Formation of Semi-crystalline State of BT16 Titanium Alloy

  When using titanium alloy, its structural strength and safety must be guaranteed. The development trend of contemporary titanium alloy applications is to reduce the metal loss of structural components and parts, and to increase the power of complete sets of equipment while meeting certain mechanical performance requirements. Although the strength of the structural parts can be improved by the traditional process, but at the same time, the plasticity is lost. Therefore, all kinds of strong plastic deformation are now commonly used, which can form a fibrous structure or a semi-microcrystalline state, and can also increase the strength. The study shows that the semi-microcrystalline BT1-0 pure titanium undergoes grain refinement and thermal stabilization, and its strength has been tripled (σ0.2 = 1010MPa, σB = 1110MPa, Hμ = 3200MPa), and the plasticity is 6% .
  It is well known that the semi-microcrystalline state and fibrous structure of alloys are easier to form than pure metals during strong plastic deformation. Therefore, this paper mainly studies the formation of semi-crystalline state of BT16 titanium alloy and the possibility of subsequent hot rolling.
  Research materials and methods
   The research material is a 50mm-long hot-extruded blank cut from (α+β) type BT16 titanium alloy Φ25mm rod. The structure of the blank is extremely uneven, and the axial part is a layered (α+β) structure of coarse grains (Figure 1α, grain size is 6); there is also a layered structure near the surface, but more is a dispersed structure (Figure 1б, The particle size is 4). The microhardness of the surface layer is 2680 MPa, and the on-axis is 2470 MPa. Products and semi-finished products made from this blank can be hot rolled to ensure a small porosity and fine grain structure. Due to the removal of scale and gas-saturated layers in the final mechanical processing, this processing method will cause greater metal loss.
At this stage, the strong plastic deformation of the original billet at this stage is three-way sequential forging (upsetting), which is similar to the multi-step abc extrusion. The billet is forged at 600°C to 20mm, then the billet is rotated 90 degrees, at 550°C to 20mm, and finally the billet is rotated 90 degrees and forged at 500°C (Figure 2α). After these steps, a bar with a length of 60 mm and a cross section of 20×20 mm can be obtained. It can be seen that the volume of the blank is reduced by 2.2%, which is due to the reduced porosity and the removal of the looseness on the shaft, which causes the metal to shrink.
in conclusion
  The research results prove that the strong plastic deformation can form the semi-microcrystalline structure of the BT16 titanium alloy blank. Strong plastic deformation is the three-way forging deformation through shaft deformation, followed by multi-pass rolling at 550-600°C with a total deformation of not more than 80%. After these deformations, a semi-microcrystalline structure with an α phase size of 25 nm is formed, which can ensure that the microhardness of the alloy is increased by 40%.
   The semi-microstructure formed at the same time contains internal stress, which can greatly reduce the plasticity of the material and even cause the blank to crack. In order to eliminate these defects, it is necessary to determine the compression rate during rolling and perform final annealing before recrystallization.