Fig。 10。 The curves of logarithmic stress versus logarithmic strain rate: (a) ε¼ 0。6 and (b) ε¼ 0。8。

Fig。 11。  Power dissipation map of 7A09 aluminum alloy: (a) ε¼ 0。6, 3D map; (b) ε¼ 0。6, isoline map; (c) ε¼ 0。8, 3D map and (d) ε¼ 0。8, isoline map。

of 7A09 aluminum alloy within the instability flow zone。 In the present study, the hot processing map is used for optimizing the process parameters to guarantee the stable flow zone of the metal material, such as the strain rate and the deformation temperature。 In the case of large plastic deformation, when the strain rate is less than

0。1 s−1,  7A09  aluminum  alloy  shall  exhibit  the  more  stable  flow。 In

addition, it seems that 7A09 aluminum alloy is characterized  by steady  flow  in  the  temperature  range  from  300 1C  to  460 1C。  The

processing map lays a certain foundation for the optimum working conditions on the basis of flow stress, strain rate and deformation temperature。 The establishment of the processing map based on the microstructural  defects  shall  be investigated  in the future。

6。Finite  element simulation

DEFORM3D commercial finite element code was used for simulating isothermal precision forging of rotating disk  based on  ring  preform  and  pentagram  preform。  The     deformation

temperature is determined as 430 1C and the die moves at the velocity  of 0。1 mm s−1   during  finite element  simulation。

Fig。 14 indicates finite element simulation results of isothermal plastic forming of rotating disk forging based on the different preforms。 Fig。 14(a) indicates the distribution of the equivalent strain derived from finite element simulation of isothermal forging of rotating disk forging based on ring preform。 For the  sake of better understanding the deformation mechanism of the forged sample, the forming process of the forged sample can be pided into three stages, namely upsetting, forming of inner ear and forming of outer ear。 At the upsetting stage,  the  metal  mainly flows along the radial direction and the ring wall of the forged sample thickens gradually。 At the second stage, the inner ear impressions are firstly filled with the metal since they are nearer to the preform, and simultaneously the metal continues to flow inside to form the flash。 At the third stage, as the upper die continues to move down, the resistance force from the flash gutter of the inner ears gets larger and larger。 As a result, the metal is forced  to  flow  into  the  outer  ear  impressions  along  the    radial

Fig。 12。  Instability processing map of 7A09 aluminum alloy: (a) ε¼ 0。6, 3D map; (b) ε¼ 0。6, isoline map; (c) ε¼ 0。8, 3D map and (d) ε¼ 0。8, isoline map。

Fig。 13。 Hot processing map of 7A09 aluminum alloy: (a) ε¼ 0。6 and (b) ε¼ 0。8。

direction to form five outer ears。 The final simulation results reveal that four outer ear impressions are not full of metal。 Fig。 14

(b)indicates the distribution of the equivalent strain derived from finite element simulation of isothermal forging of rotating disk forging based on pentagram preform。 The shape of pentagram preform causes five outer ear impressions to be covered with the sufficient metal, so the metal mainly flows into the impressions along the axial direction。 According to the flow characteristic of the metal, the forming process of the forged sample can also be pided into three stages, namely local forming, forming of outer ears and forming of inner ears。 At the local forming stage, the metal flows into the impressions along the axial direction by means of backward extrusion。 With the increase in the plastic strain, the metal flows outside along the radial direction until the outer ear impressions are completely filled and the corresponding flash is finished。 Consequently, the resistance force from the flash gutter of the outer ears causes the metal to flow into the inner ear impressions along the radial direction。 Finally, the inner ears are