A hot-pressing composite precision forming process for high-temperature alloy special bolts

Abstract

The application provides a hot-pressing composite precision forming process for high-temperature alloy special bolts, which comprises the following steps: S1, gradient density pre-compaction blank preparation, high-temperature alloy pre-alloy powder is pressed into a pre-compaction blank with gradient density, so that the density of the head region of the pre-compaction blank is lower than that of the stem region; S2, hot isostatic pressing pre-compaction blank forming, the pre-compaction blank is subjected to hot isostatic pressing densification treatment to form a pre-compaction blank with a near-final shape of a bolt; and S3, partition temperature control mold preparation, the heated pre-compaction blank is placed in a three-section gradient temperature zone composite mold, the mold is divided into an independent temperature control head forming zone, a stem extrusion zone and a transition zone along the axial direction, the temperature of the head forming zone is controlled to be higher than the gamma prime phase solvus temperature of the alloy, the temperature of the stem extrusion zone is lower than that of the head forming zone, and the temperature of the transition zone is lower than that of the stem extrusion zone.

Description

Technical Field

[0001] This invention belongs to the field of special bolt processing, and specifically relates to a hot-pressing composite precision forming process for high-temperature alloy special bolts. Background Technology

[0002] Currently, high-temperature alloy special bolts are among the most critical fasteners in the connection systems of high-temperature hot-end components such as aero-engines and gas turbines, and they serve under extreme conditions such as high temperature, high stress, and alternating loads for extended periods. These bolts are mostly made of nickel-based precipitation-strengthened high-temperature alloys, such as GH4169, Waspaloy, GH4738, and GH4141.

[0003] The common process for forming the head of high-temperature alloy bolts is hot upsetting, which involves using a bar or wire rod as a blank, and after induction heating or resistance heating, axial compression through a die to upset the end of the rod into the head shape, with the rod basically not undergoing plastic deformation. Existing hot upsetting processes suffer from the following common defects: First, the process is singular. Hot upsetting is essentially uniaxial compression deformation, with large deformation at the head while the shank remains almost undeformed. This results in the shank's microstructure not being refined through forging, leading to mixed or coarse-grained microstructures after heat treatment and poor overall performance uniformity. Second, the temperature field control precision is insufficient. Induction heating or flame heating inherently suffers from wide heating zones and long transition zones, making it difficult to precisely control the temperature within the ideal forging window. Too high a temperature causes abnormal grain growth, while too low a temperature easily leads to defects such as folding and cracking. Third, the die suffers severe wear under high-temperature and high-stress conditions, and the cavity filling precision is limited under the single upsetting roughing deformation mode. Fourth, the material utilization rate is low. After forming, multiple turning passes are required to remove excess material, resulting in a utilization rate of only 70%–80%, which is economically unfeasible for high-value high-temperature alloys. Fifth, there is a lack of proactive design capabilities for bolt service performance. There is no systematic correlation between process parameters and microstructure, making it impossible to customize performance for different working conditions such as creep and fatigue.

[0004] The root cause of these shortcomings lies in the fact that existing hot upsetting processes rely on a "whole-body heating—unidirectional upsetting—uniform cooling" process path, which fails to implement differentiated deformation and cooling control for the bolt head, shank, and underhead transition zone. Consequently, it cannot actively construct a gradient microstructure and property distribution on the same bolt. Although some research has attempted to improve material utilization by preparing high-temperature alloy billets using powder metallurgy hot isostatic pressing (HIP), the goal remains overall uniformity and density, without addressing the concept of using powder methods to construct gradient structures for subsequent differentiated forming. Multi-temperature zone molds are also used in hot forming, but these are all used for isothermal or sequential forming of integral workpieces, lacking a composite forming technology that simultaneously implements differentiated temperature control and deformation for different functional areas of a single bolt part. Furthermore, existing mold cooling systems only protect the mold itself; no technical solution has yet been found that utilizes internal mold cooling channels to implement differentiated cooling paths for different areas of the workpiece to actively regulate gradient microstructure and residual stress distribution.

[0005] There is an urgent need in this field to develop a new precision forming process that can overcome the limitations of the above-mentioned single process path, realize the integrated precision forming of powder near-net-shape preparation, gradient temperature zone composite deformation and in-mold gradient heat treatment, so as to achieve the purpose of controlling the microstructure and properties of the head, shank and transition zone on the same bolt as needed, thereby fully realizing the potential of materials and meeting the increasingly demanding service requirements of high-temperature alloy special bolts for high-end equipment. Summary of the Invention

[0006] This invention proposes a hot-pressing composite precision forming process for special high-temperature alloy bolts, which solves the problem that the existing hot upsetting process adopts a single process path of overall heating, unidirectional upsetting, and uniform cooling. This makes it impossible to implement differentiated deformation and cooling control for the bolt head, shank, and underhead transition zone, resulting in the inability to actively build a gradient microstructure and property distribution on the same bolt, low material utilization, and severe mold wear.

[0007] The technical solution of this invention is achieved as follows: a hot-pressing composite precision forming process for high-temperature alloy special bolts, the process comprising the following steps: S1: Gradient density pre-pressed billet preparation, high-temperature alloy pre-alloyed powder is pressed into a pre-pressed billet with gradient density, so that the density of the head region of the pre-pressed billet is lower than the density of its rod region; S2: Hot isostatic pressing preform forming, the pre-pressed billet is subjected to hot isostatic pressing densification treatment to form a preform with a near-net-shape of a bolt; S3: Preparation of mold with zoned temperature control. The heated preform is placed in a three-section gradient temperature zone composite mold. The mold is divided into an independently temperature-controlled head forming zone, rod extrusion zone and transition zone along the axial direction. The temperature of the head forming zone is controlled to be higher than the total melting temperature of the γ′ phase of the alloy, the temperature of the rod extrusion zone is lower than the temperature of the head forming zone, and the temperature of the transition zone is lower than the temperature of the rod extrusion zone. S4: Staged composite pressure molding, the molding process goes through the first stage and the second stage in sequence: In the first stage, the upper mold applies hot pressure to the head and back pressure to the rod to form the head shape in the cavity of the head forming area; In the second stage, the back pressure is removed or reduced, and the rod material is forced to flow downward along the rod extrusion area by the continuous pressure of the upper mold. The inner wall of the cavity of the rod extrusion area is provided with spiral grooves. When the material flows through this area, it directly forms the thread pre-profile and completes the extrusion and refinement of the rod structure; S5: In-mold gradient cooling. After molding, the preform is kept in the mold. The head, rod and transition zone are subjected to differential gradient cooling with decreasing cooling rate by using the cooling channels set in each area of ​​the mold. This allows each area to obtain a microstructure corresponding to the cooling path. Then, aging heat treatment is performed.

[0008] The core technical challenge of this process lies in how to simultaneously implement differentiated temperature fields, differentiated deformation amounts, and differentiated cooling paths for different functional areas of the same part within a single mold and a single thermal cycle, without causing mutual interference between the differentiated operations.

[0009] The technology employed in this application first constructs a gradient density pre-compressed billet; then, by filling the head and rod regions of the cold isostatic pressing die with pre-alloyed powders of different particle sizes, a gradient density distribution with a low head and a high rod is formed. This gradient density provides initial conditions for differentiated deformation in subsequent forming; the lower density at the head makes compression forming easier, while the higher density at the rod reserves space for extrusion densification, resulting in a natural difference in the amount of deformation between the two regions under the same forming pressure, without the need for additional mechanical intervention.

[0010] A three-section gradient temperature zone composite mold is used for zoned temperature control. The head zone temperature is higher than the γ′ phase complete melting temperature to ensure sufficient material softening and reduce deformation resistance. The bar zone temperature is lower, allowing for greater accumulation of deformation energy during extrusion to refine grains. The transition zone temperature is the lowest and lower than the γ′ phase precipitation peak temperature, allowing this zone to accumulate deformation energy during R-angle forming and avoid dynamic recrystallization, laying the foundation for the subsequent formation of a residual compressive stress layer. The temperature difference between the three zones is maintained by an internal thermal insulation structure, ensuring no interference between them.

[0011] Decoupling of the molding process is achieved through staged compound pressurization. In the first stage, the upper mold applies pressure to the head while the rod applies back pressure, forcing the material to fill the head cavity first. In the second stage, the back pressure is removed, and the head pressure drives the rod to flow downwards through extrusion. The spiral grooves on the inner wall of the cavity in the rod extrusion zone simultaneously form the thread pre-profile while constraining the material flow. The two mechanical paths have a clear division of labor: the head is formed by thermoforming, and the rod is formed by extrusion.

[0012] Gradient cooling is implemented using multiple independent cooling channels within the mold; rapid cooling at the head produces fine reinforcing phases to maintain hardness, while slow cooling at the rod produces coarse, uniform precipitates to maintain toughness; and medium-speed cooling in the transition zone, combined with the deformation energy accumulated during the molding stage, forms a residual compressive stress layer. This allows for full-chain zoned control of the "heating-deformation-cooling" process within the same mold.

[0013] As a preferred embodiment, the gradient density in step S1 is prepared as follows: a first high-temperature alloy pre-alloyed powder with a particle size of 45~75μm is filled into the mold cavity of the corresponding head region, and a second high-temperature alloy pre-alloyed powder with a particle size of 15~45μm is filled into the mold cavity of the corresponding rod region. Then, cold isostatic pressing is performed uniformly, with a pressure range of 200 to 350MPa, and the pressure is maintained for 60 to 180 seconds, so that the powder density in the head region and the powder density in the rod region reach the theoretical density, forming the gradient density distribution with a low head and a high rod.

[0014] In a preferred embodiment, the gradient density is achieved by filling the head region of the cold isostatic pressing mold cavity with a first powder and the rod region with a second powder. The average particle size of the first powder is greater than that of the second powder, so that the density of the head region is lower than that of the rod region under the same pressing conditions. The first powder and the second powder are obtained by sieving the same batch of high-temperature alloy pre-alloyed powder, with the upper limit of sieving being the first powder and the lower limit being the second powder.

[0015] In a preferred embodiment, the transition zone is configured as a final forming surface with a lower R-angle of the bolt head, and its temperature T3 is precisely controlled within 10~30°C below the peak precipitation temperature of the alloy γ′ phase, so that the material in this region accumulates deformation storage energy during the forming process in step S4 while avoiding dynamic recrystallization; the deformation storage energy is released in the subsequent gradient cooling and final aging heat treatment in step S5, driving the γ′ phase to preferentially precipitate in the lower R-angle region, thereby forming a residual compressive stress layer.

[0016] In a preferred embodiment, the temperature T3 of the transition zone and the peak precipitation temperature Tp of the γ′ phase of the alloy satisfy the following condition: T3 < Tp, and the difference between Tp and T3 is 10~30℃.

[0017] As a preferred embodiment, after the second stage, step S4 proceeds to the third stage, where the upper mold pressure is increased to 50-100 MPa and maintained for 1-3 seconds to complete the full filling of the head cavity and the shaping of the transition zone material. At the same time, the rod is precisely shaped under the constraint of the helical groove to achieve the thread pre-contour. The total time of the first, second, and third stages does not exceed 5 seconds to ensure that all forming actions are completed within the cooling window of the high-temperature alloy.

[0018] As a preferred embodiment, the ratio of the holding pressure in the third stage to the hot pressing pressure in the first stage is 1.2 to 2.5.

[0019] In a preferred embodiment, the gradient cooling within the mold in step S5 is achieved through at least three cooling channels located inside the composite mold with three gradient temperature zones. Specifically, high-pressure inert gas is introduced into the first channel corresponding to the head, causing it to cool rapidly at a rate of 15~25℃ / s to obtain fine and dispersed γ′ reinforcing phase; low-pressure inert gas is introduced into the second channel corresponding to the rod, causing it to cool slowly at a rate of 5~10℃ / s to obtain coarse and uniformly distributed γ′ phase; and a mixture of high-pressure and low-pressure inert gas is introduced into the third channel corresponding to the transition zone, causing it to cool at a moderate rate of 10~15℃ / s, which, together with the deformation storage energy accumulated in step S3, forms residual compressive stress.

[0020] In a preferred embodiment, the cooling channels of the mold are independent gas channels, wherein the inlet cross-sectional area of ​​the cooling channel in the head region is larger than the inlet cross-sectional area of ​​the cooling channel in the transition region, and the inlet cross-sectional area of ​​the cooling channel in the transition region is larger than the inlet cross-sectional area of ​​the cooling channel in the rod region.

[0021] As a preferred embodiment, step S6 is performed after step S5: the bolt that has completed the aging heat treatment is subjected to warm thread rolling, with the rolling temperature controlled at 500~550℃ and the rolling pressure controlled at 800~1200N, so as to finally form the thread based on the thread pre-profile and further increase the residual compressive stress at the root of the thread; at the same time, during the warm thread rolling process, the existing residual compressive stress layer in the transition zone is used as a stress barrier to prevent the stress concentration at the root of the thread from extending to the R-angle area below the head.

[0022] The beneficial effects of this invention after adopting the above technical solution are: it realizes the proactive design of bolt gradient performance. Through the coordinated operation of four links—powder gradient density, zoned temperature control, staged pressure application, and gradient cooling—high head hardness (rapid cooling to obtain fine γ′ reinforcing phase), high shank toughness (slow cooling to obtain uniform precipitated phase), and residual compressive stress in the transition zone under the head (synergistic effect of medium-speed cooling and deformation energy) are simultaneously achieved on the same bolt. The microstructure and mechanical properties of each region are customized according to service requirements, fundamentally changing the homogenized performance distribution mode of traditional processes.

[0023] Near-net-shape preforms are prepared by hot isostatic pressing of pre-alloyed powders, combined with near-net-shape forming in the mold, which significantly reduces machining allowance and improves material utilization. This is a significant improvement over the traditional hot upsetting process and has important economic significance for high-value high-temperature alloys. This significantly shortens the process flow. The bar extrusion and thread pre-profile forming are integrated into one step. While the bar material flows along the extrusion channel, the thread pre-profile is simultaneously formed by the spiral grooves on the inner wall of the channel. This eliminates the need for separate machining of the thread blank diameter and thread rolling after upsetting in the traditional process, thus achieving a substantial merger of processes.

[0024] It improves the working conditions and lifespan of the mold. In the three-stage gradient temperature zone composite mold, each temperature zone is independently controlled, and each area of ​​the mold is always in a relatively stable temperature field. This avoids the severe thermal shock and thermal fatigue caused by repeated contact with high-temperature blanks in traditional hot forging molds, and effectively extends the service life of the mold.

[0025] This process improves the uniformity of the microstructure and product consistency. The rod section undergoes extrusion deformation to achieve refined grains, while the head is hot-pressed to maintain continuous grain flow lines along the overall shape, resulting in a uniform and dense microstructure. All process parameters are precisely controlled by the equipment, ensuring excellent batch-to-batch product consistency and meeting the stringent reliability requirements of aerospace fasteners. Attached Figure Description

[0026] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0027] Figure 1 This is a process flow diagram of the present invention; Figure 2 This is a schematic diagram comparing the powder particle size and compact density of the present invention; Figure 3 This is a schematic diagram illustrating the increase in cold isostatic pressure intensity in Embodiment 2 of the present invention; Figure 4 This is a schematic diagram of the γ′ phase in 200 μm according to the present invention. Detailed Implementation

[0028] 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.

[0029] Example 1

[0030] like Figures 1-4 As shown, a hot-pressing composite precision forming process for special high-temperature alloy bolts includes the following steps: S1: Gradient density pre-pressed billet preparation, high-temperature alloy pre-alloyed powder is pressed into a pre-pressed billet with gradient density, so that the density of the head region of the pre-pressed billet is lower than the density of its rod region; S2: Hot isostatic pressing preform forming, the pre-pressed billet is subjected to hot isostatic pressing densification treatment to form a preform with a near-net-shape of a bolt; S3: Preparation of mold with zoned temperature control. The heated preform is placed in a three-section gradient temperature zone composite mold. The mold is divided into an independently temperature-controlled head forming zone, rod extrusion zone and transition zone along the axial direction. The temperature of the head forming zone is controlled to be higher than the total melting temperature of the γ′ phase of the alloy, the temperature of the rod extrusion zone is lower than the temperature of the head forming zone, and the temperature of the transition zone is lower than the temperature of the rod extrusion zone. S4: Staged composite pressure molding, the molding process goes through the first stage and the second stage in sequence: In the first stage, the upper mold applies hot pressure to the head and back pressure to the rod to form the head shape in the cavity of the head forming area; In the second stage, the back pressure is removed or reduced, and the rod material is forced to flow downward along the rod extrusion area by the continuous pressure of the upper mold. The inner wall of the cavity of the rod extrusion area is provided with spiral grooves. When the material flows through this area, it directly forms the thread pre-profile and completes the extrusion and refinement of the rod structure; S5: In-mold gradient cooling. After molding, the preform is kept in the mold. The head, rod and transition zone are subjected to differential gradient cooling with decreasing cooling rate by using the cooling channels set in each area of ​​the mold. This allows each area to obtain a microstructure corresponding to the cooling path. Then, aging heat treatment is performed.

[0031] The core equipment used in this application is a three-section gradient temperature zone composite mold, which is mounted on a hot press. The mold is divided into three functional temperature zones axially from top to bottom: a head forming zone, a transition zone, and a rod extrusion zone. The head forming zone is located in the upper section of the mold. Its cavity has a hexagonal bolt head final shape profile, surrounded by resistance heating elements, and has a first independent cooling channel whose inlet is connected to a high-pressure inert gas source. The rod extrusion zone is located in the lower section of the mold. Its cavity is a variable-diameter extrusion channel, with helical grooves machined on the inner wall of the channel matching the pitch of the thread to be formed. An induction heating coil is wound around the outside of this area, and a second independent cooling channel is provided, whose inlet is connected to a low-pressure inert gas source. The transition zone is located between the head forming zone and the rod extrusion zone. Its surface has a bolt head lower R-angle final shape profile. This area has a third independent cooling channel, whose inlet is connected to both a high-pressure gas source and a low-pressure gas source via a gas mixing valve. The cooling gas parameters entering this channel can be controlled by adjusting the mixing ratio. Thermal insulation rings are installed between the three temperature zones for thermal isolation. The temperature of each zone is monitored in real time by independent thermocouples and regulated in a closed loop by a PID controller, with a temperature control accuracy of ±5℃. The upper die is installed below the press slide and can move up and down axially under the drive of the press. A back pressure push rod is provided at the bottom of the rod extrusion zone, driven by an independent hydraulic cylinder, which can apply controllable upward or downward back pressure to the rod.

[0032] The process flow of this embodiment is as follows.

[0033] Step 1: Preparation of Gradient Density Pre-compressed Billets. GH4169 pre-alloyed powder obtained from the same batch of master alloy smelting through argon atomization was sieved using a standard sieve. Powder retained on a 325-mesh sieve (approximately 45 μm aperture) or larger was designated as the first powder, with a particle size range of 45–75 μm; powder passing through the 325-mesh sieve but retained on a 500-mesh sieve (approximately 25 μm aperture) or larger was designated as the second powder, with a particle size range of 25–45 μm. The first powder was filled into the mold cavity corresponding to the bolt head area in the cold isostatic pressing mold cavity, and the second powder was filled into the mold cavity corresponding to the rod area. After compaction, the mold was sealed and pressed in a cold isostatic press at 300 MPa for 120 seconds. Because the particle size of the first powder is larger than that of the second powder, its bulk density is lower under the same pressing conditions. Therefore, the density of the head region of the obtained pre-compressed billet is approximately 77% of the theoretical density, and the density of the rod region is approximately 83% of the theoretical density, forming a gradient density distribution with a low head and a high rod. This gradient density provides the initial conditions for subsequent staged forming: the lower density at the head makes it easier to compress and form to the final shape during the S4 hot pressing stage; the higher density in the rod reserves space for extrusion densification, allowing the rod to undergo greater plastic deformation during the extrusion stage, which is beneficial for grain refinement.

[0034] Step 2: Hot isostatic pressing (HIP) preform forming. The preform obtained in Step 1 is placed into a low-carbon steel sleeve, and a vacuum is applied to 10... -3 Electron beam welding was performed to seal the preform after Pa treatment. The sealing sleeve was placed in a hot isostatic pressing furnace and heated to 1120℃ at a rate of 15℃ / min (approximately 30℃ lower than the complete melting temperature of the γ′ phase of GH4169). Argon gas was introduced and pressurized to 150MPa, and the temperature and pressure were maintained for 3 hours, after which the preform was slowly cooled to room temperature in the furnace. After hot isostatic pressing, all regions of the preform were completely dense, with a density of not less than 99.5%. With the sleeve still attached, the obtained preform was rough machined to remove the sleeve and simultaneously machined to a near-net bolt shape with uniform machining allowance, with the allowance on one side controlled between 0.3 and 0.8 mm, thus obtaining the preform.

[0035] Step 3: Preparation of the temperature-controlled mold. Place the preform in a heating furnace and heat to 1050℃, maintaining a uniform temperature throughout. Simultaneously, activate the temperature control system of the three-stage gradient temperature zone composite mold: The resistance heating element in the head forming zone is energized, with a target temperature T1 set at 1030℃, higher than the complete solution temperature of the γ′ phase of GH4169 (approximately 980–1020℃), ensuring the material in this zone is in a fully dissolved state during subsequent forming, minimizing deformation resistance; the induction heating coil in the rod extrusion zone is energized, with a target temperature T2 set at 990℃, slightly lower than T1, allowing the rod material to accumulate more deformation energy during extrusion; the temperature T3 in the transition zone is set at 920℃, lower than the peak precipitation temperature range of the γ′ phase of GH4169 (approximately 940–980℃), with a difference of approximately 20℃, ensuring the transition zone is in a sub-solid solution state during subsequent forming, preventing the deformation energy accumulated during deformation from dissipating due to dynamic recrystallization. Once each temperature zone reaches the set temperature, the heated preform is quickly transferred into the mold cavity, and the mold is closed.

[0036] Step 4: Staged composite pressure molding. This step is completed through the coordinated operation of the upper mold, back pressure ejector pin, and three-section temperature zone mold, and the entire process takes no more than 5 seconds.

[0037] Phase 1 (0-0.5 seconds): The press drives the upper die to move downwards at a set speed, applying a hot pressing pressure of 35 MPa to the head of the preform. Simultaneously, the back pressure pusher at the bottom of the rod extrusion zone applies a back pressure of 30 MPa upwards under the drive of a hydraulic cylinder. At this point, the preform material is subjected to the combined action of pressure from above and back pressure from below. The rod material is constrained and cannot flow downwards. The upper die pressure forces the material to preferentially fill the cavity of the head forming area, and the hexagonal head contour of the bolt is basically formed in this stage. The material in the transition zone acquires a preliminary shape under the constraint of the die's R-angle profile.

[0038] Second stage (0.5–2.0 seconds): After the head cavity is filled, the hydraulic cylinder of the back pressure ejector switch to overflow mode, and the back pressure rapidly decreases to approximately 10 MPa. At this point, the pressure continuously applied by the upper die is no longer balanced by the back pressure, and the excess load drives the rod material to break through the back pressure constraint and flow downwards along the variable diameter channel of the rod extrusion zone. During the flow of the rod material along the extrusion channel, the pre-machined spiral grooves on the inner wall of the channel form a geometric constraint on the flowing material, and the material is forced to form a thread pre-profile corresponding to the shape of the groove when flowing through this zone. At the same time, the rod material undergoes severe plastic deformation in the extrusion channel, and the original grains are fully broken and refined. The R-angle final forming is completed in the lower head transition zone at this stage. Since this zone is in a temperature range below the γ′ phase precipitation peak, the deformation stored energy accumulated during the forming process is retained.

[0039] The third stage (2.0–4.5 seconds): The upper die pressure is increased to 80 MPa for pressure holding and shaping. This high-pressure holding stage ensures complete filling of the head cavity, and the R-angle of the transition zone achieves the final shape accuracy after high-pressure shaping. The thread pre-profile of the rod is precisely shaped under the constraint of the helical groove. After the pressure holding period, the press slide stops moving, and the forming stage is complete.

[0040] Step 5: Gradient Cooling within the Mold. After molding, the preform remains within the mold cavity. At this point, the independent cooling channels for each zone are activated: High-pressure nitrogen is introduced into the first cooling channel of the head forming zone, rapidly cooling the head at a rate of approximately 20°C / s. This rapid cooling causes the precipitation of fine, dispersed γ′ and γ″ reinforcing phases in the γ phase matrix of the head, imparting high hardness. Low-pressure nitrogen is introduced into the second cooling channel of the bar extrusion zone, slowly cooling the bar at a rate of approximately 8°C / s. This slow cooling provides sufficient time for the uniform precipitation and proper growth of the γ′ phase, resulting in good toughness in the bar. A gas mixture of high-pressure and low-pressure nitrogen is introduced into the third cooling channel of the transition zone, cooling the transition zone at a moderate rate of approximately 12°C / s. This moderate cooling rate, combined with the deformation storage energy accumulated in Steps 3 and 4, spontaneously generates a residual compressive stress layer on the surface of the lower R-corner region of the head. This compressive stress layer has a depth of approximately 50–150 μm and an amplitude of approximately 200–400 MPa.

[0041] Once the temperature in each zone has dropped below 600℃, the bolts are removed from the mold and air-cooled to room temperature. The bolts then undergo a two-stage aging heat treatment: holding at 720℃ for 8 hours, furnace cooling to 620℃ for 8 hours, and air cooling to room temperature. This aging treatment allows the γ′ and γ″ phases to fully separate and grow to their optimal size within their respective suitable temperature ranges, thus completing the heat treatment process for the finished bolts.

[0042] Step Six: Final Finishing. The heat-treated bolts undergo precision grinding of the shank, removing approximately 0.15mm of excess material on each side to meet the blank diameter requirements before thread rolling. Then, warm thread rolling is performed at 525℃ with a rolling pressure of approximately 1000N. The thread is ultimately formed based on the pre-defined thread profile, further increasing the residual compressive stress at the thread root.

[0043] Subsequently, after final molding, the performance of GH4169 M12 hex head bolts was compared with that of bolts of the same specification produced by traditional hot forging process, and Tables 1, 2 and 3 were obtained; Table 1 shows a comparison of room temperature mechanical properties as follows:

[0044] Table 1 Comparison of mechanical properties at room temperature The high-temperature alloy special bolts prepared using the process described in this application exhibit significantly superior head hardness, tensile strength, yield strength, and fatigue life compared to products manufactured using traditional hot forging processes. A beneficial residual compressive stress layer is formed at the underside radius, and the grain size in the shank is significantly refined, resulting in a substantial improvement in material utilization. These performance advantages are attributed to the synergistic effect of various technical features throughout the entire process, including gradient density pre-compressed billets, three-stage gradient temperature zone composite molds, staged composite pressure forming, and gradient cooling within the mold. Table 2 was obtained under fatigue test conditions where the maximum stress was 900 MPa and the ratio of the minimum stress to the maximum stress was R = 0.1 during cyclic loading.

[0045] Table 2 Comparison of Fatigue Performance Example 2

[0046] like Figure 2-4 As shown, the overall structure of the three-section gradient temperature zone composite mold used in this embodiment is the same as that in Embodiment 1. Here, we only provide supplementary explanations on the design basis of its key structural parameters. The ratio of the inlet cross-sectional area of ​​the cooling air duct in the head forming area, the inlet cross-sectional area of ​​the cooling air duct in the transition area, and the inlet cross-sectional area of ​​the cooling air duct in the rod area is set to 3:2:1 (this ratio is obtained by first calculating the required gas convection heat transfer coefficient based on the target cooling rates of each area—approximately 20℃ / s for the head, approximately 12℃ / s for the transition area, and approximately 8℃ / s for the rod—and then rounding it to obtain this cross-sectional area ratio). The resistance heating wire in the head forming module is a nickel-chromium alloy wire with a single wire power density of 8W / cm². This parameter is determined through finite element thermal simulation. In the simulation model, the ambient temperature of the mold outer wall is set to 25℃, and the target temperature of the cavity surface is set to 1030℃. Using the heating wire power density as an iterative variable, the power density value corresponding to the cavity surface temperature uniformity deviation is solved until it is less than ±5℃. The induction heating coil on the outer periphery of the rod extrusion module is a rectangular cross-section copper tube with 6 turns. The gap between the inner diameter of the coil and the outer diameter of the extrusion module is 8mm. This gap is determined by electromagnetic-thermal coupling simulation. The optimization objective is to achieve an inner wall temperature of 990℃ in the extrusion channel and a temperature deviation along the axial direction of no more than ±10℃. The coil arrangement parameters are solved iteratively.

[0047] The transition zone forming module has an annular transition surface corresponding to the lower radius of the bolt head. This surface tapers from top to bottom to the rod diameter, and its radius of curvature matches the lower radius of the bolt head specified in the product drawings. The distance between the inner wall of the transition zone forming module and the cavity surface is 5mm. This distance is smaller than the corresponding distances of the head forming module and the rod extrusion module, so that the thermocouple in the transition zone responds more sensitively and the temperature control is more precise.

[0048] The particle size range of the first powder was selected as 45–75 μm, and the particle size range of the second powder was selected as 15–45 μm. The above particle size ranges were determined by the following experiments: GH4169 pre-alloyed powder from the same atomization batch was taken and sieved into three particle size ranges of 15–45 μm, 45–75 μm, and 75–105 μm. Each group of powder was cold isostatically pressed separately under a pressure of 300 MPa. The measured compact densities were 84%, 78%, and 71% of the theoretical density, respectively. Two groups with a density difference of about 5–7 percentage points, namely the 15–45 μm group and the 45–75 μm group, were selected as the filler powders for the rod region and the head region. The cold isostatic pressing pressure was set at 300 MPa, and the holding time was set at 120 seconds. This parameter was determined through comparative experiments on the strength of the compact under different pressure levels: powders with the same particle size distribution were pressed at 200 MPa, 250 MPa, 300 MPa, and 350 MPa, and the bending strength of the compact was tested. When the pressure increased from 250 MPa to 300 MPa, the strength increase was 18%, while when it increased from 300 MPa to 350 MPa, the strength increase was only 3%. Considering both equipment energy consumption and compact strength, 300 MPa was selected. The holding time was set at 120 seconds, based on the point when the compact density no longer increased significantly.

[0049] The hot isostatic pressing (HIP) temperature was chosen to be 1120℃. This temperature was determined as follows: the complete melting temperature of the γ′ phase in GH4169 alloy, measured by differential scanning calorimetry, was approximately 1020–1050℃. Following the principle of selecting a temperature 30–50℃ below the complete melting temperature, 1120℃ was chosen to achieve the optimal balance between the billet densification rate and grain growth rate. Above 1150℃, grain growth begins significantly, and below 1100℃, the time required for complete densification exceeds 5 hours. The HIP pressure was chosen to be 150 MPa. This was chosen because it represents the minimum pressure required to achieve a density of not less than 99.5% in the pre-pressed billet within 3 hours at 1120℃. This was determined by comparing densification curves under different pressure conditions.

[0050] The head forming zone temperature T1 was set to 1030℃, and a high-temperature compression test was conducted on the GH4169 alloy at a strain rate of 0.1 s⁻¹. -1 Rheological stress was tested at different temperatures under various conditions. The results showed that when the temperature exceeded 1020℃, the rheological stress decreased to below 100MPa, a decrease of about 40% compared to 980℃. 1030℃ was chosen as the forming temperature. The extrusion zone temperature T2 of the rod was set at 990℃, which is about 40℃ lower than T1. Through hot compression experiments, it was determined that the material rheological stress at 990℃ is about 1.3 times that at 1030℃. This difference is sufficient to allow the rod to accumulate more deformation storage energy during the second stage of extrusion, while avoiding excessive temperature that could cause adhesion on the extrusion channel surface. The transition zone temperature T3 was set at 920℃. The peak precipitation temperature range of the γ′ phase of the GH4169 alloy was determined by differential scanning calorimetry to be 940~980℃. T3 was set at 920℃, with a difference of about 20℃ from Tp. This difference falls within the range of 10~30℃, ensuring that the transition zone is in a subsolid state to accumulate deformation storage energy, while avoiding insufficient material plasticity to complete the R-angle filling due to excessively low temperatures.

[0051] The first-stage hot-pressing pressure is 35 MPa, and the first-stage back pressure is 30 MPa. These values ​​were determined as follows: A finite element model of the mold cavity was established, with the goal of completely filling the cavity with the blank head. The minimum hot-pressing pressure required under the conditions of T1=1030℃ and friction coefficient 0.3 was calculated to be 32 MPa, and after taking the engineering margin, it was set to 35 MPa. The back pressure value was set to 85% of the hot-pressing pressure to ensure that the rod does not flow downward while the material fills the head first. Through molding experiments at different back pressure levels, it was verified that when the back pressure is lower than 80% of the hot-pressing pressure, the rod begins to flow downward in the first stage. When the back pressure is higher than 90%, the energy consumption of the upper mold increases significantly while the head filling efficiency does not improve significantly. Therefore, 85%, or 30 MPa, was taken.

[0052] The third-stage holding pressure is 80 MPa, which is approximately 2.3 times the first-stage hot-pressing pressure of 35 MPa. (This ratio was determined by conducting pressure replenishment experiments on the semi-finished billet after the second stage, measuring the head cavity filling rate after holding pressure, and finding that the filling rate reached over 99.5% when the holding pressure exceeded 70 MPa. 80 MPa was used to obtain a reliable safety margin.) The total duration of the three stages is approximately 4.2 seconds. The natural cooling curve of the preform at 1030℃ was recorded using a high-speed camera. The time it took for the billet center temperature to drop from 1050℃ to the forging termination temperature of approximately 930℃ was measured to be 7–8 seconds. To ensure a safety margin within the cooling window, the total forming time was controlled to within 5 seconds.

[0053] The cooling rate at the head is approximately 20℃ / s, the cooling rate in the transition zone is approximately 12℃ / s, and the cooling rate at the rod is approximately 8℃ / s. These cooling rates were determined as follows: Experiments were conducted on the continuous cooling transformation behavior of the GH4169 alloy at different cooling rates, and continuous cooling transformation curves were plotted. At a cooling rate of 20℃ / s, the γ′ phase precipitate size was mainly concentrated in the range of 10–30 nm, corresponding to a hardness of 42–45 HRC. At a cooling rate of 8℃ / s, the γ′ phase precipitate size was 50–100 nm, corresponding to a hardness of 38–41 HRC and an elongation greater than 14%. At a cooling rate of 12℃ / s, the precipitate size distribution was between the two, and this cooling rate also resulted in a residual compressive stress of approximately 300–400 MPa on the surface of the transition zone. Based on this, the target values ​​for the differentiated cooling rates in the three zones were determined. The aging heat treatment regime is 720℃ for 8 hours and furnace cooling to 620℃ for 8 hours. This regime is the standard two-stage aging regime for GH4169 alloy, obtained through literature records and the manufacturer's recommended process manual.

[0054] The thread rolling temperature was 525℃. Stress relaxation experiments on GH4169 alloy at this temperature determined that: at 525℃, the stress relaxation rate of the material was moderate, and the retention rate of residual compressive stress at the root of the thread after rolling was relatively high; above 550℃, the residual compressive stress relaxation was significant; below 500℃, the material's deformation resistance was too high, leading to accelerated wear of the thread rolling wheel. The thread rolling pressure was 1000N; this value is the experimentally measured value of the pressure required to completely fill the thread pre-profile to the standard thread profile at this temperature.

[0055] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A hot-pressing composite precision forming process for high-temperature alloy special bolts, characterized in that, The process includes the following steps: S1: Gradient density pre-pressed billet preparation, high-temperature alloy pre-alloyed powder is pressed into a pre-pressed billet with gradient density, so that the density of the head region of the pre-pressed billet is lower than the density of its rod region; S2: Hot isostatic pressing preform forming, the pre-pressed billet is subjected to hot isostatic pressing densification treatment to form a preform with a near-net-shape of a bolt; S3: Preparation of mold with zoned temperature control. The heated preform is placed in a three-section gradient temperature zone composite mold. The mold is divided into an independently temperature-controlled head forming zone, rod extrusion zone and transition zone along the axial direction. The temperature of the head forming zone is controlled to be higher than the total melting temperature of the γ′ phase of the alloy, the temperature of the rod extrusion zone is lower than the temperature of the head forming zone, and the temperature of the transition zone is lower than the temperature of the rod extrusion zone. S4: Staged composite pressure molding, the molding process goes through the first stage and the second stage in sequence: In the first stage, the upper mold applies hot pressure to the head and back pressure to the rod to form the head shape in the cavity of the head forming area; In the second stage, the back pressure is removed or reduced, and the rod material is forced to flow downward along the rod extrusion area by the continuous pressure of the upper mold. The inner wall of the cavity of the rod extrusion area is provided with spiral grooves. When the material flows through this area, it directly forms the thread pre-profile and completes the extrusion and refinement of the rod structure; S5: In-mold gradient cooling. After molding, the preform is kept in the mold. The head, rod and transition zone are subjected to differential gradient cooling with decreasing cooling rate by using the cooling channels set in each area of ​​the mold. This allows each area to obtain a microstructure corresponding to the cooling path. Then, aging heat treatment is performed.

2. The hot-pressing composite precision forming process for high-temperature alloy special bolts as described in claim 1, characterized in that: The gradient density in step S1 is prepared as follows: a first high-temperature alloy pre-alloy powder with a particle size of 45~75μm is filled into the mold cavity of the corresponding head region, and a second high-temperature alloy pre-alloy powder with a particle size of 15~45μm is filled into the mold cavity of the corresponding rod region. Then, cold isostatic pressing is performed uniformly, with a pressure range of 200 to 350MPa, and the pressure is maintained for 60 to 180 seconds, so that the powder density in the head region and the powder density in the rod region reach the theoretical density, forming the gradient density distribution with a low head and a high rod.

3. The hot-pressing composite precision forming process for high-temperature alloy special bolts as described in claim 2, characterized in that: The gradient density is achieved by filling the head region of the cold isostatic pressing mold cavity with a first powder and the rod region with a second powder. The average particle size of the first powder is greater than that of the second powder, so that the density of the head region is lower than that of the rod region under the same pressing conditions. The first powder and the second powder are obtained by sieving the same batch of high-temperature alloy pre-alloyed powder, with the upper limit of sieving being the first powder and the lower limit being the second powder.

4. The hot-pressing composite precision forming process for high-temperature alloy special bolts as described in claim 1, characterized in that: The transition zone is configured as a final forming surface with a lower R-angle of the bolt head, and its temperature T3 is precisely controlled within 10~30℃ below the peak precipitation temperature of the alloy γ′ phase, so that the material in this region can accumulate deformation storage energy during the forming process in step S4 while avoiding dynamic recrystallization; the deformation storage energy is released in the subsequent gradient cooling and final aging heat treatment in step S5, driving the γ′ phase to preferentially precipitate in the lower R-angle region, thereby forming a residual compressive stress layer.

5. The hot-pressing composite precision forming process for high-temperature alloy special bolts as described in claim 4, characterized in that: The transition zone temperature T3 and the peak precipitation temperature Tp of the γ′ phase in the alloy satisfy the following condition: T3 < Tp, and the difference between Tp and T3 is 10~30℃.

6. The hot-pressing composite precision forming process for high-temperature alloy special bolts as described in claim 1, characterized in that: Step S4 proceeds to the third stage after the second stage, increasing the upper mold pressure to 50~100MPa and maintaining the pressure for 1~3 seconds to complete the full filling of the head cavity and the shaping of the transition zone material. At the same time, the rod is precisely shaped under the constraint of the helical groove to achieve the thread pre-contour. The total time of the first, second and third stages shall not exceed 5 seconds to ensure that all forming actions are completed within the cooling window of the high-temperature alloy.

7. The hot-pressing composite precision forming process for high-temperature alloy special bolts as described in claim 6, characterized in that: The ratio of the holding pressure in the third stage to the hot pressing pressure in the first stage is 1.2 to 2.

5.

8. The hot-pressing composite precision forming process for high-temperature alloy special bolts as described in claim 1, characterized in that: In step S5, the gradient cooling inside the mold is achieved through at least three cooling channels set inside the composite mold with three gradient temperature zones. Specifically, high-pressure inert gas is introduced into the first channel corresponding to the head, which cools it rapidly at a rate of 15~25℃ / s to obtain fine and dispersed γ′ strengthening phase; low-pressure inert gas is introduced into the second channel corresponding to the rod, which cools it slowly at a rate of 5~10℃ / s to obtain coarse and uniformly distributed γ′ phase; and a mixture of high-pressure and low-pressure inert gas is introduced into the third channel corresponding to the transition zone, which cools it at a moderate rate of 10~15℃ / s, and works together with the deformation storage energy accumulated in step S3 to form residual compressive stress.

9. The hot-pressing composite precision forming process for high-temperature alloy special bolts as described in claim 1, characterized in that: The cooling channels of the mold are independent gas channels. The inlet cross-sectional area of ​​the cooling channel in the head area is larger than that of the cooling channel in the transition area, and the inlet cross-sectional area of ​​the cooling channel in the transition area is larger than that of the cooling channel in the rod area.

10. The hot-pressing composite precision forming process for high-temperature alloy special bolts as described in claim 1, characterized in that: After step S5, proceed to step S6: perform warm thread rolling on the bolts that have completed the aging heat treatment. The thread rolling temperature is controlled at 500~550℃ and the thread rolling pressure is controlled at 800~1200N. This is to finally form the thread based on the thread pre-contour and further increase the residual compressive stress at the thread root. At the same time, during the warm thread rolling process, the existing residual compressive stress layer in the transition zone is used as a stress barrier to prevent the stress concentration at the thread root from extending to the R-angle area below the head.

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