Investigation On Friction Stir Welding Of 2195-T8 Al-Li Alloy Thick Plate

2195 Al-Li alloy is the third generation of aluminum-lithium alloy,which has the advantages of high specific strength,good corrosion resistance and fatigue resistance.The alloy can effectively reduce the weight of aircrafts and spacecrafts.Friction stir welding(FSW),as an advanced solid-state joining technique,can avoid the problems caused by the traditional fusion welding techniques,such as Li burning,porosity,hot cracks and so on.At present,the aerospace field has raised the requirement for Al-Li alloy thick plate,while the Al-Li alloy FSW research is still mainly focused on the thin plate,and the thick plate is rarely studied.Therefore,it is of great significance to carry out Al-Li alloy thick plate FSW research from the points of engineering and academic research.Taking the 12 mm thickness 2195-T8 Al-Li alloy as the research object,the thesis carried out the investigations of stirring tool design,joint forming,mechanical properties,microstructure and welding thermal cycle.The results showed that the FSW joint of Al-Li alloy thick plate can be obtained by using the designed tool.The higher rotation speed(?500rpm)leaded to the hole defects,while the higher welding speed(?100mm/min)leaded to the incomplete penetration.The maximum tensile strength of the joints was 422 MPa,which was obtained at the welding speed of 75mm/min and rotation speed of 300rpm;The maximum elongation of the joints was 7.1%,which was obtained at the welding speed of 50mm/min and rotation speed of 300 rpm.Under the selected parameters,the tensile strength of the upper parts of the joints were higher than that of the lower parts.The hardness distribution of the top parts(TP)of joints were in the shape of “W”,while that of the bottom parts(BP)of joints were in the shape of “V”.The lowest hardness of the joint located at the interface of the thermal mechanical affected zone (TMAZ)and the heat affected zone(HAZ).The TP and the BP of the joint were dimple fracture,while the middle part(MP)was quasi-cleavage fracture.The average grain size of the weld nugget zone(WNZ)decreased along with thickness direction of the joint.The original precipitates in the TP of WNZ were dissolved,and the needle-shaped precipitates with dispersive distribution were precipitated after welding.The BP of WNZ consisted of the spherical and bulk-shaped precipitates.The TP of TMAZ/HAZ interface consisted of T1 phases,while The BP of TMAZ/HAZ interface consisted of the bulk-shaped precipitates.The precipitates in the HAZ were as same as the base material,which occurred coarsening.Precipitates strengthening was the main strengthening approach of the FSW joint.According to the results of the simulation,the peak temperature reached 560?.The peak temperature difference of the TP and the BP of WNZ was about 49?,and the high temperature residence time difference of those was about 15s.The peak temperature difference of the TP and the BP of TMAZ/HAZ interface was about 31?,and the high-temperature residence time difference of those was about 12.5s.The closer regions to the weld center or the shoulder had the higher peak temperature and the longer hightemperature residence time.The inhomogeneous welding thermal cycle can account for the inhomogeneous microstructure and the inhomogeneous mechanical properties.