| Magnesium alloy sheets have been widely used in the manufacture of complex thin-walled structure parts due to the low density,good mechanical properties,but the grain size of conventional magnesium alloy sheet is relatively large(above 10?m)and shaping property of which is poor.Compared with conventional magnesium alloy sheets,the fine-grained magnesium alloy sheets have been widely used in the field of transport vehicles,aerospace,electronic products due to its outstanding advantages of manufacture technology and mechanical properties such as excellent strength,high ductility,high toughness and excellent plastic formability.But few researches on the formability of fine-grained magnesium alloy sheets were reported.The ZK60 magnesium alloy with the best comprehensive mechanics property was chosen to rolled into ZK60 and ZK60-Y fine-grained magnesium alloy sheets(with average grain size under 5?m)under high strain rate rolling technology and its deformation behavior at medium temperature,high strain rate superplasticity behaviors was studied.Work hardening,dynamic recovery and dynamic recrystallization occurred during medium temperature deformation of magnesium alloy sheet.Work hardening make the alloy reach high level of mechanical properties,However,dynamic recovery and dynamic recrystallizationcan soften the alloy to eliminate or suppress the negative influence which caused by work hardening.The theoretical support on control of the strength,plasticity of forming panels is provided through exploration of the interrelation among above three kinds of effects.This research has crucial reference value in research and development of magnesium alloy forming technology at medium temperature and application of fine-grained magnesium alloy sheet.Firstly,the flow stress behavior,work hardening and softening behavior of the fine-grained ZK60 and ZK60-1.0Y sheet was researched by uniaxial tension tests at the temperature ranging from 323 K to 523K with the initial strain rates ranging from 1×10-4s-1 to 1×10-3s-1.The dynamic recovery and recrystallization critical condition was determined by calculating the strain-hardening 0 under different deformation conditions.The micro structural evolution during deforming process was studied.In addition,the micro structural evolution and properties in the following machining process was predicted by controlling the deforming condition.Secondly,thecharacteristics of superplastic deformation for the fine-grained ZK60 and ZK60-1.0Y sheet at the temperature ranging from 523 K to 723 K with the initial strain rates ranging from 1×10-3s-1 to 1×10-1s-1 was studied,and the effects of temperature and strain rate on the micro structure after superplastic deformation was investigated.The superplastic deformation mechanism was discussed.The main conclusions are listed below:(1)The shape of flow stress curve for fine-grained ZK series alloy sheet is sensitive to the temperature and strain rate,and the shape of flow stress curve are different with the variety of softening mechanisms.Typical dynamic recovery and recrystallization curve shape are shown in the flow stress curve.The critical temperature of dynamic recrystallization in fine-grained ZK60 sheet is 423K,dynamic recrystallization will be the majorsoftening mechanism above 423K.The critical temperature of dynamic recrystallization in fine-grained ZK60-1.0Y sheet is 473K,and the dynamic recrystaliization temperature increased because of rare earth element Y addition.(2)The relationship between critical values and peak values is ?c=0.984?p and?c=0.768sp when the fine-grained ZK60 alloy sheet deformed at the temperature ranging from 423K to 523K with the initial strain rates ranging from 1×10-4s-1 to 1×10-3s-1.The relationship between critical values and peak values is ?c=0.9788ap and?c=0.7613?p when the fine-grained ZK60-1.0Y alloy sheet deformed at the temperature ranging from 473 K to 523K with the initial strain rates ranging from 1×10-4s-1 to 1×10-3s-1.Tne dynamic recovery and recrystallization critical condition forward as the temperature increases and strain rate decreases.(3)The analysis of fracture shows that the main deformation mechanism of ZK60 alloy sheet was intergranular dislocation moving below 473K.And the grain boundary slidingcontrolled by diffusionbegan to dominate the deformation above 473K.The main deformation mechanism of ZK60-1.0Y alloy sheet was intergranular dislocation moving below 523K.And the grain boundary slidingcontrolled by diffusion began to dominate the deformation above 523K.(4)The fine-grained ZK60 and ZK60-1.0Yalloy sheet show excellent high strain rate superplasticity.The results show that the maximum elongation to failure of 650%is and the strain rate sensitivity index(m)of 0.53 is obtained at 648 K with the strain rate of 1 ×10-3s-1 in the as-rolled ZK60 magnesium alloy.An excellent high strain rate superplasticity with the elongation to failure of 584.5%and the strain rate sensitivity index(m)of 0.47 is obtained at 623 K with the strain rate of 1×10-2s-1.The maximum elongation to failure of 636.5%and the strain rate sensitivity index(m)of 0.55 is obtained at 723 K with the strain rate of 1×10-3s-1 in the as-rolled ZK60-1.0Y magnesium alloys.Compared the superplastic characteristics of ZK60 and ZK60-1.0Y sheet,it can be found that the addition of rare earthY in ZK60 did not improve the superplasticity,but have benefit to the organizational stability and heat resistance.(5)The analysis of microstructure and theoretical shows that,the dominant deformation mechanism in high strain rate superplasticity of fine-grained ZK60 alloy is grain boundary sliding(GBS)above 573K,which has been studied systematically.The dislocation creep controlled by grain boundary diffusion was considered the main accommodation mechanism,and some degree of inter-granular liquid phase aiding mechanismwas also observed.Superplastic deformation process of fine-grained ZK60-1.0Y sheet occurs as a consequence of multiple mechanisms,which grain boundary sliding was the main deformation mechanism.The dislocation creep controlled by grain boundary diffusion was considered to be the main accommodation mechanism,and the coalescing and moving of microcavities also play a role in regulating the deforming process. |
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