FIELD OF THE INVENTION

The invention relates to a method of producing an integrated monolithic alu minium 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 aluminium alloy structure made from an AIMgSc-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 aluminium alloy structure produced by the method of this invention and to several intermediate semi-finished products obtained by said method.

BACKGROUND TO THE INVENTION US patent no. 7,610,669-B2 (Aleris) discloses a method for producing an inte grated monolithic aluminium structure, in particular an aeronautical member, com prising the steps of:

(a) providing an aluminium alloy plate with a predetermined thickness, said plate having been stretched after quenching and having been brought to a first tem- per selected from the group consisting of T4, T73, T74 and T76, wherein said alu minium alloy plate is produced from a AA7xxx-series aluminium alloy having a com position 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 aluminium, (b) shaping said 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, said alloy plate in said 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 said shaped structure, wherein said heat-treating comprises artificially aging said shaped structure to a second temper selected from the group consisting of T6, T79, T77, T76, T74, T73 or T8,

(d) machining said shaped structure to obtain an integrated monolithic alumin ium structure as said aeronautical member for an aircraft, wherein said machining of said shaped structure occurs after said artificial ageing.

It is suggested that the disclosed method can be applied also to AA5xxx, AA6xxx and AA2xxx-series aluminium alloys.

There is a demand for forming integrated monolithic aluminium structures of more complex configuration from a rolled product.

DESCRIPTION OF THE INVENTION

As will be appreciated herein, except as otherwise indicated, aluminium alloy designations and temper designations refer to the Aluminium Association designa tions in Aluminium Standards and Data and the Registration Records, as published by the Aluminium 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 addi tion 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 com ponent to which it refers. For example, up to 0.1 % Cu may include an aluminium alloy having no Cu.

“Monolithic” is a term known in the art meaning comprising a substantially sin gle 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 aluminium 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 AIMgSc-series aluminium alloy structure of complex configuration that is machined to near-net-shape out of a rolled material.

These and other objects and further advantages are met or exceeded by the present invention providing a method of producing an integrated monolithic alumin ium structure, the method comprising the process steps of, in that order,

providing an aluminium alloy rolled product with a predetermined thickness of at least 2 mm (0.0787 inches), wherein the aluminium alloy rolled product is an AIMgSc-series alloy;

optionally pre-machining of the aluminium alloy rolled product to an intermedi ate machined structure;

high-energy hydroforming of the aluminium alloy rolled product or the interme diate 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 mono lithic aluminium structure, the high energy forming causing the plate or the interme diate machined structure to substantially conform to the contour of the forming sur face to at least one of a uniaxial curvature and a biaxial curvature;

annealing and cooling of the resultant high-energy hydroformed structure; machining or mechanical milling of the annealed high-energy formed structure to a near-final or final machined integrated monolithic aluminium structure; and optionally final annealing of the integrated monolithic aluminium structure to a desired temper to develop the required strength and other engineering properties relevant for the intended application of the integrated monolithic aluminium struc ture.

The AIMgSc-series aluminium alloy rolled product is cast, rolled to final gauge and optionally annealed. Preferably the rolling process applied comprises hot roll ing, and optionally comprises hot rolling followed by cold rolling to final gauge, and where applicable intermediate annealing is applied.

Prior to hot rolling the alloy product is homogenised or pre-heated for up to about 50 hours, preferably up to about 24 hours, at a temperature in a range of about 320°C to 470°C.

In an embodiment following the hot rolling operation 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. In an alternative embodiment 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 annealing at final gauge is to recover the microstructure and is typically performed at a temperature in the range of 200°C to 400°C, preferably in the range of 280°C to 350°C, for a time in the range of 0.5 hours to 20 hours, preferably 0.5 hours to 10 hours.

Optionally in a next process step the AIMgSc-series plate material is pre-ma- chined, such as by turning, milling, and drilling, to an intermediate machined struc ture. Preferably the ultra-sonic dead-zone is removed from a thick plate product. And depending on the final geometry of the integrated monolithic aluminium struc ture some material can be removed to create one or more pockets in the plate ma terial 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 suit able liquid environment, e.g. an oil, to allow ambient temperature forming of the aluminium alloy plate. The explosive charge can be concentrated in one spot or distributed over the metal, ideally using detonation cords. The rolled product is placed over a die and preferably clamped at the edges. In an embodiment the space between the rolled product and the die may be vacuumed before the forming pro cess.

Explosive-forming processes may be equivalently and interchangeably re ferred to as “explosion-moulding”, “explosive moulding”, “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 aluminium plate against a form (e.g. a mould) other wise 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 ap proach, an aluminium 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 dis tance from the metal workpiece to generate a predetermined shockwave in the wa ter. 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 per formed in water or another suitable liquid environment, e.g. an oil, to allow ambient temperature forming of the aluminium alloy plate. An electric arc discharge is used to convert electrical energy to mechanical energy and change the shape of the rolled 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 rolled product is placed over a die and preferably clamped at the edges. In an em bodiment the space between the rolled product and the die may be vacuumed be fore 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 aluminium alloy rolled product or an intermediate product. Preferably wherein the pre-machining and the machining to near-final or final machined structure com prises high-speed machining, preferably comprises numerically-controlled (NC) ma chining.

Following the high-energy hydroforming step the resultant structure is an nealed and cooled to ambient temperature. One of the objects is to heat the struc ture to a temperature in the range of 200°C to 400°C for a time in the range of up to about 20 hours, and preferably for about 0.5 hours to 10 hours. The annealing fol lowed by cooling is important because of obtaining an optimum recovered micro structure and a reduction of internal stresses.

In an embodiment of the method according to this invention following anneal ing treatment the intermediate product is further 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 annealed high-energy formed intermediate structure, and optionally also stress relieved, is, in that order, next machined or mechanically milled to a near-final or final machined integrated monolithic aluminium structure and followed by annealing to a desired temper to achieve final mechanical proper ties. The annealing is to a temperature in the range of 200°C to 400°C for a time in the range of up to about 20 hours, and preferably for about 0.5 hours to 10 hours. In an embodiment the final machined formed integrated monolithic aluminium structure has a tensile strength of at least 200 MPa. In an embodiment the tensile strength is at least 250 MPa, and more preferably at least 300 MPa.

In an embodiment the predetermined thickness of the aluminium alloy rolled product is a plate product of at least 5 mm (0.2 inches), and more preferably at least 12.7 mm (0.5 inches).

In an embodiment the predetermined thickness of the aluminium alloy rolled product is a plate product of at least 38.1 (1 .5 inches), and 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 aluminium alloy rolled product is a plate product of at most 127 mm (5 inches), and preferably at most 1 14.3 mm (4.5 inches).

In an embodiment the AIMgSc-series aluminium alloy has a composition com prising, in wt.%:

Mg 3.0% to 6.0%, preferably 3.2% to 4.8%, more preferably 3.5% to 4.5%, Sc 0.02% to 0.5%, preferably 0.02% to 0.40%, more preferably 0.1 % to 0.3%,

Mn up to 1 %, preferably 0.3% to 1 .0%, more preferably 0.3% to 0.8%,

Zr up to 0.3%, preferably 0.05% to 0.2%, more preferably 0.07% to 0.15%,

Cr up to 0.3%, preferably 0.02% to 0.2%,

Ti up to 0.2%, preferably 0.01 % to 0.2%,

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 0.1 %. In an em bodiment 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 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%.

To make Sc more effective, it is preferred to add also Zr in a range of up to 0.3%, and preferably is present in a range of 0.05% to 0.20%, 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%.

In an embodiment there is a purposive combined addition of Zr+Cr+Ti. In this embodiment 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.

In another embodiment there is a purposive combined addition of Zr and Ti but no purposive addition of Cr. In this embodiment 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 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. In an embodiment 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 0.2% or less, and preferably at a level of 0.1 % or less, and more preferably at a level of 0.05% or less.

Fe is a regular impurity in aluminium alloys and can be tolerated up to 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 0.3%. Preferably it is kept to a level of up to 0.2%, and more preferably up to 0.10%.

In an embodiment 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.

In a further aspect the invention relates to an integrated monolithic aluminium structure manufactured by the method according to this invention.

In a further aspect the invention relates to an intermediate semi-finished prod uct formed by the intermediate machined structure prior to the high-energy hydro forming operation.

In a further aspect the invention relates to an intermediate semi-finished prod uct formed by the intermediate, and optionally pre-machined, structure 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 prod uct formed by the intermediate, and optionally pre-machined, structure then high- energy hydroformed and having at least one of a uniaxial curvature and a biaxial curvature, and then annealed and cooled to ambient temperature. In a further aspect the invention relates to an intermediate semi-finished prod uct formed by the intermediate, and optionally pre-machined, structure then high- energy hydroformed and having at least one of a uniaxial curvature and a biaxial curvature, then annealed and cooled, and stress relieved in a cold compression operation.

The annealed and machined final integrated monolithic aluminium structure 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, pressure bulkhead, door surround, nose landing gear bay, nose fuselage, and part of a wing structure. It can also be part of a structure like an underbody structure of an armoured vehicle providing mine blast resistance, the door of an armoured vehicle, the engine hood or front fender of an armoured vehicle, a turret.

In a further aspect the invention relates to the use of a AIMgSc-series alumin ium alloy rolled product having a composition 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 impuri ties each <0.05% and total <0.15%, and with preferred narrower compositional ranges as herein described and claimed, and a thickness of at least 2 mm, prefera bly of 5 mm to 127 mm, in a high-energy hydroforming operation according to this invention, and preferably to produce an aircraft structural part.

DESCRIPTION OF THE DRAWINGS

The invention shall also be described with reference to the appended drawings, in which:

Fig. 1 shows a flow chart illustrating one embodiment of the method according to this invention; and

Fig. 2 shows a flow chart illustrating another embodiment of the method ac cording to this invention. Figs. 3A, 3B and 3C show cross-sectional side-views of an aluminium plate progressing through stages of a forming process from a rough-shaped metal plate into a shaped, near-finally shaped and finally-shaped workpiece, according to as pects of the present invention.

In Fig. 1 the method comprises, in that order, a first process step of providing an AIMgSc-series aluminium alloy rolled product having a predetermined thickness of at least 2 mm, with preferred thicker gauges. The aluminium alloy rolled product prior to the high-energy hydroforming operation can be in various conditions, in par ticular advantageous are:

the rolled product can be a solely hot rolled product;

the rolled product can be a hot rolled product and having been annealed to recover the microstructure;

the rolled product can be a hot rolled product and then cold rolled to final gauge; the rolled product can be a hot rolled product and then cold rolled to final gauge and having been annealed to recover the microstructure.

As set out herein, optionally the hot rolled product can be further very mild cold rolled or stretched to improve the flatness of the rolled product.

In a next process step the rolled product is pre-machined (this is an optional process step) into an intermediate machined structure and subsequently high-en ergy hydroformed, preferably by means of explosive forming or electrohydraulic forming, into a high-energy hydroformed structure with least one of a uniaxial cur vature and a biaxial curvature. In a next process step there is annealing and cooling of said high-energy hydroformed structure. In a preferred embodiment following an nealing and cooling the intermediate product is stress relieved, more preferably in an operation including a cold compression type of operation. Then there is either machining or mechanical milling of said annealing high-energy formed structure to a near-final or final machined integrated monolithic aluminium structure, optionally followed by a final annealing of said machined integrated monolithic aluminium structure to a desired temper to develop the required strength and other engineering properties relevant for the intended application of the integrated monolithic alumin ium structure. The method illustrated in Fig. 2 is closely related to the method illustrated in Fig. 1 , except that in this embodiment there is a first high-energy hydroforming step, followed by annealing and cooling. Then at least one second high-energy hydro forming step is performed the purpose of which is at least stress relief, followed by the annealing and machining as in the method illustrated in Fig. 1.

Figs. 3A, 3B and 3C show a series in progression of exemplary drawings illus trating how an aluminium plate may be formed during an explosive forming process that can be used in the forming processes according to this invention. According to explosive forming assembly 80a, a tank 82 contains an amount of water 83. A die 84 defines a cavity 85 and a vacuum line 87 extends from the cavity 85 through the die 84 to a vacuum (not shown). Aluminium plate 86a is held in position in the die 84 via a hold-down ring or other retaining device (not shown). An explosive charge 88 is shown suspended in the water 83 via a charge detonation line 89, with charge detonation line 19a connected to a detonator (not shown). As shown in Fig. 3B, the charge 88 (shown in Fig. 3A ) has been detonated in explosive forming assembly 80b creating a shock wave“A” emanating from a gas bubble“B”, with the shock wave“A” causing the deformation of the aluminium plate 86b into cavity 85 until the aluminium plate 86c is driven against (e.g., immediately proximate to and in contact with) the inner surface of die 84 as shown in Fig. 3C.

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 spirit or scope of the invention as herein described.