Due to the high pressure, the life of titanium rods is reduced. Therefore, when closed die forging is used to forge titanium rods, the volume of the original blank must be strictly limited, which complicates the material preparation process. Whether to use closed die forging should be considered from the perspectives of interest and process feasibility. In open die forging, the burr loss accounts for 15%-20% of the blank weight. The process waste of the clamping part (if this part must be left according to the die forging conditions) accounts for 10% of the blank weight. The relative loss of burr metal usually increases with the decrease of the blank weight. For some forgings with asymmetric structures, large cross-sectional area differences, and local parts that are difficult to fill, the burr consumption can be as high as 50%. Although there is no burr loss in closed die forging, the blank making process is complicated and more transition tool grooves need to be added, which will undoubtedly increase the auxiliary cost.
The last blank is only heat treated and cut. Forging temperature and degree of deformation are the basic factors that determine the alloy structure and performance. The heat treatment of titanium rods is different from that of steel. Die forging is usually used to make shapes and sizes close to scrap. It does not play a decisive role in the organization of the alloy. Therefore, the process specifications of the last step of titanium rods play a particularly important role. The overall deformation of the blank must not be less than 30% and the deformation temperature must not exceed the phase transition temperature. In order to make the titanium rod obtain higher strength and plasticity at the same time, the temperature and deformation degree should be distributed as evenly as possible in the entire deformed blank.
After recrystallization heat treatment, the uniformity of titanium rods and performance is not as good as that of steel forgings. In the area of intense metal flow, the low magnification is fuzzy crystal, and the high magnification is equiaxed fine crystal; in the difficult deformation area, due to the small deformation or no deformation, its organization often keeps the state before deformation. Therefore, when forging some important titanium rod parts (such as compressor discs, blades, etc.), in addition to controlling the deformation temperature below TB and the appropriate deformation level, it is very important to control the organization of the original blank. Otherwise, the coarse grain organization or some defects will be inherited into the forging, and the subsequent heat treatment cannot be eliminated, which will lead to the scrapping of the forging.
In the area of rapid deformation where the thermal effect is locally concentrated, when forging titanium rod forgings with complex shapes on the hammer. Even if the heating temperature is strictly controlled, the temperature of the metal may still exceed the TB of the alloy. For example, when forging a titanium bar blank with an I-shaped cross section, the hammer is too heavy, and the local temperature in the middle (web area) is about 100°C higher than the local edge due to the deformation heat effect. In addition, the hard-to-deform area and the area with a critical deformation level are prone to form coarse-grained structures with relatively low plasticity and durability during the heating process after forging. Therefore, the mechanical properties of forgings with complex shapes forged by hammer forging are often very unstable. However, it will lead to a sharp increase in deformation resistance. Although reducing the heating temperature for forging can eliminate the risk of local overheating of the blank, it will increase tool wear and power consumption, and it is necessary to use more powerful equipment.
The use of multiple light tapping methods can also reduce the local overheating of the blank. However, this requires increasing the heating times. To compensate for the heat lost by the blank in contact with the colder die. And when the requirements for the plasticity and durability strength of the deformed metal are not too high, it is better to use hammer forging for forgings with relatively simple shapes. However, β alloys should not be hammer forged, because multiple heating during the die forging process will have a favorable effect on the mechanical properties. Compared with the forging hammer, the working speed of the press (hydraulic press, etc.) is greatly reduced, which can reduce the deformation resistance and deformation thermal effect of the alloy. When the titanium rod is die forged on the hydraulic press, the unit die forging force of the blank is about 30% lower than that of the hammer die forging, which can increase the life of the die. The reduction of thermal effect also reduces the risk of metal overheating and temperature rise exceeding TB.
Under the same unit pressure as the forging hammer die forging, when the press is die forged. The blank heating temperature can be reduced by 50100℃. In this way, the interaction between the heated metal and the periodic gas and the temperature difference between the blank and the die are also reduced accordingly, thereby improving the uniformity of deformation, the uniformity of the structure of the die forging is also greatly improved, and the consistency of mechanical properties is also improved accordingly. Reduce the deformation speed, the numerical growth of the more obvious surface shrinkage, the surface shrinkage is more sensitive to the structural defects caused by overheating.
The friction with the tool is large and the contact surface of the blank cools too quickly. To improve the fluidity of the titanium rod and increase the life of the die. The usual practice is to increase the die forging slope and fillet radius and use lubricants: the height of the burr bridge on the forging die is larger than that of steel. The characteristic of titanium rod deformation is that it is more difficult to flow into a deep and narrow die groove than steel. This is because titanium has high deformation resistance. It is generally about 2mm larger. Sometimes, burr grooves with uneven bridge size can be used to limit or accelerate the flow of metal to a certain part of the groove. For example, in order to make the groove easy to fill. A rectangular box-shaped forging (as shown in Figure 12) has thinner front and rear side walls; thicker left and right side walls. When the burr grooves shown in B-B are used around the box-shaped part, due to the small resistance of metal flowing into the left and right side walls, the metal has difficulty flowing to the thinner front and rear side walls and cannot be filled. Later, the front and rear side walls still use the burr grooves shown in B-B, while the left and right side walls use the burr grooves shown in A-A. Due to the wide size of the bridge and the obstruction of the damping groove, the thinner front and rear side walls are completely filled, and the metal is saved compared to the aforementioned burr groove method.
It provides a feasible method for solving the forming of large and complex titanium rod precision forgings. This method has been widely used in titanium rod production. One of the more effective ways to improve the fluidity of titanium rods and reduce deformation resistance is to increase the preheating temperature of the die. Isothermal forging and hot die forging have been developed in the past two or three decades at home and abroad.
