Microstructure Evolution And Strengthening Mechanism Of TA15Titanium Alloy During Hot Power Spinning

As a typical near-α titanium alloy, TA15titanium alloy exhibits excellentproperties, such as high specific strength, good thermal stability, considerable creepresistance and great weldability. Therefore, it has been widely used to produce the keycomponents in the aviation and aerospace industry. Power spinning is a typicallocal-loading forming technology with many advantages, such as low forming load,high material utilization ratio and simple tooling. As an effective method to manufacturetubes with large diameter and thin thickness, power spinning has been widely applied inmany industries, such as aviation, aerospace, armament, marine and machinery. Thisthesis focused on the control bottlenecks in microstructure and properties of titaniumalloy during hot power spinning process. The BP neural network prediction model ofTA15titanium during hot power spinning was established, and the evolution law of themicrostructure and texture and the strengthening mechanism of TA15titanium alloyduring hot power spinning were systematically investigated.The microstructure evolution features of the plane strain thermal compressionsamples were compared with those of hot power spun tubes to identify their correlations.The grain size and recrystallization ratio of microstructure were quantitativelycharacterized using image processing software, and the microhardness was measured bythe microhardness tester. The relation between the processing parameters of the planestrain thermal compression and the microstructure and mechanical properties of TA15titanium alloy was systematically studied, and then a BP neural network predictionmodel for microstructure and mechanical properties of TA15titanium alloy during hotpower spinning was established. The prediction error were within13%compared toexperimental results. It indicates that it is feasable to simulate the microstructureevolution during hot power spinning using the plane strain thermal compressionexperiment, which can be further used to reveal the effect of the processing parametersof hot power spinning on the microstructure and mechanical properties of TA15titanium alloy.The microstructure evolution (i.e. grain size, grain aspect ratio, grain boundarydensity, etc) during hot power spinning in different passes was deeply investigated byTEM and EBSD. The results show significant changes in the microstructure shape ofTA15titanium alloy with the increasing of spinning pass. The coarse grains of theoriginal primary α phase were significantly elongated along the axial direction, resultingin the formation of a fibrous structure. The lamellar grains of the original secondary αphase evolved into a large number of small equaxial grains. Additionally, there werealso significant changes in the number and distribution of the grain boundaries with different angles in microstructure. When the thickness reduction ratio was below19.7%,most small-and medium-angle grain boundaries presented inside the original coarseprimary α phase and between the lamellas of the secondary α phase. When the thicknessreduction ratio increased to81.1%, a large number of small-and medium-angle grainboundaries evolved into large-angle ones, leading to the refinement of as-spunmicrostructure. During the hot power spinning process of the TA15titanium alloy, slipand twinning were the main deformation mechanisms of the primary α phase. When thethickness reduction ratio was below19.7%,{1012}<1011> tensile twinningpredominated the deformation processing. The globularization of the secondary α phaselamellas lead to the softenning of alloy. The spheroidization model of the secondary αphase during hot power spinning process was also proposed.The effects of the microstructure and texture on the mechanical properties of TA15titanium alloy during hot power spinning were analysed by compression test. The yieldstrength of TA15titanium alloy during hot power spinning increased with the decreaseof the grain size, the increase of the aspect ratio of the primary α phase and the increaseof the volume fraction of fresh small grains. With the increase of spinning pass andthickness reduction, the grains of TA15titanium alloy showed significantly preferredcrystal orientation distribution. When the thickness reduction ratio reached81.1%, the caxis of more than45%TA15titanium alloy grains rotated to the direction with less than20°difference relative to the ND, and the average misorientation of grains reduced fromoriginal67.5°to28.1°. Recovery happened in the coarse primary α phase, grainselongated and flattened to form the obvious {0001} bimodal or multimodal textureswith slightly diffused distribution along TD. Dynamic recrystallization happened insecondary α-phase finer grains and formed {0001} texture. The crystal orientations ofthe primary α phase grains and small secondary α-phase finer grain size were wellconsistent in some locations. The orientation consistency weakened with the increase ofthe thickness reduction ratio. The TA15titanium alloy tube have prefered orientation,the Schmid factors of the {0001}<1120> basal slip of grains in RD was obviouslysmaller than that in TD, which resulted in anisotropy of the tube, and yield stress in RDwas higher than yield stress in TD.Uniaxial, biaxial tensile tests and hydraulic bulging test were carried out to test themechanical properties of TA15titanium alloy tubes after hot power spinning. The axialtensile test showed that as-spun tubes exhibited obvious anisotropic properties: thetensile yield strength along the axial direction was higher than that along thecircumferential direction, while the elongation along the axial direction was lower thanthat along the circumferential direction. Biaxial tensile test showed that the yieldstrength of tubes under biaxial tensile test were significantly higher than that under uniaxial tensile test when the ratio between the axial stress and the circumferential stresswas1:2, and the values of bixial strengthening effect in axial and circumfentialdierection were10.2%and7.0%at room temperature,11.7%and8.9%at500℃,respectively. The result of hydraulic bulging test of as-spun tubes coincided with thebiaxial tensile test result. Therefore, the service mechanical property of TA15titaniumalloy hot power spun tubes at500℃was predicteded using the biaxial tensile test,which showed that the tube would yield with a circumferential stress of1106MPa.