Formed structural assembly and associated preform and method

A superplastically formed structural assembly is provided, as is an associated preform and method for forming such an assembly. The assembly includes a skin member and a support member that are joined to define a space between the members and between the joints. The assembly can be produced by joining the members in a flat configuration, and then forming the resulting preform to a predetermined shape of the structural assembly. The support member defines at least one aperture in communication with the space between the members. Thus, the skin member can be formed by delivering a pressurized fluid through the support member to the skin member, e.g., to superplastically form the skin member against a die that defines a contour surface corresponding in shape to the predetermined configuration of the assembly. The support member can extend in a substantially direct configuration between opposing portions of the skin member.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the forming of structural assemblies and, more particularly, relates to a method and preform for forming a structural assembly that defines at least one formed skin member and at least one support member.

2. Description of Related Art

Various methods are known for manufacturing structural assemblies having contoured configurations. According to one conventional method of manufacture, a structural assembly is produced by first forming a number of structural members and then assembling the structural members. Each of the structural members can be formed to a shape that defines a portion of the assembly, and, in this way, the structural members can be assembled to define a shape in combination that would be difficult or impossible to form as a single member. For example, in one typical method of forming the inlet of a nacelle of an aircraft engine, several portions that define the cross-section shape of the inlet are formed, e.g., by superplastic forming, and the separate portions are then assembled circumferentially so that each portion defines a portion of the inlet.

In some cases, it is desirable to form a part as a single assembly so that multiple members do not have to be connected after forming. For example, parts used in some aerospace applications require specific aerodynamic characteristics. In particular, the smoothness of parts defining the outer surfaces of aircraft and other vehicles can affect the flow of air around the aircraft. For example, if the leading edge of an engine nacelle or other body portion of an aircraft is formed of multiple members, the interfaces of the members may disrupt the flow of air around the aircraft, thereby potentially affecting the performance and efficiency of the aircraft.

Thus, there exists a need for an improved method for forming structural assemblies and an associated preform and assembly. The method should be capable of forming a variety of desired contours, and should be capable of forming the assembly with a smooth outer surface as desired for some applications. In addition, the method should be compatible with materials such as titanium.

SUMMARY OF THE INVENTION

The present invention provides a method for forming a structural assembly, and an associated preform and structural assembly. The method includes joining a skin member and a support member to form a preform with a configuration corresponding to a predetermined configuration of the structural assembly, and providing a pressurized fluid to superplastically form the skin member to a contour of the structural assembly, e.g., against a die that defines a contour surface corresponding in shape to the predetermined configuration of the assembly. In particular, the fluid is delivered through at least one aperture in the support member to the skin member such that, after forming, the support member extends in a substantially direct configuration between opposing portions of the skin member. In some cases, the support member is also formed, e.g., by stretching the support member by at least about 5%.

Each of the skin and support members can be formed of various materials including titanium. The members can be provided in a substantially flat configuration and joined in the substantially flat configuration. In some cases, the thickness of the members can be selectively reduced, e.g., by chemical milling, to facilitate forming. The connection between the members can be formed by laser welding, friction stir welding, resistance welding, diffusion bonding, or the like. In particular, first and second joints can be formed between the skin and support members so that the preform defines a space between the members and between the joints, and at least one of the apertures through the support member is in fluid communication with the space. The joints can extend annularly so that the space also extends annularly between the joints. In some cases, each joint can define a nonuniform curve, such as a sinusoidal pattern.

A structural assembly formed according to one embodiment of the present invention includes a skin member that defines an annular space, and a support member joined to the skin member by first and second annular joints. The first joint extends annularly proximate to an outer periphery of the annular space, and the second joint extends annularly proximate to an inner periphery of the space. Each joint can be circular or can define an annularly-extending sinusoidal pattern. The support member extends in a substantially direct configuration between the opposing portions of the skin member so that the space is defined between the skin and support members and between the first and second joints. The support member defines a plurality of apertures that extend to the annular space. For example, the skin member can define an inlet of an engine nacelle, and the support member can define a bulkhead within the inlet. Similarly, an airfoil for structures such as an aircraft wing, horizontal stabilizer, aircraft rudder, missile fin, helicopter blade, racecar spoiler, submarine or boat rudder, jet engine, turbine fan blade, or the like can be fabricated. The members can be formed of titanium or other materials, and each of the members can be a unitary member.

The present invention also provides a preform for forming a structural assembly. The preform includes a substantially flat skin member and a substantially flat support member joined to the skin member by first and second annular joints. The first joint is disposed radially outside the second joint so that the joints define an annular space between the members. The support member defines a plurality of apertures extending to the annular space so that the skin member is configured to be formed by delivering a pressurized fluid through the apertures. Each of the members can be a circular portion that defines a hole through the preform.

DETAILED DESCRIPTION OF THE INVENTION

The present invention now will be described more fully with reference to the accompanying drawings, in which some, but not all embodiments of the invention are shown. This invention may be embodied in many different forms and should not be construed as limited to the embodiments set forth; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout.

Referring now to the drawings, and in particular toFIG. 1, a system10for forming a structural assembly uses a preform according to one embodiment of the present invention. In particular, the preform40illustrated inFIGS. 3 and 4can be used to form the structural assembly70that is illustrated inFIGS. 8 and 9. The structural assemblies according to the present invention can define various contours and configurations. In particular, the structural assemblies can define curved contours associated with superplastic forming. The structural assemblies can be used in a variety of industries and applications including, but not limited, in connection with the manufacture of aircraft and other aerospace structures and vehicles. Further, the structural assemblies can be used individually or in combination with other structures and devices. In particular, the structural assembly70shown inFIG. 8is an inlet of a nacelle80of an aircraft engine.

The system10illustrated inFIG. 1includes a die set12having first and second dies14,16, which cooperatively define a die cavity18. The die set12is configured to be adjusted between open and closed positions so that the die cavity18can be opened to receive the preform40and then closed during the forming operation. For example, the second die16can be lifted from the first die14, or the first die14can be lowered relative to the second die16. In other embodiments of the present invention, the dies14,16can be configured in a horizontal configuration such that one or both of the dies14,16can be moved horizontally to open the die cavity18. Support structures20,22can be provided for supporting the die set12and adjusting the die set12between the open and closed positions. For example, the first and second dies14,16can be connected to the first and second support structures20,22, respectively. The first support structure20can be supported on a foundation24or other surface, and the second support structure22can be configured to be adjusted by actuators26that are hydraulic, pneumatic, electric, or otherwise powered. Thus, as the actuators26extend, the second support structure22and, hence, the second die16is lifted to open the die cavity18. As the actuators26retract, the second support structure22and the second die16are lowered to close the die cavity18. The system10can be secured in the closed position by the actuators26or otherwise so that the die cavity18is closed and sealed during a forming operation while internal pressures are achieved in the die cavity18.

The system10can also include a source30of pressurized fluid, which can be a vessel that contains pressurized fluid, a compressor device for pressurizing fluid, or the like. The source30is typically configured to provide a pressurized inert gas, such as argon, though other fluids can also be used. The source30is fluidly connected to the die cavity18, e.g., via one or more gas passages32extending through the system10to the cavity18.

The dies14,16can be formed of a variety of materials including, e.g., ceramic, metals, and the like. For example, in the embodiment illustrated inFIG. 1, the first die14is formed of a cast ceramic with a low thermal expansion and a high thermal insulation. The second die16is formed of a high temperature alloy of stainless steel. The additional support structures20,22can also be provided to maintain the shape of the dies14,16and prevent damage to the dies during operation and handling, such as is described in U.S. Pat. No. 5,683,608, entitled “Ceramic Die for Induction Heating Work Cells,” which issued on Nov. 4, 1997, and U.S. Pat. No. 6,528,771, entitled “System and Method for Controlling an Induction Heating Process,” which issued Mar. 4, 2003, both of which are assigned to the assignee of the present invention, and the entirety of each of which is incorporated herein by reference.

The die set12can be heated before or after the preform40is disposed in the die cavity18. For example, the die set12can be disposed in an oven or other heating device before or after the preform40is loaded into the die cavity18. Alternatively, the system10can include a heater, such as a heater that is integral to the die set12. In this regard, as illustrated inFIGS. 1 and 2, the system10can include a plurality of electrically conductive elements60, such as electrically resistive rods or wires that resistively heat when an electric current is passed through. Thus, the conductive elements60can be connected to an electrical power supply62and used to heat the preform40to a forming temperature in the die cavity18.

As illustrated inFIG. 1, the preform40includes a skin member42and a support member44. The skin member42is a contoured member, which typically a formed sheet that defines an outer surface of the assembly70. The members42,44of the preform40are typically flat before being formed to the desired shape of the structural assembly70ofFIG. 8. Either or both of the members42,44can be formed to various different shapes, though typically the skin member42is formed to define a desired contour of the structural assembly70, and the support member44is formed to extend in a substantially direct configuration between opposing portions of the skin member42for supporting the skin member42in its contoured configuration. That is, although the support member44can be curved, the cross-sectional shape of the support member44as shown inFIG. 9is straight between portions of an arc defined by the skin member42. Thus, the support member44can provide strength and stiffness to the skin member42to support the skin member42in the desired configuration during use of the assembly70.

The members42,44of the preform40can define a shape that corresponds to the desired shape of the structural assembly70. For example, as illustrated inFIG. 3, each member42,44can be circular and can define a hole43,45for forming the structural assembly70ofFIG. 8. Further, the support member42can be connected to the skin member42so that the support member44is disposed in a predetermined configuration after the forming operation. For example, as shown inFIG. 9, the skin member42can be formed to first and second opposite sides74,76with a space between the sides74,76, and the support member44can extend between the opposite sides74,76to support the skin member42in the desired configuration. In particular, the skin member42can have a profile defining an arc, and the support member44in cross-section can define a chord within the arc, e.g., extending substantially linearly between two points on the arced skin member42. In some cases, the position of the support member44relative to the skin member42in the preform40can be determined by analyzing the expected formation of the preform40, including which portions of the members42,44will be formed and the degree of formation that will occur throughout the members42,44. Such analysis can be performed using a computer, such as by using a computer program for conducting a finite element analysis of the preform40and the forming operation.

The support member44is connected to the skin member42by joints50,52so that the preform40defines a space54between the members42,44and between the joints50,52. That is, as shown inFIGS. 3 and 4, first and second annular joints50,52connect the members42,44. The first joint50is disposed at an outer periphery of the support member44, and the second joint52is disposed at an inner periphery of the support member44, radially within the first joint50. Thus, the space54extends annularly between the members42,44and between the joints50,52, with the joints50,52being disposed at the outer and inner peripheries of the space54, respectively. By the term “annular,” it is meant that the space54extends around an axis or other central portion that is radially inward of the space54, and each joint50,52extends proximate a circumference of the space54, i.e., with the first joint50radially inward of the space54and the second joint52radially outward of the space54. The space54and the joints50,52can define generally circular shapes, as shown inFIG. 3, or other shapes such as an ellipse, rectangle, or irregular polygon. In any case, each of the joints50,52can include one or more weld connections. For example, each joint50,52can include a pair of parallel weld connections, each of the individual weld connections being between about 0.003 inch and 0.004 inch wide and about 0.050 inch apart. The joints50,52can be formed by various welding processes, including laser welding, friction stir welding or other types of friction welding, resistance welding, diffusion bonding, brazing, fusion welding, gas arc welding, and the like.

Diffusion bonding generally refers to a bonding operation in which the members to be bonded are heated to a temperature less than the melting temperature of each member and pressed in intimate contact to form a bond between the members. Brazing generally refers to a bonding operation in which a braze material is provided between the members that are to be joined, and the members and braze material are heated to a temperature higher than the melting temperature of the braze material but lower than the melting temperature of the members being joined. Thus, a diffusion bond can be formed between the members42,44of the preform40by heating the members42,44and urging them together with sufficient pressure at the desired locations of the joints50,52. A stop-off material can be provided for otherwise preventing bonding. Brazing can be performed similarly, but generally requires that an additional braze material be provided between the members42,44, e.g., at the interface of the members42,44where the joints50,52are to be formed. The braze material can be selectively provided where joints50,52are to be formed. Diffusion bonding and brazing are further described in U.S. Pat. No. 5,420,400, entitled “Combined Inductive Heating Cycle for Sequential Forming the Brazing,” which issued on May 30, 1995, which is assigned to the assignee of the present application, and the entirety of which is incorporated herein by reference.

The support member44defines apertures56, with the apertures56extending to the space54between the members42,44. The apertures56are configured to deliver fluid so that a pressurized fluid can be delivered through the support member44to form the skin member42to the desired contour. The forming process is illustrated inFIGS. 5–7.FIG. 5illustrates the system10ofFIG. 1, with the die set12adjusted to a closed position, and a preform40partially formed. That is, the pressurized fluid is delivered from the source30to a first portion18aof the die cavity18, i.e., between the second die16and the preform40. One or more drain passages can be provided for releasing gas from a second portion18bof the die cavity18, i.e., between the first die14and the preform40, as the preform40is formed toward the first die14. It is appreciated that either portion18a,18bof the die cavity18and/or the space54can be substantially closed before or after the forming operation. That is, either portion18a,18band/or the space54can have a volume that is substantially zero. For example, if the members42,44of the preform40and the second die16are flat as shown inFIG. 1, the space54and the first portion18aof the die cavity18can be substantially closed at the start of the forming operation and then expanded during the forming operation as shown inFIGS. 5–7. The second portion18bof the die cavity18includes nearly all of the volume of the die cavity18before forming, and the volume of the second portion18bis substantially zero after forming (FIG. 7).

As the pressurized fluid is delivered to the first portion18aof the die cavity18, the pressurized fluid flows through the apertures56of the support member44and urges the skin member42toward the first die14. Thus, as shown inFIG. 6 and 7, the preform40is formed toward and against the first die14. In this regard, the preform40is also typically heated before and/or during the forming operation. For example, the power supply62can be energized and used to deliver an electric current through the conductive members, thereby heating the members and the preform40. The preform40can be heated to a forming temperature, such as the superplastic forming temperature of the material from which the preform40is made. In some cases, the power supply62can selectively heat the conductive members so that a particular distribution of heat is achieved in the preform40, e.g., to heat the preform40substantially uniformly to the forming temperature. In this regard, thermocouples or other heat monitoring devices can be configured to measure the temperature of the preform40at various positions and communicate with the power supply62so that the power supply62can responsively increase the heating in those areas of the preform40that are cooler than desired and decrease the heating of those areas of the preform40that are hotter than desired.

The skin member42is typically superplastically formed, though in some cases other types of forming can be sufficient for achieving the desired shape. Superplastic forming (“SPF”) generally refers to a process for forming plastics and metals, including titanium, aluminum, and alloys thereof, that exhibit superplastic behavior at certain temperatures, i.e., large elongations (up to about 2,000 percent). The conventional SPF process can be used for forming a single SPF sheet or an SPF pack that includes multiple layered sheets. During the SPF process, the SPF sheet or pack is placed into a shaping die and heated to a sufficiently high temperature within the superplasticity range of the material to soften the material. Pressurized gas is then injected against the material, and possibly into the pack if applicable, thereby causing the sheet or pack to be urged against the die. In some cases, contacting portions of the one or more sheets of material are joined through brazing or diffusion bonding. The formed sheet or pack is then cooled and removed from the die and final machining steps are performed, such as edge trimming. Advantageously, the SPF process can be used to form structures that can satisfy narrow shape and tolerance requirements without substantial additional machining. Such SPF and combined SPF-bonding cycles are described in U.S. Pat. No. 4,117,970, entitled “Method for Fabrication of Honeycomb Structures,” which issued on Oct. 3, 1978; U.S. Pat. No. 5,410,132, entitled “Superplastic Forming Using Induction Heating,” which issued on Apr. 25, 1995; U.S. Pat. No. 5,700,995, entitled “Superplastically Formed Part,” which issued on Dec. 23, 1997; U.S. Pat. No. 5,705,794, entitled “Combined Heating Cycles to Improve Efficiency in Inductive Heating Operations,” which issued on Jan. 6, 1998; U.S. Pat. No. 5,914,064, entitled “Combined Cycle for Forming and Annealing” which issued on Jun. 22, 1999; and U.S. Pat. No. 6,337,471, entitled “Combined Superplastic Forming and Adhesive Bonding” which issued on Jan. 8, 2002, each of which is assigned to the assignee of the present invention, and the entirety of each of which is incorporated herein by reference. In some cases, superplastic forming can affect the properties of the materials so formed. Thus, a member that is superplastically formed to a desired shape from a flat or other configuration can have material properties distinct from those of a member that is machined or otherwise formed to the same desired shape.

Alternatively, the members42,44can be formed by other conventional forming processes. However, it is appreciated that superplastic forming can produce shapes that are difficult or impossible to form using some other forming techniques. In particular, the members42,44can define deeply contoured curves, such as is illustrated inFIG. 7–9, which may be difficult or impossible to form by other forming operations. In some embodiments of the present invention, the skin member42can be elongated by 50% or more. For example, in one embodiment of the present invention, the skin member42can have an original thickness of about 0.080 inch in the preform40, and the thickness of at least a portion of the skin member42can be reduced to about 0.040 inch or less during the forming operation.

The members42,44can be formed of various materials including, but not limited to, titanium, aluminum, alloys that include titanium or aluminum, and the like. Further, the members42,44can be formed of similar or dissimilar materials. For example, according to one embodiment of the present invention, one or both of the members42,44can each be formed of Ti-6Al-4V, Ti-3Al-2.5V, or the like. The particular materials to be used for each member42,44can be selected to facilitate the manufacture of the assembly70and to provide in the finished assembly70the desired material properties and characteristics including strength, corrosion resistance, and the like. For example, the material(s) to be used for forming each member42,44can be selected according to the expected loads, operating temperatures, and other conditions. Each member42,44can be a single, unitary component, or one or both of the members42,44can be provided by joining multiple pieces of material.

While the skin member42can be formed to the desired shape of the structural assembly70, the support member44typically is formed to a configuration for extending substantially directly, i.e., substantially linearly, between the joints50,52connecting the support member44to the skin member42. That is, as shown inFIG. 9, the support member44can be bent proximate to the joints50,52so that the support member44extends in a substantially direct configuration between opposed portions of the skin member42, i.e., to define a chord between portions of an arc defined by the skin member42. The support member44can also be superplastically formed, or otherwise formed, in conjunction with the forming of the skin member42. For example, the support member44can be stretched between the joints50,52. In some cases, the support member44can be stretched by 50% or more. However, in some cases, excessive stretching of the support member44can result in mark-off of the skin member42, i.e., wrinkling or groove formation in the skin member42opposite the support member44. Also, excessive stretching of the support member44can sometimes result in enlargement of cavities, nuggets, or other features in the skin member42, such as are typically formed during laser welding of the support member44to the skin member42to form the joints50,52. Therefore, in some embodiments of the present invention, the support member44is typically stretched by about 5%–20% true strain during the forming operation.

According to one method of the present invention, the preform40is assembled as described above, such that the skin member42and support member44are connected by the joints50,52. The preform40is placed in the die cavity18, and the die set12is adjusted to a closed position so that the skin member42disposed partially against the first die14and the support member44is disposed between the skin member42and the second die16. In some cases, a parting agent such as boron nitride can be coated on the outer surfaces of the members42,44and/or the inner surfaces of the dies14,16. The first die14typically defines a contour surface15that corresponds to the desired shape of the structural assembly70. The second die16can be a substantially planar member. Alternatively, the second die16can define a contour that corresponds to the shape of the preform40in a partially formed configuration. Thus, upon closing of the dies14,16, the second die16can contact the preform40and bend the preform40to partially form the preform40to the desired contour.

The dies14,16are secured in this position, e.g., by urging the two dies14,16together with the actuators26or otherwise securing the dies14,16. The die set12and/or the preform40can be heated before or after the preform40is disposed in the die set12. In particular, the preform40can be heated to a temperature at which superplastic forming of at least the skin member42can be performed. For example, the die set12can be heated in a furnace or hot press before or after the preform40is placed in the die set12, or the preform40can be heated using the conductive elements60that are integral to the die set12. Alternatively, the preform40can be heated by providing an electromagnetic field that induces an electric current in the preform40or in a separate susceptor member disposed in thermal communication with the preform40. The use of such susceptors for heating preforms in a die is further described in U.S. Pat. Nos. 5,705,794; 5,914,064; and 6,337,471, noted above. In any case, the temperature to which the preform40is heated can be determined according to the type of material of the preform40, the type of bonding to be performed, and the like. For example, titanium typically can be superplastically formed and diffusion bonded at a temperature of between about 1425° and 1725° F. For other materials and other forming and bonding operations, a higher or lower temperature can be provided.

With the die set12adjusted to the closed position, and the first portion18aof the die cavity18sealed between the skin member42and the second die16, the pressurized fluid is configured to deliver the pressurized fluid to die cavity18. In some cases, the gas in the die cavity18around the preform40is first purged by repeatedly vacuuming gas and refilling the die cavity18with an inert gas such as argon to prevent high temperature oxidation, i.e., formation of a brittle alpha case titanium layer. Thereafter, the pressurized fluid delivered to the first portion18aof the die cavity18urges the skin member42against the contour surface15of the first die14. Thus, the members42,44are formed, i.e., so that the skin member40is formed to the desired contour of the structural assembly70and so that the support member44is positioned as desired in the assembly70and/or stretched to the desired length between the joints50,52with the skin member40.

For example, a controller90, such as a computer, programmable logic device, or other processor, can be provided for controlling the bonding operation. In particular, the controller90can be configured to communicate electrically with the pressurized fluid source30to control the pressure in the die cavity18.

After forming and/or bonding, the preform40can be removed from the die cavity18, typically after the preform40is at least partially cooled in the die set12to prevent distortion of the preform40during or after removal. Depending on the material of the preform40, it may be possible to remove the preform40with little cooling. Regardless of whether the preform40is cooled in or out of the die cavity18, the rate of cooling of the preform40can be controlled. For example, the system10can include a device for cooling the dies12,14and, hence, the preform40, such as a pump for circulating a coolant fluid through passages defined by the dies14,16. Such a cooling operation is described, e.g., in U.S. Pat. No. 6,528,771, noted above. If the preform40is removed from the die set12while hot, the preform40can be insulated to limit the rate of cooling. Alternatively, the rate of convective cooling of the preform40can be increased by inducing air circulation proximate the preform40.

The preform40can also be machined or otherwise trimmed to the desired configuration of the structural assembly70. In particular, the edges of the members42,44can be trimmed from the preform40. In some cases, the structural assembly70can also be further assembled with other similar structural assemblies. For example, as noted above, the structural assembly70illustrated inFIG. 8is an inlet of a nacelle80, or housing, for an aircraft engine. That is, the structural assembly70can be connected to the rest of the nacelle80so that the structural assembly70provides a smooth, annularly-extending inlet, with the skin member42defining a portion of an outer mold line of the nacelle80. The support member44is disposed within the skin member42and provides a bulkhead or other support feature. For example, as illustrated inFIG. 9, the support member44can be a bulkhead in the curved skin member42. In some cases, the apertures56can be used during operation of the structural assembly70, e.g., to deliver a fluid. For example, a gas can be circulated through the apertures in connection with a thermal operation for preventing or removing ice build-up on the structural assembly70.

The connection between the structural assembly70and the rest of the nacelle80can provide a smooth, continuous outer mold line surface, to promote laminar flow of air around the nacelle and to generally minimize turbulence of the air. In particular, the structural assembly70can be formed of titanium or another metal, the rest of the nacelle can be formed of a composite material, such as a graphite/epoxy composite, and the structural assembly70can be joined using sol-gel chemistry. That is, the surface of the structural assembly70can be prepared for bonding by producing a film on the surface using a solution-gelation (sol-gel) material and method, i.e., using a colloidal suspension of silica particles that is gelled to form a solid, so that a composite material can be bonded to the structural assembly70without requiring mechanical fasteners.

In some cases, a cellular core can be disposed within the inlet defined by the structural assembly70, i.e., developing a sinusoidal-pattern integral bulkhead between the opposite sides74,76of the skin member42, or otherwise within the nacelle80to increase acoustic insulation, increase structural stiffness, and/or for transmitting and directing hot gas to reduce ice formation on the inlet, as described in U.S. Pat. Nos. 6,371,411 and 6,457,676, both of which are assigned to the assignee of the present invention, and the entirety of each of which is incorporated herein by reference.

In other embodiments of the present invention, the structural assemblies can be formed to other desired contours and used in other applications. For example, the structural assembly can be formed to the shape of at least a portion of an airfoil, such as a wing, horizontal stabilizer, aircraft rudder, missile fin, helicopter blade, automobile spoiler, submarine or boat rudder, jet engine, turbine fan blade, or the like. In this regard, it is noted that a straight or arced structural assembly with a cross-section of the assembly70illustrated inFIG. 9can be used to define the leading edge of an airfoil, such as the wing82illustrated inFIG. 8. Thus, the structural assembly of the present invention can be formed with the space54between the members42,44extending substantially linearly or along an arc, and the structural assembly can be used to form the leading edge of the wing or other airfoil.

As described above in connection with FIGS.1and5–7, the contour surface15is defined by the first die14, and the skin member42is urged downwards as illustrated toward the contour surface15during forming. According to other embodiments of the present invention, the second die16can also define a contour surface so that two preforms40can be simultaneously formed in the system10. That is, a first preform40can be formed against the first die14, and a second preform40can be formed against the second die16. In this regard, the pressurized fluid can be provided between the two preforms40so that the gas flows through the apertures56of the support members44and urges each of the skin members42of the preforms40against the respective dies14,16. Thereafter, each of the preforms40can be removed from the die cavity18, and each can be used to form one of the structural assemblies70. In other embodiments, any number of structural assemblies can be formed from a single preform40. In some cases, the combined manufacture of multiple structural assemblies can reduce the average time and/or energy required for forming each structural assembly70.

The joints and/or the edges of the support member44can define a uniform curve, i.e., a circle, as shown inFIG. 3, or the joints and/or edges can define a nonuniform curve. For example, as illustrated inFIGS. 10 and 11, the joints and/or the edges of the support member44can define a sinusoidal pattern, i.e., a pattern defining a succession of waves or curve. A sinusoidal or otherwise nonuniform pattern of the joints50,52can provide an increased strength and stiffness in the structural assembly70. In addition, such a configuration can affect the vibratory characteristics of the assembly70, such that vibrations in the assembly70during use are reduced.

In addition, while the thickness of each member42,44is shown to be substantially uniform inFIG. 4, it is appreciated that each member can define a nonuniform thickness. In particular, the thickness of the skin member42and/or the support member44can be reduced in predetermined locations to increase the amount of elongation that occurs in those locations during forming. For example, as shown inFIG. 12, the thickness of the skin member42can be reduced between the joints50,52to increase the elongation of the skin member42between the joints50,52, and possibly reduce wrinkles or otherwise undesired features from forming. Further, the thickness of the members42,44can be reduced to affect the final location and configuration of the support member44relative to the skin member42in the resulting structural assembly70. Various methods can be used for changing the thickness of the members42,44. For example, the thickness of either member can be reduced by subjecting the member to a chemical milling operation in which at least a portion of the respective member is exposed to a chemical for dissolving or otherwise removing material.