Deformation behavior of the Al-5.9Zn-2.1Mg-2.1Cu (wt%) alloy during isothermal pressure-holding after pre-deformation

Load-holding after the forming load reached the limit of hydraulic press is a sufficient method to promote further forming and ensure the filling quality of large Al-5.9Zn-2.1Mg-2.1Cu (wt%) alloy die forgings. To investigate the deformation behavior of the alloy during load-holding process, the thermophysical simulation tests at the holding pressure range of 20–60 MPa, temperature range of 350–450 ℃ with the pressure-holding time of 5–30 s and/after compression tests with the strain rate of 1 s−1 and strain of 0.231 were conducted on a Gleeble-3500 isothermal simulator. The strain rate variation during pressure-holding reveals that the critical holding pressure of accelerating deformation is between 40 and 60 MPa at the temperature higher than 400 ℃. The microstructures of the tested specimens were analyzed by optical microscopy (OM) and electron back-scattered diffraction (EBSD). The pre-deformation promotes the intragranular deformation and hinders grain rotation during following pressure-holding process. Besides, the processing map based on Kumar-Prasad criterion is not as accurate as that based on Murty-Rao criterion in predicting the instability during pressure-holding. Based on the processing map and microstructure analysis, the best pressure-holding conditions are recommended to be 400 ℃&40 MPa, 450 ℃&40 MPa and 450 ℃&60 MPa.