Patent Application
20210040586
The present invention relates to an aluminum alloy fin stock for a heat exchanger excellent in strength, electric conductivity, corrosion resistance, and brazability, and a heat exchanger.
An aluminum alloy fin stock for a heat exchanger for a vehicle is required to have high thermal electric conductivity and corrosion resistance, as well as strength that can withstand repeated vibrations when mounted on a vehicle. Furthermore, there is a demand for brazability that does not cause joining defects due to buckling of the fin stock during brazing. Therefore, research on a fin stock for a heat exchanger excellent in strength, electric conductivity, corrosion resistance, and brazability has been advanced.
For example, for the purpose of realizing excellent brazability and sag resistance at low costs even with a composition having an Fe content of 0.5% or more, Patent Literature 1 proposes a fin stock including, by mass %, Si: 0.6% to 1.6%, Fe: 0.5% to 1.2%, Mn: 1.2% to 2.6%, Zn: 0.4% to 3.0%, Cu: less than 0.2%, and a balance consisting of inevitable impurities and Al, in which Mg as an impurity is limited to less than 0.05%, the fin stock has a tensile strength before brazing heating of 160 to 260 MPa, and the difference between the tensile strength before brazing heating and a 0.2% proof stress is 10 to 50 MPa.
Patent Literature 2 proposes an aluminum alloy fin stock for a heat exchanger excellent in corrugation formability and strength after brazing heating, including Si: 0.5 to 1.5 mass %, Fe: more than 1.0 mass percent and 2.0 mass % or less, Mn: 0.4 to 1.0 mass %, Zn: 0.4 to 1.0 mass %, and a balance consisting of Al and inevitable impurities, in which the size and distribution density of second phase compounds as the metallographic structure before brazing heating are defined, and a tensile strength before brazing heating, a tensile strength after brazing heating, and the sheet thickness of the fin stock are defined.
Patent Literature 3 proposes an aluminum alloy fin stock for a heat exchanger excellent in high strength, heat-transfer property, erosion resistance, sag resistance, sacrificial anode effect, and self-corrosion resistance, having a composition including Si: 0.7 to 1.4 wt %, Fe: 0.5 to 1.4 wt %, Mn: 0.7% to 1.4 wt %, Zn: 0.5% to 2.5 wt %, Mg as an impurity limited to 0.05 wt % or less, and a balance consisting of inevitable impurities and Al, in which a tensile strength and proof stress after brazing, a recrystallization grain size after the brazing, and a electric conductivity after the brazing are defined.
Patent Literature 4 describes, as an aluminum alloy fin stock for a heat exchanger excellent in strength, electric conductivity and brazability, a fin stock made of an aluminum alloy having a composition including, by mass %, Mn: 1.2% to 2.0%, Cu: 0.05% to 0.20%, Si: 0.5% to 1.30%, Fe: 0.05% to 0.5%, Zn: 1.0% to 3.0%, and a balance consisting of Al and inevitable impurities, in which the fin stock has a tensile strength of 140 MPa or more, a proof stress of 50 MPa or more, a electric conductivity of 42% IACS or more, an average grain size of 150 μm or more and less than 700 μm, and an electric potential of −800 mV or more and −720 mV or less, after brazing heating.
Japanese Unexamined Patent Application, First Publication No. 2015-218343
Japanese Unexamined Patent Application, First Publication No. 2015-14034
Japanese Unexamined Patent Application, First Publication No. 2012-211393
Japanese Unexamined Patent Application, First Publication No. 2016-121393
However, when a brazing time is further shortened to improve productivity, the ratio of jointing defects between the fin and each member increases for the reason that the Al—Si braze hardly spreads over the entire heat exchanger, and the fin is deformed due to thermal expansion from other members and cannot maintain its shape. In addition, in order to obtain required rigidity even in a case where the weight of the heat exchanger is reduced, the strength of the fin stock after brazing is necessary, and in order to sufficiently exhibit heat dissipation performance, self-corrosion resistance is also required for preventing perforation and peeling off due to corrosion of the fin is also required.
The present invention has been made in view of the above problems, and an object thereof is to provide an aluminum alloy fin stock for a heat exchanger excellent in strength, electric conductivity, corrosion resistance, and brazability, and a heat exchanger.
In the present invention, by paying attention to an alloy composition and temperature and strength in a softening process during brazing, it is possible to obtain a fin having fewer joining defects and higher brazability than in the related art.
That is, a first aspect of aluminum alloy fin stocks for a heat exchanger excellent in strength, electric conductivity, corrosion resistance, and brazability of the present invention is made of an aluminum alloy having a composition including, by mass %, Mn: 1.2% to 2.0%, Si: 0.5% to 1.3%, Cu: 0.001% to less than 0.05%, Fe: 0.1% to 0.5%, Zn: 0.5% to 2.5%, and a balance consisting of Al and inevitable impurities, in which, at a room temperature after brazing heating, the aluminum alloy fin stock has a tensile strength of 140 MPa or more, a 0.2% proof stress of 50 MPa or more, a electric conductivity of 42% IACS or more, an electric potential of −800 mV or more and −710 mV or less, and a corrosion weight loss of 120 mg/dm2 or less after 16 weeks in a neutral salt spray test.
In an invention of an aluminum alloy fin stock for a heat exchanger excellent in strength, electric conductivity, corrosion resistance, and brazability of another aspect, in the aspect, the aluminum alloy further includes, by mass %, one or two or more of Ti: 0.01% to 0.20%, Cr: 0.01% to 0.20%, Mg: 0.01% to 0.20%, and Zr: 0.01% to 0.20%.
In an invention of an aluminum alloy fin stock for a heat exchanger excellent in strength, electric conductivity, corrosion resistance, and brazability of another aspect, in the aspect, the aluminum alloy has a composition satisfying Relational Expression (i) 2.1≤[Mn content (mass %)]+[Si content (mass %)]+7.5×[Cu content (mass %)]≤3.4, and Relational Expression (ii) [Zn content (mass %)]−18.8×[Cu content (mass %)]≥0.2.
In an invention of an aluminum alloy fin stock for a heat exchanger excellent in strength, electric conductivity, corrosion resistance, and brazability of another aspect, in the aspect, the aluminum alloy fin stock has an average grain size of 100 μm or more and 2000 μm or less after brazing heating.
In an invention of an aluminum alloy fin stock for a heat exchanger excellent in strength, electric conductivity, corrosion resistance, and brazability of another aspect, in the aspect, the aluminum alloy fin stock has a 0.2% proof stress in a range of 15 to 40 MPa at each temperature in a range of 400° C. to 550° C.
In an invention of an aluminum alloy fin stock for a heat exchanger excellent in strength, electric conductivity, corrosion resistance, and brazability of another aspect, in the aspect, before brazing heating, a number density of Al—Mn-based, Al—Mn—Si-based, and Al—Fe—Si-based second phase compounds having an equivalent circle diameter of 0.01 to 0.10 μm is 1.0×105 points/mm2 or more, and a metallographic structure is a fibrous grain structure.
In an invention of an aluminum alloy fin stock for a heat exchanger excellent in strength, electric conductivity, corrosion resistance, and brazability of another aspect, in the aspect, after brazing heating, a number density of Al—Fe-based crystallized compounds having an equivalent circle diameter of 1.0 μm or more is 1.0×104 points/mm2 or less, Al—Mn-based, Al—Mn—Si-based, and Al—Fe—Si-based second phase compounds having an equivalent circle diameter of 0.01 to 0.10 μm are present in 1.0×104 points/mm2 or more, and Al—Cu-based second phase compounds having an equivalent circle diameter of 0.05 pin or more are present in 1.0×103 points/mm2 or less.
In an invention of an aluminum alloy fin stock for a heat exchanger excellent in strength, electric conductivity, corrosion resistance, and brazability of another aspect, in the aspect, the aluminum alloy fin stock has a sheet thickness of 100 μm or less.
In an invention of an aluminum alloy fin stock for a heat exchanger excellent in strength, electric conductivity, corrosion resistance, and brazability of another aspect, in the aspect, the aluminum alloy fin stock has a corrosion current density of 0.05 mA/cm2 or less.
In an invention of an aluminum alloy fin stock for a heat exchanger excellent in strength, electric conductivity, corrosion resistance, and brazability of another aspect, in the aspect, before brazing heating, the aluminum alloy fin stock has a tensile strength at a room temperature of 250 MPa or less, and a 0.2% proof stress at a room temperature of 230 MPa or less.
In an invention of an aluminum alloy fin stock for a heat exchanger excellent in strength, electric conductivity, corrosion resistance, and brazability of another aspect, in the aspect, before brazing, the aluminum alloy fin stock has a recrystallization completion temperature of 450° C. or lower.
A first aspect of heat exchangers of the present invention includes the aluminum alloy fin stock for a heat exchanger of the present invention.
Hereinafter, the reasons for limiting the chemical composition, mechanical properties, and the like in the present invention will be described. The chemical compositions are all in mass %.
Mn is added to cause an Al—Mn—Si-based intermetallic compound to precipitate and obtain strength after brazing by dispersion strengthening. When Mn is contained in less than 1.2%, the effect of dispersion strengthening by the Al—Mn—Si-based compound is insufficient, and a desired strength after brazing cannot be obtained. When Mn is added in more than 2.0%, there is concern that Al—Mn-based giant intermetallic compounds may be crystallized during casting of an ingot, leading to fractured during rolling. In addition, the solid solubility in the matrix increases, the solidus temperature (melting point) decreases, and there are cases where fins are melted during brazing, which is not preferable. Therefore, the Mn content is set to the above range.
For the same reason, it is preferable to set the lower limit of the Mn content to 1.4% and the upper limit to 1.8%.
Si is added to cause an Al—Mn—Si-based intermetallic compound to precipitate and obtain strength after brazing by dispersion strengthening. When Si is added in less than 0.5%, the effect of dispersion strengthening by the Al—Mn—Si-based compound is insufficient, and a desired strength after brazing cannot be obtained. When Si is added in more of 1.3%, the solid solubility in the matrix increases, the solidus temperature (melting point) decreases, and there are cases where fins are melted during brazing, which is not preferable. Therefore, the Si content is set to the above range.
For the same reason, it is desirable to set the lower limit of the Si content to 0.7% and the upper limit to 1.2%.
Cu: 0.001% to less than 0.05%
Cu is present as a solid solution in the Al matrix or as an Al—Cu-based compound. When Cu is contained in less than 0.001%, the solid solution strengthening contributes little to the strength after brazing. On the other hand, when Cu is contained in 0.05% or more, a θ-CuAl2 stable phase or a θ′-CuAl2 metastable phase having an electric potential higher than that of the matrix is present as a compound, becomes an origin of corrosion, and deteriorates corrosion resistance, which is not preferable. Therefore, the Cu content is set to the above range.
For the same reason, it is desirable to set the lower limit of the Cu content to 0.003% and the upper limit to 0.045%.
Fe is added to cause Al—Fe-based and Al—Fe—Si-based intermetallic compounds to crystallize and precipitate and obtain strength after brazing by dispersion strengthening. When Fe is contained in less than 0.1%, the effect is insufficient, and the desired strength after brazing cannot be obtained. In addition, since Fe is limited to the use of high-purity base metal, the cost increases, which is not preferable. On the other hand, when Fe is contained in more than 0.5%, the Al—Fe-based and Al—Fe—Si-based compounds act as an origin of corrosion and deteriorate the corrosion resistance, which is not preferable. Therefore, the Fe content is set to the above range.
For the same reason, it is desirable to set the lower limit of the Fe content to 0.15% and the upper limit to 0.4%.
Zn has an effect of being dissolved in the Al matrix and lowering the electric potential, and is added to obtain the sacrificial anode effect of the fin. However, when Zn is contained in less than 0.5%, the action of lowering the electric potential is insufficient, the desired sacrificial anode effect cannot be obtained, and the erosion depth of a combined tube becomes large. On the other hand, when Zn is contained in more than 2.5%, the electric potential becomes excessively low and the self-corrosion resistance of the fin is deteriorated, which is not preferable. Therefore, the Zn content is set to the above range.
For the same reason, it is desirable to set the lower limit of the Zn content to 0.7% and the upper limit to 2.2%.
One or two or more of Ti: 0.01% to 0.20%, Cr: 0.01% to 0.20%, Mg: 0.01% to 0.20%, and Zr: 0.01% to 0.20%
Ti, Cr, Mg, and Zr form an intermetallic compound with aluminum and improve the strength by dispersion strengthening and solid solution strengthening, so that one or more thereof are contained as desired. However, when the contents are each less than the lower limit, the effect on dispersion strengthening and solid solution strengthening is insufficient, and the effect of improving strength is insufficient. When Ti, Cr, and Zr are contained in more than the respective upper limits, there is concern that giant intermetallic compounds may be crystallized during casting of an ingot, leading to fractured during rolling. Furthermore, when Mg is contained in more than the upper limit, the brazability is deteriorated. Therefore, it is desirable that the amount of each element is within the above range.
For the same reason, it is more desirable to set the lower limit of Ti, Cr, Mg, and Zr to 0.03% and the upper limit to 0.15%.
Room Temperature Tensile Strength after Brazing Heating: 140 MPa or more
In response to the demand for a reduction in the weight of a heat exchanger, a fin stock is also required to be thin and become a high strength material. When the strength of the fin after brazing is low, repeated vibrations applied to the heat exchanger when mounted on a vehicle, and expansion and compression of cooling water cannot be suppressed, and a tube expands in a shape of an oblong cylinder or a cylinder, leading to early rupture, that is, leakage of internal cooling water. Therefore, in a case where the thickness of the fin is set to 100 μm or less, it is desirable that the fin has a tensile strength of 140 MPa or more.
Room Temperature 0.2% Proof Stress after Brazing Heating: 50 MPa or more
A 0.2% proof stress indicates the elastic limit of the fin. In a case where the proof stress after brazing is low, repeated vibrations when mounted on a vehicle cause plastic deformation even without rupturing the fin, and the core of the heat exchanger is deformed without being retained in its original shape. Even if the sheet thickness of the fin is 100 μm or less, the above deformation can be prevented when the proof stress after brazing is 50 MPa or more. Therefore, it is desirable that the 0.2% proof stress after brazing heating is 50 MPa or more.
Electric Conductivity after Brazing hHating: 42% IACS or More
In order to secure thermal electric conductivity in a case where the fin is used as a heat exchanger, it is desirable that the electric conductivity after brazing is set to 42% IACS or more.
Electric Potential after Brazing Heating: −800 mV or More and −710 mV or less (vs Ag/AgCl)
In a case where the electric potential of the fin is less than −800 mV, the electric potential of the fin is excessively low compared to other members to be joined, so that corrosion of the fin is accelerated by galvanic corrosion. In a case where the electric potential of the fin exceeds −710 mV, a sufficient electric potential difference cannot be obtained for other members to be joined, and the sacrificial anode effect cannot be obtained. In this case, for example, the corrosion of a tube is accelerated. The electric potential is more preferably set to −720 mV or less.
Therefore, it is desirable that the electric potential of the fin stock is within the above range.
Corrosion Weight Loss after 16 Weeks in a Neutral Salt Spray Test after Brazing Heating: 120 mg/dm2 or Less
In order to secure the self-corrosion resistance of the fin stock, it is desirable that the corrosion weight loss of the fin stock after 16 weeks measured by a neutral salt spray test of the method according to JIS Z 2371 (2015) is 120 mg/dm2 or less. When the corrosion weight loss after 16 weeks is 120 mg/dm2 or less, it is possible to suppress the performance deterioration and partial peeling due to the corrosion of the fin itself even in an actual use environment, and thus maintain the characteristics as a heat exchanger.
2.1≤[Mn content (mass %)]+[Si content (mass %)]+7.5×[Cu content (mass %)]≤3.4 Relational Expression (i)
[Zn content (mass %)]−18.8×[Cu content (mass %)]≥0.2 Relational Expression (ii)
By satisfying Relational Expressions (i) and (ii), an aluminum alloy fin stock for a heat exchanger excellent in strength, electric conductivity, and corrosion resistance can be obtained.
Relational Expression (i) is a relationship between the amounts of Mn and Si with respect to the amount of Cu, and represents the material strength of the fin stock. In a case where the result of Relational Expression (i) is less than 2.1, the 0.2% proof stress at a high temperature during brazing, and the tensile strength and the 0.2% proof stress at a room temperature are low, and the fin joint ratio tends to decrease. In a case where the result of Relational Expression (i) exceeds 3.4, the tensile strength and the 0.2% proof stress before brazing are high, and there are cases where it is difficult to form fins, or there are many fins that have a low solidus temperature and a large corrosion weight loss.
Relational Expression (ii) is an expression representing an electric potential in the relationship between the amount of Zn and the amount of Cu. Cu is an element that makes the electric potential of aluminum high, Zn is an element that makes the electric potential low, and each greatly contributes to the electric potential. By controlling the ratio therebetween, the electric potential can be adjusted to a target range. However, it has been found that the relationship is not linear but needs to satisfy the above relational expressions.
When the result of Relational Expression (ii) is 0.2 or more, a necessary and sufficient electric potential difference for the tube is provided, and a desired sacrificial anode effect can be obtained.
Average grain size after brazing heating: 100 μm or more and less than 2000 μm When the average grain size after brazing is as small as less than 100 μm, brazing erosion (erosion) is likely to occur through grain boundaries as paths and the fins are likely to buckle. On the other hand, in a case where the grains after brazing are as coarse as 2000 pirn or more, the strength of the fin is reduced as represented by the Hall-Petch relation (a relational expression of grain size on proof stress value). In particular, in the case of a thin fin, it is necessary to control the grain size to be in a range in consideration of brazability and high strength.
For this reason, it is desirable that the grain size after brazing heating is within the above range.
0.2% proof stress at 400° C. to 550° C.: 15 to 40 MPa
When the 0.2% proof stress value at a high temperature of 400° C. to 550° C. during brazing heating is 15 MPa or more, the fin can maintain its shape after forming even against stress generated due to thermal expansion of other members during brazing, so that the deformation of the fin stock during brazing can be prevented. On the other hand, it has been found as a result of verification that in a case where the fin has a 0.2% proof stress of more than 40 MPa in the range of 400° C. to 550° C., the strength is greatly reduced in the process of recovery and recrystallization during brazing to reach 0 temper, so that the amount of deformation against external pressure is large, and a gap is generated between the tube and the fin, which is likely to result in joining defects. Therefore, it is desirable that the 0.2% proof stress at 400° C. to 550° C. is within the above range.
Number density of Al—Mn-based, Al—Mn—Si-based, and Al—Fe—Si-based second phase compounds having an equivalent circle diameter of 0.01 to 0.10 μm: 1.0×105 points/mm2 or more
The dispersion state and metallographic structure of intermetallic compounds before brazing mainly have a great effect on the recrystallization behavior during brazing. Fine second phase compounds of 0.01 to 0.10 μm prevent the formation of dislocation cells due to recovery during an initial stage of brazing and also have an action of impeding the movement of sub-grain boundaries, so that the recrystallization temperature becomes a relatively high temperature. Accordingly, the fine second phase compounds have an effect of contributing to coarsening of the grain size. In addition, when a reduction rate before brazing is high and plastic strain is accumulated to cause the metallographic structure to become a fibrous crystal structure (in the present invention, grains having an average aspect ratio of 7.0 or more in an observation visual field are defined as having a fibrous crystal structure, recrystallization occurs at a low temperature during brazing. In the present invention, by balancing the effect of lowering the recrystallization temperature with the fibrous grain structure and the distribution state of the second phase compounds of 0.01 to 0.10 μm, the recrystallization temperature during brazing heating and the material strength are controlled. After brazing heating, Al—Mn-based, Al—Mn—Si-based, and Al—Fe—Si-based second phase compounds having an equivalent circle diameter of 0.01 to 0.10 μm are present in 1.0×104 points/mm2 or more
The state of the intermetallic compound after brazing heating affects the material strength of the fin which contributes to dispersion strengthening. In a structure in which Al—Mn-based, Al—Mn—Si-based, and Al—Fe—Si-based second phase compounds are present in 1.0×104 points/mm2 or more, high material strength can be obtained after brazing.
After brazing, the number density of Al—Fe-based crystallized compounds having an equivalent circle diameter of 1.0 μm or more is 1.0×104 points/mm2 or less, and Al—Cu-based second phase compounds of 0.05 μm or more are present in 1.0×103 points/mm2 or less
Al—Fe-based crystallized compounds and Al—Cu-based second phase compounds have a higher electric potential than the matrix, act as an origin of corrosion, and thus causes a decrease in the self-corrosion resistance of the fin. Therefore, it is desirable to control the Al—Fe-based crystallized compounds of 1.0 μm or more to 1.0×104 points/nun2 or less, and the amount of the Al—Cu-based second phase compounds of 0.05 pm or more to 1.0×103 points/mm2 or less.
Sheet Thickness: 100 μm or less
In order to achieve a reduction in the weight of the core of the heat exchanger, the sheet thickness of the fin is desirably 100 μm or less, and the effect of improving the strength becomes significant. The lower limit thereof is desirably set to 30 μm.
Corrosion Current Density: 0.05 mA/cm2 or less
When the corrosion current density exceeds 0.05 mA/cm2, the corrosion rate is high, and when the corrosion current density is 0.05 mA/cm2 or less, the fin has a low corrosion rate and excellent self-corrosion resistance. For this reason, it is desirable that the corrosion current density is 0.05 mA/cm2 or less.
Before Brazing Heating, Tensile Strength at Room Temperature: 250 MPa or less, 0.2% Proof Stress at Room Temperature: 230 MPa or less
The fins are formed into a coil shape or with a multi-slit, and then subjected to die forming, for example, formed into a corrugated shape. The formed fin stock is brazed in combination with other members for the heat exchanger. At this time, when the tensile strength at a room temperature is 250 MPa or more and the 0.2% proof stress is 230 MPa or more before brazing heating, bending deformation is not easy, and it is difficult to obtain fins having a correct shape.
A fin stock having a higher solidus temperature is more easily brazed. In the case of a normal brazing method, at 615° C. or higher, brazing can be performed without melting the fins.
By defining the distribution of the intermetallic compounds before brazing and causing the metallographic structure to be a fibrous crystal structure, the fin during brazing heating can be softened at 450° C. or lower. Under the condition that the fin is recrystallized at 450° C. or lower, the 0.2% proof stress at each temperature between 400° C. and 550° C. can be in a range of 15 to 40 MPa, so that the joining defects during brazing can be reduced.
According to the present invention, it becomes possible to provide an aluminum alloy fin stock for a heat exchanger having fewer joining defects and higher brazability than in the related art, and a heat exchanger.
Hereinafter, an embodiment of the present invention will be described.
First, a method for manufacturing an aluminum alloy fin stock will be described.
For example, the aluminum alloy fin stock may be manufactured by performing semi-continuous casting (DC method) of molten metal, a homogenization treatment of an ingot, hot rolling, and cold rolling, or can be manufactured by performing casting using continuous casting rolling (CC method) with a twin roll casting machine or the like, a homogenization treatment of a cast sheet, and cold rolling.
A molten aluminum alloy containing, by mass %, Mn: 1.2% to 2.0%, Si: 0.5% to 1.3%, Cu: 0.001% to less than 0.05%, Fe: 0.1% to 0.5%, and Zn: 0.5% to 2.5%, and as desired, further containing, by mass %, one or two or more of Ti: 0.01% to 0.20%, Cr: 0.01% to 0.20%, Mg: 0.01% to 0.20%, and Zr: 0.01% to 0.20% is produced, and an ingot or cast sheet of the aluminum alloy is obtained by a normal method such as a direct chill casting (DC) method or a continuous casting (CC) method.
In the composition, regarding the Cu, Mn, Si, and Zn contents, it is desirable that
Relational Expression (i) 2.1≤[Mn content (mass %)]+[Si content (mass %)]+7.5×[Cu content (mass %)]≤3.4, and Relational Expression (ii) [Zn content (mass %)]−18.8 ×[Cu content (mass %)]≥0.2 are satisfied.
It is necessary to perform a homogenization treatment on the ingot or cast sheet of the obtained aluminum alloy under appropriate conditions. For example, the homogenization treatment is performed under the heat treatment conditions of a heating rate of 25 to 75° C./hr, a holding temperature of 350 to 480° C., a holding time of 1 to 10 hours, and a cooling rate of 20 to 50° C./hr. By setting the composition range of Mn, Si, and Cu shown in Relational Expression (i) and performing the homogenization treatment in this range, dispersion strengthening and solid solution strengthening are well balanced, and a desired fin strength can be obtained before brazing, during brazing, and after brazing.
Thereafter, the obtained aluminum alloy is subjected to hot rolling and cold rolling in the DC method, and subjected to cold rolling in the CC method. In a case where hot rolling is performed in the DC method, the hot rolling needs be performed at a temperature equal to or lower than the temperature of the homogenization treatment, and the balance between dispersion strengthening and solid solution strengthening needs be maintained. During the cold rolling, process annealing is performed after the reduction rate reaches 60% or more. The process annealing is performed at a temperature of 200° C. to 300° C. for a holding time of 6 hours, and cold rolling is performed at a reduction rate of 10% to 25% after the process annealing, whereby an aluminum alloy fin stock which has a fibrous crystal structure before brazing heating and has a desired thickness is obtained. The sheet thickness is desirably set to 30 to 100 μm.
Through the above steps, a fin stock for a heat exchanger can be obtained.
The obtained fin stock is excellent in strength, electric conductivity, corrosion resistance, and brazability, and is suitable as a fin stock for a heat exchanger.
In particular, since the fin stock has a 0.2% proof stress in a range of 15 to 40 MPa at each temperature between 400° C. and 550° C. in the softening process during brazing, the fin stock can maintain its shape after forming even against stress generated due to thermal expansion of other members during brazing, so that deformation during brazing can be prevented.
Furthermore, in a case where the fin stock is subjected to brazing heating, the fin stock after brazing heating has, at a room temperature, a tensile strength of 140 MPa or more, a 0.2% proof stress of 50 MPa or more, a electric conductivity of 42% IACS or more, an electric potential of −800 mV or more and −710 mV or less, a corrosion weight loss of 120 mg/dm2 or less after 16 weeks in a neutral salt spray test, a corrosion current density is 0.05 mA/cm2 or less, and an average grain size of 100 μm or more and less than 2000 μm after brazing heating, and is thus excellent in strength, electric conductivity, and corrosion resistance.
Furthermore, a heat exchanger can be manufactured by corrugating the obtained fin stock into fins and performing brazing in combination with members for a heat exchanger such as a header, a tube, and a side plate. In the present invention, the heat treatment conditions and method of brazing (brazing temperature, atmosphere, presence or absence of flux, kind of brazing material, and the like) are not particularly limited, and brazing can be performed by a desired method.
Since the obtained heat exchanger includes the fin stock of the present embodiment, brazing is good and the heat exchanger is excellent in strength, electric conductivity, and corrosion resistance.
According to the present embodiment, it is possible to obtain an aluminum alloy fin stock for a heat exchanger excellent in strength, electric conductivity, corrosion resistance, and brazability and a heat exchanger.
Hereinafter, examples of the present invention will be described. An aluminum alloy ingot or a cast sheet was produced from a molten metal adjusted to have the components (Al and inevitable impurities as the balance) shown in Table 1. As shown in Table 2, the obtained ingot or cast sheet was subjected to a homogenization treatment at a heating rate of 25 to 75° C./hr, a holding temperature of 350° C. to 480° C., a holding time of 1 to 10 hours, and a cooling rate of 20 to 50° C./hr, and thereafter subjected to hot rolling and cold rolling in this order in the DC method or cold rolling in the CC method.
During the cold rolling, process annealing is performed after the reduction rate reached 60% or more. Regarding the process annealing and the subsequent cold rolling, in Examples 1 to 45 and Comparative Examples 1 to 17, 20, 22, and 24 to 37, the process annealing of performing holding at 200° C. to 300° C. for 6 hours was performed to obtain a fibrous crystal structure, and thereafter the cold rolling was performed at a reduction rate (10% to 25%) shown in Table 2. In Comparative Examples 18, 19, 21, and 23, after performing process annealing of performing holding at 350° C. for 6 hours to obtain a recrystallization structure, cold rolling was performed at a reduction rate (25% to 40%) shown in Table 2. Accordingly, H14 grade fin stocks having the sheet thicknesses shown in Table 3 were produced.
The following measurement was performed on the obtained fin stocks. The results are shown in Tables 3 and 4.
For the test material of the obtained fin stocks, the solidus temperature, the tensile strength at a room temperature, the 0.2% proof stress at a room temperature, and the number density of second phase compounds having an equivalent circle diameter of 0.01 to 0.10 μm, and the crystal structure were measured. The measurement method is as follows. The measurement results are shown in Table 3.
The solidus temperature of the fin stock was measured using a differential thermal analyzer (DTA).
(Strength before Brazing at Room Temperature)
A sample was cut out in parallel to the rolling direction to produce a JIS 13 B-shaped test piece, and the tensile strength and 0.2% proof stress of the test piece were measured by conducting a tensile test at a room temperature under the conditions of a tensile speed of 5 mm/min.
For the test material of the fin stocks, the number density (points/mm2) of the second phase compounds (with an equivalent circle diameter of 0.01 to 0.10 μm) was measured by a transmission electron microscope (TEM). As a measurement method, the material was subjected to salt bath annealing at 400° C. for 15 seconds before brazing to remove deformation strain and cause the compound to be easily observed, thereafter a thin film was produced by performing mechanical polishing and electrolytic polishing in a normal method, and a photograph was taken at a magnification of 50,000 times using a transmission electron microscope. Photographing was performed for each of five visual fields, and the size and number density of the second phase compounds were measured by image analysis of the photographs.
Assuming the strength of the fin stock during brazing heating, the 0.2% proof stress at 400° C. to 550° C. was measured. The recrystallization temperature of the fin stock was also measured. The measurement method is as follows. The measurement results are shown in Table 3.
(0.2% Proof Stress during Brazing Heating)
A sample was cut out from the fin stock before brazing in parallel to the rolling direction and machined into a JIS No. 5 shape to prepare a test piece, the test piece was put into a preheated thermostat, and a high temperature tensile test was conducted immediately after the test piece reached each of temperatures of 400° C., 450° C., and 550° C. The tensile speed in the high temperature tensile test was set to 1 mm/min, and the 0.2% proof stress at a high temperature was measured.
The temperature was raised from a room temperature to 600° C. at a constant rate (100° C./min) assuming brazing heating, and after reaching each of the predetermined temperatures, cooling was performed to a room temperature. After the cooling, the surface of the sample was observed, and the temperature at which 80% or more of the surface of the fin stock having a surface area of 300 mm2 was recrystallized was defined as the recrystallization temperature.
The fin stock was subjected to a heat treatment equivalent to brazing, and for the fin stock after the heating, at a room temperature, the tensile strength, 0.2% proof stress, electric conductivity, average grain size, electric potential, corrosion weight loss, corrosion current density, the number density of Al—Fe-based crystallized compounds having an equivalent circle diameter of 1.0 μm or more, the number density of second phase compounds having an equivalent circle diameter of 0.01 to 0.10 μm, and the number density of Al—Cu-based second phase compounds having an equivalent circle diameter of 0.05 μm or more were calculated.
Furthermore, in order to evaluate the brazability, the fin stock was corrugated and subjected to a brazing heat treatment in combination with other members, and the fin joint ratio was calculated by observing joint parts. The brazing heat treatment conditions and the measurement and evaluation methods for each item are as follows. The measurement results are shown in Table 4.
The temperature was raised from a room temperature to 600° C. at an average heating rate of 50° C./min, and held at 600° C. for 3 minutes, and then a heat treatment equivalent to brazing was performed under the conditions of a heat treatment of performing cooling at a cooling rate of 100° C./min.
(Tensile Strength and 0.2% Proof Stress after Brazing)
A sample was cut out in parallel to the rolling direction from a sample that had been subjected to the heat treatment equivalent to brazing to produce a JIS 13 B-shaped test piece. A tensile test was conducted on the test piece at a room temperature, and the tensile strength and 0.2% proof stress thereof were measured. The tensile speed was set to 5 nun/min.
The electric conductivity was measured with a double-bridge electric conductivity meter according to the electric conductivity measurement method described in JIS H 0505.
For the test material that has been subjected to the heat treatment equivalent to brazing, the sample surface was etched with a mixed solution of hydrochloric acid, hydrofluoric acid, and nitric acid to expose grains, a photograph of the surface was taken, and the average grain size was measured by a linear line intercept method using the taken photograph of the surface grain structure.
A sample for electric potential measurement was cut out from the fin stock that had been subjected to the above heat treatment equivalent to brazing, the sample was immersed in a 5% NaOH solution heated to 50° C. for 30 seconds, then immersed in a 30% HNO3 solution for 60 seconds, further washed with tap water and ion-exchanged water, and then immersed as it is without being dried in a 5% NaCl solution at 25° C. (adjusted to a pH of 3 with acetic acid) for 60 minutes, and thereafter the electric potential was measured. A silver-silver chloride electrode (Ag/AgCl) was used as a reference electrode.
A neutral salt spray test (NSS) was conducted by the method according to J1S Z 2371. A sample of 120 mm×40 mm was cut out from the fin stock, three samples were put into a corrosive environment under one condition, and the corrosion weight loss was obtained from the weight difference before and after the test. The test liquid was 5% NaCl, the pH of the test liquid was in a range of 6.5 to 7.2, and the test tank temperature was 35±2° C.
A test piece of 15 mm×60 mm was produced for the test material that had been subjected to the heat treatment equivalent to brazing. A measurement area of 1 cm2 of the produced test piece was exposed, the other area was protected by masking, the same pretreatment as in the electric potential measurement (immersion in a 5% NaOH solution heated to 50° C. for 30 seconds, subsequent inunersion in a 30% HNO3 solution for 60 seconds, and washing with tap water and ion-exchanged water) was performed, and polarization measurement was performed. In the polarization measurement, the test piece was immersed in a 5% NaCl solution (adjusted to a pH of 3 with acetic acid) at 25° C. for 5 minutes to stabilize the spontaneous electric potential, and anodic polarization measurement was thereafter performed by increasing the electric potential at a sweep rate of 0.5 mV/s to obtain an anodic polarization curve.
Furthermore, cathodic polarization measurement was performed by decreasing the electric potential from the spontaneous electric potential to obtain a cathodic polarization curve. The current density at the intersection of the anodic polarization curve and the cathodic polarization curve was defined as the corrosion current density.
For the test material that had been subjected to the heat treatment equivalent to brazing, the number densities (points/mm2) of Al—Fe-based crystallized compounds (having an equivalent circle diameter of 1.0 μm or more), Al—Cu-based second phase compounds (having an equivalent circle diameter of 0.05 μm or more), and Al—Mn-based, Al—Mn—Si-based, and Al—Fe—Si-based second phase compounds (having an equivalent circle diameter of 0.01 to 0.10 μm) were measured by a transmission electron microscope (TEM). As the measurement method, a thin film was produced by performing mechanical polishing and electrolytic polishing in a normal method, and Al—Fe-based crystallized compounds and Al—Cu-based, Al—Mn-based, Al—Mn—Si-based, and Al—Fe—Si-based second phase compounds were photographed at magnifications of 3,000 times and 50,000 times, respectively, using the transmission electron microscope. Photographing was performed for each of five visual fields, and the size and number density of the intermetallic compounds were measured by image analysis of the photographs.
The produced fin stock was corrugated, assumed in combination with other members (header plates, tubes, and side plates), and then brazed by applying a flux, whereby a heat exchanger having a size of 50 cm×50 cm was produced. Thereafter, the joint parts between the fins and the tubes of the heat exchanger were observed, the number of defective joint parts was obtained, and the good fin joint ratio was calculated as (1-(defective joint parts/all joint parts))×100 (%). A joint ratio of 95% or more was evaluated as A (good joined state), a joint ratio of 90% to 95% was evaluated as B (necessary and sufficient joined state), and a joint ratio of 90% or less was evaluated as C (defect joint).
As shown in Tables 1 to 4, all of Examples 1 to 45 having the compositions and characteristics specified in the present invention were excellent in strength, electric conductivity, corrosion resistance, and brazability (fin joint ratio), whereas, in Comparative Examples 1 to 37 which did not satisfy any one or more of the regulations of the present invention, good results were not obtained in any one or more of strength, electric conductivity, corrosion resistance, brazability, and the like.
For the above examples and comparative examples,
Regarding Relational Expression (i), the calculated values of Examples 6, 17, 24, 27, 29, 30, 35, and 36 and some of the comparative examples were less than 2.1. In addition, as shown in Tables 3 and 4, the 0.2% proof stress at a high temperature during brazing and the tensile strength and 0.2% proof stress at a room temperature were low, and the fin joint ratio tended to decrease. Furthermore, in Examples 3, 15, 18, 21, and 31 and some of the comparative examples, the calculated value of Relational Expression (i) also exceeded 3.4, so that there were cases where the tensile strength before brazing and the 0.2% proof stress were high and it was difficult to form fins, or the fin stocks had a low solidus temperature and thus had a large corrosion weight loss.
Regarding Relational Expression (ii), in Examples 3, 4, 17, 18, 20, 33, 42, and 45 and some of the comparative examples, the calculated values were less than 0.2, and as shown in Tables 3 and 4, the electric potential of the fin stocks was not in a more preferable range.
1: Heat exchanger
2: Header
3: Tube
4: Fin
5: Side plate
| Number | Date | Country | Kind |
|---|---|---|---|
| 2018-014557 | Jan 2018 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2018/046965 | 12/20/2018 | WO | 00 |
