A nickel-based brazing filler metal with in-situ generated L12 strengthening phase and a brazing method for 316L stainless steel

Abstract

The application discloses a nickel-based brazing filler metal for in-situ generation of L12 strengthening phase and a 316L stainless steel brazing method, and belongs to the technical fields of nickel-based brazing filler metal and stainless steel brazing. The composite nickel-based brazing filler metal is composed of 98.0 wt% of BNi-2 nickel-based brazing filler metal powder and 2.0 wt% of AlSi10 aluminum alloy powder. The two kinds of powders are mixed under the protection of argon to prepare the composite brazing filler metal, and the composite brazing filler metal is coated on the welding interface; the temperature is raised to 1040 DEG C at a furnace pressure of 5*10 ‑3 Pa and a temperature rising rate of 10 DEG C / min, and the 316L stainless steel brazing joint is obtained after 15 min of holding and furnace cooling. Among the three AlSi10 addition amounts of 2 wt%, 4 wt% and 6 wt% tested in the application, the 2 wt% sample obtains the best room temperature mechanical property. The welding method of the application realizes the improvement of the room temperature mechanical property of the joint on the basis of guaranteeing the wettability and gap filling capacity of the brazing filler metal, and in-situ generation of L12 ordered strengthening phase.

Description

Technical Field

[0001] This invention belongs to the field of brazing filler metal technology, specifically relating to a nickel-based brazing filler metal that generates L12 strengthening phase in situ and a brazing method for 316L stainless steel. Background Technology

[0002] Nickel-based brazing filler metals are among the most widely used brazing filler metals in aerospace applications, possessing excellent corrosion resistance and heat resistance. Furthermore, nickel-based filler metals have good compatibility with stainless steel substrates, making them the preferred material system for brazing 316L stainless steel. Nickel-based filler metals, represented by BNi-2, are often used for welding critical hot-end components in the aerospace field, such as turbine blades, combustion chambers, and guide vanes, due to their excellent heat and corrosion resistance and good compatibility with high-temperature alloys, enabling reliable joint operation at high temperatures. However, this filler metal has drawbacks: its melting point degrading elements, B and Si, easily form a continuous network of low-melting-point eutectic phases (such as Ni-Si-B compounds) in the weld, leading to increased joint brittleness and decreased high-temperature performance; the joint performance is extremely sensitive to brazing temperature, holding time, and gap, resulting in a narrow process window; simultaneously, the diffusion of B into the base metal forms a diffusion-affected zone containing boride needle-like phases, impairing the toughness of the base metal. To address the aforementioned issues, Shi Kun et al. used BNi-2 brazing filler metal to join GH4169 and GH738 high-temperature alloys. By optimizing process parameters (1110℃ / 45min / 30μm gap), they obtained a fully solid solution joint, eliminating the brittle phase in the weld center and achieving optimal microstructure and properties. However, this method relies on the full diffusion of boron, which can easily exacerbate boride precipitation in the base material and results in a long production cycle. Wang Meng et al. used BNi-2 to join tungsten and 316L stainless steel, finding that adding a copper interlayer could alleviate the residual stress caused by the mismatch in thermal expansion coefficients due to its plasticity, increasing the joint shear strength from 143MPa to 197MPa. These existing improvement schemes do indeed focus on improving the high-temperature mechanical properties of the joint or alleviating the thermal mismatch problem between dissimilar materials.

[0003] Existing research has either attempted to prevent the formation of brittle phases through narrow gap control or to reduce residual stress using an intermediate layer, but has not fundamentally addressed the inherent shortcomings of BNi-2 joints in terms of room temperature mechanical properties. The room temperature strength in the isothermal solidification zone of the brazed joint remains insufficient. The core deficiencies of existing BNi-2 brazing filler metals in brazing 316L stainless steel can be attributed to: weak interfacial bonding, insufficient room temperature strength, and low elongation.

[0004] Therefore, this invention proposes a nickel-based brazing filler metal for in-situ generation of the L12 strengthening phase and a brazing method for 316L stainless steel. Summary of the Invention

[0005] The technical problem to be solved: To avoid the shortcomings of the prior art, the present invention provides a nickel-based brazing filler metal with in-situ generation of L12 strengthening phase and a brazing method for 316L stainless steel. By adding a set amount of AlSi10 powder to BNi-2 brazing filler metal, an ordered L12 strengthening phase is generated in-situ while ensuring the wettability and gap filling ability of the brazing filler metal, thereby improving the room temperature mechanical properties of the joint.

[0006] The technical solution of the present invention is: a nickel-based brazing filler metal with in-situ generated L12 strengthening phase for brazing 316L stainless steel, characterized in that the composite nickel-based brazing filler metal comprises BNi-2 nickel-based brazing filler metal powder and AlSi10 aluminum alloy powder. At the brazing temperature, the composite nickel-based brazing filler metal generates an L12-type ordered strengthening phase in the isothermal solidification zone of the brazing seam through the in-situ reaction of Al and Ni elements, thereby achieving precipitation strengthening of the brazed joint. A further technical solution of the present invention is that the amount of AlSi10 aluminum alloy powder added is 2.0% to 6.0% of the total mass of the composite nickel-based brazing filler metal. A further technical solution of the present invention is that the BNi-2 nickel-based solder powder is composed of the following components by mass percentage: Cr: 6.0%~8.0%; Si: 4.0%~5.0%; B: 2.75%~3.5%; Fe: 2.5%~3.5%; C: ≤0.06%; The balance is Ni and unavoidable impurities.

[0007] A further technical solution of the present invention is that the AlSi10 aluminum alloy powder is composed of the following components by mass percentage: Al: 87%–90%; Si: 9%~11%; Mg: ≤0.1%; Fe: ≤0.55%; Cu: ≤0.1%; Zn: ≤0.05%; The balance represents unavoidable impurities.

[0008] A brazing method for 316L stainless steel, characterized by using the aforementioned composite nickel-based brazing filler metal, comprising the following steps: Step 1: Preparation of the workpiece to be welded: Grind, ultrasonically clean and dry the surface of the 316L stainless steel base material to be welded; Step 2: Solder preparation: BNi-2 nickel-based solder powder and AlSi10 aluminum alloy powder are mixed in proportion and ball-milled under a protective atmosphere to obtain BNi-2+AlSi10 composite solder. Step 3: Vacuum brazing: Apply BNi-2+AlSi10 composite brazing filler metal to the interface to be brazed. After assembling and fixing the parts to be brazed, place them in a vacuum brazing furnace. Heat to the brazing temperature under vacuum conditions and hold at that temperature to allow the composite brazing filler metal to melt, spread, and fill the gaps. At the same time, Al and Ni elements react in situ to generate L12 type ordered strengthening phase. Step 4: Cooling: Cool to room temperature with the furnace to obtain the brazed joint.

[0009] A further technical solution of the present invention is: in step 2, the process parameters of ball milling are: rotation speed 200 r / min, time 20 min, ball-to-material ratio 5:1; the protective atmosphere is argon.

[0010] A further technical solution of the present invention is: in step 3, the process parameters for vacuum brazing are: vacuum degree 5×10 -3 Pa, heating rate of 10℃ / min, brazing temperature of 1040℃, holding time of 15min. A further technical solution of the present invention is as follows: the composite solder coated on the interface to be soldered in step 3 is in paste form. The preparation method of the paste composite solder is as follows: the prepared BNi-2+AlSi10 composite solder powder is mixed with an organic carrier in a weight ratio of 93.5:6.5, and after thorough stirring and grinding, a paste composite solder is prepared. The organic carrier is composed of the following components by weight percentage: 22.0% polyvinyl acetate with a molecular weight of 150,000, 68.0% terpineol, 5.0% lecithin, and 5.0% butyl stearate.

[0011] A post-weld treatment method for brazed joints of 316L stainless steel, characterized by comprising the following steps: Step 1: Cut the 316L stainless steel brazed joint after vacuum brazing to obtain metallographic specimens and / or mechanical property test specimens. Step 2: Grind the sample sequentially using sandpaper of 80 grit, 240 grit, 400 grit, 800 grit, 1200 grit, 1500 grit, and 2000 grit. Step 3: Polish the ground sample until the surface is mirror-like. Step 4: Use the polished sample for microstructure observation and / or mechanical property testing; the mechanical property testing is performed using a microcomputer-controlled electronic universal testing machine ETM105D, with a testing rate of 1 mm / min.

[0012] A 316L stainless steel brazed joint, characterized in that it is prepared by the brazing method, and the brazed joint has an L12 type Ni3Al ordered strengthening phase dispersedly in the isothermal solidification zone of the brazed seam. The L12 type ordered strengthening phase is generated by the in-situ reaction of Al element in AlSi10 aluminum alloy powder and Ni element in BNi-2 nickel-based brazing filler powder at the brazing temperature. The room temperature shear strength of the brazed joint is ≥418MPa.

[0013] Beneficial effects The beneficial effects of this invention are as follows: This invention utilizes a novel brazing filler metal to significantly improve the mechanical properties of 316L brazed joints. The strengthening mechanism of the BNi-2+AlSi10 powder system belongs to second-phase strengthening, and is ordered precipitation strengthening. During the isothermal solidification process of brazing, Al atoms dissolved from the Al powder diffuse into the molten BNi-2 brazing filler metal matrix, undergoing an in-situ reaction with Ni. At the solid-liquid interface front, they preferentially nucleate and precipitate fine, dispersed L12-type intermetallic compounds Ni3Al. This L12 phase has a long-range ordered structure. When moving dislocations cut the strengthening phase, they disrupt its ordered arrangement, generating antiphase domain boundaries. Dislocations need to overcome additional antiphase domain boundary energy to continue sliding, thus significantly increasing the resistance to dislocation movement and improving the yield strength of the brazing filler metal matrix. Simultaneously, the L12 phase has good lattice matching with the Ni-based solid solution matrix, forming a coherent or semi-coherent interface, which not only lowers the nucleation energy barrier of the strengthening phase but also promotes its uniform and fine precipitation in the isothermal solidification region. The in-situ generated L12-type γ' ordered strengthening phase increases the tensile shear strength of the brazed joint by more than 15% and the elongation after fracture by 4%.

[0014] When the AlSi10 addition amount increased to 4 wt%, the introduction of excess Al promoted a further increase in the continuous network brittle phase and microcracks at the grain boundaries, resulting in a significant decrease in the tensile shear strength of the joint to 339.5 MPa and an elongation after fracture to 10%. At an addition amount of 6 wt%, due to severe Al segregation and exacerbated solidification defects, the joint strength deteriorated sharply to only 77.8 MPa, and the elongation after fracture was as low as 3%. These results indicate that within the process window defined by this invention, 2 wt% is the optimal addition amount of AlSi10. Simultaneously, the high vacuum environment effectively suppressed Al oxidation, the ball milling mixing process ensured uniform dispersion of the powder components in the solder matrix, and the heating rate of 10℃ / min, the welding temperature of 1040℃, and the holding time of 15 min provided suitable thermodynamic and kinetic conditions for the full diffusion of Al atoms and the controllable nucleation and growth of the γ′ phase. The synergistic effect of these factors ensured the uniform in-situ precipitation of the reinforcing phase within the isothermal solidification zone. The precise matching of this ratio and process parameters fundamentally solves the technical problems of insufficient room temperature strength, low elongation, and increased brittle phase caused by excessive addition of BNi-2 brazing filler metal, thus achieving a simultaneous improvement in joint strength and plasticity.

[0015] Table 1. Performance comparison of brazed joints between BNi-2 and BNi-2+AlSi10 Attached Figure Description

[0016] Figure 1 This is an Al-Ni phase diagram from an embodiment of the present invention; Figure 2 Microstructure diagram of the brazed joint with 2wt% AlSi10 added in an embodiment of the present invention; Figure 3 Microstructure diagram of the brazed joint with 4wt% AlSi10 added in an embodiment of the present invention; Figure 4 Microstructure diagram of the brazed joint with 6wt% AlSi10 added in an embodiment of the present invention. Detailed Implementation

[0017] The embodiments described below with reference to the accompanying drawings are exemplary and intended to explain the invention, and should not be construed as limiting the invention.

[0018] To address the problems existing in current technologies, this invention proposes a strengthening scheme: adding AlSi10 powder to BNi-2 brazing filler metal. During brazing, Al and Ni undergo an in-situ reaction, generating a γ′-Ni3Al intermetallic compound with an L12 structure in the isothermal solidification zone. This enhances the room temperature strength and elongation of the region through an ordered precipitation strengthening mechanism, with the strengthening effect stemming from the effective hindrance of dislocation movement by the long-range ordered characteristics of the L12 structure. This scheme aims to broaden the process window, improve the room temperature mechanical properties of the joint, and provide a new approach for joining stainless steel materials. However, directly adding AlSi10 powder to nickel-based brazing filler metals can easily lead to segregation, oxidation, and deterioration of melt fluidity. Therefore, while ensuring good wettability and filling properties of the brazing filler metal, it is crucial to accurately control the AlSi10 content through a reasonable powder addition method to achieve uniform in-situ generation of the L12 strengthening phase in the isothermal solidification zone and simultaneously improve the elongation of the joint.

[0019] This invention aims to overcome this bottleneck by adding a certain amount of AlSi10 powder to BNi-2 solder, thereby generating an L12 ordered reinforcing phase in situ while ensuring the wettability and gap-filling ability of the solder, ultimately improving the room temperature mechanical properties of the joint.

[0020] Reference Figure 1As shown, the precipitation behavior of this strengthening phase can be explained by the Al-Ni phase diagram: Al atoms and Ni atoms react through diffusion to form L12-type intermetallic compounds. Ni3Al is dispersed in the solder matrix as micron-sized particles, thus strengthening the solder matrix. In the Al-Ni phase diagram, the melting point of Ni3Al is approximately 1385–1390 °C. At 77.5 wt% Ni and 1395 °C, a peritectic reaction occurs: L + β(NiAl) → γ′(Ni3Al). The actual brazing temperature is much lower than the melting point of Ni3Al. Ni3Al mainly precipitates through solid-state ordering and desolvation: at 1050–1150 °C, Al has high solubility in γ(Ni), and solid solution is the main process, with a small amount of γ′ nucleation; at 1000–1050 °C, as the temperature decreases, the solubility of Al decreases, and a large amount of γ′ phase precipitates uniformly. The γ′ phase exists stably in the range of approximately 600 °C to 1385 °C.

[0021] The above technical solution will be further analyzed below with reference to the accompanying drawings and examples: Example 1: (I) Sample preparation Base material: 316L stainless steel, dimensions are 15mm×20mm×1mm; Solder composition (wt%): 98wt% BNi-2 powder, 2wt% AlSi10 powder; BNi-2 nickel-based solder powder is composed of the following components by mass percentage: Cr: 7%; Si: 4.5%; B: 3%; Fe: 3%; C: 0.05%; The balance is Ni and unavoidable impurities.

[0022] AlSi10 aluminum alloy powder is composed of the following components by mass percentage: Al: 89.5%; Si: 10%; Mg: 0.1%; Fe: 0.25%; Cu: 0.05%; Zn: 0.03%; The balance represents unavoidable impurities.

[0023] Powder particle size: BNi-2 is 200 mesh, AlSi10 powder is 100-300 mesh; (II) Welding process A brazing method for 316L stainless steel, using a composite nickel-based brazing filler metal, includes the following steps: Step 1: Preparation of the workpiece to be welded: Grind, ultrasonically clean and dry the surface of the 316L stainless steel base material to be welded; Step 2: Solder preparation: 98wt% BNi-2 nickel-based solder powder and 2wt% AlSi10 aluminum alloy powder were mixed. The BNi-2 and AlSi10 powders were mixed for 20 minutes at a speed of 200r / min using a QM-1SP2L ball mill. The vacuum ball mill jar of the equipment was used, and after evacuation, high-purity argon gas was filled into it as a protective atmosphere to obtain composite solder with a ball-to-solid ratio of 5:1. Step 3: Vacuum Brazing: The composite brazing filler metal obtained in Step 2 is prepared into a paste and applied to the interface to be brazed. The parts are then assembled and fixed together. The brazing gap is controlled by spot welding foil at both ends of the sample. The assembled sample is placed in a vacuum brazing furnace. Vacuum degree 5×10⁻⁶ -3 Pa, welding rate of 10℃ / min, welding temperature of 1040℃, holding temperature for 15min, and cooling with furnace.

[0024] Specifically, the process of preparing the composite solder into a paste involves adding the weighed BNi-2+AlSi10 composite solder powder and the organic carrier to a ceramic mortar, and then proceeding with the following steps: 3.1 Stir slowly in the same direction with a grinding pestle for 5-10 minutes to initially wet and mix the powder with the organic carrier. The organic carrier is composed of the following components by weight percentage: 22.0% polyvinyl acetate with a molecular weight of 150,000, 68.0% terpineol, 5.0% lecithin, and 5.0% butyl stearate.

[0025] 3.2 Once there is no obvious agglomeration of dry powder, begin manual grinding and mixing, using a method of pressing and stirring simultaneously, with a total grinding time of 30 minutes.

[0026] 3.3 During the process, every 5 to 8 minutes, use a scraper to scrape off the paste adhering to the mortar wall and pestle head, collect it in the center of the mortar and continue grinding to ensure that all materials are fully mixed and uniform.

[0027] 3.4 During the grinding process, organic carriers can be added drop by drop to adjust the viscosity according to the state of the paste. Each addition should not exceed 5% of the total mass of the carrier until a uniform, fine, particle-free, and viscous paste-like composite brazing filler metal is obtained.

[0028] 3.5 The prepared paste-like composite brazing filler metal is placed in a sealed container and left to stand at room temperature for 12 hours to allow the components to fully integrate and stabilize, thus obtaining the paste-like composite brazing filler metal.

[0029] Step 4: Cooling: Cool to room temperature with the furnace to obtain the brazed joint.

[0030] Step 5: Post-weld treatment: Cut the brazed joint sample and polish it sequentially with sandpaper of grits 80, 240, 400, 800, 1200, 1500, and 2000. Then, observe the metallographic structure using a scanning electron microscope (SEM). Mechanical properties are then tested using a computer-controlled electronic universal testing machine (ETM105D) at a testing rate of 1 mm / min.

[0031] (III) Results Analysis Reference Figure 2 As shown, the joint exhibits a typical three-layer structure: an isothermal solidification zone (ISZ) in the middle and diffusion-affected zones (DAZ) on both sides, with a continuous overall interface. The light-colored area in the middle is the isothermal solidification zone, which is uniform in thickness and dense in structure. Cubic micron-sized γ′ reinforcing phases are uniformly dispersed on the brazed joint matrix, exhibiting small phase size, high cubicity, and uniform distribution. The interface between the brazed joint and the base material has a smooth transition, and slight element diffusion traces are present on the base material side. The thickness of the interface diffusion layer is approximately 15-20 μm, which is 33.3% thicker than that of BNi-2 brazing (12-15 μm). The brazed joint obtained by adding 2wt% AlSi10 brazing filler metal has a tensile strength of 418 MPa, which is 16.7% higher than that of the BNi-2 brazed joint, and the elongation after fracture is increased by 4%.

[0032] Example 2: (I) Sample preparation Base material: 316L stainless steel, dimensions are 15mm×20mm×1mm; Solder composition (wt%): 96wt% BNi-2 powder, 4wt% AlSi10 powder; BNi-2 nickel-based solder powder is composed of the following components by mass percentage: Cr: 7.8%; Si: 4.8%; B: 2.8%; Fe: 2.6%; C: 0.04%; The balance is Ni and unavoidable impurities.

[0033] AlSi10 aluminum alloy powder is composed of the following components by mass percentage: Al: 88.5%; Si: 10.5%; Mg: 0.08%; Fe: 0.50%; Cu: 0.08%; Zn: 0.04%; The balance represents unavoidable impurities.

[0034] Powder particle size: BNi-2 is 200 mesh, AlSi10 powder is 100-300 mesh; (II) Welding process A brazing method for 316L stainless steel, using a composite nickel-based brazing filler metal, includes the following steps: Step 1: Preparation of the workpiece to be welded: Grind, ultrasonically clean and dry the surface of the 316L stainless steel base material to be welded; Step 2: Solder preparation: 96wt% BNi-2 nickel-based solder powder and 4wt% AlSi10 aluminum alloy powder were mixed. The BNi-2 and AlSi10 powders were mixed for 20 minutes at a speed of 200r / min using a QM-1SP2L ball mill. The vacuum ball mill jar of the equipment was used, and after evacuation, high-purity argon gas was filled into it as a protective atmosphere to obtain composite solder with a ball-to-solid ratio of 5:1. Step 3: Vacuum Brazing: The composite brazing filler metal obtained in Step 2 is prepared into a paste and applied to the interface to be brazed. The parts are then assembled and fixed together. The brazing gap is controlled by spot welding foil at both ends of the sample. The assembled sample is placed in a vacuum brazing furnace. Vacuum degree 5×10⁻⁶ -3 Pa, welding rate of 10℃ / min, welding temperature of 1040℃, holding temperature for 15min, and cooling with furnace.

[0035] Specifically, the process of preparing the composite solder into a paste involves adding the weighed BNi-2+AlSi10 composite solder powder and the organic carrier to a ceramic mortar, and then proceeding with the following steps: 3.1 Stir slowly in the same direction with a grinding pestle for 5-10 minutes to initially wet and mix the powder with the organic carrier. The organic carrier is composed of the following components by weight percentage: 22.0% polyvinyl acetate with a molecular weight of 150,000, 68.0% terpineol, 5.0% lecithin, and 5.0% butyl stearate.

[0036] 3.2 Once there is no obvious agglomeration of dry powder, begin manual grinding and mixing, using a method of pressing and stirring simultaneously, with a total grinding time of 30 minutes.

[0037] 3.3 During the process, every 5 to 8 minutes, use a scraper to scrape off the paste adhering to the mortar wall and pestle head, collect it in the center of the mortar and continue grinding to ensure that all materials are fully mixed and uniform.

[0038] 3.4 During the grinding process, organic carriers can be added drop by drop to adjust the viscosity according to the state of the paste. Each addition should not exceed 5% of the total mass of the carrier until a uniform, fine, particle-free, and viscous paste-like composite brazing filler metal is obtained.

[0039] 3.5 The prepared paste-like composite brazing filler metal is placed in a sealed container and left to stand at room temperature for 12 hours to allow the components to fully integrate and stabilize, thus obtaining the paste-like composite brazing filler metal.

[0040] Step 4: Cooling: Cool to room temperature with the furnace to obtain the brazed joint.

[0041] Step 5: Post-weld treatment: Cut the brazed joint sample and polish it sequentially with sandpaper of grits 80, 240, 400, 800, 1200, 1500, and 2000. Then, observe the metallographic structure using a scanning electron microscope (SEM). Mechanical properties are then tested using a computer-controlled electronic universal testing machine (ETM105D) at a testing rate of 1 mm / min.

[0042] (III) Results Analysis Reference Figure 3 As shown, the diffusion-affected zones on both sides contain a large number of network-like second phases continuously distributed along the grain boundaries; isolated micropores are also visible inside the matrix. The central light gray isothermal solidification zone contains a large area of ​​continuous dark-colored banded phases, with a large number of diffusely distributed micropores; the interface between the brazed joint and the matrix is ​​smooth, and element diffusion characteristics are obvious. The brazed joint obtained by adding 4wt% AlSi10 brazing filler metal has a tensile strength of 339.5 MPa and an elongation after fracture of 10%.

[0043] Example 3: (I) Sample preparation Base material: 316L stainless steel, dimensions are 15mm×20mm×1mm; Solder composition (wt%): 94wt% BNi-2 powder, 6wt% AlSi10 powder; BNi-2 nickel-based solder powder is composed of the following components by mass percentage: Cr: 7.8%; Si: 4.8%; B: 2.8%; Fe: 2.6%; C: 0.04%; The balance is Ni and unavoidable impurities.

[0044] AlSi10 aluminum alloy powder is composed of the following components by mass percentage: Al: 87.8%; Si: 9.8%; Mg: 0.05%; Fe: 0.15%; Cu: 0.02%; Zn: 0.01%; The balance represents unavoidable impurities.

[0045] Powder particle size: BNi-2 is 200 mesh, AlSi10 powder is 100-300 mesh; (II) Welding process A brazing method for 316L stainless steel, using a composite nickel-based brazing filler metal, includes the following steps: Step 1: Preparation of the workpiece to be welded: Grind, ultrasonically clean and dry the surface of the 316L stainless steel base material to be welded; Step 2: Solder preparation: 94wt% BNi-2 nickel-based solder powder and 6wt% AlSi10 aluminum alloy powder were mixed. The BNi-2 and AlSi10 powders were mixed for 20 minutes at a speed of 200r / min using a QM-1SP2L ball mill. The vacuum ball mill jar of the equipment was used, and after evacuation, high-purity argon gas was filled into it as a protective atmosphere to obtain composite solder with a ball-to-solid ratio of 5:1. Step 3: Vacuum Brazing: The composite brazing filler metal obtained in Step 2 is prepared into a paste and applied to the interface to be brazed. The parts are then assembled and fixed together. The brazing gap is controlled by spot welding foil at both ends of the sample. The assembled sample is placed in a vacuum brazing furnace. Vacuum degree 5×10⁻⁶ -3 Pa, welding rate of 10℃ / min, welding temperature of 1040℃, holding temperature for 15min, and cooling with furnace.

[0046] Specifically, the process of preparing the composite solder into a paste involves adding the weighed BNi-2+AlSi10 composite solder powder and the organic carrier to a ceramic mortar, and then proceeding with the following steps: 3.1 Stir slowly in the same direction with a grinding pestle for 5-10 minutes to initially wet and mix the powder with the organic carrier. The organic carrier is composed of the following components by weight percentage: 22.0% polyvinyl acetate with a molecular weight of 150,000, 68.0% terpineol, 5.0% lecithin, and 5.0% butyl stearate.

[0047] 3.2 Once there is no obvious agglomeration of dry powder, begin manual grinding and mixing, using a method of pressing and stirring simultaneously, with a total grinding time of 30 minutes.

[0048] 3.3 During the process, every 5 to 8 minutes, use a scraper to scrape off the paste adhering to the mortar wall and pestle head, collect it in the center of the mortar and continue grinding to ensure that all materials are fully mixed and uniform.

[0049] 3.4 During the grinding process, organic carriers can be added drop by drop to adjust the viscosity according to the state of the paste. Each addition should not exceed 5% of the total mass of the carrier until a uniform, fine, particle-free, and viscous paste-like composite brazing filler metal is obtained.

[0050] 3.5 The prepared paste-like composite brazing filler metal is placed in a sealed container and left to stand at room temperature for 12 hours to allow the components to fully integrate and stabilize, thus obtaining the paste-like composite brazing filler metal.

[0051] Step 4: Cooling: Cool to room temperature with the furnace to obtain the brazed joint.

[0052] Step 5: Post-weld treatment: Cut the brazed joint sample and polish it sequentially with sandpaper of grits 80, 240, 400, 800, 1200, 1500, and 2000. Then, observe the metallographic structure using a scanning electron microscope (SEM). Mechanical properties are then tested using a computer-controlled electronic universal testing machine (ETM105D) at a testing rate of 1 mm / min.

[0053] (III) Results Analysis Organizational analysis: Refer to Figure 4 As shown, the diffusion-affected zone contains a large number of network-like second phases continuously distributed along the grain boundaries. These continuous network precipitates penetrate the grain boundaries, disrupting the continuity of the matrix structure. The interface between the brazed joint and the base material is smooth, with obvious element diffusion characteristics. However, due to the poor wettability of the brazing filler metal, unbonded areas exist in the isothermal solidification zone. The brazed joint obtained by adding 6wt% AlSi10 to the brazing filler metal has a tensile strength of 77.8 MPa and an elongation after fracture of 3%.

[0054] Comparative Example 1: “Salmaliyan, M.Shamanian, M.Effect of Ti Powders Addition on Mechanical and Metallurgical Properties of IN718 / BNi-2 / 316L Diffusion Couple[J].Transactions of the Indian Institute of Metals,2020,73,(5)” This study added Ti powder to BNi-2 solder for diffusion bonding of IN718 nickel-based superalloy and 316L stainless steel. Although the addition of Ti powder increased the hardness of the joint, it had almost no effect on the shear strength at room temperature.

[0055] Comparative Example 2: "Li Yunyue, Li Zhuoxin, Li Hong, et al. Performance of brazed stainless steel joints with corrosion-resistant nickel-based foil brazing filler metal [J]. Journal of Welding, 2019, 40, (9): 30-38, I0002." This study used a new type of corrosion-resistant nickel-based foil brazing filler metal BNi685 to vacuum braze 316L stainless steel, and the maximum tensile strength of the joint was 244 MPa.

[0056] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention without departing from the principles and spirit of the present invention.

Claims

1. A nickel-based brazing filler metal that generates an L12 strengthening phase in situ, used for brazing 316L stainless steel, characterized in that, The composite nickel-based solder comprises BNi-2 nickel-based solder powder and AlSi10 aluminum alloy powder; At the brazing temperature, the composite nickel-based brazing filler metal generates an L12-type ordered strengthening phase in the isothermal solidification zone of the brazing seam through the in-situ reaction of Al and Ni elements, thereby achieving precipitation strengthening of the brazed joint.

2. The nickel-based solder with in-situ generated L12 strengthening phase according to claim 1, characterized in that: The amount of AlSi10 aluminum alloy powder added is 2.0% to 6.0% of the total mass of the composite nickel-based brazing filler metal.

3. The nickel-based solder with in-situ generated L12 strengthening phase according to claim 1, characterized in that: The BNi-2 nickel-based solder powder consists of the following components by mass percentage: composition: Cr：6.0%～8.0%； Si: 4.0%~5.0%; B：2.75%～3.5%； Fe: 2.5%~3.5%; C：≤0.06%； The balance is Ni and unavoidable impurities.

4. The nickel-based solder with in-situ generated L12 strengthening phase according to claim 1, characterized in that: The AlSi10 aluminum alloy powder consists of the following components by mass percentage: composition: Al：87%～90%； Si: 9%~11%; Mg: ≤0.1%; Fe: ≤0.55%; Cu: ≤0.1%; Zn: ≤0.05%; The balance represents unavoidable impurities.

5. A brazing method for 316L stainless steel, characterized in that, Brazing using the composite nickel-based brazing filler metal according to any one of claims 1-4 includes the following steps: Step 1: Preparation of the workpiece to be welded: Grind, ultrasonically clean and dry the surface of the 316L stainless steel base material to be welded; Step 2: Solder preparation: BNi-2 nickel-based solder powder and AlSi10 aluminum alloy powder are mixed in proportion and ball-milled under a protective atmosphere to obtain BNi-2+AlSi10 composite solder. Step 3: Vacuum brazing: Apply BNi-2+AlSi10 composite brazing filler metal to the interface to be brazed. After assembling and fixing the parts to be brazed, place them in a vacuum brazing furnace. Heat to the brazing temperature under vacuum conditions and hold at that temperature to allow the composite brazing filler metal to melt, spread, and fill the gaps. At the same time, Al and Ni elements react in situ to generate L12 type ordered strengthening phase. Step 4: Cooling: Cool to room temperature with the furnace to obtain the brazed joint.

6. The brazing method for 316L stainless steel according to claim 5, characterized in that: In step 2, the ball milling process parameters are: rotation speed 200 r / min, time 20 min, ball-to-material ratio 5:1; the protective atmosphere is argon.

7. The brazing method for 316L stainless steel according to claim 5, characterized in that: In step 3, the vacuum brazing process parameters are: vacuum degree 5×10 -3 Pa, heating rate of 10℃ / min, brazing temperature of 1040℃, holding time of 15min.

8. The brazing method for 316L stainless steel according to claim 5, characterized in that: In step 3, the composite solder coated on the interface to be soldered is in paste form. The preparation method of the paste composite solder is as follows: the prepared BNi-2 / AlSi10 composite solder powder is mixed with an organic carrier at a weight ratio of 93.5:6.5, and then thoroughly stirred and ground to form a paste composite solder. The organic carrier is composed of the following components by weight percentage: 22.0% polyvinyl acetate with a molecular weight of 150,000, 68.0% terpineol, 5.0% lecithin, and 5.0% butyl stearate.

9. A post-weld treatment method for brazed joints of 316L stainless steel, characterized in that, Includes the following steps: Step 1: Cut the 316L stainless steel brazed joint obtained by the brazing method described in any one of claims 5-8 to obtain a metallographic specimen and / or a mechanical property test specimen. Step 2: Grind the sample sequentially using sandpaper of 80 grit, 240 grit, 400 grit, 800 grit, 1200 grit, 1500 grit, and 2000 grit. Step 3: Polish the ground sample until the surface is mirror-like. Step 4: Use the polished sample for microstructure observation and / or mechanical property testing; the mechanical property testing is performed using a microcomputer-controlled electronic universal testing machine ETM105D, with a testing rate of 1 mm / min.

10. A 316L stainless steel brazing joint, characterized in that, The brazed joint is prepared by the brazing method according to any one of claims 5-8, wherein the brazed joint has an L12 type Ni3Al ordered strengthening phase dispersedly in the isothermal solidification zone of the brazed seam. The L12 type ordered strengthening phase is generated by the in-situ reaction of Al element in AlSi10 aluminum alloy powder and Ni element in BNi-2 nickel-based brazing filler powder at the brazing temperature. The room temperature shear strength of the brazed joint is ≥418MPa.

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