A cold extrusion combined strengthening method for an alloy assembly hole

Abstract

The application discloses a cold extrusion combined reinforcing method for an alloy assembly hole, and the method comprises the following steps: machining an assembly hole with a preset hole diameter on an aluminum alloy plate; performing first cold extrusion on the assembly hole along a first direction by using a core rod with a split sleeve; and performing second cold extrusion on the assembly hole after the first cold extrusion along a second direction by using the core rod with the split sleeve. The application can improve the fatigue strength, surface quality and service stability of the assembly hole.

Description

Technical Field

[0001] This application belongs to the field of alloy material assembly technology, and specifically relates to a cold extrusion combined strengthening method for alloy assembly holes. Background Technology

[0002] In the aerospace and high-end manufacturing fields, alloy materials are widely used in structural connectors, and the strength and fatigue performance of their assembly holes have a critical impact on the service life of the overall structure. Traditional assembly hole processing methods, such as drilling and reaming, can easily introduce microcracks, residual tensile stress, or rough surfaces into the hole walls, becoming crack initiation sources and seriously affecting the fatigue life and service stability of the assembly holes.

[0003] While existing methods have some effect in improving the strength of assembly holes, they generally suffer from problems such as uneven deformation, limited residual stress distribution, and difficulty in process control, making it difficult to balance strengthening efficiency and hole diameter accuracy.

[0004] Therefore, there is an urgent need for a stable process, a significant strengthening effect, and a method for strengthening assembly holes in high-strength alloy materials to improve the overall service performance of key structural components. Summary of the Invention

[0005] In view of the shortcomings of the prior art, the purpose of this application is to provide a cold extrusion combined strengthening method for alloy assembly holes, which aims to improve the fatigue strength, surface quality and service stability of the assembly holes.

[0006] To achieve the above objectives, this application provides the following technical solution: A cold extrusion combined strengthening method for alloy assembly holes, the method comprising: processing an assembly hole with a preset diameter on an aluminum alloy plate; performing a first cold extrusion on the assembly hole along a first direction using a mandrel with a slotted bushing; and performing a second cold extrusion on the assembly hole after the first cold extrusion using a mandrel with a slotted bushing along a second direction.

[0007] Optionally, the step of cold-pressing the mandrel with the slotted bushing into the assembly hole along the first direction includes: advancing the mandrel at a low speed along the first direction until the bushing is inserted into a first preset position at the depth of the assembly hole; and continuing to advance the mandrel at a high speed along the first direction until the mandrel has completely passed through the bushing.

[0008] Optionally, during the advance of the mandrel in the first direction, the mandrel rotates clockwise or counterclockwise at a first angular velocity.

[0009] Optionally, the step of performing a second cold extrusion on the assembly hole after the first cold extrusion along the second direction with the mandrel with the slotted bushing includes: advancing the mandrel at a low speed along the second direction until the bushing is inserted into the second preset position of the assembly hole depth; and continuing to advance the mandrel at a high speed along the second direction until the mandrel completely passes through the bushing.

[0010] Optionally, during the advancement of the mandrel in the second direction, the mandrel rotates counterclockwise or clockwise at a second angular velocity.

[0011] Optionally, the primary cold extrusion and secondary cold extrusion are performed according to a preset extrusion strengthening amount.

[0012] Optionally, the primary cold extrusion and the secondary cold extrusion are performed with a 7:3 ratio of extrusion strengthening.

[0013] Optionally, the slotted bushing is coated with a dry film lubricant.

[0014] Optionally, the method further includes: performing micro-shot peening on the assembly holes after secondary cold extrusion.

[0015] Optionally, the method further includes: rolling the assembly holes after micro-shot peening.

[0016] Compared with the prior art, the beneficial effects of this application are as follows: This application utilizes a directional and rotationally controlled primary and secondary cold extrusion process for alloy assembly holes. This process effectively improves the fatigue strength, corrosion resistance, and surface quality of the assembly holes while simultaneously achieving densification of the hole walls and uniform stress distribution. The method employs different advance directions and rotational angular velocities to create staggered helical strain paths, significantly improving the microstructure of the hole walls, inhibiting crack initiation and propagation, and enhancing the stability and coverage depth of the residual compressive stress layer. Combined with micro-shot peening and rolling, the orifice surface finish and service stability are further optimized, resulting in a high-performance assembly hole processing technology that offers uniform strengthening, high process controllability, and applicability to high-strength alloy materials. Attached Figure Description

[0017] Figure 1 This is a schematic flowchart of a cold extrusion combined strengthening method for alloy assembly holes provided in one embodiment of this application; Figure 2 This is a front view of a cold extrusion of an assembly hole according to an embodiment of this application; Figure 3 This is a top view of a single cold extrusion of an assembly hole provided in one embodiment of this application; Figure 4 This is a front view of a secondary cold extrusion of an assembly hole provided in one embodiment of this application; Figure 5 This is a top view of a secondary cold extrusion of an assembly hole provided in one embodiment of this application. Detailed Implementation

[0018] Specific embodiments of this application will now be described in detail with reference to the accompanying drawings. While specific embodiments of this application are shown in the drawings, it should be understood that this application can be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided to enable a more thorough understanding of this application and to fully convey the scope of this application to those skilled in the art.

[0019] It should be noted that certain terms are used in the specification and claims to refer to specific components. Those skilled in the art will understand that different terms may be used to refer to the same component. This specification and claims do not distinguish components based on differences in terminology, but rather on differences in function. The terms "comprising" or "including" used throughout the specification and claims are open-ended and should be interpreted as "comprising but not limited to." The following descriptions in the specification are preferred embodiments for carrying out this application; however, these descriptions are for the purpose of understanding the general principles of the specification and are not intended to limit the scope of this application. The scope of protection of this application shall be determined by the appended claims.

[0020] To facilitate understanding of the embodiments of this application, the following will provide further explanation and description with reference to the accompanying drawings and specific embodiments, and the accompanying drawings do not constitute a limitation on the embodiments of this application.

[0021] Figure 1 This is a schematic flowchart of a cold extrusion combined strengthening method for alloy assembly holes provided in one embodiment of this application, as shown below. Figure 1 As shown, the method includes the following steps: S100: Assembly holes with a preset diameter are machined on an aluminum alloy plate; S200: The mandrel with the slotted bushing is cold-extruded into the assembly hole in the first direction once; S300: The mandrel with a slotted bushing is used to perform a second cold extrusion on the assembly hole after the first cold extrusion along the second direction.

[0022] This embodiment achieves bidirectional plastic deformation and stress redistribution of the assembly hole wall without compromising the hole shape accuracy by pre-forming assembly holes on the aluminum alloy plate and performing cold extrusion in opposite directions. This improves the fatigue strength, residual compressive stress uniformity, and service life of the assembly holes.

[0023] In another exemplary embodiment, step S200, which involves cold-pressing the mandrel with the slotted bushing into the assembly hole along a first direction, includes the following steps: S201: As Figure 2 and Figure 3 As shown, the mandrel is advanced at a low speed along the first direction until the bushing is inserted into a first preset position of the mounting hole depth (e.g., at 60% to 70% of the mounting hole depth). In this step, the bushing is first fitted onto the mandrel and slid forward until it fits snugly against the flanged end of the top sleeve, ensuring assembly integrity. Next, the mandrel with the bushing is slowly inserted into the assembly hole from one side (the preset hole diameter is 5.72mm). At this point, the end of the plate hole naturally contacts the root of the bushing flange, forming initial positioning. The workpiece plate is fixed on both sides by a special fixture to prevent positional shift during processing. Then, the pull-gun device is activated, controlling the mandrel to slowly advance at a low speed (e.g., 0.5mm / s~1.0mm / s) in the first direction (e.g., perpendicular to the side of the assembly hole facing the pull-gun device). During this stage, the bushing gradually inserts into the assembly hole, with the insertion depth controlled at 60%~70% of the total length of the assembly hole, achieving initial strengthening of the hole wall and surface compaction through plastic deformation. Finally, at the end of the low-speed stage, the mandrel continues to push forward until it completely passes through the bushing, achieving a radial plastic expansion of approximately 0.08 mm. After the extrusion pressure is released, the bushing automatically detaches due to its own elasticity. At this point, the inner diameter of the assembly hole has been expanded from the original 5.72 mm to approximately 5.80 mm, marking the initial completion of a cold extrusion.

[0024] S202: Continue to advance the mandrel in the first direction at high speed until the mandrel has completely passed the bushing.

[0025] In this step, after the mandrel has completely passed through the bushing and completed low-speed pre-extrusion, it continues to advance at high speed in the first direction (e.g., at 3 mm / s to 10 mm / s). The main goal of this stage is to utilize the higher deformation rate to quickly complete the plastic strengthening process of the remaining portion of the hole wall. With the increased deformation rate, the internal stress and microstructure flow of the material are more complete, further improving the fatigue strength and corrosion resistance of the assembled hole.

[0026] In another exemplary embodiment, in step S200, during the advance of the mandrel along the first direction, the mandrel rotates clockwise or counterclockwise at a first angular velocity ω1 (e.g., 10° / mm).

[0027] In this embodiment, as the mandrel advances along the first direction, it rotates clockwise or counterclockwise at a first angular velocity. The purpose is to form a helical strain path by superimposing axial and rotational motions, thereby improving the material microstructure and mechanical properties of the assembly hole wall. Specifically, the rotational motion can induce a multi-directional stress field in the metal material during extrusion, which helps to refine the grain structure, homogenize the stress distribution, and effectively reduce the tendency to crack due to radial strain concentration in traditional uniaxial extrusion. In addition, this helical path can also enhance the compressive stress state of the inner surface layer of the assembly hole wall, improving its fatigue resistance and stress corrosion cracking resistance. Experiments show that, compared with the non-rotational method, rotary cold extrusion can obtain a more stable microstructure and higher pore size consistency in the orifice region, which is particularly suitable for strengthening the processing of strain-sensitive materials such as high-strength aluminum alloys.

[0028] It should also be noted that the first angular velocity ω1 of 10° / mm is only an example and can be adjusted according to the actual situation.

[0029] In another exemplary embodiment, step S300, which involves performing a second cold extrusion on the assembly hole after the first cold extrusion along the second direction using a mandrel with a slotted bushing, includes the following steps: S301: As Figure 4 and Figure 5 As shown, the mandrel is advanced at a low speed along the second direction until the bushing is inserted into the second preset position of the mounting hole depth (e.g., 70% to 80% of the mounting hole depth). In this step, the second direction is opposite to the first direction. For example, if the first direction is perpendicular to the side of the mounting hole facing the pull gun device, then the second direction is perpendicular to the side of the mounting hole facing away from the pull gun device, and vice versa. When cold extruding along the second direction, except for the opposite direction of the advance, the rest of the operation process is basically the same as the step of advancing the mandrel at low speed along the first direction, including the installation of the bushing, the control of the mandrel advance speed, the hole end positioning method, and the bushing removal mechanism, etc., which will not be repeated here.

[0030] S302: Continue to advance the mandrel in the second direction at high speed until the mandrel has completely passed the bushing.

[0031] In this step, the process of the mandrel continuing to advance at high speed along the second direction is the same as the operation of the mandrel continuing to advance at high speed along the first direction. The process parameters and effects, including the advancing speed, bushing detachment mechanism and hole wall plastic deformation, are the same, so they will not be described again here.

[0032] In another exemplary embodiment, in step S300, during the advancement of the mandrel along the second direction, the mandrel rotates counterclockwise or clockwise at a second angular velocity ω2 (e.g., 5° / mm).

[0033] In this embodiment, the mandrel is rotated at a second angular velocity ω2 (e.g., 5° / mm) different from that in the first direction when it is advanced along the second direction. The main purpose is to achieve a differentiated design of the strain path to avoid localized hardening or stress concentration caused by repeated loading of the same trajectory area. Compared with the larger angular velocity (e.g., 10° / mm) in the first cold extrusion, reducing the angular velocity can reduce the helical deformation intensity, so that the second deformation mainly acts on different depths or areas, thereby achieving more uniform tissue strengthening and residual stress distribution.

[0034] It should be noted that if the second angular velocity ω2 is greater than or equal to the first angular velocity ω1, the mandrel will rotate too quickly during the secondary cold extrusion process. This will cause the assembly hole wall to be subjected to strong shear strain again on top of the significant plastic deformation already in place. This repeated high-intensity deformation can easily lead to over-processing in local areas, manifested as grain elongation, obvious fibrous structure, and even adverse consequences such as microcracks and stress concentration. Simultaneously, excessively high rotational speed will also increase the frictional heat between the mandrel and the hole wall, leading to problems such as lubrication failure, surface scratches, and unstable bushing detachment, reducing the controllability of the extrusion process and the surface quality of the finished hole. Furthermore, a high angular velocity will cause the secondary extrusion path to overlap too much with the first, weakening the design intention of staggered strain paths and making it difficult to achieve the purpose of alternating stress distribution and multi-layer strengthening. Therefore, the second angular velocity should be appropriately lower than the first angular velocity to achieve gradual optimization of the material structure and reasonable control of residual stress, thereby improving the overall service performance of the assembly hole. It should also be noted that the second angular velocity ω2 of 5° / mm is only an example and can be adjusted according to the actual situation.

[0035] In another exemplary embodiment, the primary cold extrusion and the secondary cold extrusion are performed according to a preset extrusion strengthening amount.

[0036] In this embodiment, the extrusion strengthening amounts of the first and second cold extrusions can be set to, for example, 7:3. This ratio is based on a comprehensive optimization result of process stability and material strengthening balance. Specifically, the first cold extrusion undertakes the main plastic deformation task, achieving pore size precision control, microstructure densification, and matrix strengthening through a larger extrusion amount, thus forming the main strengthening skeleton. The second cold extrusion, on the other hand, plays a corrective and reinforcing role, aiming to further refine the grains, eliminate residual stress from the first extrusion, and optimize surface quality through a smaller deformation amount. This ratio setting avoids over-processing caused by the first extrusion while improving fatigue life and stability through appropriate secondary strengthening.

[0037] It should be noted that if the ratio of the extrusion strengthening amount between the primary and secondary cold extrusion is less than 7:3, meaning the secondary cold extrusion accounts for too large a proportion, the material may undergo excessive deformation during the second loading, damaging the initially formed microstructure and leading to problems such as hole wall cracking, out-of-tolerance hole diameter, and dimensional instability. This also increases processing difficulty and energy consumption, reducing process controllability and repeatability. Conversely, if the ratio is greater than 7:3, meaning the primary cold extrusion accounts for too high a proportion, the secondary cold extrusion becomes too slight, failing to fully utilize its role in residual stress adjustment, microstructure refinement, and hole wall surface quality repair. This results in uneven strengthening, especially at the hole ends and in the middle section, where stress concentration or fatigue weaknesses are likely to occur. Therefore, whether the ratio is too small or too large, it weakens the overall performance of the combined cold extrusion strengthening, making it difficult to balance deformation efficiency, mechanical strengthening, and hole diameter accuracy control, ultimately reducing the service stability and lifespan of the final assembled hole.

[0038] In another exemplary embodiment, in steps S200 and S300, the slotted bushing is coated with a dry film lubricant.

[0039] In this step, a dry film lubricant is applied to the surface of the slotted bushing. Its main function is to reduce the frictional resistance between the mandrel and the bore wall and bushing, preventing metal adhesion and scratches, and improving the stability and repeatability of the extrusion process. Dry film lubricants, such as molybdenum disulfide (MoS2), graphite, or fluoropolymers, maintain good lubrication performance under high pressure and high shear conditions, forming a stable interfacial protective film that effectively reduces heat accumulation and surface wear, extending the life of the die and mandrel. Simultaneously, the lubricant also helps the bushing detach smoothly after extrusion, avoiding adhesion or jamming, and improving the efficiency of automated processing. Insufficient lubrication can easily lead to increased extrusion pressure and obstructed mandrel advancement, and may also affect the bore diameter accuracy and surface quality of the assembly holes, even causing metal tearing or localized over-extrusion.

[0040] In another exemplary embodiment, the method further includes the following steps: S400: Micro-shot peening is performed on the assembly holes after secondary cold extrusion.

[0041] In this step, micro-peening refers to using metal, ceramic, or glass microspheres with a diameter typically between 0.05 mm and 0.3 mm to impact the orifice and the surface of the orifice under the action of high-pressure airflow, causing plastic deformation and dislocation accumulation on the surface of the assembly hole, forming a dense residual compressive stress layer.

[0042] Micro-shot peening effectively refines the surface grains of assembly holes, closes microcracks, eliminates minor machining defects generated during cold extrusion, and enhances the stress resistance of the hole area. Simultaneously, micro-shot peening improves the surface roughness of assembly holes, reduces corrosion susceptibility, and enhances the long-term stability and safety of assembly holes in complex service environments.

[0043] In another exemplary embodiment, the method further includes the following steps: S500: Roll forming is performed on the assembly holes after micro-shot peening.

[0044] In this step, rolling refers to using a high-hardness cylindrical roller or ball-head tool to roll along the surface of the orifice under a certain pressure, producing plastic extrusion and smoothing effects, thereby forming a mirror-like compacted surface.

[0045] Specifically, this application performs roll forming on the assembly holes using the following method: 1. The rolling head (a micro-ring MRE flexible rolling head, whose outer layer is a variable stiffness magnetorheological elastomer material (composed of silicone rubber matrix embedded with micron-sized ferromagnetic particles), and whose inner layer is a non-magnetic metal support skeleton, and integrates a micro electromagnetic coil) is installed on a high-precision CNC micro-feed device. The device is equipped with a force sensor, a displacement sensor and a rotation drive module.

[0046] 2. Align the roller head with the center of the assembly hole that has undergone micro-shot peening, and feed at an extremely low speed of 0.005 mm / s, allowing the flexible roller head to lightly touch the edge of the hole. At the same time, turn off the power to the electromagnetic coil, so that the roller head is in a low-modulus "soft state" (storage modulus of about 0.5 MPa), thereby achieving damage-free pre-contact and avoiding scratching the fine compressive stress layer formed by micro-shot peening.

[0047] 3. When approaching both ends of the hole (within 0.5 mm of the end face), the feed rate is automatically reduced to 0.002 mm / s, and the magnetic field strength is increased to locally harden the rolling head, thereby preferentially strengthening the chamfered area of ​​the hole opening and effectively suppressing stress concentration. At the same time, by switching the rotation direction (alternating between clockwise and counterclockwise), cross rolling patterns are formed to improve the isotropy of the assembly hole surface and its resistance to fatigue crack initiation.

[0048] The aforementioned rolling method, by employing a magnetorheological elastomer (MRE) flexible rolling head combined with a force-displacement closed-loop feedback system, can achieve gradient and uniform surface strengthening of assembly holes after micro-shot peening. In the initial stage of low-stress contact, it avoids damage to the shot-peened layer by maintaining a soft state. Subsequently, the stiffness of the rolling head is gradually increased through magnetic field control. Combined with alternating helical feed and rotation direction, a dense residual compressive stress layer and cross-finishing texture are formed on the hole wall. This can significantly improve the surface finish of the assembly hole (Ra reduced to 0.15μm) and suppress stress concentration at the hole opening, resulting in a fatigue life that is more than 35% higher than that of traditional processes. It can effectively solve the problems of surface damage and uneven stress distribution caused by traditional rigid rolling, thereby enhancing the service stability and structural durability of high-strength alloy assembly holes.

[0049] In summary, this application utilizes roll forming, which not only refines grains and seals surface microcracks but also introduces a stable residual compressive stress layer, effectively improving the fatigue resistance and wear resistance of the hole wall. Furthermore, roll forming can repair minor unevenness formed during cold extrusion, improving the geometric accuracy and fit quality of the hole opening, thereby contributing to extending the service life and structural integrity of the assembly hole.

[0050] Below, this application compares the assembly holes processed using the method described in this application with those processed using conventional methods (including drilling + reaming and conventional cold extrusion). The specific comparison results are shown in Table 1: Table 1

[0051] As shown in Table 1, compared with traditional methods, the assembly holes processed by the method of this application have significantly improved in terms of fatigue life, surface integrity, residual stress depth and uniformity. In particular, the fatigue life is about 7 times higher than that of conventional cold extrusion, which fully verifies the synergistic strengthening advantages of the combined process of "alternating cold extrusion + micro-shot peening + intelligent flexible rolling" of this application, and has the technical basis for large-scale application in high-reliability structural connections in aerospace.

[0052] The above are merely preferred embodiments of this application and do not limit the patent scope of this application. Any equivalent structural or procedural transformations made using the content of this application's specification and drawings, or direct or indirect applications in other related technical fields, are similarly included within the patent protection scope of this application.

Claims

1. A cold extrusion combined strengthening method for alloy assembly holes, characterized in that, The method includes: Assembly holes with a preset diameter are obtained by machining on an aluminum alloy plate; The mandrel with the slotted bushing is cold-extruded into the assembly hole along the first direction. The mandrel with the slotted bushing is subjected to a second cold extrusion along the second direction to the assembly hole after the first cold extrusion. The first cold extrusion and the second cold extrusion are performed according to a preset extrusion strengthening amount, which is 7:

3. During the advancement of the mandrel along the first direction, the mandrel rotates clockwise or counterclockwise with a first angular velocity. During the advancement of the mandrel along the second direction, the mandrel rotates counterclockwise or clockwise with a second angular velocity, and the second angular velocity is less than the first angular velocity.

2. The method according to claim 1, characterized in that, The process of cold-extruded the mandrel with the slotted bushing into the assembly hole along the first direction includes: The mandrel is advanced at a low speed along the first direction until the bushing is inserted into the first preset position of the assembly hole depth. The mandrel continues to be advanced at high speed in the first direction until it has completely passed through the bushing.

3. The method according to claim 1, characterized in that, The process of performing a second cold extrusion on the assembly hole after the first cold extrusion along the second direction using a mandrel with a slotted bushing includes: The mandrel is advanced at a low speed along the second direction until the bushing is inserted into the second preset position at the depth of the assembly hole. The mandrel continues to be advanced at high speed in the second direction until it has completely passed through the bushing.

4. The method according to claim 1, characterized in that, The slotted bushing is coated with a dry film lubricant.

5. The method according to claim 1, characterized in that, The method further includes: The assembly holes after secondary cold extrusion are treated with micro-shot peening.

6. The method according to claim 5, characterized in that, The method further includes: The assembly holes after micro-shot peening are then rolled.

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