Method for Controlling End Cracks in the Upsetting Process of Titanium Alloy Free Forging

Abstract

The method for controlling the end crack in the drawing out process of titanium alloy free forging belongs to the field of forging, and aims to control the generation of surface cracks at the end of forgings. Step 1: Calculate the total forging ratio Y of drawing out, where Y = A0 / A n ; Step 2: According to the total forging ratio Y, the initial cross-sectional area A0 of the initial blank, and the principle of forging ratio per heat, calculate the number of forging heats n, and determine the forging ratio Y i , Y i = A (i‑1) / A i , where i = 1, 2, 3...n, n is a positive integer, and the forging ratio per heat meets the principle of forging ratio per heat; when Y ≤ Y m , n = 1; when Y > Y m , n > 1, and Y i ≤ Y m ; Y m is the limit forging ratio per heat; the principle of forging ratio per heat is: when A ≥ the circular area of 400mm, Y m = 1.6 - 1.7; when 300mm ≤ A < 400mm circular area, Y m = 1.5 - 1.6; when 200mm ≤ A < 300mm circular area, Y m = 1.4 - 1.5; when 100mm ≤ A < 300mm circular area, Y m = 1.2 - 1.4. By decomposing the total forging ratio to each heat and considering the influence of cross-sectional dimensions, the number of heats and forging ratios for different specifications of drawing out are determined, effectively reducing the generation of cracks in the drawing out process of free forging.

Description

Technical Field

[0001] The present invention belongs to the technical field of forging, and specifically relates to a method for controlling end cracks in the free forging drawing process of titanium alloy. Background Art

[0002] Titanium is a rare metal material and one of the three light metals (aluminum, magnesium, titanium). Titanium and its alloys have the characteristics of low density, high specific strength, good corrosion resistance, etc., and also have good comprehensive properties. With the rapid development of the aerospace industry, structural materials are required to have lower density, longer service life, and be able to withstand more complex and harsh service conditions. Therefore, titanium alloys are increasingly widely used in aircraft, gradually replacing traditional steels, aluminum alloys, etc.

[0003] Titanium has two allotropes, represented by α and β respectively. At room temperature, it is in the α phase, and at high temperature, it is in the β phase. Their crystal structures are hexagonal close-packed and body-centered cubic structures respectively. During forging deformation, since the body-centered cubic structure has more slip systems than the hexagonal close-packed structure, the β phase is more easily deformed than the α phase. Due to the crystal structure characteristics of titanium alloys, their hot working forging is generally carried out in the two-phase region, that is, α + β forging, and at the same time, its deformation temperature range is relatively narrow, only 100 - 200 °C.

[0004] During the forging process of titanium alloys, the free forging process can be either the final process for free forgings or the intermediate process for the intermediate blanks of forged die forgings. During the production process of the free forging process, due to the crystal structure of titanium alloys, the forging temperature range, and the randomness of on-site operations, cracks often appear at the ends of forgings. As free forgings, the cracking at the ends of forgings will shorten the effective size of the forgings and not meet the dimensional delivery requirements of the forgings; as intermediate blanks for die forgings, the cracking at the ends of forgings results in smaller intermediate blank sizes, affecting die forging forming filling and the quality of forgings; when the cracking at the ends of forgings is severe, it even directly leads to product scrapping; at the same time, the cracking at the ends during the production process causes production interruption, and it is necessary to stop and eliminate the end defects before continuing production. Therefore, the cracking of forgings reduces production efficiency, increases the energy consumption of grinding, cutting, and reheating, and increases production costs. Summary of the Invention

[0005] The purpose of the present invention is to solve the problem that the ends of titanium alloy forgings are prone to cracking, and provide a method for controlling end cracks in the free forging drawing process of titanium alloy to control the generation of surface cracks at the ends of forgings and ensure the quality of forgings.

[0006] The technical solution adopted by the present invention is: a method for controlling end cracks in the free forging drawing process of titanium alloy, including the following steps:

[0007] Step 1: Calculate the total forging ratio Y of drawing, Y = A0 / A n ; A0 is the initial cross-sectional area of the initial blank; An is the final cross-sectional area after forging the blank;

[0008] Step 2: Calculate the number of forging heats n according to the total forging ratio Y, the initial cross-sectional area A0 of the initial blank, and the principle of the forging ratio per heat, and determine the forging ratio Yi per heat i , Yi i = Ai (i-1) / Ai - 1 i , where i = 1, 2, 3... n, n is a positive integer, and the forging ratio per heat satisfies the principle of the forging ratio per heat;

[0009] When Y ≤ Ylim m , n = 1;

[0010] When Y > Ylim m , n > 1, and Yi i ≤ Ylim m ; Ylim m is the limit forging ratio per heat;

[0011] The principle of the forging ratio per heat is:

[0012] When A ≥ 400 mm circular area, Yi m = 1.6 - 1.7;

[0013] When 300 mm ≤ A < 400 mm circular area, Yi m = 1.5 - 1.6;

[0014] When 200 mm ≤ A < 300 mm circular area, Yi m = 1.4 - 1.5;

[0015] When 100 mm ≤ A < 300 mm circular area, Yi m = 1.2 - 1.4.

[0016] Furthermore, the forging ratios per heat decrease successively.

[0017] Furthermore, after Step 2, determine the number of deformation steps k in each heat according to the drawing-down deformation amount in each heat, and determine the deformation amount Xi of each step j , Xi j = Aj j - Aj - 1 (j-1) , where j = 1, 2, 3... k, k is a positive integer.

[0018] Furthermore, the forging ratios per heat of each step in each heat decrease successively.

[0019] Furthermore, the cross-section of the forging after drawing-down in each heat is rectangular; and after deformation of each step in each heat, the cross-section of the forging is rectangular.

[0020] The beneficial effects of the present invention are as follows: In the present invention, considering the total forging ratio of the drawing process comprehensively, the total forging ratio is decomposed into each heat, and at the same time, considering the influence of the cross-sectional size on the maximum forging ratio, the number of heats and the forging ratio for different specifications of drawing are determined. At the same time, within a single heat, the deformation amount and the deformation mode are also controlled, effectively reducing the generation of cracks in the free forging drawing process. According to the deformation characteristics of titanium alloy in the two-phase region, the present invention decomposes the deformation from the overall to the local in sequence, layer by layer, and optimizes the deformation mode at the same time, forming a control method for preventing surface cracks at the end of the free forging drawing process of titanium alloy. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 is a curve graph of the number of heats and the forging ratio;

[0022] Figure 2 is a curve graph of the working steps and the deformation amount in each heat;

[0023] Figure 3 is a photo of the end of the forging in the comparative example;

[0024] Figure 4 is a photo of the end of the forging in the example. DETAILED DESCRIPTION OF THE INVENTION

[0025] The present invention will be further described below with reference to the accompanying drawings as follows:

[0026] A method for controlling the end crack in the free forging drawing process of titanium alloy includes the following steps:

[0027] Step 1: Calculate the total forging ratio Y of drawing, Y = A0 / A n ; A0 is the initial cross-sectional area of the initial blank; A n is the final cross-sectional area of the blank after drawing;

[0028] Step 2: According to the total forging ratio Y, the initial cross-sectional area A0 of the initial blank, and the single-heat forging ratio principle, calculate the number of forging heats n, and determine the heat forging ratio Y i , Y i = A (i-1) / A i , i = 1, 2, 3... n, and the heat forging ratio of each heat satisfies the single-heat forging ratio principle;

[0029] When Y ≤ Ym, n = 1;

[0030] When Y > Ym, n > 1, and Y i ≤ Y m ; Y m is the single-heat limit forging ratio;

[0031] The single-heat forging ratio principle is:

[0032] For a circular area with A ≥ 400 mm, Ym = 1.6 to 1.7;

[0033] The circular area of 300 mm ≤ A < 400 mm, Y m = 1.5 to 1.6;

[0034] The circular area of 200 mm ≤ A < 300 mm, Y m = 1.4 to 1.5;

[0035] The circular area of 100 mm ≤ A < 300 mm, Y m = 1.2 to 1.4.

[0036] Step 3: Determine the number of deformation working steps k in each forging pass according to the drawing-down deformation amount of each forging pass, and determine the working step deformation amount X of each working step j , X j = A j -A (j-1) , j = 1, 2, 3... k, k is a positive integer.

[0037] Since the forging ratio is a characterization of the magnitude of deformation and is also an important influencing factor for the cracking of forgings. At the same time, the forging temperature is also an important influencing factor for the cracking of forgings. For the same forging ratio, the larger the size, the slower the cooling rate during the forging process, and the forgings are not easily cracked. It has been found that the drawn-out formed forgings do not crack, and the maximum forging ratio of a single forging pass and the cross-sectional size of the material before drawing satisfy the principle of the forging ratio of a single forging pass. In the present invention, the drawing-down forging passes are determined through the principle of the forging ratio of a single forging pass, and the deformation amount of each forging pass is controlled, thereby preventing the cracking of forgings in the drawing-down process macroscopically.

[0038] Since drawing-down is a process in which the cross-section of the forging gradually becomes smaller, as the cross-section of the forging decreases, the heat dissipation is faster, that is, in chronological order, the heat dissipation rate of the forging passes gradually increases, and the risk of its cracking is greater. To reduce the cracking risk, preferably, as Figure 1 shown, the forging ratios of each forging pass decrease in sequence.

[0039] However, the cracking of forgings can occur in a certain working step of drawing-down. Therefore, in order to further prevent the cracking problem, it is necessary to control the working step deformation amount of the drawing-down working steps.

[0040] In each forging pass, as the working step progresses, the temperature of the forging gradually decreases. If the deformation amount of the subsequent working steps is large, the risk of forging cracking is higher. To avoid this problem, as Figure 2 shown, the forging ratios of each working step in each forging pass decrease in sequence.

[0041] Traditional titanium alloy open-die forgings use billets with a square cross-section. During drawing out, the cross-section also changes from a square to a square. In actual production, it is found that in this square-to-square drawing out method, cracking at the drawn end usually occurs in the central area of the drawn end. The initial cracks gather at the center of the end and then spread outwards. In the present invention, before drawing out, the cross-section of the initial billet is rectangular. After each heat of drawing out, the cross-section of the forging is rectangular; and after each step of deformation in each heat, the cross-section of the forging is rectangular. That is, in the present invention, during drawing out, it adopts the form of changing from a large-scale rectangular cross-section to a small-scale rectangular cross-section. Through practice, it is found that after adopting this form, the degree of cracking is alleviated. Through testing, it is found that when a titanium alloy part is deformed under force, shear stress will be formed along a direction at a 45° angle to the force direction. Using the square-to-square drawing out method makes the center of the forging always present a cross-shaped shear zone during the drawing out process, and its center is the geometric center of the square cross-section. The strain and stress at the center point gradually accumulate during repeated drawing out processes and finally reach the limit, resulting in cracking. While the commonly used rectangular-to-rectangular drawing out method does not have the stress at the cross-section center point superimposed, but forms a region, that is, evenly distributes the stress at the center point to the nearby area, reducing the cracking risk.

[0042] Comparative example:

[0043] A certain forging is drawn out from a round bar with a cross-sectional diameter of 250 mm into a square with a cross-section of 180 mm × 150 mm, the forging ratio is 1.8, and the number of forging heats is 1 heat. After the bar is heated, it is freely forged by an operator on a quick forging machine, as Figure 3 shown. After the deformation is completed, cross-shaped cracks appear at the tail of the forging.

[0044] Example:

[0045] The forging is exactly the same as the forging in the comparative example, and the total forging ratio Y = 1.8. According to the principle of the forging ratio per heat, for a circular area with 200 mm ≤ A < 300 mm, Y m = 1.4 - 1.5. It can be seen that the allowable limit forging ratio of a circle with a diameter of 250 is 1.4 - 1.5. Since the total forging ratio Y is greater than the allowable limit forging ratio of a circle with a diameter of 250, the drawing out is divided into two heats. The forging ratio in the first heat is 1.36, and the forging ratio in the second heat is 1.33. At the same time, the deformation in the first heat and the second heat is both large deformation first and then small deformation. After deformation, the surface quality of the forging is good, as Figure 4 shown, and there are no cracks at the end.

Claims

1. Method for controlling end cracks in the free forging drawing process of titanium alloy, characterized in that: Including the following steps: Step 1: Calculate the total forging ratio Y of drawing out, where Y = A0 / A n ; A0 is the initial cross-sectional area of the initial blank; A n is the final cross-sectional area of the blank after drawing out; Step 2: Calculate the number of forging heats n according to the total forging ratio Y, the initial cross-sectional area A0 of the initial blank, and the principle of forging ratio per heat, and determine the forging ratio Y per heat i , Y i = A (i-1) / A i , where i = 1, 2, 3... n, n is a positive integer, and the forging ratio per heat satisfies the principle of forging ratio per heat; When Y ≤ Y m , n = 1; When Y > Y m , n = 2, and Y i ≤ Y m ; Y m is the single - heat - pass ultimate forging ratio; The principle of forging ratio per heating is as follows: The area of a circle with A ≥ 400 mm, Y m = 1.6 to 1.7; The area of a circle with 300mm ≤ A < 400mm, Y m = 1.5 to 1.6; The area of a circle with 200mm ≤ A < 300mm, Y m = 1.4 to 1.5; The area of a circle with 100mm ≤ A < 300mm, Y m = 1.2 to 1.

4.

2. The method for controlling end cracks in the titanium alloy open-die forging drawing process according to claim 1, wherein: The forging ratio per heating of each heating decreases successively.

3. The method for controlling end cracks in the free forging drawing process of titanium alloy according to claim 1 or 2, characterized in that: After Step 2, according to the drawing deformation amount of each heat, determine the number of deformation working steps k in each heat, and determine the working step deformation amount X of each working step j ,X j = A j -A (j-1) , j = 1, 2, 3... k, where k is a positive integer.

4. The method for controlling end cracks in the titanium alloy open-die forging drawing process according to claim 3, characterized in that: The forging ratio per heating of each working step in each heating decreases successively.

5. The method for controlling the end crack in the free forging and drawing process of titanium alloy according to claim 1 or 2, characterized in that: The cross-section of the forging after drawing in each heating is rectangular; and after deformation of each working step in each heating, the cross-section of the forging is rectangular.

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