Abstract
Forming limits in conventional sheet metal forming are given by strain levels obtainable prior to the onset of a localized neck or tear in the sheet. While the external appearance of such a neck is not observed in superplastic metals until strains become quite large, the formation of internal cavities could dictate the tolerable levels of strain in formed components. In this paper, these useful strain limits for a superplastic 7475 Al alloy have been explored. The approach used was to establish the influence of strain state (uniaxial, plane strain, and balanced biaxial) on the inception and growth characteristics of cavities and to correlate the extent of cavitation with material properties. Based on these data, it was then possible to establish strain states for which little or no loss in properties was observed, and thereby to define forming limits for superplastic forming this material. These results, coupled with comparisons against strains developed in actual parts as well as analytically predicted strains, show that a wide range of structural parts can be superplastically formed within the constraints of the recommended forming limits.
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