CN111085796B - Fe-based multielement active high-temperature brazing filler metal for carbon fiber reinforced ceramic matrix composite - Google Patents

Abstract

The invention designs a Fe-based multielement active high-temperature brazing filler metal for a carbon fiber reinforced ceramic matrix composite. The low-expansion-coefficient stainless steel is mainly used as a master alloy, no melting-reducing element is added, and a liquid phase is obtained in an eutectic reaction mode of Fe in the brazing filler metal and carbon fiber in brazing, so that the non-metallic carbon fiber can be quickly wetted, the melting point, the brazing temperature and the joint service temperature of the brazing filler metal can be improved, and a brittle intermetallic compound formed by the melting-reducing element can be avoided; because the stainless steel master alloy and the added refractory element Mo have the advantages of small thermal expansion coefficient and high thermal strength, the obtained brazing seam has the advantages of small thermal expansion coefficient, oxidation corrosion resistance and high thermal strength, which is beneficial to reducing thermal stress, improving the service temperature of the joint and improving the comprehensive performance of the joint; the solder has the advantages of low price, high soldering temperature, fast wetting reaction, short heat preservation time and high room temperature/high temperature strength of soldered joints.

Description

Fe-based multielement active high-temperature brazing filler metal for carbon fiber reinforced ceramic matrix composite

Technical Field

The invention relates to a CMC high-temperature brazing filler metal, in particular to a high-temperature brazing filler metal for C_fCarbon fiber (C) of/SiC or the like_f) The Fe-based multielement active high-temperature brazing filler metal of the reinforced ceramic matrix composite material.

Background

A continuous fiber reinforced Ceramic Matrix Composite (CMC) obtained by implanting a continuous fiber reinforcing phase into a ceramic matrix combines the advantages of carbon fibers (high specific strength, large specific modulus, excellent high-temperature mechanical property and thermal property) and SiC ceramics (high hardness, strong corrosion resistance and strong oxidation resistance), and becomes a more stable and practical method for improving the toughness of ceramics. In the process of ceramic matrix composite bearing fracture, external force or absorbed energy can be balanced by mechanisms such as crack deflection, extremely strong fiber fracture and fiber extraction in the ceramic matrix, so that the toughness of the ceramic is greatly improved, and the advantages of high temperature resistance, oxidation resistance and corrosion resistance of the ceramic matrix are retained (reference 1). The continuous fiber reinforced ceramic matrix composite has the advantages of high temperature resistance, oxidation resistance and high specific strength, so that the continuous fiber reinforced ceramic matrix composite can be used as a high-temperature light structural material in the manufacturing of aerospace vehicles. At present, three continuous fiber reinforced ceramic matrix composite materials which are more researched are C_f/C、C_fSiC and SiC_fand/SiC. Continuous carbon fiber (C)_f) Reinforced SiC-based composite material (C)_fThe preparation cost of the/SiC) is centered among the three, the scouring resistance and the oxidation resistance thermal stability are good, the use of long service life below 1650 ℃, limited service life below 2000 ℃ and instantaneous service life below 2800 ℃ can be met, the material is an ideal high-speed aircraft thrust chamber material, and the material has important application prospects in the fields of aerospace and the like (reference 2). With C_fThe technology for preparing SiC is developed from Chemical Vapor Infiltration (CVI) to poly carbon silane Precursor Infiltration (PIP) (reference 3), and the preparation cost is reduced and the application is increased gradually.

According to the principle of material adaptation_fThe matrix densification process of the/SiC ceramic matrix composite material is long in time consumption and high in preparation cost, the matrix densification process is only used for key parts, and the rest parts are economical by adopting traditional light metal and heat-resistant alloy, so that C_fThe welding of the/SiC ceramic matrix composite material with light metal and Ni-based heat-resistant alloy is inevitable. For manufacturing large or special-shaped high-temperature structural parts, C is required_fthe/SiC ceramic matrix composite material is welded with the/SiC ceramic matrix composite material.

To date, C_fMost reports are made in the literature that the SiC ceramic matrix composite material is connected with different combinations of light metal (mainly TC 4) and Ni-based heat-resistant alloy (the high-temperature resistance is better than that of Ti alloy), and C_fRelatively few reports have been made on the welding of the/SiC ceramic matrix composites to themselves. Because the ceramic phase and the carbon fiber reinforced phase both have the characteristics of difficult deformation and difficult diffusion, and C_fthe/SiC had about 15% voids, therefore, regardless of C_fBetween SiC itself, or C_fThe welding method between the/SiC ceramic matrix composite and the light metal or the Ni-based heat-resistant alloy mainly adopts brazing. Like ceramic brazing, C_fThe solderability of the/SiC ceramic matrix composite material has the following three problems: the wettability is poor; thermal stress; the heat resistance was poor (reference 1).

The reported brazing filler metals include Ag-based active brazing filler metals (e.g., Ag-Cu-Ti), Ti-based brazing filler metals (e.g., Ti-15Cu-15Ni), Ni-based brazing filler metals (e.g., BNi-2, BNi-3), and noble metal-based brazing filler metalsAnd Ag-based or Ti-based composite solders reinforced with ceramic particles or low-expansion refractory metal particles (Pd-based or Au-based), and the like ( references 1, 4 to 7). In the aspect of improving interface wettability, three solders of Ag base, Cu base and Ti base all use Ti as an active element, and the wetting is realized through the reaction of the active element Ti with carbon fiber and a ceramic matrix. In the reported C_fIn the brazing literature of/SiC, the brazing of the combination of Cf/SiC and titanium alloy TC4 by using the traditional Ag-Cu-Ti brazing filler metal is mainly adopted. In 2010-2011, Xiong JH (Adam of bear) and Chua reported that a Cf/SiC and TC4 (references 8 and 9) are vacuum brazed by Ag-based brazing filler metal 94(Ag-28Cu) -6Ti under the conditions of (890-950) ° C x (1-35) min, and the shear strength of the joint obtained under the conditions of 900 ℃ -5 min is maximum (the shear strength at room temperature and the shear strength at high temperature of 500 ℃ are 102MPa and 52MPa respectively); microscopic Structure Observation found at C_fThe reaction layer formed on the/SiC side is C_f/SiC/TiC+Ti₃SiC₂/Ti₅Si₃+Ti₂Cu; the reaction layer formed on the TC4 side is Ti₃Cu₄/TiCu/Ti₂Cu/(Ti₂Cu+Ti)/TC4。

Regarding Ti-Cu brazing filler metal, in 2006, the Cu/Ti combined eutectic brazing filler metal pair C was adopted in the Tao of bear_fthe/SiC and Nb-based alloys were brazed, and the shear strength of the resulting joint was 34MPa (reference 10). 2011, Wangxing 2DC_fThe combination of/SiC and GH783, the nickel-based alloy widely used in the field of aerospace is selected by adding hard particles Mo with low thermal expansion coefficient (the thermal expansion coefficient of Mo is 5.1 multiplied by 10) into Cu-Ti alloy solder^-6K^-1) To solve the problems of large residual stress between the solder and the base material and low connection strength (reference 2). With the increase of the Mo content, the connection strength of the joint is continuously increased; when the Mo content is 15 percent (volume fraction), the joint connection strength reaches the maximum (141 MPa); when the Mo content is more than 15%, the joint strength of the joint begins to decrease. The addition of Mo relieves the residual stress of the joint and inhibits excessive corrosion of Ti to C/SiC, thereby effectively improving the connection strength of the joint. In 2017, the university of Beijing technology, Dongyu Fan (reference 11) used a Ti-based brazing filler metal (57Ti-13Zr-21Cu-9Ni, wt%, particle size 500 μm) with 16 vol.% TiC powder (particle size 300 μm)A composite brazing filler metal, which is prepared by mixing (C) at 930 ℃ for 30-120 min_fThe combination of/SiC)/TC 4, provides a transition liquid phase diffusion welding method, and improves the high temperature performance of the joint by fully diffusing and homogenizing Cu and Ni; when the heat preservation time is prolonged to 90min, the shearing strength of the joint at 800 ℃ reaches 137.4 MPa; when the heat preservation time is prolonged by 120min, the joint remelting temperature is increased to 1217 ℃, and the temperature is 300 ℃ higher than the melting point of the brazing filler metal.

As for the Ni-based brazing filler metal, it is generally considered that the Ni-based brazing filler metal is wetted by Cr therein as an active element (references 12, 13). However, the applicant's experiments showed that the Ni-based solder has a weak reaction and dissolution ability for carbon fibers, but has a weak reaction and dissolution ability for C_fThe matrix phase in the/SiC has over-strong dissolving capacity, and the dissolving depth can reach 100 mu m; in the case of crystallization in the deeper dissolution range, the liquid phase is transformed into Ni₂The eutectic structure of Si + NiSi, thereby causing the following two problems: (1) the matrix of the composite material is disintegrated from SiC into Ni-Si compound; (2) of which high melting point Ni₂Si will be preferentially at C_fIs formed to damage the original C_fThe combination of the/SiC interface. In early years (1996), when a Cr-containing Ni-based solder (Ni-Cr-based solder: 39Ni-33Cr24Pd-4Si, BNi-7, BNi-2, etc.) was used for vacuum brazing of C/C and Cu heat sinks by researchers at Japan Hitachi institute for manufacturing okra and the like, Cr was formed at the C/C interface₃C₂The compound has high thermal stress due to its high hardness, and has a joint shear strength of only about 5MPa (reference 13). McDERMID, BNi-5 brazing filler metal (Ni-19Cr-10Si) and SiC powder + BNi-5 composite brazing filler metal are not suitable for brazing of reaction sintered SiC/Inconel600 because the reaction of Ni and free Si in SiC causes rapid decomposition of SiC due to the contact of liquid Ni and reaction sintered SiC at 1200 ℃ to generate low-melting Ni-Si silicides and Cr₇C₃Which occurs at the ceramic/metal interface and damages the base material and degrades the high temperature performance of the joint (reference 14). Thus, for C prepared by the reaction infiltration Si method_fSiC (containing free Si), on the one hand Ni reacts with Si to damage the SiC matrix; on the other hand, Cr reacts with C to harden the interface, which aggravates the damage of thermal stress. The aged wave and the like also prove that the Ag-27.4Cu-4.4Ti solder is adopted to carry out vacuum brazing at 880 ℃ for 10minWelding C_fThe three-point bending strength (159.5MPa) of the/SiC self joint is much higher than that of the joint obtained with the Ni-based brazing filler metal (reference 15).

As for noble metal-based brazing filler metals, on the basis of their advantages of high melting point and good plasticity, Xiong Huaping developed a variety of materials for C_fPrecious metal alloy-based brazing filler metal for metal brazing/SiC, such as Pb-Co-V (reference 16), Cu-Pd-V (reference 17), Cu-Au-Pd-V (reference 18), AuNi (Cu) -Cr (reference 19), NiPdPtAu-Cr (reference 20). The alloy element palladium (Pd) has a higher melting point (1555 ℃), is in the same family with Ni, is infinitely mutually soluble with Ag, and is beneficial to improving the melting point and the heat-resistant temperature of the Ag-based solder; the copper-copper alloy is fully soluble with Cu at high temperature, and is beneficial to improving the plasticity and the thermal stress of a brazing seam at a high-temperature section. Wherein the AuNi (Cu) -Cr solder is obtained_fThe room-temperature three-point bending strength of the/SiC joints reached 154.5MPa, but the bending strength rapidly decreased to 75.2MPa at a high temperature of 600 ℃ (reference 19). When the NiPdPtAu-Cr solder and the Mo intermediate layer are adopted, the three-point bending strength at room temperature is improved from 51.7MPa to 133.2MPa, and the three-point bending strength at 900 ℃ is improved to 149.5MPa (reference 20).

In the aspect of solder design improvement, the Ag-Cu-Ti active solder has better wettability and plasticity, thereby being beneficial to ensuring C_fRoom temperature strength of SiC active metal brazed joints. Therefore, the emphasis of solder improvement design is on how to reduce thermal stress and how to improve the high temperature performance of the joint. The main improvement idea of the current solder design is the compounding of the solder (reference documents 4-7), namely ceramic powder, graphite powder and low-expansion refractory metal powder are added into Ag-Cu-Ti to obtain the composite solder, the main purpose is to reduce the thermal expansion coefficient and thermal stress of a brazing seam through the compounding of the solder, and the added powder can play a role in strengthening the brazing seam and is beneficial to improving the room temperature and high temperature performance of a joint.

The composite solder is proved to be a novel solder capable of effectively inhibiting thermal stress through experiments. Hanson, in 1999, the british institute for welding (TWI) reported increasing the solder strength and suppressing the thermal expansion coefficient of the solder by adding up to 30 vol.% SiC particles to a commercially available Ag-Cu-Ti active solder (ref 21). G.blugan In switzerland proposed In 2007 the addition of wettable SiC ceramic particles (30 vol.%) to Ag-27.25Cu-12.5In-1.25Ti (wt.%) active solders and the addition of welds In a sandwich format, while improving the room and high temperature strength of Si3N 4/tool steel joints (reference 22). In order to control the CTE of the solder and enhance the joint strength, m.c. halbiga (NASA Glenn Research Center) in the united states, vacuum brazing of SiC using an Ag-Cu-Ti solder reinforced with SiC particles, it is theorized that introduction of 45 vol% SiC reduces the CTE of the brazing seams by approximately 45 to 60% (reference 23).

The Huang Shenhua topic group of Beijing university of science and technology is C_fThe research and development aspects of the/SiC composite solder are continuously researched, and the later period of improvement effect of the composite solder is remarkable in the following reports: dongjinghe Pair of Beijing university of science and technology C in 2009_fthe/SiC composite was combined with TC4 titanium alloy using a composite braze (AgCuTi-50 Vol% W) consisting of Ag-based braze powder (67.7Ag-26.4Cu-6Ti, wt.%; 320 mesh) and 50 Vol.% low CTE tungsten powder (7000 mesh) for vacuum brazing TC 4/(C) with a composition of Cu-C alloy_f/SiC), the shear strength of the joint at the room temperature is 168MPa at 900 ℃ for 30min, which is higher than the shear strength (102MPa) without tungsten powder; the shear strength at 500 ℃ was 128MPa, which is higher than the highest value of the shear strength at 500 ℃ without W (reference 24). Thereafter, the Bing of the Bingbing of the university of Beijing technology in 2012 and 2014_fThe combination of the/SiC composite material and the TC4 titanium alloy, using a mixed powder composed of a Ti-based brazing filler metal (57Ti-13Zr-21Cu-9Ni, wt.%) having a particle size of about 200 mesh and tungsten powder having an average particle size of about 2.6 μm as a composite brazing filler metal, performing vacuum brazing at 930 ℃ for 20min, and when the content of reinforcing phase tungsten powder is 15 vol.%, the resultant joint has the highest shear strength and room temperature is 166MPa (reference 25); 96MPa at 800 ℃ and high temperature (reference 26).

As a way to reduce thermal stress, in addition to more reported composite solders, Haihai proposed C before welding in 2017_fAfter conical small holes are ablated on the surface of SiC by laser (called laser texturing process), vacuum brazing C by using paste Ag-Cu-Ti brazing filler metal_fThe shear strength of the joint of the/SiC composite material and TC4 can be improved from 63MPa to 84MPa (reference 27).

As can be seen,for C_fThe combination of the/SiC composite material and the TC4 titanium alloy utilizes that the Ag-Cu-Ti active brazing filler metal and the Ti-based brazing filler metal can wet two base metals, but refractory metal powder (such as W powder and Mo powder) or ceramic powder (such as TiC) is added, so that the thermal expansion coefficient and the interface thermal stress of a brazing seam can be reduced, the brazing seam can be strengthened, and the room-temperature shear strength (which can be as high as 160MPa) and the high-temperature strength of the joint can be improved. Further, the observation of the experimental parameters and the structure of the composite brazing filler metal in the above-mentioned documents indicates that (C) is the most effective component_fThe welding specification of the/SiC)/TC 4 combined joint, whether Ag-Cu-Ti solder or Ti-based solder Ti-Zr-Cu-Ni is similar (900 ℃ multiplied by 1 h); the wetting reaction products of the two solders and the parent metal on both sides are also very similar. At C_fOn the side of the SiC base material, Ti is formed₃SiC₂、Ti₅Si₃And a small amount of TiC (which is an intermediate reaction product and is easy to consume, so that the final residual amount is small), wherein the reaction of the SiC matrix/Ti is preferred to the reaction of the carbon fiber/Ti (the activity of the SiC matrix to the active element Ti of the brazing filler metal is larger than that of the carbon fiber); the (Ti + Ti2Cu) reaction layer formed on the TC4 side and having a thickness of about 40 μm should be formed by eutectoid reaction of beta-Ti at 790 deg.C₂Cu + alpha-Ti) eutectoid tissue; cu diffuses significantly more than Ni and excess Ni is rejected to the raffinate phase (ref 25).

The existing Ag-based solder takes Ti as an active element, has good wettability and plasticity, and is suitable for various C_fThe combination of the/SiC and the metal has good applicability, but the melting point is below 800 ℃, and the requirement of high-temperature service cannot be met. The elementary Ti in Ti-based solders (typical alloy system Ti-Zr-Ni-Cu; typical Ni-Cu content is below 30%) is itself an active element, but has the disadvantage that: however, since Ti is likely to form intermetallic compounds (IMC) with other metal base materials, it is only applicable to the brazing of CMC/TC4, and not applicable to the brazing of CMC with other metal base materials (e.g., Ni-based). ② the solder of Ti base solder has a melting point of 900 ℃ or lower (reference 28) and is inferior in heat resistance (600 ℃ or lower) similarly to Ag base solder.

In summary, the following problems exist in the existing Ag-based active solder and Ti-based active solder: (1) melting point and heat resistance: the melting point is lower (about 780-950 ℃), resulting in C_fSiC ceramic matrix compositeThe heat-resisting temperature and the remelting temperature of the solder joint are low; (2) and (3) wettability: the active elements are strong carbide forming elements Ti, although the interface reaction temperature is low, the reaction products (such as TiC, Ti)₃CSi₂Etc.) are continuously distributed along the interface, exacerbating the thermal stress hazard; (3) in the aspect of brazing seam: the absence of carbon (C) or SiC-based low cte phases in the braze (unless additional low cte phases are added) results in a large thermal expansion mismatch and large thermal stresses in cooling.

Reference documents:

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Disclosure of Invention

For carbon fiber strengthThe invention discloses a base material of a ceramic matrix composite, aims to break through the primary technical problems of low melting point and poor brazing seam heat resistance (the melting point is about 780-950 ℃) of the existing Ag-based (such as Ag-Cu-Ti) and Ti-based (such as Ti-Ni-Cu) brazing filler metal, solves the secondary problems of slow wetting of carbon fibers and overlong heat preservation time (for example, the Ni-based brazing filler metal needs to be preserved for a long time to realize isothermal solidification to improve the remelting temperature of a joint), designs a melting point reducing element without melting-down elements (without Fe, Ni, Cr and Mo) to improve the melting point of the brazing filler metal, forces the brazing filler metal to be molten into a liquid phase only through eutectic reaction with the carbon fibers, and improves the brazing temperature and the remelting temperature of the joint to Fe-C_fThe eutectic reaction temperature is 1154 ℃, the brittle intermetallic compound formed by the melting-reducing element is avoided, and cheap Fe is used as a matrix element and an active element (Fe-C is used_fEutectic reaction realizes wetting carbon fiber, so Fe is an active element for wetting carbon fiber reinforced phase).

In order to achieve the design purpose, the invention adopts the following technical scheme:

an Fe-based multielement active high-temperature brazing filler metal, in order to improve the corrosion resistance, melting point and heat strength of brazing seams (thereby determining that Cr is the primary and necessary alloy element in the Fe-based brazing filler metal provided by the invention) and reduce the thermal expansion coefficient of the brazing seams (so as to reduce the thermal stress of the interface of a ceramic matrix composite material/brazing seams), the invention provides that stainless steel with low thermal expansion coefficient (which means that the thermal expansion coefficient is lower than that of common carbon steel, low alloy steel and 3XXX series austenitic stainless steel) is adopted as a primary Fe-based master alloy (which is the primary and necessary alloy element Cr and Ni and is used as a main source to endow the brazing filler metal with corrosion resistance and certain heat strength); adding Mo and W as melting elements to improve the heat strength of the brazing seam and reduce the thermal expansion coefficient (for example, below 30%) of the brazing seam; adding less than 10% of active element Ti to improve the wettability of the ceramic matrix, and finally forming an alloy system of the Fe-based multi-element active high-temperature solder, namely Fe-Cr-Ni-Mo (-W) -Ti; two active elements of Ni (above 964 ℃, Ni is also an active element for SiC, but has the problems that the reaction product is easy to be excessive and is too brittle) and Ti containThe amount is kept low (Ti content is not more than 10%; Ni content is not more than 20-30%). The three characteristics of heat resistance, low expansibility and high melting point of the brazing filler metal are ensured and strengthened by the low-expansion stainless steel element (the main component, the low-expansion coefficient Fe-based stainless steel can be partially or completely replaced by low-expansion Fe-based stainless invar-4J 36(37Fe-32Co-11Cr) or kovar alloy-Fe-29 Ni-17 Co) and added refractory metal Mo or W (secondary component). According to the above concept, by comparing the thermal expansion coefficients, the Fe-based alloy with large thermal expansion coefficient is abandoned (for example, common carbon steel CTE is 24 × 10)^-6CTE of 17X 10/K, SUS304 or SUS321, which are commonly used austenitic stainless steels^-6K), an Fe-based alloy having a small thermal expansion coefficient (e.g., SUS630, which is a precipitation hardening type low-carbon martensitic stainless steel having a CTE of 10.8 × 10) is preferable^-6K; SUS 631; martensitic stainless steel SUS431 having CTE of 10.4X 10^-6K; duplex stainless steel). Adding one or more of elements Mo, W, Ni and Cr with high melting point, which can improve the high-temperature corrosion resistance and high-temperature creep resistance of the brazing seam, instead of adding a melting-point-reducing element to the master alloy, and reducing the Coefficient of Thermal Expansion (CTE) of the brazing seam while improving the heat strength by using one or two of the refractory Mo and W; at the same time, the brazing filler metal is forced to pass through the Fe-C_fThe eutectic reaction is melted into liquid phase, which can ensure the rapid wetting of the carbon fiber and increase the reaction melting temperature of the Fe-based brazing filler metal in the brazing process to Fe-C_fEutectic temperature is 1154 ℃, and high-temperature brazing filler metal is obtained; then adding a small amount of strong carbide and/or strong nitride forming elements into the master alloy, wherein the active elements can be deoxidized simultaneously, namely one or more of Ti, V, Nb and Zr, and improving the wettability of the brazing filler metal on the surface of a base material matrix of a ceramic matrix composite material and the surface of a metal base material by activating low-expansion Fe-based stainless steel, so as to obtain the Fe-based multi-element active high-temperature brazing filler metal, wherein the basic alloy system of the Fe-based active high-temperature brazing filler metal formed by following the principle idea comprises the following steps: Fe-Cr-Ni-Mo-Ti, Fe-Cr-Ni-W-Ti, Fe-Cr-Ni-Mo-Zr, Fe-Cr-Ni-W-Zr, Fe-Cr-Ni-Mo-Nb, Fe-Cr-Ni-W-Nb, Fe-Cr-Ni-Mo-V, Fe-Cr-Ni-Mo-W-Ti or Fe-Cr-Ni-Mo-W-Zr.

Since Cr is regarded as the primary and necessary alloy element (to ensure the corrosion resistance and heat strength of the brazing seam), the Fe-based multielement active high-temperature brazing filler metal provided by the invention can also be called Fe-Cr-based multielement active high-temperature brazing filler metal. Ni is not necessary to be added, but only is used as a candidate small-amount added element, on one hand, the addition of Ni is beneficial to improving the plasticity of the brazing filler metal and a brazing seam, on the other hand, Ni is mainly added along with Mo, so that the refractory Mo can smoothly achieve the purpose of melting Mo in a mode of dissolving into Ni (the melting point of Ni is lower than that of Fe, 1455 ℃, lower than the melting temperature of 1500 ℃, Ni-Mo has eutectic reaction at 1317 ℃, and the room-temperature solid solubility of Ni to Mo is as high as 20 wt%, so that the dissolving capacity to Mo at the melting temperature is strong).

The basic Fe-based Fe-Cr-Ni-Mo-Ti five-element active high-temperature brazing filler metal (the first three elements mainly come from stainless steel master alloy, the second two elements are added into the stainless steel master alloy, and a small amount of Ni and W can be added according to the range) created according to the idea of the invention has the main advantages of four aspects: (1) melting point and high temperature performance: the true melting point of Fe-based solder exceeds Fe-C_fThe eutectic reaction temperature (1154 ℃) is at least 374-200 ℃ higher than the melting point range (780-950 ℃) of the Ag-based brazing filler metal and the Ti-based brazing filler metal, so that even if the Ag base and the Ti base completely lose 780-950 ℃ of the bearing capacity, the brazing seam still has the austenite performance, which is the fundamental and overwhelming advantage of the invention. (2) In the aspect of wetting the carbon fiber reinforced phase, the Fe-C eutectic reaction is used for wetting the carbon fiber, so that the compactness of the interface of the brazing filler metal and the carbon fiber is ensured, and the brazing filler metal has the advantages of quick reaction, easiness in realization, low cost, no need of long-time heat preservation and the like. (3) The problem of thermal stress is more prominent along with the high brazing temperature of the Fe-based pentabasic (Fe-Cr-Ni-Mo-Ti) high-temperature brazing filler metal. For this purpose, stainless steel with low thermal expansion coefficient is preferably selected as the main component of the Fe-based brazing filler metal; and is supplemented with refractory metal (W, Mo) to further reduce the thermal expansion coefficient of the solder and improve the high temperature resistance of the solder. (4) In the aspects of preparation cost and use efficiency, the material does not contain noble metal Ag (only contains less than 5-10% of rare metal Ti), and Fe with rich resources and low price is mainly used as an element; using Fe-C_fThe eutectic reaction wets the carbon fiber quickly, and the generated carbide brittle phase is not easy to form a continuous layerThe generated carbide is brittle and has less harm to the room temperature performance of the joint due to small thermal expansion coefficient, and the red hardness of a brazing seam is improved, so that long-time heat preservation is not needed, the brazing heat preservation time can be shortened to 3-5 min, the production efficiency is greatly improved, and the time-consuming process of carrying out isothermal solidification by long-time heat preservation like Ni-based high-temperature brazing filler metal is avoided.

The design idea and advantages of the Fe-based multielement active solder are further explained as follows:

(1) one of the main master alloys in reducing the thermal stress at the "CMC/braze" interface is the use of stainless steels with low coefficients of thermal expansion (e.g., such as SUS630, a precipitation-hardened, low-carbon martensitic stainless steel with a CTE of 10.8X 10^-6K; martensite SUS431 has CTE of 10.4X 10^-6K); secondly, adding refractory metal Mo to further reduce the thermal expansion coefficient of the brazing seam metal.

(2) In the aspect of ensuring the interfacial wettability to the Ceramic Matrix Composite (CMC), the first aspect is that the wetting of the carbon fiber reinforced phase mainly utilizes Fe-C_fThe carbon fiber is wetted by eutectic reaction; secondly, the wetting of the ceramic matrix is mainly by the added strong carbon/nitride forming elements of Ti, Nb, Zr and the like. In addition, these strong carbon/nitride forming elements also can be considered as deoxidizing elements, and are advantageous for wetting the metal base material and the oxide-based ceramic matrix.

(3) Based on the utilization of Fe-C in the aspect of improving the high-temperature performance of the brazing seam_fThe idea of wetting carbon fibers by eutectic reaction is to provide a design scheme of an active low-expansion Fe-based multi-element (Fe-Cr-Ni-Mo-Ti) brazing filler metal, and the design scheme is to abandon Ag-Cu-Ti active brazing filler metal (the melting point is too low), Cu-based active brazing filler metal (the melting point is too low), Ti-based brazing filler metal (the melting point is low at 950 ℃, the brazing filler metal is slightly brittle and the interface IMC is thick) and traditional commercial Ni-based brazing filler metal BNi-x (the brazing filler metal is brittle and hard, and a CMC liquid phase infiltration area becomes brittle and hard along with the infiltration of the CMC liquid phase infiltration area); meanwhile, Mo and W are further added to improve the heat strength of the brazing seam. In view of the fact that Mo has a lower melting point than W and is more easily alloyed with the Fe matrix, Mo is preferably added preferentially, which brings about the advantage of good Mo distribution uniformity.

The addition amount of the Cr can be increased along with the improvement of the requirements on corrosion resistance and oxidation resistance; ni is synchronously increased along with the increase of Mo content so as to ensure the smooth smelting of the refractory Mo.

In order to further improve the high-temperature creep resistance of the brazing seam while adding the refractory metal and the active element, refractory and stable second-phase ceramic particles or graphite particles can be added, and the active element is utilized to ensure the wettability of the Fe-based brazing filler metal matrix and the added ceramic particles or graphite particles, so that micro gaps caused by poor wettability in the brazing seam are avoided.

A Fe-based multielement active solder comprises the following components:

(100-y)％[x％C_I-(100-x)％C_II]-y％C_III

wherein, C_IIs Fe-based Fe-Co-Ni Kovar (Kovar) master alloy, Fe-Ni Invar (Invar) master alloy or Fe-Ni Invar (Invar) master alloy with low thermal expansion coefficient less than or equal to 12 x 10^-6Fe-Ni-Cr based Fe-Ni-Cr stainless Steel master alloy, C_IIIs one or two of Mo and W, C_IIIIs one or more of Ti, Nb and Zr, wherein x is 50-99, and y is less than or equal to 10.

The melting point of the Fe-based multielement active brazing filler metal is Fe-C_fThe eutectic reaction temperature is higher.

The Fe-based Fe-Ni-Cr master alloy is selected from one or more of stainless steel or invar steel.

The stainless steel is selected from one or more of precipitation hardening martensitic stainless steels SUS630, SUS631, martensitic stainless steel SUS431 and duplex stainless steels, which have a lower CTE than that of 3XX series austenitic stainless steels and whose alloying elements promote graphitization.

The preparation method of the Fe-based multielement active brazing filler metal comprises the following steps:

melting Fe-based Fe-Ni-Cr master alloy, Fe-based Fe-Co master alloy or Fe-Ni-Co kovar alloy in a crucible under the protection of inert gas, adding Mo and/or W with high melting point into the crucible, changing the Mo and/or W into liquid phase in a dissolving mode, finally adding Ti into the crucible, homogenizing the components of the mixture in the crucible by smelting and heat preservation, and then cooling to obtain the brazing filler metal ingot.

The preparation method of the Fe-based multielement active brazing filler metal comprises the following steps:

preparing materials and alloy element range of the smelting brazing filler metal:

the types of prepared materials are three types: (1) mother alloy block: for example, SUS630(0Cr17Ni4Cu4Nb) and SUS631(17Cr-7Ni-1Al) are used as Fe, Cr, and Ni supply sources mainly for wetting carbon fibers and oxidation resistance; (2) 800-mesh Mo powder: mo is low-expansion refractory metal and is mainly used for improving the high-temperature strength of a brazed joint and reducing the thermal expansion coefficient of a brazing seam; (3) ti block: traditional active elements, mainly used to wet ceramic matrices, not carbon fibers; and can be used as an active element for reducing and removing the film of the metal base material.

The preparation combination (SUS630+ Mo) master alloy is determined by a method of adding Mo into oriented SUS630, and the component determination step comprises the steps of firstly determining that the content x% of Mo is 1-35 wt%; the balance is SUS630 content (100-x)%, i.e. the corresponding SUS630 content is 99-65 wt.%.

In the Fe-based multielement active solder, the relative contents of Ti and the above-described preparatory combination (SUS630+ Mo) master alloy are: the content of Ti is 0.5-10 wt.%; the remainder was the above preliminary combination (SUS630+ Mo) master alloy.

Smelting:

the raw materials used in the refining of the brazing filler metal are as follows: (1) the master alloy is a precipitation hardening type (precipitation hardening type) low-carbon martensitic stainless steel SUS630 block material (17Cr-4Ni-4Cu-0.3 Nb; C is less than 0.05%, Cu and Ni can promote graphitization); (2) mo powder of 800 meshes; (3) pure Ti metal blocks.

The smelting process comprises the following steps: under the protection of Ar, SUS630 was put into a crucible to be melted, heated to 1500 ℃ to ensure that SUS630 was completely melted first, and then metallic Mo powder was added. After the Mo powder is in large-area contact with the Fe liquid, the melting temperature of Mo is reduced by dissolving the Mo powder into the Fe (as known from a Fe-Mo phase diagram, when the content of Mo is below 35%, the melting point of the Fe-Mo alloy is lower than that of Fe), so that the refractory metal Mo (the melting point of Mo is 2623 ℃) is changed into a liquid phase, and the uniform distribution of the refractory metal Mo is facilitated. The active element Ti is added only after the SUS630 and Mo are smelted, otherwise the active element Ti is easy to be oxidized and cannot be added into a brazing filler metal system (namely, the active element Ti is added finally to reduce the initial surface oxidation and the later burning loss of the active element Ti as much as possible). The temperature is measured by a B-type double platinum-rhodium (platinum-rhodium 30-platinum-rhodium 6) thermocouple during the smelting process. The cast ingot after the components are homogenized is cut into slices by electric sparks, and then is manually pre-ground and thinned to 0.1-0.3 mm (the thinner the better), so that the cast ingot can be used as a brazing filler metal.

The use method (soldering method) of the Fe-based multielement active solder comprises the following steps:

presetting the Fe-Cr-Ni-Mo-Ti series Fe-based multielement active brazing filler metal sheet on C_fThe base material on one side and the other side of the/SiC base material can be stainless steel, heat-resistant steel or C_f/SiC、C_fThe carbon fiber reinforced ceramic matrix composite material such as/C and the like is pressurized to 0.1-10 MPa, and Ar protection is carried out (no harsh requirement on brazing vacuum degree); melting of solder utilizes Fe matrix and C in solder_fThe carbon fiber in the/SiC is subjected to eutectic reaction, namely the brazing temperature is Fe-C_fThe eutectic reaction temperature is higher than 1154 ℃; keeping the temperature for a short time (1-10 min); slowly cooling along with the furnace, and finishing the brazing process.

The invention has the beneficial effects that:

the invention provides a high-temperature brazing filler metal for carbon fiber (C) according to the design thought of the high-temperature brazing filler metal taking Fe as an active element_f) The reinforced ceramic matrix composite material has low cost, low expansion and high temperature resistance Fe-based multi-element (Fe-Cr-Ni-Mo-Ti system) active brazing filler metal. The invention proposes the aspect of C_fThe Fe-based multielement active brazing filler metal of the ceramic matrix composite material reinforced by the carbon fibers such as SiC and the like has the characteristics of no melting point reducing element, thereby being beneficial to improving the melting point, the brazing temperature and the joint service temperature of the brazing filler metal and avoiding the brittle intermetallic compound formed by the melting point reducing element. The brazing filler metal has the advantages of high brazing temperature, short heat preservation time, quick wetting reaction, low thermal expansion coefficient and the like, and can improve the content of C_fThe remelting temperature and the service temperature (small thermal stress, high room temperature strength and high temperature strength) of the/SiC soldered joint.

The quinary Fe-based brazing filler metal provided by the invention is actually a stainless steel brazing filler metal, and the adverse effect of the surface oxide film on the wettability can be eliminated by the following ways: the self-bursting of the oxide film in the heating process; Fe-C_fAfter eutectic reaction, the oxide film is dispersed along with the pressurized flow of the liquid phase; after eutectic reactionThe obtained liquid phase carbon atoms have reduction effect on the oxide film.

Further, the advantages of selecting SUS630 as the intermediate alloy (SUS630+ Mo) are: (1) in addition to the common advantages of SUS630 (e.g., good corrosion resistance at room temperature and high temperature), it is more critical that the SUS630 have a thermal expansion coefficient of 10.8X 10^-6The thermal expansion coefficient of the material is far lower than that of common austenitic stainless steel SUS304 (17.3 multiplied by 10)^-6K); (2) contains a small amount of strong carbide forming element Nb which is beneficial to wetting C_f(ii) a In addition, although Cu with a low melting point exists, the content is low, and the Cu does not exist in the form of simple substance Cu (precipitates are Cu-Ni), so that the melting point is not lower than that of the simple substance Cu; and Cu is the same as Ni, and is an element for promoting graphitization, thereby being beneficial to reducing the hardenability of the brazing seam matrix. (3) The SUS630 can provide Fe, Cr and Ni elements at one time, and keeps the proportion of Fe as a base and more Cr (meeting the requirement of more than 12 percent of stainless steel), thereby reducing the types of prepared materials; meanwhile, the stainless steel has better comprehensive properties of corrosion resistance, high strength, better plasticity, capability of bearing higher-temperature service conditions and the like.

In addition, the Fe-based brazing filler metal provided by the invention is 'Fe-C' in application_fCompared with the traditional brazing method of generating carbide phase wetting carbon fiber by using the reaction of active elements (the traditional Ti-containing active brazing filler metal mainly utilizes Ti + C → TiC and Ti + SiC → Ti)₃SiC₂Two reactions are in C_fTiC and Ti are formed on the surface₃SiC₂Two reaction products to achieve wetting of the carbon fibers) have the following advantages:

(1) in terms of wettability, use is made of "Fe-C_fThe eutectic reaction' process for wetting the carbon fiber reinforced phase in the ceramic matrix composite material is efficient and easy: on one hand, the liquid phase of eutectic formed by eutectic reaction has good fluidity, which is beneficial to quickly realizing self-cleaning, wetting and densification of the interface and has high welding rate; no new phase is formed on the surface of the carbon fiber (the interface phase is generally brittle); and because the eutectic liquid phase does not have a brittle phase after being slowly solidified, isothermal solidification can be realized without long-time heat preservation. Further, the SiC matrix is wetted with Cr, Ti, and Nb. On the other hand, the vacuum condition is not needed, and the operation is simple and easyUnder the protection of sexual gas (without vacuum environment), the utility model can also heat and wet quickly, which saves time and equipment investment.

(2) The method has two advantages in reducing the brazing seam thermal stress: firstly, the series solder takes SUS630 with low thermal expansion coefficient as a substrate (80%); second is in Fe-C_fAfter eutectic reaction, a large amount of dissolved C enters the liquid phase solder, so that the liquid phase is changed into a liquid phase with high carbon content, the carbon with higher content can appear in a soldering seam as a graphite phase (when the carbon content needs to reach more than 2.11 wt.%) or various types of carbides formed in situ, such as various eutectic carbides and precipitated carbides (when the carbon content is in the range of cast steel or high-carbon high-alloy tool steel, the range is 0.7-2 wt.%), and both the graphite phase and the carbide phase have very low thermal expansion coefficients; the above dual factors contribute to a reduction in the coefficient of thermal expansion of the braze joint and thus to a reduction in the risk of thermal stress. Fe-C_fThe liquid phase formed by the eutectic reaction is cooled when Fe-C_fWhen the liquid phase formed by the eutectic reaction contains a large amount of graphitization promoting elements C, Si, and Ni (the key is the concentration of C itself) and the cooling rate is slow, the thermal stress of the brazing seam is low, and graphitization is sufficient. In the brazing seam structure fully performed in the graphitization process, not only the brittle phase in the brazing seam matrix is less (Ti-containing intermetallic compounds are generated after the solidification of the traditional Cu-Ti eutectic and Ni-Ti eutectic), but also the primary graphite phase G crystallized from the hypereutectic liquid phase_IEutectic transformation (L)_C’→γ_E’+G_{Eutectic crystals}) Formed eutectic graphite (G)_{Eutectic crystals}) Can be considered as an 'in-situ formed' second phase, is naturally dense with the interface of the slow-cooling brazing seam matrix, has similar expansion physical property with carbon fiber, and has extremely low value (about 2 multiplied by 10)^-6and/K) is very beneficial to reducing the thermal expansion coefficient and the thermal stress of the brazing seam.

(3) In the near seam area of the ceramic matrix composite CMC, a liquid-phase product formed by utilizing eutectic reaction has a filling effect on original pores (residual pores in the CMC preparation state) in the near seam area, so that a 'mutual embedded type canine-teeth staggered interface' is formed at a CMC/brazing filler metal interface: the brazing filler metal periodically extends into the dissolved part of the original carbon fiber at intervals, so that the welding area is increased, and the large shrinkage stress generated by integral shrinkage when the brazing filler metal layer is distributed in a plane is reduced through segmentation, namely, a low-stress mutual embedded interface is naturally formed.

(4) In the aspect of improving the heat resistance of the joint, the remelting temperature of the brazing filler metal can be increased by more than 200 ℃ (212 ℃ -374 ℃): the melting temperature of the brazing filler metal is Fe-C_fThe eutectic reaction temperature is higher than 1154 ℃, which is far higher than the solidus of the traditional Ag-based active solder (the Ag-Cu eutectic is 780 ℃), the solidus of the Cu-based active solder (the Cu-Ti eutectic temperature is 885 ℃) and the solidus of the Ni-based active solder (the Ni-Ti eutectic temperature is 942 ℃), and the high-temperature performance of the joint is favorably improved.

Drawings

FIG. 1 is a Secondary Electron (SE) photograph and XRD pattern of a microstructure of (85SUS630-15Mo) -2Ti solder pieces (melting condition 1500 ℃ C.. times.30 min) (it is confirmed that Mo has been smoothly melted and uniformly distributed at a temperature lower than its melting point); wherein: (a) low power (100 ×) microstructural profile; (b) solder X-ray diffraction pattern.

FIG. 2 is a back-scattering photograph and XRD pattern of a microstructure of a (90SUS 630-10Mo) -2Ti solder block (melting condition 1500 ℃ C.. times.30 min); wherein: (a) a back-scattered photograph; (b) x-ray diffraction pattern.

FIG. 3 is a back-scattering photograph and XRD pattern of a microstructure of a (95SUS630-5Mo) -2Ti solder block (melting condition 1500 ℃ C.. times.30 min); wherein: (a) a back-scattered photograph; (b) x-ray diffraction pattern.

FIG. 4 is a melting curve of one of the solder materials (SUS 630-15Mo) measured (measured melting point 1478.3 ℃ C., much higher than the Fe-C eutectic temperature 1154 ℃ C.).

FIG. 5 is a view showing a CMC/stainless steel base material composition brazed with various SUS 630-Mo-Ti series brazing materials (C)_fShear strength of/SiC)/SUS 630 braze joint (brazing conditions: ar protection; 1300 ℃ 5min X3 MPa).

FIG. 6 is C_fThe/(85 SUS630-15Mo) -2Ti/SUS 630 brazing joint macrostructure, the interface reaction and the compactness microstructure of the/SiC composite material (brazing specification: 1300 ℃ C.. times.5 min. times.3 MPa): (a) c_fMacro continuous shot Back Scattering (BSE) photographs (100 x) of the/SiC composite/(85 SUS630-15Mo) -2Ti/SUS 630 braze joints; (b) a microstructure of extruded solder bead zone A (500X)(ii) a (c) b in the dotted line frame ledeburite and eutectic carbide M₆C amplifying the tissues; (d) b area (fiber dissolution area) in a, enlarging a back scattering picture; (e) d, checking the Secondary Electron (SE) picture (without white highlight edge) of the interface compactness; (f) the surface distribution result of Ti after welding (the Ti is deviated from the interface and has a beneficial effect on improving the wettability of the interfaces on two sides).

Detailed Description

The present invention will be described in further detail with reference to the accompanying drawings and examples.

Fe-based multielement active solder design

The alloy system design of the Fe-based multielement active brazing filler metal is characterized in that: (1) fe with low cost is taken as an active element (not the traditional strong carbide forming element Ti) for wetting the carbon fiber, and Fe-C is utilized_f(Fe and carbon fiber) eutectic reaction quickly wets the carbon fiber; while the traditional Ag-based Ti-containing active solder and Cu-based Ti-containing active solder mainly utilize Ti + C → TiC and Ti + SiC → Ti₃SiC₂Two reactions are carried out on carbon fiber (C)_f) TiC and Ti formed on the surface₃SiC₂The two solid-phase reaction products realize the wetting of the carbon fiber, and Fe is added into a brazing alloy system in a mode of taking stainless steel as a master alloy. (2) No melting point reducing elements aiming at Fe, Ni, Cr and Mo (or very small amount of Si: controlling the content of Si to keep the melting point of the brazing filler metal at Fe-C_fThe eutectic temperature is more than 1154 ℃) is favorable for improving the melting point of the brazing filler metal, the brazing filler metal is forced to be melted into liquid phase only through the eutectic reaction with the carbon fiber, and the brazing temperature and the remelting temperature of the joint are improved to Fe-C_fThe eutectic temperature is 1154 ℃ (far higher than the melting point and the remelting temperature of Ag-Cu-Ti and Ti-Zr-Cu-Ni), isothermal solidification caused by long-time heat preservation due to the addition of melting-reducing elements like Ni-based high-temperature solder is avoided, the heat preservation time is greatly shortened, and the production efficiency is remarkably improved. (3) The addition of Cr (also added in the form of stainless steel as master alloy) makes the brazing seam possess certain oxidation resistance, rust resistance and precipitation strengthening (by means of Cr-C compound). (4) The addition of Mo can not only reduce the expansion coefficient, but also improve the high-temperature performance of the brazing seam and improve the corrosion resistance of the brazing seam metal, and Mo in the alloy system can be partially or completely replaced by W to further improve the high-temperature performance. (5) Adding Ni (and small amount of Si)) The method can promote the graphitization of the brazing seam, and is also added in a mode of taking stainless steel as master alloy), thereby being beneficial to reducing the hard brittleness of the brazing seam matrix; the Ni and the high-melting-point Mo can perform eutectic reaction, which is beneficial to reducing the smelting temperature of the Mo, thereby being beneficial to dissolving the Mo in the Fe-Cr-Ni alloy liquid. (6) Addition of Ti for wetting the SiC matrix on the one hand and for consuming Fe-C on the other hand_fAnd excessive carbon element entering the brazing seam after eutectic reaction. (7) Using the solder of the invention and based on Fe-C_fThe brazing process of eutectic reaction has the advantages of high brazing temperature, short heat preservation time, fast wetting reaction, small brazing seam thermal stress, high joint room temperature strength, high temperature strength and the like.

Melting experiment of (II) Fe-based multi-element active solder

1. Stock preparation

The types of prepared materials are three types: SUS630 master alloy masses (0Cr17Ni4Cu4 Nb; supply of Fe, Cr, Ni, 0 refers to carbon content), 800 mesh Mo powders (low expansion refractory metals), Ti masses (conventional active elements, mainly wetting SiC matrix instead of carbon fibers), thus determining the alloy system of the solder, namely SUS630(0Cr17Ni4Cu4Nb) -Mo-Ti.

Four Fe-based Fe-Cr-Ni-Mo-Ti quinary solders were actually melted and tested. Wherein, the content of Ti is fixed at 2 wt%, and by adding Mo powder of different mass fractions to the SUS630 master alloy (changing the relative content of SUS630 and Mo), the four types of solders melted are respectively noted as: (1) (95SUS630-5Mo) -2Ti (wt.%); wherein: 2Ti means total Ti content of 2 wt.%; 95 and 5 represent the ratio of both SUS630 and Mo in the new master alloy consisting of SUS630 and Mo, i.e., 5 wt.% of Mo was added to SUS630, while the new master alloy consisting of SUS630 and Mo was 98 wt.% (1-2 wt.% Ti ═ 98 wt.%). So 95SUS630 means that in the remaining 98 wt.% (1-2 wt.% Ti 98 wt.%) solder consisting of SUS630 and Mo, the SUS630 master alloy accounts for 95 wt.%; 5Mo means Mo accounts for 5 wt.% in the remaining 98 wt.% (1-2 wt.% Ti 98 wt.%) solder consisting of SUS630 and Mo; the following are similar; (2) (90SUS 630-10Mo) -2Ti (wt.%); (3) (85SUS630-15Mo) -2Ti (wt.%); (4) (80SUS 630-20Mo) -2Ti (wt.%)

2. Melting

The melting process and parameters of the four brazing fillers are the same, and are briefly described as follows. Heating by adopting an induction power supply; flowing Ar gas for protection; because the melting temperature of SUS630 is high, a B-type double platinum-rhodium (platinum-rhodium 30-platinum-rhodium 6) thermocouple is adopted for measuring the temperature in the melting process. Laboratory melting in order to observe the melting behavior of the raw materials, a transparent glass crucible was used, and the total weight of melting was 30g per time. Firstly, the surface oxidation film of the stainless steel and the Ti block is removed by a file and a sand paper, and the Ti block is put into an ultrasonic cleaner to be cleaned by absolute ethyl alcohol. Under the protection of Ar, SUS630 was put into a crucible to be melted, heated to 1500 ℃ to ensure that SUS630 was completely melted first, and then metallic Mo powder was added. The melting temperature of Mo is reduced by dissolving Mo powder into Fe (as shown by a Fe-Mo phase diagram, when the content of Mo is below 35%, the melting point of Fe-Mo alloy is lower than that of Fe), so that the refractory metal Mo (the melting point of Mo is 2623 ℃) is changed into a liquid phase, and uniform distribution of Mo is facilitated. And observing the dissolution of the Mo powder, finally adding the Ti block, smelting and preserving heat for 30min, and naturally cooling to room temperature. The active element Ti must be added after the other various metals (including stainless steel master alloy SUS630, refractory metal Mo) are melted, otherwise the active element Ti is easily oxidized, and the active element Ti cannot be accurately added to the solder system.

(III) results of the experiment

(1) Phase analysis of microstructure photo and X-ray diffraction (XRD) of the solder:

phase analysis results of (85SUS630-15Mo) -2Ti solder microstructure photograph and X-ray diffraction (XRD) (figure 1): the structure is uniformly distributed, and blocky or needle-shaped precipitates, namely Lames phase-Lambda- (Fe, Cr), are distributed in the Fe-based solid solution matrix with the Fe content of up to 80 at%₂The Mo intermetallic compound (as can be known from an Fe-Mo binary phase diagram, the solid solubility of Mo in Fe is only 5 at% at 700 ℃, the actually measured Mo content in a block precipitate is 15-17 at%, the actually measured Mo content in a strip precipitate is 9-16 at%, and the Mo content falls in a two-phase region of (alpha Fe) + lambda); XRD diffraction analysis shows that no simple substance Mo exists, which indicates that the added Mo powder is lower than the melting point (T)_mMelting temperature (T2623 ℃) of_smeltThe solution was also successfully dissolved at 1500 ℃; meanwhile, C is not independently added in the smelting of the brazing filler metal, and the SUS630 stainless steel has ultralow carbon content(0Cr17Ni4Cu4Nb), no significant carbides were detected by XRD. It can be seen that, since the refractory Mo is dissolved smoothly, a part of Mo is formed as intermetallic compounds (Laves phase (Fe, Cr)₂Mo) exists; a part of the Fe-Cr alloy is dissolved in the Fe-Cr alloy matrix; the structure distribution of the brazing filler metal is uniform, so the melting of the brazing filler metal is successful.

As the Mo content decreases, the (. lamda. - (Fe, Cr) in the brazing filler metal (90SUS 630-10Mo) -2Ti (FIG. 2) and the brazing filler metal (95SUS630-5Mo) -2Ti (FIG. 3)₂The Mo intermetallic compound phase is reduced, and the Mo intermetallic compound phase is not easy to detect in XRD, but the structure distribution is still kept uniform.

(2) Test results of the melting point of the brazing filler metal:

according to the melting curve (FIG. 4) of one of the measured solders (85SUS630-15Mo) -2Ti, the melting point was found to be 1478.3 ℃ which is much higher than the Fe-C eutectic temperature of 1154 ℃. From the Fe-Mo phase diagram, below 40 wt.% Mo addition, Mo addition slightly reduced the Fe melting point (from 1538 ℃ to 1449 ℃), so the experimental test results agree well with the phase diagram.

(IV) brazing of_fShear strength of/SiC composite/SUS 630 dissimilar material combination

(1) Welding experiment

The base metal used for the welding experiment is C_fa/SiC ceramic matrix composite (5X 5mm) and SUS630 stainless steel (15X 2 mm); the thickness of the (SUS630+ Mo + Ti) Fe-based five-element (Fe-Cr-Ni-Mo-Ti) active high-temperature brazing filler metal sheet is 0.3 mm; (C)_fTemperature of/SiC)/(SUS 630+ Mo + Ti) solder/SUS 630 brazing: 1300 ℃; and (3) heat preservation time: 3-5 min; pressure: 3 MPa.

Different kinds of brazing filler metal pairs (C) smelted by the invention_fThe shear strength ratio of the joint obtained by high temperature brazing of the/SiC)/SUS 630 base material combination is shown in FIG. 5. The shearing strength of a soldered joint of the brazing filler metal (such as any one of the four smelted materials) can reach more than 80MPa, and the (SUS630+ Mo + Ti) system Fe-based five-membered (Fe-Cr-Ni-Mo-Ti) active high-temperature brazing filler metal provided by the invention is proved to be a low-cost Fe-based high-temperature brazing filler metal for CMC, and the principle idea of alloy system design has correctness. Wherein, when (85SUS630-15Mo) -2Ti solder is used, the obtained soldered joint has the highest shear strength and average shear strengthThe value is as high as 126.1 MPa.

(2) Brazing joint structure

In view of the highest shear strength of the solder joints obtained from the (85SUS630-15Mo) -2Ti solder, the structure of the solder joints is mainly described herein to prove the superiority and practicability of the high-temperature solder design idea for CMC proposed by the present invention.

The (85SUS630-15Mo) -2Ti brazing filler metal provided by the invention is used for brazing ceramic matrix composite (C) under the conditions of Ar protection and 1300 ℃ multiplied by 5min multiplied by 3MPa_fSiC) and stainless steel (SUS630) were combined to obtain a joint having a low-power back scattering structure, and as shown in fig. 6(a), it was found that the brazing seam structure was similar to the structure of the extruded solder bead. The extruded liquid solder beads were visible on both sides of the joint, indicating Fe-C_fThe eutectic reaction between the C fiber and the brazing seam is smoothly realized, and conditions are created for liquefying the wetting C fiber and the brazing seam. The left-side structure of the solidified extruded solder beads is shown in fig. 6(b) and 6 (c). Fe-C_fThe liquid phase composition obtained by the eutectic reaction has the characteristics of High C, High Cr and High Mo, and is very similar to the characteristics of High-carbon High alloy composition (also having higher carbon and a large amount of carbide forming elements Cr and Mo) of Fe-C-X (X ═ Cr + Mo) of High speed tool Steel (HSS Steel; used for cutting tools and rollers) and early High-Chromium Ledeburite Steel (High Chromium Ledeburite Steel; carbon content is higher than that of High speed Steel; used for cold-working dies). The solder bead crystalline structure mainly comprises primary austenite (pro-eutectic austenite) and some netlike eutectic ledeburite located at the grain boundary, the ledeburite form has obvious fishbone form (a bone morphology) of main spine (central plateau), and conforms to M₆Typical morphology of eutectic carbide type C (eutectic carbide). After 2000 times magnification (FIG. 6(C)), it was observed that the primary austenite grains were preferentially decomposed in the central region, with carbides precipitated in the form of rings and short rods (white), while no change in the decomposed precipitates from the matrix was observed in the grain periphery, which should be a mixture of martensite and retained austenite (Fe-12Cr-10C-3Mo-2Si, at.%). This uneven transformation of the primary austenite is caused by an uneven distribution of elements, i.e., alloying elements (e.g., C, Mo, Al, Cu, Mn) due to the proximity of the grain periphery to eutectic carbides,Cr) content is higher, stability of austenite grain periphery is increased, Ms point is reduced, and retained austenite appears. The presence of decomposition products (austenite decomposition products) in the central portion of the primary austenite grains and the retention of the peripheral retained austenite contribute to the improvement of plasticity. The white phase in ledeburite is "eutectic carbide" phase, the size is about 10 μ M long, and the typical "fishbone shape" and "high level Fe content" can be estimated as M₆C type carbide- (Fe, Cr, Mo)₆C。M₆Type C carbides have a complex fcc lattice structure and hardness up to 1500 Hv. However, primary graphite phases and eutectic graphite phases have not yet been observed in the solidified liquid phase, which is associated with the fact that the carbon content in the braze seams is not sufficient for the graphitization process.

It can be seen that, in contrast to the high chromium ledeburite steel structure with a high amount of reticulated eutectic carbides (eutectoid carbides), the structure of the solder beads is also very similar to the as-cast microstructure of high speed steel, consisting mainly of decomposition products of austenite, retained austenite and eutectic containing carbides. The liquid phase of the brazing seam of the invention has the composition characteristics very similar to that of high-speed steel, and both the liquid phase and primary austenite contain sufficient carbide forming elements to consume or deprive Fe-C_fThe carbon in which the eutectic reaction is dissolved reduces the content of free carbon or supersaturated carbon in the matrix, and reduces the tendency of the matrix to transform to high-carbon martensite and become brittle.

FIG. 6(a) enlarged photograph of B-zone-fiber-dissolved zone (back-scattered/BSE photograph) As shown in FIG. 6(d), the contrast of the original fiber zone changed from black to light gray, indicating an increase in Fe element content, indicating that Fe-C is present under very short soak conditions (e.g., 5min)_fThe eutectic reaction is rapid and remarkable, and reaches about 200 mu m after penetrating into the CMC, so that the wetting of the reinforcing phase of the carbon fiber is ensured; and changing the carbon fiber to F-C_fEutectic alloy fibers. At the same time, the SiC matrix in the fiber bundle also becomes Fe as the main element in composition. In the transformation mechanism of two possible SiC matrixes, dissolution and diffusion reaction, the transformation of the SiC matrixes is mainly achieved due to the fact that Fe-C solidification structures formed again after fibers are dissolved are well defined with the new matrixes in the bundleThe preparation is that under the help of medium or strong carbide forming elements such as liquid Cr, Ti and the like, the liquid Fe element is remarkably diffused into the SiC matrix, which is the main mechanism that SiC is wetted by Fe-C eutectic liquid phase and is transformed.

Fig. 6(e) is a photograph of Secondary Electrons (SE) in the same area as in fig. 6(d) (i.e., in the area B in fig. 6 a) for discriminating the density of the interface, the brazing seam, and the CMC base material dissolved region interface. The results did not show areas or interfaces with white and bright contrast (except for the small pores inherent in the CMC base material before local welding), indicating that the CMC dissolution zone front (CMC dissolution zone/diffusion affected zone — interface 1), braze seam/CMC (interface 2), braze seam/SUS 630 interface (interface 3) were dense and also had no significant microcracks (e.g., crystalline cracks or brittle cracks). The test result of element distribution by energy spectrum analysis proves that the distribution of Ti after welding is changed, and obvious segregation (including one side of the stainless steel base material) appears on the interfaces at two sides of the brazing seam, and the Ti serving as an active element has a beneficial effect of improving the surface wettability of the base materials at two sides (see fig. 6 (f)). The various interfacial wetting mechanisms can be summarized as follows: (1) wetting the C fibers of the CMC matrix mainly by Fe-C_fCarrying out eutectic reaction; (2) wetting the SiC matrix of the CMC parent material mainly depends on the diffusion of Fe assisted by Ti and Cr to SiC; (3) wetting stainless steel (SUS) base material is mainly performed by strong deoxidizing elements Ti and C (which are also melting-reducing elements and have the function of slightly dissolving SUS surface layer) in liquid phase to Cr₂O₃The reduction of the oxide film and the later deposition of C, Ti on the surface of SUS, particularly the segregation of Ti, which is a strong deoxidizing element, are significant, and a multi-carbide layer mainly containing TiC is finally formed on the surface of SUS. In consideration of the fact that the deoxidizing ability of Ti is superior to that of C (especially in the medium-low temperature range) and the deposition of Ti is significant, Ti is considered to be the primary element for achieving wetting of the SUS side. There may be two mechanisms for the segregation of Ti on SUS surfaces: (1) ti and Cr₂O₃The reaction(s) causes Ti to segregate (whether or not Cr can be completely reduced); (2) the reduced Cr of the SUS surface attracts C, C and in turn attracts Ti. However, the thickness of the TiC-rich layer should not be too thick (which can be controlled by the original Ti content, the thickness of the brazing filler metal sheet and the pressure) so as to avoid the embrittlement of the brazing seam/SUS interface.

In a word, the invention takes low expansion stainless steel as main master alloy, and adds refractory alloy element Mo, the formed alloy system is Fe-Cr-Ni-Mo-Ti multi-element active high temperature solder, which has the following characteristics:

firstly, only Fe-C can be used in the brazing process without adding melting-reducing elements_fThe liquid phase is obtained in a (Fe and carbon fiber) eutectic reaction mode, so that the nonmetal carbon fibers can be quickly wetted, the melting point, the brazing temperature and the joint service temperature of the brazing filler metal can be improved, and a brittle intermetallic compound formed by a melting-reducing element can be avoided;

secondly, because the stainless steel master alloy and the added refractory element Mo have the advantages of small thermal expansion coefficient and high thermal strength, the obtained brazing seam has the advantages of small thermal expansion coefficient, oxidation corrosion resistance and high thermal strength, which is beneficial to reducing thermal stress, improving the service temperature of the joint and improving the comprehensive performance of the joint;

and thirdly, when the brazing of the carbon fiber reinforced ceramic matrix composite material and the same or different parent metals is realized, the brazing process has the advantages of no harsh requirement on vacuum degree, high brazing temperature, short heat preservation time, fast wetting reaction and the like.

Therefore, the present invention can obtain a brazed joint having high room temperature/high temperature strength at a high speed and at a low cost.

Claims (8)

1. A design method of Fe-based multielement active high-temperature brazing filler metal for a carbon fiber reinforced ceramic matrix composite is characterized by comprising the following steps:

in order to reduce the thermal expansion coefficient of a brazing seam, reduce the thermal stress of a ceramic matrix composite material/brazing seam interface and improve the corrosion resistance and high-temperature performance of the brazing seam, stainless steel, Fe-Co alloy or invar alloy which has excellent corrosion resistance and high-temperature performance and low thermal expansion coefficient and has alloy elements capable of promoting graphitization is adopted as a master alloy of a Fe-based brazing filler metal matrix compared with common carbon steel and low alloy steel, and elementary Fe in the brazing filler metal matrix and carbon fiber C in the ceramic matrix composite material are utilized_fFe-C of_fEutectic reaction quickly wets the non-metallic carbon fiber reinforced phase; wherein the stainless steel has a thermal expansion coefficient higher than that of common carbon steel, low alloy steel and 3 XX-series austeniteThe stainless steel is low; the low expansion coefficient Fe-based stainless steel can be partially or completely replaced by low expansion Fe-based alloy 37Fe-32Co-11Cr or kovar alloy Fe-29Ni-17 Co;

secondly, not adding a melting point reducing element into the master alloy, but adding one or more of elements Mo, W, Ni and Cr with high melting point capable of improving the high-temperature corrosion resistance and the high-temperature creep resistance of the brazing seam, and reducing the coefficient of thermal expansion CTE of the brazing seam while improving the heat strength by using one or two of the refractory Mo and W; at the same time, the brazing filler metal is forced to pass through the Fe-C_fThe eutectic reaction is melted into liquid phase, which can ensure the rapid wetting of the carbon fiber and increase the reaction melting temperature of the Fe-based brazing filler metal in the brazing process to Fe-C_fEutectic temperature is 1154 ℃, and high-temperature brazing filler metal is obtained;

thirdly, adding a small amount of strong carbide and/or strong nitride into the master alloy to form one or more of active elements Ti, V, Nb and Zr which can be deoxidized, and activating the low-expansion Fe-based stainless steel to improve the wettability of the brazing filler metal on the surface of the base material matrix of the ceramic matrix composite material and the surface of the metal base material, thereby obtaining the Fe-based multi-element active high-melting-point high-temperature brazing filler metal, wherein the basic alloy system of the Fe-based active high-temperature brazing filler metal formed by following the principle idea comprises the following steps: Fe-Cr-Ni-Mo-Ti, Fe-Cr-Ni-W-Ti, Fe-Cr-Ni-Mo-Zr, Fe-Cr-Ni-W-Zr, Fe-Cr-Ni-Mo-Nb, Fe-Cr-Ni-W-Nb, Fe-Cr-Ni-Mo-V, Fe-Cr-Ni-Mo-W-Ti or Fe-Cr-Ni-Mo-W-Zr.

2. The design method according to claim 1, wherein: further comprising the steps of:

and fourthly, adding refractory metal and active elements, and simultaneously adding refractory and stable second-phase ceramic particles or graphite particles to further improve the high-temperature creep resistance of the brazing seam, and ensuring the wettability of the Fe-based brazing filler metal matrix and the added ceramic particles or graphite particles by utilizing the active elements so as to avoid the generation of micro gaps in the brazing seam due to poor wettability.

3. The design method according to claim 1, wherein: the master alloy of the Fe-based brazing filler metal matrix is selected from one or more of precipitation hardening martensitic stainless steel SUS630, SUS631, martensitic stainless steel SUS431, invar 4J36, and duplex stainless steel.

4. The Fe-based multielement active high-temperature brazing filler metal designed by the method for designing the Fe-based multielement active high-temperature brazing filler metal for the carbon fiber reinforced ceramic matrix composite material as recited in claim 1, wherein the method comprises the following steps: the Fe-based multielement active brazing filler metal comprises the following components:

(100-y)%[x% C_I-(100-x)% C_II]-y% C_III

wherein, C_IIs Fe-based Fe-Co-Ni Kovar (Kovar) master alloy, Fe-Ni Invar (Invar) master alloy or Fe-Ni Invar (Invar) master alloy with low thermal expansion coefficient less than or equal to 12 x 10^-6Fe-Ni-Cr based Fe-Ni-Cr stainless Steel master alloy, C_IIIs one or two of Mo and W, C_IIIIs one or more of Ti, Nb and Zr, x = 50-99, and y is less than or equal to 10.

5. The Fe-based multielement active solder according to claim 4, characterized in that: the melting point of the Fe-based multielement active brazing filler metal is Fe-C_fThe eutectic reaction temperature is higher.

6. The Fe-based multielement active solder according to claim 4, characterized in that: the Fe-based Fe-Ni-Cr master alloy is selected from stainless steel.

7. An Fe-based multielement active solder according to claim 6, characterized in that: the stainless steel is selected from one or more of precipitation hardening martensitic stainless steels SUS630, SUS631, martensitic stainless steel SUS431 and duplex stainless steels, which have a lower CTE than that of 3XX series austenitic stainless steels and whose alloying elements promote graphitization.

8. A method for preparing Fe-based multielement active solder according to claim 4, characterized in that: the method comprises the following steps:

melting Fe-based Fe-Ni-Cr master alloy, Fe-based Fe-Co master alloy or Fe-Ni-Co kovar alloy in a crucible under the protection of inert gas, adding Mo and/or W with high melting point into the crucible, changing the Mo and/or W into liquid phase in a dissolving mode, finally adding Ti into the crucible, homogenizing the components of the mixture in the crucible by smelting and heat preservation, and then cooling to obtain the brazing filler metal ingot.

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