Superplastic behavior of spray-deposited 5083 Al
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I.
INTRODUCTION
RECENTLY, spray deposition processing has received considerable interest as a method of producing bulk materials with uniform, refined microstructures and improved mechanical properties as compared to those of conventionally processed materials. The morphology of grains in most spray-deposited materials is generally equiaxed with grain sizes in the range of less than 50 mm.[1–6] Theoretically, the maximum segregation distance in spray-deposited materials is limited by the size of atomized droplets, which is less than 100 mm, that build up a bulk preform.[7] Moreover, grain size may be maintained to less than 20 mm by carefully controlling the thermal and solidification conditions. The aforementioned microstructural characteristics render spray-deposited materials potentially suitable for superplastic applications. Refining grain size by spray deposition processing reduces the number of thermomechanical processing (TMP) steps needed to achieve superplasticity. Another important characteristic of spray-deposited materials is their reported extension of solid solubility well beyond the equilibrium values. Moreover, it is possible to introduce dispersoids or refine intermetallic phases using spray deposition processing.[8,9] Such dispersoids help to restrain grain coarsening during elevated temperature deformation, presumably contributing to high superplasticity.[8,9] Commercial alloy 5083 Al exhibits superplasticity following thermomechanical processing.[10] Grain refinement is typically accomplished by precipitation of fine particles, which is achieved using thermomechanical treatment to produce a heavily deformed or partially recrystallized microstructure followed by an annealing treatment to produce YUE WU, Specialist, LINDA DEL CASTILLO, Graduate Research Assistant, and ENRIQUE J. LAVERNIA, Professor, are with the Department of Chemical and Biochemical Engineering and Materials Science, University of California, Irvine, CA 92697-2575. Manuscript submitted June 20, 1996.
METALLURGICAL AND MATERIALS TRANSACTIONS A
a recrystallized microstructure with fine, equiaxed grains.[10–16] The alloy concentration in 5083 Al renders difficult precipitation as a means to control grain size. Accordingly, attempts are made to investigate the possibility of using spray deposition to reduce grain size in Al-Mg alloys and to study the superplastic response of spray-deposited materials.
II.
EXPERIMENTAL
A. Material Selection A commercial 5083 Al alloy was selected as the starting material for spray atomization and deposition. The nominal composition of this alloy is Al-4.5Mg-0.7Mn (in wt pct). B. Spray Deposition Bulk, spray-deposited alloys were prepared according to the following procedure. First, the alloy was superheated to a temperature of 1173 K under N2 gas protection. Second, the melt was delivered to an atomizer and was disintegrated into a fine dispersion of micrometer-sized droplets using N2 gas at a pressure of 1.24 MPa. The spray of droplets was collected using a water-cooled Cu substrate (disk) at
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