Method for eliminating defects in forming process of variable-diameter cylindrical forgings

Abstract

The invention relates to the technical field of variable-diameter cylindrical forge piece preparation, in particular to a method for eliminating defects in a variable-diameter cylindrical forge piece forming process, which comprises the following steps: step 1, preparing a blank and preparing an allowance cylindrical metal blank, step 2, pretreating the blank, putting the blank into a compartment type resistance furnace, heating to 930 DEG C, and preserving heat for 2 hours; thirdly, after discharging, the blank is transferred and put into a mold, and the mold is subjected to first heating number forging; the cylindrical metal material is subjected to surface pre-forging, the pre-forging temperature is 920 DEG C, and the cylindrical metal material is cooled for standby application; and fourthly, second heating number forging is carried out, the blank in the second step is heated to 900 DEG C, then finish forging is carried out, and forging is completed. According to the method, the technical aims of optimizing the forming process of the variable-diameter cylindrical forge piece and producing the variable-diameter cylindrical forge piece with high performance are achieved.

Description

Technical Field

[0001] This invention relates to the field of variable diameter cylindrical forging technology, and in particular to a method for eliminating defects during the forming process of variable diameter cylindrical forgings. Background Technology

[0002] Early torsion cylinder structural components in my country were mainly manufactured using direct machining of titanium alloys or die forging. While these processes could achieve the basic shape and performance of the components, the complex shape of the torsion cylinder, composed of multiple segments of arcs and straight lines with varying curvatures, resulted in significant variations in the thickness of both the inner and outer walls along the axial and radial directions, and substantial changes in the cross-section in localized areas. Furthermore, aerospace torsion cylinders typically utilize high-strength titanium alloys, which have low plasticity and high deformation resistance, making them extremely sensitive to forming process parameters. Therefore, forming aerospace torsion cylinders presents significant challenges.

[0003] The existing forming process uses a blank with a diameter of 192mm and a height of 109mm. The forming process can be divided into three stages: extrusion stage, boss filling stage, and bottom filling stage.

[0004] During the extrusion stage, the material flow exhibits an overall radial extrusion and expansion characteristic. The metal at the bottom of the billet has a relatively low flow velocity due to the constraint of the die. The entire deformation area shows an obvious velocity gradient distribution, which causes the upper contour of the billet to begin to fit into the die cavity earlier.

[0005] As the reduction increases further, the forming process enters the boss filling stage. At this point, the metal flow towards the center intensifies, and a significant back-extrusion trend gradually appears at the boss area. Due to the sudden contraction of the die geometry, the metal needs to pass through a narrower cross-section to fill the boss area, thus requiring greater deformation resistance. The metal flow direction around the boss changes from radial flow to axial upward flow and central convergence. However, the metal in the boundary area is still affected by friction and flows lags behind, resulting in a relatively low material flow velocity around the root of the boss. This differentiated flow pattern means that boss forming is somewhat difficult, especially since local underfilling tends to occur at the top edge and the transition areas on both sides of the boss.

[0006] During the bottom filling stage, the metal in the bottom region of the billet begins to be compressed by the already formed geometry above, forcing the metal to flow downwards into the mold cavity and achieve overall filling. At this stage, the metal mainly flows downwards into the bottom of the die. Due to the relatively enclosed space at the bottom and the long metal flow path, there is still significant flow resistance during the bottom forming process. The bottom region is characterized by high-speed, concentrated metal flow; while the flow velocity in the sidewall region decreases significantly due to friction and deformation limitations. As the metal gradually fills the bottom of the die, the cavity eventually reaches saturation, and the forming process is complete. Summary of the Invention

[0007] The problem solved by this invention is to provide a method for eliminating defects in the forming process of variable diameter cylindrical forgings, thereby achieving the technical goal of optimizing the forming process of variable diameter cylindrical forgings and producing high-performance variable diameter cylindrical forgings.

[0008] To achieve the above objectives, the present invention adopts the following technical solution: A method for eliminating defects during the forming process of a variable-diameter cylindrical forging includes the following steps: Step 1: Prepare the blank. Prepare the remaining cylindrical metal blank. Step 2: Pre-treat the blank by placing it in a box-type resistance furnace and heating it to 930°C for two hours. Step 3: After exiting the furnace, the billet is transferred to a mold for the first forging; the cylindrical metal material undergoes surface pre-forging at a temperature of 920℃, followed by cooling for later use. Step four involves a second forging process, heating the billet from step two to 900°C and then performing a final forging to complete the forging process.

[0009] As an improved technical solution, in step one, the billet is a cylindrical titanium alloy billet with a diameter of 293 mm and a height of 146.5 mm.

[0010] As an improved technical solution, in step two, the pretreatment of the blank involves cleaning surface defects and applying 0.2mm of lubricant, which is a water-based glass lubricant.

[0011] As an improved technical solution, in step two, during heating, a graphite pad is placed at the bottom of the billet, and the billet is heated at a heating rate of 2.5℃ / min, and held at 200℃ for 2 hours; then held at 600℃ for 1 hour, and then held at 930℃ for another 2 hours.

[0012] As an improved technical solution, in step three, the mold includes an upper mold and a lower mold that cooperate with each other. The upper mold is a punch, and the lower mold is a die. The minimum width of the die is adapted to the blank. An adjustable limiting block for the punch forging stroke is provided between the punch and the die.

[0013] As an improved technical solution, in step three, the stroke of the pre-forging is 80mm.

[0014] The beneficial effects of the present invention are as follows: Increasing the billet volume and employing a two-stage pre-forging + final forging process improves the deformation of the metal in critical areas, resulting in a higher process tolerance. Structural analysis shows that increasing the pre-forging stroke thickens the phase transformation zone and low-temperature zone during the pre-forging stage. However, due to the reduced final forging reduction, the internal temperature difference of the forging is significantly reduced after final forging, leading to a more stable temperature distribution. The overall equivalent strain during the final forging stage is generally greater than 1, suggesting that a fine equiaxed structure can be obtained inside the forging. Under the conditions of 930–950℃, a two-stage process, and an 80mm pre-forging stroke, forming stability can be improved while ensuring forming quality, achieving the technical goal of optimizing the forming process of variable-diameter cylindrical forgings and producing high-performance variable-diameter cylindrical forgings. Attached Figure Description

[0015] Figure 1 This is a cross-sectional structural diagram of the torsion cylinder; Figure 2 This is a cross-sectional view of the mold structure in Scheme 2; Figure 3 This is a schematic diagram of the blank model dimensions and mesh generation for Scheme 1; Figure 4 This is a schematic diagram of the mold mesh division for Scheme 1; Figure 5 This is a schematic diagram of the blank model dimensions and mesh generation for Scheme 1; Figure 6 This is a schematic diagram of the mold mesh division for Scheme 2; Figure 7 This is a schematic diagram of the formation stage of Scheme 1; Figure 8 This is a schematic diagram of the forming stage of Scheme 2; Figure 9 This is a cross-sectional view of the mold structure in Scheme 2; Figure 10 This is a temperature field distribution diagram at different forging temperatures in the first stage; Figure 11 This is a schematic diagram showing the variation of the thickness of the phase transformation zone and the low-temperature zone in the first stage with forging temperature. Figure 12 This is a temperature field distribution diagram at different forging temperatures in the second stage; Figure 13 This is a temperature distribution diagram of the forgings in the second stage; Figure 14 This is a schematic diagram showing the variation of the thickness of the phase transformation zone and the low-temperature zone in the second stage with forging temperature. Figure 15 These are strain field distribution diagrams at different forging temperatures; Figure 16 These are temperature field distribution diagrams at different forging temperatures; Figure 17 This is a temperature distribution diagram inside the forging; Figure 18 Strain field distribution diagrams at different forging temperatures Figure 19 This is a diagram showing the strain field distribution during different forging strokes in pre-forging. Figure 20 These are temperature field distribution diagrams for different forging strokes; Figure 21 This is a schematic diagram showing the variation of the thickness of the phase transformation zone and the low-temperature zone with forging temperature. Figure 22 This is a diagram showing the temperature field distribution during final forging. Figure 23 This is a temperature distribution diagram inside the forging from the second firing stage; Figure 24 This is a diagram showing the strain field distribution of the forging after final forging. Figure 25 These are the forging force curves for pre-forging and final forging; Figure 26 This is a schematic diagram of the selection points for pre-forged billets; Figure 27 This is a temperature distribution diagram inside the pre-forged billet; Figure 28 This is a temperature distribution diagram inside the pre-forged billet; Figure 29 This is a schematic diagram of the selection points for the final forging billet; Figure 30 It refers to the internal temperature distribution of the pre-forged billet; Detailed Implementation

[0016] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0017] To facilitate understanding of this technical solution, a brief introduction to the torque cylinder is provided below, referring to... Figure 1The torsion cylinder is an axisymmetric rotating body structure, its geometry formed by rotating its midsection around its central axis. The overall shape is a cylindrical configuration, wider at the top and narrower at the bottom, with its outline smoothly connected by multiple arcs and straight lines, exhibiting a distinct stepped segmentation characteristic. From a cross-sectional perspective, the workpiece can be divided into three parts: an upper opening area, a middle transition area, and a lower stepped area. The outer edge of the upper opening area extends horizontally, forming a wide boundary, with large rounded corners at the edges. The inner wall is slightly inclined inwards, and the cross-section has a shallow conical structure. The maximum radius of the opening is 118mm. The middle transition area is located in the middle section of the overall shape, with an outer cylindrical shape and a diameter of 91mm. Both the inner and outer walls are composed of smooth arcs, with a minimum diameter of 70mm and a maximum inner wall thickness of 10mm. The cross-sectional lines in this area change continuously, forming a natural concave transition shape. The lower stepped area has a more complex geometry, consisting of multiple interconnected annular segments of different diameters, with rounded corners between each segment. The bottom outer contour is a trapezoidal cylinder, decreasing in diameter from 73mm to 69mm, with a 36mm diameter hole in the center. However, due to the complex shape of the torsion cylinder, its exterior is composed of multiple segments of arcs and straight lines with varying curvatures. The thickness of the inner and outer walls varies significantly along both the axial and radial directions, and the cross-sectional area varies considerably in some regions. Furthermore, aerospace torsion cylinders typically use high-strength titanium alloys, which have low plasticity and high deformation resistance, making them extremely sensitive to forming process parameters. Therefore, forming aerospace torsion cylinders presents significant challenges.

[0018] To address the aforementioned problems, this invention provides a method for eliminating defects during the forming process of variable-diameter cylindrical forgings, comprising the following steps: Step 1: Prepare the blank. Prepare a cylindrical metal blank, which is a cylindrical titanium alloy blank with a diameter of 293mm and a height of 146.5mm. Step two involves pre-treating the billet. Because titanium alloys exhibit significant adhesion at high temperatures, improper lubrication can easily cause the billet to stick to the upper die, affecting forming quality and potentially damaging the die. To avoid this problem, a water-based glass forging lubricant is uniformly coated onto the outer surface of the cylindrical billet before pre-forging. The coating thickness is approximately 0.2 mm. Figure 4-1 As shown. This lubricating layer serves two purposes: firstly, it isolates and lubricates, reducing contact friction; secondly, it provides some heat insulation and anti-oxidation effects on the surface of the billet during heating and forming. The billet is placed in a box-type resistance furnace for heating. During heating, a graphite pad is placed at the bottom of the billet, and the billet is heated at a rate of 2.5℃ / min, held at 200℃ for 2 hours; then held at 600℃ for 1 hour, and then held at 930℃ for another 2 hours.

[0019] Step three: After exiting the furnace, the hot billet is lifted out of the box furnace using a guide rail robotic arm and quickly transferred along the guide rail to the die position above the forging equipment, as per [reference needed]. Figure 2As shown, the mold includes an upper mold and a lower mold that cooperate with each other. The upper mold is a punch, and the lower mold is a die. The minimum width of the die is adapted to the blank. An adjustable limiting block for the punch's forging stroke is provided between the punch and the die. The limiting block can effectively control the mold closing depth, facilitating precise positioning at different stages. This structural design makes the process operation more controllable, and is especially suitable for forming materials with narrow high-temperature deformation windows and high plasticity sensitivity, such as titanium alloys.

[0020] To minimize heat loss, the transfer time was controlled within tens of seconds. Before the formal pressing, the initial forging temperature of the billet surface was measured again with a thermometer, finding it to be approximately 920℃. The pre-forging stroke was 80mm. Immediately after pre-forging, the forging was removed from the mold; at this point, the forging temperature was still approximately 940℃. The billet was then transferred back into the mold for the first forging pass. Surface pre-forging of cylindrical metal materials was performed at 920℃, followed by cooling for later use. Step four involves a second forging process, heating the billet from step two to 900°C and then performing a final forging to complete the forging process.

[0021] To verify the technical solution of the present invention, a comparison is made with the molding process of the prior art. For ease of description, the technical solution of the present invention is referred to as Solution 2, and the prior art solution is referred to as Solution 1. Figure 3 and Figure 4 As shown, the blank used in Scheme 1 is a cylindrical titanium alloy blank with a diameter of 192mm and a height of 109mm.

[0022] To accurately simulate the plastic deformation behavior of the billet during the forming process, a finite element mesh was generated for the billet's geometric model. Tetrahedral elements were used to discretize the billet, resulting in a total of 121,454 mesh elements with a minimum element size of 1.9 mm. Due to the large size of the mold geometry, directly using a uniform mesh could easily lead to overly coarse meshes or local distortions within the cavity, affecting computational accuracy and convergence. Increasing the global mesh count would significantly prolong computation time and increase computational load. To address these issues, a local mesh refinement technique was introduced to adaptively refine the mesh in the mold cavity region, achieving higher mesh resolution in critical deformation areas and thus improving the accuracy of the numerical simulation. After optimization, the upper pre-forging die had 67,653 mesh elements with a minimum element width of 2.8 mm; the lower pre-forging die had 76,215 mesh elements with a minimum element width of 3.2 mm. The number of grids in the upper die for final forging is 65919, and the minimum grid width is 2.4mm. The number of grids in the lower die for final forging is 68171, and the minimum grid width is 2.8mm. Figure 3 and Figure 4As shown, the mesh generation effect of the upper and lower dies for pre-forging and final forging is demonstrated. It can be seen that the mesh distribution inside the cavity is uniform and the transition is smooth, with good connection between the local refined regions and the overall region. This mesh generation strategy significantly improves the geometric fitting accuracy and numerical stability of the model while ensuring computational efficiency, providing a reliable geometric basis for the subsequent finite element analysis of the forming process.

[0023] Reference Figure 7 and Figure 9 As shown, the torsion cylinder forming and analysis of Scheme 1 reveals the typical metal flow characteristics of Scheme 1 during the pre-forging and final forging processes. Its forming process can be divided into three stages: extrusion stage, boss filling stage, and bottom filling stage. Figure 7 (a) is a vector diagram of metal flow during the extrusion stage. It can be observed that after being extruded by the upper die, the metal mainly diffuses radially outward. After the upper part of the billet contacts the die cavity, a large amount of plastic flow occurs, and the metal is rapidly pushed outward and flows downward along the side wall of the billet. The material flow in this stage exhibits an overall radial extrusion and expansion characteristic. The metal at the bottom of the billet has a relatively low flow velocity due to the constraint of the die. The entire deformation area shows a clear velocity gradient distribution, which causes the upper contour of the billet to begin to conform to the die cavity earlier.

[0024] As the reduction amount increases further, the forming process enters the boss filling stage, such as... Figure 7 As shown in (b), the metal flow towards the center region intensifies, and a significant back-extrusion trend gradually emerges at the boss area. Due to the sudden contraction of the mold geometry, the metal needs to pass through a narrower cross-section to fill the boss area, thus requiring greater deformation resistance. The flow vector shows that the metal flow direction around the boss changes from the original radial flow to axial upward flow and central convergence. However, the metal in the boundary area is still affected by friction and flows backward, resulting in a relatively low material flow velocity around the root of the boss. This differentiated flow pattern means that boss forming is somewhat difficult, especially since local underfilling tends to occur at the top edge and the transition areas on both sides of the boss.

[0025] When entering Figure 7 (c) After the bottom filling stage, the metal in the bottom region of the blank begins to be squeezed by the upper formed geometry, forcing the metal to flow downwards into the mold cavity and achieve overall filling. During this stage, the metal mainly flows downwards into the bottom of the die. Due to the relatively enclosed bottom space and the long metal flow path, there is still significant flow resistance during the bottom forming process. The flow vector in the figure is represented by the dense red area at the bottom, reflecting the high-speed concentrated flow of metal at the bottom; while the flow velocity in the sidewall area decreases significantly due to friction and deformation limitations. As the metal gradually fills the bottom of the die, the cavity eventually reaches saturation, and the forming process is complete.

[0026] Reference Figure 5 and Figure 6 As shown, in Scheme 2, the blank diameter is increased to 293mm, the height to 146.5mm, the grid is divided into 165490 grids, and the minimum grid width is 2.6mm. The mold grid still uses local grid refinement technology, referring to... Figure 8 As shown, the material flow state during the forming process of Scheme 2 can be mainly divided into two stages: the extrusion stage and the boss filling stage. In the extrusion stage ( Figure 8 (a) Under the action of the upper die, the metal deforms axially downwards, while the blanks on both sides expand radially. Material flow vectors show a relatively uniform velocity distribution in the upper part and sidewalls of the blank, while the bottom region, due to the increased blank thickness, is easier to fill, resulting in more continuous metal deformation. During the boss filling stage ( Figure 8 (b) As the mold continues to close, the metal begins to flow towards the boss area and is forced into the cavity under the action of back extrusion. Compared to Scheme 1, due to the increased billet volume, the deformation resistance is relatively reduced when flowing through the transition zone and narrow cavity opening, allowing the metal to enter the boss cavity more smoothly and significantly improving the filling capacity. Overall, Scheme 2 improves the material flow distribution during the forming process by increasing the amount of billet, resulting in more sufficient volume distribution during the extrusion stage and smoother back extrusion flow during the boss filling stage. In the continuous deformation of the two stages, not only is the material filling capacity improved, but the possibility of material strain concentration is also reduced, resulting in a more uniform material temperature distribution.

[0027] Scheme Two, the forming and analysis of the torsion cylinder, involves appropriately increasing the billet volume based on Scheme One. This increases the tolerance of the forging process by reducing material utilization. The increased billet allowance improves the filling quality of the back-extrusion area and thin-walled regions during forming, while also reducing excessive local deformation and the risk of phase transformation due to large deformation. In other words, the allowance preparation in step one of Scheme Two is more suitable than that of Scheme One, resulting in fewer defects in the formed torsion cylinder.

[0028] The influence of different forging temperatures on the temperature field distribution of the metal in the first stage is analyzed. (Refer to...) Figure 10 As shown, initial heating temperatures of 900℃, 930℃, and 950℃ were selected for the billet, and temperature field cloud diagrams were presented under these three temperature conditions. It can be observed that with increasing deformation temperature, the ranges of the high-temperature zone, phase transformation zone, and low-temperature zone inside the billet all change significantly. When the forging temperature is 900℃ ( Figure 10 (a) After the outer surface layer of the billet comes into contact with the cold mold, it cools rapidly, forming a large low-temperature zone, while the internal temperature remains at a higher level, exhibiting a distinct internally hot and externally cold structure. At this time, the phase transformation zone is relatively thin, while the low-temperature zone on the outer surface is relatively thick. As the temperature rises to 930℃ ( Figure 10(b) The overall temperature of the billet increases, the low-temperature layer near the die becomes significantly thinner, while the middle part, due to the increased temperature and contact with the upper die, flows faster, thus expanding the inner surface phase transformation zone. The temperature gradient is significantly weaker compared to 900℃, indicating that moderately increasing the forging temperature helps to slow down the heat dissipation of the billet, allowing more material to remain within the phase transformation temperature range, thereby improving its high-temperature plasticity. When the temperature is further increased to 950℃ ( Figure 10 (c) At this stage, the high-temperature zone expands towards the sidewalls and lower part, resulting in a more uniform temperature distribution within the billet. The low-temperature zone is the thinnest, while the phase transformation zone reaches its maximum thickness. (Refer to...) Figure 11 As shown, by Figure 11 (a) It can be seen that the thickness of the phase transformation zone increases significantly with the forging temperature, from about 0.2 mm at 900 °C to about 4 mm at 950 °C. The higher forging temperature brings more material to the vicinity of the β phase transformation temperature, thereby expanding the range of the phase transformation zone. Figure 11 (b) It shows that the thickness of the low-temperature zone gradually decreases with increasing forging temperature, from more than 2.5 mm at 900°C to less than 1 mm at 950°C. The higher the forging temperature, the smaller the temperature difference between the billet and the die, and the rapid cooling phenomenon of the outer surface layer is significantly alleviated, thereby effectively reducing the thickness of the low-temperature zone.

[0029] Referring to Scheme 1, where the phase transformation zone and the low-temperature zone inside the forging are in a state of equilibrium at a deformation temperature of 930℃, Scheme 2 sets the billet temperature at 930℃, while keeping all other parameters unchanged.

[0030] In this study, the first forging step of Scheme 1 consists of two consecutive hammer blows; therefore, the first stage (extrusion stage) and the second stage (boob filling stage) are both continuous. (Refer to...) Figure 12 As shown, the temperature field distribution during the second stage of boss filling process is as follows, refer to Figure 13 and Figure 14 As shown, the internal temperature distribution and the variation of the thickness of the phase transformation zone and the low-temperature zone with forging temperature during this stage are further illustrated. From Figure 12 It is evident that at 900℃, the area where the billet contacts the upper die and the sidewall area are significantly affected by the die cooling, forming a large-area low-temperature zone, while the temperature around the boss is higher but unevenly distributed. As the forging temperature rises to 930℃, the overall temperature field tends to be more uniform, the low-temperature zone shrinks significantly, and the phase transformation zone in the boss area becomes more continuous. At 950℃, the high-temperature zone expands to the entire boss and its adjacent areas, and the temperature gradient further weakens, allowing the material to maintain a higher thermal level at this stage, but its phase transformation zone also becomes thicker.

[0031] Reference Figure 13As shown, the temperature distribution curves at different inner distances from the inner wall to the outside are displayed, further revealing the temperature evolution law of the material in the second stage. It can be observed that at the three deformation temperatures, the internal temperature distribution of the material shows a gradual decrease with the increase of the inner distance. However, as the initial forging temperature increases from 900℃ to 950℃, the overall temperature level shows a significant upward trend, and the temperature difference also increases continuously, from 100℃ at the 900℃ forging temperature to 120℃ at the 950℃ forging temperature, indicating that the internal temperature gradient distribution is extremely uneven.

[0032] Reference Figure 14 As shown, Figure 14 (a) It can be seen that the thickness of the phase transition region increases significantly with increasing temperature, expanding from less than 0.5 mm at 900℃ to more than 3 mm at 950℃. Figure 14 (b) shows that the thickness of the low-temperature zone continuously decreases with increasing forging temperature, reaching nearly 4 mm at 900°C, but dropping to less than 1 mm at 950°C. This trend indicates that as the temperature increases, the billet is more likely to heat up during high-temperature deformation, making the phase transformation zone thicker. At the same time, the temperature difference between the billet and the die decreases, and the cooling rate of the outer layer of the material slows down, resulting in a significant reduction in the size of the low-temperature zone. Figure 15 The diagram illustrates the effective strain field distribution in the first stage (top row a–c) and the second stage (bottom row d–f) of the first forging of Scheme 1 under three temperature conditions (900℃, 930℃, and 950℃). Overall, as the forging temperature increases, the material softening effect intensifies, and the strain distribution gradually shifts from localized concentration to overall uniformity.

[0033] Under conditions of 900℃ ( Figure 16 (a)(d)) The strain field exhibits regional inhomogeneity. In the first stage, the main deformation is concentrated in the upper part of the billet and the inner region near the die cavity, while large areas of low strain exist on the sidewalls and bottom, indicating that the outer layer of the material is difficult to flow. After entering the second stage, the back extrusion of the boss area increases the strain in all parts, and the range of low strain areas decreases. When the forging temperature rises to 930℃ ( Figure 16 (b)(e) shows a significant improvement in strain distribution. In the first stage, the high-strain zone expands towards the sidewalls, and the degree of deformation at the bottom increases. In the second stage, due to the increased deformation temperature, the material exhibits a continuous high-strain band around the boss and in the central region, while the low-strain zone shrinks, indicating more complete and uniform material flow. At 950℃ ( Figure 16 (c)(f)) The strain distribution of the material is the most uniform. In the first stage, the strain at the center of the material is more uniform than in the previous two temperatures; in the second stage, the high strain zone covers the boss and the main bottom area, and the strain distribution shows obvious continuity and integrity, indicating that the material has stronger plastic flow capacity and the lowest deformation resistance at high temperatures, and can more effectively complete the filling of complex geometric parts.

[0034] Before final forging, the billet temperature is reheated to the specified temperature and then forging is performed. Therefore, the simulated final forging temperature is the set value (900℃, 930℃, 950℃). Figure 15 The temperature field contour maps of forgings during the final forging stage are shown when forging temperatures are 900℃, 930℃, and 950℃. Figure 16 Temperature distribution curves at different inner distances are presented. The two figures together reveal the overall pattern of temperature distribution inside the forging as a function of forging temperature and the differences in temperature uniformity, which is of great significance for understanding the thermal stability of the material in the final forging stage.

[0035] Reference Figure 16 As shown, at 900℃, the overall temperature of the forging is relatively low, and there is a clear temperature difference between the upper and lower regions due to the large deformation, maintaining a certain temperature. However, a large area of ​​low-temperature zone still forms in the upper and middle regions, resulting in poor overall temperature uniformity. When the forging temperature is increased to 930℃, the temperature of the forging becomes significantly more uniform. The high-temperature zone accumulates at the bottom, and its temperature distribution gradually changes from the original "multi-layered segmented" to "continuous distribution." The overall temperature gradient is significantly reduced, and the low-temperature zone shrinks. At 950℃, the temperature distribution exhibits the best uniformity. However, the center, bottom, and sidewalls of the forging all show continuous high-temperature zones approaching the phase transition point, with only localized cold ends appearing in the thin layer area in contact with the die. The temperature transition between layers is smoother, and there are almost no significant temperature drop zones inside, indicating that heat diffusion in the material is more complete, resulting in the highest thermal stability.

[0036] Figure 17 The temperature-internal distance distribution curves further quantify the aforementioned temperature field patterns: the 900℃ curve shows the fastest temperature decrease, with a significant linear and rapid temperature decay as the internal distance increases, indicating high heat loss, a high temperature gradient, and the worst internal temperature uniformity at this temperature, with a maximum temperature difference of approximately 100℃; the 930℃ curve is in the middle range, with a slower temperature decrease than at 900℃, indicating improved heat distribution and reduced boundary temperature drop, with a maximum temperature difference of approximately 70℃; the 950℃ curve shows the highest overall temperature and the slowest temperature decrease, with almost no low-temperature zone, and the lowest temperature is still close to 930℃, indicating that the material maintains stronger overall thermal stability at this temperature. Combining the contour plots and temperature curves, it can be seen that the higher the final forging temperature, the more uniform the internal temperature of the forging, the smaller the temperature gradient, and the easier it is to avoid insufficient plasticity and flow lag caused by uneven cooling. The 950℃ condition provides the best temperature uniformity, offering crucial thermal assurance for the full filling of complex geometric parts and subsequent structural stability.

[0037] Figure 18The effective strain field distribution of the forgings in the final forging stage is shown at forging temperatures of 900℃, 930℃, and 950℃. Overall, it can be observed that as the forging temperature increases, the strain distribution gradually transitions towards homogenization, and the material's plastic deformation capacity and cavity filling capacity are enhanced.

[0038] Under conditions of 900℃ ( Figure 18 (a) shows a distinct multi-layered strain field. Although a localized high-strain zone appears at the bottom of the forging, a large area of ​​low-strain bands exists on its sidewalls because these zones do not participate in deformation. When the forging temperature is increased to 930℃ ( Figure 18 (b) shows a slight improvement in the internal strain field of the forging. The high-strain zone expands towards the sidewalls and upper middle part, while the area of ​​the low-strain zone decreases. At 950℃ ( Figure 18 (c) The strain field exhibits the most uniform and complete deformation characteristics. The material maintains good plasticity at higher temperatures, reducing deformation resistance and allowing for more complete anti-squeeze deformation in the bottom region. The high-strain zone almost covers the entire bottom and middle region of the forging, forming a continuous isobaric band with the sidewall region.

[0039] In summary, during the pre-forging stage, the internal temperature field of the billet exhibits significant stratification. For Scheme 1, a phase transformation zone begins to appear at 900℃, but its thickness is relatively small, while a thicker low-temperature zone forms on the outer layer. As the deformation temperature increases to 930℃ and 950℃, the phase transformation zone gradually expands inward, and its thickness increases significantly; simultaneously, the low-temperature zone shrinks noticeably. This pattern is even more pronounced in the second stage. At 950℃, the phase transformation zone thickness reaches approximately 5.5mm, while the low-temperature zone almost completely disappears; at 930℃, both the low-temperature zone and the phase transformation zone are relatively thin. Regarding overall temperature uniformity, the billet still exhibits uneven internal temperature distribution after pre-forging, with the highest temperature difference reaching 120℃. In the final forging stage, since the bottom is the main deformation area, a certain degree of temperature rise occurs in some local areas, but due to the short duration of the final forging process, the phase transformation zone and low-temperature zone are relatively small and therefore not included in the statistics. However, even so, the internal temperature difference in the final forming stage still reaches 60–100℃.

[0040] Regarding the pre-forging stroke in step three, In Scheme 2, a phase transformation zone begins to appear inside the billet when the pre-forging stroke reaches 60mm in the first forging pass. Therefore, to further analyze the influence of the pre-forging stroke on the microstructure evolution and temperature distribution, this study selected three pre-forging strokes of 60mm, 70mm, and 80mm for comparative analysis. Figures 19 to 21 The effective strain field distribution, temperature field distribution, and thickness variations of the phase transformation zone and low-temperature zone are shown in the strain distribution diagram ( ). Figure 19As can be seen, with the increase of the pre-forging stroke, the contact area between the upper part of the billet and the upper die increases, resulting in greater plastic deformation of the metal at the contact interface of the upper die, leading to local temperature rise and phase transformation. When the pre-forging stroke increases from 60mm to 80mm, the thickness of the phase transformation zone increases from about 1mm to about 3mm.

[0041] As the pre-forging stroke increases, the contact time between the lower part of the billet and the die is significantly prolonged, resulting in a continuous expansion of the cooling zone in contact with the lower die. Figure 20 As shown, the thickness of the low-temperature zone increases from approximately 2 mm at 60 mm to approximately 4 mm at 80 mm with the increase in stroke. Since the total forging stroke is 120 mm, the forging stroke of the pre-forging determines the forging stroke of the final forging. Therefore, it is necessary to conduct an overall analysis of the situation after two forging cycles.

[0042] Figure 22 The figure shows the temperature field distribution inside the forging after the second forging stage. It can be seen that as the pre-forging stroke increases, the temperature at the bottom of the forging tends to be more uniform, the local high-temperature zone shrinks, and the overall temperature decreases. This indicates that appropriately increasing the pre-forging deformation helps to reduce the temperature rise effect during the final forging stage, resulting in a more uniform temperature distribution after forging.

[0043] To more intuitively analyze the internal temperature variation pattern, five measuring points were selected at equal intervals along the directions from the inner wall to the bottom and from the inner wall to the side wall of the forging at the end of the final forging, resulting in a total of six temperature distribution curves, as shown below. Figure 23 As shown. From Figure 23 (a) It can be seen that the temperature distribution trends of the three pre-forging strokes on the bottom section are basically the same, all showing the characteristics of high in the middle and low on both sides. However, the overall temperature and temperature difference vary with the pre-forging stroke: when the pre-forging stroke is 60mm, the highest temperature at the bottom is about 980℃, and the maximum temperature difference is about 60℃; when the pre-forging stroke increases to 70mm, the highest temperature drops to about 960℃, and the maximum temperature difference decreases to about 40℃; when the pre-forging stroke further increases to 80mm, the highest temperature at the bottom is about 950℃, and the maximum temperature difference is only about 20℃, and the temperature field at the bottom is significantly more gradual. Regarding the temperature distribution in the sidewall direction ( Figure 23 (b)) All three working conditions also show a trend of gradually decreasing from the inside to the outside. When the pre-forging stroke is 60mm, the maximum temperature difference between the sidewalls is about 35℃; while when the pre-forging stroke is 70mm and 80mm, the sidewall temperatures are similar, with a maximum temperature difference of about 25℃, and the temperature gradient is relatively reduced.

[0044] comprehensive Figure 19-23As shown, when the pre-forging stroke increases, although the phase transformation zone and low temperature zone inside the billet thicken during pre-forging, the temperature rise effect caused by the final forging deformation is weakened due to the corresponding reduction in the reduction amount during the final forging stage. Ultimately, this makes the temperature distribution inside the forging more stable and the temperature difference smaller after final forging, which is conducive to obtaining a more consistent microstructure and properties.

[0045] The independent variable is the first forging stroke. Under the premise that the total forging stroke remains unchanged, the deformation of the final forging varies with the forging stroke. Therefore, the overall strain of the forging remains basically the same after the final forging. Figure 24 The strain field distribution of the forging after final forging is shown. It can be seen that the forging undergoes plastic deformation to varying degrees from the bottom to the side wall and then to the upper transition area. There is no obvious low-strain "dead zone" inside, indicating that the overall forming is relatively complete.

[0046] At a strain rate of 10 s -1 Under these conditions, when the strain is approximately 0.69 during the second compression pass, adiabatic shear bands begin to appear inside the material. However, when the strain increases to approximately 1.2 during the third compression pass, the microstructure essentially transforms into equiaxed grains with significantly refined grains. (Comparison) Figure 24 It can be observed that the equivalent strain in the main load-bearing areas of the forging after final forging is generally greater than 1, with most areas approaching or exceeding 1.2, corresponding to the strain level of fine equiaxed grains observed in the third pass experiment. Therefore, it can be inferred that the internal structure of the forging after final forging under these process parameters is mainly composed of fine equiaxed grains, which is beneficial for obtaining better comprehensive mechanical properties. Therefore, Scheme 2 can be determined as the suitable torsion cylinder forming scheme.

[0047] Figure 25 As shown, by Figure 25 (a) It can be seen that the pre-forging stage mainly involves pushing the billet upwards and completing the initial filling. The die cavity is not yet completely filled, so the load change is relatively gradual throughout the deformation process, with a maximum forging force of approximately 1260t. In contrast, the final forging stage ( Figure 25 (b) It is necessary to fully fill the cavity and reinforce the key parts based on the pre-forming in the early stage. As the stroke increases, the contact area between the blank and the mold increases rapidly. When the final forming is approached, the forging force rises significantly, and the maximum load reaches about 7700t.

[0048] Regarding the billet heating in steps two, three, and four of Scheme Two, titanium alloys have low thermal conductivity at low temperatures, which gradually increases with rising temperature. When heating large billets, a significant temperature difference will occur between the inside and outside, and the maximum temperature difference during reheating should ideally be kept below 200℃. Therefore, heat preservation is necessary at each stage to reduce the internal and external temperature gradient, decrease internal thermal stress in the billet, and lower the risk of cracking. This section uses ABAQUS software to simulate the heating process to reasonably reduce the internal and external temperature difference of the billet.

[0049] The thermal conductivity and specific heat capacity of TC11 titanium alloy were imported into the ABAQUS material library. The parameters of thermal conductivity and specific heat capacity of TC11 titanium alloy are shown in the table below.

[0050]

[0051] The density of TC11 titanium alloy is 4550 kg / m³. Tetrahedral mesh is still used. Based on experience, the holding temperatures were set to 200℃, 600℃, and 940℃ (considering that a forging temperature of 930℃-950℃ is more suitable, 940℃ was selected as the forging temperature). At 200℃, no holding, holding for 1 hour, holding for 2 hours, and holding for 4 hours were selected. At 600℃, a fixed holding time of 1 hour was used, and at 940℃, a fixed holding time of 2 hours was used to eliminate internal temperature gradients. The default heating rate was 2.5℃ / min. Temperatures were recorded at four points from the inside of the billet to the surface. Figure 26 As shown.

[0052] Reference Figure 27 As shown, from Figure 27 As can be seen in (a), the internal temperature difference of the billet gradually increases before reaching 600℃ without heat preservation. After heat preservation at 600℃ for 1 hour, the internal temperature difference decreases rapidly. After heat preservation at 940℃ for 2 hours, the temperature difference is basically eliminated. Figure 27 As shown in (b), the maximum temperature difference can reach 230℃ without insulation at 200℃, and the maximum temperature difference drops to 209℃ after insulation for 1 hour, indicating that insulation at 200℃ can effectively reduce the internal temperature gradient of the billet.

[0053] Reference Figure 28 As shown, from Figure 28 As can be seen in (a), after holding at 200℃ for 2 hours, the four-point curves show that the temperature difference of the titanium alloy at low temperatures is reduced. Figure 28 As shown in (b), the temperature difference decreased from 209℃ to 190℃ after 2 hours of holding. When the holding time reached 4 hours, the internal temperature difference of the billet decreased from 190℃ to 185℃. It can be seen that as the holding time increases, the internal temperature difference of the pre-forged billet gradually decreases, but the rate of decrease also decreases with the increase of the holding time. When the holding time is 4 hours, the temperature difference is only 5℃ lower than that after 2 hours, but the holding time is doubled, which is more costly. Therefore, the heating schemes for the pre-forged billet are 200℃ for 2 hours, 600℃ for 1 hour, and 940℃ for 2 hours.

[0054] Temperature was measured at four points from the inside to the surface of the final forging billet. Figure 29 As shown. Since the billet has already taken shape after pre-forging, and the thickness of each part has been reduced, the heating rate was increased to 5℃ / min. Calculations showed that holding at 200℃ for 1 hour was the most suitable temperature. Figure 30As shown, its maximum temperature difference is 129℃, and the cost of heat preservation for 1 hour is relatively low.

[0055] A three-dimensional finite element model of the TC11 titanium alloy aerospace torsion cylinder was established based on DEFORM-3D. The calculated model was imported into the material library, and the blank and mold meshes were generated. Based on this, numerical simulations were performed on the forming processes of Scheme 1 and Scheme 2, respectively. Through systematic analysis of the evolution of the temperature field and strain field, the deformation characteristics and influencing factors at each stage were clarified.

[0056] The comparative results show that Scheme 1 has a higher material utilization rate, and the forming process can be divided into three stages: extrusion, boss filling, and bottom filling. However, the deformation in the boss and thin-walled areas is relatively high, causing the temperature to rise to the phase transformation zone. When the forging temperature increases from 900℃ to 950℃, the thickness of the phase transformation zone gradually increases, while the low-temperature zone gradually decreases. At a temperature of 930℃, the low-temperature zone and the phase transformation zone are in a good state. However, due to the large internal temperature gradient between pre-forging and final forging, it is not suitable for forging processes. Scheme 2 increases the billet volume and adopts a two-stage pre-forging + final forging process, improving the deformation of the metal in key areas and making the process more forgiving. The structure shows that increasing the pre-forging stroke from 60mm to 80mm will thicken the phase transformation zone and low-temperature zone in the pre-forging stage. However, due to the reduced final forging reduction, the internal temperature difference of the forging after final forging is significantly reduced, and the temperature distribution is more stable. The overall equivalent strain in the final forging stage is generally greater than 1, suggesting that a fine equiaxed structure can be obtained inside the forging. Based on a comprehensive analysis of the two schemes and various process parameters, this chapter concludes that, under the conditions of 930–950℃, using a two-stage forging process and an 80mm pre-forging stroke, the forming quality can be guaranteed while improving forming stability. This achieves the technical goal of optimizing the forming process of variable diameter cylindrical forgings and producing high-performance variable diameter cylindrical forgings.

Claims

1. A method for eliminating defects during the forming process of a variable-diameter cylindrical forging, characterized in that, Includes the following steps: Step 1: Prepare the blank. Prepare the remaining cylindrical metal blank. Step 2: Pre-treat the blank by placing it in a box-type resistance furnace and heating it to 930°C for two hours. Step 3: After exiting the furnace, the billet is transferred to a mold for the first forging; the cylindrical metal material undergoes surface pre-forging at a temperature of 920℃, followed by cooling for later use. Step four involves a second forging process, heating the billet from step two to 900°C and then performing a final forging to complete the forging process.

2. The method for eliminating defects in the forming process of a variable diameter cylindrical forging according to claim 1, wherein in step one, the billet is a cylindrical titanium alloy billet with a diameter of 293 mm and a height of 146.5 mm.

3. In the method for eliminating defects during the forming process of a variable diameter cylindrical forging according to claim 1, in step two, the pretreatment of the blank is to clean surface defects and apply 0.2mm of lubricant, wherein the lubricant is a water-based glass lubricant.

4. The method for eliminating defects during the forming process of a variable diameter cylindrical forging according to claim 1, wherein during heating, a graphite pad is placed at the bottom of the billet, the billet is heated at a heating rate of 2.5℃ / min, and held at 200℃ for 2 hours; then held at 600℃ for 1 hour, and then held at 930℃ for another 2 hours.

5. The method for eliminating defects in the forming process of a variable diameter cylindrical forging according to claim 1, in step three, the mold includes an upper mold and a lower mold that cooperate with each other, the upper mold is a punch, the lower mold is a die, the minimum width of the die is adapted to the blank, and a limiting block for adjusting the punch forging stroke is provided between the punch and the die.

6. In the method for eliminating defects during the forming process of a variable diameter cylindrical forging according to claim 1, in step three, the pre-forging stroke is 80 mm.

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