It has been shown that the application of hydrostatic gas pressures during superplastic deformation of fine grained 7475 Al can entirely prevent the intergranular cavitation normally encountered at atmospheric pressure. A critical ratio of hydrostatic pressure to flow stress may be defined for each superplastic forming condition above which virtually no cavitation occurs. In superplastic deformation conditions where intergranular cavitation plays a significant part in final tensile rupture, the superplastic ductility may be improved by the application of hydrostatic pressures. Similarly, detrimental effects of large superplastic strains on service properties may be reduced or eliminated by the application of suitable hydrostatic pressures during superplastic forming. In this case, superplastically formed material may have the same design allowables as conventional 7475 Al sheet.

Control of Superplastic Cavitation by Hydrostatic Pressure

This paper summarizes the results of some recent research on a thermomechanical method of refining the grain size in precipitation hardenable aluminum alloys and illustrates the infuence of grain refinement on several material properties. Grain refinement is achieved by deliberately introducing a large number of nucleation sites for recrystallization and by controlling grain growth after recrystallization. Recrystallization to a relatively small and equiaxed grain size has been achieved in a number of commercial aluminum alloys using these concepts. The influence of the fine recrystallized grain size on such properties as superplastic deformation, room temperature tensile properties, fatigue life, and exfoliation corrosion resistance is discussed. The results show that refinement to a grain size of 8–14 µm is sufficient to develop extensive superplasticity and to yield a small increase in tensile properties in alloys such as 7075 and 7475.

Characterization of Fine-Grained Superplastic Aluminum Alloys

An impression test method for characterization of the flow behavior of superplastic material

M.G. ZELIN, formerly Post Doctoral Researcher with the Department of Chemical Engineering and Materials Science, University of California, Davis, CA 95616-5294, is Senior Engineer, Concurrent Technologies Corporation, Johnstown, PA 15904. Manuscript submitted October 24, 1995. deformed surface of unfailed materials, t6,71 Since the appearance of these fibers suggests extremely high ductility of the material, the process of fiber formation has been referred to as microsuperplasticity phenomenon.tJ,z.31 Predominately, the observations of microsuperplasticity phenomenon were reported in materials subjected to the uniaxial tension. Recent s t u d y [8'91 of the deformed surface of superplastically bulged AA7475 domes demonstrated fiber formation after biaxial stretching. This article summarizes the results of the observation of fiber formation in AA7475 domes of various shapes. AA7475 blanks, 50 X 50 x 1.5 mm 3 were bulged with the punches of hemispherical, cylindrical, and conical shape. The experimental details can be found elsewhere.[8,9~ Superplastic AA7475 sheet material with a grain size of approximately 10/~m was obtained from Kaiser Aluminum and Chemical Co. (Pleasanton, CA). The rate of punch displacement and temperature corresponded to the optimum strain rate-temperature conditions for this alloy: f6,7,1~ l] ~: = 2 x 10 4 S-I and T = 516 ~ The blanks were mechanically polished before deformation with the final polishing on alumina with 0.5 /xm particle size. Marker lines were inscribed at the polished surface by using 9/~m diamond paste parallel and perpendicular to the rolling direction. The deformed surface was examined in a scanning electron microscope (SEM). Chemical analysis was performed by us-

On microsuperplasticity in aa7475 domes

Elevated temperature flow and fracture studies have been carried out on both ingot and powder processed Al-Zn-Mg-Cu (7000) series aluminum alloys. Ingot metallurgy materials were subjected to a thermomechanical processing treatment in order to produce a grain size similar to that present in the powder processed materials. Tensile specimens were subjected to high temperature testing in air in order to determine elevated temperature flow properties. Strain rate sensitivity of the flow stress, and elongation to fracture were the parameters of interest. Microstructure was determined by both optical and transmission electron microscopy methods. These results, together with those derived from scanning electron microscopy studies of the fracture surfaces, were correlated to the elevated temperature flow and fracture properties. Results of tests conducted on wrought materials and on powder materials fabricated by two different routes indicate that for a given grain size, flow stress, and strain rate sensitivity of the flow stress, may not be significantly affected by fabrication method. Ductility, as measured by strain to fracture is significantly influenced by fabrication methods, however, with the wrought material showing the greatest ductility. The extent of ductility exhibited by the powder processed material is governed by the details of fabrication method and the resultant grain boundary structure. Details will be provided in this paper.

Elevated Temperature Deformation and Fracture of Ingot and Powder Processed Aluminum Alloys

: fine grained 7475 aluminum alloy sheet was evaluated for its superplastic properties and potential for superplastic forming. The sheet alloy was specially processed by the ALCOA Technical Laboratory utilizing procedures prescribed by the contractor to achieve a fine grain size. The grain size measurements on this sheet alloy after complete recrystallization revealed a short transverse dimension of 7.8 microm and longitudinal and long transverse dimension of 14 microm. Subsequent grain growth studies showed that this fine grain size developed is quite stable at temperatures as high as 516 C . High temperature testing was conducted over the temperature range of 427 C to 516 C in order to establish the strain rate sensitivity of flow stress, strain hardening characteristics, and total elongation capabilities. Superplastic forming tests were also conducted utilizing commonly used gas pressure forming methods. Small demonstration parts were formed over a range of strain rates and thinning characteristics evaluated. Metallographic evaluation of tensile test specimens and formed parts revealed the tendency of the alloy to form internal voids, or cavities, at large strains. The cavitation appears to be sensitive to the conditions of deformation, being minimized at the higher temperature and lower strain rates.

C H Hamilton

Papers

Fine Grain Aluminum Superplasticity