing direction at ambient room temperature and when measured at S/4 according to ASTM E561 using 16-inch centre cracked panels (M(T) or CC(T)) showing an at least 20% improvement compared to its AA7050 aluminium alloy counterpart, and in the best examples an improvement of 25% or more is found.
• the alloy products have been extruded into profiles having at their thickest cross sectional point a thickness in the range of up to 10 mm, and preferably in the range of 1 to 7mm.
• the alloy product can also replace thick plate material, which is conventionally machined via high-speed machining or milling techniques into a shaped structural component.
• the extruded alloy product has preferably at its thickest cross sectional point a thickness in a range of 2 to 6 inches.
• Fig. 1 is an Mg-Cu diagram setting out the Cu-Mg range for the alloy according to this invention, together with narrower preferred ranges;
• Fig. 2 is a diagram comparing the fracture toughness vs. the tensile yield strength for the alloy product according to the invention against several references;
• Fig. 3 is a diagram comparing the fracture toughness vs. the tensile yield strength for the alloy product according to this invention in a 30 mm gauge against two references;
• Fig. 4 is a diagram comparing the plane strain fracture toughness vs. the tensile yield strength for the alloy products according to the invention using different processing routes.
• Fig. 1 shows schematically the ranges for the Cu and Mg for the alloy according to the present invention in their preferred embodiments as set out in dependent claims 2 to 4. Also shown are two narrower more preferred ranges. The ranges can also be identified by using the corner-points A, B, C, D, E, and F of a hexagon box. Preferred ranges are identified by A' to F, and more preferred ranges by A" to F". The coordinates are listed in Table 1. In Fig. 1 also the alloy composition according to this invention as mentioned in the examples hereinafter are illustrated as individual points.
• the blocks were re-heated at 410+5°C. Some blocks were hot rolled to the final gauge of 30 mm, others were hot rolled to a final gauge of 4.0mm. During the whole hot-rolling process, care was taken to mimic an industrial scale hot rolling.
• the hot- rolled products were solution heat treated and quenched. Most were quenched in water, but some were also quenched in oil to mimic the mid and quarter-thickness quenching-rate of a 6-inch thick plate.
• the products were cold stretched by about 1.5% to relieve the residual stresses.
• the ageing behaviour of the alloys was investigated. The final products were over-aged to a near peak aged strength (e.g. T76 or T77 temper).
• Tensile properties have been tested according EN10.002.
• the tensile specimens from the 4 mm thick sheet were flat EURO-NORM specimen with 4 mm thickness.
• the tensile specimens from the 30 mm plate were round tensile specimens taken from mid- thickness.
• the tensile test results in Table 1 are from the L-direction.
• the Kahn-tear toughness is tested according ASTM B871-96.
• the test direction of the results on Table 2 is the T-L direction.
• the so-called notch-toughness can be obtained by dividing the tear-strength, obtained by the Kahn-tear test, by the tensile yield strength ("TS/Rp").
• the unit propagation energy (UPE)
• UPE unit propagation energy
• EXCO exfoliation corrosion resistance
• IRC inter-granular corrosion
• the Zn-content should not be below 6.5%, and preferably not below 6.7%, and more preferably not below 6.9%.
• Mg is required to have acceptable strength levels. It has been found that a ratio of Mg/Zn of about 0.27 or lower seems to give the best strength-toughness combination. However, Mg levels should not exceed 2.2%, and preferably not exceed ⁇ o 2.1%, and even more preferably not exceed 1.97%, with a more preferred upper level of 1.95%). This upper-limit is lower than in the conventional AA-windows or ranges of presently used commercial aerospace alloys like AA7050, AA7010 and AA7075.
• Mg levels In order to have a desirably very high crack growth resistance (or UPE) Mg levels must be carefully balanced and should preferable be in the same order or slightly more 15 than the Cu levels, and preferably (0.9xMg - 0.6) ⁇ Cu ⁇ (0.9xMg + 0.05).
• the Cu- content should not be too high. It has been found that the Cu-content should not be higher than 1.9%, and preferably should not exceed 1.80%, and more preferably not exceed 1.75%.
• the dispersoid formers used in AA7xxx-series alloys are typically Cr, as in e.g. AA7x75, or Zr, as in e.g. AA7x50 and AA7x10.
• Mn is believed to be detrimental for toughness, but much to our surprise, a combination of Mn and Zr shows still a very good strength-toughness balance.
• a batch of full-size rolling ingots with a thickness of 440mm thick on an industrial scale were produced by a DC-casting and having the chemical composition (in wt.%): 7.43% Zn, 1.83% Mg, 1.48% Cu, 0.08% Zr, 0.02% Si and 0.04% Fe, balance aluminium and unavoidable impurities.
• One of these ingots was scalped, homogenised at 12hrs/470°C + 24hrs/475°C + air cooled to ambient temperature. This ingot was preheated at 8hrs/410°C and then hot rolled to about 65mm. The rolling block was then turned 90 degrees and further hot rolled to about 10mm. Finally the rolling block was cold rolled to a gauge of 5.0mm.
• variant A for 5hrs/120°C + 9hrs/155°C
• variant B for 5hrs/120°C + 9hrs/165°C.
• the tensile results have been measured according EN 10.002.
• the compression yield strength (“CYS”) has been measured according ASTM E9-89a.
• the shear strength has been measured according ASTM B831-93.
• the fracture toughness, Kapp has been measured according ASTM E561-98 on 16-inch wide centre cracked panels [M(T) or CC(T)].
• the Kapp has been measured at ambient room temperature (RT) and at -65°F.
• a high damage tolerant (“HDT”) AA2x24-T351 has been tested as well. The results are listed in Table 3.
• the exfoliation corrosion resistance has been measured according ASTM G34- 97. Both variant A and B showed EA rating.
• the inter-granular corrosion measured according MIL-H-6088 for variant A was about 70 ⁇ m and for variant B about 45 ⁇ m. Both are significantly lower than the typical 200 ⁇ m as measured for the reference AA2x24-T351.
• FCGR fatigue crack growth rate
• the tensile results have been measured according EN 10.002.
• the plane strain fracture toughness, Kq has been measured according ASTM E399-90 on CT- specimens. If the validity requirements as given in ASTM E399-90 are met, these Kq values are a real material property and called K ⁇ C .
• the K 1C has been measured at ambient room temperature ("RT").
• the EXCO exfoliation corrosion resistance has been measured according ASTM G34-97. The results are listed in Table 6. All ageing variants as shown in Table 6 showed "EA"-rating. Table 6
• Fig. 3 a comparison is given of the inventive alloy versus AA7150-T77 and AA7055-T77. From Fig. 3 it can be clearly seen that the tensile versus toughness balance of the current inventive alloy is superior to commercial available AA7150-T77 and also to AA7055-T77.
• Example 5 Another full-scale ingot taken from the batch DC-cast from Example 2 (hereinafter in Example 5 "Alloy A”) was produced to plates of 20mm thickness. Also one other casting was made (designated “Alloy B” for this example) with a chemical composition (in wt.%): 7.39% Zn, 1.66% Mg, 1.59% Cu, 0.08% Zr, 0.03% Si and 0.04% Fe, balance aluminium and unavoidable impurities. These ingots were scalped, homogenised at 12hrs/470°C + 24hrs/475°C + air cooled to ambient temperature. For further processing, three different routes were used.
• Route 1 The ingot of alloy A and B were pre-heated at 6hrs/420°C and then hot rolled to about 20 mm.
• Route 2 Ingot of alloy A were pre-heated at 6hrs/460°C and then hot rolled to about 20 mm
• Route 3 Ingot of alloy B were pre-heated at 6hrs/420°C and then hot rolled to about 24 mm, subsequently these plates were cold rolled to 20mm.
• A1 , A2, B1 and B3 The resultant plates were solution heat treated at 475°C for about 2 to 4 hrs followed by water-spray quenching. The plates were stress relieved by a cold stretching operation of about 2.1 %.
• the tensile results have been measured according EN 10.002.
• the plane strain fracture toughness, Kq has been measured according ASTM E399-90 on CT specimens. If the validity requirements as given in ASTM E399-90 are met, these Kq values are a real material property and called K C or KIC. Note that most of the fracture toughness measurement in this example failed the meet the validity criteria on specimen thickness.
• the reported Kq values are a conservative with respect to K 1C , in other words, the reported Kq values are in fact generally lower than the standard K C values obtained when specimen size related validity criteria of ASTM E399-90 are satisfied.
• the exfoliation corrosion resistance has been measured according ASTM G34-97. The results are listed in Table 8. All ageing variants as shown in Table 8 showed "EA"-rating for the EXCO resistance.
• A1 and B1 have a similar strength versus toughness behaviour.
• the best strength versus toughness could be obtained by either B3 (i.e. cold rolling to final thickness) or by A2 (i.e. pre-heat at a higher temperature).
• B3 i.e. cold rolling to final thickness
• A2 i.e. pre-heat at a higher temperature.
• Table 8 show a significant better strength versus toughness balance than AA7150-T77 and AA7055- T77 as listed in Table 7.
• alloy B represents an alloy composition according a preferred embodiment of the invention when the alloy product is in the form of a sheet product.
• Example 1 The ingots were scalped, homogenized at 12hrs/470°C + 24 hrs/475°C and then hot rolled to an intermediate gauge of 65 mm and final hot rolled to about 9 mm. Finally the hot rolled intermediate products have been cold rolled to a gauge of 4mm.
• the obtained sheet products were solution heat treated at 475°C for about 20 minutes, followed by water-spray quenching. The resultant sheets were stress relieved by a cold stretching operation of about 2%. The stretched sheets have been aged thereafter for 5 hrs/120°C + 8 hrs/165°C.
• Mechanical properties have tested analogue to Example 1 and the results are listed in Table 10. The results of this full-scale trial confirm the results of Example 1 that the positive addition of Mn in the defined range significantly improves the toughness (both UPE and Ts/Rp) of the sheet product resulting in a very good and desirable strength-toughness balance.
• alloy C represents a typical alloy falling within the AA7050-series range
• alloy D represents an alloy composition according to a preferred embodiment of the invention when the alloy product is in the form of plate, e.g. thick plate.
• the ingots were scalped, homogenized in a two-step cycle of 12hrs/470°C + 24hrs/475°C and air-cooled to ambient temperature.
• the ingot was pre-heated at 8hrs/410°C and then hot rolled to final gauge.
• the obtained plate products were solution heat treated at 475°C for about 6 hours, followed by water-spray quenching.
• the resultant plates were stretched by a cold stretching operation for about 2%.
• the stretched plates have been aged using a two-step ageing practice of first 5hrs/120°C followed by 12 hrs/165°C. Mechanical properties have been tested analogue to Example 3 in three test directions and the results are listed in Table 12 and 13.
• the specimens were taken from S/4 position from the plate for the L- and LT-testing direction and at S/2 for the ST-testing direction
• the Kapp has been measured at S/2 and S/4 locations in the L-T direction using panels having a width of 160mm centre cracked panels and having a thickness of 6.3mm after milling. These Kapp measurements have been carried out at room temperature in accordance with ASTM E561.
• the designation "ok" for the SCO means that no failure occurred at 180MPa/45days.
• the alloy according to the invention in comparison with AA7050 has similar corrosion performance, the strength (yield strength and tensile strength) are comparable or slightly better than AA7050, in particular in the ST-direction. But more importantly the alloy of the present invention showed significantly better results in elongation (or A50) in the ST-direction.
• the elongation (or A50), in particular the elongation in ST-direction, is an important engineering parameter of amongst others ribs for use in an aircraft wing structure.
• the alloy product according to the invention further shows a significant improvement in fracture toughness (both K
• alloy F represents an alloy composition according to a preferred embodiment of the invention when the alloy product is in the form of plate for wings.
• the ingots were scalped, homogenized in a two-step cycle of 12hrs/470°C + 24hrs/475°C and air-cooled to ambient temperature.
• the ingot was pre-heated at 8hrs/410°C and then hot rolled to final gauge.
• the obtained plate products were solution heat treated at 475°C for about 4 hours, followed by water-spray quenching.
• the resultant plates were stretched by a cold stretching operation for about 2%.
• the stretched plates have been aged using a two-step ageing practice of first 5hrs/120°C followed by 10 hrs/155°C.

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