Patent Application
20210381090
The invention relates to a method of producing an integrated monolithic aluminum alloy structure, and can have a complex configuration that is machined to near-net-shape out of a plate material. More specifically, the invention relates to a method of producing an integrated monolithic aluminum alloy structure made from a 7xxx-series alloy, and can have a complex configuration that is machined to near-net-shape out of a plate material. The invention relates also to an integrated monolithic aluminum alloy structure produced by the method of this invention and to several intermediate semi-finished products obtained by this method.
U.S. Pat. No. 7,610,669-B2 (Aleris) discloses a method for producing an integrated monolithic aluminum structure, in particular an aeronautical member, comprising the steps of:
(a) providing an aluminum alloy plate with a predetermined thickness, the plate having been stretched after quenching and having been brought to a first temper selected from the group consisting of T4, T73, T74 and T76, wherein the aluminum alloy plate is produced from a AA7xxx-series aluminum alloy having a composition consisting of, in wt. %: 5.0-8.5% Zn, 1.0-2.6% Cu, 1.0-2.9% Mg, <0.3% Fe, <0.3% Si, optionally one or more elements selected from the group of Cr, Zr, Mn, V, Hf, Ti, the total of the optional elements not exceeding 0.6%, incidental impurities and the balance aluminum,
(b) shaping the alloy plate by means of bending to obtain a predetermined shaped structure having a pre-machining thickness in the range of 10 to 220 mm, the alloy plate in the first temper selected from the group consisting of T4, T73, T74 and T76 to form the shaped structure having a built-in radius,
(c) heat-treating the shaped structure, wherein the heat-treating comprises artificially aging the shaped structure to a second temper selected from the group consisting of T6, T79, T78, T77, T76, T74, T73 or T8,
(d) machining the shaped structure to obtain an integrated monolithic aluminum structure as the aeronautical member for an aircraft, wherein the machining of the shaped structure occurs after the artificial ageing.
It is suggested that the disclosed method can be applied also to AA5xxx, AA6xxx and AA2xxx-series aluminum alloys.
Patent document US-2018/0230583-A1 discloses a method of forming a tubular vehicle body reinforcement, comprising providing a seam welded or extruded 7xxx aluminum tube, solution heat-treating by heating tube to at least 450° C., quenching the tube to less than 300° C. at a minimum rate of 300° C./s with no more than a 20 second delay between the heating and the quenching, preferably a pre-bending and a pre-forming operation to form the tube along its length to a desired shape, and hydroforming the tube within 8 hours of quenching, trimming and artificially ageing of the tube to provide a yield strength of more than 470 MPa. The tube may have an outer diameter of less than 5 inches and a wall thickness greater than 1.5 mm and less than 4 mm.
There is a demand for forming integrated monolithic aluminum structures of more complex configuration from a thick plate product.
As will be appreciated herein, except as otherwise indicated, aluminum alloy designations and temper designations refer to the Aluminum Association designations in Aluminum Standards and Data and the Registration Records, as published by the Aluminum Association in 2018 and are well known to the person skilled in the art. The temper designations are laid down in European standard EN515.
For any description of alloy compositions or preferred alloy compositions, all references to percentages are by weight percent unless otherwise indicated.
As used herein, the term “about” when used to describe a compositional range or amount of an alloying addition means that the actual amount of the alloying addition may vary from the nominal intended amount due to factors such as standard processing variations as understood by those skilled in the art.
The term “up to” and “up to about”, as employed herein, explicitly includes, but is not limited to, the possibility of zero weight-percent of the particular alloying component to which it refers. For example, up to 0.5% Ag may include an aluminum alloy having no Ag.
“Monolithic” is a term known in the art meaning comprising a substantially single unit which may be a single piece formed or created without joint or seams and comprising a substantially uniform whole.
It is an object of the invention to provide a method of producing an integrated monolithic aluminum alloy structure of complex configuration that is machined to near-net-shape.
It is an object of the invention to provide a method of producing an integrated monolithic 7xxx-series aluminum alloy structure of complex configuration that is machined to near-net-shape out of thick gauge plate material.
These and other objects and further advantages are met or exceeded by the present invention providing a method of producing an integrated monolithic aluminum structure, the method comprising the process steps of,
providing a 7xxx-series aluminum alloy plate with a predetermined thickness of at least 10 mm (0.4 inches), and wherein the aluminum alloy plate has been rolled, solution heat treated, cooled and stretched;
heat-treating the aluminum alloy plate in a first artificial ageing step of a plurality of ageing steps required to achieve a final temper state;
optionally, either before or after the first artificial ageing step, a pre-machining operation of the aluminum alloy plate to an intermediate machined structure;
high-energy hydroforming of the artificial aged aluminum alloy plate or the first artificial aged intermediate machined structure against a forming surface of a rigid die having a contour at least substantially in accordance with a desired curvature of the integrated monolithic aluminum structure, the high energy forming causing the aluminum alloy plate or the aged intermediate machined structure to substantially conform to the contour of the forming surface to at least one of a uniaxial curvature and a biaxial curvature;
heat-treating of the integrated monolithic aluminum structure through a remaining artificial ageing step of the plurality of ageing steps to achieve a desired final temper, preferably the desired final temper being selected from the group of a T6 or T7 temper, having the required strength and other engineering properties relevant for the intended application of the integrated monolithic aluminum structure; and
machining or mechanical milling of the high-energy formed structure to a near-final or final machined integrated monolithic aluminum structure.
It is an important feature of this invention that the 7xxx-series aluminum alloy starting plate product employed has been solution heat-treated and stretched as in well-known to the skilled person and subsequently heat-treated through a first artificial ageing step of a plurality of artificial ageing steps required to achieve a final temper state, preferably a T6 or T7 temper.
A solution heat-treatment (SHT) followed by cooling, preferably rapid cooling by means of quenching, is important for obtaining an optimum microstructure that is substantially free from grain boundary precipitates that deteriorate corrosion resistance, strength and damage tolerance properties and to allow as much solute as feasible to be available for subsequent strengthening by means of ageing. However, 7xxx-series aluminum alloys having been solution heat-treated and stretched are very susceptible to natural ageing leading to an increase in strength over time and a corresponding reduction in ductility. This leads to undesirable variations in properties over time in an individual plate and across batches of different plates. By heat-treating the SHT and stretched aluminum plate product in a first ageing step of a plurality of ageing steps required to achieve a final temper state, the further natural ageing is prevented and creates stable properties in the aluminum alloy plate.
Commonly in an industrial scale of production of 7xxx-series aluminum plate products, the time delay between SHT followed by cooling and the stretching operation is less than about 6 hours, the shorter the time delay the easier the stretching operation as very little natural ageing would have taken place allowing more successful flattening of the cooled plate. Preferably the start of the first ageing step is employed after a sufficient natural ageing period, typically of the order of 7 days or so, performing the artificial ageing immediately after quenching or when insufficient natural ageing has taken place leads to a lower strengthening capability after the SHT and cooling operation. In an embodiment, the start of the first ageing step is employed after about 168 hours after the SHT and cooling operation.
The first artificial ageing step will take solute out of the matrix by generating populations of relatively coarse GP-zones and q′ thereby preventing further natural ageing. The minimum temperature at which this occurs is somewhat 7xxx-series alloy dependent, but the first artificial ageing step is preferably performed by heating the aluminum plate product to a temperature of at least 70° C. for several hours. In an embodiment, the aluminum plate product is heated to a temperature of more than at least 90° C. for about 3 hours or more. In an embodiment, the aluminum plate product is heated at least to a temperature of 100° C. or more for about 3 to 24 hours, preferably for about 3 to 15 hours, for example for 5 hours at about 120° C. or for 7 hours at about 105° C. In an embodiment, the upper-limit for the temperature for the first artificial ageing step is about 140° C., and preferably about 130° C.
Upon performing the first artificial ageing step and prior to the high-energy hydroforming operation, the intermediate aluminum alloy plate product having stable mechanical properties may be stored in inventory or delivered or transported to another location or facility for further processing.
Optionally, either before or after the first artificial ageing step, in a next process step the 7xxx-series plate material is pre-machined, such as by turning, milling, and drilling, to an intermediate machined structure. Preferably the ultra-sonic dead-zone is removed from the plate product. And depending on the final geometry of the integrated monolithic aluminum structure, some material can be removed to create one or more pockets in the plate material and a more near-net-shape to the forming die. This may facilitate the shaping during the subsequent high-energy hydroforming operation.
In an embodiment of the method according to this invention, the high-energy hydroforming step is by means of explosive forming. The explosive forming process is a high-energy-rate plastic deformation process performed in water or another suitable liquid environment, e.g., an oil, to allow ambient temperature forming of the aluminum alloy plate. The explosive charge can be concentrated in one spot or distributed over the metal, ideally using detonation cords. The plate is placed over a die and preferably clamped at the edges. In an embodiment the space between the plate and the die may be vacuumed before the forming process.
Explosive-forming processes may be equivalently and interchangeably referred to as “explosion-molding”, “explosive molding”, “explosion-forming” or “high-energy hydroforming” (HEH) processes. An explosive-forming process is a metalworking process where an explosive charge is used to supply the compressive force (e.g., a shockwave) to an aluminum plate against a form (e.g., a mold) otherwise referred to as a “die”. Explosive-forming is typically conducted on materials and structures of a size too large for forming such structures using a punch or press to accomplish the required compressive force. According to one explosive-forming approach, an aluminum plate, up to several inches thick, is placed over or proximate to a die, with the intervening space, or cavity, optionally evacuated by a vacuum pump. The entire apparatus is submerged into an underwater basin or tank, with a charge having a predetermined force potential detonated at a predetermined distance from the metal workpiece to generate a predetermined shockwave in the water. The water then exerts a predetermined dynamic pressure on the workpiece against the die at a rate on the order of milliseconds. The die can be made from any material of suitable strength to withstand the force of the detonated charge such as, for example, concrete, ductile iron, etc. The tooling should have higher yield strength than the metal workpiece being formed.
In an embodiment of the method according to this invention, the high-energy hydroforming step is by means of electrohydraulic forming. The electrohydraulic forming process is a high-energy-rate plastic deformation process preferably performed in water or another suitable liquid environment, e.g., an oil, to allow ambient temperature forming of the aluminum alloy plate. An electric arc discharge is used to convert electrical energy to mechanical energy and change the shape of the plate product. A capacitor bank delivers a pulse of high current across two electrodes, which are positioned a short distance apart while submerged in a fluid. The electric arc discharge rapidly vaporizes the surrounding fluid creating a shock wave. The plate is placed over a die and preferably clamped at the edges. In an embodiment, the space between the plate and the die may be vacuumed before the forming process.
A coolant is preferably used during the various pre-machining and machining or mechanical milling processes steps to allow for ambient temperature machining of the aluminum alloy plate or an intermediate product. Preferably wherein the pre-machining and the machining to near-final or final machined structure comprises high-speed machining, it preferably comprises numerically-controlled (NC) machining.
In an embodiment of the method according to this invention, following the high-energy hydroforming operation the intermediate product is stress relieved, preferably by an operation including a cold compression type of operation, else there will be too much residual stress impacting a subsequent machining operation.
In an embodiment, the stress relieve via a cold compression of operation is by performing one or more next high-energy hydroforming steps. Preferably applying a milder shock wave compared to the first high-energy hydroforming step creating the initial high-energy hydroformed structure.
In one embodiment, the high-energy formed structure, and optionally also stress relieved, is, in that order, next machined or mechanically milled to a near-final or final machined integrated monolithic aluminum structure and followed by heat-treating of the machined integrated monolithic aluminum structure through a remaining ageing step of the plurality of artificial ageing steps to achieve to a desired final temper to develop the required strength and other engineering properties relevant for the intended application of the integrated monolithic aluminum structure.
In another more preferred embodiment, the high-energy formed intermediate structure, and optionally also stress relieved, is, in that order, heat-treated through a remaining artificial ageing step of the plurality of artificial ageing steps to achieve to a desired final temper to develop the required strength and other engineering properties relevant for the intended application of the integrated monolithic aluminum structure, and followed by machining or mechanical milling to a near-final or final machined integrated monolithic aluminum structure. Thus, the machining occurs after the artificial ageing to final temper.
In both embodiments, the artificial ageing to a desired final temper to achieve final mechanical properties is selected from the group of: T6 and T7. The remaining ageing step preferably includes at least one ageing step at a temperature higher than the first ageing step. In an embodiment, the ageing step includes holding the product at a temperature in the range of about 130° C. to 200° C. In an embodiment, the ageing step includes holding the product at a temperature in the range of about 130° C. to 200° C. for a soaking time in a range of about 4 to 30 hours.
In a preferred embodiment, the artificial ageing to a desired final temper to achieve final mechanical properties is to a T7 temper, more preferably a T73, T74 or T76 temper, more preferably a T7352, T7452 or T7652 temper.
In an embodiment, the artificial ageing is to a Tx54 temper and where x is equal to 3, 6, 73, 74 or 76, which represents a stress relieved temper with combined stretching and compression.
In an embodiment, the final aged near-final or final machined formed integrated monolithic aluminum structure in T6 or T7 temper has a tensile strength of at least 300 MPa. In an embodiment, the tensile strength is at least 360 MPa, and more preferably at least 400 MPa.
In an embodiment, the final aged near-final or final machined formed integrated monolithic aluminum structure in T6 or T7 temper has a substantially unrecrystallized microstructure to provide to better balance in mechanical and corrosion properties.
In an embodiment, the predetermined thickness of the aluminum alloy plate is at least 19 mm (0.75 inches), and preferably at least 25.4 mm (1.0 inches). In an embodiment, the predetermined thickness of the aluminum alloy plate is at least 38.1 mm (1.5 inches), preferably at least 50.8 mm (2.0 inches), and more preferably at least 63.5 mm (2.5 inches).
In an embodiment, the predetermined thickness of the aluminum alloy plate is at most 127 mm (5 inches), and preferably at most 114.3 mm (4.5 inches).
In an embodiment, the 7xxx-series aluminum alloy has a composition comprising, in wt. %:
and optionally one or more elements selected from the group consisting of:
impurities and balance aluminum. Typically, such impurities are present each <0.05% and total <0.15%.
This includes aluminum alloys within the compositional range of the alloys AA7010, AA7040, AA7140, AA7449, AA7050, AA7055, AA7056, AA7065, AA7075, AA7475, AA7081, AA7181, AA7085, AA7097, AA7099, and AA7199.
The Zn is the main alloying element in 7xxx-series alloys, and for the method according to this invention it should be in a range of 5.0% to 9.7%. A preferred lower-limit for the Zn-content is about 5.5%, and more preferably about 6.2%. A preferred upper-limit for the Zn-content is about 8.7%, and more preferably about 8.4%.
Mg is another important alloying element and should be present in a range of 1.0% to 3.0%. A preferred lower-limit for the Mg content is about 1.2%. A preferred upper-limit for the Mg content is about 2.6%. A preferred upper-limit for the Mg content is about 2.4%.
Cu can be present in the 7xxx-series alloy up to about 2.5%. In one embodiment, Cu is purposively added to increase, in particular, the strength and the SCC resistance and is present in a range of 1.0% to 2.5%. A preferred lower-limit for the Cu-content is 1.25%. A preferred upper-limit for the Cu-content is 2.3%.
In another embodiment, the 7xxx-series alloy has a low Cu level of up to about 0.3%, providing a slight decrease in strength and SCC resistance, but increasing fracture toughness and ST-elongation.
The iron and silicon contents should be kept significantly low, for example not exceeding about 0.15% Fe, and preferably less than 0.10% Fe, and not exceeding about 0.15% Si and preferably 0.10% Si or less. In any event, it is conceivable that still slightly higher levels of both impurities, at most about 0.25% Fe and at most about 0.25% Si may be tolerated, though on a less preferred basis herein.
The 7xxx-series aluminum alloy comprises optionally one or more dispersoid forming elements to control the grain structure and the quench sensitivity selected from the group consisting of: Zr up to 0.3%, Cr up to 0.3%, Mn up to 0.45%, Ti up to 0.15%, Sc up to 0.5%, Ag up to 0.5%.
A preferred maximum for the Zr level is 0.25%. A suitable range of the Zr level is about 0.03% to 0.25%, and more preferably 0.05% to 0.18%. Zr is the preferred dispersoid forming alloying element in the aluminum alloy product according to this invention.
The addition of Sc is preferably not more than about 0.5% and more preferably not more than 0.3%, and more preferably not more than about 0.25%. A preferred lower limit for the Sc addition is 0.03%, and more preferably 0.05%.
In an embodiment, when combined with Zr, the sum of Sc+Zr should be less than 0.35%, preferably less than 0.30%.
Another dispersoid forming element that can be added, alone or with other dispersoid formers is Cr. Cr levels should preferably be below 0.3%, and more preferably at a maximum of about 0.25%. A preferred lower limit for the Cr would be about 0.04%.
In another embodiment of the aluminum alloy wrought product according to the invention, it is free of Cr, in practical terms this would mean that it is considered an impurity and the Cr-content is up to 0.05%, and preferably up to 0.04%, and more preferably only up to 0.03%.
Mn can be added as a single dispersoid former or in combination with any one of the other mentioned dispersoid formers. A maximum for the Mn addition is about 0.4%. A practical range for the Mn addition is in the range of about 0.05% to 0.4%, and preferably in the range of about 0.05% to 0.3%. A preferred lower limit for the Mn addition is about 0.12%. When combined with Zr, the sum of Mn plus Zr should be less than about 0.4%, preferably less than about 0.32%, and a suitable minimum is about 0.12%.
In another embodiment of the aluminum alloy wrought product according to the invention, it is free of Mn, in practical terms this would mean that it is considered an impurity and the Mn-content is up to 0.05%, and preferably up to 0.04%, and more preferably only up to 0.03%.
In another embodiment, each of Cr and Mn are present only at impurity level in the aluminum alloy wrought product. Preferably the combined presence of Cr and Mn is only up to 0.05%, preferably up to 0.04%, and more preferably up to 0.02%.
Silver (Ag) in a range of up to 0.5% can be purposively added to further enhance the strength during ageing. A preferred lower limit for the purposive Ag addition would be about 0.05% and more preferably about 0.08%. A preferred upper limit would be about 0.4%.
In an embodiment, the Ag is an impurity element and it can be present up to 0.05%, and preferably up to 0.03%.
Ti can be present, in particular, to act as a grain refiner during the casting of rolling feedstock. Ti based grain refiners such as those containing titanium and boron, or titanium and carbon, may also be used as is well-known in the art. The Ti-content in the aluminum alloy is up to 0.15%, and preferably up to 0.1%, and more preferably in a range of 0.01% to 0.05%.
In an embodiment, the 7xxx-series aluminum alloy has a composition consisting of, in wt. %: Zn 5.0% to 9.8%, Mg 1.0% to 3.0%, Cu up to 2.5%, and optionally one or more elements selected from the group consisting of: (Zr up to 0.3%, Cr up to 0.3%, Mn up to 0.45%, Ti up to 0.15%, Sc up to 0.5%, Ag up to 0.5%), Fe up to 0.25%, Si up to 0.25%, balance aluminum and impurities each <0.05% and total <0.15%, and with preferred narrower compositional ranges as herein described and claimed.
In a further aspect, the invention relates to an integrated monolithic aluminum structure manufactured by the method according to this invention.
In a further aspect, the invention relates to an intermediate semi-finished product formed by the heat-treated plate in a first ageing step of a plurality of artificial ageing steps and the intermediate machined structure prior to the high-energy hydro forming operation.
In a further aspect, the invention relates to an intermediate semi-finished product formed by the intermediate structure, and optionally pre-machined, heat-treated in a first ageing step of a plurality of ageing steps and having been high-energy hydroformed formed and having at least one of a uniaxial curvature and a biaxial curvature by the method according to this invention.
In a further aspect, the invention relates to an intermediate semi-finished product formed by the intermediate structure, and optionally pre-machined, heat-treated in a first ageing step of a plurality of ageing steps, then high-energy hydroformed and having at least one of a uniaxial curvature and a biaxial curvature, then stress relieved in at least a cold compression operation, and heat-treated through a remaining ageing step of the plurality of ageing steps to achieve to a desired final temper prior to being machined into a near-final or final formed integrated monolithic aluminum structure.
The aged and machined final integrated monolithic aluminum structure in T6 or T7 temper can be part of a structure like a fuselage panel with integrated stringers, cockpit of an aircraft, lateral windshield of a cockpit, integral lateral windshield of a cockpit, an integral frontal windshield of a cockpit, front bulkhead, door surround, nose landing gear bay, and nose fuselage. It can also be as part of an underbody structure of an armored vehicle providing mine blast resistance, the door of an armored vehicle, the engine hood or front fender of an armored vehicle, a turret.
The invention shall also be described with reference to the appended drawings, in which:
In
Upon performing the first ageing step and prior to the high-energy hydroforming operation, the intermediate aluminum alloy plate product having stable mechanical properties may be stored in inventory or delivered or transported to another location or facility for further processing.
In a next process step, the aged plate material is pre-machined (this is an optional process step and on a less preferred basis can be performed prior to the first ageing step) into an intermediate machined structure and subsequently high-energy hydroformed, preferably by means of explosive forming or electrohydraulic forming, into a high-energy hydroformed structure with least one of a uniaxial curvature and a biaxial curvature.
In a preferred embodiment, the high-energy hydroformed structure is stress relieved after the high-energy hydroforming operation, more preferably in an operation including in a cold compression type of operation.
Then, there is either machining or mechanical milling of the high-energy hydroformed structure to a near-final or final machined integrated monolithic aluminum structure, followed by artificial ageing of the machined integrated monolithic aluminum structure to the desired final temper (preferably a T6 or T7 temper) to develop the required strength and other engineering properties relevant for the intended application of the integrated monolithic aluminum structure.
Or, in an alternative embodiment, there is firstly artificial ageing of the high-energy hydroformed structure to a desired final temper (preferably a T6 or T7 temper) to develop the required strength and other engineering properties relevant for the intended application of the integrated monolithic aluminum structure, for example an T7452 or T7652 temper, followed by machining or mechanical milling of the high-energy formed structure in its final temper into a near-final or final machined integrated monolithic aluminum structure.
The method illustrated in
While at least one exemplary embodiment of the present invention(s) is disclosed herein, it should be understood that modifications, substitutions and alternatives may be apparent to one of ordinary skill in the art and can be made without departing from the scope of this disclosure. This disclosure is intended to cover any adaptations or variations of the exemplary embodiment(s). In addition, in this disclosure, the terms “comprise” or “comprising” do not exclude other elements or steps, the terms “a” or “one” do not exclude a plural number, and the term “or” means either or both. Furthermore, characteristics or steps which have been described may also be used in combination with other characteristics or steps and in any order unless the disclosure or context suggests otherwise. This disclosure hereby incorporates by reference the complete disclosure of any patent or application from which it claims benefit or priority.
| Number | Date | Country | Kind |
|---|---|---|---|
| 18199078.9 | Oct 2018 | EP | regional |
This application claims the benefit of the International Application No. PCT/EP2019/076745, filed on Oct. 2, 2019, and of the European patent application No. 18199078.9 filed on Oct. 8, 2018, the entire disclosures of which are incorporated herein by way of reference.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2019/076745 | 10/2/2019 | WO | 00 |
