CN113588378A - Preparation and calculation method of brazed joint fracture toughness sample - Google Patents

Preparation and calculation method of brazed joint fracture toughness sample Download PDF

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CN113588378A
CN113588378A CN202110986664.XA CN202110986664A CN113588378A CN 113588378 A CN113588378 A CN 113588378A CN 202110986664 A CN202110986664 A CN 202110986664A CN 113588378 A CN113588378 A CN 113588378A
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fracture toughness
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brazing
unloading
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CN113588378B (en
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周帼彦
马寒阳
段鹏洋
王涛
贾云飞
涂善东
谈建平
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East China University of Science and Technology
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N1/00—Sampling; Preparing specimens for investigation
    • G01N1/28—Preparing specimens for investigation including physical details of (bio-)chemical methods covered elsewhere, e.g. G01N33/50, C12Q
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N3/00—Investigating strength properties of solid materials by application of mechanical stress
    • G01N3/32—Investigating strength properties of solid materials by application of mechanical stress by applying repeated or pulsating forces
    • G01N3/36—Investigating strength properties of solid materials by application of mechanical stress by applying repeated or pulsating forces generated by pneumatic or hydraulic means
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N3/00—Investigating strength properties of solid materials by application of mechanical stress
    • G01N3/40—Investigating hardness or rebound hardness
    • G01N3/42—Investigating hardness or rebound hardness by performing impressions under a steady load by indentors, e.g. sphere, pyramid
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N2203/00—Investigating strength properties of solid materials by application of mechanical stress
    • G01N2203/0001—Type of application of the stress
    • G01N2203/0005—Repeated or cyclic
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N2203/00—Investigating strength properties of solid materials by application of mechanical stress
    • G01N2203/0001—Type of application of the stress
    • G01N2203/0012—Constant speed test
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N2203/00—Investigating strength properties of solid materials by application of mechanical stress
    • G01N2203/0014—Type of force applied
    • G01N2203/0016—Tensile or compressive
    • G01N2203/0017—Tensile
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N2203/00—Investigating strength properties of solid materials by application of mechanical stress
    • G01N2203/0014—Type of force applied
    • G01N2203/0016—Tensile or compressive
    • G01N2203/0019—Compressive
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N2203/00—Investigating strength properties of solid materials by application of mechanical stress
    • G01N2203/003—Generation of the force
    • G01N2203/0042—Pneumatic or hydraulic means
    • G01N2203/0048—Hydraulic means
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N2203/00—Investigating strength properties of solid materials by application of mechanical stress
    • G01N2203/0058—Kind of property studied
    • G01N2203/006—Crack, flaws, fracture or rupture
    • G01N2203/0067—Fracture or rupture
    • G—PHYSICS
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    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N2203/00—Investigating strength properties of solid materials by application of mechanical stress
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Abstract

The invention provides a method for preparing and calculating a brazed joint fracture toughness test sample. The samples were prepared by applying a solder resist during the brazing process so that the initial crack tips were within the braze joint micro-zone. In the aspect of calculating the fracture toughness, the nanometer indentation size effect is comprehensively considered, and the elastic modulus, the yield strength and the hardening index of a diffusion influence area (on a fracture path) of the brazing joint are calculated. And a loading linear displacement control mode is adopted, and a J-delta a resistance curve is obtained through multiple times of loading/unloading according to an unloading flexibility method. Determination of fracture resistance J based on passivation line equation of power hardening elastoplasticity constitutive0.2BL. The invention solves the problems that in the conventional fracture toughness experiment, the initial crack size of the current processing mode is far larger than that of a brazed joint, the crack tip is difficult to form at the brazed joint, and reliable crack propagation resistance is difficult to obtainA performance parameter. The invention solves the problems of experimental and analytical methods for measuring the fracture toughness of the soldered joint and is of great importance for further researching the fracture mechanism of the soldered joint.

Description

Preparation and calculation method of brazed joint fracture toughness sample
Technical Field
The invention relates to the technical field of fracture toughness testing, in particular to a method for preparing and calculating a sample of the fracture toughness of a brazed joint.
Background
The brazing technology has the advantages of small welding deformation, high size precision, smooth welding structure and the like. Therefore, the welding method is widely applied to the welding process of the compact components. A large number of experimental researches show that the component is easy to lose effectiveness at the position of a soldered joint under extreme service conditions such as high temperature and high pressure. The braze joint is the weakest part of the overall component, which determines the safety and reliability of the component over long periods of operation. During the brazing process, the wettability of the brazing filler metal, the limitation of the brazing process and the Kirkendall phenomenon generate defects such as micro cracks, holes and the like, which are main inducers of the failure of the brazed joint. In a brazed structure, once such defects occur, the brazed joint will crack and propagate until the component is broken. Therefore, the method of testing for fracture toughness is critical to understanding the fracture mechanism of braze joints.
Brazing is a thermal joining technique. In the heating process, the liquid brazing filler metal is wetted and spread on the gaps or the surfaces of the base metals, and generates dissolution, diffusion and solidification reactions with the base metals to connect the base metals together. The braze joint can be divided into an isothermal solidification zone, a non-isothermal solidification zone, a diffusion affected zone, and a parent metal according to microstructure distribution. The diffusion influence area contains a large amount of boride, which is formed by the boron element in the brazing filler metal diffusing into the base material and reacting with elements such as chromium, molybdenum and the like in the base material. Failure occurred in the diffusion affected zone in all of the tensile, shear and peel tests. Thus, the diffusion-affected zone is the weakest point in the joint. In addition, the diffusion reaction is only carried out at the interface of tens of microns below the base material, and generally does not involve the deep structure of the base material, so that the micro-area normal temperature mechanical property is difficult to obtain. In conventional fracture toughness experiments, the wire-cut kerf size is much larger than the braze joint, it is difficult to form crack tips at the braze joint, and it is more difficult to obtain reliable performance parameters that resist crack propagation.
Disclosure of Invention
The invention aims to solve the problems of the prior art, provides a sample preparation and calculation method for the fracture toughness of a brazed joint, and aims to solve the technical problems that the brazed joint has crack propagation in a brazed structure until a component is fractured, the micro-area normal-temperature mechanical property is difficult to obtain, a crack tip is difficult to form at the brazed joint, and reliable performance parameters for resisting crack propagation are difficult to obtain.
The application also provides a preparation method of the brazed joint fracture toughness test sample, which comprises the following steps:
aligning and pressing the sample blocks, preparing an upper sample block and a lower sample block which are matched with each other up and down, wherein the butting surfaces of the upper sample block and the lower sample block are rectangles with the same shape and size, the length of one side of each rectangle is 32mm, a first rectangular area and a second rectangular area which are adjacently arranged are arranged in the extending direction of the side of each rectangle, the boundary line of the first rectangular area and the second rectangular area is parallel to the other side, and the area ratio range of the first rectangular area and the second rectangular area is 9: 10 to 1: 1, placing amorphous foil brazing filler metal in the first rectangular area, coating a brazing resistance agent in the second rectangular area, aligning the butt joint surfaces of the upper sample block and the lower sample block, and pressing the upper sample block and the lower sample block; and
and heating in a vacuum brazing furnace, wherein the upper sample block and the lower sample block are pressed and then placed in the vacuum brazing furnace for heating, and a brazed joint fracture toughness sample is prepared.
Further, an adhesive is coated around the amorphous foil brazing filler metal.
Further, the braze joint prepared and formed had initial cracks in the fracture toughness specimens, the initial cracks being located within the braze joint.
Further, the parent material of the block is austenitic stainless steel 316L; the amorphous foil brazing filler metal is made of nickel-based BNi-2 brazing filler metal.
Further, the heating temperature in the vacuum brazing furnace is 1065 ℃, the heating rate is 10 ℃/min, and the heat preservation time at 1065 ℃ is 60 min.
Further, the heating process in the vacuum brazing furnace comprises the following steps: (1) in the vacuum pumping stage, the vacuum degree is less than 0.008 Pa; (2) heating to 850 deg.C at a heating rate of 10 deg.C/min; (3) keeping the temperature at 850 ℃ for 30 min; (4) heating to 1065 deg.C at 10 deg.C/min; (5) keeping the temperature at 1065 ℃ for 60min to ensure that the amorphous foil brazing filler metal is fully diffused and reacted; (6) and cooling the vacuum brazing furnace to room temperature.
The application also provides a method for calculating the fracture toughness sample of the brazed joint, which is characterized by comprising the following steps:
preparing a brazed joint fracture toughness test sample by adopting the preparation method of the brazed joint fracture toughness test sample according to any one of claims 1 to 6;
a step of processing a brazing joint compact tensile sample, wherein the brazing sample is processed into the compact tensile sample, and a brazing seam and an initial crack are positioned in the middle; specimen thickness B was 12.7mm, width W was 25.4mm, and the ratio of initial crack length to specimen width W was 0.5; pre-restricting fatigue cracks of 1.72-8.08mm by using a high-frequency fatigue testing machine, wherein the fatigue cracks are expanded in an isothermal solidification area;
nano indentation experiment step of diffusion influence area of the soldered joint, selecting the diffusion influence area in the soldered joint as an action point; loading to a maximum load of 30mN/60mN/90mN/120mN/150mN/180mN at a loading rate of 5 mN/s; carrying for 10s to eliminate the creep effect; unloading to 10% of the maximum load at an unloading rate of 5mN/s to eliminate the influence of temperature; carrying out 100S; completely unloading;
calculating the yield strength and the hardening index of the diffusion affected zone of the braze joint, and calculating the yield strength sigma of the diffusion affected zone of the braze jointyAnd a hardening index n;
the method comprises the steps of a resistance curve experiment of the soldered joint by a single sample method, wherein the experiment is carried out in an Instron 8801 hydraulic fatigue testing machine, the test process adopts a loading linear displacement control mode, and the loading/unloading speed is 0.5 mm/min; measuring the crack length delta a and the fracture resistance J integral by using an unloading flexibility method through multiple times of loading/unloading to obtain a J-delta a resistance curve; and
calculation of fracture resistance J0.2BLStep of obtaining a calculated offset passivation line from the yield strength and the hardening index to determine the fracture resistance J0.2BL。
Further, the yield strength σ of the diffusion affected zone of the braze joint was calculatedyAnd a hardening index n, comprising:
(1) fitting parameters according to an unloading curve in a load p-displacement h curve in the unloading process, wherein the fitting range is 20% of the upper part of the unloading curve;
P＝α(h-hf)m(formula 1)
Wherein: alpha, hfAnd m is a fitting parameter;
(2) the elastic contact stiffness S and the contact area a are calculated,
Figure RE-GDA0003258815790000031
Figure RE-GDA0003258815790000032
Figure RE-GDA0003258815790000033
wherein, PmaxIs the maximum load; h ismaxIs the maximum indentation depth; ε is the indenter shape related constant, ε is 0.75 for the Berkovich indenter; c is a constant of the revised area function, C for Berkovich indenteriA value of about 150 nm;
(3) calculating the modulus of elasticity E and the reduced modulus Er，
Figure RE-GDA0003258815790000034
Figure RE-GDA0003258815790000035
Wherein β is an indenter geometry-related constant, β is 1.034 for the Berkovich indenter; e and v are the modulus of elasticity and Poisson's ratio of the sample material; eiAnd viThe elastic modulus and Poisson's ratio of the pressure head material are shown, and the values of diamond are 1140GPa and 0.07;
(4) according to the hardness values under different indentation loads, the hardness value H and the yield strength sigma which are irrelevant to the indentation depth are calculatedy，
Figure RE-GDA0003258815790000041
H0＝4.15σy(formula 8)
(5) Calculating the plastic strain epsilonpStress σ at 0.0330.033，
Figure RE-GDA0003258815790000042
Wherein the C value is the loading phase P ═ Ch2Fitting the obtained loading curvature;
(6) the hardening index n is calculated and the hardening index,
Figure RE-GDA0003258815790000043
further, a J-delta a resistance curve is obtained according to an unloading flexibility method.
Further, the calculated fracture resistance J0.2BLThe method specifically comprises the following steps:
judging the effectiveness of the J-delta a resistance curve;
the passivation line equation is obtained from the yield strength and the hardening index:
Figure RE-GDA0003258815790000044
Figure RE-GDA0003258815790000045
Dn＝0.787+1.554n-2.45n2+16.952n3-38.206n4+33.13n5(formula 13)
Obtaining fracture resistance J according to the intersection point of the 0.2mm offset passivation line and the J-delta a resistance curve0.2BL。
The invention has the beneficial effects that the preparation and calculation method of the fracture toughness sample of the brazed joint is provided, the sample is prepared by smearing the solder resist agent in the brazing process, and the initial crack tip is positioned in the range of the micro-area of the brazed joint. In the aspect of calculating the fracture toughness, the nanometer indentation size effect is comprehensively considered, and the elastic modulus, the yield strength and the hardening index of a diffusion influence area (on a fracture path) of the brazing joint are calculated. And a loading linear displacement control mode is adopted, and a J-delta a resistance curve is obtained through multiple times of loading/unloading according to an unloading flexibility method. Determination of fracture resistance J based on passivation line equation of power hardening elastoplasticity constitutive0.2BL. The invention solves the problems that in the conventional fracture toughness experiment, the initial crack size of the current processing mode is far larger than that of a brazed joint, the crack tip is difficult to form at the brazed joint, and reliable performance parameters for resisting crack propagation are difficult to obtain. The invention solves the problems of experimental and analytical methods for measuring the fracture toughness of the soldered joint and is of great importance for further researching the fracture mechanism of the soldered joint.
Drawings
FIG. 1 is a schematic flow chart of a method for preparing a fracture toughness test sample of a brazed joint according to the present application;
FIG. 2 is a schematic flow chart of a method for calculating fracture toughness of a brazed joint sample according to the present application;
FIG. 3 is a schematic illustration of the brazing sample preparation of the present application;
FIG. 4 is a cross-sectional optical microscope image of a brazed sample of the present application;
FIG. 5 is a graph of the dimensions of a compact tensile specimen of a brazed joint according to the present application;
FIG. 6 is a nano-indentation load-depth curve of the present application;
FIG. 7 is a fracture path of fracture toughness of a braze joint of the present application;
FIG. 8 is a J-R resistance curve for a braze joint according to the present application;
FIG. 9 is an overall schematic diagram of the fracture toughness test of the brazed joint according to the present invention.
In the figure: 1. a sample block is put; 2. brazing filler metal; 3. a solder resist; 4. and (4) loading a sample block.
Detailed Description
The invention is further illustrated below with reference to the accompanying drawings:
the invention provides a method for preparing a fracture toughness sample suitable for a brazing joint micro brazing seam, a resistance curve and fracture resistance J0.2BLCalculating values, wherein a flow chart of the preparation and calculation method of the brazing joint fracture toughness test piece is respectively shown in fig. 1 and fig. 2.
As shown in FIG. 1, the preparation method of the fracture toughness test sample of the braze welding joint adopts a mode of re-processing the pre-fabricated initial cracks of the solder resist, and can simultaneously prepare q fracture toughness test samples which specifically comprise S11-S12.
S11, a sample block aligning and pressing step, in conjunction with fig. 3, preparing two upper and lower sample blocks 4 and 1 fitted to each other, wherein the abutting surfaces of the upper and lower sample blocks 4 and 1 are rectangles having the same shape and size, the rectangle has a long side and a short side, the length of the short side is 32mm, a first rectangular region (refer to a region corresponding to reference numeral 2 in fig. 3) and a second rectangular region (refer to a region corresponding to reference numeral 3 in fig. 3) are provided adjacently along the extending direction of the short side, the borderlines of the first rectangular region and the second rectangular region are parallel to the long side, and the length ratio range of the first rectangular region and the second rectangular region at the position along the short side is 9: 10 to 1: 1, placing amorphous foil brazing filler metal 2 (hereinafter simply referred to as brazing filler metal) in the first rectangular area, coating solder resist 3 in the second rectangular area, aligning the butt joint surfaces of the upper sample block 4 and the lower sample block 1, so that the amorphous foil brazing filler metal 2 in the first rectangular area and the solder resist 3 in the second rectangular area can be adjacently aligned respectively, and pressing the upper sample block 4 and the lower sample block 1; an adhesive is also applied around the amorphous foil strip solder 2. The parent metal of the block is austenitic stainless steel 316L; the amorphous foil brazing filler metal is made of nickel-based BNi-2 brazing filler metal.
Referring to fig. 3, two 50mm by 16mm by 32mm blocks were prepared, 16.3mm by 32mm size solder resists (nicobraz Stop-Off Materials) were applied to the 32mm by 50mm surface sides of the blocks, 15.7mm by 50mm BNi-2 amorphous foil solder was placed on the surface sizes, and the two blocks were placed in a vacuum brazing furnace and heated. In fig. 3, 1 denotes a lower sample block, 2 denotes a brazing material, 3 denotes a solder resist, and 4 denotes an upper sample block. The 32mm 50mm surface of block is the butt joint face, is the rectangle, and the long limit of rectangle is 50mm, and the minor face of rectangle is 32 mm. It will be appreciated that 16mm is the thickness of the block. The short side of the rectangle is preferably 32mm, so that the welding quality can be ensured, and the size of the long side of the rectangle and the thickness of the block body can be set according to actual requirements. The length proportion range of the first rectangular area and the second rectangular area at the position along the short side is 9: 10 to 1: 1, this embodiment is preferably 15.7: 16.3.
the specific process of step S11 is: two blocks of 16mm by 32mm cross-section were prepared, with a thickness q times the thickness of the individual fracture toughness specimens. In addition, in order to reduce the solder wettability and the lack of penetration due to the caulking property, the thickness direction needs to be increased by 5 mm. Thus, the overall thickness d (mm) ═ q 12.7+ 5. BNi-2 amorphous foil brazing filler metal with a size of 15.7mm x thickness D was placed on the block surface (32mm x D) side, and an adhesive was added around the brazing filler metal to fix the brazing filler metal in order to prevent the brazing filler metal from slipping. The other side was coated with 16.3mm x D size solder resist (Nicrobraz Stop-Off Materials). And respectively aligning the solder resist and the solder in the two blocks, and after the solder resist and the solder are compressed, putting the blocks into a vacuum brazing furnace for heating.
And S12, heating in a vacuum brazing furnace, wherein the upper sample block 4 and the lower sample block 1 are pressed and then placed in the vacuum brazing furnace for heating, and a brazed joint fracture toughness sample is prepared. The braze joint fracture toughness test specimens prepared and formed had initial cracks that were within the braze joint range. The heating temperature in the vacuum brazing furnace is 1065 ℃, the heating rate is 10 ℃/min, and the heat preservation time is 60min at 1065 ℃.
The heating process of the vacuum brazing furnace comprises the following specific processes: (1) in the vacuum pumping stage, the vacuum degree is less than 0.008 Pa; (2) heating to 850 deg.C at a heating rate of 10 deg.C/min; (3) keeping the temperature at 850 ℃ for 30min to ensure the temperature in the furnace to be stable; (4) heating to 1065 deg.C at 10 deg.C/min; (5) keeping the temperature at 1065 ℃ for 30min to ensure that the brazing filler metal is fully diffused and reacted; (6) cooling to room temperature along with the furnace.
As shown in FIG. 2, the method for calculating the fracture toughness test sample of the brazed joint comprises the steps of S1-S6.
S1, preparing a brazing joint fracture toughness test sample, namely preparing the brazing joint fracture toughness test sample by adopting the preparation method of the brazing joint fracture toughness test sample, and specifically comprising the steps S11-S12.
Taking a brazing joint part from a brazing sample for microscopic observation, grinding and polishing the sample, and adopting corrosive liquid (10ml HNO)3 -10ml C2H4O2-15ml HCl) corrosion, observed under an optical microscope. As shown in fig. 4, the brazed joint is divided into an isothermal solidification zone, a diffusion-affected zone, and a parent metal. Furthermore, the initial cracks are located within the braze joint micro-zones.
And S2, a brazing joint compact tensile sample processing step, wherein the brazing sample is processed into a Compact Tensile (CT) sample, so that a brazing seam and an initial crack are positioned in the middle, and the geometric dimension of the specific sample is shown in the attached figure 5. The specimen thickness B was 12.7mm, the width W was 25.4mm, and the ratio of the initial crack length to the specimen width a0/W was about 0.5. The high-frequency fatigue testing machine pre-restricts fatigue cracks of 1.72-8.08mm, and the fatigue cracks are expanded in an isothermal solidification area.
S3, nano indentation experiment step of the diffusion influence area of the soldered joint, and the specific step of obtaining the conventional mechanical property of the diffusion influence area of the soldered joint by a nano indentation method: (1) selecting a diffusion influence area in a brazed joint as an action point; (2) loading to a maximum load of 30mN/60mN/90mN/120mN/150mN/180mN at a loading rate of 5 mN/s; (3) carrying for 10s to eliminate the creep effect; (4) unloading to 10% of the maximum load at an unloading rate of 5mN/s to eliminate the influence of temperature; (5) carrying out 100S; (6) and (4) completely unloading. The load-displacement curve obtained by nanoindentation is shown in fig. 6.
S4, calculating the yield strength and the hardening index of the diffusion-affected zone of the braze joint, and calculating the yield strength sigma of the diffusion-affected zone of the braze jointyAnd a hardening index n.
Wherein the yield strength sigma of the diffusion affected zone of the braze joint is calculatedyAnd a hardening index n, comprising:
(1) fitting parameters according to an unloading curve in a load p-displacement h curve in the unloading process, wherein the fitting range is 20% of the upper part of the unloading curve;
P＝α(h-hf)m(formula 1)
Wherein: alpha, hfAnd m is a fitting parameter;
(2) the elastic contact stiffness S and the contact area a are calculated,
Figure RE-GDA0003258815790000071
Figure RE-GDA0003258815790000072
Figure RE-GDA0003258815790000073
wherein, PmaxIs the maximum load; h ismaxIs the maximum indentation depth; ε is the indenter shape related constant, ε is 0.75 for the Berkovich indenter; c is a constant of the revised area function, C for Berkovich indenteriA value of about 150 nm;
(3) calculating the modulus of elasticity E and the reduced modulus Er，
Figure RE-GDA0003258815790000081
Figure RE-GDA0003258815790000082
Wherein β is an indenter geometry-related constant, β is 1.034 for the Berkovich indenter; e and v are the modulus of elasticity and Poisson's ratio of the sample material; eiAnd viThe elastic modulus and Poisson's ratio of the pressure head material are shown, and the values of diamond are 1140GPa and 0.07;
(4) according to the hardness values under different indentation loads, the hardness value H and the yield strength sigma which are irrelevant to the indentation depth are calculatedy，
Figure RE-GDA0003258815790000083
H0＝4.15σy(formula 8)
(5) Calculating the plastic strain epsilonpStress σ at 0.0330.033，
Figure RE-GDA0003258815790000084
Wherein the C value is the loading phase P ═ Ch2Fitting the obtained loading curvature;
(6) the hardening index n is calculated and the hardening index,
Figure RE-GDA0003258815790000085
in summary, the yield strength σ in the above-described calculated braze joint diffusion affected zoneyAnd a hardening index n, the step S4 including:
H0＝4.15σy；
Figure RE-GDA0003258815790000086
s5, a resistance curve experiment step of the soldered joint by a single sample method is carried out on an Instron 8801 hydraulic fatigue testing machine, the test process adopts a loading linear displacement control mode, and the loading/unloading speed is 0.5 mm/min; and measuring the crack length delta a and the integral of the breaking resistance J by using an unloading flexibility method through multiple times of loading/unloading, and obtaining a J-delta a resistance curve according to the unloading flexibility method.
FIG. 7 shows the fracture toughness fracture path where the fatigue pre-crack propagates in the isothermally solidified zone and the fracture toughness test fractures in the diffusion affected zone.
S6, calculating fracture resistance J0.2BLStep of obtaining a calculated offset passivation line from the yield strength and the hardening index to determine the fracture resistance J0.2BL。
Wherein the calculated breaking resistance J0.2BLStep S6 specifically includes:
firstly, judging the effectiveness of a J-delta a resistance curve;
then, the passivation line equation is obtained by the yield strength and the hardening index:
Figure RE-GDA0003258815790000091
Figure RE-GDA0003258815790000092
Dn＝0.787+1.554n-2.45n2+16.952n3-38.206n4+33.13n5(formula 13).
As shown in FIG. 8, fracture resistance J was obtained from the intersection of the 0.2mm offset passivation line and the J- Δ a resistance curve0.2BL。
FIG. 9 is a schematic diagram of the fracture toughness test of the brazed joint. The invention solves the problems of experimental and analytical methods for measuring the fracture toughness of the soldered joint and is of great importance for further researching the fracture mechanism of the soldered joint; the invention adopts a resistorPreparing a sample by using the soldering flux, and enabling the tip of the initial crack to be positioned in the range of a soldering joint micro-area; the invention adopts nano indentation to measure normal temperature mechanical parameters of a diffusion influence area of the soldered joint, including elastic modulus, yield strength and hardening index; the invention adopts a passivation line equation based on a power hardening elastoplasticity constitutive structure to obtain more accurate fracture resistance J0.2BL。
The invention has the beneficial effects that the preparation and calculation method of the fracture toughness sample of the brazed joint is provided, the sample is prepared by smearing the solder resist agent in the brazing process, and the initial crack tip is positioned in the range of the micro-area of the brazed joint. In the aspect of calculating the fracture toughness, the nanometer indentation size effect is comprehensively considered, and the elastic modulus, the yield strength and the hardening index of a diffusion influence area (on a fracture path) of the brazing joint are calculated. And a loading linear displacement control mode is adopted, and a J-delta a resistance curve is obtained through multiple times of loading/unloading according to an unloading flexibility method. Determination of fracture resistance J based on passivation line equation of power hardening elastoplasticity constitutive0.2BL. The invention solves the problems that in the conventional fracture toughness experiment, the initial crack size of the current processing mode is far larger than that of a brazed joint, the crack tip is difficult to form at the brazed joint, and reliable performance parameters for resisting crack propagation are difficult to obtain. The invention solves the problems of experimental and analytical methods for measuring the fracture toughness of the soldered joint and is of great importance for further researching the fracture mechanism of the soldered joint.

Claims (10)

1. A preparation method of a brazed joint fracture toughness test sample is characterized by comprising the following steps:
aligning and pressing the sample blocks, preparing an upper sample block and a lower sample block which are matched with each other up and down, wherein the butting surfaces of the upper sample block and the lower sample block are rectangles with the same shape and size, the length of one side of each rectangle is 32mm, a first rectangular area and a second rectangular area which are adjacently arranged are arranged in the extending direction of the side of each rectangle, the boundary line of the first rectangular area and the second rectangular area is parallel to the other side, and the area ratio range of the first rectangular area and the second rectangular area is 9: 10 to 1: 1, placing amorphous foil brazing filler metal in the first rectangular area, coating a brazing resistance agent in the second rectangular area, aligning the butt joint surfaces of the upper sample block and the lower sample block, and pressing the upper sample block and the lower sample block; and
and heating in a vacuum brazing furnace, wherein the upper sample block and the lower sample block are pressed and then placed in the vacuum brazing furnace for heating, and a brazed joint fracture toughness sample is prepared.
2. The method for preparing the fracture toughness test sample of the brazed joint as recited in claim 1, wherein an adhesive is further coated around the amorphous foil brazing filler metal.
3. The method of claim 1, wherein the formed braze joint fracture toughness test specimen has an initial crack therein, the initial crack being within a braze joint.
4. The method for preparing the brazing joint fracture toughness specimen according to claim 1, wherein the bulk base material is austenitic stainless steel 316L; the amorphous foil brazing filler metal is made of nickel-based BNi-2 brazing filler metal.
5. The method for preparing the sample of the fracture toughness of the brazed joint as recited in claim 1, wherein the heating temperature in the vacuum brazing furnace is 1065 ℃, the heating rate is 10 ℃/min, and the holding time at 1065 ℃ is 60 min.
6. The method for preparing the fracture toughness test sample of the brazing joint as recited in claim 1, wherein the heating process in the vacuum brazing furnace comprises the following steps: (1) in the vacuum pumping stage, the vacuum degree is less than 0.008 Pa; (2) heating to 850 deg.C at a heating rate of 10 deg.C/min; (3) keeping the temperature at 850 ℃ for 30 min; (4) heating to 1065 deg.C at 10 deg.C/min; (5) keeping the temperature at 1065 ℃ for 60min to ensure that the amorphous foil brazing filler metal is fully diffused and reacted; (6) and cooling the vacuum brazing furnace to room temperature.
7. A method for calculating a fracture toughness test sample of a brazed joint is characterized by comprising the following steps:
preparing a brazed joint fracture toughness test sample by adopting the preparation method of the brazed joint fracture toughness test sample according to any one of claims 1 to 6;
a step of processing a brazing joint compact tensile sample, wherein the brazing sample is processed into a Compact Tensile (CT) sample, and a brazing seam and an initial crack are positioned in the middle; specimen thickness B was 12.7mm, width W was 25.4mm, and the ratio of initial crack length to specimen width W was 0.5; pre-restricting fatigue cracks of 1.72-8.08mm by using a high-frequency fatigue testing machine, wherein the fatigue cracks are expanded in an isothermal solidification area;
nano indentation experiment step of diffusion influence area of the soldered joint, selecting the diffusion influence area in the soldered joint as an action point; loading to a maximum load of 30mN/60mN/90mN/120mN/150mN/180mN at a loading rate of 5 mN/s; carrying for 10s to eliminate the creep effect; unloading to 10% of the maximum load at an unloading rate of 5mN/s to eliminate the influence of temperature; carrying out 100S; completely unloading;
calculating the yield strength and the hardening index of the diffusion affected zone of the braze joint, and calculating the yield strength sigma of the diffusion affected zone of the braze jointyAnd a hardening index n;
the method comprises the steps of a resistance curve experiment of the soldered joint by a single sample method, wherein the experiment is carried out in an Instron 8801 hydraulic fatigue testing machine, the test process adopts a loading linear displacement control mode, and the loading/unloading speed is 0.5 mm/min; measuring the crack length delta a and the fracture resistance J integral by using an unloading flexibility method through multiple times of loading/unloading to obtain a J-delta a resistance curve; and
calculation of fracture resistance J0.2BLStep of obtaining a calculated offset passivation line from the yield strength and the hardening index to determine the fracture resistance J0.2BL。
8. The method of calculating the fracture toughness specimen of the braze joint according to claim 7, wherein the yield strength σ of the diffusion affected zone of the braze joint is calculatedyAnd a hardening index n, comprising:
(1) fitting parameters according to an unloading curve in a load p-displacement h curve in the unloading process, wherein the fitting range is 20% of the upper part of the unloading curve;
P＝α(h-hf)m(formula 1)
Wherein: alpha, hfAnd m is a fitting parameter;
(2) the elastic contact stiffness S and the contact area a are calculated,
Figure RE-FDA0003258815780000021
Figure RE-FDA0003258815780000022
Figure RE-FDA0003258815780000023
wherein, PmaxIs the maximum load; h ismaxIs the maximum indentation depth; ε is the indenter shape related constant, ε is 0.75 for the Berkovich indenter; c is a constant of the revised area function, C for Berkovich indenteriA value of about 150 nm;
(3) calculating the modulus of elasticity E and the reduced modulus Er，
Figure RE-FDA0003258815780000031
Figure RE-FDA0003258815780000032
Wherein β is an indenter geometry-related constant, β is 1.034 for the Berkovich indenter; e and v are the modulus of elasticity and Poisson's ratio of the sample material; eiAnd viIs a bullet of indentor materialThe sexual modulus and the Poisson ratio, and the diamond value is 1140GPa and 0.07;
(4) according to the hardness values under different indentation loads, the hardness value H and the yield strength sigma which are irrelevant to the indentation depth are calculatedy，
Figure RE-FDA0003258815780000033
H0＝4.15σy(formula 8)
(5) Calculating the plastic strain epsilonpStress σ at 0.0330.033，
Figure RE-FDA0003258815780000034
Wherein the C value is the loading phase P ═ Ch2Fitting the obtained loading curvature;
(6) the hardening index n is calculated and the hardening index,
Figure RE-FDA0003258815780000035
9. the method for calculating the fracture toughness specimen of the brazed joint according to claim 7, wherein a J- Δ a resistance curve is obtained according to an unloading compliance method.
10. The method of calculating the braze joint fracture toughness specimen of claim 7, wherein the calculated fracture resistance J0.2BLThe method specifically comprises the following steps:
judging the effectiveness of the J-delta a resistance curve;
the passivation line equation is obtained from the yield strength and the hardening index:
Figure RE-FDA0003258815780000036
Figure RE-FDA0003258815780000041
Dn＝0.787+1.554n-2.45n2+16.952n3-38.206n4+33.13n5(formula 13)
Obtaining fracture resistance J according to the intersection point of the 0.2mm offset passivation line and the J-delta a resistance curve0.2BL。
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