Effect Of Weld Processes And Post-Weld Heat Treatment On Microstructures And Properties Of 2195-T8 Al-Li Alloy During Friction Stir Welding

In this paper,rolled sheets of 2195-T8 aluminum-lithium alloy with 7.5mm in thickness were used as the base material.Control the welding process parameters(shoulder depth plunge:0-0.3mm,tilt angles of stirring pin:0.5-3.5°),and use orthogonal experiments to perform friction stir welding.The samples under optimal process parameters are subjected to post-weld artificial aging treatment(160°C×8h)and solution+artificial aging treatment(520°C×0.5h+160°C×8h).The tensile test,hardness test,and electron backscatter diffraction are used to analyze the evolution of microstructure&mechanical properties under different welding process parameters and heat treatment systems(PWHT).The main results are as follows:(1)Different welding process parameters will cause differences in the surface morphology of weld.The surface smoothness of weld increases first and then decreases,and height of flash will increase when shoulder depth plunge increases.Similarly with the increase of tilt angles of stirring pin,surface gloss of weld gradually becomes darker,and fish scales become blurred.(2)The tensile strength value is mainly concentrated at 350-390MPa.It has the highest tensile strength with 389MPa and welding coefficient is 0.65 when shoulder depth plunge is d=0.1mm,and tilt angle of stirring pin is?=2.5°.In the thermo-mechanical affected zone,the fracture shape is 45°slope.At the same time,orthogonal experiment method shows that impact of shoulder depth plunge on the tensile strength is greater than tilt angles of stirring pin.(3)The content of high-angle grain boundaries in nugget area under different shoulder depth plunge or tilt angle of stirring pin is consistent with changes of grain size.Under optimal parameters,the content of large-angle grain boundaries in nugget area is75.8%.At the same time,along white second phase distributed at the grain boundary is T_B phase;it is easy to cause holes and microcracks around grain boundary under high temperature.With the increase of heat input,second phase in nugget area is continuously reduced,and second phase in thermo-mechanical affected zone becomes coarser.(4)During FSW process,nugget area and thermo-mechanical affect zone will undergo complex texture composition changes.With the increase of shoulder depth plunge,shear texture of nugget area is named C{001}<110>changed by texture component of A{111}<uvw>.Under the action of"rolling force",retreat side of thermo-mechanical affect zone evolves into S{123}<634>rolling t_—exture.Increasing tilt angles of stirring pin,texture of nugget zone is composed of B/B{112}<110>,which meets strain coordination requirements.(5)Post-weld heat treatment effectively improves microstructure of weld zone.When heat treatment system is 160°C×8h,texture of micro-zone changes partly,and grains of Cube-ND shear texture orientation in retreat side of thermo-mechanical affect zone decrease.When heat treatment system is 520°C×0.5h+160°C×8h,grain size of nugget zone increases to reach 33.2%.The texture type and peak density of each typical area have changed significantly.(6)The weak zone of joint hardness disappears after heat treatment at520°C×0.5h+160°C×8h,without W-shaped features,which hardness value is175-195HV.Meantimes tensile strength value is 453MPa,and welding coefficient increases from 0.65 to 0.76.