| The dual-phase Mg-Li alloy simultaneously consisits of the excellent properties of both the a-Mg(HCP)and b-Li(BCC)ones.Compared with ordinary Mg alloys,the better plasticity makes it applicable to erospace,military,civil and other fields.However,the industrial application of the dual phase magnesium-lithium alloy has been limited due to its low strength.FSP(Friction Stir Processing)is a new way of severe plastic deformation technology,which can be used for surface fine-grained treatment of formed workpieces.In this paper,stiring the dual-phase Mg-Li alloy in the conditions of air,bottom water cooling and underwater respectively,the effects of processing parameters on the microstructure,texture and mechanical properties of the material were studied.In addition,the influence of subsequent annealing treatment on the microstructure and mechanical properties of normal FSPed samples was also studied,which provided a theoretical basis of fine-grained dualphase Mg-Li alloy by FSP.The effect of different processing parameters on the formability of LZ91 magnesium lithium alloy during normal friction stir processing(NFSP)were investigated.The results showed(a)LZ91 alloy was sensitive to heat accumulation;(b)the forming temperature zone was narrow;(c)the material in thermoplastic state was viscous;(d)the magnesium lithium alloy prepared without cooling medium had serious defects;(e)the formability was poor;(f)the alloy was easy to be oxidized in the air.Decreasing cooling temperature of LZ91 magnesium-lithium alloy was beneficial for uniforming materials temperature.Under the condition of bottom water cooling,the effect of rotation speed on the microstructure of the stirring zone was studied.It was found that the rotating speed could be increased to 400 r / min,and the surface of the sample was smooth without obvious defects.Different plastic flow patterns led to different structures in the advancing side and the retreating side of the stiring zone.During the friction stir processing,the initial grain as refined by severe plastic deformation,the grains in the maelstrom region were refined into uniform equiaxed fine grains,the grains were randomly oriented,and the texture strength was weakened.Reducing heat input further could increase the rotating speed to 600 r / min,thus the effect of second phase particles on the microstructure evolution and mechanical properties of the LZJX9100 alloy was studied.The results showed that the second phase particles in the alloy could be refined effectively by increasing the rotating speed,and the refining effect in the maelstrom region was the best.During the friction stir processing,the grain refinement effect of ?-Li was weaker than that of a-mg,the grains of retreating side was coarser than the advancing side.The strength and hardness of the alloy were improved significantly under the combined effect of fine grain strengthening and second phase strengthening.The transition microstructure between the advancing side and the base material,the broken second phase particles would weaken the mechanical properties of the alloy,and the fracture mode tended to brittle fracture.Increasing the rotating speed and decreasing the traveling speed could improve the microstructure of transition zone and the second phase.The effect of annealing on the microstructure and mechanical properties of LZ91 dual phase magnesium lithium alloy was studied.The results showed that initial microstructure characteristics affected organizational evolution.Uniform and small dual-phases could grow smoothly during heat treatment.Drastic organizational interface between stir zone and matrix was weakened during annealing treatment.Fine grains in ?-islands grew fast in the retreating side of samples.Uniform elongation of FSPed LZ91 was increased by 50% after annealing treatment at 250? 30 min.Stress concentration was improved in the advancing side of samples.Mechanical properties in the retreating side of samples were the lowest either before or after annealing treatment. |
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