Microstructure And Mechanical Properties Of Friction Stir Processed Magnesium Alloys

Friction stir processing（FSP）is a new severe plastic deformation technique.FSP makes the microstructure refinement,homogenization,and densification simultaneously,and thus improves both the strength and ductility of the materials.In this work,the heat-treatable AZ31 and non-heat-treatable AZ80 magnesium alloys were friction stir processed,respectively,and fine grained AZ31 and AZ80 magnesium alloys were obtained.Moreover,the effect of grain size and grain orientation on the room and high-temperature tensile and low cycle fatigue（LCF）deformation behavior of FSP AZ31 alloy was investigated.In addition,the superplastic deformation behavior and mechanism of FSP AZ80 alloy were studied.The main results were as follows:The stir zone（SZ）and thermo-mechanically affected zone（TMAZ）of FSP AZ31 alloy exhibited fine grain size and grain preferred orientation.Decreasing rotation rate made the grain refinement,and thus improved the microhardness and tensile strength.However,rotation rate had a slightly effect on the grain orientation.The soft grain orientation resulted in the fracture at the boundary between SZ and TMAZ in the advancing side（SZ/TMAZ-AS region）.Fracture surfaces exhibited the dimples and quasi-cleavage characteristics.Basal dislocation slip was the main deformation mechanism.During the LCF deformation process under an asymmetrical loading,both the BM and FSP AZ31 alloys exhibited a symmetric hysteresis loop in shape,indicating that basal slip was the main deformation mechanism for LCF.In addition,the maximum tensile stress was higher than the corresponding maximum compressive stress at all the strain amplitudes.Compared with the BM,soft grain orientation was the main reason responsible for the decrease of the LCF life for FSP AZ31 magnesium alloy.LCF fracture was located at the SZ/TMAZ-AS region,similar with the monotonic tensile deformation.Moreover,LCF behavior of both the BM and FSP AZ31 magnesium alloy was well expressed by the Coffin-Manson and Basquin relations.Grain refinement resulted from the reduced rotation rate improved the LCF life.Grain size and grain orientation had an effect on the room-temperature deformation behavior of FSP AZ31 alloy.When tensile direction was parallel to PD direction,the main deformation mechanism was basal slip associated with extension twinning,and thus the specimens had a good work hardening capacity and high elongation.When tensile direction was parallel to TD direction,the main deformation mechanism was basal and cylindrical slips,and thus the elongation was lower.In addition,grain refinement resulted in the decrease of work hardening rate,but had a slightly effect on the dynamic recovery during the third stage of work hardening.The relationship between the ratio of initial working hardening and saturation stress and uniform elongation can be well described by power function.The uniform elongation of FSP AZ31 alloy was higher to 32%,indicating that FSP is a better method to produce the room-temperature high-ductility magnesium alloys.Grain size,grain orientation and temperature had an effect on the high-temperature deformation behavior of FSP AZ31 alloy.The yield strength（YS）and ultimate tensile strength（UTS）of FSP AZ31 alloy decreased with the increase of temperature from 250 to 350℃.The YS and UTS were higher when tensile deformation direction was parallel to TD direction than that parallel to PD direction.The elongation was lower when tensile deformation direction was parallel to TD direction than that parallel to PD direction at the low temperature of 250 and 300℃.However,this difference in elongation gradually reduced with increasing temperature.The strength of FSP AZ31 alloy can be improved by refining grain size during high-temperature deformation process,which was similar with the room-temperature deformation behavior,but this strengthening effect was reduced with increasing temperature.The SZ region of FSP AZ80 alloy exhibited the same texture components with the FSP AZ31 alloy,but the texture intensity was lower.The maximum elongation of 606% was obtained when the temperature was 350 ℃and strain rate of 3×10^-4 s^-1.When superplastic deformation was conducted at the low temperature and high strain rate(300℃,3×10^-3 s^-1),or at the high temperature and low strain rate(400℃,1×10^-4 s^-1),the strain-rate sensitivity value was less than 0.3,and thus FSP AZ80 alloy did not exhibit superplasticity.The coarsening of grain and β-Mg₁₇Al₁₂ phase was the main reason for the drop of the superplasticity.Grain boundary sliding was the main superplasticity deformation mechanism.