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Publication numberUS20060032560 A1
Publication typeApplication
Application numberUS 10/976,154
Publication dateFeb 16, 2006
Filing dateOct 29, 2004
Priority dateOct 29, 2003
Publication number10976154, 976154, US 2006/0032560 A1, US 2006/032560 A1, US 20060032560 A1, US 20060032560A1, US 2006032560 A1, US 2006032560A1, US-A1-20060032560, US-A1-2006032560, US2006/0032560A1, US2006/032560A1, US20060032560 A1, US20060032560A1, US2006032560 A1, US2006032560A1
InventorsRinze Benedictus, Alfred Heinz, Christian Keidel, Alfred Haszler, Guido Weber
Original AssigneeCorus Aluminium Walzprodukte Gmbh
Export CitationBiBTeX, EndNote, RefMan
External Links: USPTO, USPTO Assignment, Espacenet
Method for producing a high damage tolerant aluminium alloy
US 20060032560 A1
Abstract
The present invention relates to a method of producing a high damage tolerant aluminium alloy rolled product a high toughness and an improved fatigue crack growth resistance, including the steps of: a.) casting an ingot having a composition selected from the group comprising AA2000, AA5000, AA6000, and AA7000-series alloys; b.) homogenising and/or pre-heating the ingot after casting; c.) hot rolling the ingot into a hot rolled product and optionally cold rolling the hot rolled product into a cold rolled product, wherein the hot rolled product leaves the hot rolling mill at an hot-mill exit temperature (TExit) and cooling the hot rolled product from the TExit to 150 C. or lower with a controlled cooling cycle with a cooling rate falling within the range defined by: T(t)=50−(50−TExit)eαt and wherein T(t) is the temperature ( C.) as function in time (hrs), t is the time (hours) and α is in the range of −0.090.05 (hrs−1).
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Claims(27)
1. Method for producing a high damage tolerant aluminium alloy rolled product a high toughness and an improved fatigue crack growth resistance, comprising the steps of
a.) casting an ingot having a composition selected from the group comprising of AA2000, AA5000, AA6000, and AA7000-series alloys;
b.) homogenising and/or pre-heating the ingot after casting;
c.) hot rolling the ingot into a hot rolled product and optionally cold rolling the hot rolled product into a cold rolled product, wherein the hot rolled product leaves the hot rolling mill at an hot-mill exit temperature (TExit) and cooling the hot rolled product from said TExit to 150 C. or lower with a controlled cooling cycle with a cooling rate falling within the range defined by:

T(t)=50−(50 −Texit)e αt
and wherein t(t) is the temperature ( C.) as function in time (hrs), t is the time (hours) and α is in the range of −0.090.05 (hrs−1).
2. Method according to claim 1, wherein α is in the range of −0.090.03 (hrs−1).
3. Method according to claim 1, wherein the hot rolled product is subjected to the controlled cooling cycle, thereby maintaining an elevated temperature for a predetermined time.
4. Method according to claim 1, wherein the hot rolled product is subjected to the controlled cooling cycle by coiling the hot rolled product alloy after hot rolling.
5. Method according to claim 1, wherein the hot rolled product is subjected to the controlled cooling cycle by coiling the hot rolled product alloy after hot rolling and hot rolling and cold rolling are in the same direction.
6. Method according to claim 1, wherein the hot rolled product is subjected to the controlled cooling cycle by coiling the hot rolled product alloy after hot rolling and hot rolling and cold rolling are in dissimilar directions.
7. Method according to claim 1, wherein the hot rolled product is subjected to the controlled cooling cycle by continuously moving the rolled product through a furnace after hot rolling, wherein said furnace is adjustable to apply heat to the rolled alloy product while passing to a cold rolling station or a coiling station.
8. Method according to claim 1, wherein the hot rolled product is subjected to the controlled cooling cycle by coiling the rolled alloy product after hot rolling in a furnace wherein said furnace is adjustable to control the cooling rate of the alloy product while coiling.
9. Method according to claim 1, wherein the hot rolled product has a gauge in a range of less than 12 mm while leaving the hot rolling mill at the hot-mill exit temperature.
10. Method according to claim 9, wherein the hot rolled product has a gauge in a range of 1 to 10 mm.
11. Method according to claim 9, wherein the hot rolled product has a gauge in a range of 4 to 8 mm.
12. Method according to claim 7, wherein the hot rolled product is cold rolled to a total cold roll reduction of 40 to 70%.
13. Method according to claim 1, wherein the method further includes one or more of the following process steps:
d.) solution heat treating of the hot rolled product after being subjected to the controlled cooling cycle or of the cold rolled product;
e.) quenching the solution heat treated alloy product;
f.) optionally stretching or compressing of the quenched alloy product;
g.) optionally ageing the quenched and optionally stretched or compressed alloy product to achieve a desired temper.
14. Method according to claim 13, comprising said stretching or compressing of the quenched alloy product.
15. Method according to claim 1, wherein the average cooling rate in the controlled cooling cycle is in a range of 12 to 20 C./hour.
16. Method according to claim 1, wherein the cast ingot comprises the following composition (in weight percent):
Si 0.6-1.3 Cu 0.04-1.1  Mn 0.1-0.9 Mg 0.4-1.3 Fe 0.01-0.3  Zr <0.25 Cr <0.25 Zn <0.6 Ti <0.15 V <0.25 Hf <0.25,
other elements each less than 0.05 and less than 0.20 in total, balance aluminium.
17. Method according to claim 1, wherein the cast ingot comprises an alloy within the compositional range of AA6013 or AA6056.
18. Method according to claim 1, wherein the cast ingot comprises the following composition (in weight percent):
Cu 3.8-5.2 Mg 0.2-1.6 Cr <0.25 Zr <0.25 Mn ≦0.50 and Mn: >0 Fe ≦0.15 Si ≦0.15,
other elements each less than 0.05 and less than 0.15 in total, balance aluminium.
19. Method according to claim 18, wherein the cast alloy comprises at least one member of the group consisting of Mn-containing dispersoids and Zr-containing dispersoids.
20. Method according to claim 18, wherein Zr of the cast ingot is in the range of 0.06-0.18 and Mn of the cast ingot is in the range of>0.15 to 0.50.
21. Method according to claim 1, wherein the casting an ingot comprising the following composition (in weight percent):
Zn 5.0-9.5 Cu 1.0-3.0 Mg 1.0-3.0 Mn <0.35 Zr <0.25 Cr <0.25 Fe <0.25 Si <0.25 Sc <0.35 Ti <0.10 Hf and/or V <0.25,
other elements each less than 0.05 and less than 0.15 in total, balance aluminium.
22. Method according to claim 21, wherein Zr of the cast ingot is in the range of 0.06-0.16.
23. Method according to claim 1, wherein the cast ingot comprises an alloy within the compositional range selected from the group consisting of AA7040, AA7050 and AA7x75.
24. An aluminium alloy sheet or plate product having a high toughness and an improved fatigue crack growth resistance made of an alloy produced according to the method of claim 1.
25. A rolled alloy sheet product according to claim 24, wherein said product is a structural member of an aircraft or an automobile.
26. A rolled alloy sheet product according to claim 24, wherein said product is a fuselage skin of an aircraft or a vehicle component part.
27. A rolled alloy product according to claim 24, wherein the rolled alloy product has a final gauge in the range of 2 to 7 mm.
Description
    CROSS REFERENCE TO RELATED APPLICATIONS
  • [0001]
    This claims priority under 35 USC 119 from European application no. 03078410.2 filed on 29 Oct. 2003 and U.S. provisional application 60/515,699 filed on 31 Oct. 2003, both incorporated herein by reference in their entirety.
  • FIELD OF THE INVENTION
  • [0002]
    The present invention discloses a method for producing a high damage tolerant aluminium rolled alloy having a good toughness and an improved fatigue crack growth resistance while maintaining good strength levels and to an aluminium alloy sheet or plate product having such a high toughness and an improved fatigue crack growth resistance. Furthermore, the invention relates to the use of an alloy product obtained by the method of this invention.
  • BACKGROUND OF THE INVENTION
  • [0003]
    It is known in the art to use heat treatable aluminium alloys in a number of applications involving relatively high strength such as aircraft fuselages, vehicular members and other applications. Aluminium alloys AA2024, AA2324 and AA2524 are well-known heat treatable aluminium alloys which have useful strength and toughness properties in T3, T39 and T351 tempers. Also aluminium alloys AA6013 and AA6056 are well-known heat treatable aluminium alloys which have useful strength and toughness properties as well as a good fatigue crack growth resistance in both T4 and T6 tempers.
  • [0004]
    It is known that the T4 temper condition refers to a solution heat treated and quenched condition, naturally aged to a substantially stable property level, whereas T6 tempers refer to a stronger condition produced by artificially aging.
  • [0005]
    Several other AA2000 and AA6000 series alloys are generally unsuitable for the design of commercial aircraft which require different sets of properties for different types of structures. Depending on the design criteria for a particular airplane component even small improvements in toughness and crack growth resistance, specifically for high ΔK-values, result in weight savings, which translate to fuel economy over the lifetime of the aircraft and/or a greater level of safety. Especially for fuselage skin or lower wing skin it is necessary to have properties such as good resistance to crack propagation either in the form of fracture toughness or fatigue crack growth resistance. A rolled alloy product either used as a sheet or as a plate with improved damage tolerance properties will improve the safety of the passengers, will reduce the weight of the aircraft and will result to a longer flight range, lower costs and less frequent maintenance intervals.
  • [0006]
    U.S. Pat. No. 5,213,639 discloses a method for producing an aluminium alloy of the AA2000-series with an aluminium base alloy which is hot rolled, heated and again hot rolled, thereby obtaining good combinations of strength together with high fracture toughness and a low fatigue crack growth rate. It is disclosed to apply an inter-annealed treatment after hot rolling the casted ingot with a temperature between 479 C. and 524 C. and again hot rolling the inter-annealed alloy. Such alloy is reported to have a 5% improvement over the conventional AA2024-series alloys in T-L fracture toughness and an improved fatigue crack growth resistance at certain Δ K-levels.
  • [0007]
    It has been reported that the known AA6056 alloy is sensitive to inter-crystalline corrosion in the T6 temper condition. In order to overcome this problem U.S. Pat. No. 5,858,134 provides a process for the production of rolled or extruded products having a defined chemical composition, and whereby the products are brought in an over-aged temper condition requiring time and money consuming processing times at the end of the manufacturer of aerospace components. Here, it is reported that in order to obtain the improved inter-crystalline corrosion resistance it is essential for the process that in the alloy the Mg/Si ratio is less than 1.
  • [0008]
    U.S. Pat. No. 4,589,932 discloses an aluminium wrought alloy product for e.g. automotive and aerospace constructions, which alloy was subsequently registered under the AA designation 6013. Such aluminium alloy has been solution heat treated at a temperature in a range of 449 C. to 582 C., approaching the solidus temperature of the alloy.
  • [0009]
    EP-A-1 143027 discloses a method for producing an Al—Mg—Si alloy of the AA6000-series having a defined chemical composition and wherein the products are subjected to an artificial aging procedure to improve the alloy and to meet high damage tolerance (“HDT”) characteristics similar to those of the AA2024-series which are preferably used for aeronautical applications but which are not weldable. The aging procedure is being optimised using a respective function of the composition.
  • [0010]
    EP-1170394-A2 discloses an aluminium alloy sheet product with improved fatigue crack growth resistance having an anisotropic microstructure defined by grains having an average length to width aspect ratio of greater than about 4. Such alloy has an improvement in compressive yield strength properties which is achieved by respective sheet products in comparison with conventional AA2524-sheet products. Throughout the high an-isotropical grain structure the fatigue crack growth resistance could be improved.
  • [0011]
    WO-97/22724 discloses a method and an apparatus for producing an aluminium alloy sheet product, typically for automotive application, with improved yield strength by continuously and rapidly heating the hot rolled and cold rolled sheet, which has been solution heat treated and quenched, to a pre-aging temperature prior to the continuous coiling step. After rapidly heating, the sheet in coil form is ambiently cooled, the rapid heating and ambient cooling improving the paintbake response of the aluminium alloy sheet. It is disclosed that it is preferred to rapidly heating the coiled sheet to between 65 C. and 121 C. and to choose an ambient cooling rate and which is preferred to be between 1.1 C./h and 3.3 C./h.
  • SUMMARY OF THE INVENTION
  • [0012]
    It is an object of the present invention to provide a method for producing an aluminium alloy product having an improved toughness and an improved fatigue crack growth resistance thereby maintaining the strength levels of conventional AA2000-, AA6000-, AA5000- or AA7000-series alloys.
  • [0013]
    More specifically, it is an object of the present invention to provide an improved method for producing high damage tolerant (“HDT”) aluminium alloys with balanced properties with regard to fatigue crack growth resistance, toughness, corrosion resistance and strength. The HDT-properties should preferably be better than those of conventional manufactured AA6013-T6, 6056-T6 alloys and preferably better than AA2024-T3 or AA2524-T3 alloys.
  • [0014]
    More specifically, there is a general requirement for rolled AA6000-series aluminium alloys preferably within the range of AA6013 and AA6056-series aluminium alloys, when used for aerospace applications, that the fatigue crack growth rate (“FCGR”) should not be greater than a defined maximum. An FCGR which meets the requirements of high damage tolerant 2024-series alloy products is for example an FCGR below 0.001 mm/cycles at ΔK=20 MPa√m and 0.01 mm/cycles at ΔK=40 MPa√m.
  • [0015]
    It is yet a further object of the present invention to provide a rolled aluminium alloy product for use to construct structural parts in the aircraft industry as well as to provide an aircraft skin material produced from such alloy or to provide a vehicle component part.
  • [0016]
    The present invention solves one or more of the above mentioned objects.
  • [0017]
    In one aspect, the present invention provides a method for producing a high damage tolerant aluminium alloy rolled product a high toughness and an improved fatigue crack growth resistance, including the steps of
      • a.) casting an ingot having a composition selected from the group comprising of AA2000, AA5000, AA6000, and AA7000-series alloys;
      • b.) homogenising and/or pre-heating the ingot after casting;
      • c.) hot rolling the ingot into a hot rolled product and optionally cold rolling the hot rolled product into a cold rolled product, characterized in that the hot rolled product leaves the hot rolling mill at an hot-mill exit temperature (TExit) and cooling the hot rolled product from the TExit to 150 C. or lower with a controlled cooling cycle with a cooling rate falling within the range defined by:
        T(t)=50−(50−T Exit)e αt
        and wherein T(t) is the temperature ( C.) as function in time (hrs), t is the time (hours) and a is in the range of −0.090.05 (hrs−1).
  • BRIEF DESCRIPTION OF THE DRAWING
  • [0021]
    FIG. 1 is a typical cooling curve of an aluminium alloy cooled down after hot rolling using the method according this invention.
  • DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
  • [0022]
    In one aspect, the present invention provides a method for producing a high damage tolerant aluminium alloy having a high toughness and an improved fatigue crack growth resistance, comprising the steps of
      • a.) casting an ingot having a composition selected from the group consisting of AA2000, AA5000, AA6000, and AA7000-series alloys;
      • b.) homogenising and/or pre-heating the ingot after casting;
      • c.) hot rolling the ingot into a hot rolled product, and optionally further cold rolling the hot rolled product into a cold rolled product, characterized in that the hot rolled product leaves the hot rolling mill at an hot-mill exit temperature (TExit) and cooling the hot rolled product from the TExit to 150 C. with a controlled cooling cycle with a cooling rate falling within the range defined by:
        T(t)=50−(50−T Exit)e αt
        and wherein T(t) is the temperature ( C.) as function in time (expressed in hours), t is the time (expressed in hours) and a (expressed in hrs−1) is a parameter defining the cooling rate and is in the range of −0.090.05 (hrs−1), and more preferably in a range of −0.090.03 (hrs−1).
  • [0026]
    It has been found that below the temperature of 150 C. the cooling rate is no longer relevant to achieve one or more of the advantages found according to this invention.
  • [0027]
    While prior art techniques teach to a skilled person to cast and hot roll an ingot to obtain a plate or sheet product, wherein the ingot is optionally preheated or homogenised before hot rolling, the hot rolled product lost its elevated temperature fairly fast, thereby compromising the performance of the product. It has been found that by maintaining the hot rolled product at an elevated temperature for a predetermined time to subject it to a controlled cooling cycle the damage tolerance properties such as toughness and crack growth resistance of such a rolled product can be improved in accordance with the present invention.
  • [0028]
    Typical hot-mill exit temperatures in an industrial scale practice are in a range of 350 to 500 C. and are alloy dependent, for example for an AA6xxx the exit temperature will be at the higher end of this range of about 420 to 500 C., whereas for AA2xxx and AA7xxx-series alloys this would be at the lower end of this range of about 350 to 425 C.
  • [0029]
    A further cold rolling of the cooled hot rolled product in coil form is optional. The cold rolling can be straight or cross rolling. Further steps of inter-annealing before, during or after cold rolling are also optional.
  • [0030]
    Furthermore, it is possible to subject the hot rolled product to coiling to obtain a coiled form and thereby achieve a controlled cooling rate until the product is cooled down to room temperature. Then, it is possible to cut the coil into blanks which are then further cold rolled. The material which is produced by this inventive processing route exhibited a better property balance than those hot rolled products which were cut into blanks during or after hot rolling without coiling (standard plate route) or those products that were coiled after cold rolling (standard sheet route).
  • [0031]
    A second alternative for subjecting the hot rolled product to a controlled cooling cycle is the step of continuously moving the alloy through a furnace after hot rolling, wherein the furnace is adjustable to apply heat and/or coldness to the alloy while passing to its cold rolling station or coiling station.
  • [0032]
    In a further alternative the rolled product is first hot rolled to a desired gauge and then cooled to room temperature using conventional cooling. Thereafter the cooled hot rolled product is reheated to a hot-mill exit temperature and then allowed to cool to below 150 C. using the controlled cooling cycle according to the invention and followed by further processing.
  • [0033]
    Depending on whether sheets or plates are produced the hot rolled product is either fed to the furnace after hot rolling or coiled after hot rolling wherein the further processing is done on coils (sheet route). If the product is cut into plates during or after hot rolling the further processing is done on thereby produced plates.
  • [0034]
    The furnace is preferably adjustable to apply various amounts of heat close to the hot rolling station and other amounts of heat at a greater distance from the hot rolling station, depending on the cooling rate, thickness and other dimensions of the hot rolled product leaving the hot rolling station.
  • [0035]
    When the hot rolled product is subjected to the controlled cooling cycle by coiling it is possible to coil the alloy after hot rolling in a respective furnace, wherein said furnace is then also preferably adjustable to apply heat to control the cooling cycle.
  • [0036]
    In an embodiment the hot rolled product has a gauge in a range of up to 12 mm while leaving the hot rolling mill at the hot-mill exit temperature, and preferably in a range of 1 to 10 mm, and most preferably in the range of 4 to 8 mm.
  • [0037]
    Where the rolled product is being further subjected to a cold rolling operation, it is preferred that the total cold roll reduction is in a range of 40 to 70% to further optimise the mechanical properties. The final gauge of the rolled alloy product is preferably in a range of about 2 to 7 mm.
  • [0038]
    The method in accordance with the present invention may further include one or more of the following steps:
  • [0039]
    d.) solution heat treating of the hot rolled product after being subjected to the controlled cooling cycle or of the cold rolled product at a temperature and time sufficient to place into solid solution soluble constituents in the alloy;
  • [0040]
    e.) quenching the solution heat treated alloy product by one of spray quenching or immersion quenching in water or other quenching media;
  • [0041]
    f.) optionally stretching or compressing of the quenched alloy product or otherwise cold worked to relieve stresses, for example leveling of sheet products;
  • [0042]
    g.) optionally ageing the quenched and optionally stretched or compressed alloy product to achieve a desired temper, which is dependent of the alloy chemistry, but includes the tempers T3, T351, T6, T4, T74, T76, T751, T7451, T7651, T77, T79.
  • [0043]
    Furthermore, it is possible to anneal and/or reheat a hot rolled ingot after a first hot rolling operation and then again hot rolling the product to a final hot-rolled gauge followed a cooling according to the invention. It is furthermore possible to inter-anneal the hot rolled product before and/or during cold rolling. These techniques, which are known from prior art, can advantageously be used in a method according to the present invention.
  • [0044]
    The average cooling rate when using the controlled cooling cycle according to the invention is in a range of 12 to 20 C./hour.
  • [0045]
    In an embodiment of the present invention the cast ingot for the processing route of the method as disclosed herein, comprises the following composition (in weight. %): Si 0.6-1.3, Cu 0.04-1.1, Mn 0.1-0.9, Mg 0.4-1.3, Fe 0.01-0.3, Zr<0.25, Cr<0.25, Zn<0.6, Ti<0.15, V<0.25, Hf<0.25, other elements, in particular impurities, each less than 0.05 and less than 0.20 in total, balance aluminium. More preferably, the cast ingot for the processing route of the method as disclosed herein, comprises alloys within the compositional range of AA6013 or AA6056.
  • [0046]
    In the present specification, elemental percentages are in weight percent unless otherwise indicated.
  • [0047]
    Another embodiment of the present invention uses an ingot comprising the following composition (in weight. %): Cu 3.8-5.2, Mg 0.2-1.6, Cr<0.25, Zr<0.25, and preferably 0.06-0.18, Mn<0.50 and Mn:>0, and preferably >0.15, Fe<0.15, Si≦0.15, and Mn-containing dispersoids, and incidental elements and impurities, each less than 0.05 and less than 0.15 in total and the balance aluminium, and preferably wherein the Mn-containing dispersoids are at least partially replaced by Zr-containing dispersoids.
  • [0048]
    According to another embodiment of the present invention the method uses an ingot comprising the following composition (in weight. %): Zn 5.0-9.5, Cu 1.0-3.0, Mg 1.0-3.0, Mn<0.35, Zr<0.25, and preferably 0.06-0.16, Cr<0.25, Fe<0.25, Si<0.25, Sc<0.35, Ti<0.10, Hf and/or V<0.25, other elements, typically impurities, each less than 0.05 and less than 0.15 in total, balance aluminium. Typical examples are alloys within the range of AA7040, AA7050 and AA7x75.
  • [0049]
    According to another aspect of the present invention an aluminium alloy sheet or plate product is disclosed which has high toughness and an improved fatigue crack growth resistance and which is made of an alloy product which is produced according to a method which has been described above and which will be described in more details herein below. More specifically, the present invention is most suitable to produce a rolled alloy sheet product which is a structural member of an aircraft or an automobile. Such rolled alloy sheet product could be used for example as a fuselage skin of an aircraft or a vehicle component part.
  • [0050]
    The foregoing and other features and advantages of the method and alloy products according to the present invention will become readily apparent from the following description of preferred embodiments, and figure, in which
  • [0051]
    FIG. 1 is a typical cooling curve of an aluminium alloy cooled down after hot rolling using the method according this invention.
  • EXAMPLES Example 1
  • [0052]
    In a first preferred embodiment of the present invention, two conventional alloys (AA6013 and AA6056) were cast and processed to a sheet product. Here, two processing variants were used:
  • [0053]
    Route 1. A normal processing route by lab-casting ingots of conventional AA6013 and AA6056-alloy compositions was used. Blocks of 8080100 mm were sawn, homogenised, preheated and hot rolled to 4.5 mm sheet. After hot rolling the hot rolled products were conventionally cooled to ambient temperature by allowing the sheet to cool to ambient air to room temperature, fed to the cold rolling station, cold rolled to 2 mm and heat treated for 20 min at 550 C., thereafter quenched and aged to a T6-temper for 4 hours at 190 C.
  • [0054]
    Route 2. Ingots of conventional AA6013 and AA6056-alloy compositions were lab-cast and sawn to a size of 8080100 mm. These blocks were homogenised, pre-heated and hot rolled to 4.5 mm. A simulation of the hot coiling in an industrial scale was incorporated by giving the hot rolled product a similar temperature history as what a coil in full-scale production would have had. The other processing steps were kept similar as with Route 1. After cold rolling the cold rolled product was heat treated at 550 C. for 20 min, quenched and consequently aged to a T6-temper at 190 C. for 4 hours. The results are given in Table 1.
    TABLE 1
    Overview of strength (Rp, Rm) using small Euronorm,
    notch toughness (TS/Rp), intergranular corrosion (IGC) in
    depths and type of 6013 and 6056-alloy compositions processed
    in accordance with route 1 and route 2 as described above, at
    two different hot rolling exit temperature settings.
    Hot rolling
    exit TS/ IGC
    No. temperature Rp Rm Rp IGC Depth
    Alloy Route ( C.) (MPa) (MPa) (μm) type
    1 6013 2 490 354 390 1.75 101 P(i)
    2 1 490 344 381 1.72 118 I
    3 2 450 345 385 1.73 97 I
    4 1 450 337 377 1.63 108 I
    5 6056 2 490 347 386 1.85 112 I
    6 1 490 349 388 1.79 177 I+
    7 2 450 328 372 1.75 103 P(i)
    8 1 450 331 375 1.70 143 I
  • [0055]
    It can be seen from Table 1 that the rolled products exhibited better notch toughness at higher hot rolling temperatures by maintaining good tensile yield strength and ultimate tensile strength levels. Furthermore, there is an improvement in intergranular corrosion so that further testing has been done with regard to the fatigue crack growth resistance (Table 2).
    TABLE 2
    Overview of the fatigue crack growth resistance (“FCGR”)
    for examples No. 1, 2 and 5, 6 of Table 1 (higher hot rolling temperatures)
    at two different ΔK-levels.
    Hot rolling
    exit
    temperature FCGR FCGR
    Alloy Route ( C.) ΔK = 30 MPa√m ΔK = 40 MPa√m
    6013 2 490 1.83E−03 5.26E−03
    1 490 1.84E−03 8.88E−03
    6056 2 490 1.62E−03 3.32E−03
    1 490 1.66E−03 4.89E−03
  • [0056]
    While the fatigue crack growth resistance of the inventive products is nearly identical to the fatigue crack growth resistance of a product produced in accordance with the standard processing route at lower ΔK values, the fatigue crack growth resistance is improved at higher ΔK values.
  • [0057]
    In accordance with another preferred embodiment of the present invention a low copper high damage tolerant AA6000-series alloy composition has been produced in a full-scale production trial. The composition is given in Table 3.
    TABLE 3
    Composition of high damage tolerant AA6000-series sheet product
    in weight-%, balance aluminium and inevitable impurities.
    Si Fe Cu Mg Mn Zn
    1.14 0.18 0.32 0.70 0.71 0.08
  • [0058]
    The alloy has been processed to a sheet product with a hot rolling gauge of 4.5 mm. The following three processing variants were then applied:
      • Route 1. A standard processing route. (No coiling step after hot rolling).
      • Route 2. The inventive processing route with coiling after hot rolling and hot rolling and cold rolling in the same direction.
      • Route 3. The inventive processing route with coiling after hot rolling and hot rolling and cold rolling in dissimilar directions (cross rolling).
  • [0062]
    All three above mentioned processing variants were applied to the following general processing route:
      • a. DC-casting of ingots of an alloy composition in accordance with Table 3.
      • b. Homogenising the cast ingots.
      • c. Preheating the homogenised ingots for 6 hours at 510 C. and subsequently hot rolling the pre-heated ingots resulting that the exit temperature is about 450 C. at a gauge of 4.5 mm.
      • d1. No coiling (=Route 1).
      • d2. Coiling, cooling and cutting into plates (=Route 2).
      • d3. Coiling, cooling and cutting into plates (=Route 3).
      • e1. Cold rolling to a final gauge of 2 mm (Route 1).
      • e2. Cold rolling in same direction as hot rolling to a final gauge of 2 mm (Route 2).
      • e3. Cold rolling in a dissimilar direction as hot rolling (cross rolling) to a final gauge of 2 mm (Route 3).
      • f. Heat treatment of 550 C. for 2 hours .
      • g. Stretching the cold rolled product by 1.5 to 2.5%.
  • [0074]
    h. Aging to a T6-temper condition at 190 C. for 4 hours.
    TABLE 4
    Overview of strength (Rp, Rm) using small Euronorm,
    notch toughness (TS/Rp) and intergranular corrosion (IGC) of
    a finished product with an alloy in accordance with Table 3 and
    using three processing Routes 1, 2 and 3 as described above.
    TS/Rp
    Rp Rm Rp Rm
    (MPa) (MPa) (MPa) (MPa) T-L IGC
    Route L-direction LT-direction direction Depth (μm)
    1 334 345 322 344 1.51 62
    2 329 344 321 341 1.60 48
    3 333 344 326 347 1.58 49
  • [0075]
    While the strength levels could be maintained the rolled products which were produced in accordance with processing Routes 2 and 3 showed a better notch toughness and a better intergranular corrosion performance. Hence, the fatigue crack growth resistance was measured also and is given in Tables 5 and 6.
    TABLE 5
    Fatigue crack growth resistance in mm/cycle for 5 different ΔK-values
    for the products produced in accordance with the processing Routes
    1, 2 and 3 as described above.
    ΔK
    (MPa√m) Route 1 Route 2 Route 3
    10 1.52E−04 1.71E−04 1.78E−04
    20 1.43E−03 8.58E−04 1.26E−03
    30 6.14E−03 3.38E−03 5.17E−03
    40 1.70E−02 9.54E−03
    50 3.73E−02 1.85E−02
  • [0076]
    TABLE 6
    Values of Table 5, relative to standard (Route 1).
    ΔK
    (MPa√m) Route 1 Route 2 Route 3
    10 100% 113%  117% 
    20 100% 60% 88%
    30 100% 55% 84%
    40 100% 56%
    50 100% 50%
  • [0077]
    The above identified examples show that the damage tolerance properties of sheet or plate products can be improved by using the inventive method and that the fatigue crack growth resistance can especially be improved for higher ΔK-values.
  • Example 2
  • [0078]
    FIG. 1 shows a typical continuous cooling down curve for an aluminium AA7050 alloy when cooled down from a hot-mill exit temperature of 440 C. to a temperature below 150 C., whereby the metal sheet has a gauge of 4.5 mm and being immediately coiled when leaving the hot-mill in accordance with an embodiment of the method of this invention. The width of the coil was 1.4 meter. The temperatures of the coil as function of time is also given in Table 7 for the hottest spot of a coil (being the centre, and indicated as HotSpt in FIG. 1) and the coldest spot (being the edge of a coil, and indicated as ColdSpt in FIG. 1)). Table 7 provides also the temperatures in case a coil having a width of 2.8 meter.
  • [0079]
    For the shown cooling curve in FIG. 1 the α is about −0.084 hrs−1.
  • [0080]
    In case a sheet with a gauge of about 4.0 to 4.5 mm was allowed to cool down from the hot-mill exit temperature to below 150 C. using conventional cooling practice, viz. leaving the plate to cool in normal stationary air after exit of the hot mill without any coiling operation or the like, the α would typically be in the range of −0.5 to −2 hrs−1, and resulting in the such a plate would cool down from the hot mill exit temperature to a temperature of 150 C. or less in a time period of less than 3 hours.
  • [0081]
    The controlled cooling cycle follows the equation set out above and in the claims, and the average cooling rate of the coiled product form from 440 to 150 C. is within the range of 12 to 20 C./hour.
    TABLE 7
    Coil temperatures as function of the time when cooled in
    accordance with the invention for an AA7050 alloy having a
    gauge when being coiled of 4.5 mm.
    Coil width 1.4 meter Coil width 2.8 meter
    Time Coldest spot Hottest spot Coldest spot Hottest spot
    (hours) ( C.) ( C.) ( C.) ( C.)
    0 431 440 431 440
    2 344 372 349 385
    6 249 266 262 287
    10 187 199 204 222
    12 165 175 182 197
    14 146 150 163 176
    16 130 137 148 159
    18 117 123 134 144
  • [0082]
    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.
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Classifications
U.S. Classification148/552
International ClassificationC22F1/04
Cooperative ClassificationC22F1/04
European ClassificationC22F1/04
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