Heating Rate Effects on Recrystallized Grain Size in Two Al-Zn-Mg-Cu Alloys
- PDF / 2,259,741 Bytes
- 6 Pages / 594 x 774 pts Page_size
- 52 Downloads / 236 Views
I.
INTRODUCTION
A
method of thermomechanical processing has previously been described ~ whereby a range of commercial heattreatable aluminum alloys may be produced in sheet form with stable grain sizes c o n s i d e r a b l y finer than conventionally processed sheet. The process is based on static, discontinuous recrystallization; that is, recrystallization occurs spontaneously when the alloy is heated, and it occurs by nucleation and growth of new grains. Further deformation at the recrystallization temperature is not required to induce recrystallization as with some aluminum alloys. 2'3 In the thermomechanical process described in Reference I, a number of recrystallization nuclei are deliberately introduced in order to obtain a fine grain size. This is achieved by precipitating coarse, overaged particles, around which are formed intense deformation zones (regions of high lattice rotation) during warm rolling. These deformation zones provide a high density of very favorable recrystallization nuclei. The overaged precipitates are generally dissolved during and after the recrystallization treatment, and the fine grain size is stabilized by the fine dispersion of insoluble intermetaiiic particles present in most high-strength aluminum alloys (Cr, Mn, or Zr/AI intermetallics). This fine grain processing technique has been shown 4 to produce microstructures in 7075 Al-type alloys that enable considerable superplastic formability to be obtained. It was shown ~ that, in the case of 7075 AI, the variation in density of particles greater than a critical size (0.75 /xm diameter) correlates well with the recrystallized grain density. This was taken as strong evidence that the overaged particles are involved in the nucleation stage of the reTable I.
crystallization process. Two further observations, however, suggested that the effective nucleation of new grains at each of the particles is not simply dependent on particle size and degree of cold work. First, it was shown that the recrystallized grain densities vary with heating rates to the recrystallization temperature, and second, only about 10 pct of the available particles appear to act as effective recrystallization nuclei, even with the most rapid heating rates. The present work was undertaken to quantify the effect of heating rate to the recrystallization temperature on grain size of 7075 and 7475 aluminum using the otherwise standard 1 fine grain processing. Metallographic studies were also carried out in order to demonstrate the microstructural mechanisms occurring in the early stages of the recrystallization process.
II.
EXPERIMENTAL
PROCEDURE
Two alloys, 7075 and 7475 A1, were studied, their chemical compositions are given in Table I. The standard fine grain thermomechanical processing route, after conventional processing to plate about 25 mm thick, is shown schematically in Figure 1, The solution treatment and aging steps were carried out in molten salt baths with the temperatures controlled to within -+2 K. Deformation was carried out by rolling at temper
Data Loading...
