Study On The Influence Of Precipitates On Microstructure And Mechanical Properties Of 2195 Al-Li Alloy Friction Stir Welding Joints

With advantages of low density,high specific strength and good low temperature property,2195 Al-Li alloy has become one of the preferred metal materials for weight reduction in aerospace and defense industries,being widely used in rocket fuel storage tank and aircraft skin.However,lack of some key technologies such as welding restricts its engineering application in China.Friction stir welding（FSW）is believed as a kind of perfect welding method for 2195 Al-Li alloy,but the problem of low joint strength is still existed.Fundamental investigation on microstructure and mechanical properties of2195 Al-Li alloy FSW joints is important to be deeply carried out to achieve high performance welding joints and promote the alloy being using widely in our country.2195 Al-Li alloy is a kind of precipitation strengthening Al alloy,which mechanical properties depend on the sort,content and size of precipitates.Thus,based on the precipitate control of 2195 Al-Li alloy,this paper will select three kinds of 2195Al-Li alloys,such as artificial aging status（T8 status）,full annealing status（O status）and natural aging status（T4 status）2195 Al-Li alloys to conduct out FSW study.By using of focus ion beam and scanning electron microscopy（FIB/SEM）,transmission electron microscopy（TEM）,micro-hardness measurement and tensile properties test,two aspects were systematically studied as bellow:（1）the change rules of the sort,content and size of precipitates including in the three kinds of 2195 Al-Li alloy FSW joints,and the effect of precipitates change on microstructure and mechanical properties of the joints;（2）effect of post-welding heat treatment（PWHT）on microstructure an mechanical properties of 2195-O and 2195-T4 Al-Li alloy welding joints.The main conclusion includes:（1）Dissolution and coarsening of T₁ precipitates mainly lead to the strength of welding joints（about 427 MPa）being lower than that of the BM（about 600 MPa）.Tensile fracture occurs in a region of the heat affected zone（HAZ）,in which the dissolution and coarsening levels of T₁ precipitates are at maximum.T₁ precipitates almost completely dissolved in the nugget zone（NZ）,resulting in the strengthening mechanism of the NZ changes from precipitates strengthening to dislocation strengthening and fine grain strengthening.However,when the rotational speed of the pin is enough（900 rpm）,T₁ precipitates can re-precipitate from Al matrix relying on the plenty of sub-structures(dislocation density about 6×10¹⁴ m^-2)in the top regions of the NZ.（2）Joint strength of 2195-O Al-Li is nearly equal to the strength of the BM,about194 MPa（joint coefficient 99%）.PWHT which includes solution and natural/artificial aging treatment can make the dissolution of equilibrium phases completely and promote the formation of many fine hard particles,improving the strength of welding joints by122.8-172.2%.Grain boundaries in retreating side of the NZ are harder to migrate as compared with that in advancing side of the NZ（AS-NZ）,which leads to grains in the AS-NZ being easy to developing into abnormal growth gains（AGG）during solution treatment process,decreasing plasticity of the heat treated welding joints.Pre-deformation rolling can make plenty of deformation structures which will develop into fine and uniform distribution recrystallization grains in the NZ after solution treatment,effectively avoiding the development of AGG and improving the plasticity of welding joints by 16.8-73.9%.The ultimate tensile strength and elongation of the welding joints with natural aging treatment are 453 MPa and 16.7%,and the ultimate tensile strength and elongation of the welding joints with artificial aging treatment（T6treatment）are 528 MPa and 7.2%.（3）Many T₁ precipitates,grain boundary precipitates（GBPs）and precipitates free zone（PFZ）with big size are developed in a region of the 2195-T4 Al-Li alloy joints where precipitation of solute atoms is at maximum,developing into the lowest hardness zone（LHZ）of welding joints.Fracture of the welding joints always occurs in the LHZ with an ultimate tensile strength of 442 MPa and elongation of 12.1%.Plenty of solute atoms can precipitate from Al matrix after T6 treatment,developing T₁ precipitates in welding joints.However,less T₁ precipitates forms in the LHZ due to lower precipitation kinetics than that in other regions of the joints after T6 treatment,resulting in less hardness increment and bigger hardness difference with other regions of welding joints.So,the strength increment of the welding joints is limited in about 10.4%.Pre-deformation rolling can further promote the precipitation of solute atoms in the LHZ after T6 treatment with high density and fine T₁ precipitates,dramatically improving the joint strength.The ultimate tensile strength of the welding joints after T6treatment are about 542 MPa,improving about 22.4%.（4）As compared with the T8 status and O status 2195 Al-Li alloys,better tensile properties and lower cost can be achieved when the FSW joints are manufactured using T4 status 2195 Al-Li alloys.A relatively simple PWHT process which includes rolling and T6 treatment can evidently improve the strength of 2195-T4 Al-Li alloy FSW joints.The PWHT process of 2195-O Al-Li alloy FSW joints are relatively complex,and the strength and plasticity of the heat treated joints are good.