Study On Deformation Behavior And Process Of Initial Extrusion For Large Size Cast TiAl Alloy

TiAl alloys with high specific strength, good creep and oxidation resistance have been taken as the potential high temperature structural materials for aerospace, energy and automotive industry. The proper deformation technique for TiAl alloys is extrusion. In general, the TiAl ingot with diameter less than90mm was extruded into bar as10-20mm in diameter and200mm in length. However, this size can not be satisfied with the applications. So it is necessary to find the extrusion method for the large sized TiAl ingot. TiAl alloys were in possess of poor hot-workability and significant sensitivity of microstructure evolution and deformation behavior, which caused it difficult to obtain extrusion with crack-free and uniform microstructure for the large sized TiAl ingot. It has been reported at aboard by one step process with extruded ratio of14, and the deformed bar with refined recrystallized grains can be obtained. Untill now, the large sized TiAl ingot deformation has not been developed in China, because with the equipment in China, it is hardly afford such one step process because of relative higher deformation resistance for TiAl alloy. On the other hand, large sized ingot need long time to finish extrusion process, which result in significant temperature loss at the surface of TiAl ingot. Then, non-uniform plastic deformation and flow behavior may happen because of difference of temperature between surface and center of the ingot, which may result in inhomogeneous microstructure in extruded bar. The large extrusion ratio can be realized by the multi-step process extrusion technique with accumulated smaller strains, which is useful for homogeneous microstructures of extruded bar.Based on the above thoughts, the title of the present study is "Study on deformation behavior and process of initial extrusion for large size cast TiAl alloy". Constitutive equation was established based on hot compression tests for TiAl alloy. The study focused on the effects from the extrusion process parameters on the plastic deformation behaviors, which were used to optimize the extrusion parameters for TiAl ingot with200mm in diameter. Extrusion with extrusion ratio of15performed to TiAl alloy ingot with diameter of200mm, the microstructure, fully lamellar microstructure, properties were also discussed. The results were show as follow: The hot deformation behavior was studied for Ti-46.0Al-2.5V-1.0Cr-0.3Ni(at.%) at elevated temperatures, which is in the range of1000℃～1200℃,10-3s-1～100s-1.The constitutive equation, lnσ=A+BlnZ,(A=1.86-73.92ε+322.2ε2-552.9ε3+326.5ε4, B=0.091+1.839s-7.993ε2+13.495ε3-7.856ε4) was established by analyzing the true stress-strain curves. The calculated active energy was493kJ/mol. The results showed that the value of prediction and experiment was quite near. The cooling temperature model was constructed and the results show that the predicted value and tested value was quite near. The constructed cooling temperature model can be embedded in the FEM software to predict the change of temperature during the transference of the ingot.The influence of extrusion ratio on plastic deformation behavior was investigated. The results showed that the strain at center of the ingot increased with increasing extrusion ratio. The strain at center of the ingot was high with extrusion ratio of15, however the extrusion load exceeded7500T, then internal equipment can not fulfill it. The flow of material was multilayer when the extrusion ratio was in excess of7. The non-uniform plastic deformation was more significant with increasing extrusion ratio. Two-step extrusion process accomplishes the high extrusion ratio with the lower extrusion load, followed by the uniform flow of material. The extrusion ratio combination of two-step extrusion was studied. It showed that the extrusion ratio combination has significant influence on the distribution of the strain and strain rate. The optimized extrusion ratio combination was3.5and4.3, which was beneficial to uniform distribution of deformation and the strain rate.The effects from die angle, extrusion rate and the preheated temperature of the workpiece on the deformation were studied. It showed that the influence of process parameter on the first step extrusion was more significant than second step extrusion. The strain and flow of material was more uniform with decreasing extrusion die angle. But the tension stress was raised with increasing die angle due to increasing friction, which caused more significant tendency of crack. The flow of material was multilayer when the die angle was in excess of70°. The plastic deformation was non-uniform when ram speed was slow, which caused concentration of the strain on the corner of the die. The plastic deformation was more uniform with increasing ram speed, but the tension stress increased. Concentration of strain was more significant with increasing of preheated temperature of the workpiece, which caused non-uniform distribution of strain. With increasing preheated temperature of workpiece, the tension stress increasing, distribution of velocity in workpiece become non-uniform. With the optimized deforming parameters by the finite element simulation, it has been succeeded for the extrusion of TiAl ingot with200mm in diameter into bar by two step processes with extrusion ratio as15, with good surface and45mm in diameter and1000mm in length. The refined microstructure of the wrought alloy was completely recrystallized and homogeneous. It proved that the extrusion ratio of15was enough to break casting microstructure at center of workpiece and the plastic deformation was uniform during extrusion.The formation of extrusion streamline was investigated, the type of extrusion texture was examined. It showed that extrusion streamline in testing TiAl alloy consisted of refined y phase, α2phase which is along the extruding direction, which is caused by the Al segregation in the cast ingot. The extruding texture components was<011>//ED,<101>//ED (ED is extrusion direction). This type of texture leaded to high elongation along axial and low elongation along radial. The refined fully lamellar microstructure was obtained and the properties were superior.