Effect of hydrostatic pressure on cavitation in superplastic aluminium alloys

Abstract

Cavitation during superplastic flow has been examined in 3 aluminium alloys (Supral 220, Al-7475E and AlCuLi alloy) strained in uni-axial and bi-axial tension with varying superimposed hydrostatic pressures. Measurements of volume fractions, population densities and size distributions of cavities have been made using precision density and/or quantitative metallographic procedures. It has been found that the rate of increase of the volume fraction of cavities with strain can be decreased by increasing the superimposed pressure. The experimental observation that cavity growth is controlled primarily by plastic flow and that cavitation should be greatly reduced by pressures equal to approximately half of the effective flow stress of the material is in broad agreement with theoretical predictions. The difference between the magnitude of the experimentally determined cavity growth rate factors and the smaller predicted values is believed to arise from the combined effects of non-equilibrium cavity growth, coalescence and the decrease in strain rate sensitivity with strain. It is envisaged that superimposed pressures substantially greater than the flow stress of the material would be required to eliminate cavitation completely.