Study On Hot Deformation Behavior And The Effects Of Friction Stir Processing On The Microstructure And Properties Of Mg-Li Alloys

As the lightest structural metallic material,magnesium-lithium(Mg-Li)alloys exhibit considerable low density,high specific strength and stiffness,superior impact resistance and electromagnetic shielding performance,which are considered as one of the ideal lightweight material for aerospace,automobile industry,weapon and equipment,electronic industry and other fields.The Mg-Li alloys with different Li contents exhibit different crystal structures,and their thermal deformation behaviors are both correlated and very different.Moreover,the addition of Li makes the absolute strength of Mg-Li alloys decrease,which restricts the development and wide applications of Mg-Li alloys.Therefore,the systematic study on the hot deformation behavior of Mg-Li alloys and the exploration of a simple and efficient strengthening processing method of which have become a hot research topic in academia and an urgent need in industry.At present,great efforts have been devoted on the investigation of thermal simulation experiment,constitutive model,strengthening method and finite element simulation of Mg-Li alloys.In addition,as a novel kind of simple,efficient and green method of severe plastic deformation(SPD)technique,friction stir processing(FSP)has been applied in the strengthening of Mg-Li alloys.However,there are still some key issues needed to be solved about the hot deformation behaviors and the friction stir strengthening process of Mg-Li alloys.The effects of hot deformation conditions on the flow behaviors,micro structure evolution,dynamic recrystallization(DRX)mechanism and deformation mechanism of Mg-Li alloys are unclear.The research on hot deformation behavior of the single and double phase Mg-Li alloys is divided to some extent.The basic data on hot deformation process,such as constitutive model of Mg-Li alloys are lacking.And it is extremely urgent to reveal the thermal-mechanical coupling mechanism,strengthening mechanism of FSP and to establish the relationships among FSP parameters,microstructure and properties of Mg-Li alloys.Focusing on the aforementioned issues,the hot deformation behaviors and the effects of friction stir processing on the microstructure and properties of Mg-Li alloys were systematically investigated using thermal simulation experiments,theoretical analysis,processing experiments and numerical simulation.The flow behaviors and microstructural evolution of the single α-Mg phase Mg-Li alloy LA43M and the α+β dual-phase Mg-Li alloy LA103Z during uniaxial hot compression tests were illuminated,and the influences of deformation conditions on grain size,grain orientation,deformation mechanism,DRX behavior and the morphology of the α-Mg phase were revealed.The Arrhenius constitutive model and the continuous dynamic recrystallization-internal state variable constitutive model were established.The dual-phase Mg-Li alloy LA103Z was selected for FSP and numerical simulation with varied conditions,the "heat generation-stress/strain-material flow" behaviors of LA103Z during FSP were investigated,the influence of the morphology and distribution of the α-Mg phase on the microstructure and properties of the friction stir processed LA103Z was elucidated,and the strengthening mechanism of FSP was clarified.The results provide data reference and theoretical guidance for the design of thermal forming process and the control of microstructure and properties of the stir friction strengthening processed Mg-Li alloys.The main research contents in this paper are shown as follows:(1)The effects of deformation temperature,strain rate and strain on the microstructure and deformation mechanism of the single α-phase Mg-Li alloy LA43M were analyzed by uniaxial hot compression tests in the temperature range of 250℃-350℃ and strain rate range of 0.0001 s-1-1 s-1.It was found that under the influenced of the initial texture and loading direction,the initial deformation mechanism of LA43M compressed under low temperature and high strain rate was dominated by extension twinning.The twinning grains then coarsened and devoured all the matrix,leading to the change of grain orientation.The generation of extension twinning delayed the nucleation of discontinuous dynamic recrystallization(DDRX)and affected the stress-strain curves of LA43M.As the deformation temperature increased and the strain rate decreased,the volume fraction of the extension twinning declined and the activity of slip systems went up,and the degree of DRX in the specimens increased,as well as the size of the DRXed grains.LA43M consisted of finer and more uniform grains when deform ed at higher temperatures and strain rates.(2)Uniaxial hot compression tests of the α+β dual-phase Mg-Li alloy LA103Z were carried out in the temperature range of 250℃-400℃,the strain rate range of 0.001 s-1-1 s-1,and the temperature range of 350℃-450℃,the strain rate range of 15 s-1 and 30 s-1.The influences of deformation conditions on the microstructure,especially the morphology and distribution of the α-Mg phase,and the DRX behavior of LA103Z were systematically investigated.The results indicated that the β-Li phase produced continuous dynamic recrystallization(CDRX)during the hot deformation process.LA103Z was sensitive to the deformation temperature,the grain size of the β-Li phase and the degree of the α-Mg phase dissolution increased with the increase of the deformation temperature.The α-Mg phase broke up and became spheroidal under 250℃.When the deformation temperature was higher than 288.5℃,α-Mg layers would spontaneously precipitate at the grain boundaries of β-Li grains.LA103Z showed good deformation ability at high temperatures and strain rates.(3)Based on the stress-strain data obtained from the hot compression tests of LA103Z,the activation energy(Q)and Zener-Hollomon(Z)parameter under different conditions were calculated and analyzed,the phenomenological Arrhenius constitutive model was established.In addition,considering the influence of grain boundary misorientation and recrystallization fraction on the microstructure evolution,the continuous dynamic recrystallization-internal state variable(CDRX-ISV)microstructure model of LA103Z was developed,and the material constants were determined using genetic algorithm technology.The correlation coefficient R of the both models were calculated as 0.994 and 0.991,indicating that the predictions were in good agreement with the experimental data.(4)The dual-phase Mg-Li alloy LA103Z sheets were subjected to single pass submerged FSP at a variety of processing directions,tool sizes,rotation rates and traverse speeds.The relationship among processing parameters,heat input,microstructure and resulting mechanical properties of LA103Z were systematically investigated,and the strengthening mechanism of friction stir processed LA103Z was clarified.The results suggested that with the increase of tool size and rotation rate,the heat input went up,the average grain size of the β-Li phase rose,and more α-Mg phase dissolved.Under the work hardening effect of FSP and the solid-solution strengthening effect of Mg atoms,the strength and hardness of LA103Z were significantly improved,while the plasticity of which was reduced.The continuous α-Mg layers precipitated along β-Li grain boundaries hindered the deformation of the β-Li phase at room temperature,which also made its plasticity decrease.In this study,after being processed by Tool A under the rotation rate of 800 rpm and the traverse speed of 200 mm/min,friction stir processed LA103Z represented the highest tensile strength of 305.53 MPa.After being processed by Tool A under the rotation rate of 800 rpm and the traverse speed of 100 mm/min,the yield strength,tensile strength and elongation of LA103Z were 238.25 MPa,268.04 MPa and 8.97%,respectively,showing good overall mechanical property.(5)Finite element simulation of the single pass submerged FSP of LA103Z under various parameters were carried out,and the macroscopic flow behaviors of the material as well as the distribution and evolution of the temperature field,strain field and stress field during FSP were investigated.The influences of the processing parameters on the "heat generation-stress/strainmaterial flow" behaviors of LA103Z during FSP and the coupling mechanism of the multiphysical fields were quantitatively analyzed.The CDRX-ISV constitutive model was used to predict the evolution of grain size and dislocation density of LA103Z during FSP.(6)In order to improve the room temperature plasticity of the friction stir processed LA103Z,the double pass submerged FSP technology was proposed.The effects of the processing direction,tool size and rotation rate of the second pass on the microstructure and properties of LA103Z were investigated.It was found that the microstructure and mechanical properties of the double pass friction stir processed LA103Z were mainly influenced by the heat input of the second pass of FSP.The hardness and strength of LA103Z basically tended to decrease as the heat input of the second pass decreased.When the heat input of the second pass was higher,the material showed higher strength and increased elongation,and the overall mechanical property of which was significantly improved.When LA103Z sheet was processed by tool A under the rotation rate of 800 rpm and the traverse speed of 100 mm/min in both the first and second passes along the same processing direction,the tensile strength of which was improved to 281.36 MPa owing to the strengthened solid-solution effect caused by the continuous dissolution of the a-Mg phase and precipitated phases.Meanwhile,the spheroidization of the remaining a-Mg phase and the fragmentation of the continuous α-Mg layers reduced the stress concentration during the room temperature deformation,and thus improved the plasticity of the material.The elongation of the double pass friction stir processed LA103Z was increased to 17.12%,showing the best overall mechanical property.