US20060230807A1

US20060230807A1 - Creep forming a work piece

Info

Publication number: US20060230807A1
Application number: US11/105,533
Authority: US
Inventors: Stephen Shultz; Peter Nash; Anthony Vinter
Original assignee: Shultz Steel Co
Current assignee: Shultz Steel Co
Priority date: 2005-04-14
Filing date: 2005-04-14
Publication date: 2006-10-19

Abstract

A method of forming a work piece using a die and a load is provided. The method comprises placing the work piece on a die having a predetermined contour and applying a load on the work piece. The work piece, the die and the load are then heated to an annealing temperature of the work piece such that the work piece, the die and the load are iso-thermal. The method also includes creep forming the work piece such that the work piece develops the predetermined contour during the annealing operation. The work piece, the die and the load are then slow cooled such that the work piece, the die and the load are iso-thermal.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention
The present invention relates to forming a work piece, and more particularly, to a method of forming a work piece using a creep forming technique.
2. Discussion of the Related Art
Today, many industries, such as the aerospace industry, employ metal structures that have complex, predefined shapes. These structures are used, for example, in constructing airplanes, including fuselages, wings and support structures. The shapes associated with these structures are typically complex and they have considerably tight tolerances in order to minimize the possibility of structural failure.
Various processes are used in forming these structures. FIG. 1A illustrates one example in accordance with the prior art. As shown, this process uses a die having a lower die 100 and an upper die 106. A work piece 102, such as one made from titanium, is placed between the lower die 100 and the upper die 106. The lower die 100 and the upper die 106 come together and, in doing so, exert a great deal of pressure on the work piece 102.
Generally, the work piece 102 is heated in a furnace to about 1700° F. before it is placed on the lower die 100. While the lower die 100 and the upper die 106 are outside of the furnace, they are both heated to about 200° F. After placing the work piece 102 onto the lower die 100, the upper die 106 is lowered onto both the work piece 102 and the lower die 100, thereby placing a load on the work piece 102. This, in turn, shapes the work piece 102, where the shape is defined by the upper and lower die. After the work piece 102 is shaped, the work piece 102 is removed from the lower and upper dies and annealed. The work piece 102 is then permitted to cool.
The above-described process causes a variety of problems. FIG. 1B illustrates one of these problems. As shown, the work piece 102 includes defects 104 which may form as a result of friction between the work piece 102 and the lower and upper dies 100 and 106. Furthermore, since the work piece 102 is at a greater temperature than the lower and upper dies 100 and 106, substantial heat loss occurs where the lower and upper dies initially make contact with the work piece 102 (see e.g., points A, B and C in FIG. 1A). This significantly reduces the pliability of the work piece 102, at points A, B and C, relative to the rest of work piece 102. Consequently, defects such as defects 104 can occur, thereby causing the work piece 102 to be outside the very tight, required physical tolerances. Furthermore, the temperature differential between the dies 100 and 106 and the work piece 102 shown in FIG. 1A allows for the formation of residual stresses. The residual stresses can cause distortion during subsequent processing of the part. Moreover, cooling of the work piece 102 leads to additional stress formation.
FIG. 1C shows a cross section of work piece 102 and lower die 100. Moreover, FIG. 1C illustrates yet another problem associated with the aforementioned prior art process. Specifically, the excessively high load applied on work piece 102 can cause the deformation of region 102 c. As is shown in FIG. 1C, during processing, the work piece 102 should have the cross-section indicated by dashed lines 102 d. Instead, the region 102 c forms where regions 102 a and 102 b of work piece 102 intersect. As one skilled in the art would readily appreciate, a deformation such as the deformation of section 102 c shown in FIG. 1C would cause the work piece 102 to be outside the required physical tolerances.
Another well known technique is the super plastic forming process. This process is specifically used for shaping thin sheets of material, such as titanium. In accordance with this process, a die is heated and then the thin sheet is placed onto the heated die. Pressure is then slowly applied until the thin sheet is formed into the shape defined by the die. While this process is useful for forming thin sheets into relatively simple shapes, this process is not applicable for forming work pieces other than thin sheets, nor is it applicable for forming complex shapes and/or contours, such as those required for manufacturing primary frame structures that are used in aircraft and other industries.
Thus, there is a substantial need for a process that can be used in forming work pieces that exhibit complex contours and/or shapes. And, in addition, there is a need for a process that can achieve this without the various problems associated with the prior art.

SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to an assembly and process for forming a work piece that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
An advantage of the present invention is that fewer defects occur as a result of the problems discussed in the prior art. As a further advantage, the present invention eliminates residual stresses.
Another advantage of the present invention is to more effectively meet tight tolerance requirements.
An advantage of the present invention is to combine annealing and forming into a simultaneous process.
As a further advantage, the annealing process produces a work piece which is free of internal stresses and residual elastic stresses, thereby increasing strength and longevity of any structure which employs the work piece. This freedom from residual stresses also minimizes any distortion during subsequent machining.
Another advantage is that the present invention forms a work piece free of wrinkles, kinks, and other non-linear distortions.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
In accordance with one aspect of the present invention, the aforementioned and other advantages are achieved with a method of creep forming a work piece. The method includes placing a work piece on a first die having a predetermined contour. The predetermined contour corresponds to a shape of the work piece after the work piece has been creep formed. After placement of the work piece on the first die, a load is applied on the work piece. The work piece, the first die and the load are then heated such that the work piece, the first die and the load are iso-thermal. The work piece is creep formed during heating using the load such that the work piece has the predetermined contour. After creep formation, the work piece, the first die and the load are cooled such that the work piece, the first die and the load remain iso-thermal.
In accordance with another aspect of the present invention, the aforementioned and other advantages are achieved by a method of hot forming a work piece. The method comprises placing a work piece on a first die having a predetermined contour and placing a load on the work piece. The work piece is then annealed while the work piece is disposed on the first die and the load is placed on the work piece. During the annealing process, the work piece, the first die and the load are iso-thermal. The method also includes creep forming the work piece with the load. Creep forming the work piece under iso-thermal conditions avoids the formation of residual stresses. In this embodiment, the creep forming occurs during annealing of the work piece such that a shape of the work piece is the same as the predetermined contour. After creep formation, the work piece, the first die, and the load are cooled where the work piece, the first die and the load are iso-thermal during cooling. The iso-thermal conditions during the cooling cycle also prevent the formation of residual stresses.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention.
In the drawings:
FIG. 1A illustrates the formation of a work piece in accordance with the prior art;
FIG. 1B illustrates the work piece shown in FIG. 1A after an upper die has been removed and the work piece has been shaped in accordance with the prior art;
FIG. 1C is a cross-sectional view of the work piece shown in FIG. 1A in accordance with the prior art;
FIG. 2 illustrates a process of forming a work piece in accordance with an embodiment of the present invention;
FIG. 3 is an embodiment of the present invention illustrating the work piece described in FIG. 2 after the work piece has been formed;
FIG. 4 is a flowchart illustrating a method of creep forming in accordance with an embodiment of the present invention;
FIG. 5 shows a further embodiment of the present invention where the assembly of FIG. 2 includes an upper die and thermocouples which monitor the temperature of the work piece during a creep formation process; and
FIG. 6 illustrates the upper die shown in FIG. 4 creep forming a work piece in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

The present invention relates to forming a work piece using a creep formation process. As will be discussed in greater detail further on, creep formation occurs when a load, which exceeds a creep limit of a work piece, is placed on the work piece during both annealing and cooling of the work piece.
FIG. 2 illustrates an arrangement of components 200 in accordance with an exemplary embodiment of the present invention used to form a work piece 208. The assembly includes a lower die 204 and a load 206 where the work piece 208 is placed on the lower die 204. The work piece 208 may be any type of metal, such as titanium or the like, and it may be formed using an extrusion process, an open or closed die forging process, or the like.
The lower die 204 may be any material such as steel, ceramic, brick, aluminum, or the like that may be cast and is capable of withstanding high temperatures associated with annealing operations. The lower die 204 has a contoured surface 204 a which is a predetermined contour which defines the desired contour that is imparted to the work piece 208 during the creep formation process. The contoured surface 204 a may exhibit a simple contour, as shown in the exemplary embodiment of FIG. 2, or a complex contour.
In the embodiment illustrated in FIG. 2, the work piece 208 has a first end 208 a and a second end 208 b. The load 206 is placed at the ends 208 a and 208 b of the work piece 208. The load 206 exceeds the creep limit of the work piece 208 such that the load 206 causes deformation of the work piece 208 such that the work piece 208 takes on a desired shape such as the contour defined by the contoured surface 204 a as illustrated in FIG. 3. It should be noted that the load 206 may also exceed a yield point of the work piece 208. When the load 206 exceeds the yield point of the work piece 208, the work piece also takes on the shape as illustrated in FIG. 3. It will be noted that the creep limit varies according to the type of metal used for the work piece 208. Thus, the load 206 varies according to the creep limit of the type of the metal used for the work piece 208.
FIG. 4 illustrates a creep formation method 300 in accordance with an embodiment of the present invention. As shown in operation 302, the work piece 208 is first placed on top of the lower die 204 when the lower die 204 is in a furnace (not shown). Next, the load 206 is placed on the work piece 208 in accordance with operation 303. The work piece 208, the lower die 204 and the load 206 are then heated inside the furnace in operation 304. Operation 304 is preferably an iso-thermal process.
Accordingly, the lower die 204, the load 206 and the work piece 208 are heated to the same temperature. The temperature within the furnace must at least reach a temperature that causes the work piece 208 to anneal. As mentioned above, the work piece 208 may be any type of metal, thus the temperature at which annealing occurs varies according to the metal. In an embodiment where the work piece 208 is titanium, the temperature of the furnace will be approximately 1200° F. to 1750° F.
During operation 304, the load 206 forms the work piece 208 as illustrated in FIG. 3. In accordance with an embodiment of the present invention, when the work piece 208 is titanium, heating, during which creep occurs, occurs during a time period lasting between about two hours and about ten hours.
Once the work piece 208 acquires the desired contour, the work piece 208, the load 206, and the lower die 204 are slow cooled, in accordance with operation 308. Preferably, operation 308 is an iso-thermal process whereby the lower die 204, the load 206 and the work piece 208 slow cool. It should be noted that slow cooling is preferred in order to ensure an iso-thermal cooling process. Where the work piece 208 is titanium, the slow cooling process should last for a period of four to twenty-four hours.
FIG. 5 illustrates a further embodiment of the present invention. As shown, the components include an upper die 210 along with the lower die 204 and the work piece 208. As may be seen in FIG. 5, the upper die 210 includes a contoured surface 210 a which complements the contoured surface 204 a of the lower die 204. The contoured surface 210 a may be used in conjunction with the contoured surface 204 a of the lower die 204 to impart a simple or complex contour on the work piece 208.
It should be noted that a thermocouple (not shown) may be placed on the work piece 208. The thermocouple can be used to monitor the temperature of the work piece 208 during the creep formation process.
The embodiment illustrated in FIG. 5 may be used in conjunction with the creep formation method 300 discussed in FIG. 4. In particular, after the work piece 208 is placed on the lower die 204 in a furnace in operation 302, the load 210 is placed on the work piece 208 in operation 303. In operation 304, the lower die 204, the work piece 208 and the load 210 are iso-thermally heated. During heating, creep occurs and the work piece acquires the desired shape, as more clearly shown in FIG. 6. After the work piece acquires the desired shape, the lower die 204, the work piece 208 and the load 210 are slow cooled during an iso-thermal cooling process.
The present invention provides an attractive option to those desiring to form a work piece so as to impart simple or complex contours or large radial contours where the requirements demand tight tolerances. Requirements such as these are necessary in many industries, including the manufacturing of aircraft and architectural applications. Although, clearly, the present invention is not limited to these industries. Furthermore, because annealing and forming are combined into a simultaneous process, time and costs are minimized.
It will be apparent to those skilled in the art that various modifications and variation can be made in the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.

Claims

1. A method of forming a work piece comprising:

placing a work piece on a first die having a predetermined contour;

applying a load on the work piece;

iso-thermally heating the work piece, the first die and the load;

forming the work piece using the load such that the work piece has the predetermined contour; and

iso-thermally cooling the work piece, the first die and the load.

2. The method of forming a work piece as recited in claim 1, wherein the work piece is titanium.

3. The method of forming a work piece as recited in claim 2, wherein the work piece, the first die and the load are heated to a temperature in a range between 1200° F. and 1750° F.

4. The method of forming a work piece as recited in claim 2, wherein the operation of iso-thermally cooling the work piece, the lower die and the load comprises:

slow-cooling the work piece, the first die and the load for up to twenty-four hours.

5. The method of forming a work piece as recited in claim 2, wherein the work piece, the first die and the load are heated for a time period lasting between two hours and eight hours.

6. The method of forming a work piece as recited in claim 1, wherein the work piece has a first end and a second end.

7. The method of forming a work piece as recited in claim 6, where the operation of applying a load on the work piece comprises:

placing a first load on the first end of the work piece; and

placing a second load on the second end of the work piece.

8. The method of forming a work piece as recited in claim 1, wherein the load exceeds a creep limit of the work piece.

9. The method of forming a work piece as recited in claim 1, wherein the load is a second die having a second predetermined contour, wherein the second predetermined contour complements the predetermined contour associated with the first die.

10. The method of forming a work piece as recited in claim 9, wherein the operation of applying a load on the work piece comprises:

placing the second die on the work piece such that the work piece is disposed between the first die and the second die.

11. The method of forming a work piece as recited in claim 1, wherein the work piece is heated to an annealing temperature.

12. The method of forming a work piece as recited in claim 11, wherein the forming operation and the heating operation are simultaneous.

13. A method of hot forming a work piece comprising:

placing a work piece on a first die having a predetermined contour;

placing a load on the work piece, wherein the load exceeds a creep limit of the work piece;

annealing the work piece while the work piece is disposed on the first die and the load is placed on the work piece wherein the work piece, the first die and the load are iso-thermal during the annealing operation;

forming the work piece with the load wherein the forming occurs during annealing of the work piece such that a shape of the work piece is the same as the predetermined contour wherein annealing the work piece and forming the work piece are simultaneous; and

iso thermally cooling the work piece, the first die, and the load.

14. The method of hot forming a work piece as recited in claim 13, wherein the work piece is titanium.

15. The method of hot forming a work piece as recited in claim 14, wherein the wherein the work piece, the first die and the load are heated to a temperature in a range between 1200° F. and 1750° F.

16. The method of hot forming a work piece as recited in claim 14, wherein the operation of iso-thermally cooling the work piece, the lower die and the load comprises:

slow cooling the work piece, the first die and the load for up to twenty-four hours.

17. The method of hot forming a work piece as recited in claim 14, wherein the work piece is annealed for a time period lasting between two hours and eight hours.

18. The method of hot forming a work piece as recited in claim 13, wherein the work piece has a first end and a second end.

19. The method of hot forming a work piece as recited in claim 18, where the operation of placing a load on the work piece further comprises:

placing a first load on the first end of the work piece; and

placing a second load on the second end of the work piece.

20. The method of hot forming a work piece as recited in claim 13, wherein the load is a second die having a second predetermined contour, wherein the second predetermined contour complements the predetermined contour associated with the first die.

21. The method of forming a work piece as recited in claim 20, wherein the operation of placing a load on the work piece comprises: