Effect Of Ageing After Cold-rolling Treatment On Mechanical Properties,corrosion Properties And Thermal Stability Of Al-Cu-Li Alloy

Al-li alloy belongs to the age hardening alloy,which has the advantages of lower density,higher specific strength and specific stiffness,etc.,and is widely used in the aerospace field.With the development of industry,traditional micro-alloying or heat treatment methods are increasingly difficult to meet the requirements of high strength and excellent corrosion resistance of alloys.In this paper,a kind of Al-Cu-Li alloy which was smelted in the laboratory was chosen as experimental material,and hardness,tensile,immersion corrosion and electrochemical tests are carried out.Firstly,the effect of different solution temperatures on the microstructure and properties of Al-Cu-Li alloy during peak ageing was studied.Secondly,the ageing after cold rolling was further studied at optimal solution temperature.Finally,the change rules of mechanical and corrosion properties of under-aged and peak-aged alloys were studied through thermal exposure experiments,and the thermal stability mechanism of the alloys was discussed.The microstructures and properties of the alloy after thermomechanical treatment was investigated by X-ray diffractometry（XRD）,texture analysis,microstructure observation,scanning electron microscopy（SEM）and transmission electron microscopy（TEM）and other analytical techniques.The purpose of this paper is to explore the internal mechanism of the alloy property improvement and optimize the deformation heat treatment process parameters.The main conclusions are as following:（1）After solution treatment at（510℃～550℃,1 h）+peak ageing at（180℃,16 h）,when solution temperature is 520℃,the mechanical properties and corrosion properties of the alloy are best matched.The tensile strength,yield strength and elongation are 582 MPa,540 MPa and 10.2%.The maximum intergranular corrosion depth is about 210μm.When the solution temperature is too low,the soluble second phase particles cannot fully dissolve into the matrix,and the strength and elongation do not reach the best value.Too high solution temperature will lead to overburning of the alloy.（2）With（520℃,1 h）solution+cold rolling with 60%thickness reduction+（120℃,140℃,160℃）ageing process,the peak strength of the alloy is significantly improved compared with conventional peak ageing T6treatment,and the alloy retains certain plasticity,and the intergranular corrosion sensitivity is significantly improved.After ageing at（160℃,12 h）,the composite properties of the alloy are better,the tensile strength,yield strength and elongation are 582 MPa,540 MPa and 4.7%,respectively.Moreover,the alloy has no IGC sensitivity,and the maximum corrosion depth is about 55μm.（3）The strength improvement of T6 alloy is mainly due to precipitation strengthening,while the strength improvement of cold rolled ageing alloy is not only attributed to precipitation strengthening,but also accompanied by dislocation strengthening,texture strengthening,refined grains and other strengthening mechanisms.During ageing,both the strength and elongation of the cold rolled alloy increase first and then decrease,and increase with the aging process,which is related to the ageing changes of T₁,S’andδ’phases and dislocation recovery.At the same time,the dislocation recovers,and the space for the matrix to accommodate dislocation slip increases,which improves the cumulative capacity of the matrix to dislocation during tensile deformation.（4）Intergranular corrosion occurs at the peak ageing stage of T6 alloy,which is related to the potential difference between the precipitated phase at the grain boundary and Precipitate Free Zone.The intergranular corrosion sensitivity of cold rolled alloy after ageing is improved,and intergranular corrosion occurs in the under-aged stage,while peak ageing and over-aged alloy change into pitting corrosion,which is related to the acceleration of grain boundary precipitated phase from continuous distribution to discontinuous distribution under the action of dislocation during ageing.（5）The strength of the under-aged alloy increases slightly after thermal exposure at 150℃for 8 days,while the strength of the peak-aged alloy decreases slightly,which is attributed to the secondary precipitation of T₁phase,the main strengthening phase,and its higher coarsening resistance.At175℃and 200℃,the strength decreases significantly,which is mainly due to the coarsening of equilibrium phase S,δand T₁.Pitting corrosion occurs in both under-aged and peaking aged alloys after thermal exposure at 150℃,175℃and 200℃,which is related to the discontinuous distribution of grain boundary precipitates,and the maximum corrosion depth gradually deepens with the increase of temperature.