A method for synchronous progressive and collaborative forming of a large-diameter-ratio large-bore hollow shaft

Abstract

This invention discloses a method for synchronous and progressive axial gradient forming of a large-diameter-ratio hollow shaft with a large inner diameter. The method involves pushing the hollow shaft blank axially. When the end of the first expansion section enters the inner hole of the hollow shaft blank, the first set of skew rolls simultaneously begins skew rolling the front end of the hollow shaft blank, causing the hollow shaft blank to undergo the first stage of plastic deformation under the bidirectional compression of internal expansion and external rolling. The hollow shaft blank continues to move axially in the same direction, sequentially passing through each local synergistic deformation zone, resulting in multi-stage continuous plastic deformation until the synergistic forming of the large-diameter-ratio hollow shaft with a large inner diameter is completed. The advantages are that this method allows the large inner diameter expansion and large-diameter-ratio external diameter reduction to be completed in stages during continuous feeding, reducing the risk of deformation concentration, sudden changes in local deformation, and instability caused by single-pass large expansion and single-pass large diameter reduction. It also improves the metal flow coordination of the large-diameter-ratio hollow shaft during the forming process and enhances the forming quality of the hollow shaft.

Description

Technical Field

[0001] This invention relates to the field of plastic forming technology for shaft parts, and in particular to a method for axial gradient synchronous progressive collaborative forming of a large-diameter-ratio, large-inner-hole hollow shaft. Background Technology

[0002] Hollow shafts are widely used in aerospace, marine engineering, and other fields due to their weight reduction, material conservation, and good service performance. In particular, the core rotating components of aerospace and marine equipment are multi-step hollow shafts with a large diameter ratio (the ratio of the initial outer diameter of the blank to the minimum outer diameter of the finished product can be greater than 2), and the diameter of the through inner hole or main functional inner hole can be greater than 50 mm. During service, these components are typically subjected to high-speed rotation, alternating loads, torsional loads, or impact loads, and some applications also involve high-temperature environments. This places high demands on the dimensional accuracy, structural uniformity, and overall reliability of multi-step hollow shafts.

[0003] Existing hollow shaft manufacturing processes, both domestically and internationally, primarily involve secondary drilling on solid shaft blanks. This approach suffers from problems such as difficulty in controlling machining accuracy, low forming efficiency, disruption of metal flow continuity, and susceptibility to fatigue cracking, making it difficult to meet the iterative upgrade requirements of lightweight, high-performance, and long-life aerospace and marine equipment. To address these issues, existing technologies have proposed using plastic forming methods such as skew rolling to prepare hollow shafts. However, for multi-step hollow shafts used in aerospace and marine equipment, the materials typically possess high deformation resistance, and the forming temperature range and process window are relatively narrow. Furthermore, these hollow shafts exhibit structural characteristics such as large diameter ratios, large inner diameters, and multi-step outer contours. If large outer diameter reduction and inner diameter expansion are achieved through single or few passes during skew rolling, problems such as excessive local deformation, uneven wall thickness distribution, unstable outer contour step forming, uncoordinated metal flow in the step transition zone, and local instability at the ends or in the transition zone can easily occur. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide a synchronous and progressive axial gradient forming method for hollow shafts with large diameter ratio and large inner hole. This method decomposes the large inner hole expansion and multi-step outer contour reduction into multiple continuous and progressive small deformation stages, so that the hollow shaft blank completes the internal hole expansion and external diameter reduction step by step in each local synergistic deformation zone, thereby improving the forming stability, wall thickness uniformity and outer contour forming accuracy of each step shaft segment and step transition zone of the multi-step hollow shaft.

[0005] The technical solution adopted by this invention to solve the above-mentioned technical problems is as follows: a method for axial gradient synchronous progressive cooperative forming of a large-diameter-ratio, large-inner-hole hollow shaft. During the forming process, a hole-expanding mandrel is integrally provided on the mandrel. The hole-expanding mandrel includes at least two hole-expanding working sections with outer diameters progressively increasing along the feed direction of the hollow shaft. Multiple sets of skew rolls are provided in the skew rolling mill. The number and position of the skew roll sets correspond to the hole-expanding working sections. Each hole-expanding working section and its corresponding skew roll set form a local cooperative deformation zone. The forming method includes the following specific steps: S1. Pre-treat the inner and outer surfaces of the hollow shaft blank; S2. Heat the hollow shaft blank to the predetermined forming temperature and preheat the expansion mandrel and the skew roll group; S3. Push the hollow shaft blank to move axially. When the end of the first hole-expanding working section enters the inner hole of the hollow shaft blank, the first set of skew rolling rolls simultaneously begins to skew roll the front end of the hollow shaft blank. The diameter of the first hole-expanding working section is larger than the inner hole diameter of the hollow shaft blank, realizing the internal hole expansion of the hollow shaft blank. At the same time, the first set of skew rolling rolls moves radially to externally reduce the diameter of the outer surface of the hollow shaft blank, so that the hollow shaft blank undergoes the first stage of plastic deformation under the bidirectional compression action of internal expansion and external rolling. S4. Propel the hollow shaft blank to continue moving axially in the same direction, passing through each local collaborative deformation zone in sequence, so that the hollow shaft blank undergoes multi-stage continuous plastic deformation, and control the radial diameter reduction of the hollow shaft blank during each stage of plastic deformation to decrease sequentially along the feed direction of the hollow shaft blank, and the axial hole expansion amount to decrease sequentially along the feed direction of the hollow shaft blank, until the collaborative forming of the large-diameter-ratio large-inner-hole hollow shaft is completed.

[0006] Furthermore, in step S1, the pretreatment specifically involves cleaning, degreasing, removing oxide scale and impurities from the outer surface and inner hole surface of the hollow shaft blank, in order to reduce the adverse effects of surface defects on the forming quality during the forming process.

[0007] Furthermore, there are three hole-expanding working sections, namely the first hole-expanding working section, the second hole-expanding working section, and the third hole-expanding working section, and there are three sets of skew rolls, namely the first skew roll set, the second skew roll set, and the third skew roll set.

[0008] Furthermore, during the first and second stages of plastic deformation, the first and second skew roll groups both reduce the diameter of the hollow shaft blank by the same diameter. During the third stage of plastic deformation, the third skew roll group moves radially to perform stepped shaft rolling on the outer surface of the hollow shaft blank.

[0009] Further, in step S4, the radial diameter reduction is represented by the reduction of area ratio η, that is: , in:i This indicates the stage at which the hollow shaft blank undergoes plastic deformation. i =1, 2, 3, where D0 represents the initial outer diameter of the hollow shaft blank, D1 represents the outer diameter of the hollow shaft blank after the first stage of external diameter reduction, D2 represents the outer diameter of the hollow shaft blank after the second stage of external diameter reduction, and D3 represents the minimum outer diameter of the hollow shaft blank after the third stage of external diameter reduction. The section shrinkage rates after the first, second, and third stages of external diameter reduction are represented as η1, η2, and η3, respectively, and η1>η2>η3. By controlling the section shrinkage rates after external diameter reduction at different stages, the high-temperature and low-deformation-resistance regions in the first stage bear a relatively large radial diameter reduction, while the regions after temperature reduction and work hardening in the later stages bear a smaller radial diameter reduction. This reduces the risk of sudden changes in wall thickness, local instability, and cracking while achieving external gradient forming of large-diameter hollow shafts.

[0010] Furthermore, η1 is 20%–35%, η2 is 12%–25%, and η3 is 6%–18%.

[0011] Furthermore, let the diameter of the forming part of the first skew roll group be M1, the diameter of the forming part of the second skew roll group be M2, and the diameter of the forming part of the third skew roll group be M3. Then M1 < 6.5D1, M2 < 6.5D2, M3 < 6.5D3, and M1 > M2 > M3.

[0012] Furthermore, in step S4, the axial hole expansion amount is represented by the hole expansion ratio ξ, that is: , Wherein: d0 represents the initial hole diameter of the hollow shaft blank, d1 represents the hole diameter of the hollow shaft blank after the first stage of internal reaming, d2 represents the hole diameter of the hollow shaft blank after the second stage of internal reaming, and d3 represents the hole diameter of the hollow shaft blank after the third stage of internal reaming. The reaming rates after the first, second, and third stages of internal reaming are represented as ξ1, ξ2, and ξ3, respectively, and ξ1>ξ2>ξ3. By distributing the total reaming amount through multiple reaming working sections, the inner hole of the hollow shaft is gradually enlarged during continuous feeding, avoiding the problems of local strain concentration, uneven wall thickness, and reaming instability caused by excessive single-stage reaming.

[0013] Furthermore, ξ1 is 1.18–1.22, ξ2 is 1.15–1.18, and ξ3 is 1.12–1.15.

[0014] Furthermore, when the front end of the hollow shaft blank moves to the predetermined position on the rolling exit side, the traction mechanism on the exit side clamps and pulls the hollow shaft blank, so that the part of the rear end of the hollow shaft blank that has not left the local collaborative deformation zone can be continuously and stably formed, thus improving the problems of insufficient tail hole expansion, incomplete end profile and local instability at the end under the condition of large hole expansion.

[0015] Compared with the prior art, the advantages of the present invention are: (1) This method uses a hollow shaft blank with an initial inner hole as the blank material. It utilizes the radial and axial synergistic effect of a hole-expanding mandrel with multiple hole-expanding working sections and multiple sets of skew rolls to complete the hole-expanding of the large inner hole and the diameter reduction of the large diameter ratio in stages during continuous feeding. This reduces the risk of deformation concentration, sudden changes in local deformation and instability caused by single-pass large hole expansion and single-pass large diameter reduction, improves the metal flow coordination of the large diameter ratio and large inner hole hollow shaft during the forming process, and improves the forming quality of the hollow shaft. (2) Since the expanding mandrel contains multiple expanding working sections of different diameters arranged along the axial direction, the expanding working section and its corresponding skew roll group form a local collaborative deformation zone. According to the forming requirements of different areas of the hollow shaft, the expansion effect on the inner hole can be applied more specifically, so that the deformation of the inner hole is no longer just passively following the outer layer flow, but can be synchronized and coordinated with the external diameter reduction process. This allows the inner hole expansion, outer contour diameter reduction and transition zone shaping of the hollow shaft to be coordinated in the same deformation process, which is beneficial to improve the wall thickness uniformity of the hollow shaft, improve dimensional accuracy and continuous forming stability. (3) This method has a short process, high material utilization, small torque required for local loading, and compact equipment layout. It is suitable for continuous plastic forming of hollow shaft components with large diameter ratio and large inner hole. Attached Figure Description

[0016] Figure 1 This is a schematic diagram of the state of the present invention before molding; Figure 2 This is a schematic diagram showing the state of the hollow shaft blank during the first stage of plastic deformation according to the present invention; Figure 3 This is a schematic diagram showing the state of the hollow shaft blank during the second stage of plastic deformation according to the present invention; Figure 4 This is a schematic diagram showing the state of the hollow shaft blank during the third stage of plastic deformation according to the present invention; Figure 5 This is a comparison diagram of the equivalent plastic strain distribution of hollow shafts obtained by the present invention and different forming processes. Detailed Implementation

[0017] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0018] As shown in the figure, a method for synchronous and progressive forming of a large-diameter-ratio hollow shaft with a large inner diameter using an axial gradient is described. During the forming process, a core rod 1 is integrally equipped with a reaming mandrel 2. The reaming mandrel 2 includes a first reaming working section 21, a second reaming working section 22, and a third reaming working section 23, whose outer diameter increases progressively along the feed direction of the hollow shaft. A skew mill is equipped with a first skew roll group 3, a second skew roll group 4, and a third skew roll group 5. The deflection angle of each roll in the first skew roll group 3, the second skew roll group 4, and the third skew roll group 5 is... The angles are set to 5° to 9°. The positions of the first skew roll group 3 and the first expansion section 21, the second skew roll group 4 and the second expansion section 22, and the third skew roll group 5 and the third expansion section 23 correspond one-to-one and maintain their axial relative positions. The first skew roll group 3 and the first expansion section 21 form a first local cooperative deformation zone, the second skew roll group 4 and the second expansion section 22 form a second local cooperative deformation zone, and the third skew roll group 5 and the third expansion section 23 form a third local cooperative deformation zone. Figure 1 As shown; the forming method includes the following specific steps: S1. The inner and outer surfaces of the hollow shaft blank 6 are pretreated by cleaning, degreasing, removing oxide scale and impurities, so as to reduce the adverse effects of surface defects on the forming quality of the hollow shaft during the forming process. S2. Heat the hollow shaft blank 6 to the predetermined forming temperature, and preheat the expansion mandrel 2, the first skew roll group 3, the second skew roll group 4 and the third skew roll group 5; S3. The hollow shaft blank 6 is pushed axially. When the end of the first expansion section 21 enters the inner hole of the hollow shaft blank 6, the first skew roll group 3 simultaneously begins to skew roll the front end of the hollow shaft blank 6. That is, the front end of the hollow shaft blank 6 enters the first local collaborative deformation zone. The diameter of the first expansion section 21 is larger than the inner hole diameter of the hollow shaft blank 6, realizing the internal expansion of the hollow shaft blank 6. At the same time, the first skew roll group 3 moves radially to perform external equal-diameter reduction on the outer surface of the hollow shaft blank 6, so that the hollow shaft blank 6 undergoes the first stage of plastic deformation under the bidirectional compression action of internal expansion and external rolling. Figure 2 As shown, in this stage, the inner diameter of the hollow shaft blank 6 is expanded in the first stage, and the outer surface is initially formed under the radial pressure of the rolls, while the continuous plastic flow of the first stage transition region is completed. S4. Propel the hollow shaft blank 6 to continue moving axially in the same direction. The hollow shaft blank 6 passes through the second local coordinated deformation zone and the third local coordinated deformation zone in sequence, causing the hollow shaft blank 6 to undergo continuous plastic deformation in the second and third stages, specifically: When the front end of the hollow shaft blank 6 enters the second local coordinated deformation zone, the second skew roll group 4 is controlled to move radially, and the second hole expansion section 22 further expands the inner hole of the hollow shaft blank 6. At the same time, the outer surface is gradually formed under the equal diameter reduction of the second skew roll group 4, such as... Figure 3As shown; when the front end of the hollow shaft blank 6 enters the third local collaborative deformation zone, the third skew roll group 5 is controlled to move radially, and the third hole expansion section 23 performs the final hole expansion on the inner hole of the hollow shaft blank 6. At the same time, the third skew roll group 5 moves radially to perform the final stepped diameter reduction forming on the outer surface, so that the inner hole of the hollow shaft blank 6 reaches the final design size, and at the same time completes the plastic forming of the third stage stepped shaft shape and its transition area, such as Figure 4 As shown; Furthermore, during the forming process of the hollow shaft blank 6, the radial reduction in diameter during plastic deformation at each stage is controlled to decrease sequentially along the feed direction of the hollow shaft blank 6. The radial reduction in diameter is represented by the reduction of area η, i.e.: , in: i This indicates the stage at which the hollow shaft blank 6 undergoes plastic deformation. i =1, 2, 3, where D0 represents the initial outer diameter of the hollow shaft blank 6, D1 represents the outer diameter of the hollow shaft blank 6 after the first stage of external diameter reduction, D2 represents the outer diameter of the hollow shaft blank 6 after the second stage of external diameter reduction, and D3 represents the minimum outer diameter of the hollow shaft blank 6 after the third stage of external diameter reduction. The section reduction rates after the first, second, and third stages of external diameter reduction are represented as η1, η2, and η3, respectively, and η1=28%, η2=17%, and η3=10%. The axial expansion amount also decreases sequentially along the feed direction of the hollow shaft blank 6. The axial expansion amount is represented by the expansion ratio ξ, that is: , Wherein: d0 represents the initial hole diameter of the hollow shaft blank 6, d1 represents the hole diameter of the hollow shaft blank 6 after the first stage of internal hole expansion, d2 represents the hole diameter of the hollow shaft blank 6 after the second stage of internal hole expansion, and d3 represents the hole diameter of the hollow shaft blank 6 after the third stage of internal hole expansion. The hole expansion rates after the first, second, and third stages of internal hole expansion are represented as ξ1, ξ2, and ξ3, respectively, and ξ1=1.20, ξ2=1.16, and ξ3=1.13. S5. When the front end of the hollow shaft blank 6 moves to the predetermined position on the rolling exit side, the traction mechanism 7 on the exit side clamps and pulls the hollow shaft blank 6, so that the part of the rear end of the hollow shaft blank 6 that has not left the local collaborative deformation zone can be continuously and stably formed until the collaborative forming of the large-diameter-ratio large-inner-hole hollow shaft is completed.

[0019] To avoid spatial interference when the three sets of skew rolls are arranged radially, and to take into account roll size, rolling torque and forming stability, the forming part diameter of the first skew roll group 3 is set to M1, the forming part diameter of the second skew roll group 4 is set to M2, and the forming part diameter of the third skew roll group 5 is set to M3, where M1 < 6.5D1, M2 < 6.5D2, M3 < 6.5D3, and M1 > M2 > M3.

[0020] The forming effect of this method on large-diameter-ratio, large-inner-hole hollow stepped shafts was compared and analyzed with that of the other two forming processes.

[0021] like Figure 5 As shown in (a), multiple sets of skew rolls and equal-diameter mandrels are used for forming. The diameter of the equal-diameter mandrel is less than or equal to the initial hole diameter of the hollow shaft blank. In this forming process, the equal-diameter mandrel mainly serves as internal support and sizing, and cannot actively expand the inner hole of the hollow shaft blank. From its equivalent plastic strain distribution, it can be seen that the plastic deformation is mainly concentrated in the outer rolling area of ​​the hollow shaft blank. The degree of material participation in plastic deformation near the inner hole is relatively low, and there is a significant strain gradient in the wall thickness direction, which easily leads to a state where the outer layer deformation is strong and the inner layer deformation is insufficient.

[0022] like Figure 5 As shown in (b), the forming method of this invention involves multiple sets of skew rolls forming multiple localized coordinated deformation zones with corresponding expansion sections on the expansion mandrel. This allows the hollow shaft blank to sequentially complete external rolling reduction and internal progressive expansion during continuous axial feeding. The equivalent plastic strain distribution shows that the material near the inner hole and within the wall thickness can participate more fully in plastic deformation. The equivalent plastic strain distribution is relatively continuous along the axial and wall thickness directions, while the high-strain regions are relatively dispersed. This indicates that by having multiple expansion sections share the total expansion amount and ensuring that internal expansion and external reduction are carried out synchronously, the coordination of plastic flow between the inner and outer layers of material is improved.

[0023] like Figure 5 As shown in (c), when using a single-stage expanding mandrel and multiple sets of skew rolls for synchronous expanding, the single-stage expanding mandrel needs to complete a large expanding deformation within one expanding stage, resulting in significant high strain concentration in the step transition region and the outer layer material. Figure 5 Compared with the method of the present invention shown in (b), the single-stage hole-expanding mandrel scheme has a higher local maximum equivalent plastic strain and a more concentrated strain change, indicating that its single-stage hole-expanding load is larger and the degree of abrupt change in local plastic deformation is stronger.

[0024] Therefore, it can be seen that the present invention is related to Figure 5 (a) shows the equal-diameter core rod scheme and Figure 5 Compared with the single-stage expansion mandrel synchronous rolling expansion scheme shown in (c), the present invention distributes the total expansion amount to multiple continuous and progressive small deformation stages through multiple expansion working sections, and makes the internal expansion and external diameter reduction of each stage proceed synchronously and in coordination, so that the material near the inner hole participates more fully in plastic deformation, disperses the single-stage forming load, reduces local strain concentration, and improves the coordination and stability of the hollow shaft forming process.

[0025] The scope of protection of this invention includes, but is not limited to, the above embodiments. The scope of protection is defined by the claims. Any substitutions, modifications, or improvements to this technology that are easily conceived by those skilled in the art fall within the scope of protection of this invention.

Claims

1. A method for synchronous and progressive axial gradient forming of a hollow shaft with a large diameter ratio and large inner hole, characterized in that: The mandrel used in the forming process is integrally equipped with a hole-expanding mandrel. The hole-expanding mandrel includes at least two hole-expanding working sections whose outer diameter increases progressively along the feed direction of the hollow shaft. The skew rolling mill is equipped with multiple sets of skew rolls, the number and position of which correspond to the hole-expanding working sections. Each hole-expanding working section and its corresponding skew roll set form a local cooperative deformation zone. The forming method includes the following specific steps: S1. Pre-treat the inner and outer surfaces of the hollow shaft blank; S2. Heat the hollow shaft blank to the predetermined forming temperature and preheat the expansion mandrel and the skew roll group; S3. Push the hollow shaft blank to move axially. When the end of the first hole-expanding working section enters the inner hole of the hollow shaft blank, the first set of skew rolling rolls simultaneously begins to skew roll the front end of the hollow shaft blank. The diameter of the first hole-expanding working section is larger than the inner hole diameter of the hollow shaft blank, realizing the internal hole expansion of the hollow shaft blank. At the same time, the first set of skew rolling rolls moves radially to externally reduce the diameter of the outer surface of the hollow shaft blank, so that the hollow shaft blank undergoes the first stage of plastic deformation under the bidirectional compression action of internal expansion and external rolling. S4. Propel the hollow shaft blank to continue moving axially in the same direction, passing through each local collaborative deformation zone in sequence, so that the hollow shaft blank undergoes multi-stage continuous plastic deformation, and control the radial diameter reduction of the hollow shaft blank during each stage of plastic deformation to decrease sequentially along the feed direction of the hollow shaft blank, and the axial hole expansion amount to decrease sequentially along the feed direction of the hollow shaft blank, until the collaborative forming of the large-diameter-ratio large-inner-hole hollow shaft is completed.

2. The method for axial gradient synchronous progressive cooperative forming of a large-diameter-ratio, large-inner-hole hollow shaft as described in claim 1, characterized in that: In step S1, the pretreatment specifically involves cleaning, degreasing, removing oxide scale and impurities from the outer surface and inner hole surface of the hollow shaft blank to reduce the adverse effects of surface defects on the forming quality during the forming process.

3. The method for axial gradient synchronous progressive cooperative forming of a large-diameter-ratio, large-inner-hole hollow shaft as described in claim 1, characterized in that: The hole expansion working section has three parts: the first hole expansion working section, the second hole expansion working section, and the third hole expansion working section. The skew roll group has three groups: the first skew roll group, the second skew roll group, and the third skew roll group.

4. The method for axial gradient synchronous progressive cooperative forming of a large-diameter-ratio, large-inner-hole hollow shaft as described in claim 3, characterized in that: During the first and second stages of plastic deformation, the first and second skew roll groups both reduce the diameter of the hollow shaft blank by the same diameter. During the third stage of plastic deformation, the third skew roll group moves radially to perform stepped shaft rolling on the outer surface of the hollow shaft blank.

5. The method for axial gradient synchronous progressive cooperative forming of a large-diameter-ratio, large-inner-hole hollow shaft as described in claim 4, characterized in that: In step S4, the radial diameter reduction is represented by the reduction of area ratio η, that is: ， in: i This indicates the stage at which the hollow shaft blank undergoes plastic deformation. i =1, 2, 3, where D0 represents the initial outer diameter of the hollow shaft blank, D1 represents the outer diameter of the hollow shaft blank after the first stage of external diameter reduction, D2 represents the outer diameter of the hollow shaft blank after the second stage of external diameter reduction, and D3 represents the minimum outer diameter of the hollow shaft blank after the third stage of external diameter reduction. The section shrinkage rates after the first, second, and third stages of external diameter reduction are represented as η1, η2, and η3, respectively, and η1>η2>η3.

6. The method for axial gradient synchronous progressive cooperative forming of a large-diameter-ratio, large-inner-hole hollow shaft as described in claim 5, characterized in that: η1 is 20%–35%, η2 is 12%–25%, and η3 is 6%–18%.

7. The method for axial gradient synchronous progressive cooperative forming of a large-diameter-ratio, large-inner-hole hollow shaft as described in claim 5, characterized in that: Let the diameter of the forming part of the first skew roll group be M1, the diameter of the forming part of the second skew roll group be M2, and the diameter of the forming part of the third skew roll group be M3. Then M1 < 6.5D1, M2 < 6.5D2, M3 < 6.5D3, and M1 > M2 > M3.

8. The method for axial gradient synchronous progressive cooperative forming of a large-diameter-ratio, large-inner-hole hollow shaft as described in claim 3, characterized in that: In step S4, the axial hole expansion amount is represented by the hole expansion ratio ξ, that is: ， Where: d0 represents the initial hole diameter of the hollow shaft blank, d1 represents the hole diameter of the hollow shaft blank after the first stage of internal hole expansion, d2 represents the hole diameter of the hollow shaft blank after the second stage of internal hole expansion, and d3 represents the hole diameter of the hollow shaft blank after the third stage of internal hole expansion. The hole expansion rates after the first, second, and third stages of internal hole expansion are represented as ξ1, ξ2, and ξ3, respectively, and ξ1>ξ2>ξ3.

9. The method for axial gradient synchronous progressive cooperative forming of a large-diameter-ratio, large-inner-hole hollow shaft as described in claim 8, characterized in that: ξ1 is 1.18 to 1.22, ξ2 is 1.15 to 1.18, and ξ3 is 1.12 to 1.

15.

10. The method for axial gradient synchronous progressive cooperative forming of a large-diameter-ratio, large-inner-hole hollow shaft as described in claim 1, characterized in that: When the front end of the hollow shaft blank moves to the predetermined position on the rolling exit side, the traction mechanism on the exit side clamps and pulls the hollow shaft blank, so that the part of the rear end of the hollow shaft blank that has not left the local cooperative deformation zone can be continuously and stably formed.

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