US20090272171A1

US20090272171A1 - Method of designing and forming a sheet metal part

Info

Publication number: US20090272171A1
Application number: US12/115,026
Authority: US
Inventors: Sergey F. Golovashchenko
Original assignee: Ford Global Technologies LLC
Current assignee: Ford Global Technologies LLC
Priority date: 2008-05-05
Filing date: 2008-05-05
Publication date: 2009-11-05
Also published as: CN101574716A; CN101574716B

Abstract

A method of forming a part and a method of designing a part to be formed from a sheet metal blank is disclosed. The part may be formed from lightweight high-strength material to an extent that would normally exceed the forming limits of the material if the part were attempted to be formed in one step in a multi-part die set. Critical areas including deep pockets and sharp radius areas of the final part are formed from a preform or intermediate shape part. The preform is further formed in a fluid pressure forming process to a final part shape wherein broad radius areas of the preform are formed into deep pockets and sharp corners.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention
The present invention relates to a method of forming and a method of designing a part from a sheet metal blank that includes drawing a sheet metal blank to an intermediate shape that is then subsequently formed to a final design shape that includes a critical region.
2. Background Art
Many manufacturing processes are available to form sheet metal blanks into parts in a wide variety of industries. Such manufacturing processes are well suited to form parts having less complex geometries. Some parts may have a critical region that may not be formed by conventional processes without exceeding the formability limits of the material to be formed. Such critical regions may exceed formability limits due to the depth draw required to form the part or due to the need to form tight radii.
To manufacture parts having critical regions, several parts having less complex geometries may be formed and then joined together by welding, riveting or other conventional fastening techniques. Another approach to forming parts having a complex geometry is to form the parts after they are heated to an elevated temperature in a warm forming or super-plastic forming process. Forming parts in an elevated temperature can create issues regarding lubrication and can result in excessive thinning of the walls of the part.
Another approach to forming complex parts having critical regions is to initially form the part, then apply a heat treatment to the part to restore material ductility before reforming the part to the desired final shape. One problem with this approach is that a substantial period of time is required to heat treat the part and the part must be heated treated according to a precise heat treatment schedule to be effective. Another variation of this method is incremental forming in which the part is partially formed and then rapidly heat treated. Any forming method, including heat treating or pre-heating the part, tends to result in excessive thinning of the blank.
Another process which has been proposed is referred to as hydro-mechanical drawing (also down as Amino technology) that provides for friction reduction in localized areas where a material enters the die cavity. The friction reduction allows additional metal to be drawn into the die cavity from the flange area of the blank. Hydro-mechanical drawing processes are not well suited to forming parts where excessive local stretching is required in control areas of a large panel that are located at a substantial distance from the edge of the die because the additional material cannot be drawn from the flange area into the central area.
Part forming processes are also impacted by the type of material that is formed. Aluminum, high-strength steel and advanced high-strength steels tend to be less ductile and have low formability, or forming limits. In particular, forming processes incorporating a heating step or heat treating step are better suited to aluminum alloys and are not generally feasible for high-strength steel and advanced high-strength steels. The need to provide the final parts that offer high-strength properties with reduced weight make it important to use these types of advanced materials for weight reduction. However, the low forming limits of such materials and alloys limit design freedom and the types of parts that may be made with these types of materials.
This development as summarized below solves the above problems and other problems that represent a long-felt need in the field that will allow more complex parts to be made from lightweight, high-strength alloys.
The improvements proposed herein are summarized below.

SUMMARY

A method is provided for forming a part into a final design shape that has at least one critical forming region, or area, from a sheet metal blank. The method comprises drawing the blank to form an intermediate shaped part that includes a bulge adjacent to the critical region. The intermediate shaped part is then further formed by forming the bulge into the critical region to form the final design shape.
According to other aspects of this development, the forming step is performed in a fluid pressure forming process. As used herein, the term fluid pressure forming process should be construed to include hydro-forming, gas-forming and electro-hydraulic forming.
The critical regions are areas of the final design shape that require drawing the blank into a sharp corner or drawing the blank to a depth that exceeds the forming limits of the blank. According to the method, in the drawing step, the blank is drawn to a depth that is within the forming limits of the blank. Then, the step of forming the part to the final design shape results in the formation of a final part design that would exceed the forming limits of a single step forming process. The method also involves providing an intermediate shaped part that has at least one bulge that is adjacent an increased radius critical region. The intermediate shaped part has substantially the same surface area compared to the surface area of the final part shape.
According to another characteristic of the method, an intermediate cavity defined by the intermediate part shape is confined within a boundary of a final cavity defined by the final part shape. According to the method, the drawing operation may be performed in a stamping die set that has at least two dies. The forming, or bonding, operation can then be performed in a fluid pressure forming tool after the initial drawing operation is completed.
A method of designing a part to be formed from a sheet metal blank is provided according to the following steps. A final part shape is defined as a computer-aided design model. The final part shape is analyzed using finite element analysis. Critical regions are identified in the final part shape where forming limits of the sheet metal blank may be approached or exceeded. A design shape is simulated to locally reduce the strain applied to the blank in developing an intermediate part shape based upon the shape of the final part. The surface area of the intermediate part shape and the final part shape are substantially the same.
A method of designing a part may further include defining an intermediate cavity within the intermediate part shape that is within a boundary of a final cavity defined by the final part shape.
In the analysis step of the method of designing a part, finite element analysis is used to determine nodal forces in the critical regions of the part. Nodal forces are imposed to transform the final shape into the intermediate shape in the simulating step.
In the step of simulating the design shape, the final part shape is elastically deformed to develop the intermediate part shapes so that the surface area relationship of the intermediate part shape and the final part shape is maintained and the intermediate shape is within the final part shape.
The method of designing a part may further include designing at least one bulge in the blank that is formed in the intermediate part shape and is then reformed to provide material from the bulge to the critical region. The intermediate part shape is designed to be formed and then reformed to allow the bulge to flow into a sharp corner. Development of the intermediate part shape is an iterative process and the final part is tested to verify that the forming limits are not exceeded as the final part is formed.
These and other aspects of the methods will be better understood in view of the attached drawings and the following detailed description of the illustrated embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of one half of a symmetrical part design with cross-hatching indicating finite element analysis cells;

FIG. 2 is a perspective view of one half of a symmetrical part that is similar to FIG. 1 and further includes a representation of designated areas on the part that can be used to develop an intermediate shape;

FIG. 3 is a fragmentary perspective view of one half of a symmetrical intermediate part shape marked with finite element analysis cells;

FIG. 4 is a fragmentary perspective view of one half of a symmetrical final part designed and made according to the method;

FIG. 5 is a diagrammatic cross-sectional view showing a three-part die with a sheet metal blank in position for forming;

FIG. 6 is a diagrammatic cross-sectional view of the die shown in FIG. 5 with the workpiece being drawn by the upper punch to form the intermediate shape;

FIG. 7 is a diagrammatic cross-sectional view similar to FIG. 5 showing the workpiece having a bulge formed therein by a lower punch;

FIG. 8 is a diagrammatic cross-sectional view of a one-sided die that may be used in a fluid pressure forming process to form an intermediate part shape; and

FIG. 9 is a diagrammatic cross-sectional view showing a pressure forming tool including a one-sided die with the final part being shown in solid lines formed against the one-sided die.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

The die design method as illustrated in the above drawings and described below facilitates forming complex sheet metal stampings using lightweight material, such as high-strength steels, advanced high-strength steels, and aluminum alloys. Strain distribution tends to be non-uniform in the majority of sheet metal stampings. Strain is greatest where the material is stretched to the greatest extent in sharp corners and in deep pockets. For example, vehicle parts such as license pockets and door handle pockets may have sharp corners and deep pockets that cannot be formed in a single drawing operation to the desired shape.
Referring to FIG. 1, the first step in the design process is to develop a numerical mesh of the final shape of the part 10. Computer-aided design (CAD) math data representing the surface of the part is loaded into an existing meshing software system, for example, Altair Hypermesh software. A finite element analysis (FEA) mesh is applied to the numerical mesh. The mesh is used to calculate how the part should be deformed backwards (relative to the forming process) from the final shape to create a facilitating preform shape. The preform is designed to have smooth radii at the entrance to a cavity while stretching the bottom of the part (the bottom of the part is not normally subjected to substantial strain during forming).
The second step in the process is to determine what forces will be required to deform a simulated elastic blank according to the planned strategy. Structural code capable of simulating contact interaction between the blank and the die is used to simulate the sheet metal forming process. Examples of structural code include MATRAS, LS DYNA, ABAQUS and AUTOFORM, or the like which may be used to simulate sheet metal forming processes. The material is modeled as a thin elastic membrane to develop an equivalent surface.
Numerical analysis is used to calculate and distribute nodal forces that would be required to form the blank from the final shape to a preform shape based upon the FEA data.
Referring to FIG. 2, numerical analysis is used to calculate the location where nodal forces have been adjusted to calculate the shape of a preform that is readily formed by stamping or other metal forming processes. Metal is pulled out of the sharp corners without stretching to design the preform so that it has an equivalent surface area to the final shape of the part.
The distribution of nodal forces for deforming the blank “backwards” from the final shape is defined based upon the numerical analysis. The resulting distribution of nodal forces is illustrated in FIG. 2 by the cross hatch 16. This is what is known as deforming the blank backwards because the design of the final shape is used as the starting point and the analysis of nodal force is used to design the preform shape 18, which is shown in FIG. 3.
The preform shape 18 includes a bulge 24 that is located adjacent to a critical area 20 that is a deep draw pocket or sharp radius area of the part. The preform shape is designed to facilitate forming the final shaped part that is shown in FIG. 4. Metal made available by the bulge 24 may be bent without substantial stretching into the final part shape 10. As used herein “without substantial stretching”, should be understood to mean that the surface area of the intermediate, or preform shape, is less than 5% of the surface area of the final part. It follows that the intermediate preform shape part 18 is subjected to less than 5% stretching as it is formed into the final part shape 10. Reforming the preform shape 18 into the final part shape 10 requires only a limited amount of pressure since almost no stretching is required. The critical regions 26 (i.e., deep pockets and sharp corners) are filled with material located in the bulge 24 on the preform 18 in the second forming step. According to the method, the precise amount of additional metal that is initially formed as the preform is subsequently formed or fed into the critical region 26.
According to the method, a mathematical tool manipulates the CAD data of the final part by distributing artificial nodal forces required to pull the material out of the sharp corners, assuming that the panel is elastic, to provide a smooth surface preform 18. Also according to the method, in the backwards simulation very high pressure is applied to the flange area to prevent it from moving as the preform shape is calculated to be moving backwards from the final shape data. Metal is drawn from the sharp corners of the final part shape 10 to form the preform part shape 18 with substantially no stretching from the sharp corners into the reserve areas, or bulge 24. In the method, the degree of stretching in the blank 40 as it is formed from the preform shape 18 to the final shape 10 is managed by shrinking the mesh of the final part backwards into the shape of the preform shape 18. When the blank is actually formed from the preform 18 to the final shape 10, the bulge 24 may be formed to a greater extent than the other parts of the preform shape 18.
Referring to FIGS. 5, 6 and 7, a tool 30 is shown that may be used to form a preform shape 18. The tool 30 includes an upper punch 32 that cooperates with a binder ring 34. The tool also includes a lower die 36 and a lower punch 38. The tool 30 is exemplary and many other tools may be provided that include additional punches or differently shaped forming surfaces. As shown in FIG. 5, blank 40 is retained on the lower die 36 by the binder ring 34. The upper punch 32, as shown in FIG. 5, is poised above the blank 40 and the lower punch 38 is retracted into the lower die 36.
Referring to FIG. 6, the blank 40 is partially formed by the upper punch 32 that forms wide radius areas on the peripheral edge 42 of the lower die 36. A wide radius curve is also formed by the preform critical area 46. The wide radius of the peripheral edge 42 and the preform critical area 46 are easily formed in the drawing operation and are not subject to splits or wrinkling.
Referring to FIG. 7, the lower punch 38 is moved into engagement with the blank and completes formation of the preform shape 18 by stretching the previously unformed area of the blank 40, as shown in FIG. 6, to create the bulge 24 in the preform shape 18.
The procedure illustrated in FIGS. 5-7 is performed with a conventional multi-part die set forming operation. However, it should be understood that the preform shape could also be formed in a fluid pressure forming operation.
Referring to FIG. 8, a one-sided forming die suitable for a fluid pressure forming process is illustrated. The one-sided die 50 is intended to be used in conjunction with a hydro-forming, gas-forming, electro-hydraulic forming, super plastic-forming, rubber-forming or explosive-forming die. For example, in fluid pressure forming processes, fluid pressure is applied to the opposite side of a blank 40 from the one-sided die 50 to form the preform shape 18. The preform 18 includes the peripheral edge 42 and the bulge 24 that are smooth radius areas and are simple to form in the fluid pressure forming process.
Referring to FIG. 9, a one-sided final die 54 is shown that is adapted to be used in a pressure-forming tool 56. Pressure is applied normal to the surface of the preform shape 18 by an appropriate fluid pressure-forming process, as previously described, to form it into the final part shape 10 against the one-sided final die 54. The peripheral edges 42 are formed from a relatively broad radius to a narrow radius and the preform critical area 46 is formed into the deep pocket, tight corner area comprising the critical area 20 in the final part 10.
The boundary of the intermediate part shape shown in FIG. 8 is within the boundary of the final cavity defined by the final part shape, as shown in FIG. 9. Forming the preform shape 18 into the final part shape 10 in a multi-part die would be very difficult or impossible due to the fact that the intermediate part shape would be crushed or wrinkled in a multi-part die set that does not apply pressure equally and normal to the surface of the preform 18. The fluid pressure-forming process essentially bends the preform 18 into the final part shape 10 with a minimum of stretching.
By assuming an elastic blank is being formed, the surface area of the preform shape 18 and the final part shape 10 within the cavity may be maintained substantially equal to each other. By keeping the surface area of the preform and the final part shape the same, the preform may be bent, instead of being stretched, to the final part shape.
While the best mode for carrying out the invention has been described in detail, those familiar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention as defined by the following claims.

Claims

1. A method of forming a part from a sheet metal blank, wherein the part has at least one critical region that is formed to a final design shape, the method comprising:

drawing the blank to form an intermediate shaped part that includes at least one bulge adjacent to the critical region;

forming the intermediate shaped part to form the at least one bulge into the critical region to form the final design shape.

2. The method of claim 1 wherein the forming step is performed in a fluid pressure forming process.

3. The method of claim 1 wherein the critical regions are areas of the final design shape that require drawing the blank into a sharp corner that exceeds the forming limits of the blank.

4. The method of claim 1 wherein the critical regions are areas of the final design shape that require drawing the blank to a depth that exceeds the forming limits of the blank.

5. The method of claim 1 wherein in the drawing step the blank is drawn to a depth that is within the forming limits of the blank, and wherein the step of forming the part to the final design shape results in forming a part that would exceed the forming limits of a single step process.

6. The method of claim 1 wherein the intermediate shaped part has at least one bulge and an increased radius critical region and wherein the intermediate shaped part has substantially the same surface area compared to the surface area of the final part shape.

7. The method of claim 1 wherein an intermediate cavity defined by the intermediate part shape is within a boundary of a final cavity defined by the final part shape.

8. The method of claim 1 wherein the drawing operation is performed in a stamping die set that has at least two dies.

9. A method of designing a part to be formed from a sheet metal blank comprising:

defining a final part shape as a computer aided design model;

analyzing the final part shape using finite element analysis;

identifying critical regions in the final part shape where forming limits of the sheet metal blank are approached or exceeded;

simulating a design shape to locally reduce the strain applied to the blank in developing an intermediate part shape based upon the shape of the final part, wherein the surface area of the intermediate part shape and the final part shape are substantially the same.

10. The method of designing a part of claim 9 wherein an intermediate cavity defined by the intermediate part shape is within a boundary of a final cavity defined by the final part shape.

11. The method of designing a part of claim 9 wherein in the analysis step finite element analysis is used to determine nodal forces in the critical regions and imposing the nodal forces required to transform the final design shape into the intermediate shape in the step of simulating the design shape.

12. The method of designing a part of claim 9 wherein in the step of simulating the design shape the final part shape is elastically deformed to develop the intermediate part shape so that the surface area relationship of the intermediate part shape and the final part shape is maintained and the intermediate shape is within the final part shape.

13. The method of designing a part of claim 9 wherein at least one bulge of the blank is designed to be formed in the intermediate part shape that is reformed to form the at least one bulge into a deep cavity.

14. The method of designing a part of claim 9 wherein at least one bulge is designed to be formed in the intermediate part shape that is reformed to form the at least one bulge into a sharp corner.

15. The method of designing a part of claim 9 wherein the intermediate part shape is developed in an iterative process wherein at least one bulge is designed to be added in at least one critical region, and further comprising testing the final part to verify that forming limits are not exceeded to form the final part.

16. A method of designing a part to be formed from a sheet metal blank comprising:

defining a final part shape as a computer aided design model;

analyzing the final part shape using finite element analysis;

simulating a design shape to locally reduce the strain applied to the blank in developing an intermediate part shape based upon the shape of the final part, wherein an intermediate cavity defined by the intermediate part shape is within a boundary of a final cavity defined by the final part shape.

17. The method of designing a part of claim 16 wherein the step of simulating the design shape further comprises elastically deforming the final part shape in the simulation to develop the intermediate part shape so that the surface area relationship of the intermediate part shape and the final part shape is maintained.

18. The method of designing a part of claim 16 wherein at least one bulge of the blank is designed to be formed in the intermediate part shape that is reformed to form the at least one bulge, wherein the at least one bulge is formed into the critical region.

19. The method of designing a part of claim 16 wherein the intermediate part shape is developed in an iterative process wherein at least one bulge is designed to be added in at least one critical region, and further comprising testing the final part to verify that forming limits are not exceeded to form the final part.

20. The method of designing a part of claim 16 wherein the forming step is performed in a fluid pressure forming process.