periphery. A sacrificial overlay sheet is then placed on top of it and clamped to the tool periphery. As gas is injected into the upper die cavity, the overlay sheet forms down over the lower die, forming the part blank simultaneously with it. While overlay forming does help to minimize thickness variations, it requires a sacrificial sheet for each run that must be discarded. Two other forming methods, shown in Fig. 2.22, were developed to reduce thickness non-uniformity during forming. However, both of these methods require moving rams within the pressure chamber which increases capital equipment costs.
The hard particles at the grain boundaries that help control grain growth may contribute to the formation of voids in aluminum alloys, a process called cavitation. Cavitation on the order of 3% can occur after about 200% of superplastic deformation. Cavitation can be minimized, or eliminated, by applying a hydrostatic back pressure to the sheet during forming, as shown schematically
Superplastic Forming Parameters for Aluminum1
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