GR5 Titanium Alloy Annealing Process Selection
In order to choose a reasonable annealing process, we first observe the heating temperature and cooling mode on the GR5 titanium microstructure and mechanical properties.
Test materials 920 ℃ hot rolled GR5 titanium alloy bar, hot rolling total deformation rate of about 80%, a + β / β transformation point of 980 ~ 990 ℃. The samples were heated at 1000 ℃, 950 ℃, 930 ℃ and 830 ℃ for 1 hour, then air-cooled, water-cooled and furnace-cooled respectively. Different annealing methods have an impact on the microstructure and mechanical properties.
The cooling rate has a significant influence on the microstructure and mechanical properties of the above four temperatures. When water-cooled, martensite transformation takes place in the β-phase components which are in equilibrium at 1000 ° C, 950 ° C and 930 ° C, and the β phase transforms into a martensitic a pin. At 1000 ℃, it shows obvious W-shaped microstructure. The mechanical properties are similar to the data of 1000 ℃ air-cooling. Microstructures were similar to air-cooled specimens at 950 ° C and 930 ° C water-cooled specimens, but β + martensite was noticed between the equiaxed primary a phases. This corresponds to the highest overall performance, but also better than the air-cooled tissue creep resistance. The equilibrium β-phase composition at 830 ℃ did not reach the M line, but very small needle-like transition products were also found in the intergranular β phase after water cooling, which can only be resolved by electron microscopy. However, the structure of the acicular product has not been measured. At this point the tensile strength and reduction of area are very low. As for the furnace cooling, all the a phases became coarse due to the slow cooling of the sample, the long residence time at high temperature, and the diversification of the polymorphism. After the furnace is cooled at 1000 ° C, a thick a-phase and an inter-sheet β phase are formed in the primary β grains, and thick strips are also formed on the primary β-grain boundaries in the form of a-strips, generally referred to as basket-like structures. After cooling at 950 ℃, 930 ℃ and 830 ℃, the a phase tends to nucleate and grow at the interface of the original a phase, and the microstructure is equiaxed a and intergranular β phase. Tensile strength after furnace cooling at 1000 ℃ is higher than that of air cooling and water cooling. After the other temperature furnace cold performance are also lower than the water-cooled and air-cooled.
In summary, in order to make GR5 titanium alloy get the best comprehensive strength and plastic properties, but also has good creep resistance and fracture toughness, can be used 950 ℃ for 1 hour air cooling (or water cooling) after the annealing process. In order to facilitate the subsequent processing, metallurgical plant factory, GR5 titanium alloy are used at 700 ~ 800 ℃ 1 hour air cooling process. For some large size forgings, in order to ensure uniformity of performance, sometimes using furnace cooling.
