Due to the high pressure, the service life of the medical titanium bar is reduced. Therefore, when using the closed die forging method to die forge the titanium bar rod, the closed die forging must strictly limit the volume of the original blank, which makes the preparation process complicated. Whether closed die forging is adopted, both interest and technological feasibility should be considered. Open die forging, the burrs of blank weight loss of 15% to 20% of the clamping part manufacturability waste (if the forging conditions must leave this part) accounted for 10% of the weight of the blank flash metal relative loss is usually increases with blank weight reduction, some asymmetric structure, area difference is bigger, and there are difficult to fill local forgings, burrs consumption as much as 50% for closed die forging are no burrs losses, but the blocking process is complex, you need to add more transitional type slot, will no doubt increase the ancillary expenses.
Then only heat treatment and machining of the final blank. The forging temperature and deformation degree are the basic factors that determine the microstructure and properties of the alloy. The heat treatment of titanium rods is different from that of steel, and die forging is usually used to create a shape and size close to the scrap. It does not determine the structure of the alloy. Therefore, the specification of the final working step of medical titanium bar plays an important role. It is necessary to make the overall deformation of the blank not less than 30% of the deformation temperature not exceed the phase transition temperature. In order to obtain high strength and plasticity of the titanium rod at the same time, the temperature and deformation degree should be evenly distributed in the entire deformation of the blank as far as possible.
After recrystallization heat treatment, titanium rod and performance uniformity is not as good as steel forging. In the intense flow zone, the low power is fuzzy crystal and the high power is equiaxed fine crystal. In the difficult deformation area, due to the small amount of deformation or no deformation, the tissue is often stored in the pre-deformation state. So in die forging some important titanium rod parts (such as compressor disk, blade, etc.), in addition to control deformation of the deformation temperature under the TB and the appropriate level, it is important to control the original organization of blank otherwise, coarse grain structure or certain defects inherited forgings, and subsequent heat treatment and cannot eliminate, will lead to the forging scrap.
In the sharp deformation area where the heat effect is concentrated locally, when the titanium bar is forged with complicated shape by hammer up and die forging. Even if the heating temperature is strictly controlled, the temperature of the metal may still exceed the TB of the alloy. For example, when the blank titanium rod with i-shaped cross section of die forging is hit too hard, the local temperature in the middle (web area) is about 100℃ higher than the local temperature at the edge due to the thermal effect of deformation. In addition, the hard deformation zone and the horizontal zone with critical deformation are easy to form the coarse grain structure with low plastic and durable strength during the heating process after die forging. Therefore, the mechanical properties of the forging with complex shape are often very unstable. However, it will lead to a sharp increase in deformation resistance, and reducing the heating temperature of die forging can eliminate the danger of local overheating of the blank. Increased tool wear and power consumption also necessitates the use of more powerful equipment.
Multiple taps can also reduce the local overheating of the blank. But it is necessary to increase the heat times when hammering on the die forging. To compensate for the heat lost when the blank comes into contact with the cooler die. Forgings with simple die forging shapes when plasticity and durability of deformed metals are not required too much. Hammer forging is preferred. However, hammer forging is not recommended for alloys, as repeated heating during die forging will have a favorable effect on mechanical properties. Compared with forging hammer, the working speed of press (hydraulic press, etc.) is greatly reduced, which can reduce the deformation resistance and deformation thermal effect of alloy. The unit die forging force of the blank is about 30% lower than that of the hammer. The reduction in thermal effects also reduces the risk of metal overheating and temperature rising beyond TB.
Under the same condition of unit pressure and hammer die forging, die forging with press machine. The heating temperature of blank can be reduced from 50 ℃ to 100℃. In this way, the interaction between the heated metal and the periodic gas and the temperature difference between the blank and the die are reduced accordingly, so as to improve the uniformity of deformation, the uniformity of the structure of the die forgings and the consistency of the mechanical properties. The surface shrinkage rate is the most sensitive to the tissue defects caused by overheating.
The friction with the tool is greater and the contact surface of the blank cools too fast. In order to improve the fluidity of titanium rods and prolong the die life. The usual approach is to increase the die skew and fillet radius and to use lubricants: the burr bridge on the die is higher than the steel, and titanium rods are deformed in such a way that it is more difficult to flow into deep, narrow die slots than steel. This is due to the high deformation resistance of titanium. About 2mm in general. In some cases, non-uniform brim grooves at the bridge can be used to limit or accelerate the flow of metal to some part of the groove.
It provides a feasible method to solve the forming of large and complex medical titanium bar rod precision forging. This method has been widely used in titanium rod production. One of the most effective ways to improve the fluidity of titanium rods and reduce the deformation resistance is to increase the preheating temperature of molds. Isothermal die forging and hot die forging developed in the past 20 to 30 years at home and abroad.
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