Method for regulating and controlling molybdenum steel brazed joint intermetallic compound based on transition elements

Abstract

The invention relates to the technical field of dissimilar material connection, in particular to a method for regulating and controlling molybdenum steel brazed joint intermetallic compounds based on transition elements. According to the technical scheme, the method comprises the following steps that a transition element coating is deposited on the surface of a molybdenum matrix, transition elements are selected from one or more of Ti, V, Nb, Zr, Hf and Ta, and the molybdenum matrix and a steel matrix are brazed through boron-containing Ni-based brazing filler metal; in the brazing process, the transition element and the boron element preferentially react to generate a transition element boride, and therefore generation of MoNiBintermetallic compounds is inhibited. According to the method, the transition element coating is introduced, a brazing interface reaction path is actively intervened and reconstructed from the thermodynamic angle, and the problem of continuous generation of the brittle MoNiBintermetallic compound in the molybdenum / steel brazing joint is fundamentally solved.

Description

Technical Field

[0001] This invention relates to the field of dissimilar material joining technology, and in particular to a method for regulating the intermetallic compound of molybdenum steel brazed joints based on transition elements. Background Technology

[0002] In the neutral beam injection system of nuclear fusion experimental devices (such as the EAST device), the electrode is the core component of the ion source, whose function is to extract and accelerate a high-energy ion beam from the plasma. This component must withstand bombardment with extremely high heat flux densities during operation, thus requiring materials with excellent electrical conductivity, high-temperature strength, and resistance to thermal fatigue. Therefore, electrodes often employ a composite structure of dissimilar materials: high-melting-point, high-sputtering-threshold molybdenum is used as the plasma-facing working surface material, while steel (such as austenitic stainless steel) with good structural strength and thermal conductivity is used as the backing material to achieve structural support and efficient cooling. Therefore, achieving a high-strength, high-reliability connection between molybdenum and steel has become a core technological challenge in manufacturing such critical components.

[0003] However, molybdenum and steel differ significantly in their physical and chemical properties, making direct welding between them extremely difficult, mainly in the following three aspects:

[0004] (1) Mismatch in thermophysical properties: the coefficient of thermal expansion of molybdenum (approximately 4.8 × 10⁻⁶) -6 / K) and the coefficient of thermal expansion of stainless steel (approximately 17 × 10) -6 The difference in thermal expansion ( / K) exceeds three times. During the brazing heating and cooling process, the huge difference in thermal expansion will generate severe residual thermal stress in the joint interface area, which can easily lead to cracks and premature joint failure.

[0005] (2) Poor wettability of brazing filler metal: Conventional nickel-based brazing filler metals have poor wettability and spreadability on the surface of molybdenum substrates, which can easily lead to incomplete filling of the brazing seam, resulting in defects such as incomplete welding, porosity and inclusions, which seriously weaken the tightness and load-bearing capacity of the joint.

[0006] (3) Formation of brittle interfacial phases: This is the most critical factor restricting the quality of the joint. When using widely used boron-containing (B)Ni-based solders (such as BNi2), the B element in the solder will diffuse strongly into the molybdenum matrix at high temperatures and undergo interfacial reactions to generate intermetallic compounds such as Mo2NiB2. These compounds are usually hard and brittle, and tend to form a continuous network or layered distribution at the interface. They are not only brittle fracture sources themselves, but also significantly aggravate stress concentration, causing the mechanical properties of the joint (especially toughness and thermal shock resistance) to deteriorate sharply, seriously threatening the service life and reliability of the components.

[0007] Currently, in order to alleviate the above problems, existing technologies mainly adopt the following measures:

[0008] Firstly, optimizing brazing process parameters, such as adjusting brazing temperature and holding time, can control the thickness and morphology of the brittle phase to some extent, but it cannot fundamentally change the thermodynamic driving force for its formation, resulting in limited effectiveness and a narrow process window.

[0009] Secondly, an intermediate transition layer can be introduced, such as pure copper or pure nickel foil. This method mainly releases some thermal stress through the plastic layer and improves the spread of the solder, but it is not effective in suppressing the reaction between B and Mo, and cannot prevent the formation and growth of the brittle Mo2NiB2 phase.

[0010] In summary, existing technologies cannot fundamentally solve the problem of continuous brittle Mo2NiB2 phase formation in molybdenum / steel brazed joints. This has become a technical bottleneck restricting the manufacturing of high-performance molybdenum / steel composite structures. Therefore, there is an urgent need in this field for an interface control method based on new principles, capable of actively intervening in and altering the interface reaction pathway during the brazing process, suppressing the formation of harmful brittle phases from an energy perspective, thereby providing a reliable technical solution for achieving high-performance and high-reliability connections between molybdenum and steel. Therefore, this application proposes a method for controlling intermetallic compounds in molybdenum-steel brazed joints based on transition elements. Summary of the Invention

[0011] The purpose of this invention is to address the problem that existing technologies cannot fundamentally solve the formation of continuous brittle Mo2NiB2 phase in molybdenum / steel brazed joints, and to propose a method for regulating intermetallic compounds in molybdenum / steel brazed joints based on transition elements.

[0012] The technical solution of this invention: A method for regulating the intermetallic compound of molybdenum steel brazed joints based on transition elements, comprising the following steps:

[0013] S1. Deposit a transition element coating on the surface of a molybdenum substrate, wherein the transition element is selected from one or more of Ti, V, Nb, Zr, Hf, and Ta;

[0014] S2. The molybdenum substrate and the steel substrate are brazed using a boron-containing Ni-based solder;

[0015] S3. During the brazing process, the transition element reacts preferentially with the boron element to form transition element borides, thereby inhibiting the formation of Mo2NiB2 intermetallic compounds.

[0016] Optionally, the thickness of the transition element coating is 50 nm to 5 μm.

[0017] Optionally, the thickness of the transition element coating is 100 nm to 2 μm.

[0018] Optionally, the transition element coating is prepared by physical vapor deposition, chemical vapor deposition, magnetron sputtering, or electroplating.

[0019] Optionally, the brazing temperature is 1000–1060°C, and the holding time is 2–30 min.

[0020] Optionally, the steel matrix is ​​austenitic stainless steel or an iron-based alloy.

[0021] Optionally, the transition element coating may be a single-layer or multi-layer structure.

[0022] Optionally, the multilayer structure is formed by sequentially depositing different transition elements.

[0023] Optionally, the boron-containing Ni-based solder is BNi2 solder, the composition of which includes: Ni balance, Cr 6.8 wt.%, Si 4.5 wt.%, Fe 2.8 wt.%, B 3.1 wt.%.

[0024] Optionally, the transition element borides are one or more of the following: TiB2, VB2, NbB2, ZrB2, HfB2, and TaB2, which are dispersed in a single element.

[0025] Compared with the prior art, this application includes at least one of the following beneficial technical effects:

[0026] By utilizing the lower binding energy between transition elements and boron, stable borides are preferentially reacted to form dispersed distributions, actively cutting off the formation path of harmful brittle phases and transforming the interface structure from a continuous brittle layer into a strong and tough dispersed reinforced structure.

[0027] By eliminating the main crack initiation points, the joint's strength, toughness, thermal shock resistance, and connection reliability are significantly enhanced, thereby extending its service life under harsh operating conditions.

[0028] This invention introduces a transition element coating to actively intervene in and reconstruct the reaction path of the brazing interface from a thermodynamic perspective, fundamentally solving the problem of continuous formation of brittle Mo2NiB2 intermetallic compounds in molybdenum / steel brazed joints. Attached Figure Description

[0029] Figure 1 This is a flowchart of a method for regulating intermetallic compounds in molybdenum steel brazed joints based on transition elements;

[0030] Figure 2 Microstructure of the molybdenum and steel joint interface in Example 1, as shown in the schematic diagram of the welding process. Detailed Implementation

[0031] The following specific examples illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that, unless otherwise specified, the following embodiments and features described therein can be combined with each other.

[0032] This invention provides a method for regulating the intermetallic compound of molybdenum / steel brazed joints based on transition elements. The core of this method is to pre-deposit a specific transition element coating on the surface of the molybdenum substrate. By utilizing the lower binding energy between the transition element and boron (B), the B element from the Ni-based brazing filler metal is preferentially "captured" during the subsequent brazing process to form a stable transition element boride. This thermodynamically inhibits the continuous generation of brittle Mo2NiB2 intermetallic compounds at the interface, ultimately obtaining a molybdenum / steel brazed joint with excellent microstructure and reliable performance.

[0033] Example 1: Molybdenum / steel brazing with Ti coating

[0034] like Figure 1 This embodiment takes the deposition of a Ti coating as an example to illustrate the implementation process of the present invention in detail.

[0035] Preparation of base material and brazing filler metal:

[0036] High-purity molybdenum sheets with dimensions of 60mm × 25mm × 1mm and 316L austenitic stainless steel sheets with dimensions of 60mm × 25mm × 10mm were selected as the base materials.

[0037] The solder used is commercially available Ni-based amorphous solder foil BNi2 (compliant with GB / T 10859 standard, with the following composition: Ni as balance, Cr 6.8 wt.%, Si 4.5 wt.%, Fe 2.8 wt.%, B 3.1 wt.%), with a thickness of approximately 40 μm.

[0038] Surface pretreatment for welding:

[0039] The surfaces of the molybdenum sheet and steel sheet to be welded are mechanically polished sequentially using metallographic sandpaper of #800 to #2000 specifications to remove the surface oxide layer and obtain a uniform roughness.

[0040] Place the polished specimen into a beaker containing ethanol and clean it in an ultrasonic cleaner for 10 minutes to thoroughly remove surface oil and abrasive particles.

[0041] Remove the specimen and dry it with hot, dry air for later use.

[0042] Transition element coating deposition:

[0043] A Ti coating was deposited on the pretreated molybdenum sheet surface to be soldered using a magnetron sputtering device.

[0044] The deposition process parameters are: background vacuum level better than 5.0 × 10⁻⁶. -3 Pa, the working gas is high-purity argon, the flow rate is set to 30 sccm, the sputtering power is 200 W, and the deposition time is 30 minutes.

[0045] By controlling the deposition time, a Ti coating with uniform thickness, density, and good adhesion was obtained, with a thickness of approximately 500 nm.

[0046] Assembly and brazing:

[0047] A molybdenum sheet with a Ti coating, a BNi-2 solder sheet, and a steel sheet are assembled in sequence to form a sandwich structure of "steel / BNi-2 solder / Ti-Mo plating".

[0048] Apply slight pressure (approximately 0.5 MPa) to the assembly using a specialized brazing fixture to ensure tight contact between the layers.

[0049] Place the clamped parts into the vacuum brazing furnace, close the furnace door, and start the vacuum system. Once the vacuum level inside the furnace stabilizes at 5 × 10⁻⁶,... -3 When the pressure is below Pa, heating begins according to the set process curve.

[0050] The brazing process parameters are as follows: heat up to 1020 ℃ at a rate of 15 ℃ / min, hold at this temperature for 10 minutes, and then cool to room temperature with the furnace before removing from the furnace.

[0051] Joint microstructure and performance analysis: The cross-section of the brazed joint was cut by wire cutting. After standard metallographic sample preparation (mounting, grinding and polishing), the interface microstructure and composition were observed and analyzed by scanning electron microscope (SEM) and its attached energy dispersive spectroscopy (EDS).

[0052] The results are as follows Figure 2 As shown, SEM observation reveals a dense interface bond in the brazed joint, free from defects such as continuous cracks and pores. No continuous plate-like or layered brittle Mo2NiB2 phases, commonly found in existing technologies, were observed near the interface region close to the original molybdenum matrix. EDS point analysis and surface scanning results confirm the formation of a large number of dispersed, fine-grained TiB2 phases at the interface. B element is effectively captured by Ti, preventing its diffusion into the deeper Mo matrix and reaction with Mo-Ni. Mo, Ni, Fe, and other elements form a good interdiffusion zone at the interface, resulting in high interfacial bonding strength. This joint exhibits excellent mechanical properties and service reliability.

[0053] Example 2: Molybdenum / steel brazing with Nb coating

[0054] The difference between this embodiment and Embodiment 1 is that the transition element coating is Nb, and the deposition method is electron beam physical vapor deposition (EB-PVD).

[0055] Base material and brazing filler metal: Same as in Example 1.

[0056] Surface pretreatment: Same as in Example 1.

[0057] Coating deposition: An Nb coating was deposited on the surface of a molybdenum wafer using an EB-PVD instrument. By controlling the deposition rate and time, an Nb coating with a thickness of approximately 1.2 μm was obtained.

[0058] Assembly and Brazing: The assembly method is the same as in Example 1. The brazing process parameters are adjusted to: vacuum degree better than 5×10 -3 Pa was heated to 1040 ℃ at a rate of 10 ℃ / min, held for 15 minutes, and then cooled in the furnace.

[0059] Example 3: Molybdenum / steel brazing with Ti / Zr multilayer coating

[0060] This embodiment demonstrates the use of multilayer transition element coatings to achieve synergistic effects.

[0061] Base material and brazing filler metal: Same as in Example 1.

[0062] Surface pretreatment: Same as in Example 1.

[0063] Coating deposition: Using a multi-target magnetron sputtering device, a Ti coating with a thickness of 200 nm was first deposited on the surface of the molybdenum wafer. Then, without breaking the vacuum, a Zr coating with a thickness of 300 nm was deposited to form a Ti / Zr multilayer coating with a total thickness of about 500 nm.

[0064] Assembly and brazing: The assembly method is the same as in Example 1. The brazing process parameters are: vacuum degree better than 5 × 10⁻⁶. -3 Pa, heat to 1030 ℃, hold for 12 minutes, then cool with the furnace.

[0065] This invention cleverly alters the interfacial reaction path during brazing by introducing a transition element coating, shifting from "passive mitigation" to "active inhibition," fundamentally solving the technical challenge of the brittle Mo2NiB2 phase in molybdenum / steel brazed joints. This method is simple, highly effective, and suitable for reliable joining of dissimilar materials like molybdenum and steel in high-end equipment such as nuclear fusion device electrodes and aerospace applications.

[0066] This invention, by introducing a transition element coating, actively intervenes in and reconstructs the reaction pathway at the brazing interface from a thermodynamic perspective, fundamentally solving the problem of continuous formation of brittle Mo2NiB2 intermetallic compounds in molybdenum / steel brazed joints. By selecting transition elements with higher binding energy to boron (such as Ti, V, Nb), stable transition element borides (such as TiB2) are preferentially reacted with boron in the brazing filler metal during the brazing process. This significantly reduces the driving force for the reaction of boron with molybdenum and ni to form Mo2NiB2 from a thermodynamic perspective, thereby actively preventing the formation of a continuous, coarse Mo2NiB2 brittle layer. This transforms the joint interface structure from a brittle layered structure to a dispersed, reinforced structure, greatly improving the interface toughness.

[0067] It is worth noting that the joint's strength, toughness, and thermal shock resistance are significantly enhanced by eliminating the continuous brittle phase that serves as a source of crack initiation and propagation. Simultaneously, the dispersed boride particles at the interface help strengthen the interface, while the interdiffusion layer formed by elements such as Mo and Ni ensures good metallurgical bonding. This results in higher connection reliability and longer service life for the joint under harsh conditions of high heat load and thermal cycling (such as electrodes in nuclear fusion devices). The method of this invention does not require the development of a complex new brazing system; it only adds a mature coating deposition process (such as magnetron sputtering or electroplating) to the existing molybdenum substrate. This process is seamlessly compatible with existing vacuum brazing processes, does not change the core brazing parameters, is easy to integrate and promote based on existing production equipment and processes, has low implementation costs, and is suitable for engineering-scale mass production. The transition element coating can be a single layer or a multilayer structure composed of multiple different transition elements. By designing multilayer gradient coatings, the differences in the reaction sequences and kinetics between different elements and B can be utilized to achieve a wider process window and better interface structure control, obtaining a synergistic enhancement effect greater than the sum of its parts (1+1>2). This method is highly versatile and applicable not only to the joining of molybdenum and austenitic stainless steel, but also to the reliable joining of molybdenum and other iron-based alloys. It has broad application prospects in fields with stringent requirements for joining dissimilar materials, such as nuclear industry, aerospace, and high-end manufacturing.

[0068] The above specific embodiments are merely several optional embodiments of the present invention. Based on the technical solutions of the present invention and the relevant teachings of the above embodiments, those skilled in the art can make various alternative improvements and combinations to the above specific embodiments.

Claims

1. A method for controlling intermetallic compounds in a molybdenum steel brazed joint based on transition elements, characterized in that, The method comprises the following steps: S1, depositing a transition element coating on the surface of the molybdenum substrate, the transition element being selected from one or more of Ti, V, Nb, Zr, Hf and Ta; S2, brazing the molybdenum substrate and the steel substrate by using a boron-containing Ni-based brazing filler metal; S3, during the brazing process, the transition element and the boron element preferentially react to generate a transition element boride, thereby inhibiting the generation of Mo2NiB2 intermetallic compound.

2. The method for controlling intermetallic compounds of molybdenum steel brazed joint based on transition elements according to claim 1, characterized in that, The thickness of the transition element coating is 50 nm to 5 μm.

3. The method of claim 2, wherein the transition element is selected from the group consisting of titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, and combinations thereof. The thickness of the transition element coating is 100 nm to 2 μm.

4. The method of claim 1, wherein the transition element is selected from the group consisting of titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, and combinations thereof. The transition element coating is prepared by physical vapor deposition, chemical vapor deposition, magnetron sputtering or electroplating method.

5. The method of claim 1, wherein the transition element is selected from the group consisting of titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, and combinations thereof. The brazing temperature is 1000 to 1060 ℃, and the holding time is 2 to 30 min.

6. The method of claim 1, wherein the transition element is selected from the group consisting of titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, and combinations thereof. The steel substrate is austenitic stainless steel or iron-based alloy.

7. The method of claim 1, wherein the transition element is selected from the group consisting of titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, and combinations thereof. The transition element coating is a single-layer structure or a multi-layer structure.

8. The method of claim 7, wherein the transition element is selected from the group consisting of titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, and combinations thereof. The multi-layer structure is formed by sequentially depositing different transition elements.

9. The method of claim 1, wherein the transition element is selected from the group consisting of titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, and combinations thereof. The boron-containing Ni-based brazing filler metal is BNi-2 brazing filler metal, and the composition thereof comprises: Ni balance, Cr 6.8 wt.%, Si 4.5 wt.%, Fe 2.8 wt.%, and B 3.1 wt.%.

10. The method of claim 1, wherein the transition element is selected from the group consisting of titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, and combinations thereof. The transition element boride is one or more of TiB2, VB2, NbB2, ZrB2, HfB2 and TaB2 in a dispersed distribution.

Images