Application of rotary reverse bending straightening technology in the production of titanium alloy precision forged bars

In 1940, Kroll perfected the method of "magnesium reduction of titanium tetrachloride" to extract titanium. Titans, Sons of the Earth, began to devote themselves to industrial production from the laboratory. Since then, the industrial production of titanium and titanium alloys has developed rapidly. In recent years, the world's titanium industry and titanium material processing technology have become increasingly mature, and the production and consumption of titanium sponge, deformed titanium alloys and titanium alloy processed materials have reached a very high level. It is increasingly widely used in the aerospace field, warships, weapons and other military products manufacturing, and it also has huge application potential in the automotive, chemical and energy industries.

Due to the needs of modern industrial production, various types of titanium materials have been developed: titanium rods, titanium plates, titanium wires, titanium strips, titanium tubes, titanium foils, etc. Among them, the production of small-sized titanium alloy bars has begun to take shape. This type of bars has the characteristics of small size and large number of counts. Generally, precision forging machines are used to produce them to ensure production efficiency and product stability. Our company has an SXP-13 precision forging machine, a JBLR-130 straightening machine, and corresponding heating equipment, with an annual production of about 600t of precision forging bars.

Theory and development of bar straightening

As the old saying goes, "overcorrection is too much." However, in the field of metal straightening, overcorrection must be overcorrection. The traditional simple reverse bending straightening method uses this theory. As shown in Figure 1, we define the curvature radius of the original bending state of bar ab as ρ0, the reverse bending radius used during straightening as ρ1, and the reverse bending reaches a'b' state. At this time, the external force is removed and the bar is free to spring. Return to the state of a"b". If a"b" is a straight line, the purpose of straightening is achieved. However, the relationship between the amount of reverse bending and the original amount of bending is nonlinear, and is affected by the plasticity and cross-sectional shape of the material. Therefore, straightening in actual production often uses multiple reverse bending to reduce the bending state of the bar and make it Meet the requirements.

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Figure 1 Principle diagram of reverse bending straightening method

Based on the simple reverse bending straightening method, the rotating reverse bending straightening theory was developed. As shown in Figure 2, the inclined roller straightening machine used for straightening small-sized pipes and bars adopts this rotation straightening theory. The surface of the straightening roller of the inclined roller straightener is a hyperboloid surface formed by the rotation of a hyperbola around the axis. During straightening, the bar rotates forward under the friction of the straightening roller. After the axial fibers of the bar undergo large elastic-plastic deformation, The rebound ability gradually becomes consistent. Even if the original bending state is different, since each fiber has been extruded and deformed from small to large and then from large to small many times, the final resilience of the bar tends to be the same. The more times this deformation is repeated, the greater the resilience. The closer the abilities are, the better the straightening effect will be. Research has shown that the straightening effect of the inclined roller straightener is related to the following conditions: the number of points creating the bending zone is at least four, that is, two effective straightening and reverse bending zones are produced; the longer the curve length of the bending zone, the straightening The better the effect; a reasonable geometric relationship must be maintained between the straightening roller bevel angle, roller distance and roller gap.

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Figure 2 Principle diagram of rotational reverse bending straightening

Research and improvement of titanium alloy precision forged bar straightening technology
experiment method
This test is divided into two groups: test one focuses on the relationship between the straightening effect of titanium alloy precision forged bars of the same specifications and different brands and the straightening roller bevel angle, and test two explores the effect of curvature length on the straightening of titanium alloy precision forged bars. effect. The straightened bar is judged using a correction platform and a feeler gauge: if the curvature of the bar after correction is not greater than 3mm/1000mm, it will be marked as qualified, and if the curvature of the bar is not greater than 1mm/1000mm, it will be marked as excellent. The test equipment is the JBLR-130 (2-2-2) straightening machine of Baoji Titanium Industry Company, and the test materials are φ45mm TA2, TC4 and Ti55511 titanium alloy precision forged bars.
In the first test, TA2, TC4 and Ti55511 precision forged bars with φ45mm were selected. The straightening machine adjusted three sets of roll gaps and straightening roll bevel angles (roller gap φ44 ~ φ44.5mm, straightening roll bevel angle 37.0°; roll gap φ44.5～ φ45mm, straightening roller bevel angle 37.2°; roll gap φ43.5 ~ φ44mm, straightening roller bevel angle 36.8°) for straightening, and record the straightening effect. In the second test, TC4 precision forged bars with a diameter of 45 mm were selected for single straightening. According to the feeding sequence, the three pairs of straightening rollers of the 2-2-2 straightening machine are defined as straightening roller 1, straightening roller 2, and straightening roller 3. In the test, the bevel angle of straightening roller 1 and straightening roller 2 adopts the optimal data in test 1, and the bevel angle and roll gap of straightening roller 3 are increased appropriately to increase the radius of curvature of the second straightening reverse bending zone. Record Straightening effect.
Test Data
⑴The first data of test one: roll gap φ44 ~ φ44.5mm, straightening roll bevel angle 37.0°
⑵The second data of test one: roll gap φ44.5 ~ φ45mm, straightening roll bevel angle 37.2°
⑶The third data of test 1: roll gap φ43.5 ~ φ44mm, straightening roll bevel angle 36.8°
⑷ For the second test, the angle of straightening roller 1 and straightening roller 2 is selected as 37.0°, which has the best straightening effect in test one, and the angle of straightening roller 3 is appropriately increased for the test.
Experimental analysis
Through the statistical data analysis of the three tests in Experiment 1, it can be seen that the straightening effect of different brands of titanium alloy precision forged bars is closely related to the roll gap and straightening roller bevel of the straightening machine. After analyzing the straightening roller bevel, The angle is the dominant factor affecting the straightening effect, while the roll gap affects more the surface quality of the straightened bar. During the test, we found that the straightening effect is related to the spiral curve formed when the blank passes through the straightening machine. From TA2 to TC4 to Ti55511, as the plasticity of the material becomes worse, the hardness increases. In order to achieve the required straightening For curved curves, the straightening bevel angle must be reduced. Therefore, even if the blanks have the same specifications, in order to achieve a good straightening effect, the influence of the plasticity and hardness of the blank itself must be considered. From the data of Experiment 2, we can see that appropriately increasing the bevel angle of straightening roller 3 makes the reverse bending radius of the second straightening reverse bending zone larger, and correspondingly increases the curve length of the reverse bending zone, which can improve straightening. Effect. However, when the bevel angle of straightening roller 3 exceeds a certain limit angle, the straightening quality will drop sharply. The analyzed reason is that when the bevel angle of straightening roller 3 is excessively increased, an effective straightening reverse bending area cannot be formed, and the straightening effect cannot be guaranteed. .
in conclusion
⑴ When straightening titanium alloy precision forged bars, there is no strict linear relationship between the straightening roller bevel angle and the bar diameter. Considering the influence of material plasticity and hardness on the straightening reverse bending line, for grades with poor plasticity and high hardness, it is necessary to Appropriately reduce the straightening roller bevel angle to ensure the required reverse bending line for straightening. ⑵ For the (2-2-2) type pipe and rod straightening machine, appropriately increase the bevel angle of the straightening roller to increase the reverse bending radius and improve the straightening effect. However, when the angle is too large, an effective straightening and rebending area cannot be formed, and the straightening effect drops sharply.

--Article selected from《Forging and Stamping》Issue 5, 2023