Patent Application
20060260373
Not Applicable.
1. Field of the Invention
The present invention relates generally to superplastic forming of a workpiece; and, more specifically to a forming tool and method used to form a workpiece.
2. Description of Related Art
Superplastic forming (SPF) takes advantage of a material's superplasticity or ability to be strained past its rupture point under certain elevated temperature conditions and strain rates. Superplasticity in metals is defined by very high tensile elongations, ranging from two hundred to several thousand percent. SPF is a process that can be used to produce parts that takes advantage of the high elongation behavior of certain superplastic materials.
SPF typically includes the steps of heating a sheet of material to a point in which superplastic deformation is possible, clamping the material within a sealed die and then using gas pressure to force the material to stretch and take the shape of a forming surface located in the die cavity. Controlling the gas pressure during the forming process controls the deformation rate of the material and maintains superplasticity at the elevated temperature.
Superplastic forming process can lead to part thinning and strain localization in certain areas, such as corners of the mold cavity. Typically, the workpiece stretches or thins in a uniform manner in the open area of the die cavity. Once the workpiece engages the mold surface of the die cavity, deformation or thinning in the contact area is restricted resulting in a greater amount of workpiece stretching or thinning in the last area to contact the die surface. To increase control of the thinning problem and correspondingly control thickness distribution the workpiece can be pre-stretched. That is, the workpiece is pre-stretched or pre-thinned in certain areas depending upon the ultimate configuration of the die cavity. Once the workpiece is successfully pre-stretched, pressure is applied to form the workpiece by pressing it against the mold surface of the forming die.
One process for pre-stretching a workpiece entails blow forming the workpiece away from the molding surface to create a pre-form bubble. After sufficiently stretching the workpiece, reverse pressure then forces the workpiece into the die cavity and against the mold surface. In order to perform such a process, the forming tool must be designed to have a sealed pressure system on both sides of the workpiece and include a pre-form cavity located in the forming tool opposite the molding surface.
The present invention is a superplastic forming tool for forming a workpiece. The forming tool includes a forming die having first and second die members that move between a first open position and a second closed position. At least one of the first and second die members having a cavity including a mold surface. The die member further having a recess. A cap attached to the die member covers the recess such that the cap cooperates with the recess to create a plenum. An aperture in the die member enables communication with the plenum and provides an entryway for supplying pressure to the plenum. A plurality of passageways extend from the plenum to the cavity. The passageways allow passage of fluid between the cavity and the plenum.
In a further embodiment of the invention, both the first and second die members include a cavity and a plenum. A plurality of passageways extend between the plenum and the cavity to allow fluid communication between the plenum and mold cavity.
The present invention further includes a method for reverse gas pressure superplastic forming. The method includes providing a superplastic forming tool, the forming tool having first and second die members each of the die members having a plenum, a mold cavity and a plurality of passageways extending between the plenum and the mold cavity. Further steps include placing a workpiece between the first and second die members and closing the die members to sandwich the workpiece in a sealed relationship between the first and second die members. A pressure source supplies pressure to the plenum and correspondingly through said passageways on said first die member to one side of the workpiece to urge the workpiece toward said second die member. Once the workpiece is sufficiently pre-stretched, applying pressure through said plenum and correspondingly said passageways of said second die member against the opposite side of said workpiece to urge said workpiece toward the first die member. Continuously applied pressure against one side of the workpiece forces the workpiece against the mold cavity of the first die member to ultimately form the workpiece.
The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:
Referring now to the drawings,
The upper die member 16 includes a mold cavity 20 having a mold surface 22. The upper die member 16 further includes a recess or hollow portion 24. Connected to the upper die member 16 is a cap 26 that extends over and covers the recess 24 to create a chamber or plenum 28. Placed between the cap 26 and the die member 16 is a gasket 30 that seals the cap 26 to prevent leakage from the chamber or plenum 28. The gasket 30 is made of a material such as aluminum or copper that withstands superplastic forming temperatures. An aperture 32 in the upper die member 16 communicates with the plenum 28. A pressure source supplies pressure through the aperture 32 to the plenum 28. Typically, the pressure source is a supply of pressurized gas used in the superplastic forming process. Various gases are used, typically depending upon the composition of the material being formed.
A plurality of passageways 36 located in the upper die member 16 extend from the plenum 28 to the mold cavity 20. The passageways 36 are placed in those areas or regions of the mold surface 22 of the upper die member 16 that may form an enclosed area within the mold cavity 20 when the workpiece 12 contacts the mold surface 22. Accordingly, for a mold surface 22 having a complex geometry there may be many passageways 36. The passageways 36 all connect to the plenum 28 to produce a sealed pressure cavity.
The lower die member 18 also includes a cavity 38 having a forming surface 40. The lower die member 18 further includes a recess or hollow portion 42. Connected to the lower die member 18 is a cap 44 that extends over and covers the recess 42 to create a chamber or plenum 46 in the lower die member 18. Once again, a gasket 45 placed between the die 44 and the lower die member 18 seals the cap 44 to prevent leakage from the chamber or plenum 46. An aperture 48 in the lower die member 18 communicates with the plenum 46 whereby fluid pressure from a pressure source is supplied to the plenum 46. Specifically, the fluid pressure in the plenum 46 may be increased or decreased by transferring fluid either in or out of the plenum 46 through the aperture 48.
A plurality of passageways 50 located in the lower die member 18 extend from the plenum 46 to the cavity 38. The passageways 50 are placed in those parts of the lower die member 18 that may form an enclosed area 54 within the cavity 38. Depending upon the geometry of the cavity 38 formed in the lower die member 18 multiple passageways 50 may extend from the cavity 38 to the plenum 46. As with the upper die member 16 the passageways 50 all communicate with the plenum 46 located in the lower die member 18 to produce a sealed pressure cavity.
As shown, the recess 24 formed in the upper die member 16 and the recess 42 formed in the lower die member 18 each include a plurality of pockets 62 separated by a plurality of web members 64 and interconnected by channels or grooves 66 located in each of the web members 64. Thus, each recess 24, 42 includes a plurality of upright members or lands 68. The upright members 68 engage the respective cap members 26, 44 and help support the cap members 26, 44 on the upper die member 16 and lower die member 18. Forming the recesses 24, 42 of the upper and lower die members 16, 18 and a lattice or web arrangement is one way to reduce the overall weight of the die members 16, 18 while maintaining structural strength. Another way is to form the recesses 24, 42 in the upper and lower die members 16, 18 as an open area or large cavity without the lattice or web arrangement.
The superplastic forming die or forming tool 10 according to the present invention is useful for reverse gas-pressure superplastic forming. Reverse gas pressure superplastic forming involves applying pressure to both sides of the workpiece during the forming process and is particularly useful to pre-stretch the workpiece before the final forming step. During the superplastic forming process, the forming tool 10 is generally heated to and maintained at a superplastic forming temperature. The most common heat source is electrical heating; typically, resistance type heating elements located in the press platens that provide a heat source to maintain the forming tool 10 at forming temperature. In some instances, heating units are placed directly in the forming tool 10. Accordingly, the forming tool 10 is heated to forming temperature and is subjected internally to gas forming pressures.
Specifically, as shown in
As the workpiece 12 is driven downward and contacts the of forming surface 40 of the lower die member 18, the gas or fluid contained in the cavity 38 in the lower die member 18 escapes or vents through the passageways 50 into the plenum 46 and correspondingly through the aperture 48. The aperture 48 is connected to a pressure source (not shown.) A vent valve, connected to a line extending between the pressure source and the plenum 46, operates to vent the pressure from the plenum 46 to the atmosphere. Using an inert gas during the forming process enables venting the gas to the atmosphere. While in the preferred embodiment the gas is vented to the atmosphere, in an alternative embodiment, the pressure or gas contained in the plenum 46 can be contained and returned to the pressure source or other storage means. In this manner, the gas is contained in the plenum 46 and pressure source and is not vented to the atmosphere. While some pressurized gases or fluids can be vented to the atmosphere, others due to either cost or toxicity need to be contained.
As illustrated in
Accordingly, the reverse gas-pressure superplastic forming process has two main elements, first applying pressure on one side of the workpiece 12 to create a pre-stretched or pre-formed workpiece 12 in the cavity 38 located in the lower die member 18. Second, applying pressure to the opposite side of the workpiece 12 and driving the workpiece 12 into the molding or forming cavity 20 and against the mold surface 22. Accordingly, both the upper and lower die members 16, 18 have plenums 28, 46 and passageways 36, 50 that enable both die members 16, 18 to both supply and vent pressure in their respective die cavities 20, 38.
In a further embodiment, the apparatus according to the present invention is suitable for use with differential forming pressures. Specifically, the apparatus enables an operator to control the pressure in both the upper and lower cavities 20, 38. The difference between the pressures in the upper and lower cavities 20, 38 drives the workpiece 12 towards one of the mold surfaces 22, 40. Use of differential pressures imposes a hydrostatic pressure on the workpiece 12 that helps to prevent the onset of cavitation and delay failure in the workpiece 12.
While the cavity 38 located in the lower die member 18 is shown with a forming surface 40, it should be understood that the cavity 38 in the lower die member 18 need not have a forming surface 40. Instead, in some instances, a cavity or space suitable to create a pre-form bubble that enables pre-stretching of the workpiece 12 is all that is required. When forming complex shapes with geometric configurations, however, it may be necessary to pre-stretch the workpiece 12 over a specifically configured forming surface prior to forming the workpiece 12 against the mold surface 22.
An additional embodiment of the present invention is illustrated in
Accordingly, the above-disclosed superplastic forming tool and superplastic forming process expands forming limits, reduces part-to-part cycle time and produces a part with more uniform part thickness. This process can be applied to new dies or by retrofitting existing dies to include a plurality of passageways connected to a common plenum.
The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.
