Abstract:
A method for forming sheet metal articles, such as automotive body panels, having complex curvatures. Opposing, complementary, pre-form and finish-form tools are used in a single press. A sheet of hot blow-formable sheet metal alloy, heated to a forming temperature, is placed between the tools. The pre-forming tool is lowered to a first stage closed position thereby trapping edges of the sheet between binder surfaces of the tools. The sheet is first stretched against the pre-form tool using pressurized gas to form a pre-form that undergoes most of the metal stretching required for the final part shape. Then the finish-form tool is pivotably advanced into a second stage closed position. The pre-form is displaced away from the pre-form tool and hot plastically formed against the opposing finish-form tool with pressurized gas to obtain the final sheet metal part shape. The finish-form tool is then pivotably retracted so as to avoid die lock or part interference conditions.

Description:
TECHNICAL FIELD  
       [0001]     The present invention generally pertains to hot-gas blow-forming of metal alloy sheet blanks into articles of complex curvature such as automotive body panels. More specifically, this invention pertains to hot blow-forming of a sheet metal blank with double-action tooling having a first, pre-form stage and a second, finish-form stage, wherein a form tool is movable along a curvilinear path so as to enable complex part geometry to be formed and extracted from the tooling while avoiding a die lock condition.  
       BACKGROUND OF THE INVENTION  
       [0002]     Sheet metal articles can be made by hot stretch-forming processes that use complementary forming tools in a press under the pressure of a working gas to stretch-form a preheated sheet metal blank against forming surfaces on the forming tools. Such processes are particularly applicable to forming sheet metal into products of complex three-dimensional curvature such as automobile body panels.  
         [0003]     In sheet metal hot blow forming processes, a hydraulic press is often used to support and move opposing forming tools that form a flat, pre-heated sheet metal blank into a three-dimensional contoured component. The hydraulic press is also preferred in order to provide balancing forces to oppose the high fluid pressures that build up between the forming tools. Hydraulic presses for shaping large parts typically open and close along a vertical axis. A vertically oriented hydraulic press, thus, has a lower platen for supporting one of the forming tools, often a punch or male finish-form tool, and an upper platen for carrying a complementary, opposing forming tool with a concave cavity, typically a female pre-form tool. The forming tools may be individually heated to maintain a suitable forming temperature for the sheet metal blank.  
         [0004]     With hot blow forming processes, as well as with more conventional mechanical stamping processes, an engineer in charge of designing a manufacturing process will often choose a two-stage, two-station forming operation if the shape of the final body panel is impossible to form in a single stage. Such a two-stage process involves a first, pre-form stage and a second, finish-form stage that, together, minimize forming time and divide the total amount of material elongation to minimize the severity of panel deformation to yield a high-quality body panel. If, however, manufacturing economics dictate use of only a single station, then the engineer will often use double-action tooling. To operate such double-action tooling, the press can be a double-action press, wherein a secondary action of the press is generally used to clamp a sheet blank and perform first-stage forming.  
         [0005]     In double-action hot stretch-forming, the sheet metal blank is inserted between the forming tools while in their open position and the press moves the forming tools from the open position to a first stage pre-forming position. Here, the edges of the blank are gripped between opposed binder surfaces of the opposed forming tools and gas pressure is applied to one side of the blank so that a central part of the blank is stretched against a pre-forming surface of the pre-form tool. Then, the opposing finish-form tool is moved closer to the now pre-formed blank in a second stage forming position. Gas pressure is now applied to an opposite side of the blank so that the central part of the blank is stretched against a finish-forming surface of the finish-form tool to complete the shaping of the blank. The press then opens for removal of the formed component and insertion of a new blank.  
         [0006]     Hot stretch-forming of certain automobile body panels, however, poses unique challenges for tooling designers. For example, a deck lid, which, when mounted to an automobile, has a generally horizontal surface for covering a top of an automobile trunk and has a generally vertical surface for defining a rear end of the trunk. Both surfaces usually have complex curved shapes or features, such as at the corners thereof or deep pockets in the vertical surface for license plate or stop lamp recesses. One challenge is that such complex features are not amenable to the traditional vertical motion of the press and forming tools, because the complex features are disposed at a negative draft angle with respect to the vertical axis of the press motion. Accordingly, there exists a die lock condition between the finish-form tool and the finish-form blank. In other words, after second stage forming, the finish-form tool cannot be vertically retracted away from the finish-form blank, or vice-versa, without binding or interference between the complex features on the finish-form tool and the complex features of the pre-form blank.  
         [0007]     Thus, there is a need for hot stretch-forming tooling designs that better accommodate part features disposed at a negative draft angle to avoid die-lock conditions.  
       SUMMARY OF THE INVENTION  
       [0008]     The present invention meets the above-mentioned needs by providing an improved method of forming a sheet metal article. The method is particularly applicable to forming sheet metal into a stretch-formed product of complex three-dimensional curvatures with recessed, pocket-like, regions of high elongation. For example, the invention is applicable to the forming of automotive vehicle body panels.  
         [0009]     In general, this invention is a method of using complementary, internally or externally heated, double-action forming tools in a single press and the pressure of a working gas to form a superplastically or quick plastically formable metal alloy sheet metal blank into a sheet metal product of complex shape. In a first stage, the sheet metal blank is given a pre-form shape involving substantial elongation of the sheet. In a second stage, the pre-form is then shaped into the final product.  
         [0010]     Panels of complex shape can be formed in a single press using usually self-heated, complementary, but not mating, forming tools in the two stage forming process. Preferably, a concave or pre-form tool defines a generally concave cavity and an opposing punch or finish-form tool has a generally convex punch surface. The blank is inserted between the forming tools with a front side facing the pre-form tool and a back side of the blank engaging the finish form surface of the finish-form tool. The pre-form tool is shaped to accomplish a major portion of the stretching and elongation of the sheet. The finish-form tool completes bends and recessed corners and defines the final shape of the sheet metal produced in this forming operation. But, preferably, most of the metal stretching is accomplished in the pre-form step.  
         [0011]     In accordance with an aspect of the present invention, the forming tools are in opposing relationship and movable from an open position for insertion of a sheet metal blank therebetween. The blank is externally preheated to its forming temperature or heated by radiation and conduction from the forming tool surfaces. In the pre-form stage of the process, the forming tools are moved to a first stage forming position in which the edges of the blank are gripped between binder surfaces of the forming tools. Then pressure of a suitable working gas, such as air or nitrogen, is applied to one side of the sheet blank to push and stretch the sheet against a pre-form tool surface of the pre-form tool. Alternatively, the pre-form stage of the process may be accomplished by mechanically imparting a pre-form shape to the sheet metal blank by mechanical stretching. Accordingly, the punch is moved toward the concave tool so as to stretch the sheet metal blank thereover.  
         [0012]     In a second stage of the process, the opposing finish-form tool is then moved closer to the pre-formed sheet in accordance with a curvilinear path. Gas pressure is then applied to the opposite side of the blank to force it against the finish-form tool to complete the shaping of the sheet metal part. The press is then opened, the finish-form tool retracted in accord with the curvilinear path, and the formed part removed to accommodate insertion of a new blank.  
         [0013]     As discussed previously, some part and finish-form tooling geometries pose a negative draft angle condition whereby it is impossible to vertically separate the part from the finish-form tooling without damage to the part after the part has been formed against the finish-form tooling. Such a die lock condition may be remedied by providing a finish-form tool that can be retracted in accordance with a path that is not necessarily straight, nor vertical, but rather is curvilinear. Accordingly, the finish-form tool is pivotably mounted about a fixed pivot axis, such that the finish-form tool may separate from the part in accordance with the negative draft angle to avoid the die lock condition therebetween.  
         [0014]     This two stage forming process enables parts with complex curvatures to be formed in a single press using a double-action forming tool, wherein die lock conditions and attendant part damage are avoided. Other objects and advantages of the invention will be understood from a detailed description of a preferred embodiment which follows. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]     These and other features and advantages of the invention will become apparent upon reading the detailed description in combination with the accompanying drawings, in which:  
         [0016]      FIG. 1  is an elevational view in cross section of a press and tooling assembly having an upper pre-form tool and a lower pivotable finish-form tool with a sheet metal blank draped over a binder ring, wherein the forming tools are open for blank loading;  
         [0017]      FIG. 2  is an elevational view in cross section of the press and tooling assembly of  FIG. 1 , wherein the pre-form tool has been lowered into a first stage forming position and the sheet metal blank has been formed against the pre-form tool;  
         [0018]      FIG. 3  is an elevational view in cross section of the press and tooling assembly of  FIG. 2 , wherein the finish-form tool has been pivoted into a second stage forming position and the sheet metal blank has been formed against the finish-form tool; and  
         [0019]      FIG. 4  is an elevational view in cross section of the press and tooling assembly of  FIG. 3 , wherein the pre-form tool has been raised and the finish-form tool has been pivotably retracted such that the now finished blank may be removed. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0020]     In general, the present invention has application in two-stage stretch-forming of a heated sheet metal workpiece in a process where pressurized air or nitrogen is applied first to one side of the workpiece and then the other side to first stretch it against a heated pre-form tool and then against a heated finish-form tool.  
         [0021]     Articles of complex shape such as automobile body panels can be made by such a practice using suitable high elongation alloys. For purposes of illustration the practice of this invention will be described in the super or quick plastic forming of fine grained, superplastically formable AA5083 sheet material about 1.5 mm in thickness. Typically, superplastic metal alloy blanks are processed to be about 0.7 to 3 mm in thickness. The aluminum alloy sheet metal blank will have been produced by a combination of hot rolling and cold rolling to a desired sheet thickness. The cold worked sheet is subjected to a static thermal re-crystallization operation to produce a suitable fine grained microstructure for superplastic or quick plastic forming of the sheet at an elevated temperature of, for example, 925° F. or 850° F., respectively. The sheet may also have at least one surface that has a high quality finish acceptable as an external visible surface of an assembled vehicle. Of course, the quality of such a sheet metal blank surface must be preserved throughout panel forming operations. When a forming analysis of the part indicates to the manufacturing engineers that the part cannot be formed in one stretching operation without producing surface folds or tears, use of the subject process may be imperative. Below, suitable press and tooling apparatus will be described for the practice of a preferred embodiment of the method of this invention.  
         [0022]     In general,  FIGS. 1 through 4  are schematic elevational illustrations in cross section of press platens and two complementary, but not mating, forming tools useful in a preferred embodiment of the invention, which illustrate the forming of an automotive body closure panel such as a deck lid outer panel.  
         [0023]     Referring to  FIG. 1 , the press and tooling assembly is indicated generally and schematically at  100  and is shown in an open position prior to pre-forming of a sheet metal blank  102 . Blank  102  is shown in cross section in its pre-form position, and has a first side or upper surface  104  and a second side or lower surface  106 .  
         [0024]     The press and tooling combination  100  comprises an upper press platen  108  (the full press structure and hydraulic actuating mechanisms are conventional and not shown to reduce the complexity of the illustration). In accordance with well known mechanisms and techniques, there is securely attached to the upper press platen  108 , a base plate  109  and a concave or pre-form tool  110  which is generally concave in configuration. The pre-form tool  110  may also be known as an upper female die, and the like. An insulation layer  112  thermally isolates pre-form tool  110  from the base plate  109  and upper platen  108 . The upper base plate  109  may be cooled, such as by passing water through cooling passages therein, to prevent excessive heating of the upper platen  108 . Similarly, the sides of the pre-form tool  110  may be wrapped in insulation layers (not shown) if desired. The pre-form tool  110  includes a pre-form surface  116  for use in shaping the blank  102 . The pre-form surface defines a pre-form part geometry or configuration for the blank  102 .  
         [0025]     In accordance with known Quick-Plastic-Forming (QPF) techniques, the pre-form tool  110  is internally heated and it is thermally insulated from the upper press structure. Thus, pre-form tool  110  may include a plurality of heating elements (not shown) distributed therein for maintaining the tool  110  and surface  116  at a temperature suitable for forming of the AA5083 sheet material. An illustrative pre-form tool temperature for this magnesium containing aluminum alloy is, for example, 500° C. Heating elements are suitably commercially available electrical resistance heaters that are connected to suitable available electric power supply and electrical control units (not shown). While the specific heating elements may be of like construction and function, it is often preferred to connect them for electrical control purposes in several different control zones. It is preferred to closely control the temperature of pre-form tool  110  and pre-form surface  116  at a specified uniform temperature.  
         [0026]     The pre-form tool  110  also includes a gas port  120  for admitting a working gas under pressure for a forming operation to be described below. Air or nitrogen is typically used as the working gas. The working gas is vented through gas port  120 , or other venting port, when the forming operation is completed.  
         [0027]     A lower press platen  130  carries a binder ring  132  and a punch or finish-form tool  134 . The punch tool  134  may also be known as a male finish-form die, and the like. Lying on lower press platen  130  is a base plate  136  that supports a water cooled support structure  138  for binder ring  132 . Unlike the upper base plate  109 , the lower base plate  136  is not provided with cooling water passages, because the binder ring support structure  138  is already water cooled. The support structure  138  carries an insulation layer  141  and cylindrical columns  140  for supporting the binder ring  132 . The binder ring  132  may be enclosed by an insulation ring (not shown). The binder ring  132  preferably contains heating elements (not shown). The finish-form tool  134  likewise may contain heating elements (not shown) for maintaining the finish-form tool  134  at the specified forming temperature of the sheet metal blank  102 . In the finish-forming of the AA5083 pre-form, finish-form tool  134  is suitably maintained at a uniform temperature of about 440° C.  
         [0028]     The sheet metal blank  102  is preferably preheated, externally of the press, and initially positioned over raised side portions  142  of the binder ring  132  when the press/tool assembly  100  is in its open position. Thus, the hot flexible blank  102  is draped over the binder ring  132  and above the finish-form tool  134 . When the press is closed for pre-forming, or first stage forming, edges  103  of the draped sheet blank  102  become gripped between flat binder surfaces  111  of the pre-form tool  110  and flat binder surfaces  133  of the binder ring  132 . The edges  103  of the blank  102  remain gripped between the pre-form tool  110  and the binder ring  132  throughout the two stage forming process and until the press is opened for removal of the formed part.  
         [0029]     A gas port  144  extends through the binder ring  132  and permits the introduction of a working gas against the back side  106  of the sheet blank  102  during the pre-form step as will be described below. A sealing ring  146  is disposed between the binder ring  132  and the support  138  to seal the working gas within the press and tool assembly  100  during the pre-form step as seen in  FIG. 1 .  
         [0030]     With the preheated, flat sheet metal blank  102  loaded in the open press/tool assembly  100 , the forming process proceeds as follows.  
         [0031]     Referring now to  FIG. 2 , the upper press platen  108  and pre-form tool  110  assembly is now moved toward and against the binder ring  132  into a first stage closed position. Relative movement of upper platen  108  and lower platen  130  closes the press and tool assembly  100  to the  FIG. 2  position. The pre-form tool  110  is now positioned closer to the punch tool  134 . In this closed position of the press and tool assembly  100 , the pre-form tool  110  and binder ring  132  tightly secure the periphery or edges  103  of the sheet metal blank  102  between the opposed binder surfaces  111 ,  133 . The secured blank  102  thus closes the press space around the finish-form tool  134  so that working gas pressure can be maintained against lower side  106  of blank  102 . There is an additional sealing feature in the press/tool assembly  100  which is described below.  
         [0032]     In accord with one preferred aspect of the pre-form stage of the present invention, a working gas under suitable pressure is introduced through gas port  144  so that gas pressure is applied to the lower side  106  of blank  102 . Air that is trapped between the blank  102  and the cavity surface  116  is vented through a multitude of holes (not shown) in the cavity surface  116  that are fluidically connected to the gas port  120 . The pressure of the working gas forces the preheated blank  102  against the cavity surface  116  and stretches or balloons it into desired compliance with the pre-form tool pre-form shaping surface  116 . The preheat softened blank  102  and the relatively high temperature of the internally heated tools  110 ,  134  permit the blank  102  to be stretched at a gas pressure and strain rate suitable for practical and efficient forming cycles.  
         [0033]     The air pressure is suitably applied in appropriate increasing increments as described, for example, in the Rashid et al. U.S. Pat. No. 6,253,588, Quick Plastic Forming of Aluminum Alloy Sheet Metal, which is incorporated by reference herein. Within a short period (e.g., 20 to 100 seconds) the heated blank  102  has assumed the shape of the pre-form tool  110  as illustrated in  FIG. 2 . When the pre-form stretching and shaping of the blank  102  has been completed, the working gas is released through gas port  144  or other venting port. In general, much of the metal stretching required to produce the final part shape is introduced in the pre-form step. Final bending and corner details and the like are accomplished in the second forming stage, described below in reference to  FIG. 3 .  
         [0034]     As shown in  FIGS. 1 and 2 , punch tool  134  is carried by the lower press platen  130  but is movable separately therefrom. The punch tool  134  is carried on a mounting plate  150  with an insulation layer  152  positioned therebetween. The plate  150  rests on the cooled support structure  138 , with an O-ring or seal  154  mounted in a groove therein to provide a gas seal for the above described pre-form operation.  
         [0035]     The plate  150  is connected to the finish-form tool  134  by rods, posts, or cylinders  156  which extend through insulation  152 . The finish-form tool  134  is pivotably mounted to the lower platen  130  by means of a swing arm  158  that is pivotably supported by a stanchion  160  by a pivot point or pin  162 . The stanchion  160  is preferably fixed to the lower base plate  136 . Preferably, there are two laterally opposed swing arms  158  for balanced support of the finish-form tool  134 .  
         [0036]     The finish-form tool  134  and swing arm  158  are pivotably actuated by means of a cylinder and cam device  164  that is positioned preferably between the laterally opposed swing arms  158 . The cylinder and cam device  164  includes a cylinder  166  that may be mounted to the lower base plate  136  in any manner desired. The cylinder  166  may be hydraulic, electric, pneumatic, or the like. A piston  168  mounts within the cylinder  166  and is fastened at a distal end thereof to a drive cam  170 . Accordingly, the cylinder  166  and piston  168  are actuatable to traverse the drive cam  170  back and forth. The drive cam  170  has a cam surface  172  that cooperates with a cam surface  174  on a driven cam  176  that is mounted to the mounting plate  150  under the finish-form tool  134 . Therefore, the finish-form tool  134  may be pivoted about the pivot pin  162  when the cylinder  166  is activated to drive the piston  168  and drive cam  170  toward the driven cam  176 , such that the cam surfaces  172 ,  174  slidingly engage one another to lift the finish-form tool  134 . Thus, the finish-form tool  134  moves independently of the motion of lower press platen  130 . This independent motion of finish-form tool  134  takes the form of curvilinear motion and initiates a “second stage” operation of the subject tooling and forming process. Curvilinear motion means any motion along a curved path or an otherwise non-straight path.  
         [0037]     In accord with another preferred aspect of the pre-forming stage of the present invention, the punch tool  134  may be used to mechanically impart a pre-form shape to the sheet metal blank  102 . After the press is closed for pre-forming, the punch tool  134  may be pivoted from its lowered position toward its raised position so as to mechanically stretch the sheet metal blank  102  thereover and, if desired, into contact with the concave tool  110 .  
         [0038]     Referring now to  FIG. 3 , after sheet metal blank  102  has been subjected to the pre-form step as illustrated in  FIG. 2 , the working gas is vented from between the finish-form tool  134  surface and the sheet metal  102  through port  144 , or other venting port, in the binder ring  132 . Then, the internally heated punch tool  134  is raised for the final sheet metal forming step. In  FIG. 3  it is seen that mounting plate  150  and finish-form tool  134  have been raised into closer proximity with the pre-form tool  110 , such that the finish-form tool  134  is in a second stage closed position. As can be seen, the cylinder  166  has driven the piston  168  and drive cam  170  to a fully distal position such that the driven cam  176  is locked into position above the drive cam  170 . As can also be seen, the swing arm  158  is pivoted to an upper position.  
         [0039]     After the finish-form tool  134  is in forming position, gas pressure is introduced through the pre-form tool  110  through gas port  120 . The sheet metal  102  is forced away from the forming surface  116  of the pre-form tool  110  and is stretched into contact with a finish forming surface  178  of the punch tool  134  as shown in  FIG. 3 . The finish forming surface  178  defines a predetermined finished part geometry or configuration for the finished blank  102 . The back surface  106  of sheet metal blank  102  is now in full contact with the finish forming surface  178  of finish-form tool  134 .  
         [0040]     The temperature of the finish-form tool  134  is significantly lower than the temperature of pre-form tool  110 . This lower temperature is possible because each tool is separately and internally heated. And, as described, each tool is insulated from the supporting press structure and, except for their opposing surfaces, they are insulated from each other. The lower temperature of this finish-form tool is suitable for lower strain rate finish shaping of the workpiece and to reduce the temperature of the sheet to facilitate prompt removal of the heat softened part from the tool when the press is opened.  
         [0041]     Again, the air pressure is gradually increased in increments for finish-forming and within a short period of, e.g., 80 to 200 seconds the pre-formed sheet metal has been stretched against the surface of the punch tool  134  so that it assumes the final product configuration obtained in this tool/press assembly  100 . An additional exhaust port  180  represents a manifold to supply an escape path for air trapped between the blank  102  and the tool  134 . A multitude of small vent holes (not shown) are provided in the forming surface of the tool  134  and are formed in fluidic communication with the exhaust port  180 . After the forming step, the air pressure is then released through gas port  120  or other suitable venting port.  
         [0042]     As shown in  FIG. 4 , the pre-form tool  110  may now be retracted by activation of the platen  108  for removal of the finish-formed blank  102  from the press. Before the finished blank  102  may be removed, however, the finish-form tool  134  must be pivotably retracted away therefrom. As shown in  FIG. 4 , the finished part  102  includes deep drawn portions  182  that were formed by projections  184  of the finish-form tool  134 . Moreover, the binder ring  132  closely follows the contour of the finish-form tool  134  such that a projection  186  of the binder ring  132  fits closely with a projection  188  of the finish-form tool  134 .  
         [0043]     Vertical retraction of the finish-form tool  134  would thus be impossible with the part geometry and tool geometry shown in  FIG. 4 . Vertical retraction of the finish-form tool  134  would result in deformation of the deep drawn portions  182  of the finished blank  102  and interference of the finish-form tool  134  and the projections  186 ,  188  of the binder ring  132  and finish-form tool  134 . Accordingly, the finish-form tool  134  must be pivoted out of second stage position as provided herein. The location of the pivot pin  162  is selected so as to enable pivotable clearance between the finish-form tool  134  and the binder ring  132 . The finish-form tool  134  is pivoted out of position by retracting the piston  168  back into the cylinder  166  so as to retract the drive cam  170  away from the driven cam  176 . Accordingly, the finish-form tool  134  pivotably descends in a controlled curvilinear motion by virtue of the swing arm  158  mounting arrangement.  
         [0044]     The finish blank  102  is then removed and suitably cooled. Any trimming operations and the like are accomplished to finish the making of the part. The press is now in its open position and the tooling is ready for the insertion of a new blank  102  so that the process starts again to form the next part.  
         [0045]     It should be understood that the invention is not limited to the embodiments that have been illustrated and described herein, but that various changes may be made without departing from the spirit and scope of the invention. For example, the practice of the present invention has been described in reference to hot stretch-forming an aluminum alloy AA5083 sheet metal blank into an automotive deck lid outer panel. However, the present invention is also applicable to conventional forming operations for producing a wide variety of sheet metal products. Moreover, it is contemplated that some part geometry may necessitate motion that is not strictly linear or strictly curved along a single radius. Rather, the present invention contemplates that the punch may be moved in a non-uniform motion in any combination of pivoting and linear retraction or compound curvilinear retraction. Accordingly, it is intended that the invention not be limited to the disclosed embodiments, but that it have the full scope permitted by the language of the following claims.