Titanium plate and titanium alloy plate heat treatment in the process of crystallization

In addition to the recovery and recrystallization process of the cold-worked structure, the high-quality titanium alloy plate and the titanium plate also have the solution of the compound and the polymorphic transformation of a→β. In order to improve the properties of titanium alloys and Grade 7 Titanium Sheet, in addition to necessary alloying, appropriate heat treatment is generally used. The recovery process of titanium alloys and titanium plates is also a process of eliminating most of the second type of internal stress generated during deformation through the movement of vacancies and dislocations at a certain temperature. The temperature at which the recovery process occurs is lower than the recrystallization temperature, generally 500 to 650°C.

Like other metals, the recrystallization process of high-quality titanium alloy plates and titanium plates is also in the nucleation and growth process of crystal grains in the deformed structure. At this time, the lattice type does not change, but there is a change in mechanical properties. This process is affected by the degree of cold deformation, heating temperature and holding time, and can be recrystallized by three-dimensional recrystallization of cold deformation rate, heating temperature and recrystallized grain size.

The effect of alloying elements on the recrystallization temperature of pure titanium has been described in the previous section. In addition to niobium and cobalt, commonly used alloying elements and impurity elements can increase the recrystallization temperature of titanium.

Determination of recrystallization mainly adopts a combination of metallographic observation and X-ray diffraction. When recrystallization occurs, fine equiaxed grains appear on the deformed fibrous structure, and the diffraction rings on the X-ray back-reflection Laue diagram phase begin to become disconnected spots. For heat-treatable β alloys, incomplete aging (500°C/4-8 hours, air cooling) can also be used to display the recrystallized structure, and the unrecrystallized grains after incomplete aging appear dark after corrosion. It has been determined that the initial recrystallization temperature of TA2 pure titanium is about 550 °C, TA7 titanium alloy is about 600 °C, TC4 titanium alloy is about 700 °C, and TB2 alloy is about 750 °C.

It should be pointed out that in grade 7 titanium alloy plate and titanium sheets, the recrystallization process is often accompanied by some other structural changes. For example, in near-a alloys and a+β alloys with a small content of β-stabilizing elements, the dissolution of a phase and the change of β composition are accompanied by the recrystallization process in heat-treatable β alloys. Or have a gestational effect on subsequent aging. In addition, different types of alloys have different microstructures at room temperature, different alloy phases involved in deformation, and different recrystallization processes and characteristics. The recrystallization of the a-alloy is mainly carried out in the a-phase. In addition to industrial pure titanium, the cold deformation ability of a alloy is small, so the grain refinement effect is difficult, and the recrystallization in the β-type alloy is mainly carried out in the β phase. The cold deformation ability of β alloy is great, the degree of grain breakage is great, and the original structure can be changed by recrystallization. However, due to the large tendency of grain growth in β alloys, grain refinement is still difficult. As for the a+β alloy, it depends on the main phase involved in the deformation, and it is analyzed according to the specific situation. For example, the recrystallization of the TC4 alloy is mainly the recrystallization of the a phase.