Forming Limit Diagram for a Superplastic 5083 Aluminum Alloy

Abstract

The superplastic blow forming limits for a 5083 aluminum alloy were investigated at a strain state from the plane strain to the balanced biaxial at a temperature of 773 K and a strain rate of about 1 � 10� 3 s� 1. It was discovered that the equivalent strain at the forming limit was in a range of 1:3 � 0:1 for every strain state and was consistent with that obtained by the uniaxial tensile tests. Namely, the forming limit for the blow forming could be estimated from the equivalent strain-to-fracture in the uniaxial tensile test. The cavity volume fraction and the cavity growth rate at the forming limit, however, increased by changing the strain state from uniaxial to balanced biaxial through a plane strain. The relationship between the forming limit and the cavity volume fraction is dependent upon the strain state. Most research on the deformation characteristics of super- plastic materials has been carried out using uniaxial tensile tests to evaluate the flow stress, strain rate sensitivity, elongation and cavity volume fraction at various temper- atures and strain rates. The uniaxial tensile tests at high temperatures facilitated a comparison of the phenomena of creep and superplasticity and enabled the elucidation of the scientific characteristics of superplasticity. However, it is difficult to accurately estimate the capability of the super- plastic blow forming from the results of the uniaxial tensile test. The superplastic blow forming is usually performed under the biaxial tensile conditions. Therefore, the deforma- tion characteristics of the materials under the biaxial tension at high temperatures are required in order to compare them with those under the uniaxial tension. For instance, the prediction of the forming limit for the biaxial tension from the uniaxial tension is expected. There are many studies on the experimental and theoretical research regarding the cold press-forming limit for steels and aluminum alloys. 1-7) The theoretical and experimental results for the cold press- forming limit diagram showed that the minor strain at the plane strain state was lower than that at the biaxial strain state because the effect of the strain rate sensitivity on the plastic stability was not expected at room temperature. There are a few reports on the superplastic-forming limit. 8) Mahoney et al. carried out the superplastic deformation for the 7475 aluminum alloy at the uniaxial tension, the plane strain and the balanced biaxial strain state. 8) They showed that a forming limit diagram was developed to relate the damage due to cavitation during forming to a corresponding decrease in the mechanical properties and that a major strain at the plane strain was as large as that at the biaxial strain state. They also described that the strain state has no measurable influence on cavity growth rate during the superplastic forming. The result is somewhat surprising since the mean stress is known to influence cavitation in creep and super- plasticity. 9) In this study, a superplastic blow-forming machine was constructed for laboratory use in which the pressure-time path was precisely controlled by a personal computer. Using this equipment, the superplastic forming for a 5083 aluminum alloy was performed in a range of strain state from a plane strain to a balanced biaxial strain. The purpose of this study is to show the forming-limit diagram for a 5083 alloy and a relationship between the forming limit and cavitation.