Effects of alloy modification and thermomechanical processing on recrystallization of Al-Mg-Mn alloys
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INTRODUCTION
THE A1-Mg-Mn alloy 5083 is of interest for superplastic forming (SPF) because of its moderate strength, corrosion resistance, weldability, and because it has been shown to be sufficiently superplastic to permit SPF of components, t~61 Elongations of over 300 pctt2.3] have been achieved using commercial grades of the 5083 alloy. Development of SPFgrade 5083 alloys, chiefly by reducing the Fe and Si impurity content, has led to higher elongations of up to 630 pct. H,51Other modifications have involved additions of Cu, Zr, and Mn.t4.61 Vetrano et al.t3] also studied the effect of thermomechanical processing (TMP), aimed at reducing grain sizes, on superplastic elongation in commercial alloys. This study, which focused on the effect of cold rolling reductions, concluded that thickness reductions of at least 5:1 gave the best superplastic tensile elongations. In most cases, the strain rates necessary to obtain acceptable ductility are in the range of 2 • 10 4 S-I to 4 • 10 -4 S- I , requiring relatively long forming times, of the order of 30 minutes to a few hours. There is, therefore, interest in increasing the superplastic strain rate of this type of alloy through further alloy development and TMP, while retaining the low-cost casting methods for the material. There has been some success in achieving elongations of 500 to 600 pct at high strain rates of 10 -2 s -1 in the A15Mg-0.8Zr alloy system processed by powder metallurgy (PM). t71 It is easier to obtain a finer dispersion of secondphase particles and finer grain sizes by PM than by ingot casting, but the former is substantially more expensive. It is well known that a primary factor in producing superplasticity is a fine grain size, and the finer the micros-
K. KANNAN, Graduate Student, and C.H. HAMILTON, Professor, are with the School of Mechanical and Materials Engineering, Washington State University, Pullman, WA 99164-2920. J.S. VETRANO, Senior Research Scientist, is with the Structural Materials Research Section, Pacific Northwest National Laboratory, Rlchland, WA 99352. Manuscript submitted October 30, 1995. METALLURGICAL AND MATERIALS TRANSACTIONS A
tructure, the higher the strain rate for superplastic deformation (SPD). Two types of microstructures are believed to be suitable for improved SD rates: (1) stabilized subgrain structure leading to continuous dynamic recrystallization during SPD and (2) statically recrystallized equiaxed microstructure with grain sizes less than about 5 /xm, which is stable at temperatures greater than 0.5Tin (in degrees kelvin). The former has been demonstrated in the 8xxx A1-Li-Zd 8'91 and the Supral 2xxx t~~ series of alloys (by ingot casting), but there have not been many such studies with the 5083 series. Almost all studies reported for this alloy have been with (2), i.e., statically recrystallized microstructures, with grain sizes of about 8 to 10/zm. Recrystallization characteristics and resulting grain sizes in A1 alloys, arising out of a particular TMP, are related to the amount and distribution of s
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