The Effect of Grain Size on Hot Deformation Behavior of a New High Strength Aluminum Alloy

Abstract:

The effects of grain size inhomogeniety in large size ingot on hot deformation behaviors of a new high strength Al-7.68Zn-2.12Mg-1.98Cu-0.12Zr alloy were investigated using Gleeble-3500 test machine at 300–450℃ with strain rates from 0.1 to 10 s-1.SEM observation showed that the grain size in surface layer of ingot was finer than that in central layer. During hot deformation, the low stress of the sample with fine grain(surface layer) was lower than that of the sample with coarse grain(central layer)at high temperature and low strain rate conditions. The hot deformation activation energies(Q) of 140kJ/mol and 125.4kJ/mol were obtained for fine grain microstructure and coarse grain microstructure respectively. Thelow stresses were predicted by a two-stage type constitutive model based on the dislocation density theory, and the dynamic recrystallization soften equations were established for the microstructures of fine grain and coarse grain. Electron Back-Scattered Diffraction(EBSD) observations revealed that the alloy exhibited dynamic recovery at temperatures ranging from 300 to 400℃ and dynamic recrystallization(DRX) at 450℃ with low strain rate(≤0.1s-1).The DRX grains nucleated at the original grainSboundaries. It was found that the DRX fracture was higher in finer grain (surface layer)microstructure resulting from the higher grain boundary density.

Key words:Al-Zn-Mg-Cu alloy;grain size;hot deformation;dynamic recrystallization