A forming and strengthening treatment method for titanium alloy fasteners

Abstract

The present invention relates to a forming and strengthening treatment method for titanium alloy fasteners, comprising the following steps: successively performing solution treatment, aging treatment and mechanical rolling extrusion on the titanium alloy fasteners; placing the titanium alloy fasteners after mechanical rolling extrusion in different ultra-low temperature environments, and continuing to keep warm for a first set time after they are completely cooled to the target ambient temperatures; putting the titanium alloy fasteners after cooling treatment into a high-temperature drying oven and keeping warm for a second set time. The purpose of the forming and strengthening treatment method for the titanium alloy fasteners is to solve the problem of low fatigue life of titanium alloy fasteners during long-term service in a complex working environment.

Description

Technical Field

[0001] The present invention relates to the technical field of titanium alloy strengthening treatment, and particularly relates to a forming and strengthening treatment method for titanium alloy fasteners. Background Art

[0002] As a typical aerospace light alloy material, titanium alloy has the advantages of low density, high specific strength, high temperature resistance, easy machining, high specific elastic modulus, etc. At present, it has become an important material for key load-bearing complex components in aerospace. However, under complex service conditions, titanium alloy is prone to fatigue fracture, creep failure and other phenomena. Therefore, further improving the comprehensive performance of titanium alloy and prolonging its fatigue life have become extremely important factors for improving the service performance of titanium alloy aerospace components, such as fasteners, engines, etc.

[0003] Traditional titanium alloy heat treatment processes, including ultra-low temperature or thermal aging processes, can make the material obtain the expected mechanical properties by changing the tissue morphology, type, etc. of the material. For example, the patent application with the application number 201610794400.3 proposed a heat treatment process for TC4 titanium alloy fasteners, which adopted solution treatment at a temperature about 50°C below the phase transformation point of TC4 titanium alloy forgings, with water cooling as the cooling method. After solution treatment, annealing was carried out at 760°C - 780°C. After annealing and air cooling, aging treatment was carried out at 530 ± 10°C for not less than 6 hours. This invention effectively solved the problems of excessive α phase, unqualified high-temperature creep performance, and too low room temperature strength in the traditional ordinary annealing heat treatment process of TC4 titanium alloy fasteners. The patent application with the application number 201810278998.X proposed a cryogenic treatment process for TC4 titanium alloy fasteners, which adopted advanced upsetting and rolling forging treatment of TC4 titanium alloy, followed by solution quenching treatment, with water cooling as the cooling method. After solution treatment, double low-temperature annealing treatment was carried out. After annealing and air cooling, aging treatment was carried out, and finally, cryogenic treatment was carried out. This invention improved the uniformity and stability of the internal organizational structure of TC4 titanium alloy fasteners and eliminated the thermal stress generated during the heat treatment process.

[0004] However, although these methods have improved the strength of the material to a certain extent, they do not consider the most important problem of prolonging the service life of the material during service. Although some surface strengthening processes, such as shot peening, laser shot peening, etc., can refine the surface grains of the material and improve the fatigue life of titanium alloy fasteners to a certain extent, there are problems of environmental pollution and high cost. Therefore, finding a technology that can economically and environmentally improve the performance of materials such as fatigue life has become a major problem that urgently needs to be solved in the aerospace manufacturing field for titanium alloy to serve in a complex environment for a long time.

[0005] Therefore, the inventor provides a forming and strengthening treatment method for titanium alloy fasteners. Summary of the Invention

[0006] (1) Technical problem to be solved

[0007] The embodiment of the present invention provides a forming and strengthening treatment method for titanium alloy fasteners, which solves the technical problems of how to form titanium alloy fasteners at low temperature efficiently and how to have a relatively low fatigue life during long-term service in a complex working environment.

[0008] (2) Technical solution

[0009] The present invention provides a forming and strengthening treatment method for titanium alloy fasteners, including the following steps:

[0010] Successively perform solution treatment, aging treatment and cryogenic mechanical rolling extrusion on the titanium alloy fasteners;

[0011] Place the titanium alloy fasteners after cryogenic mechanical rolling extrusion in different ultra-low temperature environments, and continue to keep warm for the first set time after they are completely cooled to each target environmental temperature;

[0012] Put the titanium alloy fasteners after cooling treatment into a high-temperature drying oven and keep warm for the second set time.

[0013] Further, for the solution treatment of the titanium alloy fasteners, specifically: keep the titanium alloy fasteners at a temperature of 850 - 950 °C for 0.5 - 2 h, and perform water quenching at room temperature.

[0014] Further, the average heating rate of the solution treatment is 15 °C / min.

[0015] Further, for the aging treatment of the titanium alloy fasteners, specifically: keep the titanium alloy fasteners at a temperature of 500 - 600 °C for 2 - 6 h.

[0016] Further, for the cryogenic mechanical rolling extrusion of the titanium alloy fasteners, specifically: perform 1 - 2 passes of mechanical rolling extrusion on the titanium alloy fasteners in a cryogenic environment, where the cryogenic treatment temperature is -150 °C to -196 °C.

[0017] Further, the step of placing the titanium alloy fasteners after cryogenic mechanical rolling extrusion in different ultra-low temperature environments and continuing to keep warm for the first set time after they are completely cooled to each target environmental temperature is specifically:

[0018] Starting from room temperature, cool the titanium alloy fasteners to a low temperature of -150 °C at a set cooling rate and store them for 4 - 6 h, then cool them to -196 °C and keep warm for 24 - 48 h.

[0019] Further, the set cooling rate is 1 - 10 °C / min.

[0020] Further, putting the heat-treated titanium alloy fasteners into a high-temperature drying oven and keeping them warm for a second set time specifically includes:

[0021] Taking out the titanium alloy fasteners and putting them into a high-temperature drying oven at 170-190°C and keeping them warm for 2-4 hours.

[0022] Further, the average heating rate of the high-temperature drying oven is 10°C / min.

[0023] Further, after successively performing solution treatment, cryogenic mechanical rolling extrusion, and aging treatment on the titanium alloy fasteners, it further includes: performing anodic oxidation on the treated titanium alloy fasteners.

[0024] (3) Beneficial effects

[0025] In summary, the present invention performs solution + cryogenic mechanical rolling extrusion forming and strengthening + aging treatment + cryogenic heat treatment on titanium alloy materials. This method performs mechanical rolling extrusion under cryogenic conditions to form and strengthen the solution-treated titanium alloy, and then performs crystallographic orientation aging treatment and subsequent cryogenic surface strengthening, which can improve the forming limit of titanium alloy during the rolling extrusion process, reduce the probability of defect generation, and through subsequent cryogenic surface treatment, can effectively increase the dislocation density, induce a large number of ultra-fine precipitates, and greatly promote grain and tissue refinement, thereby improving the fatigue performance of metal materials and effectively enhancing the fatigue life of titanium alloy fasteners. In addition, the effect of the present invention on titanium alloy is to improve the forming limit of fasteners, reduce the forming temperature, and strengthen the entire metal material. This technology is convenient for application on titanium alloy aviation components and has a large-scale industrialization prospect. Description of the drawings

[0026] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following will briefly introduce the drawings required to be used in the embodiments of the present invention. Obviously, the following described drawings are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0027] Figure 1 is a schematic flow chart of a forming and strengthening treatment method for a titanium alloy fastener provided by an embodiment of the present invention;

[0028] Figure 2 is a microscopic structure diagram of the surface layer of the fastener after upsetting deformation;

[0029] Figure 3 is a microscopic structure diagram of the surface layer after solution treatment in Embodiment 2 of the present invention;

[0030] Figure 4 is a microscopic structure diagram of the surface layer of the fastener after cryogenic rolling extrusion + aging treatment + cryogenic treatment;

[0031] Figure 5 It is a partial morphology diagram of position 1 of the static tensile fracture surface layer under the heat treatment process of Embodiment 1 of the present invention;

[0032] Figure 6 It is a partial morphology diagram of position 2 of the static tensile fracture surface layer under the heat treatment process of Embodiment 1 of the present invention;

[0033] Figure 7 It is a partially enlarged morphology diagram of the static tensile fracture under the heat treatment process of Embodiment 1 of the present invention;

[0034] Figure 8 It is a full morphology diagram of the static tensile fracture under the heat treatment process of Embodiment 1 of the present invention;

[0035] Figure 9 It is a partial morphology diagram of the fatigue tensile fracture surface layer under the heat treatment process of Embodiment 1 of the present invention;

[0036] Figure 10 It is a partial morphology diagram of position 1 of the fatigue tensile fracture under the heat treatment process of Embodiment 1 of the present invention;

[0037] Figure 11 It is a partial morphology diagram of position 2 of the fatigue tensile fracture under the heat treatment process of Embodiment 1 of the present invention. Detailed implementation manners

[0038] The following further describes the implementation manners of the present invention in detail with reference to the drawings and embodiments. The detailed description and drawings of the following embodiments are used to exemplarily illustrate the principle of the present invention, but cannot be used to limit the scope of the present invention, that is, the present invention is not limited to the described embodiments, and covers any modifications, substitutions, and improvements of parts, components, and connection manners without departing from the spirit of the present invention.

[0039] It should be noted that, without conflict, the embodiments in the present application and the features in the embodiments can be combined with each other. The following will describe the present application in detail with reference to the drawings and embodiments.

[0040] Figure 1 It is a flow schematic diagram of a forming and strengthening treatment method for a titanium alloy fastener provided by an embodiment of the present invention. The method includes the following steps:

[0041] S100. Perform solution treatment, aging treatment, and cryogenic mechanical rolling extrusion on the titanium alloy fastener in sequence;

[0042] S200. Place the titanium alloy fastener after cryogenic mechanical rolling extrusion in different ultra-low temperature environments, and continue to keep warm for the first set time after it is completely cooled to each target environmental temperature;

[0043] S300. Place the heat-treated titanium alloy fasteners in a high-temperature drying oven and keep them at a constant temperature for a second set time.

[0044] In the above embodiment, cryogenic treatment is introduced after the mechanical rolling surface strengthening process. Due to the volume shrinkage effect, the material will further undergo plastic deformation, resulting in dislocation strengthening, making the size of the second phase more uniform and the distribution more dispersed. More importantly, mechanical rolling strengthening is performed on the surface of the forged parts to introduce a compressive stress layer on the surface. However, due to differences in surface roughness and local deformation, the stress in the compressive stress layer after mechanical rolling strengthening fluctuates. Especially in some positions with a relatively high dislocation density, cracks are likely to initiate when subjected to fatigue loads, and failure first occurs from the surface of the parts. Through cryogenic treatment (using liquid nitrogen as the cooling medium, reducing environmental pollution) or cryogenic thermal cycling treatment (up to 3 cycles), the average compressive stress may not be effectively increased, but the local stress concentration area can be effectively adjusted, making the stress fluctuation smaller and the stress distribution more uniform. Especially in the area near the surface, the time for crack initiation can be effectively extended, thereby greatly improving the fatigue life of the parts.

[0045] Among them, 1 - 3 cryogenic thermal cycles can be adjusted according to different grades of titanium alloy.

[0046] The titanium alloy fasteners are uniformly heated to the solution temperature. After the solute elements are completely dissolved into the matrix, water quenching treatment is carried out. Then, the aging treatment promotes the precipitation of a large number of secondary phases in the supersaturated solid solution structure of the material. During the mechanical rolling process, the organizational structure near the surface layer of the titanium alloy is refined. Next, by adjusting the process parameters of the cryogenic heat treatment, the uniformity and stability of the organizational structure of the titanium alloy can be effectively improved, the grain structure can be optimized, and the dislocation density can be increased, so as to enhance the fatigue performance of the material.

[0047] In some alternative embodiments, in step S100, the solution treatment of the titanium alloy fasteners is specifically as follows: Keep the titanium alloy fasteners at a temperature of 850 - 950 °C for 0.5 - 2 h and perform water quenching at room temperature.

[0048] In some alternative embodiments, in step S100, the average heating rate of the solution treatment is 15 °C / min. Among them, this average heating rate ensures the temperature uniformity of different fasteners during batch processing of fasteners.

[0049] In some alternative embodiments, in step S100, the aging treatment of the titanium alloy fasteners is specifically as follows: Keep the titanium alloy fasteners at a temperature of 500 - 600 °C for 2 - 6 h. Among them, this aging treatment can eliminate internal stress, prevent or reduce deformation caused by internal stress, and stabilize its shape and size.

[0050] In some alternative embodiments, in step S100, cryogenic mechanical rolling extrusion is performed on the titanium alloy fastener, specifically: the titanium alloy fastener is subjected to 1 to 2 passes of mechanical rolling extrusion in a cryogenic environment, where the cryogenic treatment temperature is -150°C to -196°C.

[0051] Among them, the number of rolling extrusion passes cannot be too many, as too many may reduce the fatigue life, or increase the production cycle, reduce the efficiency, and increase the cost.

[0052] In some alternative embodiments, in step S200, the cryogenically mechanically rolled titanium alloy fastener is placed in different ultra-low temperature environments, and after it is completely cooled to each target environmental temperature, it is kept warm for a first set time, specifically:

[0053] Starting from room temperature, the titanium alloy fastener is first cooled to a low temperature of -150°C at a set cooling rate and stored for 4 to 6 hours, and then it is cooled to -196°C and kept warm for 24 to 48 hours. Among them, it is ensured that sufficient micro-plastic deformation occurs and local high-density dislocations are generated in the fastener under low-temperature conditions, facilitating the precipitation of strengthening phases in subsequent processes.

[0054] In addition, the transfer of the titanium alloy fastener does not exceed 3 minutes to reduce the adverse effects of the environment on the structure and performance of the fastener during the transfer process.

[0055] In some alternative embodiments, the set cooling rate is 1 to 10°C / min. Among them, this cooling rate can ensure that the fasteners have good temperature uniformity during mass production.

[0056] In some alternative embodiments, in step S300, the cooled titanium alloy fastener is placed in a high-temperature drying oven and kept warm for a second set time, specifically:

[0057] The titanium alloy fastener is taken out and placed in a high-temperature drying oven at 170 to 190°C and kept warm for 2 to 4 hours.

[0058] Among them, this treatment process is to remove the adsorbed gas on the surface, facilitating the deposition of a corrosion-resistant coating on the surface during the subsequent anodizing process.

[0059] In some alternative embodiments, the average heating rate of the high-temperature drying oven is 10°C / min. Among them, this heating rate is conventionally selected.

[0060] In some alternative embodiments, after the titanium alloy fastener is successively subjected to solution treatment, aging treatment, and mechanical rolling extrusion, it further includes: anodizing the treated titanium alloy fastener. Among them, the anodizing treatment is to improve the electrochemical corrosion resistance of the fastener.

[0061] Example 1

[0062] In this embodiment, TC4 titanium alloy is used as the sample to perform solution quenching + aging + cryogenic mechanical rolling extrusion + cryogenic heat treatment of the above embodiments of the present invention. The specific steps are as follows:

[0063] 1) Heat the TC4 titanium alloy bar fastener to 950 °C for solution treatment. The average heating rate is 15 °C / min. After reaching the temperature, hold for 0.5 hours and then water quench.

[0064] 2) Put the TC4 titanium alloy bar fastener after solution quenching treatment into a high-temperature drying oven. The average heating rate is 10 °C / min. After reaching the temperature, hold for 4 hours and then take it out and air cool to room temperature.

[0065] 3) Perform 1 or 2 passes of reciprocating mechanical rolling extrusion on the material. The rolling extrusion is carried out in a cryogenic environment, where the cryogenic treatment temperature is -150 °C to -196 °C.

[0066] 4) Put the material into a cryogenic box and cool it to -150 °C at a rate of 5 °C / min. After storing for 3 h, further cool it to -196 °C at a rate of 5 °C / min and hold for 24 h.

[0067] 5) Take the material out of the cryogenic box and directly put it into a high-temperature drying oven that has been heated to 180 °C. After holding for 3 h, take it out.

[0068] 6) According to different grades of titanium alloy, 1 to 3 cryogenic heat cycles can be adjusted and then air cooled.

[0069] The partial morphologies of the static tensile fracture and fatigue tensile fracture of the titanium alloy fasteners prepared in this embodiment are as Figures 5 - 11 shown.

[0070] Example 2

[0071] In this embodiment, TC4 titanium alloy is used as the sample to perform solution quenching + aging + cryogenic mechanical rolling extrusion of the above embodiments of the present invention. The specific steps are as follows:

[0072] 1) Heat the TC4 titanium alloy bar fastener to 950 °C for solution treatment. The average heating rate is 15 °C / min. After reaching the temperature, hold for 0.5 hours and then water quench.

[0073] 2) Put the TC4 titanium alloy bar fastener after solution quenching treatment into a high-temperature drying oven. The average heating rate is 10 °C / min. After reaching the temperature, hold for 4 hours and then take it out and air cool to room temperature.

[0074] 3) Perform 1 or 2 passes of reciprocating mechanical rolling extrusion on the material. The rolling extrusion is carried out in a cryogenic environment, where the cryogenic treatment temperature is -150 °C to -196 °C.

[0075] It can be seen that Figure 4The microstructure diagram of the TC4 titanium alloy fastener that has not been treated by the present invention is shown. Although the grains are refined by plastic deformation, there are more impurities after cooling to room temperature. Figure 3 This is the microstructure diagram of the TC4 titanium alloy treated in Example 2. After solution quenching + artificial aging, the solute atoms are dissolved into the matrix during the solution treatment, and artificial aging causes the supersaturated solid solution formed during quenching to precipitate a large amount of secondary phase.

[0076] In contrast, after adding mechanical rolling extrusion + deep cold heat treatment process, Figure 2 It can be seen that, overall, the titanium alloy grain size is Figure 3 , 4 The average grain size is smaller, the precipitated phase is evenly distributed, and the near-surface layer is obviously refined. This fully demonstrates that the grain structure of the material is optimized after the treatment process described in the present invention; the data in Table 1 demonstrates that the process effectively improves the fatigue strength of the material.

[0077] Table 1 Fatigue performance comparison

[0078]

[0079] It should be clear that the various embodiments in this specification are described in a progressive manner, and the same or similar parts between the various embodiments can be referred to each other, and each embodiment focuses on the differences from other embodiments. The present invention is not limited to the specific steps and structures described above and shown in the figures. In addition, for the sake of brevity, a detailed description of known methods and technologies is omitted here.

[0080] The above are only embodiments of the present application and are not limited to the present application. For those skilled in the art, the present application may have various changes and variations without departing from the scope of the present invention. Any modification, equivalent substitution, improvement, etc. made within the spirit and principle of the present application should be included in the scope of the claims of the present application.

Claims

1. A forming and strengthening treatment method for titanium alloy fasteners, characterized in that, The method comprises the following steps: Successively perform solution treatment, aging treatment and cryogenic mechanical rolling extrusion on the titanium alloy fasteners; Place the titanium alloy fasteners after cryogenic mechanical rolling extrusion in different ultra-low temperature environments, and keep them insulated for a first set time after they are completely cooled to each target ambient temperature; Put the titanium alloy fasteners after cooling treatment into a high-temperature drying oven and keep them insulated for a second set time; Perform cryogenic mechanical rolling extrusion on the titanium alloy fasteners, specifically: perform 1 to 2 passes of mechanical rolling extrusion on the titanium alloy fasteners in a cryogenic environment, wherein the cryogenic treatment temperature is -150°C to -196°C; The step of placing the titanium alloy fasteners after cryogenic mechanical rolling extrusion in different ultra-low temperature environments and keeping them insulated for a first set time after they are completely cooled to each target ambient temperature is specifically: Starting from room temperature, cool the titanium alloy fasteners to a low temperature of -150°C at a set cooling rate and store them for 4 to 6 hours, then cool them to -196°C and keep them insulated for 24 to 48 hours.

2. The forming and strengthening treatment method of the titanium alloy fastener according to claim 1, characterized in that Perform solution treatment on the titanium alloy fasteners, specifically: keep the titanium alloy fasteners at a temperature of 850 to 950°C for 0.5 to 2 hours and perform water quenching at room temperature.

3. The forming and strengthening treatment method of the titanium alloy fastener according to claim 2, characterized in that, The average heating rate of the solution treatment is 15°C / min.

4. The forming and strengthening treatment method of the titanium alloy fastener according to claim 1, characterized in that, Perform aging treatment on the titanium alloy fasteners, specifically: keep the titanium alloy fasteners at a temperature of 500 to 600°C for 2 to 6 hours.

5. The forming and strengthening treatment method of the titanium alloy fastener according to claim 1, characterized in that, The set cooling rate is 1 to 10°C / min.

6. The forming and strengthening treatment method of the titanium alloy fastener according to claim 1, characterized in that, The step of putting the titanium alloy fasteners after cooling treatment into a high-temperature drying oven and keeping them insulated for a second set time is specifically: Take out the titanium alloy fasteners, put them into a high-temperature drying oven at 170 to 190°C and keep them insulated for 2 to 4 hours.

7. The forming and strengthening treatment method of the titanium alloy fastener according to claim 6, characterized in that, The average heating rate of the high-temperature drying oven is 10°C / min.

8. The forming and strengthening treatment method of the titanium alloy fastener according to claim 1, characterized in that, After successively performing solution treatment, cryogenic mechanical rolling extrusion and aging treatment on the titanium alloy fasteners, it further includes: performing anodic oxidation on the treated titanium alloy fasteners.

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