Study On Temperature Field、Microstructure And Properties Of Dissimilar Linear Friction Welded Titanium Alloy Joint

Linear friction welding (LFW) has become a key technology in manufacturingand repairing aeroengine blade disks (blisks). At present, LFW of dual performanceblisks begins to be studied in China. Therefore it is necessary to evaluate thetemperature field, microstructure, texture and mechanical properties of linear frictionwelded (LFWed) titanium joints, which can provide a theoretical basis andexperimental data for the manufacturing of the dual performance titanium blisks.LFWed dissimilar TC4to TC11titanium alloys joints were investigated in this study.The main results are as follows:According to the Fourier heat conduction equation, the heat conduction modelwithin the thermomechanically affected zone (TMAZ) and base metal (BM) wasobtained, and the factor of heat conduction speed was corrected. The nominal frictioncoefficient at the quasi-stationary phase was obtained by the measured power, andthe average friction power was calculated within a cycle along the direction offriction. Based on those results, the mathematical model of temperature field in theweld zone (WZ) was established. The temperatures of the LFWed TC4joint at theweld interface, TMAZ and BM were tested using the specially designed temperaturemeasurement system. The errors resulted from thermal inertia of thermocouples werecorrected, and the thermal histories of WZ were obtained. The temperature curvescalculated by the heat conduction model corresponded well with the experimentalresults, which verifies the accuracy and feasibility of the model.In order to avoid the influence of alloying elements on the microstructure andtexture during the welding, the microstructure and texture of LFWed pure titanium(TA2) joints were analyzed, which is expected to provide a theoretical basis for theLFWed TC4/TC11joints. The microstructure of LFWed TA2joints in the TMAZpresents large deformation, and there are many low-angle boundaries (LABs) withinthe grains. In the WZ, the sizes of grains (about3μm) are smaller than those in theBM (about6.7μm) and the fraction of LABs is about70%. In addition, there are alarge number of subgrains and recrystallized nuclei at the weld center. Theorientation of texture in the WZ and the end of flash is the normal direction of {0001}, which is perpendicular to the texture at the friction interface. The slip plane,slip direction and the movement of Ti atom from BM to WZ of the LFWed TA2jointwas analyzed, which explains the texture change at different zones and differenttimes.The texture at different times was studied. The texture orientation is the same atdifferent times by analyzing EBSD of the LFWed TA2joint, and the texture of jointis along the normal direction of {0001}, which is perpendicular to the texture at thefriction interface, and the{1010}plane rotates toward C-axis during welding. Alarge number of dislocations are formed in a grain under the shear force. Somedislocations with opposite sign annihilate, and dislocations with same sign arrangeand form dislocation wall under the stress, which tangle together and form subgrains.The disharmony of microstructural deformation is eliminated under the axial stressand shear stress, resulting in the rotation of grain boundary. The subgrain rotationproduces the recrystallization nuclei. During welding, recovery and recrystallizationexist simultaneously. The evolution mechanism of LFWed tiatanium joint wasinvestigated.The microstructure and the texture evolution in WZ of TC4/TC11joint and TA2joint are similar by analyzing their EBSD maps and microstructure.The mechanical properties of LFWed TC4/TC11joints were studied. Theultimate tensile strength of the joint is higher than parent TC4, and the tensilefracture is predominantly characterized by the ductile fracture characteristics. Theimpact toughness of the joint is80%of the parent TC4. The fatigue properties wereanalyzed. The Young’s modulus of the joint is almost the same as the parent TC4. Asthe strain amplitude is higher than0.6%, cyclic softening basically occurs in joints.The fatigue life and fatigue parameters are almost the same as the BM, whichsuggests that the fatigue properties of joint are good.Based on fatigue strength, tensile strength and impact toughness, the joints showanisotropy of mechanical properties. When the direction of fatigue and tensile load isperpendicular to the interface, and it is the same as the normal direction of basaltexture {0001}, the strength is high. However when the direction of the impact loadis parallel to the interface, and is perpendicular to the normal direction of basal texture {0001}, the impact toughness is low. The relationship between the textureand the properties was discussed using the Schmid factor.