Effect Of Heat Treatment And Pre-rolling On Microstructures And Mechanical Properties Of 2195-O Al-Li Alloy Fsw Joints

Al-Li alloy is a typical heat-treatable strengthened aluminum alloy.2195 Al-Li alloy has the characteristics of low density,high specific strength and high elastic modulus,so it is widely used in aerospace field.2195 Al-Li alloy that adopts friction stir welding(FSW)technology can avoid pores,hot cracks and other defects,but in the subsequent heat treatment process,the abnormal grain growth(AGG)often occurs in the nugget zone(NZ)of the welded joint,which leads to the decrease of the mechanical properties of the alloy.In order to restrain the appearance of AGG,the joint was cold rolled before heat treatment.In this paper,the 7.5mm thick 2195-O Al-Li alloy was made to be FSW,solution treatment and aging treatment.Besides,the welded joint was cold rolled with the deformation of 10% to 40% before heat treatment.The effects of heat treatment and pre-rolling deformation on the microstructure and properties of the joint were investigated by optical microscope(OM),scanning electron microscope(SEM)and transmission electron microscope(TEM),as well as hardness and tensile properties tests.The main conclusions are as follows:(1)Due to the heat input and material flow in the FSW process,the size and distribution of the second phase particles in the NZ of 2195-O Al-Li alloy are uneven,showing a "dendritic" shape.Compared with NZ near the advanceing side,the second phase particles near the retreating side are thicker and more distributed on the grain boundary.(2)AGG occurs in NZ during solid solution treatment,and the range of AGG expands gradually with the increase of solution time.AGG starts from the top of NZ and the root of NZ near AS-TMAZ,and spreads to the whole nugget zone.The main strengthening phase of natural aging in NZ is δ ’phase,and the strengthening phase of artificial aging is T1 phase,δ’ phase and θ’ phase.(3)The solution treatment increases the hardness distribution of the joint as a whole,and the shape like the Chinese character "几" becomes relatively flat.After artificial aging treatment,the hardness of the joint continues to increase,and the shape of "几" appears again.It is determined that the heat treatment process is solution treatment(520℃,30min)+ water quench in room temperature + aging treatment(160℃,60h).At this time,the ultimate tensile strength is 502.9MPa and the elongation is 4.2%.(4)The pre-rolling deformation has a great influence on microstructure of the joint in NZ during solid solution treatment.When the pre-rolling deformation is 10%,AGG of NZ particles are more prominent.When the deformation is 20%,AGG phenomenon disappears.With the continuous increase of pre-rolling deformation,the size of grains in NZ becomes tinier and tinier.(5)The effect of pre-rolling deformation on the mechanical properties of solution treatment and aging treatment of joints is similar.When the deformation is 10%,the tensile properties of the welded joints decrease after heat treatment.When the deformation is more than 20%,the ultimate tensile strength and elongation of the joint increase with the increase of deformation.After 40% deformation and peak aging treatment,the ultimate tensile strength of the joint is 527.6 MPa and the elongation is7.2%.