| Superplastic forming(SPF)is a promising hot processing technology which enables the formation of complex geometrical parts and can improve the utilization rate of materials and cost savings compared to other conventional manufacturing technics.The material is prone to obtain excellent superplasticity with stable and fine equiaxed microstructure which is generally obtained by large plastic deformation in α+β region and subsequent heat treatment.SP700 alloy,a new type β-rich α+β titanium alloy,exhibits excellent mechanical properties,good workability and pronounced superplastictity compared with Ti-6Al-4V alloy.However,there are a larger number of unspheroidized a lamellae in the sheet,which is harmful to the mechanical properties,especially superplasticity of the sheet.Therefore,it is necessary to get more information of the recrystallization behavior,especially the spheroidization of lamellae structure.In order to systematically study the microstructure evolution,especially the spheroidization behavior of grain boundary a phase,the single/double-pass plane strain hot compression tests and annealing were carried out which could provide theoretical basis for the optimization of the rolling process.Upon this,superplastic tensile behavior and deformation mechanism of SP700 titanium alloy sheet were explored,which provided the experimental and technical foundation for the application of the superplastic forming.The main conclusions are as follows:The single-pass plane strain hot compression test was carried out at a temperature range of 800-880℃,strain rate range of 1-10 s-1 and compression strain of 30-75%,in order to illustrate the effects of deformation parameters on the spheroidization behavior of lamellar structure in SP700 titanium alloy.The results showed that the β phase had undergone dynamic recrystallization and the degree of recrystallization of β phase increased with the increase of deformation compression.For a lamellar,it was gradually kinked under compression stress.With the decrease of temperature,the increase of strain rate and deformation strain,the aspect ratio of a lamellae decreased and the degree of spheroidization increased.When compressed at 800℃ with strain over 50%and strain rate over 5 s-1,bothβ phase and a lamellae showed obvious recrystallization behavior.In actual,the multi-pass rolling deformation is adopted in the sheet production process of SP700 alloy.Therefore,isothermal and cooling-thermal double-pass plane strain hot compression were carried out to further explore the effects of parameters on the spheroidization behavior of lamellae structure.The results showed that substructures and dynamic recrystallized grains of β phase firstly generated at the prior β grain boundaries during the first pass compression,where provided more preferential nucleation site in the second pass compression,which was conducive to the recrystallization.With the same total strain,the degree of recrystallization increased with the increase of the compression passes.Compared with the cooling-thermal compression,isothermal compression was more conducive to dynamic recrystallization.Annealing temperature is one of the important factors during the sheet production.The effect of annealing process on the microstructure after single-or double-pass compression was systematically studied.The annealing treatments were carried out at the temperature below,equal to and above the compression temperature.It was found that the static recrystallization degree increased with the increase of annealing temperature.Due to the influence of compression parameters,the static recrystallization still occurred at an annealing temperature below compression temperature when deformed at high strain rate(higher than 5 s-1)and compression strain(over 50%),which could avoid the grain growth during annealing treatment.During the heating process before compression,α lamellae precipitated at β grain boundary(GBα),and there is a Burgers orientation relationship(BOR)between GBα and βphases.Deformation behavior of the grain boundary a phase is not only affected by the thermal deformation parameters but also related to the orientation characters.There were 5 types GBα according to BOR and the angle between c-axis and compression direction.Spheroidization could occur in the GBα lamellae of type Ⅰ-Ⅳ after compression and annealing.However,when the GBα lamellae maintained BOR with both the adjacent βgrains with the c-axis approximately paralleled to the compression axis,the dislocation slip was not easy to active inner the GBα lamellae.At the same time,the dislocation slip was hardly activated across the α/α interface boundary with the slip direction quite different between the secondary a and GBα lamellae.As a result,the a lamellae maintained an aspect ratio over 15 even after compression and annealing treatment.While,decrease of the deformation temperature could reduce the precipitation probability of this uneasily spheroidized grain boundary a phase and obtained the qualified SP700 titanium alloy sheet for superplastic forming.In the actual superplastic forming process,the deformation strain varies at different deformation positions,resulting in different microstructure characteristics.In this paper,the effects of deformation parameters on superplastic behavior and microstructure of SP700 titanium alloy sheet were systematically studied.The results showed that the SP700 titanium alloy exhibited excellent superplasticity at temperature of 740-800℃ and strain rates of 1×10-4-1×10-2 s-1.The maximum elongation was obtained at 760℃/1×10-3 s-1.With the increase of temperature,strain rate and strain,the average grain size increased and α→βtransfer occurred due to the diffusion of elements.When deformed at 740-780℃ and medium strain rates(approximately 5×10-4-5×10-3 s-1),the m-values were all over 0.45 and GBS accommodated by dislocation slip was the main deformation mechanism.As temperature increased to 800℃ with strain rates of 1× 10-4 s-1,the m-value was only 0.1,and deformation would be controlled by β grains owing to the high volume fraction of β phase and the quickly growth at a higher temperature. |
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