Effect Of Initial Microstructure On Hot Deformation Behavior Of ZK60 Alloy

In this paper,ZK60 magnesium alloy was selected as the study object.ZK60 alloy rods with different microstructures(with the extrusion ratio were 15 and 60)were prepared by hot extrusion.The uniaxial thermal compression tests of two as-extruded and one as-cast magnesium alloy were carried out.The constitutive equations of ZK60 magnesium alloy with different initial microstructure were established and the relationship between the flow stress and the Z factor was analyzed.The mechanism of dynamic recovery and recrystallization in the deformation process was analyzed by means of optical microscope,scanning electron microscope and electron backscatter diffraction(EBSD)technique.The results were shown as follow.In the process of hot compression deformation,the true stress-true strain curves of ZK60 magnesium alloy with different initial microstructure all showed obvious dynamic recovery and dynamic recrystallization characteristics.The flow stress was affected by the strain rate and the deformation temperature.The flow stress increased with increasing strain rate and decreasing the deformation temperature(i.e.increasing Z-factor).The average activation energy of ZK60 alloy,Q,for hot compression deformation decreased as the grain size of initial microstructure decreases,which were 147.99 1KJ/mol,143.025KJ/mol and 122.884KJ/mol,respectively.The constitutive equations of as-cast and as-extruded(with the extrusion ratio were 15 and 60)ZK60 magnesium alloys were expressed as follows:ε = 3.9116 ×1011[sinh(0.0098σ)]5.7081exp(-147991/RT)as-castε = 8.3584 ×1010[sinh(0.0129σ)]3 0739exp(-143025/RT)extrusion ratio was 15ε = 3.2432 ×109[sinh(0.0157σ)]5 0960exp(-122884/RT)extrusion ratio was 60 The dynamic recrystallization is activated for both as-cast and as-extruded ZK60 alloys,but the mechanisms are different.The mechanism is discontinuous dynamic recrystallization for as-cast samples,but continuous dynamic recrystallization for as-extruded samples.The critical flow stress characterizing the onset of dynamic recovery and dynamic recrystallization increased with increasing strain rate and decreasing of temperature and grain size.For as-cast ZK60 alloy,dynamic recrystallization occurs in part of regions and the coarse primary grains still existed.For as-extruded ZK60 alloy,the fully dynamic recrystallization took place at high deformation temperature and low strain rate.When uniaxial thermal simulation compression was carried out at high strain rate and low deformation temperature,fine dynamic recrystallization grains could be obtained.But the dynamic recrystallization was insufficient and the recrystallization grains were unevenly distributed.In addition,the grain size and orientation of compressed samples were affected by the initial microstructure of the grains.The final grains size decreased and the basal texture weakened as the initial grains size decreased.