sample exhibits the dominant dynamic recrystallization at the deformation temperature of 460 1C, as shown in Fig。 6。 It can be

observed from Fig。 6(a) that in the as-received microstructure of 7A09 aluminum alloy, the grains are obviously elongated along the axial direction, which means that aluminum alloy bar is subjected to plastic working。 However, it can be found from Fig。 6(b) that in the microstructure of 7A09 aluminum alloy subjected to compres- sion at the high strain rate of 10 s−1, the grains exhibit  the equiaxed ones, which means that dynamic recrystallization pre- vails。 It can be generally accepted that dynamic recrystallization is difficult to take place in the aluminum alloy with high fault energy。 However, there have been the literatures [24–26] demonstrating that  dynamic  recrystallization   can   occur   during   hot working of  aluminum   alloy  at   the   high   strain   rates。  The  mechanism

Fig。 3。 3D model of isothermal precision forging die: (a) top die and (b) bottom die。

Fig。 4。  Scheme of rotating disk preform: (a) ring preform and (b) pentagram  preform。

Fig。 5。  The true stress–strain curves of 7A09 aluminum alloy under hot compression: (a) ε_ ¼ 0。01 s−1; (b) ε_ ¼ 0。1 s−1; (c) ε_  ¼ 1 s−1  and (d) ε_ ¼ 10 s−1。

of dynamic recrystallization of 7A09 aluminum alloy during hot compression deformation can be described as follows。 It is gen- erally accepted that the recrystallized grains firstly nucleate at the local region which possesses a high density of dislocations in the initial microstructure subjected to a certain plastic deformation degree, and then they grow up with the increase in the plastic strain。 With the progression of the plastic deformation, the recrystallized  grains  stop  growing  and  undergo  a  certain plastic

deformation。 Consequently, the new crystal nuclei occur at the local region with a high density of dislocations in the recrystallized grains formed previously, and thus the new recrystallized grains arise repeatedly until the plastic deformation finishes。 It can be concluded that dynamic recrystallization is characterized by repeated nucleation and finite growth of the recrystallized grains。 On the one hand, the high strain rates lead to the rapid increase in the   dislocation    density   and   consequently   have   an     adverse

Fig。 6。  Microstructures of 7A09 aluminum alloy samples: (a) as received and (b) dynamic  recrystallized。

alloy。 The constitutive equation of 7A09 aluminum alloy is based on the  Arrhenius type  equation  [27,28]:

ε_ ¼ A½sinhðαsÞ]  exp   − RT

where ε_ is the strain rate, s is the flow stress, T is the absolute temperature,  Q   is  the  activation  energy,  R  is  the  universal  gas

constant ð8:314 J mol constants。

Þ,  and  A,  α  and  n   are  the   material

In order to further obtain the material constants in Eq。 (1) according to the experimental  data,  it  is  necessary  to  simplify Eq。 (1) mathematically  [29]。

When the low stress level leads to αso1, Eq。 (1) can be simplified as

Fig。 7。 The curves of the temperature rise versus the time of 7A09 aluminum alloy under hot compression at different strain rates。

influence on climb and cross-slip of the dislocations。 Therefore, dynamic recovery cannot be performed sufficiently and thus  the