1. Billet
Its initial forging (blanking) temperature is 150-250°C above the β transformation point, at this time, the plasticity of the casting structure is good. At the beginning, light and fast blows should be used to deform the ingot until the primary coarse-grained structure is broken. The degree of deformation must be kept within the range of 20% to 30%. The ingot is forged to the desired cross-section and then cut to size blanks.
The plasticity increases after the casting structure is broken. Aggregation recrystallization is aggravated with the increase of temperature, holding time and grain refinement. In order to prevent aggregation recrystallization, the forging temperature must be gradually reduced with grain refinement, and the heating and holding time should also be strictly controlled.
2. Multi-directional repeated upsetting
It starts forging at 80-120°C above the β-transformation temperature, and alternately upsetting and drawing for 2-3 times, while changing the axis and edge alternately. In this way, a very uniform recrystallized fine-grained structure with β-region deformation characteristics can be obtained throughout the section of the blank. If the blank is rolled on the rolling mill, such multi-directional upsetting is not necessary.
3. The second multi-directional repeated upsetting
It is the same as the first multi-directional repeated upsetting method, but the initial forging temperature depends on whether the semi-finished product after forging is the blank of the next process or the delivery product. If the blank for the next process is used, the initial forging temperature can be 30-50°C higher than the beta transition temperature; if the product is to be delivered, the initial forging temperature is 20-40°C below the beta transition temperature. Due to the low thermal conductivity of titanium, in free forging equipment When upsetting or drawing a billet, if the preheating temperature of the tool is too low, the striking 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 or cross-section. This is especially true for non-isothermal upsetting on hydraulic presses. This is because the temperature of the tool is low, and the contact between the billet and the tool causes the surface layer of the metal billet to be chilled. During the deformation process, the deformation heat generated by the metal has no time to conduct heat to the surroundings, and a large temperature gradient is formed from the surface layer 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 cracks are formed under the action of tensile stress of opposite sign. Therefore, when free forging titanium alloys, the striking speed should be faster, the contact time between the blank and the tool should be shortened as much as possible, the tool should be preheated to a higher temperature as much as possible, and the degree of deformation in one stroke should be properly controlled.
When forging, the edges and corners cool the fastest. Therefore, the blank must be turned over several times when drawing, and the hammer force must be adjusted to avoid acute angles. Forging on the hammer, the initial stage should be lightly beaten, and the deformation degree should not exceed 5% to 8%, and then the deformation amount can be gradually increased.
Die forging is usually used to produce final blanks that are close in shape and size to the finished product, followed by only heat treatment and machining. The forging temperature and the degree of deformation are the basic factors that determine the structure and properties of the alloy. The heat treatment of titanium alloy is different from that of steel, and it does not play a decisive role in the structure of the alloy. Therefore, the process specification of the final step of die forging of titanium alloys plays a particularly important role.
