JP7726569B2 - Brazing material formed from metal powder and brazing method for manufacturing fin-tube heat exchangers using the same - Google Patents

Description

The present invention relates to a brazing material.
The present invention also relates to a brazing method for manufacturing a fin-tube heat exchanger using the brazing material of the present invention.
A fin-tube heat exchanger is a structure that has a heat sink made of multiple multi-layered fins made of metal or ceramic, with tubes that pass through it, and the fins and tubes are joined by brazing.

Below, (1) a conventional brazing material and (2) a method for manufacturing a fin-tube heat exchanger according to the prior art will be described in order.
(1) Conventional brazing materials Conventional brazing materials are based on metals suitable for brazing, such as copper or copper alloys, aluminum or aluminum alloys, nickel or nickel alloys, and may contain trace elements. Their melting points are 450°C or higher, and when heat is applied, they melt when the temperature reaches the melting point or higher, and solidify when the temperature falls below the melting point. Some brazing materials contain flux as a solvent.
As conventional molded brazing materials, two types are known: a rod-shaped brazing material that is a metal block; and a paste-like brazing material that contains powdered metal and has flowability.

[Metal rod-shaped brazing material]
The first compact, a rod-shaped brazing material, is a metal ingot made by melting and sintering the raw metal. It is often provided as a rod with a rectangular or circular cross-section. The metal is melted and sintered, resulting in a dense, high-density state with a packing ratio (the ratio of apparent density to true density) of 95% to 100%.

(Patent Document 1)
For example, JP 2016-540644 A (Patent Document 1) is known. This Patent Document 1 discloses a molding method for obtaining a brazing material preform by pressure molding a metal powder. The document discloses the composition of the metal powder of the brazing preform that is the subject of the document, the addition of an organic binder, the particle size distribution, and the density corresponding to the packing rate, and a person skilled in the art will understand the molding method and the resulting brazing preform suggested by the scope of the disclosure.
However, Patent Document 1 uses powder press molding, a so-called compression molding method using a mold, and the molded product is required to be uniform and high-density from the standpoints of preventing breakage during molding, high strength, and high quality. Therefore, the brazing powder is subjected to a freeze-aggregation process or the like to form large-grained spherical agglomerates, which are then uniformly filled into a mold cavity and pressed under a high molding pressure of more than 300 MPa to 1000 MPa to obtain a uniform, high-density product. However, the high compression molding pressure causes deformation, destruction, and pulverization of the powder itself, as well as the accumulation of residual stress.

(Patent Document 2)
Also, for example, Japanese Patent Laid-Open No. 07-308794 (Patent Document 2) is known. This Patent Document 2 aims to melt at a low temperature in a short time, and discloses a brazing material formed by using a rolling method involving hot pressing using metal powders of Al and Cu.
Patent Document 2 attempts to lower the liquidus temperature by mixing different metal powders and forming a pressure-molded body without using an organic binder. However, Patent Document 2 describes the method of forming the brazing material, which includes pressurizing powder using a hot press or the like (paragraph 0019), using a pressure rolling method (paragraph 0020), using a conform extrusion method (paragraph 0021), being in a pressurized solidified state (paragraph 0022), and using a hot pressing method (paragraph 0028). Furthermore, as is clear from the fact that no organic binder is used, what is suggested to those skilled in the art is the same processing method as in Patent Document 1 above, that is, powder press molding under strong pressure that sufficiently deforms the Al and Cu metal powders, that is, a molding method using a compression molding method.
That is, the method of forming the brazing material disclosed in Patent Document 2 is similar to Patent Document 1 in that it is formed by applying a high forming pressure, such as a (cold or hot) forming press, which melts and solidifies the metal particles while deforming them.

[Paste-type brazing material]
Next, paste-type brazing materials contain the metal that will be used as the brazing base material in the form of metal powder, which is then mixed with a binder to create a highly viscous, fluid paste. Silver brazing paste containing flux is widely used. Because it is a fluid paste, it can be squeezed out in a fixed amount using a dispenser, making it widely used for brazing. It is also convenient for applying to flat substrates using screen printing.

In addition, brazing filler metals such as nickel alloys, which have been attracting attention in recent years, are hard and difficult to process, making wire drawing and rolling difficult. As a result, a technology has become widespread in which the material is powdered using an atomization method, mixed with a binder, and used as a paste. However, the melting properties are still poor, and this has not yet been put to practical use.

This paste-like brazing material is relatively easy to handle, as it can be simply squeezed out of a dispenser and applied to the area to be brazed, but it also has many disadvantages.
For example, it is difficult to apply paste brazing material directly from the dispenser outlet to the inside of the equipment or to hidden areas where the brazing area is not exposed. Therefore, the only option is to apply it to the area near the brazing area, apply heat to melt the brazing material, and then use capillary action to transport it to the inside of the equipment or the hidden brazing area. As a result, there are disadvantages such as residual brazing material adhering to the traces of the material, which can mar the appearance and result in loss of brazing material.

[Other brazing materials using metal powder]
Next, other brazing materials using metal powders in the prior art will be described.
FIG. 11 is a diagram showing a conventional technique disclosed in Japanese Patent Laid-Open No. 55-92288 (Patent Document 3).
As shown in Figure 11, the technology disclosed in Japanese Patent Laid-Open No. 55-92288 (Patent Document 3) involves brazing a hole-shaped portion of a part to be brazed, such as a camshaft in an automobile, by fitting a metal tube into the hole and brazing it to form an oil supply path. During brazing, a metal powder is filled into the metal tube as a brazing material. A slit is formed in the metal tube, and the brazing material inside is dissolved and supplied to the outer surface through the slit, thereby brazing the metal tube itself. In other words, the metal tube shown in Patent Document 3 is not a jig (tool), but the actual component to be brazed.
Special table 2016-540644 publication Japanese Patent Application Publication No. 07-308794 Japanese Unexamined Patent Publication No. 55-092288 "Powder and Powder Metallurgy" Vol. 13, No. 3 "Compression Molding of Powder"

However, the above-mentioned conventional techniques have the following problems.
Patent Document 1 uses powder press molding, a so-called compression molding method using a mold, which involves high-temperature and high-pressure processing, and press processing is performed at a high molding pressure of more than 300 MPa to 1000 MPa.
The brazing material to be produced in Patent Document 1 is intended to be uniform and high-density from the viewpoints of preventing damage to the molded product during molding, and achieving high strength and high quality. Therefore, it is disclosed that brazing powder is subjected to a freeze-aggregation process or the like to form large-grained spherical agglomerates, which are then uniformly filled into a mold cavity and then pressed at a high molding pressure of more than 300 MPa to 1000 MPa to be processed into a uniform and high-density product.
However, due to the high compression molding pressure, the metal powder used as the raw material does not remain in powder form, but is deformed, broken, and refined, and residual stress accumulates. In other words, the metal mass aggregates under high temperature and pressure, and becomes a sintered body.

The content disclosed in Patent Document 2 is that the powder press molding, or so-called compression molding using a mold, of Patent Document 1 is used in a similar processing method. In other words, it involves high-temperature, high-pressure processing, and although Patent Document 2 does not disclose a numerical range of pressure, it would suggest to a person skilled in the art that the press processing is performed in the same pressure range as Patent Document 1, that is, at a high molding pressure of more than 300 MPa to 1000 MPa.
As a result, when using the technology of Patent Document 2, a high compression molding pressure is required, and the metal powder used as the raw material does not remain in a powder state, but is subjected to deformation, destruction, and pulverization from the powder, as well as the accumulation of residual stress. In other words, the metal mass aggregates under high temperature and pressure, and becomes a sintered body, just like the technology of Patent Document 1.

In other words, brazing materials formed using the technology of Patent Document 1 or Patent Document 2 have a major disadvantage: reduced heat input efficiency during the brazing process. As mentioned above, during the forming process, high temperature and pressure are applied to the metal powder material in a mold, and it is sintered using a compression molding method. This results in the internal structure of the brazing material after forming becoming a dense metal structure. As a result, the brazing heat input during the brazing process is only input directly to the outer surface, and heat transfer from the outer surface to the metal inside occurs via thermal conduction, so the heat input efficiency cannot be said to be good. As a result, the specific surface area for heat input is not large, which has the disadvantage of reduced thermal efficiency.

Next, the brazing material technology disclosed in Patent Document 3 has a problem.
The first problem is that it is not a technology that can be used generally. The metal tube disclosed in Patent Document 3 is merely the part itself that is incorporated into the cam, and is not like a jig or tool that is removed by pulling out after brazing; it is the brazed part itself that is left in the cam as is. For those skilled in the art who read Patent Document 3, this technology can only be applied when the part to be brazed happens to have a cavity inside so that a slit can be formed, and the part can simply be fitted and left in place. There is no disclosure or suggestion whatsoever about general-purpose uses or applicability.
The second problem is that it is difficult to ensure the amount of brazing filler metal used. In other words, the slit width cannot be made too large in order to keep the metal powder inside, so the brazing filler metal that dissolves due to heat input remains inside the metal tube due to surface tension and capillary action.
The third problem is that the brazing process cannot be performed uniformly depending on the direction of the slits. As described above, even when heat is input and the brazing process is performed, the metal tube remains and the brazing material leaks only from the slits. Therefore, the brazing material does not leak in all directions, and the brazing process cannot be performed uniformly depending on the direction of the slits.

Next, the paste-type brazing material used in the prior art also had a problem with applicability. That is, the paste-type brazing material can be applied without any problems to areas that are easily accessible from the outside, but it is difficult to apply to areas that are difficult to access.
When the brazing point is inside an equipment structure or when the accessible space is narrow, access is difficult with a highly viscous paste brazing material. In such cases, the brazing process must be performed by applying a large amount of paste brazing material near the brazing point, applying heat, and then conducting and flowing the melted brazing material into the brazing point.
Furthermore, when using a paste-like brazing material, after the brazing material has spread to the brazing point, a process of evaporating and drying the binder, etc. is required, which is time-consuming and costly.

In view of the above problems, the brazing material of the present invention, which is formed from metal powder, aims to improve the heat input efficiency during brazing treatment and to improve applicability to places that are difficult to access.
The brazing material of the present invention, which is formed from metal powder, aims to simultaneously solve two problems that at first glance seem difficult to achieve.
The first challenge, improving the heat input efficiency during brazing, is achieved by using an extrusion molding method, but in a manner completely different from Patent Documents 1 and 2, in which the extrusion pressure is extremely low, within a range that does not cause physical changes such as aggregation of the metal powder or stress deformation, while producing a brazing material molded body with the desired particle size distribution and porosity, thereby making heat transfer more efficient and improving melting properties, etc.
Furthermore, in order to improve applicability to difficult-to-access locations, which is the second issue, paste properties that are highly fluid and unable to maintain their shape, such as paste-like brazing materials, are not desirable, and a material with rigidity that allows it to maintain a certain shape is required.
In other words, the brazing material is not a dense metal mass like a sintered one, but is made by molding metal powder in a state where the metal powder has solidified as powder, that is, in a state where the metal powder is loosely bonded with many gaps (voids) inside.

In order to achieve the above object, the brazing material of the present invention is a brazing material that uses a metal powder obtained by powdering a metal to be used as a brazing material as a raw material, and extrudes the powder at an extrusion pressure equal to or less than the yield stress to form a molded body of a predetermined shape.
In other words, the metal powder is extruded at an extrusion pressure below the yield stress, and is therefore compressed into a predetermined shape while remaining in the form of metal powder, without ever being crushed or melted.
With the above-mentioned structure, the inside of the brazing material is made of metal powder with many gaps, so the heat input area during brazing is extremely large. In other words, in the case of a brazing material made of a metal block in the prior art, the heat input area is limited to the outer surface of the metal block, but in the brazing material made of molded metal powder of the present invention, the heat input area is the sum of the outer surfaces of the individual metal powders, so the specific surface area is extremely large and the heat input efficiency is greatly improved.

It's easier to understand if we use another physical phenomenon as an analogy. For example, if we compare it to the process of sucrose crystals being immersed in hot water, heated, and melted, the brazing material of the prior art metal block is immersed and heated as a single crystal, like rock candy, and gradually melts from its outer surface. However, the brazing material formed from the metal powder of the present invention is a molded block of powdered sucrose with an appropriate particle size, like a sugar cube, and can melt instantly when immersed in hot water and heated. This is because the area available for heat input is the sum of the outer surfaces of each powder.

Here, the extrusion pressure range below the yield stress is preferably 2 to 18 MPa.
In the compression molding of Patent Documents 1 and 2, the molding pressure range of the prior art is 300 to 1000 MPa, which is clearly a pressure much higher than the yield stress, and such high pressure would cause the metal powder to deform, aggregate, and be crushed into a metal mass. However, the extrusion pressure of the present invention (2 to 18 MPa) is clearly a pressing force much lower than the yield stress, so the metal powder does not deform and aggregate, and remains lightly solidified.

The inventors produced a prototype and observed the internal structure, and found that it was formed with many gaps inside.
When the density of the brazing material formed within these pressure ranges was examined, it was found that the material formed with a density of about 4.0 to 6.0 was good.
More preferably, the density after extrusion is 4.9 to 5.4.
It was found that within these ranges, the raw metal powder remains in a metal powder state as a brazing material formed from metal powder, while maintaining a constant external shape, such as a rod shape, as a brazing material, in a good condition.

The composition of the brazing material obtained by molding the metal powder of the present invention may contain an organic binder in addition to the metal powder.
Although it would be ideal to use no binder, the use of an organic binder improves the molding state and also makes it possible to reduce the metal oxide that is the metal powder.

Here, in the brazing material of the present invention formed from metal powder, it is preferable that the particle size range of the metal powder is 20 to 75 μm, accounting for 90% or more, and that the packing rate of the metal powder (the ratio of apparent density to true density) is in the range of 50% to 80%.
The particle size range of the metal powder can be set as above because a certain particle size allows for fast melting and a good molten state. Also, the filling rate of the metal powder can be set as above because if it is too low, it becomes difficult to maintain the molded state, and if it is too high, the improvement in heat input efficiency, which is the technical effect of the present invention, is reduced.

Next, in the brazing material obtained by molding the metal powder of the present invention, the metal can be any one of nickel alloy, copper, and aluminum, or a combination thereof. These have excellent physical properties as base materials for brazing materials, and since there is accumulated knowledge about them, they are easy to handle.
In the case of nickel alloys, chromium, silicon, phosphorus, and boron may be included in the composition of nickel.

In the brazing material obtained by molding metal powder according to the present invention, the molded body may be any one of a molded body by extrusion molding, a molded body by vacuum extrusion, a molded body by compression molding, or a combination thereof. In addition, a molding method in which the metal powder is placed in a mold and dried may also be used.
The shape of the molded body is not limited, but it can be molded into, for example, a rod shape, a ring shape, or a sheet shape.

Next, the brazing method for manufacturing a fin-tube heat exchanger using the brazing material formed from the metal powder of the present invention can be as follows.
First, the fin tube heat exchanger is assumed to include a fin multi-layer structure in which a number of fins are stacked and arranged at predetermined intervals, and tubes inserted into through holes that are drilled through the fin multi-layer structure and formed to run continuously through it, and the brazing points are assumed to be configured as contact points between the inner peripheries of the through holes in the fin multi-layer structure and the outer peripheries of the tubes.
In the above configuration, a groove is provided by cutting out a portion of the through hole toward the outer periphery, and a brazing material formed from metal powder shaped into a rod is inserted through the groove, and a predetermined heat is input to the brazing material to perform the brazing process.

The brazing material of the present invention has a high heat input efficiency, which solves the problem of low heat input efficiency that was an issue with conventional processing methods, shortens the heat input processing time in the brazing process, and allows the brazing process to be completed quickly.
In other words, the brazing method of the present invention requires only a short brazing process time, and therefore the metal powder in the brazing process can be melted well in a short time even when radiant heat is used from the surroundings, resulting in uniform and good wetting during the brazing process.

FIG. 1 is a diagram simply showing an example of the composition of a brazing material formed from metal powder according to Example 1 of the present invention. 1 is a photograph showing a brazing material obtained by molding a metal powder produced by vacuum extrusion molding. 10 is a photograph showing a brazing material of a metal block made by another company and having the same composition. FIG. 10 is a diagram showing density depending on the amount of added element. FIG. 1 is a diagram showing the results of differential thermogravimetric analysis (time changes of DTA (uV) and TG (wt%)) of a brazing material formed from metal powder according to the present invention and a brazing material made from a metal block according to the prior art. FIG. 1 is a graph showing the relationship between extrusion pressure and density. 1 is a diagram simply showing the configuration of a fin-tube heat exchanger 100 that is brazed using a brazing material formed from the metal powder of the present invention. FIG. 1 is a diagram (part 1) showing the procedure of a brazing method between the fins 110 and the tubes 120 in the fin-tube heat exchanger 100 of the second embodiment. FIG. 2 is a diagram (part 2) showing the procedure of the brazing method between the fins 110 and the tubes 120 in the fin-tube heat exchanger 100 of the second embodiment. FIG. 3 is a diagram showing the procedure of the brazing method between the fins 110 and the tubes 120 in the fin-tube heat exchanger 100 of the second embodiment (part 3). FIG. 1 is a diagram showing the prior art disclosed in Japanese Patent Laid-Open Publication No. 55-92288.

Below, examples of the brazing material of the present invention will be described with reference to the drawings. Note that the present invention is not limited to these examples.

A brazing material 100 according to a first embodiment of the present invention will be described. As an example, the brazing material 100 will be described, focusing on a material that uses a nickel alloy as a base material and contains a small amount of binder.
FIG. 1 is a diagram simply showing an example of the composition of a brazing material formed from metal powder according to Example 1 of the present invention.
In the examples shown in FIG. 1, the base material is a nickel alloy, but the composition of the brazing material formed from the metal powder of the present invention can be varied, and the composition shown in FIG. 1 is merely one example.
In the example shown in FIG. 1, the compounding ratio after drying is 97.6% nickel brazing metal powder and 2.4% binder.
There are various other metals that can be considered other than nickel alloys, such as copper alloys. Copper alloys are widely used in conventional technology, but the present invention can also be applied to Cu-Mn-Ni copper alloys and Cu-Sn-Ti copper alloys, which are considered to be difficult to process.
Other metal powders such as gold, silver, tin, aluminum, lead, phosphorus, chromium, tungsten, molybdenum, titanium, platinum, palladium, zinc, indium, molybdenum, and manganese, as well as combinations thereof, can also be used.

The prototype is shown below.
The prototype was manufactured using nickel brazing filler with the No. 1 formulation listed in Figure 1.
The nickel brazing material, nickel, was powdered by atomization or other methods to form a metallic powder of nickel brazing material. The powder was not melted, but was compressed into a rod shape by vacuum extrusion molding to obtain a molded body.

FIG. 2 is a photograph showing a brazing material molded from a metal powder experimentally produced by vacuum extrusion molding using the composition of List No. 1 shown in FIG.
Figure 2(a) shows the appearance of the brazing material molded from the prototype metal powder. Several prototypes are shown lined up. Each is molded into a rod shape with a circular cross section.
Figure 2(b) shows a photomicrograph of the cross section of a prototype brazing material formed from the metal powder shown in Figure 2(a) at 30x magnification.
Fig. 2(c) is a magnified microscope photograph at a magnification of 300 times higher than that of Fig. 2(b).
As shown in FIG. 2(c), it can be seen that even when formed into a rod shape by vacuum extrusion, the nickel alloy is not melted and remains in a granular state as a metal powder.

For comparison, the cross section of a brazing material made from a metal block manufactured by another company and having the same composition was also observed.
FIG. 3 is a photograph showing a brazing material made of a metal block from another manufacturer with the same composition.
Figure 3(a) shows the appearance of a metal ingot brazing material made by another company with the same composition. Only one piece is shown.
Figure 3(b) shows a micrograph of a cross section of a metal ingot brazing material made by another company and having the same composition as Figure 3(a). The cross section is a longitudinal section. The magnification is 30 times.
Fig. 3(c) is a magnified microscope photograph at a magnification of 300 times higher than that of Fig. 3(b).
As shown in Figures 3(b) and 3(c), the brazing material of the metal block is a molten nickel alloy that has been formed into a block. When observed under a microscope at magnifications (30x and 300x), the three-dimensional structure is not visible, and it can be confirmed that the metal block is a uniform, homogeneous metal block.
The state of the brazing material formed from the metal powder of the present invention shown in FIGS. 2(b) and 2(c) is clearly different from the state of the brazing material in the form of a metal block shown in FIGS. 3(b) and 3(c), and the powdery, particulate metal powder can clearly be seen. It can be seen that the brazing material formed from the metal powder of the present invention has never been melted, and is in a physically compressed state while maintaining its granular powder form.

Next, the particle size range of the metal powder in the brazing material formed from the metal powder of the prototype according to the present invention shown in Figure 2 was investigated. It was confirmed that 90% or more of the powder had a median diameter of 20 to 75 μm. Specifically, this is within the range shown in Table 1 below, and it can be said that the overall diameter is essentially 63 μm or less.

Next, the packing ratio of the brazing material obtained by molding the metal powder according to the present invention was measured as shown in Figure 2. In the present invention, the packing ratio represents the ratio of the apparent density to the true density.
For comparison, a brazing material made by another company and having the same composition as shown in FIG. 3 is also shown.
Table 2 shows the numerical values of the filling rate of the brazing material made of metal ingots manufactured by other companies having the same composition and the filling rate of the brazing material molded from the prototype metal powder according to the present invention.

The packing ratio was calculated by the ratio of the actually measured density to the theoretical value using the calculated density values of each metal shown in FIG.
In Table 2, the fill factor of the competitor's metal block brazing material is in the range of approximately 82% or 90%, but it is thought that such a low value range is calculated because flux and binder are also mixed in. If only the metal block portion of the competitor's metal block brazing material is measured, the fill factor is expected to be 95% or more.

The results shown in Table 2 show that the packing ratio of the brazing material molded from the metal powder of the present invention is clearly smaller than that of the brazing material made from metal blocks by other companies, at about 60%. This packing ratio can be adjusted by the pressure applied during extrusion molding.
In the present invention, the metal powder filling rate in the compact is preferably in the range of 50% to 80%. If the filling rate is less than 50%, the binder content is low, resulting in low hardness, and the brazing material formed from rod-shaped metal powder is likely to break when handled during brazing. If the filling rate is greater than 80%, depending on the binder content, the product may become similar to conventional metal ingot products, which may result in reduced heat input efficiency. Therefore, here, the metal powder filling rate in the compact is assumed to be in the range of 50% to 80%.

Next, the heat input efficiency of the brazing material formed from the metal powder produced experimentally according to the present invention was investigated.
The heat input efficiency was evaluated using a differential thermogravimetric analyzer. This analyzer is capable of performing thermogravimetry (TG), which continuously measures the change in weight of a sample while heating it at a constant rate, and differential thermal analysis (DTA), which measures the change in temperature between the sample and a reference material when the sample is heated together. Using this differential thermogravimetric analyzer, it is possible to verify the timing of the melting of a sample material, which is determined to occur when the thermogravimetry (TG) value remains constant and the differential thermal analysis (DTA) value peaks downward.

A brazing material formed from the metal powder of the present invention and a brazing material in the form of a metal block according to the prior art were prepared as samples, and differential thermogravimetric analysis was carried out under the following measurement conditions.
Measurement equipment: Differential thermogravimetric analyzer (NEXTA STA-300, manufactured by Hitachi High-Tech Science Corporation)
Temperature conditions: room temperature to 1200℃
Temperature increase condition: 10°C/min
Sample amount: 10±1 mg

FIG. 5 is a diagram showing the results of differential thermogravimetric analysis (time-dependent changes in DTA (uV) and TG (wt%)) of the brazing material formed from the metal powder according to the present invention and the brazing material of the metal block according to the prior art.
FIG. 5(a) is a graph showing the time change of DTA (uV) and TG (wt%) of a brazing material formed from the metal powder of the present invention, and FIG. 5(b) is an enlarged view showing the change in the vicinity of where the powder melts in FIG. 5(a).
FIG. 5(c) is a graph showing the time changes in DTA (uV) and TG (wt%) of a brazing material for a metal block according to the prior art, and FIG. 5(d) is an enlarged view showing the changes in the vicinity of where the metal block melts in FIG. 5(c).

The results are summarized in Table 3.

As shown in Table 3, the brazing material formed from the metal powder of the present invention began to melt after 97.4 minutes, and the sample was completely melted after 99.7 minutes. In other words, it took 2.3 minutes for the brazing material formed from the metal powder to start melting and reach a state where it could be used for brazing.
On the other hand, the brazing material of the metal block manufactured by Company A started to melt after 97.4 minutes, and the sample was completely melted after 102.2 minutes. In other words, it took 5.2 minutes for the brazing material of the metal block to start to melt and become ready for brazing.
The reason for this clear difference in the time it takes to melt can be analyzed as follows: the brazing material formed from the metal powder of the present invention has a large total area of the outer surface of the granular metal powder, resulting in a large heat input area, while the brazing material made from the metal block product of Company A only has a heat input area from the end area of the rod-shaped outer surface, and only metal thermal conduction occurs toward the center.
Thus, it was demonstrated that the brazing material of the molded body obtained by molding the metal powder of the present invention into a predetermined shape has an extremely high heat input efficiency.

Next, we will consider the extrusion pressure conditions for producing a brazing material formed from the metal powder of the present invention, which are conditions for extrusion pressure that result in the metal powder remaining in a granular state as shown in Figure 2 as a result of forming.
According to a paper that studied compression molding of metal powder (Non-Patent Document 1: "Compression Molding of Powder"), compression molding (compression molding by cold press), in which metal powder is placed in a mold and pressurized, progresses through the following four stages.
[Stage 1]: Powder particles press against each other, and most of the pressure energy is lost due to friction between particles. [Stage 2]: Bridges within the powder break down, allowing smaller particles to fill the gaps between larger particles, and the particles themselves begin to deform under pressure. Pressure energy is lost due to friction between the powder and mold walls, etc.
[Third stage]: The uneven surfaces of the particles break down and interlock due to friction and pressure, creating firm contact between the particles. The loss of pressure energy is consumed in particle deformation, and some of it is stored as internal residual stress.
[Fourth stage]: When the work hardening of the powder particles reaches its limit, further pressure causes the particles to break and the crystals to become finer. All of the pressure energy is consumed in deforming and breaking the particles.

The conditions of extrusion pressure and density for producing the brazing material formed by extruding the metal powder according to the present invention relate to the flow resistance that occurs when the metal powder is extruded, and if we apply this to the above-mentioned compression molding of the metal powder in an enclosed space in the cited example, it falls below the first stage because it does not involve deformation of the metal powder.
In conclusion, the extrusion pressure condition of the present invention is equal to or less than the yield stress at which no deformation occurs.

Next, the numerical range of the extrusion pressure is determined.
Figure 6 shows the change in density when the pressing pressure is increased using the metal powder used in this prototype. Observation of the brazing material formed at each of the values shown in Figure 6 reveals that the outer shape of the brazing material is maintained overall, but the internal structure is such that the metal powder remains granular, as shown in Figure 2. In other words, it was found that an extrusion pressure of 2 to 18 MPa is still below the yield stress and is within a suitable range for a brazing material formed from the metal powder according to the present invention.

In addition, since the present invention requires that the brazing material be in a state suitable for application, i.e., that the overall outer shape of the brazing material is maintained appropriately and the internal structure is such that the metal powder remains in granular form as shown in Figure 2, there is no need to apply any more pressure, and therefore the best mode is the extrusion pressure range (2 to 18 MPa) shown in Figure 6.

It was found that the density of the brazing material obtained within the range of extrusion pressure was about 4.0 to 6.0, more precisely 4.9 to 5.4, given the composition of the metal powder used.
As described above, it has been possible to obtain a brazing material according to the present invention that allows heat input treatment in a shorter time than brazed bodies according to the prior art.

Next, as a second embodiment of the present invention, a brazing method for manufacturing a fin-tube heat exchanger using a brazing material formed from the metal powder of the present invention will be described.
FIG. 7 is a diagram simply showing the structure of a fin-tube heat exchanger 100 to be brazed using the brazing material formed from the metal powder of the present invention.
Fig. 7(a) is a perspective view of the fin 110 viewed from a slight angle with the surface of the fin 110 through which the tube 120 is inserted facing the front, and Fig. 7(b) is a longitudinal cross-sectional view taken along the longitudinal centerline of the fin 110 with the surface through which the tube 120 is inserted facing the front. In other words, it is a cross-sectional view taken vertically through the center of the tube 120 and the groove 112 (described later). Note that the longitudinal cross-section in Fig. 7(b) is shown hatched.

The fins 110, which serve as heat sinks, are fabricated by pressing a thin plate of copper, stainless steel, aluminum, or the like to form tube through-holes 111, and then cutting the plate to a predetermined size. The tube through-holes 111 are provided to match the outer shape of the tubes 120. They may be circular or flat. In this example, they are circular.
The fin material can be any of those conventionally used. For example, copper, stainless steel, and aluminum alloys are often selected from the viewpoints of formability, heat transfer, and light weight. The brazing method for manufacturing the fin-tube heat exchanger of the present invention can be applied to any of these materials.
As shown in Figures 7(a) and 7(b), the fins 110 in the finned tube heat exchanger 100 are configured with multiple fins 110 stacked together. There is no limit to the number of fins 110, and for example, several hundred fins are possible. In this example, 36 fins are stacked together. The spacing between the fins 110 is also not limited, and they may be stacked with a gap of a few millimeters between them. When these multi-layered fins 110 are stacked together, all of the tube through-holes 111 drilled in the fins 110 are aligned in the same position, forming a continuous hole space.
Note that there may be one or more tube through-holes 111. It is also possible to insert multiple tubes 120, provide U-shaped curved portions at the ends of the tubes, and fold the circulation path so that the tubes travel back and forth across the fins multiple times. For simplicity's sake, the example in Figure 7 shows only one tube through-hole 111 and one tube 120 inserted therethrough.

Furthermore, the fin 110 according to the second embodiment has a groove 112 formed on the upper outer side of the tube insertion hole 111. As will be described later, this groove 112 is intended to provide a space for inserting and storing the brazing material 200 formed from the metal powder of the present invention in a continuous manner at a position adjacent to the tube 120.
In this way, the groove 112 is provided on the upper outer side of the tube insertion hole 111, and is integrally connected to and adjacent to the tube insertion hole 111. Therefore, when the brazing material 200 formed from the metal powder of the present invention inserted into this groove 112 melts, it immediately reaches the tube 120 directly below, which has the advantage of expanding the "wetting" area.

Tube 120 is a hollow tube and can be any metal tube with high thermal conductivity. There are no particular restrictions on the material, but copper or copper alloy tubes are often used due to their workability, heat transfer, ease of installation, and corrosion resistance.

The brazing process between the fins 110 and the tubes 120 in the fin-tube heat exchanger 100 according to the second embodiment is carried out in the following manner.
8 to 10, as with Fig. 7, (a) is a perspective view seen from a slight angle with the surface of the fin 110 through which the tube 120 is inserted facing the front, and (b) is a longitudinal cross-sectional view cut longitudinally along the center line with the surface of the fin 110 through which the tube 120 is inserted facing the front. In (b), the longitudinal cross-section is shown hatched.

First, from the state shown in FIG. 7 (where the tubes 120 serving as heat transfer tubes are inserted into the tube through holes 111 of the multilayered fins 110), as shown in FIG. 8, a rod-shaped brazing material 200 formed from the metal powder of the present invention is inserted into the space formed by the groove 112 on the upper outer side of the tube insertion hole 111.
It is preferable that the length of the brazing material 200 formed from the metal powder of the present invention be approximately the same as the penetration length of the multi-layered fin 110 .

Next, as shown in Fig. 9, a heat source of a predetermined temperature is brought into contact with the end of the brazing material 200 formed from the metal powder of the present invention, and heat is input. For example, a heat source of 200 degrees is used.
The brazing material 200 formed from the metal powder of the present invention has a high heat input efficiency as shown in Example 1, and the brazing material melts efficiently in a short time, and the brazing material melts not only at the ends but also at the center in a short time.
As a result, as shown in Figure 10, the brazing material 200 formed from the metal powder of the present invention quickly melts in its entirety, and the brazing material 200 that melts and becomes liquid in the space of the groove 112 reaches the tube insertion hole 111 directly below in an extremely short time and spreads to the outer periphery of the tube 120 that is in contact with the inner periphery of the tube insertion hole 111, so that so-called "wetting" spreads in a short time to the contact point between the fin 110 and the tube 120, i.e., the brazing point, and good "wetting" is achieved throughout the brazing point.

FIG. 10 is a diagram showing the fin-tube heat exchanger 100 in a state where the brazing process between the fins 110 and the tubes 120 has been completed.
The brazing material 200 formed from the metal powder of the present invention contains a small amount of binder and requires a short drying process, so it only takes a short time to go from the state shown in Figure 8 to the state shown in Figure 10. Nickel alloy brazing material easily solidifies into a metallic state when the temperature drops below 140°C, so it can quickly cool from the heat input temperature of 200°C and reach the state shown in Figure 8.
When the state shown in Figure 10 is reached, a strong metal bond is formed between the fins 110 made of aluminum metal material and the tubes 120 made of copper metal material using a brazing material made of nickel alloy metal material, resulting in an extremely strong fin-tube heat exchanger 100.

The above has illustrated and explained preferred embodiments of the brazing material formed from the metal powder of the present invention and the brazing processing method for manufacturing a fin-tube heat exchanger using the same, but it will be understood that various modifications are possible without departing from the technical scope of the present invention.

The brazing material of the present invention, which is formed from metal powder molded into a rod shape, can be used as a wide variety of brazing materials.
Furthermore, the brazing method for manufacturing a finned tube heat exchanger using a brazing material formed from a rod-shaped metal powder according to the present invention can be applied to the brazing process in the manufacture of a wide variety of finned tube heat exchangers, and can also be applied to a method for brazing two metal or ceramic members in a variety of mechanical devices, not limited to finned tube heat exchangers.

100 Fin-tube heat exchanger 110 Fin 111 Tube through-hole 112 Groove 120 Tube 200 Brazing material

Claims (15)

A brazing material formed from metal powder, which is made by powdering a metal to be used as a brazing material and extruding a material containing an organic binder at an extrusion pressure equal to or lower than the yield stress of the metal powder to form a molded body of a predetermined shape . A brazing material formed from the metal powder according to claim 1, wherein the extrusion pressure below the yield stress is 2 to 18 MPa. A brazing material formed from the metal powder according to claim 2, having a density of 4.0 to 6.0 after extrusion molding. A brazing material formed from the metal powder according to claim 3, having a density of 4.9 to 5.4 after extrusion molding. 3. The brazing material formed from metal powder according to claim 1, wherein 90% or more of the metal powder has a particle size of 20 to 75 μm. 6. The brazing material formed from metal powder according to claim 5 , wherein the metal is a nickel alloy containing nickel as a base material and containing one or a combination of chromium, silicon, and phosphorus in its composition . 7. The brazing material formed from metal powder according to claim 6 , wherein the predetermined shape is a rod shape. A method for manufacturing a brazing material formed from a molded metal powder, in which a material containing a powdered metal used as a brazing material and an organic binder is extruded at an extrusion pressure equal to or lower than the yield stress of the metal powder to form a molded body of a predetermined shape . The method for producing a brazing material formed from metal powder according to claim 8, wherein the extrusion pressure equal to or less than the yield stress is 2 to 18 MPa . The method for producing a brazing material formed from a metal powder according to claim 9, wherein the density after the extrusion molding is 4.0 to 6.0 . The method for producing a brazing material formed from a metal powder according to claim 10, wherein the density after the extrusion molding is 4.9 to 5.4 . The method for producing a brazing material formed from a molded metal powder according to claim 8 or 9, wherein 90% or more of the metal powder has a particle diameter of 20 to 75 μm . 13. The method for producing a brazing material formed from metal powder according to claim 12, wherein the metal is a nickel alloy having nickel as a base material and containing chromium, silicon, phosphorus, or a combination thereof in its composition . The method for producing a brazing material formed from molded metal powder according to claim 13, wherein the predetermined shape is a rod shape . A brazing method for manufacturing a fin-tube heat exchanger using a brazing material formed from the rod-shaped metal powder according to claim 8,
The fin-tube heat exchanger includes a fin multilayer structure in which a number of fins are stacked and arranged at predetermined intervals, and a tube inserted into a through-hole formed in the fin multilayer structure so as to extend through the structure,
In a configuration in which the brazing portion is a contact portion between an inner peripheral edge of the through hole of the fin multilayer structure and an outer peripheral edge of the tube,
A groove is provided by cutting out a part of the through hole toward the outer periphery,
This is a brazing method for manufacturing a fin-tube heat exchanger, in which the brazing material is inserted through the groove and brazed by inputting a predetermined heat into the brazing material.