Method for improving fatigue performance of additive structure

Abstract

The invention belongs to the technical field of advanced manufacturing, and particularly relates to a method for improving the fatigue performance of an additive structure. The method comprises the steps that the surface of a metal part for additive manufacturing is cleaned, and residual metal powder in the additive manufacturing process is removed; residual compressive stress is introduced to the surface of the part, the introduction process of the residual compressive stress is performed once or multiple times, and the compressive stress level can be adjusted according to the performance requirement of the part in the multiple-time introduction process; the part is placed in a high-temperature environment protected by vacuum or inert gas to be subjected to heat treatment; the residual compressive stress is introduced again, heat treatment is not carried out any more, and the strength of the residual compressive stress reaches the highest fatigue performance level of the material; and surface vibration finishing treatment is adopted, so that the surface roughness of the part is reduced. According to the method, the grain size of the surface of the structure is refined through a combination method, meanwhile, residual compressive stress is introduced between the surface and the subsurface of the part in a combined mode, and the fatigue resistance of the additive manufacturing part is improved by 30 MPa or above.

Description

Technical Field

[0001] The present invention belongs to the field of advanced manufacturing technology, and specifically relates to a method for improving fatigue performance of an additive structure. Background Art

[0002] The additive manufacturing process is highly flexible and can manufacture components of various topological structures. It is widely used in weight-sensitive fields, manufacturing small quantities and a wide variety of parts, product development and rapid iteration stages. In recent years, its application range has expanded from traditional stators and non-critical structures to load-bearing and critical structures, such as missile rudders, airfoils, engine high-pressure compressor rotor blades, low-pressure turbine blades, etc.

[0003] Most load-bearing and key structures need to withstand alternating loads, requiring the structure to have good fatigue resistance. However, in additive manufacturing of metal parts, the forming process of the parts is also the material preparation process. Powdered and filamentary materials are melted at high temperature and stacked layer by layer. The grain structure of the formed parts material has columnar crystal characteristics, and the macroscopic properties of the material show obvious anisotropy, high surface roughness, and inevitably produce more microporous defects inside the material. Since fatigue cracks always initiate from high stress areas on the surface or areas with weak material properties, how to improve the fatigue resistance of additive structures / parts has become one of the difficult problems that need to be solved in the development of such parts. Summary of the invention

[0004] 1. Technical issues to be resolved

[0005] The present invention mainly aims at the above problems and proposes a method for improving the fatigue performance of additively manufactured structures, the purpose of which is to solve the problem of relatively low fatigue strength of additively manufactured component structures.

[0006] (II) Technical solution

[0007] To achieve the above object, the present invention provides a method for improving fatigue performance of an additive structure, the method comprising the following steps:

[0008] Step 1: Clean the surface of the additively manufactured metal parts to remove the metal powder remaining in the additive manufacturing process;

[0009] Step 2: introducing residual compressive stress on the surface of the part. The process of introducing residual compressive stress is performed once or multiple times. The compressive stress level can be adjusted according to the performance requirements of the part during the multiple times.

[0010] Step 3: Place the parts in a high temperature environment protected by vacuum or inert gas for heat treatment to allow the surface material to undergo dynamic recrystallization and refine the grains;

[0011] Step 4: Residual compressive stress is introduced again without heat treatment, so that the strength of the residual compressive stress reaches the highest level of fatigue performance of the material;

[0012] Step 5: Use vibration finishing to treat the surface and reduce the surface roughness of the parts.

[0013] Furthermore, before performing step 4, steps 2 and 3 are repeated, and during the repetition of step 3, the temperature parameters of the heat treatment remain unchanged.

[0014] Further, additive manufacturing processes include, but are not limited to, using powders, wires, or rods as material sources.

[0015] Furthermore, the heat source used in the additive manufacturing process includes but is not limited to laser, arc and electron beam; the surface of the metal part is the original state after additive manufacturing, or the state after polishing or machining.

[0016] Furthermore, methods of introducing residual compressive stress include shot peening, laser shock, rolling, and ultrasonic rolling.

[0017] Furthermore, in step 2, for titanium alloy, the residual compressive stress intensity value is calibrated using an N-shaped Almen test piece, and the intensity of the compressive stress is gradually increased. The intensity increment of the compressive stress is in units of 0.1 mmN, and the maximum value does not exceed 1.0 mmN.

[0018] Furthermore, in step 2, for high-temperature alloys and structural steels, the residual compressive stress strength value is calibrated using an A-shaped Almen test piece, and the strength of the compressive stress is gradually increased. The strength increment of the compressive stress is in units of 0.1 mmA, and the maximum value does not exceed 1.0 mmA.

[0019] Further, in step 3, the heat treatment temperature is 800-1000° C. for titanium alloys and 900-1200° C. for high-temperature alloys, and the heat treatment process is carried out in air, vacuum or inert gas protection environment.

[0020] Furthermore, in step 4, for titanium alloy, the residual compressive stress intensity value is calibrated using an N-shaped Almen test piece, and the compressive stress intensity is between 0.4-0.7 mmN.

[0021] Furthermore, in step 4, for high temperature alloys and structural steels, the residual compressive stress intensity value is calibrated using an A-shaped Almen test piece, and the compressive stress intensity is between 0.1-0.25 mmA.

[0022] (III) Beneficial effects

[0023] Compared with the prior art, the present invention provides a method for improving the fatigue performance of additive structures, which introduces compressive residual stress on the material surface multiple times and combines multiple heat treatments to change the surface microstructure of the material, refine the surface grains of additively manufactured parts, and generate compressive residual stress on the surface. That is, by refining the grains and introducing residual stress, the fatigue resistance of additively manufactured metal parts is improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 Microstructure of a material when creating a metal part for additive manufacturing.

[0025] Figure 2 The microstructure of the surface of a material is altered for the methods disclosed herein.

[0026] Figure 3 A map of the variation of compressive residual stress on a surface is generated for the method disclosed in the present application.

[0027] Figure 4 This is a structural schematic diagram of the method disclosed in this application applied to the manufacture of disc-shaped parts. DETAILED DESCRIPTION

[0028] The present invention will be described in detail below in conjunction with the accompanying drawings, and the technical solutions in the embodiments of the present invention will be clearly and completely described. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0029] The present invention provides a method for improving fatigue performance of additive structures, which refines the grain size of the structure surface by a combined method, and / or introduces residual compressive stress between the surface and sub-surface of the part to increase the fatigue strength of the additively manufactured part by more than 30 MPa. The specific method includes the following steps:

[0030] Step 1: Clean the surface of the additively manufactured metal parts to remove the metal powder remaining in the additive manufacturing process;

[0031] In additive manufacturing, especially when using powder bed melting technology (such as laser or electron beam melting), the surface of the part is often covered with a layer of incompletely melted powder. These residual metal powders not only affect the appearance quality of the part, but may also become the initiation point of fatigue cracks, affecting the mechanical properties and fatigue life of the structure. Therefore, it is first necessary to thoroughly clean the surface of the part to remove these residues. This step can usually be achieved through mechanical, chemical or thermal treatment methods, such as using high-pressure water guns, air jets or ultrasonic cleaning to ensure that the surface of the part is clean and flat, laying the foundation for subsequent surface modification treatments (such as shot peening, polishing or heat treatment). When additive manufacturing parts are manufactured using methods other than powder, go directly to step 2.

[0032] Step 2: introducing residual compressive stress on the surface of the part. The process of introducing residual compressive stress is performed once or multiple times. The compressive stress level can be adjusted according to the performance requirements of the part during the multiple times.

[0033] In this step, residual stress can effectively inhibit the initiation and expansion of fatigue cracks, thereby significantly improving the fatigue resistance of parts. If the performance of parts needs to be further improved, residual compressive stress can be applied multiple times, and the compressive stress level can be appropriately adjusted in each process to meet specific needs.

[0034] Step 3: Place the parts in a high temperature environment protected by vacuum or inert gas for heat treatment to allow the surface material to undergo dynamic recrystallization and refine the grains;

[0035] The purpose of this step is to promote the dynamic recrystallization process of the surface material through thermal action, thereby achieving grain refinement. During this process, the parts are placed in a high-temperature environment protected by vacuum or inert gas. The temperature setting is usually determined according to the type of material (such as titanium alloy, high-temperature alloy or structural steel, etc.). Such a temperature can cause changes in the microstructure inside the material. Dynamic recrystallization can reduce the grain boundary energy of the material, leading to grain refinement, which helps to break the columnar crystal and coarse grain structure introduced by additive manufacturing, increase the uniformity and anisotropy of the material, and thus improve its fatigue resistance. Grain-refined materials usually exhibit better mechanical properties and crack resistance. This is because fine grains can more effectively block the propagation of cracks and enhance the overall durability of the material.

[0036] Step 4: Residual compressive stress is introduced again without heat treatment, so that the strength of the residual compressive stress reaches the highest level of fatigue performance of the material;

[0037] In this step, by precisely controlling the parameters of shot peening or impact (such as intensity, coverage and number), a consistent and stable compressive stress layer can be formed from the surface of the material to a certain depth. These parameters are usually calibrated and verified through standardized tests (such as using Almen test pieces) to ensure that the intensity and distribution of stress are optimal, thereby maximizing the fatigue performance of the material.

[0038] Step 5: Use surface vibration finishing to reduce the surface roughness of the parts.

[0039] Vibratory finishing is a surface treatment technology that refines the surface of parts by using high-frequency vibration tools in combination with grinding or polishing media. It can effectively smooth the surface and remove microscopic defects, scars and other irregularities formed during the additive manufacturing process, thereby significantly reducing surface roughness.

[0040] During the surface vibration finishing step, almost no material is removed in order to maintain the near-net shape characteristics of the additively manufactured parts. Near-net shape refers to the direct manufacture of parts close to the final shape and size through additive manufacturing technology, which can greatly reduce the need for subsequent machining. By using surface vibration finishing technology, the surface quality can be effectively improved and the roughness can be reduced without affecting the overall geometry and dimensional accuracy of the parts.

[0041] Before the final residual compressive stress is introduced in step 4, steps 2 and 3 are repeated. This repetition is to further enhance and stabilize the structural properties of the material, ensuring that the material is uniformly subjected to compressive stress throughout the compression area, thereby optimizing its fatigue performance.

[0042] When repeating step 2, the strength of the introduced residual compressive stress is gradually increased each time. This can affect the microstructure of the material more deeply and obtain a finer and thicker surface modification layer.

[0043] Repeating step 3 involves placing the part in a high temperature environment for heat treatment again to promote deeper dynamic recrystallization and grain refinement. Importantly, although step 3 is repeated, the temperature parameters of the heat treatment remain unchanged. This is done to ensure the consistency and controllability of the heat treatment process, so that the material's microstructure can stably achieve the desired refinement effect in repeated treatments. Maintaining the same temperature settings helps achieve repeatability of material properties, ensuring that each heat treatment can effectively improve the material's fatigue resistance.

[0044] In this embodiment, the material sources for the additive manufacturing process method include but are not limited to powders, metal wires, and rods, and the heat sources used in the additive manufacturing process include but are not limited to lasers, electric arcs, and electron beams.

[0045] In this embodiment, the target part surface may be in the original state after additive manufacturing, or in the state after polishing or machining.

[0046] In step 2, the methods of introducing residual compressive stress include dry shot peening, wet shot peening, laser shock, and rolling.

[0047] In step 2, in order to introduce and accurately control the intensity of residual compressive stress, two different standard Almen test pieces are used: N-shaped and A-shaped. For titanium alloys, the N-shaped Almen test piece is used to calibrate the intensity value of residual compressive stress. The N-shaped test piece is suitable for lighter materials and thinner covering layers. The intensity of residual compressive stress is gradually increased in increments of 0.1mmN until it reaches a maximum value of no more than 1.0mmN.

[0048] For high-temperature alloys and structural steels, A-shaped Al men test pieces are used to calibrate the residual compressive stress intensity value. A-shaped test pieces are suitable for heavier materials and thicker covering layers, reflecting the ability of these materials to withstand stronger stress. Similarly, the intensity of compressive stress is gradually increased in units of 0.1mmA, and the maximum does not exceed 1.0mmA. In this way, it can be ensured that the residual compressive stress is evenly and effectively distributed on the entire surface and underlying areas of the part, thereby improving the performance stability and fatigue life of the material under high load and high temperature environments.

[0049] During the heat treatment of the parts in step 3, the temperature settings are precisely determined according to the properties and requirements of different materials. For titanium alloys, the heat treatment temperature range is set at 800 to 1000°C, and for high-temperature alloys, the heat treatment temperature is set in a higher range, that is, 950 to 1200°C.

[0050] In addition, the optimal intensity and distribution of stress will vary depending on the material type (such as titanium alloy, high-temperature alloy or structural steel). For example, in titanium alloy, the intensity of the final residual compressive stress is between 0.4 and 0.7 mmN, while for high-temperature alloys and structural steel, it is between 0.1 and 0.25 mmA. By precisely adjusting these parameters, it can be ensured that each material type can achieve its optimal fatigue performance.

[0051] In this embodiment, a method for improving the fatigue resistance of additively manufactured metal parts is proposed, mainly targeting the performance limitations caused by the long columnar crystal structure formed during the manufacturing process of these parts. Figure 1 As shown, the structure shows obvious length and width under microscopic observation (several millimeters in length and about 20 microns in width), and the structural mechanical properties bear transverse anisotropy characteristics, resulting in poor fatigue resistance of the material.

[0052] In order to solve this problem, this embodiment applies compressive residual stress to the material surface multiple times by surface modification, and combines multiple heat treatments to change the microstructure of the material surface and refine the surface grains of the additively manufactured parts (such as Figure 2 , under microscopic conditions, the grains appear to be fine equiaxed). And compressive residual stress is generated on the surface ( Figure 3 The residual stress between the surface and the subsurface is compressive stress. The compressive stress first increases and then decreases with the thickness, and finally turns into tensile stress. The thickness of the compressive stress area is 0.1mm to several millimeters). That is, by refining the grains and introducing residual stress, the fatigue resistance of additively manufactured metal parts is improved.

[0053] The method of the present invention is applied to the manufacture of a certain type of disc-shaped parts ( Figure 4 ), the parts are made of high-temperature alloy GH4169 manufactured by laser selective melting. After the parts are formed, they are first treated with hot isostatic pressing, and then the surface is treated with dry shot blasting. The shot blasting intensity is 0.20mmA and the coverage is 200%. Then, solid solution strengthening is carried out at 800℃. Next, the surface of the parts is polished. Finally, the surface of the parts is strengthened by dry shot blasting ceramic shot. The shot blasting intensity is 0.15mmA and the coverage is 200%. The rotating bending fatigue test is carried out with the furnace parts. At 1×10 7 Under cyclic conditions, the fatigue strength of the material increased from 500MPa to 540MPa.

[0054] Therefore, no matter from which point of view, the embodiments should be regarded as illustrative and non-restrictive, and the scope of the present application is limited by the appended claims rather than the above description, so it is intended that all changes falling within the meaning and scope of the equivalent elements of the claims are included in the present application. Any reference signs in the claims should not be regarded as limiting the claims involved. In addition, it is obvious that the word "comprising" does not exclude other units or steps, and the singular does not exclude the plural.

[0055] The above embodiments are only used to illustrate the technical solution of the present application and are not intended to limit it. Although the present application has been described in detail with reference to the preferred embodiments, a person skilled in the art should understand that the technical solution of the present application may be modified or replaced by equivalents without departing from the spirit and scope of the technical solution of the present application.

Claims

1. A method for improving fatigue performance of an additive structure, characterized in that: The steps include: Step 1: Clean the surface of the additively manufactured metal parts to remove the metal powder remaining in the additive manufacturing process; Step 2: introducing residual compressive stress on the surface of the part. The process of introducing residual compressive stress is performed once or multiple times. The compressive stress level can be adjusted according to the performance requirements of the part during the multiple times. Step 3: Place the parts in a high temperature environment protected by vacuum or inert gas for heat treatment to allow the surface material to undergo dynamic recrystallization and refine the grains; Step 4: Residual compressive stress is introduced again without heat treatment, so that the strength of the residual compressive stress reaches the highest level of fatigue performance of the material; Step 5: Use vibration finishing to treat the surface and reduce the surface roughness of the parts.

2. A method for improving fatigue performance of additive structures according to claim 1, characterized in that: Before performing step 4, the method further includes repeating steps 2 and 3. During the process of repeating step 3, the temperature parameters of the heat treatment remain unchanged.

3. The method for improving fatigue performance of additive structures according to claim 1, characterized in that: Additive manufacturing processes include, but are not limited to, using powders, wires, or rods as material sources.

4. The method for improving fatigue performance of additive structures according to claim 1, characterized in that: The heat sources used in the additive manufacturing process include but are not limited to laser, arc and electron beam; the surface of the metal part is the original state after additive manufacturing, or the state after polishing or machining.

5. The method for improving fatigue performance of additive structures according to claim 1, characterized in that: Methods for introducing residual compressive stress include shot peening, laser shock, rolling, and ultrasonic rolling.

6. A method for improving fatigue performance of additive structures according to claim 1 or 2, characterized in that: In step 2, for titanium alloy, the residual compressive stress intensity value is calibrated using an N-shaped Almen test piece, and the intensity of the compressive stress is gradually increased. The increment of the compressive stress intensity is in units of 0.1 mmN, and the maximum value does not exceed 1.0 mmN.

7. A method for improving fatigue performance of additive structures according to claim 1 or 2, characterized in that: In step 2, for high-temperature alloys and structural steels, the residual compressive stress strength value is calibrated using an A-shaped Almen test piece, and the strength of the compressive stress is gradually increased. The strength increment of the compressive stress is in units of 0.1 mmA, and the maximum value does not exceed 1.0 mmA.

8. A method for improving fatigue performance of additive structures according to claim 1 or 2, characterized in that: In step 3, the heat treatment temperature is 800-1000° C. for titanium alloys and 900-1200° C. for high-temperature alloys, and the heat treatment process is carried out in an environment protected by air, vacuum, and inert gas.

9. A method for improving fatigue performance of additive structures according to claim 1 or 2, characterized in that: In step 4, for titanium alloy, the residual compressive stress intensity value is calibrated using an N-shaped Almen test piece, and the compressive stress intensity is between 0.4-0.7 mmN.

10. A method for improving fatigue performance of additive structures according to claim 1 or 2, characterized in that: In step 4, for high temperature alloys and structural steels, the residual compressive stress intensity value is calibrated using an A-shaped Almen test piece, and the compressive stress intensity is between 0.1-0.25 mmA.

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