CN115255714B

CN115255714B - Strong magnetic particle reinforced flux-cored solder and preparation method thereof

Info

Publication number: CN115255714B
Application number: CN202211007041.4A
Authority: CN
Inventors: 钟素娟; 龙伟民; 裴夤崟; 黄俊兰; 张冠星; 路全彬; 董媛媛; 周许升
Original assignee: Henan University of Science and Technology; Zhengzhou Research Institute of Mechanical Engineering Co Ltd
Current assignee: Henan University of Science and Technology; Zhengzhou Research Institute of Mechanical Engineering Co Ltd
Priority date: 2022-08-22
Filing date: 2022-08-22
Publication date: 2023-04-28
Anticipated expiration: 2042-08-22
Also published as: CN115255714A

Abstract

The invention provides a ferromagnetic particle reinforced flux-cored solder and a preparation method thereof, and relates to the technical field of brazing. Specifically, the flux-cored solder comprises a solder skin and an inner core, wherein the solder skin wraps the inner core; wherein the inner core is mixed powder comprising neodymium iron boron and soldering flux, and the surface of the neodymium iron boron particles is uniformly coated with the soldering flux; optionally, a plurality of scale rings are arranged on the brazing filler metal skin at equal intervals. The invention solves the technical defect that the flux core in the flux core solder is discontinuous, the feeding amount of the solder can not be accurately controlled and quantified, and the joint strength is poor finally; the strong magnetism of the neodymium iron boron is utilized, the flux is coated outside the neodymium iron boron to form magnetic microspheres, and the magnetic microspheres are mutually adsorbed and connected into a flux linkage by virtue of magnetic force between the magnetic microspheres, so that the discontinuous phenomenon of the flux is avoided; meanwhile, strong magnetic particles are instantaneously demagnetized and dispersed in the joint in the high-temperature welding process, so that the high-temperature welding process plays a role in dispersion strengthening, is beneficial to enhancing the strength of the joint, and has good application prospect.

Description

Strong magnetic particle reinforced flux-cored solder and preparation method thereof

Technical Field

The invention relates to the technical field of brazing, in particular to a ferromagnetic particle reinforced flux-cored solder and a preparation method thereof.

Background

The flux-cored brazing filler metal is a novel composite brazing material with brazing filler metal and soldering flux combined into a whole, has the characteristics of simple brazing process, easiness in operation, no waste of soldering flux, controllable dosage and the like, thereby reducing the waste and pollution of the soldering flux and meeting the requirements of efficient, high-quality and clean production. However, in the preparation process of flux cored solder, the plasticity and fluidity of the powder flux as the flux core are poor, and cannot be matched with the plasticity of the external metal coating; therefore, the discontinuous flux core of the flux core solder is very easy to be caused in the drawing and reducing process.

The discontinuous flux core of the internal soldering flux, namely, a plurality of parts in the solder have no soldering flux or have small soldering flux, so that the soldering flux is not enough to remove the film in the soldering process, and the solder has poor joint filling property or cannot be welded at all, and finally, the problem of poor joint strength is caused.

In view of this, the present invention has been made.

Disclosure of Invention

The first object of the present invention is to provide a ferromagnetic particle reinforced flux-cored solder, which solves many of the shortcomings of the existing flux-cored solders; the flux-cored brazing filler metal comprises the inner layer of neodymium iron boron ferromagnetic particles and brazing flux, the outer layer is a brazing filler metal skin, and the defects of discontinuous brazing flux powder and low joint strength in the conventional flux-cored brazing filler metal can be overcome by utilizing the magnetic properties of neodymium iron boron; the scale ring is optionally arranged, so that a worker can accurately control the solder consumption of a single welding spot, and the quality consistency of the welding spot is improved.

The second aim of the invention is to provide a preparation method of the ferromagnetic particle reinforced flux-cored solder.

In order to achieve the above object of the present invention, the following technical solutions are specifically adopted:

a ferromagnetic particle reinforced flux cored solder comprising a solder skin and an inner core, the solder skin surrounding the inner core; the inner core is mixed powder comprising neodymium iron boron and soldering flux, and the surface of the neodymium iron boron particles is uniformly coated with the soldering flux.

The neodymium-iron-boron ferromagnetic particles have strong remanence and isotropy, and the neodymium-iron-boron particles are adsorbed with each other; according to the invention, a brazing flux layer is uniformly coated outside the neodymium iron boron particles to form magnetic microspheres, the magnetic microspheres are mutually adsorbed and connected into a flux linkage in the metal brazing filler metal by virtue of magnetic force between the magnetic microspheres, and gaps between the magnetic microspheres are filled with the brazing flux filled in the other part of the brazing flux layer, so that the discontinuous phenomenon of the brazing flux is avoided; in addition, strong magnetic particles are instantaneously demagnetized and dispersed in the joint in the high-temperature welding process, so that the dispersion strengthening effect is achieved, and the strength of the joint is enhanced.

Preferably, the solder includes at least one of silver solder or copper solder; more preferably, the silver solder comprises BAg50CuZnNi and the copper solder comprises BCu60ZnSn.

The specific kind of the flux is not limited in the present invention; according to the type of the adopted brazing filler metal, based on the brazing flux existing in the prior art, any brazing flux which is suitable for the brazing filler metal is correspondingly selected to be used as the brazing flux of the invention; preferably, the flux corresponding to the silver solder includes a silver flux QJ308, and the flux corresponding to the copper solder includes a copper flux QJ301.

Preferably, in the flux-cored solder, the mass content of the mixed powder is 20-30%, and the mass content of the solder is 70-80%;

preferably, in the mixed powder, the mass content of the neodymium iron boron is 8% -12%, and the mass content of the neodymium iron boron is 88% -92%.

Preferably, the ratio of the grain size of the brazing flux to the grain size of the neodymium iron boron is 1/3-2/3; when the particle size ratio is based, the particles of the brazing flux are small enough and are not easy to agglomerate, and can be uniformly adhered to the outer surfaces of the particles of the neodymium iron boron, so that the magnetic neodymium iron boron microsphere with a coating structure is formed. If the ratio of the grain sizes is less than 1/3 (namely, the grain size of the brazing flux is too small), the surface energy of the brazing flux is large, and the brazing flux is easy to agglomerate and cannot form uniform coating; if the ratio of the grain sizes exceeds 2/3 (namely, the grain size of the brazing flux is too large), the surface energy of the brazing flux is too small, the adsorption capacity is poor, and the brazing flux is not easy to adsorb and adhere to the surfaces of the NdFeB grains;

more preferably, the particle size of the neodymium iron boron is 200-500 μm;

more preferably, the particle size of the flux is 120 μm to 180 μm.

The inventors found in the practice of production that: the feeding amount of the brazing filler metal in manual welding of the flux-cored brazing filler metal by workers cannot be accurately controlled and quantified, so that the material amount of each welding point is quite different, if the welding point is lack of material, the material amount of the welding point is redundant, the quality consistency of the welding point is poor, and even the brazing filler metal is wasted; therefore, as a preferable technical scheme, a plurality of scale rings with equal intervals can be arranged on the brazing filler metal skin; the interval of the scale rings outside the solder can be set according to the solder consumption of the solder joints, so that a user is helped to control the consistency of the solder consumption of each solder joint, and the solder waste and the instability of solder joint quality are avoided;

preferably, the pitch of the scale ring satisfies the following relation:

h＝m/(πr ² ρ); h is the distance (cm) between the scale rings, m is the mass (g) of the flux-cored solder for a single welding point, r is the radius (cm) of the flux-cored solder, and ρ is the average density (g/cm) of the flux-cored solder ³ )；

More preferably, the diameter of the flux-cored solder is 1 mm-3 mm;

more preferably, the interval of the scale rings is 2 mm-10 mm.

The preparation method of the ferromagnetic particle reinforced flux-cored solder comprises the following steps:

(1) Fully mixing the adhesive and part of the brazing flux to paste, adding neodymium iron boron, fully mixing until all the surfaces of the neodymium iron boron particles are coated with the brazing flux paste, and then carrying out heat treatment to obtain prefabricated dry powder; adding the rest brazing flux into the prefabricated dry powder, and fully and uniformly mixing to obtain mixed dry powder;

(2) Preparing solder metal foil from solder;

(3) And placing the mixed dry powder and the brazing filler metal foil into a flux-cored wire forming machine, starting the flux-cored wire forming machine, and performing seaming, rolling and reducing to obtain the flux-cored brazing filler metal.

Preferably, the binder is an ethanol solution comprising hydrogenated rosin; more preferably, the mass content of the hydrogenated rosin is 5-10%.

Preferably, the mass ratio of the partial brazing flux to the brazing flux is 40% -60%.

Preferably, the heat treatment comprises a hot air treatment; more preferably, the time of the hot air treatment is 10 min-15 min; more preferably, the hot air treatment is performed with shaking, vibration, or the like, so that the heated air is more uniformly heated.

Preferably, before step (1), the method further comprises: the steps of calculating and weighing are carried out according to the preset mass content of each component, and the dosage of each component in the preparation method is strictly implemented according to the weighing amount and cannot be used or increased; for example: when mixing the neodymium iron boron particles and the soldering flux paste, the coating can be carried out in batches for multiple times, but the paste coating and the particles are ensured to be just used up.

Preferably, in step (2), the preparation process of the solder metal foil further includes: in the rolling process of the brazing filler metal foil, marking equidistant scale marks on the surface of the brazing filler metal foil by a marking knife; optionally, enabling the solder metal foil to continuously pass through a special scribing cutter with a certain depth in a rolling process, sliding the scribing cutter back and forth along the solder foil, and scribing equidistant scale marks on the surface of the metal foil;

more preferably, the depth of the graduation marks is 1/10 to 3/10 of the thickness of the brazing filler metal foil;

more preferably, YG8 hard particles are uniformly distributed on the cutter body of the dividing cutter.

Compared with the prior art, the invention has the beneficial effects that:

(1) According to the invention, a layer of soldering flux is coated outside the neodymium iron boron strong magnetic particles to form magnetic microspheres, and magnetic flux is mutually adsorbed and connected into a flux linkage in the metal solder by virtue of magnetic force among the magnetic microspheres, so that continuity of the soldering flux in the flux-cored solder is ensured, discontinuous phenomenon of soldering flux powder in the flux-cored solder is avoided, and meanwhile, a part of soldering flux powder is further filled in gaps of the strong magnetic particles coated with the soldering flux, so that the soldering flux content is ensured, and quantitative addition of the soldering flux is realized conveniently; meanwhile, the neodymium iron boron ferromagnetic particles in the invention are demagnetized instantaneously at high temperature and dispersed in the brazing seam, thereby being beneficial to enhancing the strength of the brazing seam.

(2) The scale ring outside the flux-cored solder is beneficial to workers to accurately regulate and control the consumption of the solder at the welding point, and avoids the waste of the solder and the instability of the quality of the solder joint.

Drawings

In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings that are needed in the description of the embodiments or the prior art will be briefly described, and it is obvious that the drawings in the description below are some embodiments of the present invention, and other drawings can be obtained according to the drawings without inventive effort for a person skilled in the art.

FIG. 1 is a schematic structural view of a flux cored solder of the invention;

FIG. 2 is a schematic diagram of the core microstructure of the flux cored solder of the invention;

FIG. 3 is a schematic illustration of a process of scoring a scale ring on the surface of a solder foil in accordance with the present invention; fig. 3 (a) is a front view, and fig. 3 (b) is a top view;

fig. 4 is a physical diagram of the flux-cored silver solder of example 1 of the present invention; fig. 4 (a) is an external view, and fig. 4 (b) is a sectional view;

FIG. 5 is a diagram of a welded article of example 1 and comparative example of the present invention; fig. 5 (a) is a physical diagram of example 1, and fig. 5 (b) is a physical diagram of comparative example.

Reference numerals:

1-NdFeB; 2-soldering flux;

3-brazing filler metal skin; 4-a scale ring;

5-a scoring blade matrix; 6-hard alloy particles.

Detailed Description

The technical solution of the present invention will be clearly and completely described below with reference to the accompanying drawings and detailed description, but it will be understood by those skilled in the art that the examples described below are some, but not all, examples of the present invention, and are intended to be illustrative of the present invention only and should not be construed as limiting the scope of the present invention. All other embodiments, which can be made by those skilled in the art based on the embodiments of the invention without making any inventive effort, are intended to be within the scope of the invention. The specific conditions are not noted in the examples and are carried out according to conventional conditions or conditions recommended by the manufacturer. The reagents or apparatus used were conventional products commercially available without the manufacturer's attention.

In the description of the present invention, it should be noted that the directions or positional relationships indicated by the terms "inner", "outer", "inner core", "outer", etc. are based on the directions or positional relationships shown in the drawings, are merely for convenience of describing the present invention and simplifying the description, and do not indicate or imply that the elements referred to must have a specific direction, be constructed and operated in a specific direction, and thus should not be construed as limiting the present invention. Furthermore, the terms "(1)", "(2)", "step one", "step two", and the like are used for descriptive purposes only and are not to be construed as indicating or implying relative importance.

The invention is implemented by the following steps: a ferromagnetic particle reinforced flux-cored solder, the flux-cored solder comprising a solder skin 3 and an inner core, the solder skin 3 wrapping the inner core; the inner core is mixed powder comprising neodymium iron boron 1 and brazing flux 2, and the brazing flux 2 is uniformly coated on the particle surfaces of the neodymium iron boron 1. Fig. 1 is a schematic structural view of the flux cored solder of the present invention.

As a preferred embodiment, in the inner core, the particle surface of each neodymium iron boron 1 is uniformly coated with the brazing flux 2; namely, the positional relationship between each particle of the neodymium iron boron 1 and the brazing flux 2 is shown in fig. 2, and the particles of the neodymium iron boron 1 do not have agglomeration phenomenon except for a very small number of individual phenomena which are unavoidable due to the preparation process.

As a preferred embodiment, in the flux cored solder, the mass content of the mixed powder includes, but is not limited to, 20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, 28%, 29%, 30%; as a preferred embodiment, the mass content of the neodymium iron boron 1 in the mixed powder includes, but is not limited to, 8%, 9%, 10%, 11%, 12%.

As a preferred embodiment, the diameter of the flux cored solder includes, but is not limited to, 1mm, 1.5mm, 2mm, 2.5mm, 3mm; as a preferred embodiment, the particle size of the NdFeB 1 comprises, but is not limited to, 200 μm, 250 μm, 300 μm, 350 μm, 400 μm, 450 μm, 500 μm; as a preferred embodiment, the grain size of the flux 2 includes, but is not limited to, 120 μm, 130 μm, 140 μm, 150 μm, 160 μm, 170 μm, 180 μm.

As a preferred embodiment, a plurality of scale rings 4 are arranged on the brazing filler metal skin at equal intervals; the spacing of the scale ring 4 includes, but is not limited to, 2mm, 3mm, 4mm, 5mm, 6mm, 7mm, 8mm, 9mm, 10mm.

Example 1

Step one, calculating and weighing dry powder (1840 g) of the neodymium iron boron 1 magnetic particles (160 g in mass) and the QJ308 brazing flux 2 according to the mass percentage (20%) of the mixed powder in the brazing filler metal and the mass percentage (8%) of the strong magnetic particles of the neodymium iron boron 1 in the mixed powder; wherein, the particle size of the NdFeB 1 used in the embodiment is 200-300 μm, the particle size of the brazing flux 2 is 120-150 μm (the particle size is distributed in the above interval based on the limitation of the particle preparation technology);

step two, mixing half brazing flux powder with a binder (ethanol solution containing 5% of hydrogenated rosin ester) to prepare a paste, then putting all the paste into neodymium iron boron 1 ferromagnetic particles, and uniformly stirring to uniformly coat a brazing flux solution on the surface of each particle;

step three, the ferromagnetic particles in the step two are dried for 10 minutes by hot air of a blower while shaking, and then are uniformly mixed with the dry powder of the residual brazing flux 2 to form mixed powder for standby;

preparing a solder metal foil by adopting a smelting, extruding and rolling method in sequence, wherein the solder adopts silver solder (BAg 50 CuZnNi); the solder metal foil is interrupted in the rolling process and passes through a special scribing cutter with a certain depth, the scribing cutter slides back and forth along the solder foil, and scale marks with equal intervals (10 mm) are scribed on the surface of the metal foil; YG8 hard alloy particles 6 are uniformly distributed on the scoring blade matrix 5, and the depth of the hard alloy particles 6 pressed into the brazing filler metal foil is 1/10 of the thickness of the brazing filler metal foil; fig. 3 shows a schematic view of the process of scribing the scale ring 4 in this step, wherein fig. 3 (a) is a front view and fig. 3 (b) is a top view;

and fifthly, placing the metal foil in the fourth step and the mixed powder in the third step on a flux-cored wire forming machine, and finally obtaining the ferromagnetic particle reinforced flux-cored silver solder with the wire diameter of 1.0mm and the scale ring 4 through seaming, rolling and reducing.

Fig. 4 shows a physical diagram of the flux-cored silver solder of this example; fig. 4 (a) is an external view, and fig. 4 (b) is a sectional view.

Example 2

Substantially the same as in example 1, the only difference is that:

step one: the mass percentage of the mixed powder in the flux-cored brazing filler metal is 25%, the mass percentage of the strong magnetic particles of the neodymium iron boron 1 in the mixed powder is 10%, the particle size of the neodymium iron boron 1 is 300-400 mu m, and the particle size of the brazing flux 2 is 150-160 mu m;

step two: the binder is ethanol solution containing 6% of hydrogenated rosin ester;

step three: drying for 12min;

step four: the spacing between the graduation marks is 8mm, and the depth of hard particles pressed into the brazing filler metal foil is 2/10 of the thickness of the brazing filler metal foil;

step five: finally obtaining the ferromagnetic particle reinforced flux-cored silver solder with the wire diameter of 2.0mm and the scale ring 4.

Example 3

Substantially the same as in example 1, the only difference is that:

step one: the mass percentage of the mixed powder in the flux-cored brazing filler metal is 30%, the mass percentage of the strong magnetic particles of the neodymium iron boron 1 in the mixed powder is 12%, the particle size of the neodymium iron boron 1 is 400-500 mu m, and the particle size of the brazing flux 2 is 160-180 mu m;

step two: the binder is ethanol solution containing 8% of hydrogenated rosin ester;

step three: drying time is 13min;

step four: the spacing between the graduation lines is 6mm, and the depth of hard particles pressed into the brazing filler metal foil is 3/10 of the thickness of the brazing filler metal foil;

step five: finally obtaining the ferromagnetic particle reinforced flux-cored silver solder with the wire diameter of 2.5mm and the scale ring 4.

Example 4

Substantially the same as in example 1, the only difference is that:

step one: the mass percentage of the neodymium iron boron 1 ferromagnetic particles in the mixed powder is 18%;

step two: the binder is ethanol solution containing 9% of hydrogenated rosin ester;

step three: blow-drying for 15min;

step four: the interval between the graduation marks is 4mm;

step five: finally obtaining the ferromagnetic particle reinforced flux-cored silver solder with the wire diameter of 2.8mm and the scale ring 4.

Example 5

Substantially the same as in example 1, the only difference is that:

step one: the mass percentage of the mixed powder in the flux-cored brazing filler metal is 30%, the mass percentage of the strong magnetic particles of the neodymium iron boron 1 in the mixed powder is 11%, the particle size of the neodymium iron boron 1 is 300-400 mu m, and the particle size of the brazing flux 2 is 150-160 mu m;

step two: the binder is ethanol solution containing 10% of hydrogenated rosin ester;

step four: the spacing between the graduation lines is 2mm, and the depth of hard particles pressed into the brazing filler metal foil is 3/10 of the thickness of the brazing filler metal foil;

step five: finally obtaining the ferromagnetic particle reinforced flux-cored silver solder with the wire diameter of 3.0mm and the scale ring 4.

Comparative examples 1 to 5

Placing the silver solder metal foil and the soldering flux on a flux-cored wire forming machine respectively, and finally obtaining the conventional flux-cored silver solder with wire diameters of 1.0mm, 2.0mm, 2.5mm, 2.8mm and 3.0mm through seaming, rolling and reducing; wherein, the metal component of the flux-cored silver solder in each comparative example is BAg50CuZnNi, the brazing flux component is QJ308 silver brazing flux, and the brazing flux proportion is 22%.

Test example 1

In order to examine the continuity degree of the inner soldering flux powder in the ferromagnetic particle reinforced type flux-cored silver solder and the traditional flux-cored silver solder, two flux-cored silver solders with the same specification are respectively taken, 3 samples with the length of 5 meters and one section are respectively cut off by sharp scissors (1000 sections in total) at intervals of 5mm, whether the flux-cored silver solders exist in the sections or not is checked, and the powder breakage rate (the percentage of the broken powder sections to the total sections) of the two flux-cored silver solders is compared.

TABLE 1 powder break test results table

Proved by verification, the traditional flux-cored silver solder has the defects of increased powder breaking rate and serious powder breaking phenomenon along with the reduction of the wire diameter, because the thinner the wire diameter, the more processing passes and the smaller the powder diameter, the lower the matching degree of the flux powder and the flow velocity of the external metal skin, and the more easy powder breaking. The special flux-cored silver solder has extremely low powder breakage rate (about 3%) only for flux-cored silver solders with wire diameters of 1.0, and the powder breakage rates of the flux-cored silver solders with other wire diameters are all 0, so that the special flux-cored silver solder has the advantages of being beneficial to improving the continuity of brazing flux powder in the flux-cored silver solder and reducing the powder breakage rate.

Test example 2

In order to examine the brazing performance of the ferromagnetic particle reinforced flux-cored silver brazing filler metal and the traditional flux-cored silver brazing filler metal, lap joint flame brazing tests (lap joint length 5 mm) of 304 stainless steel wires with wire diameters of 2.0mm are respectively carried out in examples 1-5 and comparative examples 1-5, 50 brazing filler metals are welded in each brazing filler metal, the average value is taken, and the maximum breaking force and the brazing seam consistency of each welding spot are compared in a joint tensile test.

TABLE 2 braze joint strength test results

Through verification, the effect is better when the joint solder with the stainless steel wire lap joint of 5mm is about 0.3g, the solder quantity requirement can be met by using 1 unit of scale ring solder quantity in each of examples 1-5, and the feeding length is only measured by naked eyes in the traditional flux-cored silver solder welding process, so that the difference of the solder quantity of the stainless steel wire joint is larger and the consistency is poorer.

FIG. 5 is a diagram of a welded article of example 1 and comparative example of the present invention; fig. 5 (a) is a physical diagram of example 1, and fig. 5 (b) is a physical diagram of comparative example 1. As can be seen from Table 2 and FIG. 5, the solder joint strength of the invention is higher than that of the traditional flux-cored silver solder, and the solder joint quality consistency is good.

While the invention has been illustrated and described with reference to specific embodiments, it is to be understood that the above embodiments are merely illustrative of the technical aspects of the invention and not restrictive thereof; those of ordinary skill in the art will appreciate that: modifications may be made to the technical solutions described in the foregoing embodiments, or equivalents may be substituted for some or all of the technical features thereof, without departing from the spirit and scope of the present invention; such modifications and substitutions do not depart from the spirit of the corresponding technical solutions; it is therefore intended to cover in the appended claims all such alternatives and modifications as fall within the scope of the invention.

Claims

1. A ferromagnetic particle reinforced flux-cored solder, characterized in that the flux-cored solder comprises a solder skin and an inner core, the solder skin wrapping the inner core;

the inner core is mixed powder comprising neodymium iron boron and soldering flux, and the surface of the neodymium iron boron particles is uniformly coated with the soldering flux;

(1) Fully mixing a binder and part of the brazing flux into paste, adding the neodymium iron boron, fully mixing until the surfaces of the particles of the neodymium iron boron are coated with the brazing flux paste, and then carrying out heat treatment to obtain prefabricated dry powder; adding the rest brazing flux into the prefabricated dry powder, and fully and uniformly mixing to obtain mixed dry powder;

(2) Preparing solder metal foil from solder;

(3) And placing the mixed dry powder and the brazing filler metal foil into a flux-cored wire forming machine, and starting the flux-cored wire forming machine to obtain the flux-cored brazing filler metal.

2. The ferromagnetic particle reinforced flux cored solder of claim 1, wherein the flux cored solder comprises at least one of the following features a-c:

a. in the flux-cored solder, the mass content of the mixed powder is 20% -30%;

b. in the mixed powder, the mass content of the neodymium iron boron is 8% -12%;

c. the solder skin comprises at least one of silver solder or copper solder.

3. The ferromagnetic particle reinforced flux-cored solder of claim 1, wherein the ratio of the particle size of the flux to the particle size of the neodymium iron boron is 1/3-2/3.

4. The ferromagnetic particle-reinforced flux-cored solder of claim 1, wherein the solder skin is provided with equally spaced scale rings;

the distance between the scale rings is 2 mm-10 mm.

5. The ferromagnetic particle-reinforced flux-cored solder of claim 1, wherein the binder is an ethanol solution comprising hydrogenated rosin.

6. The ferromagnetic particle reinforced flux-cored solder of claim 1, wherein the mass ratio of the partial flux to the flux is 40% -60%.

7. The ferromagnetic particle-reinforced flux-cored solder of claim 1, wherein the heat treatment comprises a hot air treatment.

8. The ferromagnetic particle-reinforced flux-cored solder of claim 1, wherein in step (2), the solder metal foil is prepared by: in the rolling process of the brazing filler metal foil, equidistant scale marks are marked on the surface of the brazing filler metal foil through a scribing cutter.

9. The ferromagnetic particle reinforced flux-cored solder of claim 8, wherein the graduation marks have a depth of 1/10 to 3/10 of the thickness of the solder metal foil.