Effect Of Li Addition On Microstructure Evolution And Mechanical Properties Of Al-Mg-Si Alloys

With its high specific strength,specific stiffness and excellent comprehensive performance,Al-Li alloy has been favored by aerospace manufacturing industry since its birth.At present,the third-generation Al-Li alloy,which is the 2XXX series aluminum alloy as the base alloy to add Li element,has been widely used.They maintain a high level of Cu/Li ratio,and take T1 phase(Al2CuLi phase)as the main precipitate strengthening phase,which have strong mechanical properties.However,due to the high Cu/Li ratio and relative low content of Li,the density of this generation of Al-Li alloys is about 2.7 g/cm3,which greatly reduces the weight loss advantage compared with the traditional 7XXX series of high-strength aluminum alloys.Al-Mg-Si series(6XXX series aluminum alloy)alloys are widely used as medium strength alloys with low density,good formability and high corrosion resistance.If Al-Mg-Si alloy as the base alloy to add Li element,in the artificial ageing stage can not only form ?'-Al3Li phase,but also can obtain the needle shaped ?"-Mg2Si strengthening phase of Al-Mg-Si alloy,so as to achieve dual phases strengthening,and the density of this new type of Al-Li alloy will be more low.The specific strength,specific stiffness and other performance will further enhance,which has a significant contribution to the development and application of Al-Li alloy.This article is based on the possibility of ?'-Al3Li phase and ?"-Mg2Si phase precipitation strengthening in Al-Mg-Si alloy with proper amount of Li element,and on the basis of thermodynamic phase diagram calculation results.Different microstructure characterization methods,with microhardness and tensile properties test at room temperature,systematically studied the effect of high content Li element(>2.0 wt.%)addition to the Al-Mg-Si alloy on the microstructure evolution and mechanical properties.The influence of Li,Mg and Si were also analyzed as three main alloy elements on the microstructure of the experimental alloys.The composition conditions and heat treatment process for the dual-phases precipitation of ?' and ?" phases were investigated.The main research contents are as follows:The microstructure of the alloys with different Mg/Si ratios and the evolution characteristics of the precipitation phase during homogeneous heat treatment were studied.Compared with the base alloy,there are Mg2Si phase at the grain boundary,Al2LiMg phase and ?'-Al3Li phase in the Li-containing alloy,while for the Si rich alloy,the free Si element will combine with Li element to form AlLiSi ternary compound.Through the comparative observation of the components of solidified precipitated phases,it can be found that during the solidification process,some Li element will dissolve into Mg2Si phase and replace Mg element,which changes the composition of the phase and increased the melting temperature of the phase in the ?-Al matrix,while increasing the content of Mg element can inhibit such behavior of Li element.However,in the homogeneous heat treatment process,all the Li-containing alloy exist replacement phenomenon in the Mg2Si phase to a greater extent,and combine with Si atoms to form AlLiSi ternary compound.The strong binding between Li and Si elements can make AlLiSi ternary compound have a high melting point,resulting in that the residual phase cannot be eliminated by homogeneous heat treatment.The influence of different content of Li element on the microstructure and solution heat treatment of the alloys after hot extrusion was studied.The recrystallization degree of the alloy with the highest Li and Si content and the average grain size after deformation are the smallest in the extruded microstructure of the three Li alloys,which is mainly due to the presence of more Si-rich residual second phase in the microstructure.The suitable solution heat-treatment of the four alloys are determined respectively.After the solid solution heat treatment,the Mg/Si ratio of the residual second phase of the three Li alloys is lower than that of the second phase in the corresponding as-cast structure,but the reduction degree is much lower than that after the homogeneous heat treatment due to the short holding time.The ageing precipitation behavior of the alloys with different Li content(0,2.0,2.5,3.0%)in artificial ageing stage was studied.After 100 h ageing at 170?,the needle shaped?"-Mg2Si phase is precipitated in the alloy with the smallest Li content,thus forming ?'-Al3Li and ?"-Mg2Si dual phase precipitation.However,no ?" phase precipitation is observed in the alloys with high Li content at each ageing stage,and the main precipitates is ?' phase.It has been observed that,in the artificial ageing stage,some Li atoms will dissolve into the?" phase to replace Mg atoms,and gradually deepen with the extension of ageing time.The variation of ?'-PFZs at grain boundary of three Li alloys during the ageing stage is also studied.The half width of ?'-PFZs of the three alloys increases significantly with the ageing time,and decreases with the increase of Li content.The constant values of the growth rates of ?'-PFZs of the three alloys are 0.31,0.23 and 0.21 respectively by the formula fitting.The effect of different Li content on the density and mechanical properties of the alloys was studied.In the aged state,the density of Al-Mg-Si alloys decrease significantly with the addition of Li,and the weight loss effect improve with the increase of Li content.The addition of Li greatly improves the strength and elastic modulus of Al-Mg-Si alloy,but at the same time the elongation of Al-Mg-Si alloys decreases obviously,especially the alloys with high Li content show typical intergranular fracture characteristics in the middle and late ageing stage.The effect of pretensile deformation on ageing precipitation behavior of alloys was studied.For the three kinds of Li-containing alloys,the time needed to reach the peak hardness of 170? single stage ageing can be greatly shortened after pre-stretching deformation.After 2%prestretching,the time needed to reach the peak is reduced from 192 h to 72 h,while after 4%prestretching,the time is 60 h.For the alloy with the lowest Li content,2%pretensile deformation can promote the nucleation and precipitation of GP region phase of ?"-Mg2Si phase in the early ageing stage,but can not significantly increase the precipitation amount and promote the growth of ?" phase in the late ageing stage.Before ageing,the introduction of dislocation by 2%pre-stretching deformation can refine the phase of ?' in the Li-containing alloys,make its distribution more uniform and a relatively slow growth rate of ?'-PFZs,thus improving the mechanical properties of the alloy.For the alloy with the lowest Li content,the dynamic elastic modulus under ageing state can reach 77.7 GPa;after 2%pre-tensile deformation+170?/72 h ageing process,the tensile strength and yield strength of the alloy can reach 358 MPa and 269 MPa respectively,which are 34.6%and 24.0%higher than that of the base alloy,and the elongation can reach 11.0%.