The influence of the deformation degree of titanium alloy forging on the microstructure and mechanical properties of titanium rods and titanium forgings

The influence of deformation degree and deformation temperature on the microstructure and mechanical properties of titanium rods, medical titanium rods, and titanium alloy forgings is mutually interactive.

The deformation degree of titanium alloys has a significant impact on the high and low magnification microstructures. When the deformation degree is greater than 30%-40%, the microstructure of the titanium alloy becomes significantly refined. To fully refine the coarse needle-like and lamellar microstructure and transform it into a spherical structure, the deformation temperature must be controlled within the two-phase region, and the deformation degree should be no less than 60%-70%. The deformation degree should be ensured to form an intermediate microstructure between the needle-like and equiaxed structures. Above the β transformation temperature, for plastic deformation in the β phase region, a sufficient deformation degree (30%-40%) is required to achieve microstructure refinement. The reason is that after forging in the (α+β) phase region, the alloy reserves sufficient deformation energy and more dislocations, and during deformation in the β phase region, a recrystallization occurs, making the grain refinement in the β phase region more effective.

However, in the forged die-casting parts above the β transformation temperature, the above effect of refining grains is often not achieved. The reason lies in the large size of the die-casting parts and the long time that the metal stays in the β phase region. Especially in the difficult-to-deform areas of the die-casting parts, coarse grain structures often occur because the deformation degree is small, no recrystallization (β grain recrystallization) occurs, while the original grains undergo intense growth.

The change in deformation degree not only affects the grain size, but also has an impact on the needle-like (plate-like) microstructure within the grains. An increase in deformation degree will lead to the refinement of the microstructure within the grains. Like the influence of deformation temperature, the deformation degree has the most significant effect in the (α+β) phase region temperature, because there is an α phase present and this α phase undergoes plastic deformation.

At the temperature of the (α+β) phase region, the change in deformation degree has a more significant effect on the mechanical properties, especially the plasticity, than in the β single-phase region. During plastic deformation in the β phase region, the increase in deformation temperature will reduce the influence of deformation degree on the mechanical properties.

For example, when the TC6 alloy ingot is forged at a temperature above the β transformation temperature (1050°C), as the deformation degree increases, the low-magnification microstructure gradually becomes finer; at a deformation degree of 15%, the cast titanium microstructure is not fragmented; at a deformation degree of 30%, the cast titanium microstructure is slightly fragmented and slightly fibrous; at a deformation degree of 60%, it is in a clear fibrous state, but still leaves a relatively clear cast titanium microstructure; when the deformation degree increases to 80%, it is in a fine fibrous microstructure, but still leaves traces of the cast titanium microstructure.