Not Applicable.
1. Field of the Invention
The present invention relates generally to material forming; and more particularly, to a method for forming a metal sheet.
2. Description of Related Art
Various methods are known for forming a metal sheet. One method involves a draw process wherein a punch pulls a portion of the metal sheet through a shaped die. During the process, the metal sheet typically undergoes a reduction or change in the cross-sectional area or wall thickness of the sheet. Such processes are typically limited by the material's ability to be strained past its rupture point. Thus, depending upon the complexity of the part, the forming stresses on the metal sheet during the forming process may result in metal failure or fatigue and correspondingly an unusable or scrap part.
Superplastic forming (SPF) is a process that takes advantage of a material's superplasticity or ability to be strained past its rupture point under certain elevated temperature conditions. Superplasticity in metals is defined by very high tensile elongation and is the ability of certain materials to undergo extreme elongation at proper temperature and strain rate. SPF is a process used to produce parts that are difficult to form using conventional fabrication techniques.
During the superplastic forming process, the metal sheet, or as often referred to the blank, is heated to a point of superplasticity after which a predefined gas pressure is applied to one side of the sheet. The pressure forces the sheet into a die cavity while maintaining a target strain rate for deforming the sheet throughout the forming cycle. The superplasticity of the material enables forming of complex components that normally cannot be formed by conventional room temperature metal forming processes. Use of a superplastic forming process enables forming a workpiece with a deep cavity or one formed over very small radii. Superplastic forming does have a disadvantage in that it normally requires relatively long forming cycle times. Specifically, a conventional SPF process used to manufacture a complex part can require a forming cycle time as high as 30 minutes.
Further, superplastic forming cannot always be used to obtain a complex part in a single step and therefore may require two or more forming steps. U.S. Pat. No. 6,581,428 illustrates one method for forming a part that uses a single die capable of preforming both a mechanical draw process and superplastic forming process. The '428 patent utilizes a pre-forming punch disposed on one of the die members, wherein the punch pre-forms the blank prior to an application of gas pressure to the blank to complete the forming process. While this die structure and corresponding process is well suited to many applications, the die structure is somewhat complex and may not accommodate forming some aspects of a complex part such as small radii and corners without causing wrinkling during the drawing process.
In addition, such die sets can be somewhat expensive, relatively complex and in some instances, suitable material from the binder area may not be pulled into the die during the draw process. Accordingly, there exists a need for a process for forming metal sheets or blanks that reduces the complexity of the die components while making use of both mechanical and superplastic forming processes.
The present invention is a method for forming a workpiece, typically a metal sheet or blank. The method includes a multistage hot metal forming operation wherein the metal sheet or blank undergoes a first, pre-forming stage prior to undergoing a second, gas pressure or superplastic forming stage. The method includes a double-action hot draw die used to create a preform. Once created, the preform is mechanically or manually transferred to a conventional single-sided superplastic forming tool that forms the final part geometry utilizing a superplastic forming process.
The method includes developing a preform geometry that reduces forming time and improves panel mobility during the gas or superplastic forming cycle by improving the forming aspect ratio, requiring less part or metal stretch to form over small radii and the ability to draw additional material from the binder area to reduce the amount of thinning required to form the part.
Referring now to the drawings,
A first step in the forming process is to place or load the blank 14 into the draw die 10. According to one embodiment of the present invention, the blank 14 is heated to a suitable forming temperature, typically the superplastic forming temperature of the particular material, prior to being loaded into the draw die 10. Preheating the blank 14 may be accomplished by a variety of methods, including a preheat assembly having upper and lower platens that contact and heat the blank 14. In addition, heated press platens or heaters embedded within the draw die 10 operate to heat the draw die 10 to the superplastic forming temperature of the blank 14. Such preheat and heating assemblies for use with a superplastic forming tool are known to individuals of skill in the art.
The blank 14 and corresponding draw die 10 are heated to the superplastic forming temperature of the blank material since the blank 14 ultimately undergoes a superplastic forming step in the second stage of the forming process. By preheating the blank 14 prior to undergoing the first stage of the process, the preform 12 is already at a superplastic forming temperature and is therefore ready to undergo the second stage of the process thereby eliminating any wait time necessary to heat the preform 12. Eliminating wait time correspondingly decreases the overall forming time necessary to form the finished workpiece 18.
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It should be understood that the flange 32 of the preform 14 need not be a planar surface. For an example, the ultimate configuration of the preform 12 includes a flange 32 having a complex curvature, see
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The present invention provides for a preform 12 design that makes use of the advantages of the draw die 10 to rapidly create a heated preform in a predetermined configuration. By making the preform 12 in a predetermined configuration, the advantages of a conventional superplastic forming process, such as forming of complex components that require drawing metal over a very small radii or forming deep cavities are now available in a single step at a reduced cycle time. As disclosed herein, the preform 12 may take a multitude of shapes including a non-planar flange or binder.
The preform 12 is manually or mechanically transferred from the draw die 10 to the superplastic forming tool 40 wherein the final workpiece 18 is formed using a superplastic forming process. As set forth above, the preform 12 geometry or configuration is engineered to reduce forming time and to improve formability during the superplastic forming cycle by improving gas forming aspect ratios, requiring less stretch to form over radii and drawing additional material from the binder area to reduce the amount of thinning required to form the workpiece 18. In addition, the preform 12 is formed in a hot draw die 10 such that the flow stress of the material during the draw process is very low which, substantially reduces the load requirements on the press ram and cushion system. Subsequently the draw die 10 can be constructed from lower strength material. It should be understood that the present invention provides for a process whereby deep draw components, such as inner door panels for automotive vehicles made from an aluminum or magnesium alloy, can be manufactured at a reduced cycle time over conventional superplastic forming processes. The disclosed process may also take advantage of the fact that both the draw die 10 and superplastic forming tool 40 can be placed in the same press whereby the heated press platens heat, through conductivity, both the draw die 10 and the superplastic forming tool 40.
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.
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