1. Open billet
Its initial forging (opening) temperature is 150 to 250°C above the β transition point, at this time, the plasticity of the cast structure is the best. At the beginning, the ingot should be deformed with light and quick blows until the primary coarse grain structure is broken. The degree of deformation must be maintained in the range of 20% to 30%. Forging the ingot to the required cross-section, and then cutting into a blank of fixed size.
The plasticity increases after the casting structure is broken. Aggregation recrystallization is exacerbated with increasing temperature, longer holding time and grain refinement. In order to prevent aggregation recrystallization, the forging temperature must be gradually reduced with grain refinement, and heating and holding time must be strictly controlled.
2. Multi-directional repeated upsetting
It starts forging at 80～120℃ above the temperature of β transition point, and alternately upsets and pulls out 2-3 times, and changes axis and edge alternately. In this way, a very uniform recrystallized fine-grained structure with β-region deformation characteristics is obtained throughout the blank section. If the blank is rolled on a rolling mill, this multi-directional upsetting is not necessary.
3. Repeated upsetting for the second time
It is the same as the first multi-directional upsetting repeatedly, but the initial forging temperature depends on whether the semi-finished product after forging is the blank of the next process or the product is delivered. For the blank for the next process, the initial forging temperature may be 30-50°C higher than the β transformation temperature; if the product is delivered, the initial forging temperature will be 20-40°C below the β transformation temperature. Due to the low thermal conductivity of titanium, the free forging equipment When upsetting or drawing long blanks, if the tool preheating temperature is too low, the blow speed of the equipment is low, and the degree of deformation is large, and an X-shaped shear band is often formed on the longitudinal section or cross section. This is especially the case when non-isothermal upsetting is performed on hydraulic presses. This is because the tool temperature is low, and the contact between the blank and the tool causes the surface of the metal blank to be chilled. During the deformation process, the deformation heat generated by the metal is too late to conduct heat to the surroundings, forming a large temperature gradient from the surface to the center. As a result, the metal forms a strong flow Strain band. The greater the degree of deformation, the more obvious the shear band, and finally the crack is formed under the tensile stress of the opposite sign. Therefore, when forging titanium alloy freely, the striking speed should be faster, try to shorten the contact time between the blank and the tool, and preheat the tool to a higher temperature as much as possible, while also properly controlling the degree of deformation within one stroke.
During forging, the corners cool down the fastest. Therefore, it is necessary to turn over the blank many times during the drawing and adjust the hammering force to avoid sharp angles. Forging on the hammer, it should be tapped at the beginning, and the degree of deformation does not exceed 5% to 8%, and then the amount of deformation can be gradually increased.
Die forging is usually used to make the final blank with shape and size close to the finished product, and then only heat treatment and cutting. Forging temperature and deformation degree are the basic factors that determine the structure and performance of the alloy. The heat treatment of titanium alloy is different from the heat treatment of steel, and it has no decisive effect on the structure of the alloy. Therefore, the process specification of the last step of titanium alloy die forging has a particularly important role.