Mechanical behavior of aluminum deformed under hot-working conditions
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I.
INTRODUCTION
MOST hot-rolling operations of commercial nonheattreatable wrought aluminum alloys at industrial scale start at relatively high temperatures of the order of 823 K and low strain rates of about 1 s -1. By contrast, at the end of the processing schedule, the deformation temperature can drop as low as 673 K whereas the strain rate can rise up to values of the order of 30 s 1. Typically, for these materials, ingots of about 283 x 845 X 1710 mm are rolled to final thicknesses of about 5 to 6 mm in 9 to 11 passes, where the effective strain varies approximately from 0.2 to 1.1 per pass, increasing progressively toward the end of the rolling schedule. Figure 1 illustrates typical hot-rolling programs for both commercial-purity aluminum and aluminum-1 pct magnesium alloy, which describe some of the processing conditions under which such materials are manufactured. In recent years, there has been an important development in the modeling of dynamic material behavior under hot-deformation conditions initially applied to Ti6242 alloytq and later extended to many other materials including aluminum of different purities. [2,3,4] E.S. PUCHI and M.H. STAIA, Professors, are with the School of Metallurgical Engineering and Materials Science, Central University of Venezuela, Caracas 1045, Venezuela. Manuscript submitted June 10, 1994.
METALLURGICAL AND MATERIALS TRANSACTIONS A
An interesting aspect of such a model is that it considers the workpiece as a dissipator of power in the total processing system. Accordingly, the dissipator co-content, J, which has been related to the metallurgical mechanisms that occur dynamically to dissipate power, has been defined as J = f
~do-
[1]
o
The subsequent approach formulated by Prasad and coworkers u-4] presents two major inconsistencies. First, as far as the calculation of J is concerned, it does not take into consideration that the flow stress of the material (o-) is also a function of strain (e), besides temperature (7) and strain rate (~). Second, it assumes the validity of the power-law relationship of Tegart and co-workersES.6] for any strain applied to the material, in spite of the fact that such a relationship has only been employed for the analysis of "steady-state flow stress" data under low-stress conditions (o- < 18 MPa approximately for aluminum and aluminum alloys). However, it is a well-known fact that under hotdeformation conditions, the flow stress of the material is also a function of the applied strain,iS ,8] which can in turn have a particular relevance since, as pointed out before, some important manufacturing operations conducted in the hot-rolling range, such as the industrial rolling schedules shown in Figure 1, take place at decaying temperatures;
VOLUME 26A, NOVEMBER 1995--2895
5.5
rities deformed under hot-working conditions from data previously published in the current literaturd 21following, if possible, an approach similar to the one earlier developed. tl6j Second, to use such a constitutive equation to reassess the calculation of the dis
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