Metal deforming – By application of fluent medium – or energy field – Using fixed die
Reexamination Certificate
2003-03-13
2004-06-08
Jones, David B. (Department: 3725)
Metal deforming
By application of fluent medium, or energy field
Using fixed die
C072S350000, C072S453040, C029S421100
Reexamination Certificate
active
06745604
ABSTRACT:
FIELD OF THE INVENTION
The present invention concerns seals for high temperature applications, tools having such seals, and forming processes carried out in sealed cavities.
BACKGROUND OF THE INVENTION
Metal articles are often forming at elevated temperature at which the metal is more ductile. “Quick plastic forming” and “superplastic forming” refer to processes for forming metallic materials such as aluminum alloys of aluminum, magnesium, and titanium at temperatures at which the materials have exceptional ductility. Metal alloys suitable for superplastic forming generally have tensile ductility ranging from 200% to 1000%. For example, certain aluminum alloys such as SP Aluminum Alloy 5083 can be deformed with air at temperatures of about 450 to about 600° C., depending on the specific composition, to take the shape of a die surface inside of one-half of a sealed tool cavity. C. H. Hamilton and A. K. Ghosh, “Superplastic Sheet Forming,”
Metals Handbook, Ninth Edition,
Vol. 14, pages 852-868, incorporated herein by reference, provides a background description of practical superplastic metal alloys and SPF processes. The authors describe several superplastic aluminum and titanium alloys that are suitably fine grained for SPF processes.
Stretch forming is one SPF process that is adaptable to forming relatively large sheets of superplastic metal, e.g. superplastic aluminum alloys, into automobile body panels or other large parts. In stretch forming a flat sheet blank is gripped or clamped at its edges by two complementary tool parts that, when closed together, meet around a periphery of an inner, forming volume within which the sheet is held. The tool parts are sealingly closed together and the sheet is heated to its SPF temperature. One of the tool parts has a cavity with inner shaping surface [“die surface”] opposite one face of the sheet. The second tool part, opposite the other face of the sheet, forms a pressure chamber, with the sheet as one wall, to contain the working gas for the forming step. The tool parts and the sheet are maintained at an appropriate forming temperature, for example by using electric resistance heating elements located in press platens or embedded in ceramic or metal pressure plates located between the parts and the platens. The chamber formed by the second tool part is pressurized using a suitable gas such as air or argon. The central, unclasped portion of the heated sheet is forced by the pressure to stretch and plastically deform into conformity with the die surface to make an article of the desired shape. The rate of pressurization is controlled so the strain rates induced in the sheet being deformed are consistent with the required elongation for part forming. Suitable strain rates are usually 0.0001 to 0.01 s
−1
. The part is then cooled and removed from the tool. Vehicle body panels and other articles of complex shape may be formed by this process.
Rashid et al., U.S. Pat. No. 6,253,588, incorporated herein by reference, describes another representative superplastic forming process, quick plastic forming, using a magnesium-containing aluminum alloy having a particular microstructure. The superplastic forming (SPF) process is usually a relatively slow, controlled deformation process that yields complicated products. The quick plastic forming (QPF) process is a variant of SPF in which a higher strain rate allows a part to be formed in a much shorter time. An advantage of SPF and QPF processes is that they often permit the manufacture of large single articles that cannot be made by other processes such as conventional sheet metal stamping. Sometimes a single SPF part can replace an assembly of several parts made from non-SPF materials and processes.
When the periphery of the sheet is held in a fixed position between binding surfaces of the two complementary tool parts (i.e., the edges of the tool parts that contact the sheet) for an SPF process, the binding surfaces grip the sheet in a gas tight seal to allow for pressurization on one side of the sheet. The sheet does not flow over the binding surface as is typical in a conventional deep drawing operation. It is common to use a raised seal bead to grip the periphery of the sheet. For instance, male rectangular cross-section beads may be employed on one tool surface while the opposing binding surface is flat. A typical seal bead has a raised rectangular or trapezoidal cross-section approximately 10-15 millimeters wide and 0.5-1 mm tall. Because the tool parts must be well sealed in the superplastic forming process to achieve the necessary pressure to force the metallic material against the die, it would be desirable have a seal bead that forms as gas-tight of a seal as possible during use of the tool. If the tool is not adequately sealed, the article may not be fully formed. One problem encountered in superplastic forming is poor sealing due to slight misalignment of the tool parts or inadequate clamping pressures. It would thus be desirable to provide a seal bead that can be sealed with less pressure and/or can form a good seal even if the complementary tool parts are slightly misaligned.
Another problem encountered in superplastic forming is sticking of the formed sheet to the tool in the vicinity of the seal bead during part extraction. Because the sheet components are very deformable at the forming temperature, sticking can distort the panel during panel extraction due to uneven forces that may be applied in dislodging the part. Distortion is undesirable when a class A surface or dimensionally accurate part is required. Sticking also excludes effective use of robots for handling the parts during production because the amount of sticking is not predictable and individualized care must be taken in extraction.
The problem is particularly acute with aluminum sheet metal and severely slows the effective removal of an SPF-formed article from the binding portions of the tools. The aluminum sheet sticks primarily on the raised bead binding surface, but also may stick on the opposing flat binding surface. The sticking is due to reaction of the binding surfaces with freshly exposed, unoxidized aluminum at forming temperatures. This unoxidized, reactive aluminum is exposed at the sheet surface as a result of plastic deformation of the aluminum sheet during the clamping process prior to sheet forming. As the die is closed, aluminum is extruded (locally) away from the volume clamped between the bead and the opposing flat tool binding surface. As a result, the protective aluminum oxide film on the aluminum sheet surface is ruptured, and highly reactive aluminum is brought into intimate contact with the tool binding surface. The SPF forming tools are often made of, e.g., P20 steel, ductile cast iron or tool steel. For most such tool materials, local reaction or microwelding occurs to locally bond the aluminum sheet to the tool and cause sticking and tearing during part removal. This sticking problem may be tolerable when low volume production articles can be carefully pried from the tool, but the problem cannot be tolerated when high production rates are required. To adapt SPF to the production of automotive panels, practices must be developed that facilitate fast removal of an SPF-formed article from the forming tools.
One method that has been used to reduce sticking of the article to the toolsurfaces has been to apply a generous amount of a lubricant in the area of contact. This approach is not desirable for a number of reasons. First, applying a lubricant adds expense and preparation time to the forming process. Second, lubrication may need to be reapplied between parts. Additionally, the lubricant must be cleaned from the formed article and in some cases from the forming tool. These drawbacks make lubrication an unattractive solution. Another method that has been used to reduce sticking of the article to the tool has been to use a shallower seal bead to limit sheet deformation, as described by Scroth, U.S. Pat. No. 6,047,583, incorporated herein by reference. This reduces the surface ar
General Motors Corporation
Jones David B.
Marra Kathryn A.
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