A machining method of a TC4 titanium alloy surface layer

Abstract

The application provides a processing method of a TC4 titanium alloy surface layer, and innovatively adopts a surface composite treatment mode combining nanocrystallization and vacuum chromizing, uses a mechanical grinding method to prepare a nanocrystallization layer, and uses vacuum chromizing to prepare a diffusion layer on the surface of a substrate, so that the diffusion layer prepared by the preparation method of the application not only has the characteristics of high bonding strength and low wear of a traditional diffusion layer, but also has a nanometer rear surface mechanism improvement, greatly improves the surface performance of the titanium alloy, and effectively improves the comprehensive performance and service life of the substrate. In the process of chromizing, ammonium chloride and ammonium iodide are decomposed to generate ammonia gas as a nitrogen source for nitriding, and nitriding is introduced at the same time of chromizing, so that the surface hardness of the titanium alloy is improved, and HV0.5 is increased from 342 to 609.

Description

Technical Field

[0001] This invention belongs to the field of heat treatment technology, and specifically relates to a processing method for the surface layer of TC4 titanium alloy. Background Technology

[0002] Marine engineering is a crucial foundation for marine research and development. Compared to corrosion-resistant materials like stainless steel and aluminum alloys, titanium alloys, with their high specific strength and low density, can significantly reduce the weight of structural components and offer superior resistance to pitting corrosion, crevice corrosion, intergranular corrosion, and uniform corrosion. Their corrosion resistance in neutral and oxidizing atmospheres and numerous harsh environments surpasses that of other commonly used metallic materials, making them the preferred material for marine engineering equipment. Titanium alloys are ideal for constructing the pressure hulls of deep-sea submarines and submersibles. The French SM97 uses TC4 titanium alloy, as do the hulls of Japan's "Shinkai 2000," and the American submersibles Aivin and Sea-Cliff. Replacing stainless steel with titanium can extend the service life of marine engineering equipment and reduce maintenance and repair costs. According to the "2020 China Titanium Industry Development Report," China's titanium processed material output in 2020 was 97,029 tons, with the marine engineering sector accounting for 7.7% of that output. Research institutions, represented by the Northwest Institute of Nonferrous Metals, have developed a variety of high-performance titanium alloys for marine engineering, such as Ti75, Ti31 and Ti91, which are now widely used in marine engineering equipment such as ships, submarines and detectors.

[0003] However, the marine environment is complex and harsh. Besides the high hydrostatic pressure, the deep sea exhibits significant differences in temperature, salinity, pH, and dissolved oxygen compared to shallower waters, posing unknown corrosion and failure risks to deep-sea equipment. Simultaneously, the harsh deep-sea environment presents challenges to the application of deep-sea equipment and the research on deep-sea corrosion of materials. Statistics show that economic losses caused by marine corrosion reach 700 billion RMB annually, and this figure is increasing year by year. Its application in deep-sea equipment is growing daily, gradually shifting from components to key structures. Although its use is increasing, many problems remain, such as insufficient high-temperature oxidation resistance, heat corrosion resistance, and poor resistance to reducing solutions, limiting its further application. Therefore, the research on the deep-sea corrosion behavior of titanium alloys and the evaluation of their corrosion resistance have become research hotspots.

[0004] Seawater is a highly corrosive natural electrolyte. The highly corrosive chloride ions present can penetrate and destroy the oxide film on metal surfaces. It can also form soluble chlorides with some metal elements, accelerating the dissolution of the oxide film and leaving the metal matrix unprotected and susceptible to corrosion. Furthermore, deep-sea stress is also a major cause of titanium alloy failure. External stress generated in the service environment or uneven stress caused by the volume effect of corrosion products are all sources of stress. The higher the stress level, the shorter the time to stress corrosion cracking. Studies have shown that as seawater depth increases, corrosion of active metals decreases, while the activation and passivation properties of passive metals change, resulting in a lower uniform corrosion rate. However, localized corrosion forms such as pitting and stress corrosion cracking become more pronounced.

[0005] Many domestic users have prepared TiN, CrN, and VALCN coatings on titanium alloy surfaces using thermal spraying and physical vapor deposition (PVD) technologies. However, during use, different levels of peeling occur at the film-substrate interface. Furthermore, some researchers have studied double-glow plasma chromizing on TC4 alloy surfaces, but this method is difficult to implement for parts with complex deep-hole structures, especially since the ion source cannot be sputtered into the inner wall of slender tubes. Additionally, chromium and nitrogen cannot be simultaneously co-diffused; achieving chromium-nitrogen co-diffusion requires nitriding the chromated parts, adding a heat treatment step. Moreover, industrial pure titanium nitrided at 900℃ and above also suffers from reduced surface crack initiation resistance. Poor load-bearing capacity and a sharp increase in roughness also lead to increased surface wear. Improving the stability and safety of titanium alloy components during service is a pressing issue that needs to be addressed. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings and deficiencies of the existing technology and to provide a method for processing the surface layer of TC4 titanium alloy.

[0007] This invention provides a method for processing the surface layer of TC4 titanium alloy, comprising the following steps:

[0008] S1. The surface of the TC4 substrate is subjected to shot peening and nano-processing.

[0009] S2. Clean the substrate surface and dry it;

[0010] S3. The substrate is embedded in a crucible containing chromizing agent, sealed, and placed in a vacuum furnace. It is heated and cooled to obtain the chromizing layer.

[0011] Preferably, in step S1, the shot peening pressure is 0.5~0.8 MPa, the vibration frequency is 40~60 Hz, and the shot diameter is 0.5~1 mm.

[0012] Preferably, in step S3, the chromium diffusion agent comprises, by weight, 50-60 parts chromium powder, 34-40 parts filler, 1-2 parts catalyst, 2-3 parts NaF, and 3-5 parts Y2O3.

[0013] Preferably, the filler is spherical alumina.

[0014] Preferably, the catalyst comprises NH4I and NH4Cl, wherein the mass ratio of NH4I to NH4Cl is (1.5-2.5):1.

[0015] Preferably, in step S3, the temperature is raised to 550-600℃ at a rate of 20-100℃ / h and held for 1-2 hours, during which diffusion chromium infiltration is carried out; then the temperature is lowered from 550-600℃ to 300-200℃ at a rate of 60-70℃ / h, the heat source is cut off and the workpiece is cooled with the furnace, and then removed from the furnace to cool below 100℃.

[0016] The beneficial effects of this invention are as follows:

[0017] (1) This invention innovatively employs a combination of nano-sizing and vacuum chromizing to perform composite treatment on the surface of TC4 titanium alloy. A nano-layer is prepared using nanotechnology. Since the growth rate of the compound layer on the material surface depends on both chemical reaction and atomic diffusion, the surface nanocrystals exhibit high activity after nano-sizing, accelerating surface chemical reactions. Furthermore, the high volume fraction of the interfaces between the surface nanocrystals provides an ideal channel for atomic diffusion. The combined effect of these two factors significantly accelerates the diffusion kinetics. Innovatively, chromizing is implemented around the nano-sizing of the titanium alloy. A chromizing layer is prepared on the substrate surface using vacuum chromizing. This method ensures that the chromizing layer maintains the high bonding strength and low wear characteristics of traditional chromizing layers while also possessing improved surface structure after nano-sizing, greatly enhancing the surface properties of the titanium alloy and effectively improving the overall performance and service life of the substrate. During chromizing, ammonium chloride and ammonium iodide decompose to generate ammonia, providing a nitrogen source for nitriding. Nitriding is also introduced during chromizing, increasing the surface hardness of the titanium alloy; HV0.5 increases from 342 to 609.

[0018] (2) This invention addresses the issue that the inner wall of a slender titanium alloy tube cannot be treated by means of ion nitriding, spraying, physical vapor deposition, etc. due to size limitations. It uses powder filling to prepare a diffusion layer to improve the performance of the part. It introduces surface nano-sizing to reduce the temperature of preparing the diffusion layer and sets the temperature of preparing the diffusion layer to 550~600℃, which can effectively avoid the phase transformation that easily occurs in titanium alloys above 900℃ and the deformation of the part caused by high temperature.

[0019] (3) Using a vacuum heat treatment furnace for chromium diffusion can avoid the oxidation problem of traditional chromium diffusion technology. Adding NaF to the diffusion agent can activate the surface of the parts and solve the problem of rough surface caused by powder sticking after chromium diffusion. Adding rare earth Y2O3 can increase the chromium diffusion speed, shorten the chromium diffusion time, and reduce energy consumption. This method can avoid the disadvantages of insufficient wear resistance and corrosion resistance in preparation by a single technology. Attached Figure Description

[0020] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, obtaining other drawings based on these drawings without creative effort still falls within the scope of the present invention.

[0021] Figure 1 This is a graph showing the test results of the elemental content of the infiltration layer with TC4 as the matrix in Example 1 of the present invention as a function of depth.

[0022] Figure 2 This is a three-dimensional contour diagram of the wear marks in a TC4 substrate according to Embodiment 1 of the present invention.

[0023] Figure 3 This is a three-dimensional contour diagram of the wear marks on the TC4 sample in Example 2 of the present invention;

[0024] Figure 4 This is a graph showing the test results of the elemental content of the infiltration layer with TC4 as the matrix as a function of depth in Embodiment 2 of the present invention.

[0025] Figure 5 The bar chart shows the average surface hardness HV5 of the chromium-nitrided layer in Examples 1, 2, and 3 of this invention. Detailed Implementation

[0026] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings.

[0027] This invention first employs surface nano-sizing technology and a surface mechanical grinding treatment device. The main factors affecting surface self-nano-sizing include: shot peening pressure P, vibration frequency f, shot diameter D, and shot peening time t. Among these, the shot peening time t has a significant impact on surface self-nano-sizing. The process parameters for surface self-nano-sizing of TC4 test material in this invention embodiment are as follows: shot peening pressure 0.5~0.8 MPa, vibration frequency 40~60 Hz, shot diameter 0.5~1 mm, and treatment time 20~40 min.

[0028] A method for preparing a chromium-impregnated layer on TC4 titanium alloy material employs vacuum chromium impregnation technology. The vacuum chromium impregnation uses Cr, Al₂O₃, NaF, Y₂O₃, NH₄Cl, and NH₄I as chromium impregnating agents. The chromium impregnation formula contains 50-60% chromium powder (impregnating agent), 34-40% spherical alumina (filler), 2-3% sodium fluoride, 3-5% yttrium oxide, and 1-2% ammonium chloride + ammonium iodide (catalyst), with an ammonium iodide to ammonium chloride mass ratio of (1.5-2.5):1. The chromium powder and spherical alumina have a particle size of 300-400 mesh; the mixture is stirred until homogeneous using a powder mixer.

[0029] Using high-purity Ar as a protective gas, a chromium-impregnating agent is embedded in a substrate after surface cleaning and activation treatment. The substrate is then placed in a vacuum furnace and heated and kept at a constant temperature to prepare an impregnation layer on the substrate surface. The characteristic feature is that the thickness of the impregnation layer is controlled by adjusting the temperature and time during the preparation process.

[0030] The preparation process of the infiltration layer consists of the following two steps:

[0031] 1. After cleaning and degreasing the substrate and performing surface activation treatment, bury it in a container containing chromium diffusion agent, cover it, seal the container and load it into the furnace. Start the vacuum pump to begin evacuation. When the vacuum level reaches below 10 Pa, raise the temperature to 550-600℃ at a rate of 20-100℃ / h and hold it at that temperature for 1-2 hours. During this process, diffusion chromium diffusion is carried out.

[0032] 2. Cool the workpiece from 550-600℃ to 300-200℃ at a rate of 60-70℃ / h. Disconnect the heat source and cool the workpiece with the furnace until it reaches below 100℃, then remove it from the furnace and air cool.

[0033] Example 1

[0034] In this embodiment, the above-mentioned diffusion layer is prepared on the TC4 substrate, and the specific preparation method is as follows:

[0035] 1) The process of preparing a nano-layer on the surface of TC4 sample using a sandblasting machine is as follows: shot blasting pressure is 0.5 MPa, vibration frequency is 40 Hz, shot diameter is 0.5 mm, and processing time is 20 min.

[0036] 2) Pre-infiltration treatment

[0037] The substrate was placed in petroleum ether and ultrasonically stirred and cleaned for 15 minutes to remove oil stains from the surface of the substrate. Then it was cleaned in ethanol for 15 minutes and finally dried.

[0038] 3) Vacuum chromizing

[0039] The chromizing process consists of the following two steps:

[0040] The TC4 sample substrate was embedded in a crucible containing a chromizing agent (50% chromium, 40% alumina, 3% sodium fluoride, 5% yttrium oxide, and 2% NH4Cl and NH4I). The crucible was covered and sealed, then placed in a vacuum furnace. Heating began when the vacuum level reached below 10 Pa. The temperature was then increased to 600℃ at a rate of 20℃ / h and held for 2 hours.

[0041] The temperature is reduced from 600℃ to 300~200℃ at a rate of 70℃ / h. The heat source is cut off and the workpiece is cooled with the furnace until it reaches below 100℃, then it is removed from the furnace and air-cooled to obtain the infiltrated layer.

[0042] The above-prepared infiltration layer was subjected to the following performance tests:

[0043] (1) The surface hardness of the sample was tested using the HV-10IS series Vickers microhardness tester according to the method in GB / T 4340.1-2009. The test results are as follows: Figure 5 As shown, the average HV0.5 value of the measured results is 609.

[0044] (2) The surface chemical analysis glow discharge emission spectroscopy method was used to detect the distribution of Ti, Cr, and N elements from the sample surface along the vertical direction according to GB / T 19502-2004. The results are as follows: Figure 1 The curve in the middle indicates that the thickness of the chromium nitriding layer is 10 μm.

[0045] (3) The friction and wear life of the infiltrated layer sample and the raw material sample under atmospheric conditions was evaluated using a UMT-3 multifunctional friction and wear testing machine. The specific method was as follows: the infiltrated layer sample and the raw material sample were used in a reciprocating sliding manner with the friction pair. The sliding frequency was 5 Hz, the load was 5 N, the ambient temperature was (25±3)℃, the relative humidity was (75±5)%, the test time was 30 min, and a steel ball with Φ=6mm was used as the friction pair. The three-dimensional contours of the wear tracks of the infiltrated layer sample and the raw material sample are shown below. Figure 2 and Figure 3 As shown, the wear marks on the infiltrated layer sample are relatively small, while the original sample morphology shows obvious pitting. The calculated wear rates are 1.7 × 10⁻⁶. -5 / mm 3 / Nm and 2.1×10 -4 / mm 3 / Nm, the infiltrated layer sample has a low wear rate and good wear resistance.

[0046] Example 2

[0047] In this embodiment, the specific steps for preparing the diffusion layer on the TC4 titanium alloy substrate are as follows:

[0048] 1) The process of preparing a nano-layer on the surface of TC4 sample using a sandblasting machine is as follows: shot blasting pressure is 0.7 MPa, vibration frequency is 50 Hz, shot diameter is 0.8 mm, and processing time is 30 min.

[0049] 2) Pretreatment before chromizing

[0050] The steps are the same as in Example 1, step 1).

[0051] 3) The specific steps of vacuum chromizing are as follows:

[0052] (1) is basically the same as step (1) in Example 1, except that the infiltrator (chromium mass fraction is 60%, alumina mass fraction is 34%, sodium fluoride is 2.5%, yttrium oxide is 3%, NH4Cl and NH4I mass fraction is 1.5%) is heated to 550℃ at a rate of 60℃ / h and kept at that temperature for 1h.

[0053] (2) is basically the same as step (2) in Example 1, except that the cooling rate is 65℃ / h.

[0054] The chromium-nitrided layer prepared above was subjected to the following performance tests:

[0055] (1) The surface hardness of the sample was tested using the HV-10IS series Vickers microhardness tester according to the method in GB / T 4340.1-2009. The test results are as follows: Figure 5 As shown, the average HV5 value of the measured results is 555.

[0056] (2) The surface chemical analysis glow discharge emission spectroscopy method was used to detect the distribution of Ti, Cr, and N elements from the sample surface along the vertical direction according to GB / T 19502-2004. The results are as follows: Figure 3 The curve in the middle indicates that the thickness of the chromium nitrided layer is 7.5 μm.

[0057] Example 3

[0058] In this embodiment, the specific steps for preparing the diffusion layer on the TC4 titanium alloy substrate are as follows:

[0059] 1) The process of preparing a nano-layer on the surface of TC4 sample using a sandblasting machine is as follows: shot blasting pressure is 0.8 MPa, vibration frequency is 60 Hz, shot diameter is 1 mm, and processing time is 40 min.

[0060] 2) Pretreatment before chromizing

[0061] The steps are the same as in Example 1 and 2, step 1).

[0062] 3) The specific steps of vacuum chromizing are as follows:

[0063] (1) Same as step (1) in Examples 1 and 2, except that the infiltrator (chromium mass fraction of 56%, alumina mass fraction of 35%, sodium fluoride 3%, yttrium oxide 4%, NH4Cl and NH4I mass fraction of 2%) is heated to 580℃ at a rate of 100℃ / h and kept at that temperature for 1.5h.

[0064] (2) is basically the same as step (2) in Example 1, except that the cooling rate is 70℃ / h.

[0065] The chromium-nitrided layer prepared above was subjected to the following performance tests:

[0066] The surface hardness of the samples was tested using the HV-10IS series Vickers microhardness tester according to the method in GB / T 4340.1-2009. The test results are as follows: Figure 5 As shown, the average HV5 value of the measured results is 532.

[0067] The above description discloses only preferred embodiments of the present invention and should not be construed as limiting the scope of the present invention. Therefore, equivalent variations made in accordance with the claims of the present invention are still within the scope of the present invention.

Claims

1. A method for processing the surface layer of TC4 titanium alloy, characterized in that, Includes the following steps: S1. The surface of the TC4 substrate is subjected to shot peening and nano-processing. S2. Clean the substrate surface and dry it; S3. Embed the substrate into a crucible containing chromizing agent, seal it, put it into a vacuum furnace, heat it to 550~600℃ and hold it for 1~2 hours, and then cool it to obtain the chromizing layer. In step S3, the chromium diffusion agent comprises, by weight, 50-60 parts chromium powder, 34-40 parts filler, 1-2 parts catalyst, 2-3 parts NaF, and 3-5 parts Y2O3; The catalyst comprises NH4I and NH4Cl, with a mass ratio of NH4I to NH4Cl of (1.5-2.5):1; In step S3, the temperature is increased to 550-600℃ at a rate of 20-100℃ / h and held for 1-2 hours, during which diffusion chromium infiltration is carried out; then the temperature is reduced from 550-600℃ to 300-200℃ at a rate of 60-70℃ / h, the heat source is cut off and the workpiece is cooled with the furnace, and then removed from the furnace to cool when the temperature drops below 100℃.

2. The method for processing a TC4 titanium alloy surface layer according to claim 1, characterized in that: In step S1, the shot peening pressure is 0.5~0.8MPa, the vibration frequency is 40~60Hz, and the shot diameter is 0.5~1mm.

3. The method for processing a TC4 titanium alloy surface layer according to claim 1, characterized in that: The filler is spherical alumina.

Images