Reverse Bulging in Hydro/Pneumatic Sheet Metal Forming Operations: Is it Worth It?

The merits of warm and elevated temperature hydro/pneumatic sheet metal forming operations, most prominently superplastic and quick plastic forming, have been ever counteracted by two major drawbacks: slow forming rates and non-uniform thickness distribution with potentially severe thinning. Trying to resolve one of the two issues has generally led to escalating the other, so a compromise based on the nature of the part being formed is often targeted. To tackle the latter of the two issues, imposing a pre-thinning reverse bulging step has been shown to ease the problem with specific part geometries that involve large plastic strains and intricate details. The aerospace industry, however, is the prime sector that is able to afford the “seemingly” prolonged forming times associated with this approach. Yet with the lack of adequate details on the implications of utilising reverse bulging, this effort explores some of the hidden merits of the approach. A recently-developed simple monitoring technique for providing a direct feedback on the sheet’s advancement during pneumatic forming operations, coupled with an interrupted testing methodology, are utilised to have a closer look at the process. The results reveal significant time-savings that can be achieved with the proper use of reverse bulging, for both simple and complex part geometries.