US20080173057A1

US20080173057A1 - Stamping of work hardenable aluminum alloy sheets

Info

Publication number: US20080173057A1
Application number: US11/625,861
Authority: US
Inventors: Gary A. Kruger; Paul E. Krajewski
Original assignee: GM Global Technology Operations LLC
Current assignee: GM Global Technology Operations LLC
Priority date: 2007-01-23
Filing date: 2007-01-23
Publication date: 2008-07-24

Abstract

Work hardenable aluminum alloy sheet material is progressively stamped to a final stamped shape in a succession of stamping operations. A blank sheet of annealed starting material is stamped into a least a first preform shape between heated stamping tools to form a preform shape and subsequently anneal the worked preform shape. After one or more annealed preform shapes have been progressively attained, the last preform shape is cooled, if necessary, and stamped to its final stamped shape between unheated stamping tools.

Description

TECHNICAL FIELD

This invention pertains to the stamping of work hardenable aluminum sheet metal alloys into automotive vehicle body panels and other articles of complex shape. More specifically, this invention pertains to the progressive stamping of work hardenable magnesium-containing aluminum alloy sheet materials into such products.

BACKGROUND OF THE INVENTION

Automotive body panels and other sheet metal products have been made of suitable steel alloys by stamping processes at ambient temperatures. The edges of a steel sheet blank are gripped by a binder mechanism and a punch pushes and draws the metal against a generally concave forming surface. Often a steel alloy is available that is suitably formable and the metal is stretched into a complex shape, such as a body panel, without tearing, wrinkling or otherwise marring the sheet. A progressive sequence of stamping and, for example, piercing and trimming operations may be successively performed on a single steel sheet to make a panel with each step being completed in a matter of seconds.
Aluminum sheet alloys would be substituted for steel in many applications to save weight. For example, some Aluminum Association alloys of the 5xxx and 6xxx series have been used in sheet stamping operations. But such aluminum stamping alloys are not as ductile and formable as steel alloys and the aluminum sheet often tears if it is stamped to the same shape. The aluminum alloy work-hardens at stamping strain rates and some portion of the sheet yields and tears. This property of aluminum alloys has limited the product shapes to which they can be formed by high production rate stamping. Complex panel shapes often have to be made in multiple pieces and welded together. This usually results in higher manufacturing cost and may complicate dimensional control of the composite product.
It is an object of this invention to provide a method for using work-hardenable, magnesium containing aluminum sheet alloys in relatively high production rate stamping and annealing operations to form one-piece products whose stamped shape includes areas of deformation that exceed the strain limits of the starting sheet material.

SUMMARY OF THE INVENTION

A method is provided for forming a stamped configuration in a sheet of work hardenable aluminum alloy material where the sheet is drawn and/or stretched by a punch tool closing into conformance with a complementary stamping tool surface. The punch tool used in sheet metal stamping typically has convex or protruding surface regions and the opposing stamping tool surface has complementary concave or valley-like regions into which metal must be stretched to achieve the desired shape. The method is useful where the desired stamped configuration contains regions of elongated and deformed metal that would be torn or otherwise damaged if the sheet metal were stretched to form the local shape in a single closure of unheated stamping tools.
An aluminum sheet metal alloy is selected for the stamping of a particular part, the material having known tensile-strain properties in its cold rolled and tempered condition preparatory for stamping. A sheet metal blank of suitable area profile and thickness is specified for the physical requirements of the part. The capability of the blank to be stamped into the desired product shape using punch/concave cavity type cooling is assessed. Such assessment may be conducted experimentally and/or by a suitable computational model using, for example, a finite element analysis method. If it is found that the proposed part cannot be made in a stamping operation without tearing or wrinkling the sheet material, the following stamping/anneal process may be used.
The strategy of the practice of this invention is to stamp a work hardenable aluminum alloy sheet blank into a preform shape that embodies a substantial portion of the deformation required to acquire the final stamped shape of the part without marring the aluminum sheet and without excessive thinning of the sheet. In some instances a second, more severely formed preform shape may be required before a final stamped shape may be attained.
In accordance with this invention, a first set of stamping tools are prepared, comprising a punch tool and an opposing complementary concave stamping surface tool, for stretching and/or drawing the aluminum sheet blank between and against the tools into a first predetermined preform shape. The opposing tools, supported and actuated by a stamping press, open and close along a common axis. They open for placement of a sheet metal workpiece and close on the interposed sheet stretching and/or drawing the sheet metal between their facing surfaces to deform the sheet material into conformance. The tools are preferably made of unitary metal blocks (steel or like tool material) with provision for internal heating. The internal heating system is constructed and arranged to heat the opposing forming surfaces of the stamping tools to temperatures at which they can heat, by thermal conduction, a stamped sheet metal workpiece to its annealing temperature. The annealing takes place just as the metal has been deformed and is still confined between the opposing stamping tools.
The function of the stamping tools for a first predetermined preform shape is to close on a blank of the sheet metal and, thus, immediately stretch the metal into the preform shape. Typically, the preform shape is formed in a second or two. Some regions of the initially soft sheet material will be extensively deformed (elongated). Such regions will be work hardened and resist further easy elongation. But the preform shape is retained for a brief annealing period (a period, for example, of five to ten seconds after the stamping tools are closed) between the hot faces of the stamping tools.
The design strategy in providing internal heating for the two tools is to heat at least those tool surface regions contacting work hardened portions of the stamping to an annealing temperature for the sheet material. For example, the relatively massive tool surfaces are heated to temperatures suitable for quickly heating, by thermal conduction, the stamped metal (or just work hardened regions of the preform) to about 350° C. to temper hardened regions in the material. After a short but suitable period, the stamping press is opened and the stamping tools release the annealed preform shape. Preferably, the hot preform shape is cooled by impingement of forced air circulation, or the like.
If a second preform structure is required before a final stamping shape can be attained, a second preform/anneal step is performed to obtain a second, more extensively deformed, annealed sheet metal preform shape. A second stage of internally heated stamping tools is used (substantially as described above) for this second stage of the stamping process.
When a suitable preform shape of the aluminum alloy sheet material has been obtained, it is stamped into a final stamped shape between unheated complementary stamping tools. Preferably the annealed preform shape is cooled, if necessary, to about ambient temperature for acquiring the final stamping shape in the deformed metal. In general, the final stamped shape is left in a work hardened condition that contributes to the strength of the final part. The stamped part is then subjected to any finishing steps to ready it for its intended use.
The method of this invention may be practiced in a continuous manufacturing line where initially soft aluminum alloy blanks are fed continually and successively through each preform stamping/anneal stage(s), each cooling stage(s), and through a final stamping shape stage. In this embodiment, the time that the workpiece spends in each stage should be about the same to permit uninterrupted, progressive, stepwise movement of workpieces through the manufacturing line.
Other objects and advantages of the invention will be apparent from a description of preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic plan view of a preform stamping of an aluminum alloy inner panel for a tailgate of an automobile vehicle.

FIG. 1B is a schematic plan view of a final shape stamping of a one-piece aluminum alloy inner panel for a liftgate of an automotive vehicle.

FIG. 2 is a schematic diagram, partly in cross-section, showing the progressive and continuous process flow of blanks of work hardenable aluminum alloy sheet material as they are moved through two stamping/anneal stages and a final stamping stage at ambient temperature. In this embodiment, preformed and annealed sheets are air cooled following stamping/anneal stages.

DESCRIPTION OF PREFERRED EMBODIMENTS

Automotive vehicle body panels such as hoods have been stamped using commercial aluminum alloy sheet materials such as Aluminum Association alloy (AA) 6111. AA 6111 has a nominal composition, by weight, of 0.9% silicon, 0.4% iron, 0.7% copper, 0.3% manganese, 0.75% magnesium, and the balance substantially aluminum. Panels such as liftgate inner panels have been stamped using AA 5182 and AA 5754. AA 5182 has a nominal composition, by weight, of 4.50% magnesium, 0.35% manganese 0.20% silicon, 0.15% copper, 0+10% chromium, 0.25% zinc and the balance substantially aluminum. AA5754 has a nominal composition, by weight, of 2.7% magnesium, 0.80% manganese, 0.12% chromium and the balance substantially aluminum. These materials are cast, hot rolled and cold rolled to a desired sheet thickness, typically 1 to 2.5 mm, and annealed or tempered to recrystallize the work strained microstructure so that the coiled sheet material is in the most formable state (O temper condition) obtainable from the alloy composition.
Generally, aluminum sheet alloys are not as formable as available low carbon steel sheet alloys. Designers of automobiles now seek body panel surfaces and configurations with relatively deep pockets and other sharp bends. Given a specific panel design the engineers of the stamping process favor making the part in a single stamping operation to save tooling costs and manufacturing time. A sheet metal blank of suitable thickness and two dimensional shape is developed together with complementary two-part (punch/concave cavity) tooling for obtaining part shape from the flat blank. Trimming and piercing of the shaped sheet metal may follow the stamping steps.
Aluminum alloys have been successfully stamped into vehicle hoods because hoods are often rather simple closure panels to make. However, vehicle door and tailgate inner panels are parts of more complex shape and they have been difficult to stamp as a single piece. Often, the aluminum alloy cannot be shaped into the configuration of the desired part, even with heavy lubrication, without tearing, wrinkling or otherwise marring the sheet material. In the usual case of tearing, the sheet is damaged because the strain limit of the material is exceeded at one or more locations of the geometry of the sheet as it is being stretched between the stamping tools.
FIG. 1B is a schematic plan view of a stamping 10 for an inner tailgate panel for a sport utility vehicle. Stamping 10 was made from a developed blank of AA5182-O sheet metal. The blank was about four feet across, three and one-half feet wide and about one millimeter thick. It was trapezoidal in plan view. Material in the location of the intended tailgate window hole was removed from the blank.
As illustrated in FIG. 1B, stamping 10 has not been trimmed for assembly with an outer panel. An inner panel like that illustrated in FIG. 1B will be hemmed or otherwise attached to an outer panel in the assembly of the tailgate. The face of the stamping shown in FIG. 1B will lie adjacent the inner face of the outer panel and the two panels when assembled provide a compartment between them for window and tailgate latching mechanisms, window wiper drives, wiring for lights, and other components carried by the tailgate.
Illustrated is the aluminum alloy sheet stamping 10 before trimming of edge material 12 and trim material 14 at the window opening 16. Stamping 10 comprises an upper portion 18 for receiving a window glass that typically contains an electrical resistance heating element for defrosting and a lower portion 20 for defining one side of a compartment for the mechanisms described above. Upper portion 18 and lower portion 20 are bent at beltline 22. Upper portion 18 and lower portion 20 extend below the plane of the drawing figure and belt line 22 above the plane. Thus, the belt line 22 in the completed stamping 10 may lie 4 to 5 inches above the plane of the edges 24, 26 of the upper portion 18 and lower portion 20. Lower portion 20 of stamping 10 has a box section defined by side walls 28, 30 and 32 and bottom 34. The box section defined by walls 28, 30, and 32 is about four inches deep and provides one side of the above described compartment for hardware and mechanisms to be contained in a tailgate. There is another prominent box section in lower portion 20 of stamping 10 defined by walls 36, 38, 40, 42 and floor 44. This box section provides a complementary depression for a license plate pocket formed in the outer panel of the tailgate, not shown.
Upper portion 18 of stamping 10 also has a box section defined by walls 46, 48, 50 and floor 52. This box portion of stamping 10 provides metal for enclosing a glass window in the assembled tailgate. There are other shaped features of stamping 10 illustrated in FIG. 1B which are used in the assembly of a tailgate and components retained between the inner and outer panels, but these other features are not as critical in the making of stamping 10 as those which have been described.
Dashed line circles 54 and 56 indicate sharp bend corner portions between stamped walls 28, 30 and bottom 34, and walls 30, 32 and bottom 34, respectively in which splits and tears occur in the aluminum alloy metal of stamping 10 if it is formed in a single step. The window area of the upper portion 18 of stamping 10 also contains a stamped box section, as described, and tears occur at the circled areas 58, 60, 62 and 64 near corners of the window opening portion 16 of stamping 10.
Thus, the six dashed line circled areas 54, 56, 58, 60, 62, and 64 indicate portions of the finished shape of the illustrated tailgate inner panel stamping 10 that are difficult or impossible to form in a single stamping operation without splits or tears when panels are to be repetitively stamped at a commercially acceptable strain rates and stamping rates. The tailgate panel illustrated by stamping 10 has significantly strained portions in many of the stamping and is hard to form in a single step from an AA5182 sheet at ambient temperatures.
Suitable two-piece complementary punch/concave cavity tooling was available for making the one piece stamping from low carbon sheet material one millimeter thick. The stamping illustrated in FIG. 1B is readily formable from steel blanks on a continuous production basis. But it is desired to make the large inner panel structure from a lighter aluminum alloy. An attempt was made to make the stamping from a blank of AA5754-O temper, using heavy lubrication and a slowed stamping stroke, but tears occurred in the product at circled regions 54-64. A second attempt was made using a blank of AA 5182-O with the same result. This led to the formulation of an embodiment of this invention.
Several blanks were prepared with a trapezoidal perimeter and a cut-out window opening portion 16 as can be perceived from the shape of the finished stamping in FIG. 1B. The next step was to partially form the panel by cycling the press some distance from the bottom of the stroke (“off-bottom”). The punch was stopped at different distances from its bottom position in which the sheet metal is fully pressed against the die surface. The amount of preforming was varied to determine the deepest part possible without necking or tearing. The preform stamping 100 for the tailgate panel that satisfied this condition is illustrated in FIG. 1A. It was formed by stopping the punch motion 19 millimeters off-bottom. Cycling the press deeper produced splits in the sheet material at the circled locations of FIG. 1B.
Preform stamping 100 contains the beginnings of many of the shape features of final shape stamping 10 in FIG. 1B. Shape features of preform stamping 100 that correspond to shape features of final stamping 10 are indicated by numerals 1xx where the xx values correspond to features described with respect to stamping 10.
As illustrated in FIG. 1A the shape of preform stamping 100 has progressed such that upper portion 118, lower portion 120 and beltline 122 are perceptible. The bent shape of the final panel has been started in the preform stamping 100. In upper portion 118 of preform 100, walls 146, 148 and 150 and bottom 152 of the window receiving box section have been substantially formed without edge tears in the sheet metal portion 114. In lower portion 120 of preform stamping 100, walls 128, 130, 132 and bottom 34 of the lowed box section have been substantially formed but not to final edge sharpness and wall angle. However, there is no equivalent forming of license plate pocket defined by walls 36, 38, 40 and 42 in the preform stamping 100. Regions 136, 138, 140 show only minimal initial forming of the license pocket. Preform stamping 100 is a result of the punch having traveled only about 75% of its stroke in pushing the sheet material toward the opposing die surface.
Sheet metal preform 100 was removed from the die and placed in a furnace sized to hold a single preformed blank. The standard heating and annealing cycle for the cold worked AA5182 preform blank was 10 minutes in the oven which was maintained at 350° C. The ten minute heating and annealing period was known to completely remove the effect of the preform stamping cold work from the sheet metal and restore its original O temper condition for the final shape stamping step.
The annealed panel was removed from the furnace, allowed to cool in ambient air to room temperature, and then placed back in the press. The annealed preform was re-lubricated with boron nitride prior to the redraw operation. The stroke of the punch was completed to its bottom position and the finish shape stamping of FIG. 1B produced without defect.
This invention provides a method of making a finished part like that illustrated in FIG. 1B in a continuous process. A process for continuously and progressively forming annealed, work hardenable, aluminum alloy blanks into articles such as the tailgate panel stamping (FIGS. 1A and 1B) is illustrated schematically in FIG. 2. The embodiment illustrated in FIG. 2 uses a first stamping/anneal stage to make a first preform shape, a second stamping/anneal stage to make a second preform shape, and a ambient temperature stamping stage to achieve a completed stamping shape in the workpieces. In this embodiment the annealed aluminum alloy workpieces leaving the first two stamping/annealing stages are cooled before entering the next stamping operation.
In FIG. 2, seven work hardenable, aluminum alloy, sheet workpieces are illustrated in seven stages of the forming process of this embodiment. Workpiece 200 is a flat, annealed, aluminum alloy (e.g., AA 5182) blank awaiting transfer into a first stamping/anneal equipment stage 220. Workpiece 200 may be pre-bent for easier placement into the first stamping/anneal equipment stage. Workpiece 200 represents an aluminum alloy blank at the start of the forming process Such a blank typically has a thickness of about one to three millimeters and a predetermined plan view (not shown) sized and shaped to make the desired final stamping shape.
Workpiece 202 is the aluminum alloy sheet material which has been stamped in the first stamping/anneal stage with the closing of the internally heated complementary tools (described below) and is being heated and annealed by thermal conduction from the internally heated tools.
Workpiece 204 is the next downstream workpiece in the progressive flow sequence. Workpiece 204 has acquired the first preform stamping shape and has been annealed. It is being cooled by a fan propelled air stream (“fan cool” with wavy arrows) before being transferred to the second stamping/anneal stage 230.
Workpiece 206 is the aluminum alloy sheet material which has been stamped in the second stamping/anneal stage 230 with the closing of the internally heated complementary tools (described below) and is being heated and annealed by thermal conduction form the internally heated tools. Workpiece 206 has acquired a second stamping preform shape and at least the work hardened regions of the preform are being re-softened by conductive heat transfer from the complementary tools. The aluminum alloy sheet material has undergone further stretching and deformation as it is transformed from the first stamping preform shape (workpiece 202) to the second stamping preform shape (workpiece 206).
Workpiece 208 is the next downstream workpiece in the progressive flow sequence. Workpiece 208 has acquired a second preform stamping shape and has been annealed. Again, it is being cooled by forced air circulation (“fan cool” with wavy arrows) before being transferred to the final stamping stage 240.
Workpiece 210 has been stamped to its final stamping shape in the final stamping stage 240. Stamping stage 240 is performed at ambient temperature with unheated tools and the stamped workpiece/article is left in a partially work hardened condition. Further deformation of portions of the aluminum alloy sheet material has occurred in arriving at the final stamping shape.
Workpiece 212 is a finish-stamped workpiece that has been removed from finish stamping stage 240. Workpiece 212 would now be moved for any finishing operations on the part such as trimming, piercing for functional openings, or attachment to another part. When the part is a tailgate panel it will be subjected to finishing steps as described above and attached to a complementary inner or outer vehicle closure panel.
In general, it is preferred that the time for each stamping and cooling stage be of about that same duration so that a continuous series of stampings are progressively formed in a continuous process.
The successive stamping stages 220, 230, and 240 play an important role in the practice of the forming method.
The first stamping stage 220 comprises an internally heated lower punch tool 221 and an internally heated upper complementary stamping surface tool 222. In the practice of the method of this invention, lower punch tool 221 would be carried by and attached in a known manner to a lower platen of a stamping press, not shown in FIG. 2 for simplification of the illustration. Since punch tool 221 is internally heated, a layer of suitable thermal insulation 224 is interposed between the back side (with respect to its punch surface) of punch tool 221 and the lower press platen. In a similar manner, a layer of thermal insulation 225 is interposed between the back side of stamping surface tool 222 and the upper press platen. Depending upon the attachment mechanisms for attaching punch tool 221 to the lower press platen and for attaching upper complementary stamping surface tool 222 to the upper press platen, insulation layers 224, 225 may be load bearing layers. Insulation layers 226 are also applied to the lateral sides of the tools 221, 222 for better control of the tool surface temperatures and to provide a cooler working environment around the press.
A least one of the press platens is movable to open the stamping tools for removal of a stamped and annealed workpiece 202 and the insertion of a blank workpiece 200. A blank sheet 200 introduced between the open tools would be placed over the surface of punch tool 221. The facing surfaces of the stamping tools 221, 222 may include a binder ring (not shown) or other sheet clamping device for gripping and restraining the edges of a sheet workpiece as the press is closed and the punch surface of punch tool 221 stretches the aluminum alloy sheet metal into suitable conformance with the complementary stamping surface of tool 222. In first stamping stage 220, the blank metal is stretched into its preform shape (like pieces 202, 204) in, for example, a second or two as the press is closed. But the preformed sheet is retained between the facing complementary surfaces of the stamping tools 221, 222 for an additional period of, for example, five to ten seconds until work hardened regions of the preform shape are annealed and tempered.
In a preferred embodiment, both tools 221, 222 are made from unitary, cast and machined blocks of steel. Holes are bored in each of the steel blocks for the insertion of electrical resistance heating rods 223. Commercially available heating rods (cal rods) are suitable for this purpose. Other methods of heating tools 221, 222 may be employed, such as with the circulation of heated oil or other fluids through machined passages in the tools.
The number, locations, and heating capacities of the heating rods are predetermined to heat the blocks of the complementary stamping tools 221, 222 so that their respective forming surfaces are at temperatures for conductively heating and annealing at least the work hardened portions of aluminum alloy workpiece 202 while the stamped preform sheet is confined between complementary surfaces of the heated tools. The heating rods are connected with suitable leads to a electrical heating control and power delivery system.
As illustrated in FIG. 2, the heating rods 223 in punch tool 221 and complementary stamping surface tool 222 are located close to, but spaced apart from, their forming surfaces (which are in contact with opposite sides of the preformed sheet, workpiece 202). For many aluminum alloys of the 5XXX family an annealing temperature of about 350° C. or so is suitable. Achieving this temperature in timely fashion in at least the hardened regions of the work piece may mean the contacting surfaces of the tools 221, 222 are heated to a higher temperature. As stated, selected work hardened portions of the first preform shape 202 may be heated, or the entire preform stamping may be heated.
In general, it is preferred to cool stamped and annealed preform workpiece 204 before it is subjected to a second preform stamping step and anneal. Cooling of workpiece 204 close to ambient temperatures places the metal in condition for a second stamping operation even though the second preform stamping is also to be annealed. Forced air cooling is usually suitable for this cooling step. Cooling with water spray may be used in place of or in addition to air cooling.
Punch tool 231 and complementary stamping surface tool 232 are likewise mounted in a stamping press, not shown. The stamping press is actuated to open and close the tools for forming and annealing a second preform shape as illustrated with workpieces 206 and 208. The punch tool 231 and complementary stamping surface tool 232 of the second stamping/anneal stage 230 are suitably formed, heated, and insulated like tools 221 and 222 as described above. The forming surfaces of tools 231 and 232 are shaped to convert the first preform stamping shape (202, 204) to the second preform stamping shape as illustrated on workpieces 206 and 208. But tools 231 and 232 contain suitable strategically located heating rods 223 for heating their forming surfaces to anneal a succession of second preform stampings 206. And tools 231 and 232 have insulation blocks 224, 225, respectively, on the back sides opposite their forming surfaces, and insulation blocks 226 on their lateral sides for facilitation of the annealing process and stamping press operation.
The cooling operation performed on annealed workpiece 208 is for the same purpose as described above with respect to the cooling of workpiece 204. And the cooling may be accomplished with forced air circulation. But it is desired that workpiece 208 be cooled close to ambient temperature for a final stamping step to be performed with unheated stamping tools in forming stage 240.
Punch tool 241 and complementary stamping surface tool 242 are mounted to platens of a stamping press as described with respect to the tools used in stamping/annealing stages 220 and 230. And the press for stage 240 moves tools 241 and 242 between open and closed positions for receiving workpiece 208 in the second preform shape and stretching it further into a final stamping shape as illustrated by workpieces 210 and 212, as well as the corresponding shapes of tools 241 and 242 as they are closed on workpiece 210. Tools 241 and 242 are not heated; the final stamping step is conducted at ambient temperatures. However, insulation blocks, such as insulation blocks 224, 225, and 226, may still be applied to these tools 241, 242 in the same manner as insulation was used in the stamping/annealing stages 220 and 230.
The cross-sectional shapes of workpieces 204, 208, and 212 are progressively, increasingly curved and complex. In order to achieve the final shape in the work hardenable aluminum alloy sheet material in a practical period of manufacturing time, it was necessary to successively stamp predetermined shapes with increasing deformation and elongation of portions of the sheet material. Some or all of the preform shapes are annealed to soften work hardened regions immediately after the stamping has occurred with the sheet material still held between heated stamping tool surfaces. The annealed and softened sheet material is preferably cooled before a subsequent preform stamping operation. An annealed preform is cooled before a final stamping operation.
In a preferred embodiment of the invention the successive stamping stages are operated “in-line” in a continuous flow of aluminum alloy sheet metal workpieces to and from the stages. In such a mode of operation, the rate of flow of the workpieces is generally set by the slowest stage. The stamping/anneal stages are likely to be the most time consuming stages. Although the stamping presses may be opened in a second or two, and closed in a like period of time, the requirement for heating a stamped piece to an annealing temperature will require more time than press actuation and metal stretching. It is expected that annealing will require about three to ten times the time required to stamp the work hardenable aluminum alloy sheet material. This means that several seconds will be available for the final unheated stamping stage and for any cooling of annealed stamped preforms between stamping stages.
The invention has been described by illustrative examples which are not intended to limit the scope of the invention.

Claims

1. A method of forming an aluminum alloy sheet into an article with a final stamped shape in a sequence of at least two stamping steps, the sheet being progressively deformed in each of the stamping steps by stretching between successive sets of a punch tool and a complementary stamping surface tool, at least one of each punch tool and complementary forming surface tool in a set being movable between an open position for receiving an aluminum alloy sheet or preform and a closed position for stamping the blank or preform, the method comprising:

heating the punch surface and complementary stamping surface of a first stamping stage of punch tool and forming surface tool so that their respective surfaces are maintained at a temperature for annealing the aluminum alloy blank material;

placing an unheated blank of an aluminum alloy sheet between the heated first stamping stage tools and closing the tools to form the blank into a preform shape and to anneal the preform shape;

removing the annealed preform shape from the first stamping stage tools;

transferring the preform shape to a set of unheated final stage stamping tools, and

forming the preform shape to a final article stamped shape between the unheated final stage stamping tools.

2. A method of forming an aluminum alloy sheet as recited in claim 1 in which the punch tool and complementary stamping surface tool of the first stamping stage are insulated and internally heated for attaining the annealing temperatures of their respective surfaces.

3. A method of forming an aluminum alloy sheet as recited in claim 1 in which the preform shape is stamped to a second preform shape and annealed in a second set of heated stamping tools before being transferred to the unheated final stage stamping tools.

4. A method of forming an aluminum alloy sheet as recited in claim 1 in which an annealed preform shape is cooled to ambient temperature before being transferred to the unheated final stage stamping tools.

5. A method of forming an aluminum alloy sheet as recited in claim 1 in which each step is performed in about the same time period so that a continuous series of like aluminum alloy sheets are successively stamped to like final stamped shapes in a continuous flow sequence.

6. A method of forming an annealed, work hardenable, aluminum alloy sheet into a formed article with a final stamped shape in a sequence of at least two stamping steps, the sheet being progressively deformed in each of the stamping steps by stretching between successive sets of a punch tool and a complementary stamping surface tool, each punch tool and stamping surface tool comprising a unitary metal block providing the punch surface or respective complementary stamping surface, at least one of each punch tool and complementary forming surface tool in a set being movable between an open position for receiving an aluminum alloy sheet blank or preform shape and a closed position for stamping the blank or preform shape, the following steps being of substantially equal duration, the method comprising:

heating the punch surface and complementary forming surface of a first stamping stage of punch tool and forming surface tool so that their respective surfaces are maintained at a temperature for annealing the aluminum alloy blank material;

placing an unheated blank of an aluminum alloy sheet between the heated first stamping stage tools and closing the tools to stamp the sheet metal blank into a preform shape and retaining the preform shape between the hated and closed tools for a period to anneal the preform shape;

removing the annealed preform shape from the first stamping stage tools;

stamping the preform shape to a final stamped shape between the unheated final stage stamping tools.

7. A method of forming an aluminum alloy sheet as recited in claim 6 in which the punch tool and complementary stamping surface tool of the first stamping stage are insulated and internally heated for attaining the annealing temperatures of their respective surfaces.

8. A method of forming an aluminum alloy sheet as recited in claim 6 in which the preform shape is stamped to a second preform shape in a second set of heated stamping tools before being transferred to the unheated final stage stamping tools.

9. A method of forming an aluminum alloy sheet as recited in claim 6 in which an annealed preform shape is cooled to ambient temperature before being transferred to the unheated final stage stamping tools.

10. A method of forming an aluminum alloy sheet as recited in claim 6 in which each step is performed in about the same time period so that a continuous series of like aluminum alloy sheets are successively stamped to like final stamping shapes in a continuous flow sequence.