A method of forming a fastener

Abstract

The present application relates to a kind of forming method of fastener, the longitudinal section of fastener is T-shaped, including vertically arranged stud and the head of top in the stud, the diameter M of stud ≥30mm, this forming method sequentially includes the following steps: 1) laser shot processing, 2) quenching treatment, 3) multiple penetration treatment and 4) micro-arc oxidation treatment.Process synergy is made by laser shot, quenching, multiple co-permeation and micro-arc oxidation, so that fastener has high hardness while obtaining high corrosion resistance and fatigue performance.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of material forming and surface treatment, and particularly relates to a forming method of a fastener. BACKGROUND

[0002] As one of the key basic components, high-strength fasteners are widely used in offshore wind power, drilling platforms, railway engineering, bridges, 5G base stations, high-end equipment and other industries. The connection of high-strength and high-toughness weather-resistant fasteners has become the main means of structural engineering installation, and the product quality and reliability play a crucial role in the safety of engineering structures. At present, more than 90% of the high-strength fasteners required above 10.9 grade and the steel used for them still need to be imported from abroad.

[0003] At present, the high-strength fasteners with a strength level greater than 10.9 on the market often have poor corrosion resistance and generally have a delayed fracture problem. The stainless steel fasteners with excellent corrosion resistance generally have a low strength grade. The high-strength of the fastener can effectively reduce the size of the fastener, which is conducive to the miniaturization and compactness of the device. The high fatigue performance can effectively ensure the safety of the fastener in use. The high corrosion resistance can effectively increase the service life of the fastener, thereby reducing the update cost of the device.

[0004] At present, there are many surface treatment methods for improving the corrosion resistance of high-strength fasteners. Common protection methods include electroplating technology, coating technology, hot dip galvanizing technology, thermal spraying technology, QPQ technology, powder zinc infiltration technology, etc. Common coatings include Zn-Ni plating layer, Cd plating layer, Cd-Ti plating layer, Ni plating layer, Cu plating layer, Dacromet coating, Teflon polytetrafluoroethylene coating, etc. There are also many surface treatment methods for improving fatigue life, such as mechanical shot blasting, water jet, multi-element co-diffusion, etc., but all have defects. Among them, multi-element co-diffusion improves corrosion resistance and fatigue performance, but the microstructure of multi-element co-diffusion is a sudden change without gradual transition, so the effect of resisting delayed fracture is poor in the later stage. The Dacromet coating has strong fatigue life, only improves corrosion resistance, and the corrosion resistance rapidly decreases after the coating is damaged.

[0005] Therefore, it is necessary to further improve the existing forming method of the fastener. SUMMARY

[0006] The technical problem to be solved by the present application is to provide a forming method of a fastener which can improve the fatigue performance and corrosion resistance while ensuring the high strength of the fastener in view of the status of the prior art.

[0007] The technical scheme adopted by the present application to solve the above technical problems is: a forming method of a fastener, the longitudinal section of the fastener is T-shaped, including a vertically arranged stud and a head arranged at the top of the stud, characterized in that the diameter M of the stud is greater than or equal to 30 mm, and the forming method comprises the following steps in sequence:

[0008] 1) Laser shot processing: at least one layer of laser shot processing is performed, the connection between the stud and the head of the fastener is the root of the stud, the starting point of the laser shot is from the root of the stud, the laser shot path is arranged on the outer peripheral wall of the stud in a spiral shape along the axial direction of the stud, the shot path has multiple passes, and is arranged in sequence along the circumferential direction of the stud in a clockwise direction, and the last pass of the shot path at least partially overlaps with the first pass of the shot path;

[0009] 2) Quenching treatment: the fastener after step 1) is subjected to quenching treatment;

[0010] 3) Multi-element permeation treatment: the surface of the fastener after step 2) is doped with yttrium, and then placed in a furnace, vacuumized, and the voltage and current are adjusted before the gas is introduced; after the furnace temperature reaches a preset temperature T0, a small amount of ammonia gas is introduced for ionization; after the furnace temperature continues to rise to T1, the current is reduced, and ammonia gas is introduced again, and a mixed gas of ethanol and carbon dioxide is introduced as an auxiliary;

[0011] 4) Micro-arc oxidation treatment: the fastener after step 3) is subjected to micro-arc oxidation treatment.

[0012] The above-mentioned yttrium doping on the surface of the fastener is to catalyze the permeation, so as to make the fastener obtain better surface performance. The above-mentioned quenching treatment releases part of the residual stress and improves the performance of the fastener.

[0013] The fastener after micro-arc oxidation treatment is washed: deionized water is used to wash the fastener after micro-arc oxidation, and then a drying box is used for drying.

[0014] In order to further improve the fatigue performance and corrosion resistance of the fastener, the laser shot processing has at least two layers, the starting point of the second layer of laser shot is located at the root of the stud, and is located at the lap joint between the adjacent two passes of the first layer of laser shot, the second layer of laser shot is along the path of the first layer of laser shot, and each spot in the second layer of laser shot is at the lap joint of the adjacent two spots at the corresponding position of the first layer of laser shot. The first layer of laser shot processing forms a continuous micro-deformation on the surface of the fastener, and the second layer of laser shot processing smoothes the protrusions at the lap joint of the first layer, so that the surface of the fastener is more uniform.

[0015] Preferably, in the first layer of laser shot, the process parameters of each laser shot are 10-20 GW / cm 2The process parameters of each laser shot in the second layer of laser shot peening are 5-10 GW / cm 2 .

[0016] Preferably, the distance D between each thread satisfies: D=3.1415*M.

[0017] Preferably, the overlapping rate of the laser shot spot in the first layer of laser shot peening is 45-55%, and the laser shot spot in the second layer of laser shot peening is not overlapped. The overlapping in the first layer of laser shot peening is selected because it is necessary to make the surface micro-deformation continuous, while the second layer of laser shot peening is mainly used to smooth the protrusions after the impact, so there is no need for overlapping.

[0018] Preferably, in the first layer of laser shot peening, the diameter of the stud is 30-50 mm, and the diameter of the laser spot is 3-5 mm; the diameter of the bolt is greater than 50 mm, and the diameter of the laser spot is 5-6 mm; in the second layer of laser shot peening, the diameter of the spot is 1-3 mm. If the size of the first layer of spot does not match the diameter of the fastener, i.e. the circumference of the cylinder, and if the diameter of the spot is greater than the circumference by a certain value, the fastener product will be severely formed into a polygon, and cannot form an effective lifting; and since the diameter of the first layer of spot is controlled, the protrusions formed at the overlapping position will be smaller, so a suitable spot diameter is selected for impact to smooth the protrusions.

[0019] Preferably, in step 3), before the ventilation, the voltage is adjusted to 450-600 V, the current is 10-20 A, T0 is 100-120℃; T1 is 500-550℃, and after T2, the flow rate of ammonia gas is 1.8-2.2 L / min, the flow rate of the mixed gas of ethanol and carbon disulfide is 0.1-0.3 L / min, and the volume ratio of ethanol to carbon disulfide is 5:1.

[0020] Specifically, the multi-element co-permeation adopts a segmented permeation layer, T1 lasts for t1 time, then the temperature is lowered to T2 and lasts for t2 time, then the temperature is raised to T3 and lasts for t3 time, then the temperature is lowered to T3 and lasts for t3 time, wherein T2=T1-50℃, T3=500-550℃, T4=T3-50℃, t1=3-5h, t2=10-20min, t3=3-5h, t4=10-20min, t1+t3≥8h. If the multi-element co-permeation is performed once, the surface-permeated atoms and the stress layer formed on the surface will be uneven, while the gradient segmented co-permeation can make the permeated atoms more uniform and the stress layer formed on the surface more uniform.

[0021] Before the micro-arc oxidation treatment, the fastener is placed in a dilute nitric acid solution with a concentration of 5-8 ml / L for 3-5 min, then taken out and cleaned, and after the micro-arc oxidation treatment, the fastener is cleaned and dried.

[0022] Preferably, in step 4), the electric parameters of the micro-arc oxidation treatment are as follows: positive voltage 700-900V, negative voltage 200-400V, frequency 1000-3500Hz, positive-negative duty cycle 5-30%, and the waveform is sine; the electrolyte of the micro-arc oxidation treatment is prepared from Na2AlO2, NaH2PO2, Na2WO4 and NaOH, the concentration of Na2AlO2 is 20-50g / L, the concentration of NaH2PO2 is 5-15g / L, the concentration of Na2WO4 is 10-20g / L, and the concentration of NaOH is 1-5g / L; the temperature of the electrolyte is kept at 25-50℃, and the oxidation time is 25-50min. The aforementioned high concentration of aluminate can increase the content of Al2O3 in the micro-arc oxidation film layer of the steel, and NaH2PO2 can induce the deposition of chlorate on the surface of the steel.

[0023] Compared with the prior art, the fastener forming method has the advantages that: the fastener forming method comprises laser shot processing, quenching treatment, multi-element permeation treatment and micro-arc oxidation treatment in sequence. The laser shot processing causes high-speed micro-plastic deformation of the surface of the fastener, forms a uniform gradient hardening and plastic layer on the surface, improves the fatigue strength, and forms a variable gradient and modified surface together with the multi-element permeation treatment. The surface quality of the mechanical shot is poor, and the formed hardening layer and plastic layer are not uniform. The quenching treatment can appropriately reduce the super-high pressure stress of the surface treated by the laser shot processing, rebound the pressure stress layer and plastic layer, and provide conditions for the multi-element permeation treatment (if the surface hardening layer is too dense, it is not conducive to the penetration of atoms, which affects the uniformity of the multi-element permeation treatment and the effect of the multi-element permeation treatment). The multi-element permeation treatment can change the types and contents of surface elements, improve the surface corrosion resistance, form a variable gradient and modified surface together with the laser shot, and provide conditions for the later micro-arc oxidation. The increase of the contents of N and C can improve the quality of the micro-arc oxidation. The micro-arc oxidation can form a wear-resistant and high-corrosion-resistant film layer on the outermost surface, greatly improve the corrosion resistance, and finally make the fastener maintain high hardness while obtaining high fatigue performance and corrosion resistance. The above forming method can be used on fasteners with a diameter of more than 30mm, and thus can be applied to wind power, drilling platforms and railways. BRIEF DESCRIPTION OF DRAWINGS

[0024] Fig. 1(a) is a structural schematic view of a fastener with a first layer of shot paths distributed on a stud in the embodiment of the application;

[0025] Fig. 1(b) is a sectional view in the direction of A-A in Fig. 1;

[0026] Fig. 1(c) is a structural schematic view of the stud in Fig. 1(a) from another angle; Figure 2

[0027] Figure 2 Fig. 1(c) is a structural schematic view of the stud in Fig. 1(a) from another angle; ​

[0028] Figure 3 Figure 1(c) shows a schematic diagram of the structure with multiple shot peening starting points distributed in the second layer of shot peening treatment;

[0029] Figure 4 This is a graph showing the temperature versus time relationship in the multi-element co-diffusion treatment. Detailed Implementation

[0030] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0031] Example 1:

[0032] As shown in Figure 1, the fastener 1 has a T-shaped longitudinal section, including a vertically arranged stud 11 and a head 12 located at the top of the stud 11. The diameter M of the stud is ≥30mm. The connection between the stud and the head of the fastener is referred to as the root of the stud.

[0033] The fastener forming method of this embodiment includes the following steps in sequence:

[0034] 1) Laser shot peening:

[0035] ① First layer of laser shot peening: The laser shot peening starts from the root of the stud 11 and is located on the outer peripheral wall of the root. The laser shot peening path is arranged spirally along the axial direction of the stud 11. There are multiple shot peening paths, which are arranged clockwise along the circumference of the stud 11 on the outer peripheral wall of the stud. The last shot peening path intersects and overlaps with the first shot peening path. That is, when there are N shot peening paths, the N shot peening paths intersect and overlap with the first shot peening path. See details. Figure 3 As shown in Figures 1(a) and (b), in the first layer of laser peening, the starting point of the first pass is A. The process parameters for each pass in the first layer of laser peening are: 10GW / cm². 2 The stud diameter is 30mm, the laser spot diameter is 3mm, and the spot overlap rate is 45%; the overlap rate between two adjacent studs is 30%; the spacing between the studs of each stud is D = 3.1415 × 30mm = 94.245mm.

[0036] ② Second layer laser shot peening treatment: such as Figure 3 As shown, the starting point of the second laser peening layer also begins at the root of the stud. The starting point of the second laser peening layer is located at the overlap between two adjacent passes in the first laser peening process. The second laser peening layer also has N peening paths. As shown in Figure 1(c), the starting point of the first laser peening pass is B. Figure 3As shown, the arrow points to the start of each pass of the second layer of laser peening. In the second layer of laser peening, each pass follows the helical path of the first layer, and the pitch is the same as that of the helical path of the first layer. Each spot in the second layer of laser peening hits the overlap between two adjacent spots in the first layer. The process parameters of each pass of the second layer of laser peening are 5 GW / cm 2 , the spot is 1 mm, and there is no overlap between the spots. Figure 3 Compared with Fig. 1(b), after the second layer of laser peening, the protrusions at the overlap of the first layer are smoothed and smaller.

[0037] 2) Quenching treatment: The high-frequency furnace for quenching and tempering is selected to have a frequency of 20 kHz and a power of 30 kW. The high-frequency quenching temperature (oil cooling) + tempering (oil cooling) is determined according to different fastener materials. The quenching temperature is affected by the material of the fastener, and different materials have different quenching temperatures. The material needs to be tested by DSC, and the quenching temperature is determined according to the actual selected material. The quenching and tempering heating time is 3 h, so that part of the residual stress is released, and the performance of the fastener is improved.

[0038] 3) Multi-element co-diffusion treatment: The surface of the fastener is doped with yttrium, which can catalyze the diffusion of yttrium, a rare earth element, so that the fastener has better surface performance. Then the fastener is placed in the furnace. Before the gas is passed, the vacuum degree in the vacuum furnace needs to be less than 75 Pa, the voltage is adjusted to 450 V throughout the process, and the current is 10 A; when the furnace temperature T0 reaches 100℃, a small amount of ammonia gas is introduced for ionization; when the temperature T1 in the furnace reaches 500℃, and the current is reduced to less than 10 A, the ammonia gas is started to be introduced, and the flow rate of the ammonia gas is 1.8 L / min, and the flow rate of the mixed gas of ethanol and carbon disulfide (volume ratio of 5:1) is 0.1 L / min. In order to make the performance of the fastener better, the multi-element co-diffusion effect is better, and the multi-element co-diffusion adopts segmented diffusion layer, as Figure 4 shown, the treatment is carried out according to the curve, wherein T1 = 500℃, T3 = 500℃, T2 = 450℃, T4 = 450℃, t1 = 3h, t3 = 5h, t2 = 10min, t4 = 10min.

[0039] 4) Micro-arc oxidation pretreatment: 5 ml / L of dilute nitric acid solution is configured, and the fastener is soaked in the dilute nitric acid solution for 3 min, and then washed clean after taking out.

[0040] 5) Micro-arc oxidation treatment: the electrical parameters of the micro-arc oxidation treatment: positive voltage 700 V, negative voltage 200 V, frequency 1000 Hz, positive and negative duty ratio 5%, and the waveform is sine. The electrolyte for the micro-arc oxidation treatment is prepared by Na2AlO2, NaH2PO2, Na2WO4 and NaOH, the concentration of Na2AlO2 is 20 g / L, the concentration of NaH2PO2 is 5 g / L, the concentration of Na2WO4 is 10 g / L, and the concentration of NaOH is 1 g / L. The electrolyte temperature is kept at 25℃, and the oxidation time is 25 min.

[0041] 6) Washing and drying: the fastener after the micro-arc oxidation treatment is washed by deionized water, and then dried by a drying box.

[0042] Example 2:

[0043] Compared with the above example 1, the difference is only that the diameter of the stud is 40 mm, and in step 1), in the first layer of laser shot peening treatment: the spot diameter is 4 mm, the spot overlap rate is 50%, and the overlap rate between adjacent two passes is 35%; the pitch D of each spiral is 3.1415 x 40 mm = 125.66 mm; in the second layer of laser shot peening treatment: the process parameters of each laser shot peening are 8 GW / cm 2 , and the spot is 2 mm.

[0044] In step 2), the heating time of quenching and tempering is 4 h.

[0045] In step 3), before aeration, the full-process voltage is adjusted to 500 V, the current is 15 A, the furnace temperature T0 is 110℃, T1 is 520℃, the flow rate of the ammonia gas is 2 L / min, and the flow rate of the mixed gas of ethanol and carbon disulfide is 0.2 L / min. The multi-element co-permeation adopts segmented permeation layer, and is processed according to the curve, wherein T1 = 520℃, T3 = 520℃, T2 = 470℃, T4 = 470℃, t1 = 4 h, t3 = 4 h, t2 = 15 min, and t4 = 15 min.

[0046] In step 4), a dilute nitric acid solution of 6 ml / L is configured, and the fastener is soaked in the dilute nitric acid solution for 4 min.

[0047] In step 5), the electrical parameters of the micro-arc oxidation treatment: positive voltage 800 V, negative voltage 300 V, frequency 2500 Hz, positive and negative duty ratio 20%, and the waveform is sine; the concentration of Na2AlO2 in the electrolyte is 35 g / L, the concentration of NaH2PO2 is 10 g / L, the concentration of Na2WO4 is 15 g / L, and the concentration of NaOH is 3 g / L. The electrolyte temperature is kept at 35℃, and the oxidation time is 40 min.

[0048] Example 3:

[0049] The difference between this embodiment and the above-mentioned embodiment 1 is only that the diameter of the stud is 60 mm, and in the first layer laser shot peening process in step 1), the process parameters of each laser shot are 20 GW / cm 2 , the spot diameter is 5 mm, the spot overlap rate is 55%, and the overlap rate between adjacent two shots is 40%; the pitch D of each spiral is 3.1415 x 60 mm = 188.49 mm; in the second layer laser shot peening process, the process parameters of each laser shot are 10 GW / cm 2 , and the spot is 3 mm.

[0050] In step 2), the quenching and tempering heating time is 5 h.

[0051] In step 3), before aeration, the full-range voltage is adjusted to 600 V, the current is 20 A, the furnace temperature T0 is 120℃, T1 is 550℃, the flow rate of the ammonia gas is 2.2 L / min, and the flow rate of the mixed gas of ethanol and carbon disulfide is 0.3 L / min. The multi-element co-diffusion adopts segmented diffusion layer, and is processed according to the curve, wherein T1 = 550℃, T3 = 550℃, T2 = 500℃, T4 = 500℃, t1 = 5 h, t3 = 3 h, t2 = 20 min, and t4 = 20 min.

[0052] In step 4), 8 ml / L of dilute nitric acid solution is configured, and the fastener is soaked in the dilute nitric acid solution for 5 min.

[0053] In step 5), the electrical parameters of the micro-arc oxidation treatment are as follows: positive voltage 900 V, negative voltage 400 V, frequency 3500 Hz, positive and negative duty ratio 30%, and waveform sine; the concentrations of Na2AlO2, NaH2PO2, Na2WO4 and NaOH in the electrolyte are 50 g / L, 15 g / L, 20 g / L and 5 g / L respectively; the electrolyte temperature is kept at 50℃, and the oxidation time is 50 min.

[0054] In addition, the diameter of the stud can also be 50 mm, and in the above-mentioned first layer laser shot peening process, the process parameters of each laser shot are 15 GW / cm 2 .

[0055] Comparative Example 1:

[0056] The ordinary fastener is directly used, i.e. the fastener before the above-mentioned forming method.

[0057] Comparative Example 2:

[0058] The fastener is only treated by multi-element co-diffusion and Dacromet process, i.e. compared with Comparative Example 1, the difference is only that the multi-element co-diffusion and Dacromet process are performed.

[0059] The above comparative examples and examples were tested and analyzed, the salt spray test was performed by using S-ICIA type salt spray corrosion box, and the test was performed according to GB10125-1997 "artificial atmosphere corrosion test salt spray test" standard: 50g / L NaCl solution was used, the PH value was 6.5-7.2, and HCl or NaHCO3 dilute solution was used for pH adjustment, the test temperature was (35±2)℃, the test was stopped when the protection level reached 9, and then the required time was recorded. The hardness test was performed by using HR-150A Brinell hardness tester, a quenched steel ball with a diameter of 10mm was pressed into the surface of the material at a load of 3000kg for a period of time, after unloading, the ratio of load to indentation area was calculated, 3 times of measurement were taken and the average value was the Brinell hardness value (HB); the SO2 acid corrosion test was strictly set according to the provisions of national standard GB / T9789-2008, and the corrosion time was 12h; the fatigue life test was completed on MTS Landmark 370.10 fatigue test system, the stress level was 100MPa, the stress ratio was 0.06, and when the service life of the sample reached 1.0×10 8 , the test was stopped, and the test results are shown in Table 1.

[0060] Table 1 stress corrosion and fatigue properties of examples and comparative examples

[0061]

[0062] As can be seen from Table 1, examples 1-3 did not break, comparative examples 1-2 broke, and the fatigue life was less than 1.0×10 8 . The hardness and fatigue stress of examples 1-3 were significantly improved, and the corrosion resistance was better.

[0063] In the specification and claims of the present application, directional terms are used, such as "vertical", "top", etc., to describe various example structural parts and elements of the present application, but these terms are used only for the convenience of description and are determined based on the example orientation shown in the drawings. Since the disclosed embodiments of the present application can be arranged in different directions, these directional terms are only for description and should not be considered as limitation, for example, "vertical" is not necessarily limited to the direction opposite or consistent with the direction of gravity.

Claims

1. A method for forming a fastener, the fastener having a T-shaped longitudinal section, comprising a vertically arranged stud and a head disposed at the top of the stud, characterized in that, The diameter M of the stud is ≥ 30 mm, and the forming method includes the following steps in sequence: 1) Laser peening: At least one layer of laser peening is performed. The connection between the stud and the head is called the root of the stud. The starting point of the laser peening is from the root of the stud. The laser peening path is arranged in a spiral shape along the axial direction of the stud on the outer peripheral wall of the stud. There are multiple peening paths, which are arranged clockwise along the circumference of the stud. The last peening path overlaps with the first peening path at least partially. 2) Quenching treatment: The fasteners processed in step 1) are quenched. 3) Multi-component co-infiltration treatment: Yttrium is incorporated into the surface of the fasteners treated in step 2), and then the fasteners are placed in a furnace. Before the gas is introduced, the voltage and current are adjusted. After the furnace temperature reaches the preset temperature T0, a small amount of ammonia is introduced for ionization. After the furnace temperature continues to rise to T1, the current is reduced, ammonia is introduced again, and a mixture of ethanol and carbon dioxide is introduced as an auxiliary gas. T0 is 100-120℃ and T1 is 500-550℃. 4) Micro-arc oxidation treatment: Perform micro-arc oxidation treatment on the fasteners after step 3).

2. The forming method according to claim 1, characterized in that: The laser peening process has at least two layers. The starting point of the second laser peening layer is located at the root of the stud and at the overlap between two adjacent laser peening passes of the first layer. The second laser peening layer follows the path of the first laser peening layer, and each spot in the second laser peening layer hits the overlap between two adjacent spots at the corresponding position of the first laser peening layer.

3. The forming method according to claim 2, characterized in that: In the first layer of laser peening, the process parameters for each laser peening pass are 10–20 GW / cm. 2 In the second layer of laser peening, the process parameters for each laser peening pass are 5–10 GW / cm. 2 .

4. The forming method according to claim 2, characterized in that: The thread pitch D for each thread satisfies: D = 3.1415 × M.

5. The forming method according to claim 3, characterized in that: The overlap rate of the first layer of laser peening spots is 45-55%, while the overlap of the second layer of laser peening spots is non-overlapping.

6. The forming method according to claim 3, characterized in that: In the first layer of laser peening, if the stud diameter is 30-50 mm, the laser spot diameter is 3-5 mm; if the stud diameter is greater than 50 mm, the laser spot diameter is 5-6 mm; in the second layer of laser peening, the spot diameter is 1-3 mm.

7. The forming method according to any one of claims 1 to 6, characterized in that: In step 3), before the gas is introduced, the voltage is adjusted to 450-600V and the current to 10A-20A throughout the process. T0 is 100-120℃; T1 is 500-550℃; after T2, the flow rate of ammonia is 1.8-2.2L / min, the flow rate of the mixed gas of ethanol and carbon disulfide is 0.1-0.3L / min, and the volume ratio of ethanol to carbon disulfide is 5:

1.

8. The forming method according to claim 7, characterized in that: The multi-element co-infiltration adopts a segmented infiltration layer. T1 is maintained for time t1, then the temperature is lowered to T2 and maintained for time t2, then the temperature is raised to T3 and maintained for time t3, then the temperature is lowered to T3 and maintained for time t3. Wherein, T2 = T1 - 50℃, T3 = 500~550℃, T4 = T3 - 50℃, t1 = 3~5h, t2 = 10~20min, t3 = 3~5h, t4 = 10~20min, t1 + t3 ≥ 8h.

9. The forming method according to claim 7, characterized in that: Before micro-arc oxidation, the fasteners are placed in a dilute nitric acid solution with a concentration of 5-8 ml / L for 3-5 minutes, then removed and rinsed clean. After micro-arc oxidation, they are cleaned and dried.

10. The forming method according to claim 7, characterized in that: In step 4), the electrical parameters for micro-arc oxidation are as follows: positive voltage 700–900V, negative voltage 200–400V, frequency 1000–3500Hz, positive and negative duty cycle 5–30%, and waveform is sinusoidal. The electrolyte for micro-arc oxidation is composed of Na2AlO2, NaH2PO2, Na2WO4, and NaOH, with Na2AlO2 concentration of 20–50 g / L, NaH2PO2 concentration of 5–15 g / L, Na2WO4 concentration of 10–20 g / L, and NaOH concentration of 1–5 g / L. The electrolyte temperature is maintained at 25–50℃, and the oxidation time is 25–50 min.

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