Texture Evolutions In The Tube Spinning And Subsequent Heat Treatment Of Pure Titanium And Bt20 Alloy

Tube spinning is a typical local-loading forming process with the advantages such as material saving, low forming load and simple tooling. Considered as an effective method to manufacture large-diameter, thin-thickness and seamless tubular part the process has been applied widely in aeronautics, aerospace, weapon, shipbuilding and mechanical industries, etc. Until now most attention has been paid on how to fabricate a workpiece without defects and improve the geometrical precision because spinning deformation is very complex and influenced by many factors. However it was rarely considered to control the mechanical property of as spun part. The mechanical property of a single crystal is usually anisotropic. Therefore material mechanical property is strongly influenced and feasibly controlled by crystal texture. Texture evolution could occur in plastic deformation and heat treatment, so the topic of the paper is oriented as texture evolutions in titanium and BT20 alloy tube spinning and heat treatment of as-spun workpiece. Finite element simulation based on crystal plasticity theory, which is an important method to investigate plastic deformation of polycrystalline aggregate, was applied to study spinning texture. Considering the plastic mechanisms of pure titanium are various at low temperature and spinning deformation is very complex, texture evolution only in single-pass spinning of pure titanium at high temperature, in which only slip systems were expected to be activated, was simulated in the paper. Texture evolutions in multi-pass spinning and subsequent heat treatment were investigated by experimental method.Initial texture, deformation systems and deformation history in forming process are necessary in deformation texture simulation. Discretized macro-texture was adopted as initial texture, and deformation systems of pure titanium were slips in basal, prismatic, pyramidal plane, and slips in first-order pyramidal plane{1010}, second-order pyramidal plane{1122} . Polycrystalline model was established and crystal plasticity constitutive equations were coded in material subroutine UMAT of commercial software ABAQUS, then the parameters in the equations were obtained by fitting stress-strain curves and texture evolution of pure titanium simple compression at 750℃. Spinning deformation was represented by deformation gradient, and calculated from FEM simulation of pure titanium tube spinning using isotropic constitutive relationship. In order to realize spinning deformation of polycrystalline model the deformation gradient as a function of time was transferred to the displacements of surface nodes on the model as boundary condition. Therefore the procedure makes it possible to investigate the influences of feed rate, thickness reduction and initial texture on spinning texture. The results show that {0002}-type texture which contains {0002}<1010> -type texture and {0002}-type fiber texture components is evolved in both outer and inner surfaces of pure titanium as-spun workpiece by different feed rates and thickness reductions. In outer surface {0002}<1010> -type texture component is enhanced by spinning with large thickness reduction and small feed rate.Texture analyses of multi-pass and large deformed tube spinning show that double peaks, the intensities of which are different, tend to appear on {0002} pole figure around the surface normal of pure titanium as-spun part, and the line through the two peaks deviates the hoop direction of tube. However, just unsteady single or several peaks appear on {0002} pole figure of BT20 alloy as-spun workpiece, and steady single peak would come into being when total thickness reduction reaches a quite large level (84.8% in the paper).As-spun workpieces of pure titanium and BT20 alloy after multi passes and large deformation were heat treated, and optical microstructures were observed to determine reasonable temperatures for primary recrystallization annealing and grain growth. Crystal orientations in heat treatment samples were recognized using electron back scattered diffraction technology. Number-based and volume-based pole figures and their difference were calculated to figure out the mechanisms of texture formation, namely oriented nucleation or growth, in primary recrystallization. Texture evolution during grain growth was also studied. The results show that oriented nucleation mechanism plays an important role in the recrystallization texture formation of both as-spun pure titanium and BT20 alloys, and oriented growth tendency in BT20 alloy is larger than that in pure titanium. Textures in the primary recrystallizations of two titanium materials were randomized in grain growth, and now components would occur at near orientations.