Resonant vibration behavior of an Al-3.8Cu-0.8Li-0.3Mg alloy
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also more homogeneous. This is consistent with the even greater homogeneity found in DC-cast AA5754, which, for the same amount of final cold work as the strip-cast material, has seen much larger amounts of thermomechanical processing.[9] The less homogeneous grain structure of the strip-cast material probably explains why under certain conditions it has been observed to be more sensitive to abnormal grain growth than the DC-cast material. For example, during our work, 90 pct cold-rolled strip-cast AA5754 showed abnormal grain growth during annealing at 525 °C. The material that experienced primary warm rolling yielded a slightly larger grain size after intermediate and final annealing. The amount of deformation required to store the equivalent deformation energy certainly increases with the warm rolling temperature.[14] Thus, for a given degree of deformation, a higher working temperature results in a coarser grain size and a higher recrystallization temperature. Also, the sheets that first underwent warm rolling exhibit fewer potential nucleation sites during intermediate annealing, which promotes the growth of a few large grains and a less homogeneous grain structure. Thus, there appears to be no benefit attained from warm rolling, from a grain size perspective. Casting of AA5754 strip using the Alcan Belt Caster results in an equiaxed structure with clusters of large Fe-based intermetallic particles and centreline porosity. During thermomechanical processing, the final grain size and its distribution depend sensitively on the amount of secondary deformation. Secondary rolling and annealing also promote homogenization of the grain structure. Increased deformation and higher iron content both induce a finer structure, which suggests the importance of particle-stimulated nucleation as a major recrystallization mechanism. This study also shows that starting with a warm rolling step induces a coarser and less homogeneous grain structure after both intermediate and final annealing. In terms of grain size, our results are quite comparable to those reported for DC cast alloys, the main difference being that the more heavily worked DC-cast AA5754 develops a more homogeneous grain structure in the last stages of processing. The fact that more processing steps favor a more homogeneous grain structure represents an ongoing challenge to the development of formable strip-cast AA5754 for the automotive industry. REFERENCES 1. W.S. Miller, L. Zhuang, J. Bottema, A.J. Wittebrood, P. De Smet, A. Haszler, and A. Vieregge: Mater. Sci. Eng., 2000, vol. A280, p. 37. 2. P.A. Friedman and A.M. Sherman: in Automotive Alloys II, S. Das, ed., TMS, Warrendale, PA, 1998, p. 147. 3. Y.G. Kim, B. Farouk, and D. Apelian: in Modelling of Casting and Welding Processes—IV, A.F. Giamey and G.J. Abbaschian, eds., TMSAIME, Warrendale, PA, 1988, p. 265. 4. P.C. Reagan: Light Met. Age, 1971, Apr., p. 10. 5. C.J. Petry: in Aluminum Alloys—Physical and Mechanical Properties, E.A. Starke and T.H. Sanders, eds., Engineering Mat
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