Effect of recrystallization annealing temperature on microstructure and mechanical properties of large plastic thermal deformation Ti-6Al-4V alloy

Biphasic Ti-6Al-4V alloy is the most widely used titanium alloy, accounting for more than half of the market. This is mainly due to its excellent mechanical properties: high strength, fatigue strength and fracture toughness. And its mechanical properties depend on its microstructure. The application of deformation-combined heat treatment can effectively change the microstructure of the alloy, such as the complete martensite structure, the equiaxed + structure, the two-phase structure, and the chip + structure. Among the four kinds of tissue structures mentioned above, the +B tissue with equal axis has better plasticity, higher fatigue strength, and better high-temperature formability. The Ti-6Al-4V was refined by means of hot rolling.

The grain can be refined to hundreds of nanometers, and the and grains can be obtained in the same axial state. However, when Ti-6Al-4V grains are significantly refined by hot rolling, the texture will be formed, leading to the optimization of most grain orientation distribution and anisotropy of material properties. Due to the inconsistency between the surface and the center strain of sheet rolling, the surface strain is larger than the center strain, forming grains with varying sizes from the surface to the center. In order to obtain better mechanical properties, the size of the thickness of the material is limited, and the size of the general material is small. The process of thermal forging can remarkably refine the grain of the material. Generally, the grain of Ti-6Al-4V refined by multidirectional forging is a fine grain of equiaxed sub-micron scale. Moreover, the grain is more uniform without significant anisotropy and has better comprehensive mechanical properties. A process by which the -phase or + -phase thermal deformation is followed by a subsequent heat treatment to refine the grain and recombine the product. The recrystallization temperature, time and cooling rate all affect the microstructure and mechanical properties of the materials.

Ti-6Al-4V biphasic titanium alloy was heated to + phase zone, and then subjected to recrystallization heat preservation heat treatment at different temperatures by means of large plastic thermal deformation by means of pile-drawing. The results show that after the heat treatment, the grain is transformed into fine equi-axis grain and strip A + structure. With the increase of recrystallization annealing temperature, the grain size of equi-axis grows gradually, the strip + structure also grows gradually, the volume fraction of recrystallization grain increases, and the proportion of small Angle grain boundary decreases. After recrystallization at 800 ℃, the elongation and section shrinkage of the material were up to 19.9% and 42.5%, and the comprehensive mechanical properties of the material showed a decreasing trend with the increase of recrystallization temperature, which was attributed to the coarsening of the grain and the deterioration of the uniformity of the grain.