Development of forming limits for superplastic formed fine grain 7475 AI
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I.
INTRODUCTION
IT is of considerable interest to be able to impart a high degree of formability (e.g., superplasticity) to high strength aluminum alloys for aerospace applications. To achieve this objective, a thermomechanical process for developing a fine grain size in precipitation hardenable aluminum alloys was established, l,Z,3This method consists of overaging the alloy to generate large precipitates, rolling to produce significant deformation around these precipitates, and finally recrystallizing to produce a fine grain size. Using this thermomechanical process, grain sizes of the order of 10 to 15 ~m have been achieved in the high strength A1-Zn-Mg alloys (7000 Series). It has been shown that the development of this fine grain size and relatively equiaxed grain morphology permits substantial superplastic deformation? Ghosh and Hamilton 4 observed cavitation in a 7075 A1 alloy after superplastic deformation and concluded that cavitation rate (1) increased with increasing temperature, (2) increased with larger grain sizes, and (3) showed a maximum at an intermediate strain rate of 1.76 • 10-4s -l. The work presented here extends these efforts to include the fracture tough 7475 alloy and evaluates the effect of stress state on cavitation rate during gas pressure forming of highly strained parts. 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 correlate the extent of cavitation with tensile and fatigue properties. Based on these results, a forming limit diagram is presented which predicts the design penalty associated with different levels of cavitation. II.
MATERIALS
All tests were performed using a conventional composition of 7475 aluminum with composition limits shown in Table I. Per procedures discussed in References 1 through 3, this material was thermomechanically processed to sheet M.W. MAHONEY, Senior Research Specialist, C.H. HAMILTON, Director, and A.K. GHOSH, Group Manager, are all with Rockwell International Science Center, Thousand Oaks, CA 91360. Manuscript submitted August 17, 1982. METALLURGICALTRANSACTIONS A
- 2 . 5 mm thick resulting in a dL grain size of --14 Ixm and an aspect ratio less than 2. Microstructure of the thermomechanically processed sheet as compared to as-received plate is shown in Figure 1.
III.
EXPERIMENTAL PROCEDURES
A. Tensile Tests Evaluation of the superplastic potential of fine grain 7475 A1 was conducted by determining the flow stress over a strain rate range of 10-5 to 2 x 10-2s -I at temperatures of 499, 516, and 527 ~ as well as by constant strain rate tests to failure within the same range of parameters. Superplastic flow characteristics were determined by a step strain rate test method. The test technique has been described elsewhere 5 and is a modification to Backofen's method. 6 In this method, a tensile test is started with a slow strain rate and each time load attains a maximum, the strain rate is incremented to a higher value. The
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