EP3887073B1

EP3887073B1 - Method of producing a high-energy hydroformed structure from an al-mg-sc alloy

Info

Publication number: EP3887073B1
Application number: EP19797293.8A
Authority: EP
Inventors: Achim BÜRGER; Philippe Meyer; Andreas Harald BACH; Philipp Daniel RUMPF; Sabine Maria Spangel
Original assignee: Airbus SAS
Current assignee: Airbus SAS
Priority date: 2018-11-26
Filing date: 2019-11-06
Publication date: 2024-08-28
Anticipated expiration: 2039-11-06
Also published as: WO2020108932A1; NL2024300B1; EP3887073A1

Description

FIELD OF THE INVENTION

The invention relates to a method of producing an integrated monolithic aluminium 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 AlMgSc-series alloy, and can have a complex configuration, that is machined to near-net-shape out of a plate material.

BACKGROUND TO THE INVENTION

EP2948571 , on which the preamble of claim 1 is based, discloses a method of forming an AL MG alloy plate comprising the steps of providing an AIMG alloy plate, forming said plate, heat treating and cooling said formed plate. US patent no. 7,610,669-B2 (Aleris ) discloses a method for producing an integrated monolithic aluminium structure, in particular an aeronautical member, comprising 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 temper selected from the group consisting of T4, T73, T74 and T76, wherein said aluminium alloy plate is produced from a AA7xxx-series aluminium 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 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 aluminium 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 designations 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 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 0.1% Cu may include an aluminium alloy having no Cu.
"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 AlMgSc-series aluminium alloy structure of complex configuration that is machined to near-net-shape out of a rolled material.
The above object is achieved by a method according to the features of claim 1 which is a method of producing an integrated monolithic aluminium 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 AlMgSc-series alloy;
forming the rolled product;
heating and cooling the formed structure;
characterized in that the method producing an integrated monolithic aluminium structure comprises the steps of:
optionally pre-machining of the aluminium alloy rolled product to an intermediate machined structure;
high-energy hydroforming of the aluminium alloy rolled product or the 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 aluminium structure, the high energy forming causing the plate or the 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;
annealing and cooling of the resultant high-energy hydroformed structure;
machining of the annealed high-energy formed structure to a near-final or final machined integrated monolithic aluminium structure; and
optionally annealing of the near-final or final integrated monolithic aluminium structure to a desired temper.

The AlMgSc-series aluminium alloy rolled product is preferably cast, rolled to final gauge and optionally annealed. Preferably the rolling process applied comprises hot rolling, 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 receives 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 AlMgSc-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 a thick plate product. And depending on the final geometry of the integrated monolithic aluminium 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 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 process.
Explosive-forming processes may be equivalently and interchangeably referred 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) 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 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 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 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 embodiment the space between the rolled product 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 aluminium alloy rolled product or an intermediate product. Preferably wherein the pre-machining and the machining to near-final or final machined structure comprises high-speed machining, preferably comprises numerically-controlled (NC) machining.
Following the high-energy hydroforming step the resultant structure is annealed and cooled to ambient temperature. One of the objects is to heat the structure 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 followed by cooling is important because of obtaining an optimum recovered microstructure and a reduction of internal stresses.
In an embodiment of the method according to this invention following annealing 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 properties. 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 114.3 mm (4.5 inches).

In an embodiment the AlMgSc-series aluminium alloy has a composition comprising, 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 AlMgSc-series alloys, and for the method according to this invention it should be in a range of 3.0% to 6.0%. A preferred 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 embodiment the Sc-content is up to about 0.4%, and preferably up to about 0.3%.
Mn may be added to the AlMgSc-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 AlMgSc alloy as strengthening element or for improving the corrosion resistance or for grain refiner purposes. A preferred 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 precipitates.
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 AlMgSc-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 AlMgSc-series aluminium alloy has a composition consisting 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.
An annealed and machined final integrated monolithic aluminium structure manufactured by the method according to the invention 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 AlMgSc-series aluminium 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 impurities each preferably <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, preferably of 5 mm to 127 mm, in the method according to this invention, and still in a further aspect 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 according 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 aspects of the present invention.

In Fig. 1 the method comprises, in that order, a first process step of providing an AlMgSc-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 particular 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.

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-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 next process step there is annealing and cooling of said high-energy hydroformed structure. In a preferred embodiment following annealing 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 aluminium 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 illustrating 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 scope of the invention as defined by the appended claims.

Claims

A method of producing an integrated monolithic aluminium structure, the method comprising the steps of:
- providing an aluminium alloy rolled product with a predetermined thickness of at least 2 mm, and wherein the aluminium alloy rolled product is an AlMgSc-series aluminium alloy;

- forming the rolled product;

- heating and cooling the formed structure;
characterized in that the method producing an integrated monolithic aluminium structure comprises the steps of:
- optionally pre-machining of the aluminium alloy rolled product to an intermediate machined structure;

- high-energy hydroforming of the rolled product or optional intermediate machined structure against a forming surface of a rigid die having a contour in accordance with a desired curvature of the integrated monolithic aluminium structure, the high energy forming causing the plate or the intermediate machined structure to conform to the contour of the forming surface to at least one of a uniaxial curvature and a biaxial curvature;

- annealing and cooling of the high-energy hydroformed structure;

- machining of the annealed high-energy formed structure to a near-final or final machined integrated monolithic aluminium structure; and

- optionally annealing of the near-final or final integrated monolithic aluminium structure to a desired temper.
Method according to claim 1, wherein the high-energy hydro-forming step is by explosive forming.
Method according to claim 1, wherein the high-energy hydro-forming step is by electrohydraulic forming.
Method according to any one of claims 1 to 3, wherein following annealing and cooling of the high-energy hydroformed structure, in that order, the annealed high-energy formed structure is machined to a near-final or final machined integrated monolithic aluminium structure and then final annealed to a desired temper.
Method according to any one of claims 1 to 3, wherein following annealing and cooling of the high-energy hydroformed structure, in that order, the annealed high-energy formed structure is final annealed to a desired temper and then machined to a near-final or final machined integrated monolithic aluminium structure.
Method according to any one of claims 1 to 5, wherein following annealing and cooling of the high-energy hydroformed structure, said structure is stress-relieved, preferably by compressive forming, followed by machining into the integrated monolithic aluminium structure.
Method according to any one of claims 1 to 6, wherein following annealing and cooling of the high-energy hydroformed structure, said structure is stress-relieved, preferably by compressive forming in a next high-energy hydroforming step, followed by machining and final annealing to a desired temper of the integrated monolithic aluminium structure.
Method according to any one of claims 1 to 7, wherein the predetermined thickness of the aluminium alloy plate is at least 5 mm, preferably at least 12.7 mm, and preferably at least 38.1 mm.
Method according to any one of claims 1 to 8, wherein the predetermined thickness of the aluminium alloy plate is at most 127 mm, and preferably at most 114.3 mm.
Method according to any one of claims 1 to 9, wherein the annealing following the high-energy hydroforming step is by holding the structure at a temperature in the range of 200°C to 400°C, preferably for a time in a range of up to 20 hours.
Method according to any one of claims 1 to 9, wherein the final annealing of the integrated monolithic aluminium structure is by holding the structure at a temperature in the range of 200°C to 400°C, preferably for a time in a range of up to 20 hours.
Method according to any one of claims 1 to 11, wherein the AlMgSc-series aluminium alloy has a composition comprising, in wt.%: Mg 3.0% to 6.0%, preferably 3.2% to 4.8%, Sc 0.02% to 0.5%, preferably 0.02% to 0.40%, Mn up to 1%, Zr up to 0.3%, preferably 0.05% to 0.2%.
Method according to any one of claims 1 to 12, wherein the AlMgSc-series aluminium alloy has a composition comprising, in wt.%: Mg 3.0% to 6.0%, preferably 3.2% to 4.8%, Sc 0.02% to 0.5%, preferably 0.02% to 0.40%, Mn up to 1%, preferably 0.3% to 1.0%, Zr up to 0.3%, preferably 0.05% to 0.2%, Cr up to 0.3%, Ti up to 0.2%, preferably 0.01% to 0.2%, Cu up to 0.2%, Zn up to 1.5%, Fe up to 0.4%, Si up to 0.3%,

impurities and balance aluminium.
Method according to any one of claims 1 to 13, wherein the pre-machining and final machining comprises high-speed machining, preferably comprises numerically-controlled (NC) machining.
Use of an AlMgSc-series aluminium 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 impurities, and a gauge in a range of at least 2 mm in a method according to any one of claims 1 to 14.
Use of an AlMgSc-series aluminium 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 impurities, and a gauge in a range of at least 2 mm in a method according to any one of claims 1 to 14 to produce an aircraft structural part.