WO1998002277A1

WO1998002277A1 - Method for forming a bi-metallic structural assembly

Info

Publication number: WO1998002277A1
Application number: PCT/US1997/012038
Authority: WO
Inventors: Kevin D. Walters; James M. Hannah
Original assignee: Mcdonnell Douglas Corporation
Priority date: 1996-07-12
Filing date: 1997-07-11
Publication date: 1998-01-22
Also published as: AU3723997A

Abstract

A method for forming a bi-metallic structural bulkhead assembly (10) formed from at least two sections. A thin strip of metal (16), metallurgically similar, and weldable to, a plurality of lower metal sections (14) is explosion bonded to an upper metal section (12). The lower section(s) (14) are then welded to the thin metal strip (16), thus forming the structural assembly (10). The structural bulkhead assembly (10) may then be properly machined and subjected to an appropriate heat teatment process.

Description

METHOD FOR FORMING A

BI-METALLIC STRUCTURAL ASSEMBLY

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The present invention relates generally to explosively joined bi-metallic structures and associated methods for forming such structures and, more particularly, to methods for joining structural members, formed from different metals, without the use of mechanical fasteners.

Bttc-cσround of the invention

Modern, advanced aircraft are required to meet several performance requirements including, the ability to navigate long ranges without re-fueling, the ability to cruise at supersonic speeds, and the ability to maneuver within the range of *9 to -3 Gs. Additionally, it is desired that these aircraft have increased survivability along with long, affordable, useful lives. In order to meet these desired performance goals, advanced aircraft must use lightweight, yet high strength, designs, which typically incorporate both aluminum and titanium components.

The wing carry-through bulkhead of the fuselage of such an aircraft is typically located such that it is subjected to the most demanding loads. The wing carry-through bulkhead is placed in the location within an aircraft structure in which wing bending loads are transferred through the aircraft and where fuselage and landing gear loads are at their peak. In addition, the structural bulkheads must be capable of accommodating large pass-through regions for, as examples, air inlet ducts, engines, large fuel tanks, and possibly integral weapon bays and, therefore, are constrained in size and shape.

The upper region of the bulkhead is subjected to significant compression loading, which can be in excess of 250,000 pounds. At the same time, the lover region of the bulkhead may be subjected to tensile loading in the range of about 200,000 pounds. The bulkhead may also be subjected to bending and other torsional load stresses at points in which the bulkhead interfaces with other structural elements, such as longerons, stringers, keels, and skins. Conventionally, bulkhead designs used single or multi- piece titanium structures or, in order to achieve a weight- efficient design and to meet the loading requirements described above, preferred bulkheads used aluminum upper sections attached to one or more titanium lover sections. The single titanium material designs resulted in a structure that did not meet weight considerations and typically required time-consuming and expensive machining, thereby producing a large amount of scrap material. In addition, due to the size requirements in certain bulkhead designs, no single piece billet or plate of material of sufficient size was readily available. Any multi-piece design typically required the two or more pieces to be bolted or otherwise attached together into a single assembly. The attached interfaces typically increased the weight of the overall structure, were time-consuming to assemble and were prone to corrosion and fatigue. In addition, these interfaces were difficult to inspect and, if located in a integral fuel tank, increased the likelihood of fuel leakage.

Accordingly, there is a continuing need for a method to fabricate a preferred bulkhead design that would use an aluminum upper section (sufficient to withstand relatively large compression loads) and a titanium lover section (s) (which has the advantages of decreased weight and the ability to withstand large tensile loads) . In order to avoid the above-mentioned problems inherent in mechanically fastened multi-part designs, the upper section of the preferred bulkhead would be welded to the lower section (s) . However, as is known, welding dissimilar materials is generally metallurgically impossible or results in poor material properties at the weld joint. Therefore, the preferred method would provide a method to join the upper and lower sections fabricated from different metals, without the need for mechanical fasteners, and avoiding the problems of welding dissimilar metals together.

nummary of the Invention

In accordance with the method of the present invention, a bi- etallic structural assembly having an upper section and a plurality of lower sections, in which the upper and lower sectιon(s) are formed from metals, which are incapable of welding together, may be readily fabricated. One or more thin, metal strips, which are capable of being welded to the lover section(s), are explosion bonded to the lover end of the upper section. The upper section, with the metal strip attached, may then be rough machined into near the final shape. The lower section(s) is attached to the upper section by welding the lover section to the metal strip(s) , thus forming a fastener-free bimetallic structural assembly. The structural assembly may then be properly machined and subjected to an appropriate heat treatment process.

Brief Description of the Drawings These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings wherein:

Figure 1 is an perspective view of a three-piece carry- through bulkhead in which the lower two sections have been joined to the upper section using the method of the present invention;

Figure 2 is an elevational viev of an aluminum forging to which strip (s) of titanium has been joined using an explosion bonding process; and

Figure 3 is an perspective view of a structural assemoly machined from the aluminum forging and the attached strip of titanium of Figure 2.

Detailed Description of the invention The preferred method of the present invention provides a method for producing a large multi-piece structural body, such as a bulkhead for use in an aircraft structure, in which the various attached sections of the structural body may be formed from dissimilar metals. Preferably, as shown in Figure 1, the three-part structural bulkhead 10 incorporates a machined aluminum upper section 12 and tvo lover machined or cast titanium sections 14. The upper section 12 is initially a thick aluminum forging or plate fabricated from a preferred aluminum alloy such as, for example, 7050 Aluminum. The two lover near-net shape titanium sections 14 (either machined or cast) are preferably formed from a weldable titanium alloy such as, for example, Ti- 6A1-4V, Ti-6-2-2-2-2, or any other suitable titanium alloy. The carry-through bulkhead 10 is a structural assembly located in the center fuselage of an aircraft adjacent the wing assemblies and is designed to accommodate large pass-through regions 11 for hosting subassemblies, such as, for example, engines, armament, and fuel tanks. The method of the present invention preferably comprises the steps of explosion welding a strip(s) of metal, which is metallurgically similar to the metal comprising the lower section, to the aluminum forging or plate, forming a subassembly. After the strip (s) is joined, the aluminum forging and strip may preferably be rough-machined into the near final shape. Thus, the subassembly may be machined to near desired structural dimensions and with required features such as caps, stiffeners and webs. By explosion welding the strip to the upper section prior to machining the subassembly, any distortion introduced in the upper section is minimized and may be subsequently machined off. The lover sections are then attached to the subassembly by velding the lover sections to the strip attached to the upper section thereby creating a single structural body.

By way of example of the method of the present invention. Figure 2 illustrates a large metal billet 18, for example, an aluminum forging, with thin strip (s) 16 of a dissimilar metal, such as, for example, titanium, joined to the lower end of the aluminum billet 18. For the presently preferred bulkhead, the aluminum billet 18 is generally rectangular and is preferably approximately four feet in height, eleven feet in length and seven inches in width. The metal strips 16 are preferably approximately ten feet in length and seven inches in width and, preferably, as discussed below, are built up to approximately one inch in height. In order to reduce the number of explosion bonding processes required, it is preferred to use metal strips 16, which are sized to cover the entire lower end of the aluminum billet 18. However, during the explosion joining process, stresses may build up in the aluminum billet 18 and, therefore, it may be necessary to explosion join two (or more depending on the number of interface regions required to connect the upper section to the lower section(s)) separate and shorter titanium strip sections. The strip sections, which may be formed using multiple strips 16, are shaped to cover the entire subsequent interface region between the upper and lower section(s) , thus forming the largest possible bi-metallic joint(s). Each strip section may be built up to the desired height by explosively bonding multiple strips together, as discussed below. Additionally, an interlayer of material, such aβ, for example, tantalum, may be placed betveen the aluminum billet 18 and the metal strip(s) 16 in order to facilitate bonding during the explosive joining process.

Preferably, a series of one or more bi-metallic joints 22 are produced by explosion velding the strip(s) 16 of titanium to the aluminum billet 18. It is preferred to explosion veld a strip of titanium to aluminum because of the ability to easily secondarily weld titanium alloys together. While it may be possible to explosion bond a strip of aluminum to a titanium section, and then weld the aluminum forging to the aluminum strip, only certain aluminum alloys exhibit desirable welding characteristics. Moreover, weldable aluminum alloys are generally structurally inferior to non-weldable aluminum alloys.

The number of bi-metallic joints is dependant on the number of interfaces between the upper and lower sections. For example, for the preferred bulkhead 10 illustrated in Figure l, two bi-metallic -joints 22 would be created, one for each interface connecting the upper section 12 to each of the two lower sections 14. Preferably, each titanium strip 16 is formed from the same weldable titanium alloy as the lover sections 14. Alternatively, the strip 16 and the lover sections 14 may be formed from different titanium alloys, vhich may be sufficiently welded together.

As is known in the art, explosion welding is a form of cold pressure welding in which tvo metallic parts may be joined together by pressure produced by the shock wave from a controlled detonation of an explosive. Typically, explosion welding is used to clad a base metal such as, for example, steel, with a corrosion-resistant metal cladding, such as aluminum. The explosion bonding process allows combinations of metals having widely divergent mechanical properties to be bonded together. As is known, generally an explosion bonding process can be used to bond together substantially all metals. A bi-metallic welded joint is created by the explosively-driven, high-velocity, angular impact of the two metal surfaces. As is known, aluminum is capable of being used in an explosion welding process because during impact, the surfaces of the aluminum and its oxide layers are stripped off and ejected away by the closing angle of the impact. During the process, the two metallurgically pure surfaces are pressed into contact by the explosive pressure. Additionally, because there is no heat-affected zone, as exists in typical fusion joining processes, the parent metal retains its original strength and fatigue resistance properties. Preferably, the amount and type of explosive, as well as the angle of explosion, are all determined in conventional ways and are adjusted according to the size and type of materials used. Preferably, the amount and type of explosive should be chosen so that the strips 16 are sufficiently bonded to the aluminum billet 18, but should not substantially deform the aluminum billet 18. The strip(s) of titanium 16 that are explosion bonded to the aluminum billet 18 must be relatively thin. If the titanium strip 16 is too thick, the explosion may not contain sufficient force to achieve bonding. However, preferably, the height of the titanium strip 16 is of reasonable thickness, for example, one inch, to protect the heat treat properties of the aluminum billet 18 during the subsequent velding process used to join the aluminum upper section 12 to the titanium lover section (s) 14. If required to increase the distance from the titanium-to-titanium weld joint to the aluminum section 12, additional titanium strips may be joined to the original strip 16 by subsequent explosion bonding processes, thereby increasing the distance between the welding area and the aluminum section 12. The explosion bonding step mechanically bonds the titanium metal strip 16 and the aluminum billet 18 together and creates a bi-metal joint 22. Preferably, the bi-metallic joint 22 is positioned within the bulkhead such that aircraft-induced loads are minimized on the joint 22 itself.

After the explosion welding step, the strips 16, as well as the aluminum billet 18, are machined as a sub-assembly 20, as shown in Figure 3, to the near desired shape and size. Preferably, the strip 16 is dimensioned to allow it to be welded to the lower section 14 using, preferably, an electron-beam welding process. As is known in the art, the explosion bonding process does not create a desirable bond along the perimeter of the bonded parts and, therefore, the edges of the strip 16, as well as the corresponding sections of the aluminum billet 18 are machined off. For this reason, the aluminum billet 18 and the titanium strips 16 must be initially larger than the desired interface region. During this rough machining step, caps 26, stiffeners 27, and webs 28, as illustrated in Figure 3, may be formed as desired into the aluminum upper section 12 of the bulkhead 10.

In the preferred representative bulkhead 10 illustrated in Figure 1, the subassembly 20, as shown in Figure 3 is machined such that there are two bi-metal regions 24, one to join to each lower titanium section 14 to the subassembly 20. Each bi-metal region 24 is sized to mate with the corresponding region on the lower titanium section 14 and is approximately at most seven inches in width (the width dimension varies with the size and shape of the various stiffeners 27 and webs 28 machined into the upper section) , and at least thirty-six inches in length.

After the aluminum upper section 12, along with the attached titanium strip 16, are properly rough machined to the approximate desired shape and, preferably, the titanium lower section(s) 14 may be joined. The titanium sections 14 are preferably formed from any suitable, weldable titanium alloy and are pre-formed into the desired shapes by any suitable method, such as casting, forging, machining, or similar methods.

Preferably, the titanium sections 14 are welded, such as, for example, by electron beam welding, to the titanium strip(s) 16 attached to the upper section 12 to create a structural bulkhead 10. Preferably, after the welding process, and after any machining necessary to clean up any distortion surrounding the weld joint, the structural bulkhead 10 may preferably undergo a heat treatment designed to fully age the aluminum upper section 12 and to partially relieve the stress of the election beam weld. For example, the structural bulkhead 10 may be heated to a temperature of 250*F (121 °C) , held for between 3 and 6 hours, and then heated to a temperature of 350^βF (177 °C) and held for between 6 and 10 hours. The bulkhead 10 may then be final machined to achieve the required tolerances and desired final shape. The method of the present invention may also be used to fabricate a larger multi-piece component comprised of smaller sections. For example, the component may be a large aluminum part (which is too large to be formed from a single billet) formed from smaller, non-weldable, high-strength aluminum sections. For example, for a two-piece component, titanium strips may be explosively welded to each of the tvo aluminum sections and then these subassemblies may be joined by velding the titanium strips of each subassembly together. The previously described method of the present invention has many advantages. The method results in a structural, lightweight bulkhead that has desirable compressive loading characteristics in the upper section and tensile loading capabilities in the lower section (s). The amount of raw material for each section is reduced, as is the time for machining the part. The joints eliminate the need for mechanical fasteners, thereby eliminating local stress points surrounding the mechanical joints, the possibility of fuel leakage, and the possibility of corrosion around the joints. The method of the present invention also eliminates the weight and assembly time associated with conventional mechanical fasteners. Additionally, the reliability and maintainability of the bulkhead are increased because of the reduced inspection time and repair time.

Although the present invention has been described in considerable detail with reference to certain preferred versions thereof, other versions are possible without departing from the spirit and scope of the present invention. Therefore the spirit and scope of the appended claims should not be limited to the description of the preferred versions contained therein.

Claims

What is claimed is:

1. A method for forming a wing carry-through bulkhead assembly comprising an upper section fabricated from a first metal having desired compressive loading characteristics and a lower section fabricated from a second metal having desired tensile loading characteristics, wherein the first metal is not weldable to the second metal, the method comprising the steps of:

(a) explosion welding a plurality of metal strips to the upper section forming mechanical bonds between the strips and the upper section; and

(b) velding the lover section to the metal strips, thereby forming the bulkhead assembly.

2. The method of claim l further comprising the step of rough machining the metal strips and the upper section prior to velding the lover section to the metal strips.

3. The method of claim 1 vherein the metal strips are built up to a thickness to protect the heat treat properties of the upper section during velding step (b) .

4. The method of claim 1 vherein the lover section is electron-beam welded to the metal strips.

5. The method of claim 1 further comprising the step of placing an interlayer of material between the upper section and the strips to facilitate bonding during the explosive joining step.

6. The method of claim 1 further comprising the step of subjecting the bulkhead to heat treatment after step (b) .

7. The method of claim 6, wherein the heat treatment comprises the steps of:

(a) heating the bulkhead assembly to a temperature of approximately 250 *F and holding the temperature for approximately between 3 and 6 hours; and

(c) heating the bulkhead assembly to a temperature of approximately 350 *F and holding the temperature for approximately between 6 and 10 hours.

8. The method of claim 1 further comprising the step of final machining the bulkhead assembly.

9. The method of claim 1 vherein the upper section is formed from aluminum, and the lover section and metal strips are formed from weldable titanium alloys.

10. The method of claim 1 wherein the mechanical bonds are positioned within the bulkhead assembly such that aircraft- induced loads are minimized on the mechanical bond.

11. A method for forming a structural assembly comprising a plurality of sections, wherein each section is fabricated from a metal that is non-weldable , the method comprising the steps of:

(a) explosion welding a plurality of metal strips to each section forming mechanical bonds between the strips and the sections; and

(b) welding the metal strips attached to each section together, thereby forming the structural assembly.

12. A wing carry-through bulkhead assembly comprising:

(a) an upper section fabricated from a first metal having desired compressive loading characteristics;

(b) a lover section fabricated from a second metal having desired tensile loading characteristics, vherein the first metal is not weldable to the second metal; and

(c) a fastenerless interface joining the upper section to the lover section.

13. The bulkhead assembly of claim 12 wherein the fastenerless interface comprises a plurality of metal strips that are explosively joined to the upper section forming mechanical