Investigation On The Deformation Behaviors And Dynamic Microstructure Evolution Machanism Of The Metals During Hot Shear-compression Deformation

The flow behavior and dynamic microstructure evolution mechanism at high temperature of metal and alloys have been the hot topics in the research field of metal thermal deformation,and they are also the theoretical basis for the establishment of the thermal deformation processing technology and the control of microstructure.Now,the hot tensile,compression,torsion and some simple uniaxial physical model are widely used experimental method in the above problems.However,these methods cannot meet the need of physical simulation with composite stress and strain state in some factual machining processes.In present,two typical metals,7050 aluminum alloy and Fe-38 Mn alloy,were used as model materials,and the shear-compression specimen was employed to realize the thermal physical simulation.On the premise of detailed understanding of the hot shear-compression deformation law,the flow behavior and dynamic microstructure evolution mechanism of the two model materials were investigated.In addition,on the basis of the above research,the influence of multi-directional shear deformation on the microstructure refinement during friction stir processing was explored,and then its microstructure refinement mechanism was revealed.The investigations on the numerical simulation of hot shear-compression deformation showed that the deformation was mainly concentrated in the gage region of the specimen,and that the deformation in the center of the gage region was uniform but the stress concentration phenomenon existed in the transition zone between gage region and cylindrical end.In the initial stage of the hot shearcompression deformation,the axial compression deformation was the main one.Then,due to the sharp increase of shear deformation in the middle stage of the deformation,the deformation was converted into a process that combined with compression and shear.But in the last stage,the axial compression deformation reached the limit,resulting in the dominant position of shear deformation.The investigations on the hot deformation activation energy showed that the hot deformation activation energy of 7050 aluminum alloy decreased with the increase of deformation temperature and strain rate,and that the hot deformation activation energy under the hot shear-compression deformation was higher than that under uniaxial deformation.Furthermore,based on the classical and optimized hyperbolic-sine constitutive laws,the constitutive equations of two model materials were constructed,and the optimized one is more suitable for the analysis of the hot shear-compression deformation.The investigations on the flow behavior and dynamic microstructure evolution mechanism of 7050 aluminum alloy during hot shear-compression deformation revealed that the flow curves of the 7050 aluminum alloy during hot shear-compression deformation exhibited a plateau due to the combined deformation in the middle stage of deformayion,but then had different dropping tends resulting from the shear localization and dynamic softening in the last stage.In addition,the microstructure of the 7050 aluminum alloy during hot shear-compression deformation was characterized by typical shear bands,and its continuous dynamic recrystallization had the characteristics of geometric dynamic recrystallization.Furthermore,with the increase of deformation temperature and strain rate,the dynamic recrystallization degree is improved.The investigations on the flow behavior and dynamic microstructure evolution mechanism of Fe-38 Mn alloy during hot shear-compression deformation revealed that the flow curves of the Fe-38 Mn alloy showed a significant continuous work hardening in the investigated strain rate-temperature domain.The interaction between twin and dislocation induced by the large strain and combined deformation lead to the increase of working hardening rate.The dynamic microstructure evolution mechanism of the Fe-38 Mn alloy during hot shear-compression deformation included dynamic recrystallization mechanism,microshear band mechanism and twinning mechanism.Notably,due to the severe shear deformation,the particular step-like structure of twin boundary and deformation twins induced by twinning provided favorable conditions for microstructure refinement.In addition,from the comparative analysis of the microstructures between hot shear-compression specimen and friction stir processing,it indicated that the microstructure refinement of Fe-38 Mn alloy during the friction stir processing was the result of the combined effect with dynamic recrystallization mechanism and twinning mechanism.