Microstructure and Texture Evolution in Linear Friction Welded TC11 Titanium Alloy Joint

In recent years，aircraft engines are continuously developed towards lightweight. As an important component of the aerospace engine，blisk’s performance is very important for improving the stability of the aircraft and the engine thrust-weight ratio. TC11 （Ti-6.5Al-3.5Mo-1.6Zr）is a titanium alloy that is very suitable for blisk manufacturing. At present，the titanium alloy blisk is manufactured using linear friction welding technology overseas. The microstructure and texture change greatly due to thermal-mechanical coupling during welding. Therefore，studying the microstructure and texture evolution of welded joints is necessary. Equiaxed TC11 titanium alloy was used in the experiment. The specimen size was 130 mm×75 mm×20 mm（L×W×H=length×width×height），and W×H was the welding section. In the experiment，the samples were welded with a frequency of 30 Hz，amplitude of 3 mm，and friction pressure of 52 MPa. After welding，a wire cutting machine was used to cut off the flash edges，and the metallographic samples were obtained. Subsequently，the samples were mechanically polished and etched with a solution of HF∶HNO3∶H2O=1∶3∶7（volume ratio）. The microstructure was observed with Olympus PMG3 optical microscope（OM）to obtain metallographs. Then the samples were observed with JSM-7900F scanning electron microscope（SEM）. The samples were electrolytically polished，and then electron backscattered diffraction（EBSD）experiments were performed with JSM-7900F electron microscope. OIM ANALYSIS software was used for subsequent data analysis. The Vickers hardness tester was used to test the metallographic hardness. The loading pressure was 1000 g and the pressing time was 15 s. The tensile test of the welded joint was carried out by INSTRON-3382 testing machine. The loading speed was 0.5 mm·min-1. The tensile samples were made according to GB/T228-2002 standard. The results showed that the boundary between the thermos-mechanically affected zone（TMAZ）and weld zone（WZ）was not obvious，the closer to the weld，the larger the deformation. Grain size became fine and compact near the boundary between WZ and TMAZ. There was no equiaxed grain in TMAZ near WZ，instead，there were deformed grains with streamlined patterns. In TMAZ，when the temperature exceeded β transformation temperature in the welding process，the primary α phase gradually disappeared and transformed into β phase. Due to the short welding time and the rapid heating and cooling processes，the primary α phase did not have time to completely transform，and some primary α phase remained. At the same time，a large amount of needle-like secondary α phase was precipitated during the cooling process，and a little of β phase was not decomposed in time during the rapid cooling process，forming the metastable β phase. A complete α→β→α/α´ transformation process took place in the center of the weld during welding. The central microstructure consisted of fine β equiaxed grains with distinct grain boundaries. Martensite and acicular secondary α lamella were distributed in the central weld. The broken α-phase structure could also be seen inside the β fine grains in WZ. The microtexture results showed that the relative intensity of α-phase texture in base metal（BM）was 19.342 mud，which was the lowest among the three joint regions. The main type of α-phase texture was P texture，and there were other orientations. There was no serious preferred orientation in BM. The texture of β phase was similar to that of α phase and there was no preferred orientation. The type of α-phase texture in TMAZ was P texture，and the strength was greatly improved to 48.033 mud，showing a strong preferred orientation. The main type of β-phase texture was F texture. Although E texture existed，its strength was low. Compared with the base material，the texture type changed and the preferred orientation appeared to a certain extent. This phenomenon was related to the strong deformation of grains in TMAZ. In addition to P texture，several high-density points appeared in α-phase pole figure（PF）of WZ，and the texture strength also decreased compared with TMAZ，with a strength of 34.745 mud. β-phase texture in WZ was similar to α-phase texture，with many high-density points and many types of textures. The correspondence between β-phase texture and α-phase texture proved the Burgess orientation relationship between α phase and β phase. In addition to the random grain orientation caused by dynamic recrystallization in WZ，the transformation of β phase led to the randomization of α-phase texture. Due to the similar force and metal flow direction in the welding process，P texture in WZ was similar to that in TMAZ. Based on the above results，it could be found that the microstructure and texture evolution during linear friction welding of TC11 titanium alloy were inseparable. Phase transformation，grain deformation，and metal flow all led to texture types and orientation changes. The microhardness showed a W-shaped distribution，and the highest hardness was located at the center of the weld，reaching HV 435. The tensile results showed that the average tensile strength and the average yield strength were 996.9 MPa and 920.9 MPa respectively，and the average elongation was 10.2%. The mechanical properties of the welded joint were not weaker than those of BM. © 2023 Editorial Office of Chinese Journal of Rare Metals. All rights reserved.