Control of Superplastic Cavitation by Hydrostatic Pressure
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I.
INTRODUCTION
EXTENSIVE superplastic formability has been demonstrated I in fine grain processed z 7475 A1--a high strength aerospace alloy. Practical and theoretical studies 3 have shown substantial advantages for fabrication of aerospace components by superplastic forming (SPF) of fine grained 7475 AI compared with conventionally fabricated components. These benefits are in terms of weight savings, cost savings, and in some cases, more efficient design. An undesirable characteristic of the fine grained 7475 AI is the tendency to cavitate (form small intergranular voids) during superplastic stretch-forming.4 This cavitation may limit the superplastic ductility of the material, as well as reduce its subsequent mechanical properties. 5 The possible degradation of service properties may be particularly serious in the case of the 7475 A1 type alloys, since they are often used for structural aerospace components. However, a method has now been demonstrated that eliminates this cavitation by the simultaneous imposition of a hydrostatic pressure during the superplastic deformation. In earlier studies of cavitation during metal deformation, hydrostatic gas pressures were used to reduce high temperature creep cavitation,6'7 to increase the rate of sintering of small grain boundary cavities, 8'9'~~and to eliminate cavitation during room temperature ductile tensile fracture.'1 The present work describes the first published account of the application of hydrostatic gas pressure to control superplastic cavitation. Experiments were carried out to determine the effects of hydrostatic pressures on superplastic cavitation, superplastic ductility, and subsequent room temperature tensile properties.
- 1 4 / x m diameter in the rolling plane and - 8 / z m in the short transverse direction. Two methods of superplastic deformation were used with the application of various hydrostatic gas pressures. The first involved uniaxial tensile testing, at constant true strain rate, inside a pressurized retort. The second involved blow forming into a die, at constant true strain rate, against a restraining gas pressure. These methods will now be more fully described.
A. Uniaxial Tensile Testing Superplastic tensile testing was carried out in an Instron modified to allow constant true strain rate tests. The retort, designed to provide a controlled temperature and pressure environment, is shown schematically in Figure 1. The load cell was placed inside the retort in order to measure flow stresses accurately and avoid corrections for friction forces from sliding pressure seals. Pressures up to 7 MPa could be obtained by using compressed argon. The temperature control was more difficult at high pressures because of convection currents and because of the high elongations of up to 1200 pct which required a long uniform temperature zone. However, the variation was kept below +5 ~ along the specimen length. The specimen shape and dimensions are shown in Figure 2. With this design, and appropriate grips, it was possible to test simultaneously up to
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