Patent Publication Number: US-6910358-B2

Title: Two temperature two stage forming

Description:
TECHNICAL FIELD 
     This invention pertains to hot stretch forming of a sheet metal blank between a preform tool (first stage) and then a final-form tool (second stage). More specifically, this invention pertains to such two stage stretch forming where the preform tool is maintained at a higher forming temperature than the final-form tool. This enables faster forming in the preform stage and distortion-free removal of the part from the final-form tool. 
     BACKGROUND OF THE INVENTION 
     Automotive body panels can be made by sheet metal stretch forming processes that use complementary, double action forming tools in a press and the pressure of a working gas to stretch form a preheated blank against the forming surfaces. In one embodiment, the process is applicable to stretch forming of a superplastically formable or quick plastically formable metal alloy blank into a sheet metal product of complex shape. The metal alloy may, for example, be a magnesium-containing, aluminum alloy having a fine-grained microstructure (grain size suitably less than ten micrometers) for high elongation plastic forming. Typically the aluminum alloy sheet has a thickness in the range of about 0.7 to 4 mm. The sheet metal blank is given a preform shape involving substantial elongation of the sheet. In a second action of the tools the preform is then shaped into the final product. Such a process is described in U.S. patent application Ser. No. 10/274,493, filed Oct. 17, 2002, entitled “Gas Pressure Preforming Double Action Superplastic or Quick Plastic Forming Tool and Method”, and assigned to the assignee of this invention. That specification, including the drawing figures, is incorporated by reference into this application for its description of the two-stage forming process. 
     The method is particularly applicable to forming the sheet metal into a stretch formed product of complex three-dimensional curvature and regions of sharp corners and high elongation. For example, the invention is applicable to the forming of automotive vehicle body panels. 
     In accordance with two stage forming using gas pressure, the sheet metal is usually formed in a single press using complementary, but not mating, heated forming tools. The tools are in opposing (facing) relationship and movable from an open position, for insertion of a sheet metal blank, to their forming positions. Preferably, the blank is externally preheated to a desired forming temperature. After insertion of the preheated blank, the tools are moved to a first stage preforming position. The edges of the blank are gripped by a binder mechanism and gas pressure is applied to one side of the heated sheet to stretch it against a preform tool surface. The opposing, finish-shape tool is then moved closer to the preformed sheet in a second stage forming position. Gas pressure is applied to the opposite side of the sheet to force it back against the finish-form tool to complete the shaping of the sheet metal part. The press is then opened for removal of the formed part and insertion of a new blank. 
     The preform tool is shaped to accomplish a major portion of the stretching and elongation of the sheet in forming it toward the final part shape. The finish tool completes bends and recessed corners and defines the final detailed shape of the sheet metal produced in this press operation. In each forming stage, the pressure of a suitable working gas, such as air or nitrogen, is used to push and stretch the sheet against the respective tool surfaces. The pressure is applied to opposite sides of the sheet in the successive preform and finish-form steps. Thus, the necessary elongation lines or stretch directions in the sheet to form the part are predetermined. A substantial part of the elongation is accomplished in the preform step and is introduced nearly evenly over the preform shape. The final elongation is accomplished by forcing the preformed sheet away from the preform tool against the shaping surfaces of the finish shape tool. 
     This stretch forming process is efficient in its utilization of a single press with two forming tools. However, the working gas must be applied and vented from each side of the metal workpiece and the forming must be done at a strain rate that does not introduce defects in the visible surface of the formed part. The overall process has remained slow for high volume production operations. Accordingly, it is an object of this invention to increase the forming speed of the two stage stretch forming process and minimize localization of the strain in the formed part. 
     SUMMARY OF THE INVENTION 
     The practice of this invention focuses on control of the respective temperatures of the preform tool and the finish-form tool in the two stage stretch forming of suitable sheet metal blanks. Both tools are preferably insulated from the supporting press structure and independently, internally heated to provide different uniform temperatures across their respective forming surfaces. The blanks are typically preheated for the two stage forming process. 
     Briefly stated, the preforming tool is maintained at a relatively high temperature to facilitate rapid plastic elongation of the sheet material as it is stretched under suitable working gas pressure and ballooned against the surface of the preform tool. The preform tool preferably has a concave surface to receive the ballooning blank. The relatively high temperature of the preform tool surface permits the preheated blank to be initially shaped at a relatively high strain rate for the sheet metal alloy. A purpose of the higher preform temperature is to use a lower working gas pressure, consistent with a high strain rate, which permits more rapid venting of the preform gas. Thus, the preform step introduces substantial elongation in the blank by establishing a gross shape approximating the final shape of the part. Such preforming permits the final forming of the detailed bends, curvatures and other shape features in the final part without tearing or marring of the formed part. 
     The temperature of the finish-form tool surface is lower than the surface of the preform tool. This means that the preform part experiences some cooling as it is pushed from the preform tool to the finish form tool. The finish-form step is carried out at a somewhat lower temperature at which the sheet metal retains suitable ductility for final forming but also achieves more rigidity for distortion-free removal of the part from the finish-form tool. 
     The practice of this invention is useful in the two stage stretch forming of any sheet metal having suitable ductility at an elevated temperature for such plastic deformation. Various aluminum, magnesium titanium and ferrous alloys can be processed into sheets having a ductile metallurgical microstructure. Usually the sheets are formed by hot rolling a cast billet to a strip and then cold rolling the strip to a sheet of desired thickness and surface finish. Depending upon the material, the cold worked sheet may be heat treated to obtain the necessary ductility. 
     In one illustrative embodiment, this invention is used in the stretch forming of magnesium containing aluminum alloys (such as AA5083) that have been cold rolled and recrystallized to a very fine grain structure. These alloys display tensile elongations in excess of 300% at forming temperatures in the range of 450° C. to about 550° C. and have been formed into automotive body panels such as deck lid outer panels. In such an embodiment, this invention is practiced, for example, by preheating the blank to about 500° C. and maintaining the preform tool at the same temperature and the finish-form punch at about 440° C. 
     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 
         FIG. 1  is an elevation view in cross section of separately heated and temperature controlled upper preform tool and lower finish form tool with a sheet metal workpiece shown stretched against the preform tool. 
         FIG. 2  is an elevation view in cross section like  FIG. 1  with the sheet metal workpiece shown stretched against the finish form tool. 
     
    
    
     DESCRIPTION OF A PREFERRED EMBODIMENT 
     This invention has application in the two-stage stretch forming of a heated sheet metal work piece 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 preform tool and then against a heated finish form tool. As described in the above referenced U.S. patent application, 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 quick plastic forming of fine grained, superplastically formable AA5083 sheet material about 1.5 mm in thickness. Suitable press and tooling apparatus will be described for the practice of a preferred embodiment of the method of this invention. 
       FIGS. 1 and 2  are schematic illustrations in cross section of an elevation view of press platens and two complementary, but not mating, forming tools useful in a preferred embodiment of the invention. They illustrate the forming an automotive body closure panel such as a deck lid outer panel preform configuration as illustrated in  FIG. 1  of the above referenced patent application and then a deck lid panel final configuration as seen in  FIG. 2  of that U.S. application. 
     Referring first to  FIG. 1  of this specification, the press and tooling assembly is indicated generally and schematically at  100  and is shown in an operative position for the preforming of a sheet metal blank  102 . Blank  102  is shown in cross section and on edge in full line depiction in its preform position as will be described shortly. Sheet metal blank  102  (with a dashed lead line) is also shown in a preliminary position before preforming. As best seen on the blank  102  preliminary position, the blank has an upper surface  104  and a lower surface  106 . 
     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). Securely attached to upper press platen  108  is a preform tool  110  which is generally concave in configuration. An insulation layer  112  thermally isolates preform tool  110  from upper platen  108 . Similarly, the sides of preform tool  110  are wrapped in insulation layers  122 . Preform tool  110  includes a preform surface portion  116  for use in shaping the workpiece preform from blank  102 . 
     Preform tool  110  is internally heated and it is thermally insulated from the upper press structure. Thus, preform tool  110  comprises a plurality of heating elements  118  for maintaining the tool and surface  116  at a temperature suitable for forming of the AA5083 sheet material. An illustrative preform tool temperature for this magnesium containing aluminum alloy is, for example, 500° C. In addition to the insulation layer  112  between press platen  108  and preform tool  110 , the four sides of preform tool  110  are enclosed in insulation blocks  122  (two blocks shown in the sectional views of  FIGS. 1 and 2 ). 
     Heating elements  118  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 (zone boundaries  119 ,  121 ) as indicated on tool  110  in  FIG. 1 . It is preferred to closely control the temperature of preform tool  110  and preform surface  116  at a specified uniform temperature. Depending on the size and shape of the tool  110 , the heater current draw requirements in different heater element  118  zones can vary due to differences in heat losses. As suggested by the spacing of heating zone boundary lines  119 ,  121  in  FIGS. 1 and 2  for tool  110 , the central heating zone between boundaries  119  and  121  may be larger and its heater elements maintained at an appropriate temperature for the heating of preform surface  116  to a specified uniform temperature. The heater zones outside the boundaries  119  and  121 , outside the preform surface  116 , may require different current draws or duty cycles to contribute to the uniform temperature of preform surface  116 . 
     Preform 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. 
     The press lower platen  130  carries a binder ring  132  and a punch tool  134 . Lying on press lower platen  130  is a layer of insulation material  136 . Insulation layer  136  carries a water cooled support structure  138  for binder ring  132 . The water passages are indicated at  139 . Support structure  138  carries cylindrical columns  140  for carrying binder ring  132 . Enclosing binder ring  132  is an insulation ring  142 . Binder ring  132  contains heating elements  133 . Punch  134  likewise contains heating elements  137  for maintaining the punch tool at the specified forming temperature of the sheet metal blank  102 . In the finish-forming of the AA5083 preform, punch  134  is suitably maintained at a uniform temperature of about 440° C. 
     A preheated sheet metal blank is initially deposited on convex punch  134  when the press/tool assembly  100  is in its open position (not shown in the drawing figures). The hot flexible sheet drapes itself over punch  134  and binder ring structure  132 . When the press is closed for preforming, or first stage forming, the edges of the draped sheet  102  are gripped between the edges of the preform tool  110  and the binder ring  132 . The position of the blank at that time is as indicated at its outline position  102  in  FIG. 1 . The edges of the blank remain gripped between the preform 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. 
     Gas port  144  extending through insulation  142  and binder ring  132  permits the introduction of working gas against the back side  106  of sheet blank  102  during the preform step as will be described below. Sealing ring  141  between binder ring  132  and support  138  helps seal the working gas within the press/tool assembly  100  during the preform step as seen in  FIG. 1 . 
     With the preheated, flat sheet metal blank  102  loaded in the open press/tool assembly  100 , the forming process proceeds as follows. 
     Referring to  FIG. 1 , the upper press platen  108 /cavity tool  110  assembly is now closed against binder ring  132 . Relative movement of upper platen  108  and lower platen  130  closes the press/tool assembly  100  to the  FIG. 1  position. Cavity tool  110  is now positioned close to the punch tool  134 . In this closed position of the press/tool assembly  100 , cavity tool  110  and binder ring  132  tightly secure the periphery of the sheet metal blank  102 . The secured blank  102  thus closes the press space around punch  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. 
     Air under suitable pressure is introduced through gas port  144  so that air pressure is applied to the lower side  106  of blank  102 . This pressure forces the preheated blank  102  against the cavity surface  116  and stretching or ballooning it into desired compliance with the cavity tool, preform shaping surface as seen in cross-section in  FIG. 1 . The preheat softened blank and the relatively high temperature of the internally heated tool permit the blank to be stretched at a gas pressure and strain rate suitable for practical and efficient forming cycles. 
     The air pressure is suitably applied in appropriate increasing increments as described, for example, in the Rashid et al patent, U.S. Pat. No. 6,253,588, Quick Plastic Forming of Aluminum Alloy Sheet Metal. Within a short period (e.g., 20 to 100 seconds) the heated blank  102  has assumed the shape of the preform tool  110  as illustrated in  FIG. 1 . When the preform 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 make the final part shape is introduced in the preform step. Final bending and corner details and the like are accomplished in the next forming stage. 
     As shown in  FIGS. 1 and 2 , punch tool  134  is carried by the lower press platen  130  at support ring  138  but is movable separately from platen  130 . Punch tool  134  is carried on cylindrical supports  150  which are carried on water cooled plate  152 . In  FIG. 1 , plate  152  rests on support ring  138 . O-ring  153  mounted in a groove in water cooled support ring  138  provides a gas seal for the above described preform operation when plate  152  rests on it. 
     Plate  152  is connected to punch platen  154  by rods  156  which extend through insulation plate  136  and press platen  130 . Rods  156  are based on platen  154 . Punch platen  154  is actuated by means, not shown, to move punch  134  independently of the motion of press lower platen  130 . This independent motion of punch  134  provides the “second stage” operation of the subject tooling and forming process. 
     After sheet metal blank  102  has been subjected to the preform step as illustrated in  FIG. 1 , the internally heated punch tool  134  is raised for the final sheet metal forming step. In  FIG. 2  it is seen that punch platen  154  has been raised and the surface of the punch  134  is now in closer proximity with the cavity tool  110 . Air is vented from between the punch  134  surface and the sheet metal  102  (now in its preform shape) through port  144 , or other venting port, in the binder ring  132 . Air pressure is now introduced through the cavity tool  110  through gas port  120 . The sheet metal  102  is forced away from the surface of the cavity tool  110  and it is stretched into contact with the surface of punch tool  134  as shown in  FIG. 2 . Back surface  106  of sheet metal  102  is in full contact with the surface of punch  134 . 
     The temperature of this final-form tool, punch  134  is significantly lower than the temperature of preform 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 final-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. 
     Again, the air pressure is gradually increased in increments for final-forming and within a short period of, e.g., 80 to 200 seconds the preformed sheet metal has been stretched against the surface of the punch tool  134  so that it assumes the final product configuration,  FIG. 2 , obtained in this tool/press assembly  100 . The air pressure is then released through gas port  120  or other suitable venting port. 
     The cavity tool  110  and punch tool  134  are now separated (not shown in the drawing figures) by activation of their respective platens  108 ,  130  and  154  for removal of the finish formed part from the press. The part is 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. 
     The above described two-stage forming of a heat softened metal sheet requires, among other process and equipment parameters, careful control of the temperature of the workpiece if it is to be formed in a practical time without tearing or other damage to the formed part. The practice of this invention results in new process efficiencies by focusing on the control of the temperatures of the shaping tools. The method utilizes separately and internally heated preform and finish-form tools to shorten the duration of the forming steps while making defect-free parts. The tools are maintained at different temperatures with the preforming tool at the higher temperature. 
     The hotter preform tool increases the formability of the workpiece. Such increased formability enables the sheet to be stretched to its preform shape at a higher strain rate and lower working gas pressure. Stretching at a higher strain means that the sheet can be stretched faster. For example, by increasing the temperature of AA5083 sheet material by 50° C., from 425° to 475° C., the useful strain rate can be increased from 0.004s −1  to 0.01 s −1  at a gas pressure of 83 psi. Use of lower gas pressure enables the gas to be vented from the preform stage faster. 
     There are also advantages from use of a cooler finish-form tool. The final forming is preferably done at a lower strain rate to assure the detailed shaping of a defect-free part. And the cooler part is easier to remove from the finish-from tool without distortion. 
     The practice of the invention has been described by an illustrative example. But the scope of the invention is broader and not limited by the example.