Study On Effects Of Friction Stir Processing On Microstructure And Properties Of LA103Z Mg-Li Alloy

Magnesium lithium alloy is the lightest metallic structural materials,not only has specific strength,but also has good damping performance and thermal conductivity.However,the low tensile strength of Mg-Li alloy at room temperature limits its engineering applications in aerospace,rail transit and other fields.It is of great theoretical and practical significance to carry out research on friction stir processing(FSP)of Mg-Li alloy and explore efficient and green strengthening methods for expanding Mg-Li alloy engineering applications,enriching modification methods of severe plastic deformation of metal materials,and promoting structural lightweight of aerospace and rail transit.In this study,the dual-phase Mg-Li alloy LA103Z was taken as the research object,and the effect mechanisms of processing parameters,processing passes,overlapping ratio(OR)and shoulder size on the microstructure and properties of Mg-Li alloy were systematically explored through processing experiments,numerical simulation and other methods.Efficient strengthening of Mg-Li alloy plate was realized.The main research contents are as follows:(1)FSP experiments were carried out on LA 103Z,and the effects of processing parameters on microstructure and mechanical properties in the stirred zone(SZ)and the thermomechanically affected zone were investigated.The relationship among processing parameters,grain size and tensile strength was revealed.The results show that with the increase of the rotational speed,the grain size rises,the hardness and strength decrease,and the plasticity increases.Under the research conditions of this paper,two groups of process parameters with good processing effect are obtained.When the rotational speed is 400 rpm and the traveling speed is 100 mm/min,the average hardness of processing center,tensile strength and elongation of material are 82.10 HV0.2,251.4 MPa and 13.2%,respectively,its hardness and tensile strength than the base material increased 52.9%,58.2%,respectively.When the rotational speed is 600 rpm and the traveling speed is 100 mm/min,the average hardness of processing center,tensile strength and elongation of material are 79.17 HV0.2,243.6 MPa and 17.9%,respectively,its hardness and tensile strength than the base material increased 47.5%,53.3%,respectively.(2)The finite element modeling method of Mg-Li alloy FSP was studied,and the numerical simulation model was established.The distribution and evolution of material flow and physical fields in FSP were simulated and studied,as well as the influence of processing parameters.It is found that the material flow intensity of the advancing side and the retreating side are different.The change of the rotational speed has a greater influence on the value of the temperature field.The larger rotational speed is,the higher temperature of the plate is.The temperature field shape is more affected by the change of traveling speed.When the traveling speed is higher,the high temperature zone in front of the tool.For the strain field,the larger the rotational speed,the larger material deformation per unit time,and the larger the material strain.The smaller traveling speed,the longer processing time on the same position,the larger strain.(3)Aiming at the problems of material plasticity reduction and limited size of modified plate after single pass FSP,the multi-pass reprocessing FSP and multi-pass overlapping FSP of Mg-Li alloy were studied.The effects of processing pass,and OR on the microstructure and properties of plate were investigated.For multi-pass reprocessing FSP,double and triple pass processing are respectively carried out at the original position.The results show that the shape of the stirring zone changes from bowl shape to barrel shape and the grain is refined with the increase of reprocessing times.The α-Mg phase is dissolved after double pass,the hardness decreases to 68.39 HV0.2 and the elongation increases to 24.0%.After triple pass,the sphericalα-Mg phase precipitates at the grain boundary of β-Li phase,the hardness increases again,and the elongation increases to 26.3%,which is 46.9%higher than that of single pass.And with OR of 0 and 0.5 were used to multi-pass overlapping FSP on Mg-Li alloy,it is found that grains in the SZ are further refined and the dispersive α-Mg phase is precipitated.After multi-pass overlapping FSP,the hardness of the plate presents certain distribution law.The plate hardness with OR=0 is roughly periodic distribution,and the plate hardness with OR=0.5 gradually increases with the increase of the processing pass.The elongation along processing direction with OR=0 and 0.5 is 25.3%and 25.4%,respectively,and the plasticity is improved compared with single pass.However,the plasticity along transverse direction is poor due to the existence of transition zone in multi-pass overlapping FSP.(4)In view of the low temperature of the bottom of the plate and the low heat of the multipass FSP in the medium thickness of plate,a measure was presented to increase the heat production in the FSP of large shoulder.The heat input of large shoulder is higher,which will cause the grain to grow.For different processing parameters,the average grain size and elongation is increased by 41.5%and 219.5%after single pass processing compared with small shoulder.After OR=0 processing,the grain size of the SZ is reduced to 18.71 μm,the plasticity of the material is greatly improved,and the elongation in processing direction is increased to 63.6%,which is 109.2%higher than that of the base material.