Development and Validation of an Analytical Seal Bead Design Model for Automotive Superplastic Forming

With the increasing demand for fuel efficient vehicles, technologies like superplastic forming (SPF) are being developed and implemented to allow for the utilization of lightweight automotive sheet materials. While forming under superplastic conditions leads to increased formability in lightweight alloys, such as aluminum, the slower forming times required by the technology can limit the technology to low to mid production levels. One problem that can increase forming time is the reduction of forming pressure due to pressurizing (forming) gas leaks, during the forming cycle, at the die/sheet/blankholder interface. Traditionally, such leaks have been successfully addressed through the use of a seal bead. However, for advanced die technologies that result in reduced cycle times (such as hot draw mechanical performing, which combine aspects of mechanical preforming of the sheet metal followed by SPF), the use of seal beads can restrict the drawing of sheet material into the forming die. Therefore, it is desirable to design and implement a bead that can provide the required sealing at the interface, yet allow for the material to flow past the seal bead and into the die. The objective of this paper is to develop and validate an analytical method of determining the required seal bead geometry that is capable of allowing material to draw over the bead while sealing and eliminating leaks during forming. Comparisons between numerical and analytical methods are presented along with a seal bead which was developed based on the analytical method and tested on a pre-existing die.