A series of AZ80 billets were compressed with 60%height reduction on hot process simulator at 250,300,350,400℃ under strain rates of 0.01,0.1,1 and 10 s- 1.In order to predict the occurrence of surface fracture,the values of the Cockcroft-Latham equation were calculated by the corresponding finite element numerical algorithm developed.A concept about damage incremental ratio in plastic deformation was defined as the ratio of damage increment at one step to the accumulated value.A method of finding the intersection of incremental ratio varying curve and simulation step axis was brought forward to make the fracture step certain. Then,the effects of temperature and strain rate on critical damage value were achieved.The results show that the critical damage value is not a constant but changes in a range of 0.021 8-0.378 0.It decreases significantly with the increase of strain rate at a certain temperature.While under a certain strain rate,the critical damage value has little change with the increase of temperature.
The dynamic recrystallization refinement of magnesium alloy AZ80 by compression tests was studied,and its effect on the mechanical properties was investigated.It is observed that the microstructure of the as-cast billet with grain size of 240μm becomes refined to about 120,110,94 and 50μm after upsetting at 350℃ under strain rates of 0.01,0.1,1 and 10 s -1 respectively.The changes in the mechanical properties according to grain size show that yield strength significantly decreases with grain size increasing,while strain hardening exponent and micro hardness increase very sharply.Further,the grain size vs strain rate and change in Vickers micro hardness according to the various strain rates show that grain size and micro hardness decrease with strain rate increasing.
