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
  • C22F1/047—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 magnesium as the next major constituent
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
  • B22D21/00—Casting non-ferrous metals or metallic compounds so far as their metallurgical properties are of importance for the casting procedure; Selection of compositions therefor
  • B22D21/02—Casting exceedingly oxidisable non-ferrous metals, e.g. in inert atmosphere
  • B22D21/04—Casting aluminium or magnesium
• • 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
  • C22C21/06—Alloys based on aluminium with magnesium as the next major constituent
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
  • B21B—ROLLING OF METAL
  • B21B3/00—Rolling materials of special alloys so far as the composition of the alloy requires or permits special rolling methods or sequences ; Rolling of aluminium, copper, zinc or other non-ferrous metals
  • B21B2003/001—Aluminium or its alloys
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
  • B21B—ROLLING OF METAL
  • B21B3/00—Rolling materials of special alloys so far as the composition of the alloy requires or permits special rolling methods or sequences ; Rolling of aluminium, copper, zinc or other non-ferrous metals
  • B21B2003/006—Powder metal alloys

Definitions

• the invention relates to a method of manufacturing an AIMgSc-series alumin- ium alloy product.
• the resultant product provides an improved corrosion resistance.
• the aluminium alloy product can be in the form of a rolled product (sheet or plate), an extruded product, a forged product or a powder-metallurgy product.
• AIMg-series aluminium alloys which may optionally or mandatorily have Sc as alloying element are known in the art, for example from the following documents:
• US-6, 695, 935-B1 discloses an alloy in the form of a rolled or extruded product and having the composition of: 3.5-6.0% Mg, 0.4-1 .2% Mn, 0.4- 1 .5% Zn, up to 0.25% Zr, up to 0.3% Cr, up to 0.2% Ti, up to 0.5% Fe, up to 0.5% Si, up to 0.4% Cu, one or more selected from the group of (0.005-0.1 % Bi, 0.005- 0.1 % Pb, 0.01 -0.1 % Sn, 0.01 -0.5% Ag, 0.01 -0.5% Sc, 0.01 -0.5% Li, 0.01 -0.3% V, 0.01 -0.3% Ce, 0.01 -0.3% Y, 0.01 -0.3% Ni), others each 0.05%, total 0.15%, balance aluminium.
• the alloy is said to offer improved long-term corrosion resistance in both soft temper (O-temper) and work- or strain-hardened temper (H-temper)
• EP-1917373-B1 discloses an aluminium alloy product having 3.5-6.0% Mg, 0.4-1 .2% Mn, up to 0.5% Fe, up to 0.5% Si, up to 0.15% Cu, 0.05-0.25% Zr, 0.03-0.15% Cr, 0.03-0.2% Ti, 0.1 -0.3% Sc, up to 1 .7% Zn, up to 0.4% Ag, up to 0.4% Li, optionally one or more dispersoid-forming elements selected from the group consisting of (Er, Y, Hf, V) each up to 0.5%, impurities or incidental elements each ⁇ 0.05%, total ⁇ 0.15%, and the balance aluminium.
• RU-2280705-C1 discloses an alloy containing 4.2-6.5% Mg, 0.5-1 .2% Mn, up to 0.2% Zn, up to 0.2% Cr, up to 0.15% Ti, up to 0.25% Si, up to 0.30% Fe, up to 0.1 % Cu, 0.05-0.3% Zr, at least one element selected from the group consisting of
• the aluminium alloy is said to have improved ballistic proper ties.
• the Zn and Si content are reduced to improve the weldability and to increase the corrosion resistance of the aluminium alloy.
• RU-2268319-C1 discloses an alloy containing 5.5-6.5% Mg, 0.10-0.20% Sc,
• WO-01/12869-A (Kaiser Aluminum) discloses an alloy comprising 4.0-8.0% Mg, 0.05-0.6% Sc, 0.1 -0.8% Mn, 0.5-2.0% Cu or Zn, 0.05-0.20% Hf or Zr, and the balance aluminium and incidental impurities.
• WO-98/35068 discloses an aluminium alloy product comprising 3-7% Mg, 0.03-0.2% Zr, 0.2-12% Mn, up to 0.15% Si, and 0.05-0.5% of a dispersoid- forming element selected from the group consisting of (Sc, Er, Y, Ga, Ho, Hf), the balance being aluminium and incidental elements and impurities, and wherein the aluminium alloy product is preferably Zn-free and Li-free.
• WO-2018/073533-A1 discloses a method for producing a hot- worked product, in particular a sheet product having a thickness of less than 12 mm, made of an aluminium alloy composed, of Mg: 3.8-4.2%, Mn: 0.3-0.8%, Sc: 0.1 - 0.3%, Zn: 0.1 -0.4%, Ti: 0.01 -0.05%, Zr: 0.07-0.15%, Cr: ⁇ 0.01 %, Fe: ⁇ 0.15%, Si
• the homogenisation is carried out at a temperature of between 370°C and 450°C, for between 2 and 50 hours, such that the equivalent time at 400°C is between 5 and 100 hours, and the hot deformation is carried out at an initial temperature of between 350°C and 450°C.
• the products are said to be ad vantageous as they offer a better compromise in terms of mechanical strength, toughness and hot-formability.
• aluminium alloy and temper designations refer to the Aluminium Association designations in Aluminum Standards and Data and the Registration Records, as published by the Aluminium Association in 2018 and are well known to the persons skilled in the art.
• up to 0.10% Zn may include an alloy having no Zn.
• the present invention providing a method of manufacturing an AIMgSc-series alumin ium alloy product, the method comprising the step of cooling said AIMgSc-series aluminium alloy product from a final annealing temperature or a set annealing tem perature to below 150°C, wherein the cooling down in a first temperature range of about 250°C to about 200°C is at an equivalent time of more than 4 hours, preferably more than 6.5 hours and more preferably more than 26 hours, and wherein the cool ing down in a second temperature range from about 200°C to about 150°C is at an equivalent time of more than 0.2 hours, preferably more than 0.4 hours and more preferably more than 0.8 hours, and wherein the equivalent time ( t( eq ) ) is defined as
• T in degrees Kelvin
• T ref in degrees Kelvin
• the method according to the invention provides AIMgSc-series aluminium al loy products have a good strength, preferably Rp> 200 MPa, in combination with a good corrosion resistance, in particular a good exfoliation corrosion resistance in combination with a good intergranular corrosion resistance.
• the cooling rates ap plied are economical feasible in an industrial environment of manufacturing the AIMgSc-series aluminium alloy products.
• the AIMgSc-series aluminium alloy product manufactured in accordance with the invention are resistant to exfoliation corrosion.
• "Resistant to exfoliation corro sion” means that the aluminium alloy product passes ASTM Standard G66-99 (2013), entitled “Standard Test Method for Visual Assessment of Exfoliation Corro sion Susceptibility of 5XXX Series Aluminium Alloys (ASSET Test)”.
• N, PA, PB, PC and PD indicate the results of the ASSET test, N representing the best result.
• the aluminium alloy products manufactured in accordance with the invention achieve before and after being sensitised a PB result or better.
• the AIMgSc-series aluminium alloy product manufactured in accordance with the invention are also resistant to intergranular corrosion.
• "Resistant to intergranular corrosion” means that, both before and after the AIMgSc-series aluminium alloy has been sensitized, the aluminium alloy product passes ASTM Standard G67-13, enti- tied "Standard Test Method for Determining the Susceptibility to Intergranular Cor rosion of 5XXX Series Aluminium Alloys by Mass Loss After Exposure to Nitric Acid” (NAMLT Test)”. If the measured mass loss per ASTM G67-13 is not greater than 15 mg/cm 2 , then the sample is considered not susceptible to intergranular corrosion.
• the sample is considered susceptible to intergranular corrosion. If the measured mass loss is between 15 mg/cm 2 and 25 mg/cm 2 , then further checks are conducted by microscopy to determine the type and depth of attack, whereupon one skilled in the art may determine whether there is intergranular corrosion via the microscopy results.
• the AIMgSc-series aluminium alloy products manufactured in accordance with the invention achieve a measured mass loss per ASTM G67-13 not greater than 15 mg/cm 2 , both before and after being sensitized.
• the measured mass loss is not greater than 12 mg/cm 2 , and more preferably not greater than 9 mg/cm 2 .
• “Sensitized” means that the AIMgSc aluminium alloy product has been annealed to a condition representative of at least 20 years of service life.
• the aluminium alloy product may be continu ously exposed to elevated temperature for several days (e.g. , a temperature in the range 100°C to 120°C for a period of about 7 days/168 hours).
• the AIMgSc-series aluminium alloy product may realize resistance to stress corrosion cracking and intergranular corrosion as a result of, at least in part, due to the absence of a continuous film of b-phase particles at the grain boundaries.
• Alu minium alloy products are polycrystalline.
• a “grain” is a crystal of the polycrystalline structure of the aluminium alloy, and “grain boundaries” are the boundaries that connect the grains of the polycrystalline structure of the aluminium alloy, "b-phase” is A Mg2, and "a continuous film of b-phase” means that a continuous volume of b- phase particles is present at the majority of the grain boundaries.
• the continuity of the b-phase may be determined, for example, via microscopy at a suitable resolution (for example at a magnification of at least 200X).
• a very fast cooling rate for example by means of quenching from the final annealing temperature to below 150°C has an adverse effect on the corrosion resistance of the AIMgSc-series alu minium alloy product, in particular on the corrosion resistance tested according to the NAMLT-test after being sensitized.
• a slower cooling rate results in an enhanced intergranular corrosion resistance.
• the equivalent time should be longer than 4 hours, preferably longer than 6.5 hours, more preferably longer than 26 hours and in the second temperature range of about 200°C to about 150°C, the equivalent time should be longer than 0.2 hours, prefer ably longer than 0.4 hours, more preferably longer than 0.8 hours.
• the relative slow cooling rate is important for the precipitation of discontinuous b-phase particles at the grain boundaries and to avoid the precipitation of a continuous film of b-phase particles, both after cooling to ambient temperature and after the Al-Mg-Sc alloy has been sensitized.
• the cooling down is preferably performed in a continuous mode such that the metal temperature is continuously reduced over time.
• the cooling down from the final annealing temperature to the first temperature range starting at about 250°C is not critical. When employing the method according to the invention on an industrial scale it can be useful or convenient to apply about the same cooling rate as for the first temperature range.
• the further cooling down from about 150°C to below about 85°C is less critical and can be done at a higher cooling rate to minimize the coarsening of precipitates.
• the cooling rate for the cooling down from about 85°C to ambient temperature is not critical.
• the AIMgSc-series aluminium alloy product is in a form se lected from the group consisting of a rolled product (sheet or plate), an extruded product, a forged product and a powder-metallurgy product. In a further embodiment any of these products are in a welded condition or in a formed condition.
• the AIMgSc-series aluminium alloy product is in the form of a rolled product.
• the rolled product has been welded or formed.
• the thickness of the AIMgSc-series aluminium alloy rolled product is at most 25.4 mm (1 inches), and preferably at most 12 mm (0.47 inches), and more preferably 6 mm (0.24 inches), and most preferably 4.5 mm (0.18 inches).
• the thickness of the AIMgSc-series aluminium rolled product is at least 1 .2 mm (0.047 inches).
• the AIMgSc-series aluminium alloy product is in the form of an extruded product.
• the AIMgSc-series aluminium alloy rolled product is cast, subsequently rolled to final gauge and annealed.
• the alloy can be provided as an ingot or slab for fabrication into rolling feedstock using casting techniques regular in the art for cast products, e.g. Direct Chill DC-casting, and preferably having an ingot thickness in a range of about 220 mm or more, e.g. 400 mm, 500 mm or 600 mm.
• thin gauge slabs resulting from continuous casting e.g. belt casters or roll casters, also may be used, and having a thickness of up to about 40 mm.
• the thick as-cast ingot is commonly scalped to remove segregation zones near the cast surface of the ingot.
• the rolling process applied comprises hot rolling, and optionally comprises hot rolling followed by cold rolling to final gauge, and where applicable an intermediate annealing is applied either before the cold rolling operation or during the cold rolling operation at an intermediate cold rolling gauge.
• the AIMgSc-series aluminium alloy product Prior to hot rolling the AIMgSc-series aluminium alloy product is homogenised or pre-heated for up to about 50 hours, preferably up to about 24 hours, at a tem perature in a range of about 320°C to 470°C, preferably of about 320°C to 450°C.
• the hot rolled product re ceives a very mild cold rolling step (skin rolling or skin pass) with a reduction of less than about 1 %, preferably less than about 0.5%, to improve the flatness of the rolled product.
• the hot rolled product can be stretched. This stretching step can be carried out with a reduction of up to 3%, preferably between about 0.5% to 1 %, to improve the flatness of the hot rolled product.
• the final annealing or annealing heat-treatment at final gauge is to recover the microstructure and is typically performed at a set annealing temperature in the range of 250°C to 400°C, preferably in the range of 260°C to 375°C, and more preferably in a range of about 280°C to 350°C, for a time in a range of about 0.5 hours to 20 hours, and preferably of about 0.5 hours to 10 hours.
• the method of cooling the aluminium alloy product is applied immediately following a high-temperature forming operation for shaping the AIMgSc-series aluminium alloy product into a single- or double-curved shape product.
• the high-temperature forming operation is performed at the final annealing temperature in the range of 180°C to 500°C, preferably in the range of 250°C to 400°C, more preferably in a range of 260°C to 375°C, and most preferably in a range of 280°C to 350°C, for example at about 300°C or at about 325°C.
• a particular preferred embodiment of such a high-temperature forming operation at the final annealing temperature is by means of a creep forming operation or a relax ation forming operation.
• Creep forming is a process or operation of restraining a component to a specific shape during heat treatment, allowing the component to relieve stresses and creep to contour, for example fuselage shells with a double curvature. This creep forming process is for example explained in the paper by S. Jambu et al. , “Creep forming of Al Mg Sc alloys for aeronautic and space applica tions”, published at the occasion of the ICAS-2002 congress.
• a rolled AIMgSc-series aluminium alloy product is being employed.
• the AIMgSc-series alu minium alloy product can be provided in an annealed condition also manufactured by the method according to this invention.
• extruded AIMgSc-series aluminium alloy products are being employed, for example as extruded stringers as part of a fuselage panel.
• the method of cooling the aluminium alloy product is applied on a welded product or panel incorporating the AIMgSc-series aluminium alloy product immediately following a post-weld heat-treatment to recover some strength in particular by reprecipitating AlScZr dispersoids.
• the post-weld heat-treatment is performed at a temperature similar as for the final anneal heat- treatment and is in the range of 250°C to 400°C, preferably in the range of 260°C to 375°C, and more preferably in a range of about 260°C to 350°C, for a time in a range of about 0.5 hours to 20 hours, and preferably of about 0.5 hours to 10 hours.
• the method of cooling the aluminium alloy product is applied on a cold-formed and shaped product from the AIMgSc-series aluminium alloy whereby an annealing heat-treatment is performed to reduce resid ual stress in the cold-formed and shaped product or to recover certain engineering properties such as elongation or damage tolerance.
• Such an annealing heat-treat ment is performed at a temperature similar as for the final anneal heat-treatment and is in the range of 250°C to 400°C, preferably in the range of 260°C to 375°C, and more preferably in a range of about 280°C to 350°C, for a time in a range of about 0.5 hours to 20 hours, and preferably of about 0.5 hours to 10 hours.
• the AIMgSc-series aluminium alloy has a composition com prising, in wt.%:
• Mg 3.0% to 6.0%, preferably 3.2%-4.8%, more preferably 3.5% to 4.5%
• Cu up to 0.2%, preferably up to 0.1 %, more preferably up to 0.05%,
• Zn up to 1 .5%, preferably up to 0.8%, more preferably 0.1 % to 0.8%,
• Fe up to 0.4%, preferably up to 0.3%, more preferably up to 0.20%,
• Si up to 0.3%, preferably up to 0.2%, more preferably up to 0.1 %, impurities and balance aluminium. Typically, such impurities are present each ⁇ 0.05% and total ⁇ 0.15%.
• the Mg is the main alloying element in the AIMgSc-series alloys, and for the method according to this invention it should be in a range of 3.0% to 6.0%.
• a pre ferred lower-limit for the Mg-content is about 3.2%, more preferably about 3.8%.
• a preferred upper-limit for the Mg-content is about 4.8%. In an embodiment the upper- limit for the Mg-content is about 4.5%.
• Sc is another important alloying element and should be present in a range of 0.02% to 0.5%.
• a preferred lower-limit for the Sc-content is about 0.05%, and more preferably about 0.1 %.
• the Sc-content is up to about 0.4%, and preferably up to about 0.3%.
• Mn may be added to the AIMgSc-series aluminium alloys and may be present in a range of up to about 1 %. In an embodiment the Mn-content is in a range of about 0.3% to 1 %, and preferably about 0.3% to 0.8%.
• Zr in a range of up to about 0.3%.
• the Zr is present in a range of 0.05% to 0.30%, pref erably in a range of about 0.05% to 0.25%, and more preferably is present in a range of about 0.07% to 0.15%.
• Cr can be present in a range of up to about 0.3%. When purposively added it is preferably in a range of about 0.02% to 0.3%, and more preferably in a range of about 0.05% to 0.15%. In an embodiment there is no purposive addition of Cr and it can be present up to 0.05%, and preferably is kept below 0.02%.
• Ti may be added up to about 0.2% to the AIMgSc alloy as strengthening ele ment or for improving the corrosion resistance or for grain refiner purposes.
• a pre ferred addition of Ti is in a range of about 0.01 % to 0.2%, and preferably in a range of about 0.01 % to 0.10%.
• the combined addition is at least 0.15% to achieve sufficient strength, and preferably does not exceed 0.30% to avoid the formation of too large precipi tates.
• the combined addition of Zr+Ti is at least 0.08%, and preferably does not exceed 0.25%, and wherein Cr is up to 0.02%, and preferably only up to 0.01 %.
• Zinc (Zn) in a range of up to 1.5% can be purposively added to further enhance the strength in the aluminium alloy product.
• a preferred lower limit for the purposive Zn addition would be 0.1 %.
• a preferred upper limit would be about 0.8%, and more preferably 0.5%, to provide a balance in strength and corrosion resistance.
• the Zn is tolerable impurity element and it can be present up to 0.15%, and preferably up to 0.10%.
• Cu can be present in the AIMgSc-alloy as strengthening element in a range up to about 2%. However, in applications of the product where the corrosion resistance is a very critical engineering property, it is preferred to maintain the Cu at a low level of about 0.2% or less, and preferably at a level of about 0.1 % or less, and more preferably at a level of 0.05% or less. In an embodiment the Cu-content is 0.01 % or less.
• Fe is a regular impurity in aluminium alloys and can be tolerated up to about 0.4%. Preferably it is kept to a level of up to about 0.3%, and more preferably up to about 0.20%.
• Si is also a regular impurity in aluminium alloys and can be tolerated up to about 0.3%. Preferably it is kept to a level of up to 0.2%, and more preferably up to 0.10% .
• the AIMgSc-series aluminium alloy has a composition con sisting of, in wt.%: Mg 3.0% to 6.0%, Sc 0.02% to 0.5%, Mn up to 1 %, Zr up to 0.3%, Cr up to 0.3%, Ti up to 0.2%, Cu up to 0.2%, Zn up to 1 .5%, Fe up to 0.4%, Si up to 0.3%, balance aluminium and impurities each ⁇ 0.05% and total ⁇ 0.15%, and with preferred narrower compositional ranges as herein described and claimed.
• the method can be employed to a wide range of AIMgSc-series aluminium alloys.
• the aluminium alloy product is a single or double curved panel, in particular a single or double curved aircraft fuselage panel.
• Sheet products of 4.5 mm have been manufactured on an industrial scale com prising the steps of DC-casting of a rolling ingot, scalping, milling, preheating to hot rolling temperature between 400°C and 450°C, hot rolling, cold rolling to 4.5 mm and with intermediate annealing during the cold rolling operation, and final annealing at a set temperature of 325°C (598K) for 2 hours and followed by different controlled cooling rates according to Table 1 and whereby specimen A, B and C are according to the invention, and specimen D is comparative.
• the AIMgSc aluminium alloy cast has the following composition, in wt.%, 4.0% Mg, 0.55% Mn, 0.2% Sc, 0.3% Zn, 0.1 % Zr, 0.07% Cr, 0.07% Ti, 0.02% Si, 0.02% Fe, 0.006% Cu, balance aluminium and inevitable impurities.
• Table 1 lists the measured mass loss per ASTM G67-13 for each specimen having different cooling regimes from the final annealing temperature after sensitis ing at 120°C for 168 hours. Table 1.
• the AIMgSc-series aluminium alloy rolled product manufactured in accord ance with the invention are resistant to intergranular corrosion.
• "Resistant to inter- granular corrosion” means that, both before and after the AIMgSc-series aluminium alloy has been sensitized, the aluminium alloy product passes ASTM Standard G67- 13, (NAMLT Test)”. All sensitized specimen had a PA performance, and all non- sensitized specimen had also a PA performance.
• the method according to the invention results in an aluminium alloy prod uct having a good intergranular corrosion resistance in combination with a good exfoliation corrosion resistance.
• Similar corrosion performance of the aluminium alloy product will be achieved in the cooling down from a high-temperature forming operation performed at the final annealing temperature, for example a creep forming operation performed at 310°C or 325°C.

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