Patent Application
20190153576
The present invention relates to an aluminum alloy material for monolayer heat-joining and also to a method for producing the same. It specifically relates to an aluminum alloy material having a heat-joining function in monolayer, which has improved deformation resistance at the time of brazing.
Brazing is often used in a method for producing a product made of an aluminum material and having a large number of metal joints, such as a heat exchanger or a heat sink. As an aluminum material for brazing, a brazing sheet, which is formed of a core material made of an aluminum material and a brazing material cladded on the core material, or a preplaced brazing material has been used. However, the use of a clad material such as a brazing sheet, in which a plurality of layers are lap-joined, or an additional joining material such as an preplaced brazing material has been, in relation to its production cost or material cost, the factor that increases the cost of a heat exchanger or the like.
Thus, in recent years, an aluminum alloy material capable of heat-joining in monolayer has been proposed to replace the above brazing sheet or brazing material (e.g. Patent Literature 1). This aluminum alloy material is made of an Al—Si-based alloy, and a liquid phase generated inside the alloy material upon heating is used for joining. Because the liquid phase functions as a brazing material, such an aluminum alloy material in monolayer can be joined to other components without the use of a joining material, such as a preplaced brazing material. Incidentally, in the present invention, when joining is possible by heating without a joining material, such a function is referred to as “heat-joining function”. In addition, joining by such an aluminum alloy material having a heat-joining function in monolayer is referred to as “brazing”, and the heating temperature at that time is referred to as “brazing temperature” about the brazing temperature.
In the aluminum alloy material having a heat-joining function in monolayer, because the material is turned into a semi-molten state during brazing, it is important to ensure deformation resistance at the brazing temperature. As a technique for improving the deformation resistance of the aluminum alloy material, for example, in Patent Literature 2, the coarsening of recrystallized grains during brazing has been proposed, and an aluminum alloy material having a heat-joining function in monolayer having such a function has been clarified.
However, according to the examination by the present inventor, a conventional aluminum alloy material having a heat-joining function in monolayer may be insufficient in terms of deformation resistance at the time of brazing. In particular, in an aluminum alloy material subjected to working with a high reduction ratio, a decrease in brazing deformation resistance is sometimes seen. Generally, an aluminum alloy material that forms as a component of a heat exchanger, a heat sink, or the like is subjected to working, such as corrugating or pressing, before brazing. Then, depending on the form of the component, an increase in the reduction ratio is inevitable. Accordingly, in an aluminum alloy material having a heat-joining function in monolayer, it is necessary to ensure deformation resistance at the time of brazing independent of the presence of working or the level of reduction ratio.
The present invention has been accomplished against the above background. An object of the present invention is to provide an aluminum alloy material having a heat-joining function in monolayer, in which a decrease in deformation resistance caused by working before brazing is suppressed.
In order to solve the above problems, the present inventor has conducted research on the metal structural characteristics of an aluminum alloy material having a heat-joining function in monolayer and also on the influences of working before brazing.
The aluminum alloy material having a heat-joining function in monolayer, which is the subject matter of the present invention, is made of an Al—Si-based alloy containing a relatively high concentration of Si. For example, as compared with a 3000-series aluminum alloy, which is a core material of a general brazing sheet, the Si concentration of this aluminum alloy material having a heat-joining function is high. In such an aluminum alloy containing a high concentration of Si, the density of second phase particles tends to be high. In an aluminum alloy containing a high concentration of Si, a crystallized product is abundantly generated in a casting step, and the crystallized product forms the second phase particles described above in an aluminum alloy material. Then, it has been confirmed that such crystallized product-derived second phase particles have a relatively large particle size and are likely to become recrystallization nuclei. That is, it can be said that an aluminum alloy material having a heat-joining function in monolayer has a tendency that, due to its composition, intrinsically, recrystallization nuclei are likely to appear at a high density.
In addition, in the production process of a conventional aluminum alloy material having a heat-joining function in monolayer, the material after a casting step is subjected to a rolling step and an annealing step. This annealing step is usually performed under conditions where recrystallization takes place. For example, in Patent Literature 1 mentioned above, in the Examples, annealing is performed at 380° C. for 2 hours, which is an annealing step accompanied by recrystallization. In addition, in Patent Literature 2 mentioned above, it is clearly shown that an annealing step that forms a recrystallized structure is provided (paragraph 0073 of Patent Literature 2). Like this, in the production process of a conventional aluminum alloy material having a heat-joining function in monolayer, an annealing step has been generally performed under conditions accompanied by recrystallization.
Like this, it can be said that a conventional aluminum alloy material having a heat-joining function in monolayer has a metal structure in which, based on its composition and production process, the crystallized product-derived second phase particles described above are distributed at a high density in a matrix having a recrystallized structure.
Then, in such an aluminum alloy material having a recrystallized structure, at the time of working before brazing, strains are likely to be localized around second phase particles, and, with an increase in the reduction ratio, the number of recrystallization nuclei rapidly increases. Therefore, at the time of brazing, refined recrystallized grains appear, resulting in a decrease in the crystal grain size. Due to the refinement of the crystal grain size, boundary sliding frequently occurs, resulting in a decrease in deformation resistance.
The present inventor has considered that in a conventional aluminum alloy material having a heat-joining function in monolayer, due to the above factors, the deformation resistance at the time of brazing decreases with an increase in the reduction ratio. Then, the present inventor has found that in order to alleviate the localization of recrystallization nuclei caused by working before brazing described above, it is preferable to make the material structure of the aluminum alloy material not a recrystallized structure but a unrecrystallized, fibrous structure.
According to the present inventor, in an aluminum alloy material having a fibrous structure, strains caused by working before brazing are unlikely to be localized around second phase particles and tend to be dispersed also in other regions. Accordingly, the induction of recrystallization nuclei around second phase particles can be reduced. Therefore, presumably, when an aluminum alloy material is provided with a fibrous structure, the occurrence of fine recrystallization at the time of brazing can be suppressed, and the deformation resistance can be maintained.
However, it is difficult to ensure deformation resistance at the time of brazing only by a fibrous structure. This is because a fibrous structure itself has a high driving force for recrystallization, and its material structure is prone to recrystallization at the time of heating. That is, although a fibrous structure can deal with problems caused by working before brazing (localization of strains), problems of recrystallization at the time of brazing (at the time of heating) cannot be dealt with by a fibrous structure alone. That is, in the aluminum alloy material having a heat-joining function in monolayer, which is the subject matter of the present invention, while employing a fibrous structure, it is further necessary to add an element for suppressing the refinement of recrystallized grains at the time of heating.
Thus, as a result of extensive research, the present inventor has found that with respect to second phase particles dispersed in an aluminum alloy material, it is necessary to specify the particle size and the number density of particles attributable to the refinement of recrystallized grains. In addition to this, the present inventor has found that when the predetermined mechanical properties of the aluminum alloy material is optimized, such a material is insusceptible to the reduction ratio and has excellent deformation resistance at high temperatures, and thus accomplished the present invention.
The present invention that achieves the above object is an aluminum alloy material including Si: 1.5 mass % to 5.0 mass %, Mn: 0.05 mass % to 2.0 mass %, Fe: 0.01 mass % to 2.0 mass %, and balance: Al and inevitable impurities and having a heat-joining function in monolayer. The aluminum alloy material has a fibrous structure, the number density of second phase particles including an Si-based compound or an AlMnFeSi-based compound and having an equivalent circle diameter of 5.0 μm to 10.0 μm is 1,000/mm2 or less, the work hardening exponent n between two points from a nominal strain that is 0.9 times the nominal strain at maximum load to the nominal strain at maximum load is 0.03 or more, and the local elongation is 1% or more.
In addition, the present invention is the aluminum alloy material having a heat-joining function in monolayer according to claim 1, further including at least one of the following elements:
Zn: 0.05 to 6.0%
Mg: 0.05 to 2.0%
Cu: 0.05 to 1.5%
Ni: 0.05 to 2.0%
Cr: 0.05 to 0.3%
Zr: 0.05 to 0.3%
Ti: 0.05 to 0.3%
V: 0.05 to 0.3%.
In addition, the present invention is also the aluminum alloy material having a heat-joining function in monolayer according to claim 1 or 2, further including at least one of the following elements:
In: 0.005 to 0.3%
Sn: 0.005 to 0.3%.
Furthermore, the present invention may be the aluminum alloy material having a heat-joining function in monolayer according to any one of claims 1 to 3, further including at least one of the following elements:
Be: 0.0001 to 0.1%
Sr: 0.0001 to 0.1%
Bi: 0.0001 to 0.1%
Na: 0.0001 to 0.1%
Ca: 0.0001 to 0.05%.
In addition, a method for producing the aluminum alloy material according to the present invention includes: a casting step of producing a slab by DC casting; after the casting step, a step of hot rolling after performing a homogenization treatment step or after performing, without a homogenization treatment, a heating step of heating before hot rolling; a step of cold rolling after the hot rolling step; and an annealing step performed at least once during the cold rolling step. The casting speed in the casting step is 20 to 100 mm/min, the homogenization treatment step or the heating step before hot rolling is performed under conditions including a heating temperature of 350° C. to 480° C. and a maintenance time of 0 to 30 hours, and, in the annealing step, annealing is performed at a temperature for a period of time such that recrystallization is not caused.
Furthermore, a method for producing the aluminum alloy material according to the present invention may include: a casting step of producing a plate-shaped ingot by continuous casting; a homogenization treatment step performed after the casting step; a step of cold rolling after the homogenization treatment step; and an annealing step performed at least once during the cold rolling step. The casting speed in the casting step is 500 to 3,000 mm/min, the homogenization treatment step is performed under conditions including a heating temperature of 350° C. to 480° C. and a maintenance time of 1 to 10 hours, and, in the annealing step, annealing is performed at a temperature for a period of time such that recrystallization is not caused.
Incidentally, in the above two methods, including the case where cold rolling is performed after the annealing step, it is preferable that the working ratio in the cold step is 20% or less.
The aluminum alloy material according to the present invention has a heat-joining function in monolayer, in which a decrease in deformation resistance caused by working before brazing is reduced. According to the present invention, improvements can be expected in the product dimension or the yield of heat exchangers or the like. In addition, even when stronger working than conventional is applied before brazing, deformation resistance during brazing can be maintained.
Hereinafter, the aluminum alloy material for monolayer heat-joining and the method for producing the same according to the present invention will be described in further detail. First, the constituent elements, material structure, and mechanical properties of the aluminum alloy material according to the present invention will be described. Incidentally, in the specification of the present application, simple “%” in the description of the composition of an alloy means “mass %”.
I. Constituent Elements of Aluminum Alloy Material according to the Present Invention
The aluminum alloy material according to the present invention includes Si, Mn, and Fe as essential elements. The technical significance of these essential elements and their added amounts are as follows. Incidentally, the aluminum alloy material according to the present invention is composed of the following essential elements and, as balances other than selective alloying elements, aluminum and inevitable impurities.
Si
Si is an element that generates an Al—Si-based liquid phase and contributes to joining. However, when the Si concentration is less than 1.5%, a sufficient amount of liquid phase cannot be generated, and the exudation of a liquid phase decreases, resulting in incomplete joining. Meanwhile, when the concentration is more than 5.0%, the amount of Si particles in the aluminum alloy material increases, and an increased amount of liquid phase is generated. Accordingly, the material strength during heating decreases too much, making it difficult to maintain the shape as a fin material. Therefore, the Si concentration is specified to be 1.5% to 5.0%. The Si concentration is preferably 1.6% to 3.5%, and more preferably 2.0% to 3.0%. Incidentally, the amount of liquid phase exuded increases with an increase in the plate thickness and an increase in the heating temperature. Therefore, for the amount of liquid phase necessary at the time of heating, it is preferable to adjust the amount of Si or the brazing temperature required according to the structure or dimension of the fin of a heat exchanger to produce.
Mn
Mn forms an AlMnFeSi-based intermetallic compound (second phase particles) together with Si and Fe and functions as dispersion hardening. In addition, Mn is also dissolved in an aluminum parent phase to function as solid solution hardening. Like this, Mn is an important alloying element that improves the strength of the alloy material. When the Mn content is more than 2.00%, a coarse intermetallic compound is likely to be formed, resulting in a decrease in corrosion resistance. Meanwhile, when the Mn content is less than 0.05%, the above effects are insufficient. Therefore, the Mn content is specified to be 0.05 to 2.00%. The Mn content is preferably 0.10 to 1.50%.
Fe
Fe has the effect of being slightly dissolving in the matrix to improve strength. In addition, Fe also has the effect of being dispersed as an AlMnFeSi-based crystallized product or precipitate to prevent strength reduction particularly at high temperatures. When the content of Fe is less than 0.01%, not only that the above effects are not sufficiently obtained, but also that it is necessary to use high-purity ingots, leading to an increase in the material cost. Meanwhile, when the Fe content is more than 2.00%, a coarse intermetallic compound is generated at the time of casting, resulting in problems with productivity. In addition, when the joined body is exposed to a corrosive environment (particularly a corrosive environment where a corrosive liquid flows), the corrosion resistance decreases. Further, because crystal grains recrystallized by heating at the time of joining are refined to increase the grain boundary density, the amount of deformation during joining increases, whereby the dimensional change before and after joining increases. Therefore, the Fe content is specified to be 0.01 to 2.00%. The content of Fe is preferably 0.10% to 0.60%.
The aluminum alloy material according to the present invention may selectively contain, in addition to the above essential elements, at least one of Mg, Cu, Ni, Cr, Zr, Ti, V, Be, Sr, Bi, Na, Ca, Zn, In, and Sn.
Mg
Mg turns into Mg2Si after brazing and causes age hardening to improve strength. Therefore, Mg is an alloying element that exerts a strength-improving effect. When the amount of Mg added is more than 2.0%, it reacts with the flux and forms a high-melting-point compound, resulting in a significant decrease in joining properties. Therefore, the amount of Mg added is preferably 2.0% or less. The preferred amount of Mg added is 0.05% to 2.0%. In addition, in the present invention, not only for Mg but also for other alloy components, the description “a predetermined added amount or less” also includes 0%.
Cu
Cu is an alloying element that is dissolved in the matrix to improve strength. However, when the amount of Cu added is more than 1.5%, the corrosion resistance decreases. Therefore, the amount of Cu added is preferably 1.5% or less. The preferred amount of Cu added is 0.05% to 1.5%.
Ni
Ni has the effect of being crystallized or precipitated as an intermetallic compound to cause dispersion hardening, thereby improving strength after joining. The amount of Ni added is preferably 2.0% or less. The preferred added amount is 0.05% to 2.0%. When the content of Ni is more than 2.0%, a coarse intermetallic compound is likely to be formed, resulting in a decrease in workability. In addition, the self-corrosion resistance also decreases.
Cr
Cr causes solid solution hardening to improve strength. In addition, an Al—Cr-based intermetallic compound is precipitated, acting on the coarsening of crystal grains after heating. When the added amount is more than 0.3%, a coarse intermetallic compound is likely to be formed, resulting in a decrease in plastic workability. Therefore, the amount of Cr added is preferably 0.3% or less. The more preferred added amount is 0.05% to 0.3%.
Zr
Zr exerts the effect of being crystallized as an Al—Zr-based intermetallic compound to cause dispersion hardening, thereby improving strength after joining. In addition, an Al—Zr-based intermetallic compound acts on the coarsening of crystal grains during heating. When the added amount is more than 0.3%, a coarse intermetallic compound is likely to be formed, resulting in a decrease in plastic workability. Therefore, the amount of Zr added is preferably 0.3%. The preferred added amount is 0.05% to 0.3%.
Ti, V. Ti and V are dissolved in the matrix to improve strength, and also have the effect of being distributed in layers to prevent the progress of corrosion in the plate thickness direction. When the added amount is more than 0.3%, a huge crystallized product appears, which inhibits moldability and corrosion resistance. Therefore, the amounts of Ti and V added are each preferably 0.3% or less. The more preferred added amount is 0.05% to 0.3%.
Zn
Zn is an element that is effective in improving corrosion resistance by the sacrificial protection function. Zn has the function of being almost uniformly dissolved in the matrix to lower the self-potential. For example, when the aluminum alloy material according to the present invention is used as a fin material, and its potential is lowered, the sacrificial protection function that relatively suppresses the corrosion of the joined tube can be exerted. When the Zn content is less than 0.05%, the effect of lowering the potential is insufficient. Meanwhile, when the Zn content is more than 6.00%, the corrosion rate increases, and the self-corrosion resistance decreases, resulting in a decrease in the sacrificial protection function. Therefore, the Zn content specified to be 0.05 to 6.00%. The Zn content is preferably 0.10% to 5.00%.
Sn, In
Sn and In have the effect of exerting the sacrificial anode function. When the added amount is more than 0.3%, the corrosion rate increases, and the self-corrosion resistance decreases. Therefore, the amounts of these elements added are each preferably 0.3% or less. The more preferred added amount is 0.05% to 0.3%.
Be, Sr, Bi, Na, Ca
Be, Sr, Bi, Na, and Ca achieve improvements in the characteristics of a liquid phase, thereby making the joining properties even better. The preferred ranges of these elements are Be: 0.0001% to 0.1%, Sr: 0.0001% to 0.1%, Bi: 0.0001% to 0.1%, Na: 0.0001% to 0.1%, and Ca: 0.0001% to 0.05%, and at least one of them is added as necessary. These trace elements can achieve the fine dispersion of Si particles, the improvement of the fluidity of a liquid phase, and the like, thereby improving joining properties. When the amounts of these trace elements are less than the more preferred ranges specified above, their effects are small, while when the amounts are more than the more preferred ranges specified above, adverse effects, such as a decrease in corrosion resistance, may be caused. Incidentally, when at least one of Be, Sr, Bi, Na, and Ca is added, it is necessary that the added components are all within the above preferred or more preferred ranges.
As described above, the aluminum alloy material having a heat-joining function in monolayer according to the present invention is characterized by its metal structure and has a fibrous crystal grain structure (hereinafter referred to as “fibrous structure”). Further, the density of second phase particles within a specified particle size range is limited. Hereinafter, the technical significance of these features will be described.
The aluminum alloy material for monolayer heat-joining according to the present invention is characterized by having a fibrous structure. Thanks to the fibrous structure, strains caused by working before brazing can be uniformly dispersed without being localized around second phase particles. Then, accordingly, an increase in the number of recrystallization nuclei at the time of working is suppressed, and a decrease in deformation resistance due to fine recrystallization at the time of brazing is suppressed. Here, a fibrous structure is a metal structure composed of crystal grains elongated in the rolling direction. Specifically, in the present invention, a fibrous structure is a metal structure including crystal grains having an aspect ratio of 10 or more.
When the aspect ratios of crystal grains are measured for the determination of a fibrous structure, it is preferable that crystal grains are observed by an anodic oxidation method on a cross-section of the aluminum alloy material perpendicular to its width direction, and the shape of crystal grains is measured by image analysis. As conditions for the anodic oxidation method, for example, a 3.3% aqueous HBF4 solution is used, and a current at a voltage of 30 V is applied for 60 sec, thereby forming an oxide film. The sample after anodic oxidation can be observed under an optical microscope by use of a polarizing filter. The observation magnification at this time is not particularly limited, and, according to the size of crystal grains, a magnification of about 50 to 200 is selected.
When the aspect ratios of crystal grains are measured for the determination of a fibrous structure, it is preferable to observe at least five fields of view per sample. At this time, it is preferable that an image including an alloy material having a length of at least 300 μm is analyzed as one field of view. It is preferable that the image is prepared such that at least five fields of view are measured. In the image analysis, the aspect ratios of crystal grains are determined based on an image photographed under an optical microscope.
Then, by the image analysis of an image photographed under a microscope, or by the direct measurement of an image, the presence of a fibrous structure can be determined. For the image analysis, a suitable software may be used, and the average aspect ratio is calculated by analysis. When the average aspect ratio is calculated to be 10 or more, such a field of view is determined to have a fibrous structure. When the presence of a fibrous structure is determined in 80% or more of the number of fields of view observed, such a sample is defined as an alloy material having a fibrous structure. In addition, in the case where an image is directly measured, when the number of crystal grains having an aspect ratio of 10 or more measurable per field of view is 10 or more, such a field of view is determined to have a fibrous structure. When the presence of a fibrous structure is determined in 80% or more of the number of fields of view observed, such a sample is defined as an alloy material having a fibrous structure.
The aluminum alloy material for monolayer heat-joining according to the present invention includes second phase particles composed of an Si-based compound or an AlMnFeSi-based compound. Then, the present invention is configured such that the density of second phase particles having an equivalent circle diameter of 5 μm to 10 μm is 1,000/mm2 or less. Second phase particles are particles of a phase whose composition is different from the matrix Al or Al alloy. Second phase particles are composed of an Si-based compound or an AlMnFeSi-based compound. Specifically, Si-based compounds include elemental Si and Si compounds. An Si compound is a compound containing Si, such as a compound containing Ca, P, or like elements in part of elemental Si, for example. In addition, an AlMnFeSi-based compound is an intermetallic compound generated from Al and an alloying element in the alloy. Specific examples thereof include an Al—Fe-based compound, an Al—Fe—Si-based compound, an Al—Mn—Si-based compound, an Al—Fe—Mn-based compound, and an Al—Mn—Fe—Si-based compound.
Second phase particles of the present invention are derived from a crystallized product or precipitate generated in the production process of an aluminum alloy material. A crystallized product is mainly generated from a liquid phase in a casting step, and a precipitate is mainly generated from a solid phase in a step after the casting step, such as a homogenization treatment. However, in the present invention, with respect to second phase particles observed in the observation of the metal structure of an aluminum alloy material, whether the second phase particles are from a crystallized product or a precipitate is not questioned. Regardless of which the second phase particles are from, as long as the particle size is within the specific range, such particles are the second phase particles specified in the invention of the present application. The meanings of the terms “crystallized product” and “precipitate” herein are distinguished based only on the timing of production. The meanings are the same in that they both become “second phase particles” in the aluminum alloy material.
In the present invention, the density of second phase particles of a specific particle size range is limited in order to suppress the proceeding of recrystallization of the aluminum alloy material at the time of brazing. That is, as described above, the aluminum alloy material of the present invention is characterized by having a fibrous structure. This fibrous structure has, at the time of working before brazing, the effect of uniformly dispersing strains to suppress the formation of recrystallization nuclei. Meanwhile, at the time of brazing (at the time of heating), the material structure has a higher driving force for recrystallization as compared with a conventional aluminum alloy material. Thus, in the present invention, the density of second phase particles which may become recrystallization nuclei at the time brazing is restricted to reduce the influence. As a result, recrystallization that occurs during brazing is suppressed.
Then, second phase particles that may become recrystallization nuclei at the time of brazing are relatively large-size second phase particles having an equivalent circle diameter of 5 μm to 10 μm. According to the present inventor, when the number density (surface density) of second phase particles having an equivalent circle diameter of 5 μm to 10 μm is more than 1,000/mm2, recrystallized grains are refined, and the deformation resistance decreases. Thus, the density of such second phase particles has been specified to be 1,000/mm2 or less. For the suppression of refinement of recrystallized grains at the time of brazing, the number density of second phase particles having an equivalent circle diameter of 5 μm to 10 μm is preferably low, and it is preferable that there are no such second phase particles.
Incidentally, in the present invention, it is not necessary to specify the presence and the number density of second phase particles having an equivalent circle diameter of less than 5 μm. This is because, according to the examination by the present inventors, fine second phase particles having an equivalent circle diameter of less than 5 μm are unlikely to become recrystallization nuclei. In addition, it is also unnecessary to particularly specify second phase particles having an equivalent circle diameter of more than 10 μm. This is because such coarse second phase particles are hardly present under the preferred production conditions of the aluminum alloy material according to the present invention described below.
In the present invention, an equivalent circle diameter of a second phase particle means the diameter of an equivalent circle. The equivalent circle diameter of a second phase particle can be measured by the SEM observation of an arbitrary cross-section of an aluminum alloy material. By the image analysis of an SEM image obtained by SEM observation, the equivalent circle diameter of a second phase particle can be calculated. Incidentally, with respect to the distinction in configuration of second phase particles, second phase particles of Si and second phase particles of an AlMnFeSi-based compound can be distinguished based on the contrast density of an SEM image. The exact composition can be confirmed by EPMA (Electron Probe Micro Analyser).
The aluminum alloy material for monolayer heat-joining according to the present invention has the above characteristics in composition and material structure. Then, because of such characteristics, the aluminum alloy material for monolayer heat-joining according to the present invention shows characteristic tendencies in the n-value and the local elongation.
(a) n-Value (Work Hardening Exponent)
In the aluminum alloy material for monolayer heat-joining according to the present invention, the n-value between two points from a nominal strain 0.9 times the nominal strain at maximum load to the nominal strain at maximum load is specified to be 0.03 or more. Here, the n-value herein is an index called “work hardening exponent” and shows the ease of work hardening at the time of material deformation. The n-value can be calculated from the true stress and the true strain obtained based on the nominal stress-nominal strain curve obtained from a tensile test. The numerical value of the n-value varies depending on the range of nominal strains applied to calculation. The n-value within a range from a nominal strain a times the nominal strain at maximum load to a nominal strain b times the nominal strain at maximum load can be calculated by the following formula (1).
[Equation 1]
n=(ln σb−ln σa)/(ln εb−ln εa) equation (1)
where
σa=(1+εNa)×σNa
σb=(1+εNb)×σNb
εa=ln(1+εNa)
εb=ln(1+εNb)
In the above formulae, the meanings of σa, σb, σNa, σNb, εa, εb, εNa, and εNb are as follows. In is a natural logarithm.
σa: True stress at a times the nominal strain
σb: True stress at b times the nominal strain
σNa: Nominal stress at a times the nominal strain
σNb: Nominal stress at b times the nominal strain
εa: True strain at a times the nominal strain
εb: True strain at b times the nominal strain
εNa: Nominal strain at a times the nominal strain
εNb: Nominal strain at b times the nominal strain
The range of nominal strains for specifying the n-value of the aluminum alloy material according to the present invention (formulas (1), a and b) is from a nominal strain 0.9 times the nominal strain at maximum load to the nominal strain at maximum load (a=0.9, b=1.0). Generally, an aluminum alloy has a tendency that the n-value is high in an area where the nominal strain is low, while the n-value is low in an area where the nominal strain is high. In the aluminum alloy material according to the present invention, it is important that even in an area near the maximum load where the nominal strain increases, a high n-value is maintained, and the localization of strains due to working is prevented. Therefore, an n-value between two points from a nominal strain 0.9 times the nominal strain at maximum load to the nominal strain at maximum load (1.0 time) is applied.
Then, in the present invention, the n-value within a range from a nominal strain 0.9 times the nominal strain at maximum load to the nominal strain at maximum load is specified to be 0.03 or more. The n-value, which is a work hardening exponent, is an index that indicates the ease of work hardening. When a material is easy to work harden, deformation spreads peripherally, and uniform deformation is possible. When the n-value is less than 0.03, at the time of working before brazing, in the case where a strain smaller than the nominal strain at maximum load is applied, deformation is locally introduced. Therefore, the recrystallized grain size at the time of brazing is refined, resulting in a decrease in deformation resistance. For this reason, the n-value is specified to be 0.03 or more. The n-value is preferably 0.04 or more, and still more preferably 0.05 or more. The upper limit is not set on the n-value, but is substantially preferably 0.98 or less.
In the aluminum alloy material for monolayer heat-joining according to the present invention, the local elongation is specified to be 1% or more. Local elongation is a difference between the strain at maximum load and the strain at break in a nominal stress-strain curve obtained from a tensile test. When the local elongation is less than 1%, at the time of working before brazing, in the case where a strain larger than the strain at maximum load is applied, deformation is locally introduced. Therefore, recrystallized grains at the time of brazing may be refined, resulting in a decrease in deformation resistance. The local elongation is preferably 2% or more, and still more preferably 4% or more. The upper limit should not be set on the local elongation, but is substantially preferably 30%.
Next, the method for producing an aluminum alloy material for monolayer heat-joining according to the present invention will be described. Basically, the aluminum alloy material of the present invention can be produced in the usual manner. However, attention has to be paid to the conditions of heating after casting. This is because, as described above, in the present invention, the presence of a fibrous structure, which is an unrecrystallized structure, is required. Hereinafter, as the method for producing an aluminum alloy material of the present invention, the production by DC casting and the production by continuous casting (CC) will be described.
A slab cast by a DC casting method becomes the aluminum alloy material of the present invention through a heating step before hot rolling, a hot rolling step, a cold rolling step, and an annealing step. Here, a crystallized product may be generated in the casting step. In order to suppress the formation of second phase particles having an equivalent circle diameter of 5 μm to 10 μm that become recrystallization nuclei at the time of brazing, it is preferable that the casting speed is 20 to 100 mm/min. Incidentally, it is also possible to perform a homogenization treatment before hot rolling.
The slab produced by a DC casting method may be, after or without a homogenization treatment, subjected to a heating step before hot rolling. It is desirable that the conditions for the homogenization treatment and the heating step are each such that the heating maintenance temperature is 350 to 480° C., and the maintenance time is within a range of 0 to 30 hours. When the maintenance temperature is less than 350° C., the deformation resistance of the slab during hot rolling may be so high that cracking occurs. Meanwhile, in the homogenization treatment and the heating step, a fine precipitate may be generated, and a heat treatment at a high temperature for a long period of time causes the coarsening of second phase particles. When the maintenance temperature is more than 480° C., or when the maintenance time is more than 30 hours, the coarsening of second phase particles occurs, and the density of second phase particles having an equivalent circle diameter of 5 μm to 10 μm increases. In addition, as a result of a heat treatment at a high temperature for a long period of time, the dissolution amount of alloying elements increases, and recrystallized grains after brazing are refined. Incidentally, when the maintenance time is 0 hours, this means that heating is finished immediately after the above heating maintenance temperature is reached.
Then, hot rolling is performed after the above heating step, and, according to the content of tempering, the hot rolled material is subjected to a cold working step or an annealing step. In the case of H1n tempering, after the completion of the hot rolling step, the hot rolling material is subjected to a cold rolling step. The conditions for the cold rolling step are not particularly limited. Then, in the course of the cold rolling step, an annealing step of annealing the cold rolled material is performed at least once. The annealing step is performed under conditions where recrystallization does not take place. Specifically, the material is heated at 150 to 300° C. for 1 to 5 hours. When the temperature is less than 150° C., the softening of the material is insufficient, and the subsequent workability decreases, resulting in a decrease in elongation in the final plate thickness. When the temperature is more than 300° C., recrystallization is likely to take place. Even at a temperature of 300° C. or less, recrystallization may take place depending on the alloy composition or the production process. Accordingly, in such a case, within the above temperature range, a temperature at which recrystallization does not take place is selected.
After the annealing step, the cold rolled material is subjected to final cold rolling to a final plate thickness. When the working ratio in the final cold rolling (working ratio=(plate thickness before working−plate thickness after working)/plate thickness before working (%)) is too high, the driving force for recrystallization during brazing increases, and crystal grains are reduced in size, resulting in an increase in deformation during brazing. Incidentally, the final cold rolling is arbitrary and does not necessarily have to be performed, and the plate thickness after the annealing step may serve as the final plate thickness. In addition, when the final cold rolling is performed, its working ratio is preferably about 3 to 20%.
When an aluminum alloy material is produced by H2n tempering, there are the case where after the completion of the hot rolling step, the hot rolled material is subjected to a cold rolling step to the final plate thickness and then subjected to an annealing step (final annealing), and the case where, as in the case of H1n tempering, an annealing step is performed at least once in the course of the cold rolling step, and final annealing is performed after final cold rolling. In each case, the annealing step is performed under conditions where recrystallization does not take place as above.
The continuous casting method is not particularly limited as long as it is a method of continuously casting a plate-shaped ingot, such as a twin-roll type continuous casting rolling method and a twin-belt type continuous casting method. A twin-roll type continuous casting rolling method is a method in which an aluminum molten metal is fed between a pair of water-cooled rolls from a feed nozzle made of a refractory material, thereby continuously casting and rolling a thin plate. As such methods, the Hunter process, the 3C process, and the like are known. In addition, a twin-belt type continuous casting method is a continuous casting method in which a molten metal is poured between upper and lower opposed, water-cooled rotary belts, then the molten metal is solidified by cooling from the belt surfaces to form a slab, and the slab is continuously drawn from the not-pouring side of the belts and wound up into a coil. As such methods, the Hazelett process and the like are known.
In the step of casting a plate-shaped ingot by a continuous casting method as described above, in order to suppress the formation of second phase particles having an equivalent circle diameter of 5 μm to 10 μm that become recrystallization nuclei at the time of brazing, it is necessary that the casting speed is 500 to 3,000 mm/min.
The plate-shaped ingot cast by a continuous casting method is subjected as it is to a homogenization treatment. Subsequently, during the step of cold rolling to the final plate thickness, an annealing step is performed. This annealing step has to be performed at least once.
In order to form a fine precipitate and obtain a suitable metal structure, it is preferable that the homogenization treatment is performed at 350 to 480° C. for 1 to 10 hours. When the temperature is less than 350° C., the formation of a fine precipitate is insufficient. Meanwhile, the fine precipitate generated here may also be distributed as second phase particles in the aluminum alloy material. When the homogenization treatment temperature is more than 480° C., second phase particles are coarsened, and the density of second phase particles having an equivalent circle diameter of 5 μm to 10 μm increases. In addition, when the homogenization time is less than 1 hour, the above effects are not sufficient, while when the homogenization time is more than 10 hours, the above effects are saturated, leading to an economic disadvantage.
In the annealing step, the material is softened to make it easy to obtain the desired material strength in final rolling. This annealing step needs to be performed under conditions where recrystallization does not take place. Specifically, the step is performed at 150 to 300° C. for 1 to 5 hours. When the temperature is less than 150° C., the softening of the material is insufficient, and thus the subsequent workability decreases, resulting in a decrease in elongation in the final plate thickness. When the temperature is more than 300° C., recrystallization is likely to take place. Even at a temperature of 300° C. or less, recrystallization may take place depending on the alloy composition or the production process. Accordingly, in such a case, within the above temperature range, a temperature at which recrystallization does not take place is selected.
After the annealing step, the rolled material is subjected to final cold rolling, thereby giving an aluminum alloy material having the final plate thickness. In the case of H1n tempering, when the working ratio in the final cold rolling stage is too high, the driving force for recrystallization during brazing increases, and crystal grains are reduced in size, resulting in a decrease in deformation resistance during brazing. Incidentally, the final cold rolling is arbitrary and does not necessarily have to be performed, and the plate thickness after the annealing step may serve as the final plate thickness. In addition, when the final cold rolling is performed, its working ratio is preferably about 3 to 20%.
When an aluminum alloy material is produced by H2n tempering, there are the case where after the completion of the hot rolling step, the hot rolled material is subjected to a cold rolling step to the final plate thickness and then subjected to an annealing step (final annealing), and the case where, as in the case of H1n tempering, an annealing step is performed at least once in the course of the cold rolling step, and final annealing is performed after final cold rolling. In each case, the annealing step is performed under conditions where recrystallization does not take place as above.
Hereinafter, the present invention will be described through examples in comparison with comparative examples.
In the Examples, the aluminum alloys shown in Table 1 to Table 4 were dissolved and DC cast, and then subjected to a hot rolling step and a cold rolling step. In the course of the cold rolling step, intermediate annealing was performed under conditions where recrystallization did not take place, and final cold rolling was performed to give an aluminum alloy material having a final plate thickness of 0.1 mm. In the Examples, in the heating step before hot rolling, the alloy was heated to 450° C. and maintained at such a temperature for 10 hours. In addition, in the annealing step in the course of the cold rolling step, the alloy was maintained at 200° C. for 3 hours so as not to cause recrystallization. Further, the final cold rolling ratio was 10%.
However, in the alloy A69 in Table 4, the final cold rolling rate was 20%. In addition, in the alloy A70 in Table 4, the hot rolling temperature was 420° C. In addition, in the alloy A71 in Table 4, in order to perform H2n tempering, the plate thickness was made 0.1 mm by cold rolling after hot rolling, followed by final annealing, thereby preparing a test specimen. In the final annealing, the alloy was maintained at 200° C. for 3 hours.
In addition, in the alloy A72 in Table 4, the same aluminum alloy as the alloy A1 in Table 1 was dissolved and continuously cast to produce a plate-shaped ingot. Then, the plate-shaped ingot was subjected to a homogenization treatment and cold rolling, then to an annealing step (intermediate annealing) under conditions where recrystallization did not take place, and to final cold rolling, thereby giving an aluminum alloy material having a final plate thickness of 0.1 mm. In the homogenization treatment, the alloy was heated to 450° C. and maintained at such a temperature for 10 hours. In the intermediate annealing, the alloy was maintained at 200° C. for 3 hours. In addition, the final cold rolling rate was 10%.
In the same manner as in the Example, the aluminum alloys shown in Table 5 were dissolved and DC cast, and then subjected to a hot rolling step and a cold rolling step. In the Comparative Examples, the alloys B1 to B5 in Table 5 were produced under the same conditions as in the Examples (heating step before hot rolling, annealing step), and in the alloys B6 to B10, production conditions different from the Examples were applied. Specifically, in the alloy B6, the annealing temperature was 350° C. for 3 hours. In the alloy B7, H2n tempering was performed, and, without the intermediate annealing step, the final annealing step was performed at an annealing temperature of 350° C. for 3 hours. In the alloy B8, the heating temperature in the heating step before hot rolling was 520° C., and the maintenance time was 5 hours. In the alloy B9, the heating temperature before hot rolling was 350° C. In the alloy B10, the final cold rolling rate was 30%.
As the observation of the metal structures of the aluminum alloy materials of the above examples and comparative examples, the crystal grain structure (fibrous structure) was determined, the number density of second phase particle was measured, and the physical properties relating to mechanical characteristic (n-value, local elongation) were measured.
The crystal grain structure of each plate material (blank) produced was determined by observing a cross-section perpendicular to the width direction under an optical microscope. In the observation, a test specimen was resin-embedded and polished, and then surface-treated by an anodic oxidation method to make it easy to observe the crystal grain structure. In the examples and comparative examples, the crystal grain structure of a sample of the present invention was observed as follows. Each test specimen was photographed in five fields of view under an optical microscope, and subjected to image analysis by use of a commercially available image analysis software (trade name: AZOKUN, manufactured by Asahi Kasei Corporation) to measure the average aspect ratio of crystal grains. A crystal grain structure having an average aspect ratio of 10 or more was determined as a fibrous structure, and a structure having an average aspect ratio of less than 10 was determined as a recrystallized structure. Then, when 80% of the five fields of view (four fields of view) were determined to have a fibrous structure, the crystal grain structure of such a sample was determined as an alloy material having a fibrous structure, and a sample in other cases was determined as an alloy material having a recrystallized structure.
In addition, in some test materials, crystal grains were observed in lines, and it was difficult to measure the aspect ratio by the image analysis. For such a test material, a fibrous structure was judged based on direct measurement by visual determination utilizing a crystal grain sample having an aspect ratio of 10.
An example of the crystal grain structure determination method by visual observation will be described.
In the case of such a crystal grain structure, based on the thickness of the thickest crystal grain among the observed crystal grains, a rectangle having an aspect ratio of 10, whose shorter side is such a thickness, was drawn for the determination. Specifically, a rectangular parallelepiped having an aspect ratio of 10 as shown in the enlarged image in the lower part of
Incidentally, in the determination of a crystal grain structure by the visual observation of an image, there may be a region where the second phase particles appear enlarged by an anodic oxidation method, and it is difficult to observe crystal grains. Therefore, in a sample where a large amount of second phase particles appear, it is difficult to accurately measure the exact aspect ratios of crystal grains. In particular, crystal grains observed with near-black contrast in polarization observation are difficult to distinguish from second phases. From such a reason, it is preferable that the object of aspect ratio measurement is crystal grains that are observed with white contrast in crystal grain structure observation.
Next, with respect to each aluminum alloy plate material (blank), the number density (surface density) of second phase particles having an equivalent circle diameter of 5 μm to 10 μm was measured. This measurement was performed by the SEM observation (reflection electron image observation) of a cross-section perpendicular to the plate thickness direction. Each test specimen was observed in five fields of view, and the SEM photograph of each field of view (magnification: 1,000) was subjected to image analysis, thereby examining the surface density of second phase particles having an equivalent circle diameter of more than 5 μm.
The n-value and the local elongation of each aluminum alloy plate material (blank) were measured. A fin material cut into a strip shape (dimension: 35 mm wide×200 mm long) was molded and subjected to a tensile test at ordinary temperature, and the measurement of these physical property values was performed based on the results of the tensile test. For the n-value, a true stress-true strain was prepared based on a nominal stress-nominal strain curve obtained from the tensile test, and, according to the above formula (1), the n-value between two points from a nominal strain 0.9 times the nominal strain at maximum load to the nominal strain at maximum load was calculated. The local elongation was calculated from a nominal stress-nominal strain curve obtained from the tensile test. Incidentally, in the measurement test, when a plate material broke without showing a maximum load, its elongation at break was defined as the nominal strain at maximum load.
After the evaluation of metal structure and the measurement of physical property values described above were performed, each aluminum alloy plate material (blank) was subjected to an evaluation test. The evaluation test was performed for the evaluation of deformation resistance due to working and the evaluation of brazability.
For the evaluation test of deformability, an aluminum alloy plate material was subjected to 10% cold working to imitate working before brazing. Subsequently, the plate material was cut into a sag test specimen having a width of 16 mm and a protruding length of 50 mm, and a sag test was performed under conditions of 600° C.×3 min. As a result of the sag test, a sag amount of 20 mm or less was rated as “⊙”, more than 20 mm and 30 mm or less as “◯”, more than 30 mm and 40 mm or less as “Δ”, and more than 40 mm as “x”. In the Examples, when the result was “Δ” or higher, such a sample was determined to have excellent deformation resistance.
In the evaluation test of brazability, the produced aluminum alloy plate material was cut into a strip shape 20 mm in width and 300 mm in length and corrugated. The corrugated fin material was combined with an extruded flat tube of JIS A1050 and assembled in a minicore shape, and a fluoride-based flux was sprayed to the minicore, dried, and then maintained in a nitrogen atmosphere at a temperature of 600° C. for 3 minutes to perform brazing joining. A cross-section of the minicore after brazing was observed, and the presence of fillets at the joint between the fin and tube was examined to evaluate the brazability. At this time, a fillet formation of 100% was rated as “O”, less than 100% and 98% or more as “0”, 90% or more and less than 98% as “Δ”, and less than 90% as “x” In the Examples, when the result was “Δ” or higher, such a sample was determined to have excellent brazability. Incidentally, in this evaluation test of brazability, only for the alloys A24 to A26, brazing joining was performed in a vacuum without applying a flux, and the resulting brazability was evaluated.
In addition, in the aluminum alloy plate materials in the examples and comparative examples, the productivity in plate material production was previously evaluated. In the evaluation of productivity, when the load exceeded the equipment capacity in the hot rolling step, making it impossible to produce a plate material, or when cracking occurred in the cold rolling step, making it impossible to produce a plate material, the productivity was rated as “x”. When plate material production was possible, the productivity was rated as “O”.
The measurement results and the evaluation results of the aluminum alloy plate materials of the examples and comparative examples are shown in Table 6 to Table 10. From these tables, regarding the configuration and effects of the present invention, the following examination results were obtained.
From Table 6 to Table 9, in all the aluminum alloy materials of Examples 1 to 72 having the various conditions specified in the present invention, the crystal grain structure is a fibrous structure, and the number density (surface density) of second phase particles having an equivalent circle diameter of 5 μm to 10 μm, the n-value, and the local elongation all satisfy the conditions.
Then, in these examples, the deformation resistance after working was excellent. In addition, there was no problem with productivity, and the brazability was also excellent.
Meanwhile, from Table 10, in Comparative Example 1 to Comparative Example 10 (alloys B1 to B10), the composition or the metal structure or physical property value (n-value) attributable to the production conditions was outside the range specified by the present invention. As a result, they were poor in terms of deformation resistance evaluation or brazability, specifically as follows.
In Comparative Example 1, the amount of Si is too small. Accordingly, the liquid filler metal was insufficient, and the brazability was x.
In Comparative Example 2, the amount of Si is too large. Accordingly, the melting or erosion of the core material was conspicuous, and the deformation resistance after working was x.
In Comparative Example 3, the amount of Mn is too small. Accordingly, the coarsening of crystal grains after brazing was insufficient, and the deformation resistance after working was x.
In Comparative Example 4, the amount of Mn is too large. Accordingly, a coarse intermetallic compound was generated at the time of casting. Thus, it became difficult to perform rolling, and the productivity was x.
In Comparative Example 5, the amount of Fe is too large. Accordingly, a coarse intermetallic compound was generated at the time of casting. Thus, it became difficult to perform rolling, and the productivity was x.
As described above, in Comparative Example 1 to Comparative Example 5, the characteristics were insufficient due to the composition.
In addition, in Comparative Example 6, the temperature of the annealing step (intermediate annealing) was high. Accordingly, the crystal grain structure was not a fibrous structure but a recrystallized structure. As a result, the deformation resistance after working was x. In addition, the local elongation of this alloy was as low as less than 1%.
Also in the alloy of Comparative Example 7, the temperature of the annealing step is high. This temperature is the final annealing temperature in this comparative example. Because of the high annealing temperature, the structure is a recrystallized structure, and the deformation resistance after working was x. The local elongation of this alloy was also low.
From these comparative examples, it was confirmed that when the temperature of an annealing step is high, and recrystallization takes place, the crystal grain structure of such an alloy material changes from a fibrous structure (unrecrystallized) to a recrystallized structure, and the deformation resistance after working decreases.
In Comparative Example 8, the heating temperature in the heating step before hot rolling was high, and the number density (surface density) of second phase particles having an equivalent circle diameter of 5 μm to 10 μm exceeded the suitable range. Because of the excessive generation of coarse second phase particles, recrystallized grains were refined during brazing, and the deformation resistance after working was x.
Considering the results of Comparative Example 8 together with the results of Comparative Examples 6 and 7 described above, it can be seen that in an aluminum alloy material having a heat-joining function in monolayer, in order to ensure deformation resistance at the time of brazing after working, the adjustment of the crystal grain structure (fibrous structure) and the suppression of the density of second phase particles having an equivalent circle diameter of 5 μm to 10 μm (1,000/mm2 or less) are both necessary.
Then, in Comparative Example 9, the heating temperature of the heating step before hot rolling is too low. Accordingly, it becomes difficult to perform rolling, and the productivity was x. In Comparative Example 10, the cold rolling ratio (final cold rolling ratio) is high, and the n-value is too low. Accordingly, the deformation resistance after working was x.
As described above, the present invention is an aluminum alloy material having a heat-joining function in monolayer, which is a material having improved deformation resistance at the time of brazing after working as compared with the conventional art. This deformation resistance is maintained even when the reduction ratio of working before brazing is high. The present invention is useful as a constituent material for various aluminum material products, such as heat exchangers and heat sinks. Then, because the material has a heat-joining function in monolayer, products can be supplied at lower cost as compared with the application of a brazing sheet or a preplaced brazing material.
| Number | Date | Country | Kind |
|---|---|---|---|
| P2017-222428 | Nov 2017 | JP | national |
