Flat pinch hemming of aluminum panels

#This invention pertains to making flat hems or modified flat hems, sometimes called pinch hems, for aluminum panels. More specifically this invention pertains to a method of making flat, sharp pinch hems on aluminum automotive body panels that have been formed in a superplastic or other forming operation that involves stretching of an aluminum sheet.

#In a continuing effort to reduce weight in automotive vehicles, aluminum alloys are substituted for steels in many applications. Aluminum sheet alloys are not as easy to form or hem or weld, as are low carbon steel sheets. Considerable effort has been expended to develop aluminum alloys for sheet metal forming and welding. For example, U.S. Pat. No. 6,253,588, Rashid, et al, entitled “Quick Plastic Forming of Aluminum Alloy Sheet Metal” describes methods for stretch forming large sheets of superplastically formable (SPF) aluminum alloys into automotive body panels. Cold rolled Aluminum Alloy 5083 sheet that has been recrystallized to a very fine grain structure, sometimes called a pseudo-single phase material, is an example of a suitable SPF alloy.

#Automobile body panels are usually of stylish three-dimensional curvature and require a commercial quality outer surface for painting or other finishing. The methods of the '588 patent have been used by the assignee of this invention to make inner and outer deck lid panels and inner and outer lift gate panels. Other candidate vehicle closure panels include door and hood panels. Automobile closure panels have to be formed with commercially acceptable appearance and with suitable dimensional accuracy for fitting with adjacent body structures. Furthermore, flange portions of the outer panel must be capable of bending around the edges of an assembled inner panel in a hem that secures the panels in a rattle-free and attractive bond.

#FIG. 1 shows an outer deck lid panel stretch formed by an SPF process with flanges for hemming to an inner panel.

#FIG. 2 is a cross-section of a stretch forming tool and a formed deck lid like that of FIG. 1, showing the formed hem flange.

#FIG. 3 is an enlarged and exploded view, taken at region 3 of FIG. 2, of a formed flange region with a thinned hemline.

#The practice of the invention will be illustrated in connection with stretch forming of a sheet of superplasticly formable (SPF) aluminum alloy 5083. A sheet of this magnesium containing aluminum alloy is heated to a temperature of about 400° C. to 510° C. The sheet metal is formed by stretch forming. A heated sheet blank is held between two opposing forming tool members that clamp the sheet at its edges. A working gas under suitable pressure (for example air, nitrogen or argon) is introduced against one side of the sheet to progressively force it into conformance with the forming surface of a forming tool. In stretch forming, the edges of the sheet are held fixed and sealed between the complimentary forming tool halves, and the interior of the heated sheet is literally stretched into conformance against the shaping surface of a tool by the gas pressure applied to the opposite side of the sheet. The stretch forming of a complex panel for an automotive vehicle in an SPF process may require a few minutes to several minutes. Of course, the stretching results in localized thinning of the original sheet as it is stretched.

#In the case of forming an automobile body panel such as a hood, a door, or a deck lid, it is recognized that these body components typically comprise two sheets, that is, inner and outer panels. In order to attach the sheets into an assembled body closure panel it is a common practice to form a flange at one or more edges of the outer panel. After both panels have been separately formed, the inner panel is laid against the back surface of the outer panel and one or more flanges of the outer panel is folded around the edge of the inner panel in a hemming operation.

#For years vehicle body panels have been made of low carbon steel and the hemming operation was readily accomplished because of the excellent malleability or formability of the steel sheet material. However, when aluminum alloy sheet materials are stamped, stretch formed or otherwise shaped into body panels, the hemming operation is more difficult because the aluminum is not as formable as low carbon steel. The hemming operation tends to produce cracks or fractures in the bend of the hemming flange. In accordance with the subject invention, the hem flange is formed and bent at an angle to the adjacent panel surface as the aluminum alloy body panel is being formed. The panel is formed from a sheet blank using a working fluid at a suitable pressure to shape the blank against a forming surface. Preferably the forming is done at an elevated temperature, e.g. below or at a superplastic forming temperature. And the flange is bent and formed with a suitable, thinned hemline as will be described.