Method for strengthening CoCrFeNiMo laser cladding layer by inducing TCP phase precipitation through high-temperature aging

Abstract

The invention discloses a method for strengthening a CoCrFeNiMo laser cladding layer by inducing TCP phase precipitation through high-temperature aging, and belongs to the technical field of metal surface engineering and remanufacturing. According to the invention, the pre-alloyed spherical powder is adopted to blend the required component high-entropy alloy powder, so that the powder preparation time and cost are saved, and the thickness of a single cladding layer is controlled to be 0.5-2mm. The cladding layer is placed in a vacuum furnace to be heated, argon is selected as the protective atmosphere, and the temperature rises along with the furnace; firstly, a sample block is heated to 950 DEG C, heat preservation is conducted for 1 h, then the sample block is cooled to 500 DEG C, 700 DEG C and 900 DEG C along with a furnace, heat preservation is conducted for 5 h, and furnace cooling is conducted after heat preservation is completed so that alloy elements in a cladding layer can be fully diffused in a solid solution, and a TCP phase is induced to be generated in situ. According to the method, the appropriate high-temperature aging heat treatment temperature and heat preservation time are selected, fine TCP phases are induced to be separated out from the CoCrFeNiMo laser cladding layer, the hardness and strength of the cladding layer are greatly improved under the condition that exogenous elements are not added, the method has the advantages of being simple in process, low in cost, high in applicability and the like, and application of the high-entropy alloy in the field of surface strengthening and remanufacturing can be promoted.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of metal materials and their surface remanufacturing, and relates to a method for in-situ inducing TCP phase strengthening of a CoCrFeNiMo high-entropy alloy cladding layer by high-temperature aging heat treatment. BACKGROUND

[0002] Unlike the traditional alloy which is designed with a single element as the main component and supplemented by a small amount of other elements, the high-entropy alloy can accommodate multiple elements at the same time, and usually exhibits excellent high strength, high hardness, good corrosion resistance and excellent high-temperature performance. Therefore, the high-entropy alloy has shown great application potential in the high-temperature field of aerospace, nuclear energy and the like. During the solidification process of the high-entropy alloy, a large number of intermetallic phases are usually not formed, but a simple FCC, BCC or HCP solid solution matrix structure is formed. The formation of such a solid solution matrix greatly improves the strength of the material, so that the high-entropy alloy is superior to many traditional alloys in terms of mechanical properties. However, due to the diversity and complexity of the elements in the high-entropy alloy, the plasticity thereof is relatively low, which limits its wide application in some applications. Especially after laser cladding of the high-entropy alloy, the performance of the cladding layer is often affected by the high melting point and complex phase composition, making it difficult to optimize the mechanical properties.

[0003] As a kind of efficient surface strengthening and repairing means, the laser cladding technology can be used to melt and clad an alloy with compactness, good bonding strength and designable composition on the surface of a substrate material, and is widely applied to the repair and remanufacturing of parts. For the high-entropy alloy with high melting point and high hardness, if specific strengthening phases can be further generated through subsequent heat treatment, the comprehensive performance of the cladding layer can be significantly improved. However, different temperature and time conditions in the heat treatment process can affect the organizational structure and phase composition of the cladding layer, which can either promote the precipitation of strengthening phases or cause the problem of plasticity reduction. The TCP phase (Topologically Close-Packed Phases) is a common precipitated phase in iron-based, cobalt-based and nickel-based high-temperature alloys and austenitic stainless steel. The sigma phase, mu phase and R phase Laves phase all belong to this category. The TCP phase (such as the sigma phase and mu phase) has extremely high hardness but outstanding brittleness. For the laser cladding strengthening layer or repair layer, relatively high hardness and strength are usually required to resist external wear, and the requirement for plasticity is relatively low, and a certain plasticity is acceptable. Therefore, if a dispersed TCP phase can be generated in-situ, the hardness and strength of the cladding layer can be effectively improved, and although the plasticity is reduced, the influence of this change is acceptable in the repair and strengthening occasions.

[0004] In the prior art, the research on performance optimization of high-entropy alloys mostly relies on complex heat treatment processes. For example, CN111809126A discloses a method for eliminating the amplitude modulation structure in FeCrNiMn quaternary high-entropy alloy, eliminating the FeCr phase in FeCrNiMn quaternary high-entropy alloy, and improving its toughness and impact resistance, thereby improving the mechanical properties of the high-entropy alloy. However, this method does not involve the directional regulation of specific nanophases. In addition, traditional strengthening methods, such as the generation of sigma phase or carbides to improve strength, often cause brittleness problems, and the generation of carbides requires additional elements, increasing the complexity of the process. CN109457197A discloses a high-entropy alloy heat treatment technology assisted by ultrasonic and pressure, which refines the grain through ultrasonic. The heat treatment process includes: fixing the high-entropy alloy sample by applying appropriate pressure without causing plastic deformation of the sample, then placing the sample in a heat treatment furnace at 800°C, and treating it in an argon atmosphere while starting the ultrasonic environment. The sample is kept in this environment for 5 hours and cooled in the furnace. This process can effectively promote the transformation of high-entropy alloy grains and refine the grain structure, thereby improving the overall mechanical properties of the alloy. Although this technology has a significant effect on improving the performance of the alloy, the process flow is relatively complex, and each process parameter has a relatively sensitive effect on the performance of the alloy, making it difficult to operate and control, which is not conducive to practical application. Other high-entropy alloy impact heat treatment technologies (CN110106457A) can eliminate casting defects, but also face the problem of low efficiency of multi-stage temperature control and impact treatment. In addition, CN113817971A discloses a heat treatment method for NbMoTaW-based refractory high-entropy alloy, which increases the hardness and wear resistance of the high-entropy alloy by 0.5-1 times through heat treatment. However, this heat treatment process is relatively complex and takes a long time, and does not combine with forging technology, resulting in poor cutting performance of the alloy.

[0005] In summary, the prior art generally relies on exogenous elements (such as C, B) to generate strengthening phases (such as carbides), increasing the difficulty of composition control; at the same time, it lacks the ability to directionally regulate specific nanophases (such as ultra-fine grains or layered structures), and the organizational optimization often has a high degree of randomness. More importantly, the process flow of the prior art is generally long and highly sensitive to parameters, which has certain limitations in industrial application.

[0006] To solve the above problems, the present application proposes a simple and efficient method for preparing high-strength and high-hardness CoCrFeNiMo high-entropy alloy laser cladding layer. Through this innovative process, TCP phases can be generated in situ in the high-entropy alloy cladding layer, thereby optimizing the microstructure of the cladding layer, significantly improving its strength and hardness, and avoiding the complexity of relying on exogenous elements to generate strengthening phases, providing a new solution for repair and remanufacturing applications of high-entropy alloys. SUMMARY

[0007] The purpose of the present application is to provide a heat treatment method of high-strength and high-hardness CoCrFeNiMo high-entropy alloy laser cladding layer with proper plasticity, which innovatively uses two kinds of pre-alloyed spherical powder to adjust the required powder composition, saves the time and cost of the past specific powder atomization, generates TCP phase in situ through hierarchical heat treatment, greatly strengthens the hardness and strength of the cladding layer, and does not affect its wear resistance and corrosion resistance.

[0008] The present application designs a high-entropy alloy based on Co, Cr, Fe, Ni and Mo, and the main effects of each element in the present application are briefly described as follows.

[0009] Co can control element diffusion and precipitation kinetics, play a solid solution strengthening effect, and promote the nucleation and uniform distribution of leaf-shaped phase. Studies have shown that Co can significantly improve the hot corrosion resistance of high-temperature alloy. The addition of Co can reduce the precipitation of carbides on the grain boundary, improve the hot working property, plasticity and impact toughness of high-entropy alloy.

[0010] Cr not only has excellent oxidation resistance and hot corrosion resistance, but also helps to control the morphology and interface characteristics of leaf-shaped phase, has good oxidation resistance and hot corrosion resistance, and is an important element indispensable in high-entropy high-temperature alloy.

[0011] Fe as a base element helps to build a stable alloy matrix and participate in the precipitation reaction of leaf-shaped phase, thereby improving the overall mechanical properties.

[0012] Ni, with its high solubility and diffusivity, promotes the complete transformation of traditional gamma phase to leaf-shaped phase during heat treatment, realizing the synergistic effect of solid solution strengthening and precipitation strengthening, and its main role is to form a face-centered cubic crystal structure matrix.

[0013] Mo plays a key role in significantly improving the high-temperature strength and creep resistance of the alloy by controlling the nucleation and growth of leaf-shaped phase. It plays a crucial role in improving the hardness and wear resistance of the alloy.

[0014] The technical solution adopted by the present application includes the following steps.

[0015] (1) Steel plate pretreatment. Use sandpaper to roughen the surface of the substrate to remove the oxide film on the surface of the substrate to prevent rusting, and put it into an oven for preheating at 200℃~400℃ to reduce the tendency of cracking.

[0016] (2) High-entropy alloy powder preparation. The pre-alloyed powder of CoCrFeNi and CoCrFeNiMo with a purity of 99.9% or above is used as raw material, and the powder particle size is 45-150m; the elements and atomic ratio of CoCrFeNiCo pre-alloyed powder are Co:Cr:Fe:Ni=a:b:c:d, and the chemical composition of each element is 20≤a≤30, 20≤b≤30, 20≤c≤30, and 20≤d≤30, a+b+c+d=100. The elements and atomic ratio of CoCrFeNiMo pre-alloyed powder are Co:Cr:Fe:Ni:Mo=a:b:c:d:e, and the chemical composition of each element is 20≤a≤30, 20≤b≤30, 20≤c≤30, 20≤d≤30, 1≤e≤13, and a+b+c+d+e=100; the preparation ratio of CoCrFeNi pre-alloyed powder:CoCrFeNiMo alloy is 1%-99% (mass fraction), and the mixed powder is prepared according to the ratio, mixed by a horizontal planetary mixer for 4-10h, and the mixer speed is 100-250r / min; the mixed powder is dried at 200℃ for 3h to ensure the fluidity of the material, and is ready for use.

[0017] (3) Laser cladding layer preparation. The prepared CoCrFeNiMo high-entropy alloy powder is selected to clad the surface of the Q235 substrate. The laser power, scanning speed, powder feeding speed and other parameters during the cladding process should be optimized according to the requirements of the cladding layer to ensure that the cladding layer is dense, crack-free and pore-free, and the single-layer thickness is generally controlled between 0.5-2mm.

[0018] (4) High-temperature aging heat treatment process. The sample block with the cladding layer is placed in a vacuum atmosphere furnace for heating, and the protective atmosphere is selected to be argon, and the furnace is heated. First, the sample block is heated to 950℃ and kept for 1h, then cooled to 500℃, 700℃ and 900℃ respectively, and kept for 5h, and then cooled in the furnace, so that the alloy elements in the cladding layer can fully diffuse in the solid solution, and induce the in-situ generation of TCP phase.

[0019] Compared with the prior art, the advantages of the present application are that: the high-entropy alloy powder of the required composition is prepared by mixing the pre-alloyed powder, which saves the time and cost of the past specific powder atomization process, and then the cladding layer is prepared, and the TCP phase is in-situ precipitated in the CoCrFeNiMo high-entropy alloy cladding layer by high-temperature grading aging, which can significantly improve the hardness and strength of the cladding layer; the heat treatment system is relatively simple, the cost is low, and it is easy to popularize and apply in industrial production; the generation of TCP phase has a certain negative impact on plasticity, but it is acceptable in the application of repair and remanufacturing cladding layer, and will not significantly weaken the service life; the present application provides a new idea for high-entropy alloy surface strengthening and remanufacturing technology, which can be combined with other surface engineering methods to further expand the application range of high-entropy alloy. BRIEF DESCRIPTION OF DRAWINGS

[0020] Figure 1 Mo after aging at different temperatures in the present invention 0.1 XRD curve of the cladding layer.

[0021] Figure 2 Mo after aging at different temperatures in the present invention 0.3 XRD curve of the cladding layer.

[0022] Figure 3 Mo after aging at different temperatures in the present invention 0.5 XRD curve of the cladding layer.

[0023] Figure 4 Mo after aging at different temperatures in the present invention 0.1 SEM images of the cladding layer, where (a) as-deposited cladding; (b) aging at 500°C; (c) aging at 700°C; (d) aging at 900°C.

[0024] Figure 5 Mo after aging at different temperatures in the present invention 0.3 SEM images of the cladding layer, where (a) as-deposited cladding; (b) aging at 500°C; (c) aging at 700°C; (d) aging at 900°C.

[0025] Figure 6 Mo after aging at different temperatures in the present invention 0.5 SEM images of the cladding layer, where (a) as-deposited cladding; (b) aging at 500°C; (c) aging at 700°C; (d) aging at 900°C.

[0026] Figure 7 Mo after aging at different temperatures in the present invention 0.1 Average hardness of the cladding layer.

[0027] Figure 8 Mo after aging at different temperatures in the present invention 0.3 Average hardness of the cladding layer.

[0028] Figure 9 Mo after aging at different temperatures in the present invention 0.5 Average hardness of the cladding layer.

[0029] Figure 10 Mo after aging at different temperatures in the present invention 0.1 Tensile curve of the cladding layer.

[0030] Figure 11 Mo after aging at different temperatures in the present invention 0.3 Tensile curve of the cladding layer.

[0031] Figure 12 The Mo 0.5 Tensile curve of the cladding layer. DETAILED DESCRIPTION

[0032] The present application is described in detail from three aspects of component design, alloy preparation and performance test.

[0033] 1. Component design: specifically, three different components of Co a Cr b Fe c Ni d Mo e , wherein the component range of each element is 20≤a≤30, 20≤b≤30, 20≤c≤30, 1≤e≤12, a+b+c+d+e=100. The specific components of the three alloys are shown in Table 1. The purpose of designing the three alloy components is mainly to study the influence law of each element on the mechanical properties of the alloy. In addition to Ni, Co, Cr, Fe, Mo is selected as an additional element to further enhance the strength and hardness.

[0034] Table 1: Component ratio of three CoCrFeNiMo high-entropy alloys (atomic percentage).

[0035] Alloy name Co Cr Fe Ni Mo Mo 0.1 ]] 24.39 24.39 24.39 24.39 2.44 Mo 0.3 ]] 23.26 23.26 23.26 23.26 6.96 Mo 0.5 ]]> 22.22 22.22 22.22 22.22 11.12

[0036] 2. Alloy preparation.

[0037] 1) batching: using CoCrFeNi and CoCrFeNiMo finished powder with purity of more than 99.9% as raw material, powder particle size is 45~150m; according to the proportion, mixed powder is obtained to obtain the required component, mixed powder is mixed by horizontal planetary mixer for 4~10h, the rotation speed of the mixer is 100~250r / min; the mixed powder is dried at 200℃ for 3h to ensure the flowability of the material, and is ready for use.

[0038] 2) preparation of high-entropy alloy cladding layer: laser cladding is used to prepare the cladding layer, the laser cladding power is 1800~2500W, the scanning speed is 8~12mm / s, the powder feeding speed is 15~25g / min, the spot diameter is 3~4mm, the powder feeding gas and the protection gas are high-purity argon, the cladding overlap rate is 30%~70%, and the high-entropy alloy layer is cladded on the surface of Q235 steel material by multi-cladding, the single-layer cladding layer thickness is 0.5~2mm.

[0039] 3) High temperature aging heat treatment process: the sample with cladding layer is placed in a vacuum atmosphere furnace for heating, and argon is selected as the protective atmosphere. First, the sample is heated to 950℃ for 1h, then cooled to 500℃, 700℃ and 900℃ respectively, and kept for 5h, and then cooled in the furnace to make the alloying elements in the cladding layer fully diffuse in the solid solution and induce in-situ TCP phase.

[0040] 3, Alloy microstructure and performance test.

[0041] 1) X-ray diffraction (XRD) test: the phase composition of the sample is analyzed by using an X-ray diffractometer, and a Cu target XRD device is used for testing, the scanning range is 20°-90°, and the scanning speed is 5° / min. Figure 1 Mo cladding layer after aging at different temperatures in the present application 0.1 The XRD curve of the cladding layer shows that the original cladding layer is an equiaxed FCC structure. Figure 2 Mo cladding layer after aging at different temperatures in the present application 0.3 The XRD curve of the cladding layer shows that the original FCC phase has been precipitated in the cladding layer, and the μ phase (Co7Mo6 phase) peak begins to appear after aging at 900℃. Figure 3 Mo cladding layer after aging at different temperatures in the present application 0.5 The XRD curve of the cladding layer shows that the σ phase and μ phase peaks become obvious. It can be seen that the increase of Mo content and aging temperature will promote the precipitation of σ phase and μ phase.

[0042] 2) Scanning electron microscope observation: the microstructure of the cladding layer is observed by using a scanning electron microscope. Figure 4 Mo cladding layer after aging at different temperatures in the present application 0.1 The SEM image of the cladding layer shows that Mo 0.1 The microstructure of the as-deposited cladding layer is fine and uniform, most of the grains are equiaxed, and the microstructure is equiaxed FCC and intergranular net-like divorced eutectic FCC+σ. With the increase of aging temperature, the net-like divorced eutectic structure gradually decreases. When the temperature reaches 900℃, the characteristics of the net-like divorced eutectic have basically disappeared. Figure 5 Mo cladding layer after aging at different temperatures in the present application 0.3 The SEM image of the cladding layer shows that Mo 0.3 The microstructure of the as-deposited cladding layer is composed of FCC phase, σ phase and intergranular net-like divorced eutectic FCC+σ. Similarly, with the increase of aging temperature, the net-like divorced eutectic gradually decreases. When the aging temperature rises to 900℃, the original microstructure structure changes obviously, and a large amount of granular TCP phase is precipitated in the sample. Figure 6 Mo cladding layer after aging at different temperatures in the present application 0.5SEM images of the cladding layer show that the network-shaped divorced eutectic gradually decreases with increasing aging temperature. When the aging temperature rises to 900°C, the original microstructure undergoes a significant transformation, with a large number of massive, dot-shaped, and granular TCP phase precipitates forming in the sample.

[0043] 3) Hardness test: The sample was tested using a HV-1000 small load Vickers hardness tester with a load of 0.5 kg and a load holding time of 15 s. Ten test points were randomly selected in the cladding layer and the average hardness was calculated. Figure 7 The Mo after aging treatment at different temperatures in the present invention 0.1 Average hardness of the cladding layer. It can be observed that the hardness of the cladding layer increased by approximately 20% after aging at 500°C and 700°C, with the hardening effect of 500°C slightly better than that of 700°C. When the aging temperature reached 900°C, the hardness increased by approximately 40%. Figure 8 The Mo after aging treatment at different temperatures in the present invention 0.3 The average hardness of the cladding layer. Its hardness change law is similar to that of Mo 0.1 The hardness of the sample aged at 900℃ can reach 485 HV. 0.5 . Figure 9 The present invention is different temperature aging treatment Mo 0.5 The average hardness of the cladding layer. It can be seen that the hardness change law is similar to that of Mo 0.1 When the aging temperature reaches 900℃, the hardness increases to 540HV 0.5 .

[0044] 4) Tensile mechanical properties test: The prepared high entropy alloy cladding layer was machined into a standard size plate sample and subjected to room temperature tensile test. The tensile test was conducted using an electronic universal tensile testing machine (MTS E45.305) with a tensile rate of 1×10 -3 s -1 At least three samples were selected for testing for each alloy composition. The tensile stress-strain curves of the three composition cladding layers are shown in Figure 2. Figures 10-12 As shown. Figure 10 It can be seen that aging treatment significantly improves Mo 0.1 The strength and plasticity of the cladding layer. Figure 11 It can be seen that aging treatment at 500℃ and 700℃ can achieve Mo 0.3 The strength and toughness of the cladding layer are significantly improved. Aging at 900℃ significantly increases the strength, with the yield strength increasing from about 500MPa to about 800MPa, and the tensile strength increasing from about 800MPa to about 1150MPa. At this time, although the plasticity decreases significantly, it still has more than 5% plasticity. Figure 11 It can be seen that high temperature aging treatment significantly improves the Mo 0.5The strength of the cladding layer is increased, but the plasticity is reduced. After aging at 500℃ and 700℃, the yield strength is increased from about 500MPa to about 700MPa, the tensile strength is increased from about 800MPa to about 1080MPa, and the plasticity is more than 5%. After aging at 900℃, Mo 0.5 The yield strength of the cladding layer increased to 900 MPa, the tensile strength exceeded 1200 MPa, and it still had about 2% plasticity. The above structure shows that the high-temperature graded aging heat treatment of the present invention can effectively promote the precipitation of TCP phase, thereby enhancing the strength and hardness of the cladding layer.

Claims

1. A method for high-temperature aging-induced TCP phase precipitation strengthening of a CoCrFeNiMo laser cladding layer, the high-entropy alloy of the invention being characterized in that it comprises five alloying elements Co, Cr, Fe, Ni, Mo, the alloying chemical composition of which is designed as follows in atomic ratio: Co 20-30% Cr 10-20% Fe 20-30% Ni 10-20% Mo 5-15% wherein, a Cr b Fe c Ni d Mo e and wherein, 20≤a≤27, 20≤b≤27, 20≤c≤27, 20≤d≤27, 1≤e≤12, a+b+c+d+e=100; the preparation method of the laser cladding layer comprises the following steps: (1) rough grinding the surface of the substrate with sandpaper to remove the oxide film on the surface of the substrate to prevent rusting of the substrate surface, and placing the substrate into an oven for preheating treatment at 200-400 DEG C to reduce the crack tendency; (2) using CoCrFeNi and CoCrFeNiMo pre-alloy powders with a purity of more than 99.9% as raw materials, and the powder particle size is 45-150m; the elements contained in the CoCrFeNiCo pre-powder and the atomic ratio are as follows: Co:Cr:Fe:Ni=a:b:c:d, the chemical composition of each element is 20≤a≤30, 20≤b≤30, 20≤c≤30, 20≤d≤30, a+b+c+d=100; the elements contained in the CoCrFeNiMo pre-alloy powder and the atomic ratio are as follows: Co:Cr:Fe:Ni:Mo=a:b:c:d:e, the chemical composition of each element is 20≤a≤30, 20≤b≤30, 20≤c≤30, 20≤d≤30, 1≤e≤13, a+b+c+d+e=100; the preparation proportion of the CoCrFeNi pre-alloy powder to the CoCrFeNiMo alloy is 1%-99% (mass fraction), and the mixed powder is prepared according to the proportion; the mixed powder is mixed by using a horizontal planetary mixer for 4-10h, and the rotation speed of the mixer is 100-250r / min; the mixed powder is dried at 200 DEG C for 3h to ensure the fluidity of the material, and the mixed powder is prepared; (3) laser cladding is used to prepare the cladding layer, the laser cladding power is 1800-2500W, the scanning speed is 8-12mm / s, the powder feeding speed is 15-25g / min, the spot diameter is 3-4mm, the powder feeding gas and the protective gas are high-purity argon, the cladding overlap rate is 30%-70%, multi-pass cladding is carried out, the high-entropy alloy layer is cladded on the surface of the carbon steel, and the thickness of the single-layer cladding layer is 0.5-2mm; the high-temperature aging method comprises the following steps: (1) placing the sample block with the cladding layer into a vacuum atmosphere furnace for heating, the protective atmosphere is selected to be argon, and the furnace is heated; (2) heating the sample block to 950 DEG C and keeping for 1h; (3) then cooling to 500-900 DEG C with the furnace and keeping for 5h, and after the end, the furnace is cooled to make the alloy elements in the cladding layer fully diffuse in the solid solution and induce in-situ generation of TCP phase.

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