JP7469072B2 - Aluminum alloy forgings and their manufacturing method - Google Patents

Description

The present invention relates to an aluminum alloy forging material used to manufacture aluminum alloy forgings with excellent heat resistance, and a method for manufacturing the same.

Various proposals have been made for aluminum alloy forging materials. For example, Patent Document 1 proposes an aluminum alloy that contains 0.1-2.0 wt% Si, 1.0-2.0 wt% Fe, 2.0-6.0 wt% Cu, 1.0-3.0 wt% Mg, 3.0 wt% or less Ni, 0.01-0.2 wt% Ti, where Si+Fe+Mg≧3.1 wt%, the balance being Al and unavoidable impurities, and has electrical conductivity of 25.0% IACS or more and 40.0% IACS or less.

JP 2017-214655 A

In general, many rotating and linear components, such as engines, compressors, and turbocharger impellers, are used continuously at high temperatures. Therefore, the aluminum alloy forgings that are the raw materials for these components are particularly required to have good creep properties. The aluminum alloy proposed in Patent Document 1 is classified as an AA2618 aluminum alloy, which has excellent heat resistance. It is indicated that the tensile strength of the aluminum alloy proposed in Patent Document 1 at 250°C is greater than that of JIS 4032 alloy. However, there is still room for improvement in terms of improving the creep properties of the aluminum alloy proposed in Patent Document 1.

The objective of the present invention is to provide an aluminum alloy forging material with excellent creep properties and a method for manufacturing the same.

As a result of intensive research and development into aluminum alloy forgings made of AA2618 aluminum alloy to provide excellent creep properties, the inventors discovered that the above problems could be solved by controlling the total content of Fe and Ni and the total content of Mn, Cr, and Zr, as well as controlling the size of the intermetallic compounds and the uniformity of their dispersion, and thus completed the present invention.

In other words, the aluminum alloy forging material according to the present invention is an aluminum alloy forging material consisting of Si: 0.10-0.25 mass%, Fe: 0.9-1.3 mass%, Cu: 1.9-2.7 mass%, Mg: 1.3-1.8 mass%, Zn: 0.10 mass% or less, Ni: 0.9-1.2 mass%, Ti: 0.01-0.1 mass%, the balance being Al and unavoidable impurities, in which the total content of Fe and Ni is 2.2 mass% or less, the total content of Mn, Cr, and Zr is 0.20 mass% or less, the average circle equivalent diameter of the intermetallic compounds is 4.5 μm or less, and the variation in the distance between the intermetallic compounds in the ST direction is 2.3 or less.

The method for producing an aluminum alloy forging according to the present invention is a method for producing an aluminum alloy forging having the above alloy composition, in which a molten metal having the above alloy composition is cooled at a cooling rate of 1.2°C/sec or more to cast an ingot, and the ingot is subjected to homogenization treatment, hot forging, solution treatment, quenching, and aging treatment to produce the aluminum alloy forging.

The aluminum alloy forged material according to the present invention has excellent creep properties.
Furthermore, the method for producing an aluminum alloy forging according to the present invention can produce an aluminum alloy forging having excellent creep properties.

FIG. 1 is a schematic diagram illustrating a state in which a molten metal temperature is measured in the case of a semi-continuous casting method (DC casting method (Direct Chill casting process)). FIG. 2 is a schematic cross-sectional view illustrating a process for determining a linear approximation between a mold temperature and a cooling rate of a molten metal in a mold casting method. 1 is a graph showing an example of a mold temperature, a cooling rate of a molten metal, and a linear approximation equation, in which the horizontal axis indicates the mold temperature (° C.) and the vertical axis indicates the cooling rate of the molten metal (° C./sec.). FIG. 2 is an explanatory diagram illustrating a mold, an ingot removed from the mold, and a portion from which a sample for a forging test is prepared from the ingot. 1 is a plan view of a forged material produced by hot forging, in which the hatched areas indicate the positions at which test pieces used for tensile tests and creep tests were cut out. 1 is a side view of a forged material produced by hot forging, in which the hatched areas indicate the positions at which test pieces used for tensile tests and creep tests were cut out. 1 is a graph showing the mold temperature, the cooling rate of the molten metal, and a linear approximation equation, in which the horizontal axis shows the mold temperature (° C.) and the vertical axis shows the cooling rate of the molten metal (° C./sec.). 1 is a graph showing the relationship between the total content (mass%) of Mn, Cr, and Zr and the total content (mass%) of Fe and Ni for examples and comparative examples. In the figure, examples are plotted with "◆" and comparative examples are plotted with "□". 1 is a graph showing the relationship between the average equivalent circle diameter (μm) of intermetallic compounds and the variation in the distance between intermetallic compounds in the ST direction for examples and comparative examples. In the figure, examples are plotted with "◆" and comparative examples are plotted with "□". 1 is a graph showing the relationship between the variation in the distance between intermetallic compounds in the ST direction and the rupture time (hr) in a creep test for the examples and the comparative examples. In the figure, the examples are plotted with "◆" and the comparative examples are plotted with "□". 1 is a graph showing the relationship between the average equivalent circle diameter (μm) of intermetallic compounds and the rupture time (hr) in a creep test for examples and comparative examples. In the figure, examples are plotted with "♦" and comparative examples are plotted with "□".

One embodiment of the present invention will be described below with reference to the drawings as appropriate. Note that the drawings referred to in the following description are schematic illustrations of one embodiment of the present invention, and therefore the scale, spacing, and positional relationships of each component may be exaggerated, or some components may not be shown. In this specification, "~" is used to mean that the numerical values before and after it are the lower and upper limits. The upper and lower limits described in stages in this specification may be replaced with other upper and lower limits described in stages, or may be replaced with numerical values shown in the examples.

[Aluminum alloy forgings]
First, one embodiment of an aluminum alloy forging will be described.
The aluminum alloy forging according to the present embodiment has a predetermined alloy composition as an aluminum alloy and is used to manufacture a forged product (aluminum alloy part). The aluminum alloy forging has the following alloy composition and structural characteristics.

That is, the aluminum alloy forging material is composed of 0.10-0.25 mass% Si, 0.9-1.3 mass% Fe, 1.9-2.7 mass% Cu, 1.3-1.8 mass% Mg, 0.10 mass% or less Zn, 0.9-1.2 mass% Ni, 0.01-0.1 mass% Ti, the balance being Al and unavoidable impurities, the total content of Fe and Ni is 2.2 mass% or less, and the total content of Mn, Cr, and Zr is 0.20 mass% or less. Furthermore, the aluminum alloy forging material is configured such that the average circle equivalent diameter of the intermetallic compounds is 4.5 μm or less, and the variation in the distance between the intermetallic compounds in the ST direction is 2.3 or less. The variation in the distance between the intermetallic compounds in the ST direction applies to all observable intermetallic compounds.

(Alloy Composition)
The aluminum alloy having the above alloy composition belongs to the AA2618 aluminum alloy. The AA2618 aluminum alloy is used for structural members that require high-temperature properties. The alloy composition will be described below. These components are generally known, as described in, for example, Japanese Patent No. 6348466 and Japanese Patent No. 5879181, and have the following effects.

(Si: 0.10 to 0.25 mass%)
Si, together with Mn, precipitates fine dispersed phases such as Al-Mn-Si compounds, enhances the pinning effect of dislocations, and inhibits the coarsening of recrystallized grains during solution treatment, thereby improving the strength of the aluminum alloy forging. The Si content is in the range of 0.10 to 0.25 mass%. If the Si content is less than 0.10 mass%, the effect of improving strength is not sufficiently obtained. If the Si content exceeds 0.25 mass%, compounds of Mg and Si are formed, and the heat resistance is reduced. The lower limit of the Si content is preferably 0.13 mass%, more preferably 0.15 mass%. The upper limit of the Si content is preferably 0.23 mass%, more preferably 0.21 mass%.

(Fe: 0.9 to 1.3 mass%)
Fe forms Fe-Ni compounds together with Ni, improving the heat resistance of the aluminum alloy forging. The Fe content is in the range of 0.9 to 1.3 mass%. If the Fe content is less than 0.9 mass%, the effect of improving heat resistance is not sufficiently obtained. If the Fe content exceeds 1.3 mass%, Fe compounds such as Al-Fe and Al-Fe-Cu compounds dispersed in the matrix are significantly formed, so that the effect of improving heat resistance is reduced. The lower limit of the Fe content is preferably 1.0 mass%. The upper limit of the Fe content is preferably 1.2 mass%.

(Cu: 1.9 to 2.7 mass%)
Cu contributes to improving the strength of aluminum alloy forgings at room temperature and high temperature. Cu is an essential element for ensuring room temperature yield strength and heat resistance, which are mainly required for Al alloys in the applications of the present invention, due to the action of both solid solution strengthening and precipitation strengthening. More specifically, Cu combines with Al and Mg during high temperature artificial aging treatment, and causes fine and dense precipitation of G.P.B. zones and S' phases, etc., to improve the strength of the Al alloy after artificial aging treatment. The Cu content is in the range of 1.9 to 2.7 mass%. If the Cu content is less than 1.9 mass%, the effect of improving strength cannot be sufficiently obtained. If the Cu content exceeds 2.7 mass%, the eutectic melting start temperature decreases and the solution treatment temperature must be lowered, so the amount of solid solution in the parent phase decreases and the effect of improving strength cannot be expected. The lower limit of the Cu content is preferably 2.0 mass%, more preferably 2.1 mass%. The upper limit of the Cu content is preferably 2.6 mass %, and more preferably 2.5 mass %.

(Mg: 1.3 to 1.8% by mass)
Mg coexists with Cu and contributes to improving the strength of aluminum alloy forgings at room temperature and high temperature. Mg is an essential element for ensuring room temperature yield strength and heat resistance, which are mainly required for Al alloys in the applications of the present invention, due to the effects of both solid solution strengthening and precipitation strengthening. More specifically, Mg combines with Al and Cu during high temperature artificial aging treatment, and precipitates G.P.B. zones and S' phases finely and densely, thereby improving the strength of the Al alloy after artificial aging treatment. The Mg content is in the range of 1.3 to 1.8 mass%. If the Mg content is less than 1.3 mass%, the effect of improving strength is small. If the Mg content exceeds 1.8 mass%, the deformation resistance of the material increases during hot working such as forging, and productivity decreases. The lower limit of the Mg content is preferably 1.4 mass%, more preferably 1.5 mass%. The upper limit of the Mg content is preferably 1.7 mass%, more preferably 1.6 mass%.

(Zn: 0.10 mass% or less)
Zn is an element that is often contained as an unavoidable impurity. Zn is also one of the additive elements that can improve the room temperature and high temperature strength of aluminum alloy forgings by solid solution strengthening and precipitation strengthening. However, Zn is particularly harmful to corrosion resistance. Therefore, the Zn content is set to 0.10 mass% or less. Since the effect of improving the room temperature and high temperature strength of aluminum alloy forgings can be sufficiently obtained by adding Cu and Mg, Zn does not need to be contained (the Zn content may be 0 mass%). The upper limit of the Zn content is preferably 0.09 mass%, more preferably 0.08 mass%, and even more preferably 0.05 mass%.

(Ni: 0.9 to 1.2 mass%)
Ni forms Fe-Ni compounds together with Fe, improving the heat resistance of the aluminum alloy forging. The Ni content is in the range of 0.9 to 1.2 mass%. When the Ni content is less than 0.9 mass%, the effect of improving heat resistance is not sufficiently obtained. When the Ni content exceeds 1.2 mass%, Ni compounds such as Al-Ni and Al-Ni-Cu compounds dispersed in the matrix are formed, so that the effect of improving heat resistance is reduced. When the Ni content exceeds 1.2 mass%, coarse intermetallic compounds such as Fe-Ni compounds are formed. Therefore, in this case, cracks are likely to occur during hot working such as forging, and productivity is reduced. The lower limit of the Ni content is preferably 0.95 mass%, more preferably 1.0 mass%. The upper limit of the Ni content is preferably 1.18 mass%, more preferably 1.1 mass%.

(Ti: 0.01 to 0.1 mass%)
Ti, like Zr, is added to stably obtain a fine crystal grain structure. The Ti content is in the range of 0.01 to 0.1 mass%. If the Ti content is less than 0.01 mass%, the effect of stabilizing the fine crystal grain structure is not sufficiently obtained. If the Ti content exceeds 0.1 mass%, huge Zr-Ti compounds are formed during casting, reducing the strength. The lower limit of the Ti content is preferably 0.04 mass%. The upper limit of the Ti content is preferably 0.09 mass%.

(Remainder)
The balance is Al and inevitable impurities. In actual operation, inevitable impurities may be inevitably contained due to raw materials used. In addition to the above-mentioned Zn, inevitable impurities include, for example, Mn, Cr, Zr, V, etc. V does not cause any particular problem if it is 0.05 mass% or less. Mn, Cr, and Zr will be described later. Note that Mn, Cr, Zr, and V do not interfere with the effects of the present invention not only when they are contained as inevitable impurities but also when they are actively added, as long as they do not exceed the predetermined content or total content described in this specification.

(Total content of Fe and Ni: 2.2 mass% or less)
Fe and Ni improve the heat resistance of aluminum alloy forgings, but when the total content exceeds 2.2 mass%, a large number of intermetallic compounds such as Fe-Ni intermetallic compounds are formed. Therefore, the area ratio and number density of the intermetallic compounds in a given surface (cross section) are high. Examples of intermetallic compounds that appear when the total content of Fe and Ni exceeds 2.2 mass% include Al-Cr-Cu-Fe-Mn-Si, Fe-Ni, Mg-Si, and Al-Cu-Fe intermetallic compounds. In addition, when the total content of Fe and Ni exceeds 2.2 mass%, the dispersion of the intermetallic compounds becomes biased due to segregation. Then, under the influence of this, the variation in the distance between the intermetallic compounds in the ST direction increases. The specific numerical range of the variation in the distance between the intermetallic compounds in the ST direction will be described later. A large variation in the distance between the intermetallic compounds in the ST direction means that the dispersion of the intermetallic compounds is not uniform, which reduces the creep characteristics. Therefore, the total content of Fe and Ni is set to 2.2 mass% or less. The upper limit of the total content of Fe and Ni is preferably 2.1 mass%, more preferably 2.0 mass%. The predetermined plane (cross section) may be, for example, a cross section perpendicular to the metal flow (L direction) of the aluminum alloy forging or test piece. The ST direction refers to the thickness direction in which the aluminum alloy is most deformed by the forging process.

(Total content of Mn, Cr, and Zr: 0.20 mass% or less)
As described above, Mn, Cr, and Zr are included as unavoidable impurities. Mn, Cr, and Zr refine the metal structure (microstructure) of the aluminum alloy forging material and suppress recrystallization. However, if the total content of Mn, Cr, and Zr exceeds 0.20 mass%, they segregate during casting or promote the nucleation and growth of crystallized matter, so that the dispersion of the intermetallic compounds becomes biased. As a result, for example, the variation in the distance between the intermetallic compounds in the ST direction increases. And, the creep characteristics are deteriorated because the dispersion of the intermetallic compounds is not uniform. Therefore, the total content of Mn, Cr, and Zr is set to 0.20 mass% or less. The upper limit of the total content of Mn, Cr, and Zr is preferably 0.15 mass%.

(Microstructure)
(Average circle equivalent diameter of intermetallic compounds: 4.5 μm or less)
If the average equivalent circle diameter of the intermetallic compound exceeds 4.5 μm, the creep properties are degraded. Therefore, the average equivalent circle diameter of the intermetallic compound is set to 4.5 μm or less. The average equivalent circle diameter of the intermetallic compound is preferably 4.2 μm or less, and more preferably 4.0 μm or less. The average equivalent circle diameter of the intermetallic compound is significantly affected by the cooling rate of the molten metal when casting the ingot. The cooling rate of the molten metal when casting the ingot will be explained in the section on the manufacturing method.

(Variation in distance between intermetallic compounds in ST direction: 2.3 or less)
The variation (standard deviation) of the distance between intermetallic compounds in the ST direction is calculated and evaluated as the variation of the shortest distance between adjacent intermetallic compounds in the ST direction in the observed cross section. In other words, this variation represents the uniformity of the dispersion of intermetallic compounds in the ST direction. The smaller this variation, the more uniformly the intermetallic compounds are dispersed, which means a good quality microstructure. The variation of the distance between intermetallic compounds in the ST direction can be calculated using formula (1).

If the variation in the distance between intermetallic compounds in the ST direction exceeds 2.3, the uniformity of the dispersion of the intermetallic compounds is low (i.e., the intermetallic compounds are unevenly distributed), and the creep properties deteriorate. Therefore, the variation in the distance between intermetallic compounds in the ST direction is set to 2.3 or less. This variation is preferably 2.13 or less, and more preferably 2.02 or less.

(Observation of microstructure (average circle equivalent diameter, etc.))
The average equivalent circle diameter of the intermetallic compounds and the distance between the intermetallic compounds in the ST direction can be grasped by observing the microstructure as follows.
For example, to observe the microstructure, a test piece of about 10 mm square is cut out from a forged material that has been subjected to T61 tempering treatment, mechanically polished and buffed, and then the metal structure of a cross section perpendicular to the metal flow (L direction) is observed in 2 to 3 fields of view at a magnification of 100 times using an optical microscope (Nikon Eclipse MA200). From the observed microstructure photograph, the grain size in the ST direction is measured using a sectioning method, and the average circle equivalent diameter, area ratio in the field of view, number density, etc. of the intermetallic compounds are calculated by image analysis using analysis software.
In addition, the distances between all the intermetallic compounds that can be observed are observed and measured in the ST direction, and the variation in the distances is evaluated in terms of the standard deviation. The intermetallic compounds that can be observed are, for example, those with a grain size of 0.4 μm or more.
In the embodiment, WinROOF2018 manufactured by Mitani Shoji Co., Ltd. was used as the analysis software, but the analysis software is not limited to this.

As explained above, the aluminum alloy forging material has a specified alloy composition, the total content of Fe and Ni and the total content of Mn, Cr, and Zr are each set to a specified value or less, and the average circle equivalent diameter of the intermetallic compounds and the variation in the distance between the intermetallic compounds in the ST direction are each set to a specified value or less, thereby achieving excellent creep properties.

[Production method]
Next, an embodiment of a method for producing an aluminum alloy forging will be described. In the following description of the production method, detailed description of items that have already been described will be omitted.
The method for producing an aluminum alloy forging according to this embodiment is a method for producing an aluminum alloy forging having the alloy composition described above.
In this manufacturing method, a molten metal having the above-mentioned alloy composition is cooled at a cooling rate of 1.2°C/sec or more to cast an ingot, and the ingot is subjected to homogenization treatment, hot forging, solution treatment, quenching, and aging treatment within a preset temperature range, thereby producing an aluminum alloy forging.

(Cooling rate of molten metal: 1.2°C/sec or more)
The cooling rate of the molten metal when casting an ingot has a large effect on the size of the intermetallic compounds (e.g., Fe-Ni intermetallic compounds). The cooling rate of the molten metal is 1.2°C/sec or more. If the cooling rate of the molten metal is less than 1.2°C/sec, it takes too long for the molten metal to solidify, so that the dissolved Fe, Ni, etc. precipitate at grain boundaries, causing the intermetallic compounds to grow larger. As a result, the size of the intermetallic compounds increases. The cooling rate of the molten metal is preferably 1.8°C/sec or more, more preferably 3°C/sec or more, and even more preferably 10°C/sec or more.

(Measurement and calculation of the cooling rate of molten metal)
The cooling rate of the molten metal can be measured or calculated, for example, by the following methods in {1} to {3} in the case of the mold casting method and the DC casting method. In this manufacturing method, the cooling rate of the molten metal calculated in advance by the methods in {1} to {3} can be used as the cooling rate condition of the molten metal when casting an ingot.

<<1>> Method for measuring and calculating the cooling rate of molten metal in the case of metal mold casting One method for measuring and calculating the cooling rate of molten metal in the case of metal mold casting involves directly measuring the temperature of the poured molten metal using a digital recorder connected to a thermocouple and calculating the cooling rate of the molten metal from the results.

<2> Method for measuring and calculating the cooling rate of the molten metal in the case of DC casting Methods for measuring and calculating the cooling rate of the molten metal in the case of DC casting include the following: Here, Fig. 1 is a schematic diagram for explaining the state of measuring the molten metal temperature in the case of DC casting.
As shown in FIG. 1, a thermocouple 12 is installed in a DC casting device 11. The thermocouple 12 is installed about 10 mm away from a mold 13 of the DC casting device 11, at a position 16 (a position slightly below the surface of the molten metal 14 in the mold 13) where the molten metal 14 is molten aluminum before it solidifies to become an ingot 15. The mold 13 is a water-cooled mold, and water W is injected from inside the mold 13 toward the molten aluminum. A wire 18 is fixed to the bottom of a casting table 17. The wire 18 is fixed so that its length is parallel to the direction in which the casting table 17 is pulled down. The thermocouple 12 is fixed to the wire 18 to prevent the thermocouple 12 from shifting in the horizontal direction. The thermocouple 12 is connected to a digital recorder 19, and the temperature of the molten metal 14 is directly measured. That is, the thermocouple 12 is pulled down by the casting table 17 together with the wire 18 fixed to the bottom of the casting table 17, and the temperature is continuously and directly measured until the molten metal 14 solidifies to become the ingot 15. The cooling rate (°C/sec) is calculated from the change in temperature per unit time when the molten metal temperature drops to the solidus (550°C).

<3> Other embodiments of the method for measuring and calculating the cooling rate of the molten metal in the case of the mold casting method The methods listed in <1> and <2> above use a thermocouple to directly measure the temperature of the molten metal, so that it is possible to accurately measure and calculate the cooling rate of the molten metal. Here, in the case of the mold casting method, in addition to the method listed in <1>, the cooling rate of the molten metal may be calculated (estimated) from the mold temperature and a preset correlation equation (linear approximation equation), as described below.

The linear approximation formula is obtained, for example, as follows: Here, Fig. 2 is a schematic cross-sectional view for explaining how to obtain a linear approximation formula between the mold temperature and the cooling rate of the molten metal in the mold casting method.
As shown in FIG. 2, a thermocouple 24 is arranged so that the temperature of the center position 23 of the space 22 into which the molten metal is poured in the mold 21 can be measured. Next, the mold 21 is heated to a predetermined temperature by a gas burner, and the mold temperature around the opening 25 of the mold 21 is measured by a contact thermometer. Then, the molten metal (for example, 720°C) melted and adjusted to the above-mentioned alloy composition is poured into the mold 21 to produce an ingot. The molten metal temperature is continuously recorded by a digital recorder 26 connected to the thermocouple 24. Then, the cooling rate (°C/sec) is calculated from the change in temperature per unit time when the molten metal temperature drops to the solidus (550°C). This is performed for three mold temperatures, for example, 200°C, 246°C, and 296°C, and the cooling rate of the molten metal at each mold temperature is calculated.
The cooling rate when the molten metal reaches the solidus is, for example, 3.25° C./sec when the mold temperature is 200° C.
The cooling rate when the molten metal reaches the solidus is, for example, 2.7° C./sec when the mold temperature is 246° C.
The cooling rate when the molten metal reaches the solidus is, for example, 2.2°C/sec when the mold temperature is 296°C.

Figure 3 is a graph showing an example of the mold temperature, the cooling rate of the molten metal, and the linear approximation formula. As shown in Figure 3, the linear approximation formula is obtained from the three points marked with x (the cooling rates of the three molten metals shown above). In the example shown in Figure 3, the linear approximation formula is y = -0.0109x + 5.4185. In this formula, y is the cooling rate of the molten metal (°C/sec), and x is the mold temperature (°C). The linear approximation formula obtained in this manner can be used to estimate the cooling rate of the molten metal. The estimated value of the cooling rate of the molten metal calculated using this linear approximation formula is approximately the same as the cooling rate calculated from the temperature of the molten metal measured using the direct measurement method described in {1} above.

The above describes the methods for measuring and calculating the cooling rate of the molten metal, but this embodiment is not limited to these, and the measurement and calculation may be performed using methods other than those described above.

(Other treatments in this manufacturing method)
Other treatments in the present manufacturing method, i.e., melt adjustment of the alloy composition of the molten metal, homogenization treatment, hot forging, solution treatment, quenching, and aging treatment, can be performed under general conditions that are performed, for example, on aluminum alloy forgings made of AA2618 aluminum alloy. For example, melt adjustment of the alloy composition of the molten metal can be performed at 700 to 760°C. Homogenization treatment can be performed at 450 to 550°C. Hot forging can be performed at 300 to 355°C. Solution treatment can be performed at 500 to 550°C. Quenching can be performed using water or oil at a temperature of 90 to 100°C, but can also be performed using water or oil at room temperature (about 25°C). Aging treatment can be performed at 170 to 220°C. In other words, the present manufacturing method can be performed by T6 treatment or T61 treatment, but is preferably performed by T61 treatment. By setting such manufacturing conditions, the present manufacturing method can manufacture the aluminum alloy forgings according to this embodiment. It should be noted that the present manufacturing method is not limited to the general conditions exemplified here.

[Aluminum alloy parts]
The aluminum alloy forged material according to this embodiment can be forged (hot or cold) to obtain a near-net shape, and then cut to produce rotating parts and linear parts (aluminum alloy parts) for engines, compressors, turbocharger impellers, and the like.

Next, the aluminum alloy forging material and its manufacturing method according to the present invention will be specifically described using examples. Figure 4 is an explanatory diagram illustrating a mold, an ingot removed from the mold, and a portion from which a sample for a forging test is made from the ingot. Figure 5 is a plan view of the forging material made by hot forging. Figure 6 is a side view of the forging material made by hot forging. The hatched areas in Figures 5 and 6 indicate the positions from which test pieces are cut out for use in tensile tests and creep tests, respectively.

Molten AA2618 aluminum alloy (720°C) adjusted to the alloy composition shown in No. 1 to 9 in Table 1 was used to produce ingots by die casting using a copper mold or by DC casting. The ingots produced using the copper mold (simply referred to as "mold" in Table 1) had a mass of approximately 8.5 kg, while the ingots produced using the DC casting method (simply referred to as "DC" in Table 1) had masses of approximately 17 kg and approximately 42 kg. Note that this embodiment is not limited to the general conditions exemplified here, and the same applies to ingots with a mass of 2 tons or more.

Then, as shown in FIG. 4, the ingot 42 removed from the mold 41 was cut and machined to prepare a sample 43 having a diameter of 100 mm and a height of 120 mm from approximately the center of the ingot.
Next, the sample 43 was subjected to homogenization heat treatment in an air furnace at 520° C. for 20 hours, and then cooled to room temperature.
Thereafter, the sample 43 was heated to 300 to 340° C. in an air furnace, removed from the furnace, and hot forged in a hydraulic forging press to a height of 30 mm (diameter approximately 200 mm) as shown in FIGS. 5 and 6 to produce a forged material 56.
Thereafter, the steel was subjected to a solution treatment at 530° C. for 6 hours in an air furnace, and then quenched in boiling water.
Next, the specimens were subjected to artificial aging treatment at 197° C. for 22 hours in an air furnace to obtain T61-treated test specimens.
From the test material thus prepared, test pieces for microstructure observation, tensile test, and creep test were cut out and used for observation and testing. The test pieces for the tensile test and creep test were cut out so as to be in the hatched portion in the L direction shown in FIG. 5 and the hatched portion in the center of the ST direction shown in FIG. 6. Here, the L direction refers to the direction of metal flow of the test material. In this embodiment and comparative example, the L direction corresponds to the length direction (radial direction) of the test material in a plan view. The ST direction refers to the thickness direction in which the aluminum alloy is most deformed by forging. In this embodiment and comparative example, the ST direction corresponds to the thickness direction of the test material.

In the case of permanent mold casting, the mold temperature was measured with a contact thermometer immediately before pouring the molten metal into the copper mold. The mold temperatures are shown in Table 1. Note that for DC casting, the mold temperature was not measured because a water-cooled mold was used.

The cooling rate (°C/sec) was calculated from the change in temperature per unit time when the molten metal temperature dropped to the solidus (550°C). Here, based on the contents explained in the embodiment (3), the cooling rates of the molten metal in the mold casting method were calculated for three mold temperatures of 200°C, 246°C, and 296°C.
As a result, the cooling rate when the molten metal (720°C) dropped to the solidus was calculated to be 3.25°C/sec when the mold temperature was 200°C, 2.7°C/sec when the mold temperature was 246°C, and 2.2°C/sec when the mold temperature was 296°C.

Figure 7 is a graph showing the mold temperature, the cooling rate of the molten metal, and the correlation equation (linear approximation equation). The calculated cooling rates of the three molten metals are indicated by x marks in Figure 7. A linear approximation equation was then obtained from these cooling rates of the three molten metals. As shown in Figure 7, the linear approximation equation was y = -0.0109x + 5.4185. Note that y is the cooling rate of the molten metal (°C/sec), and x is the mold temperature (°C). The cooling rate of the molten metal in the mold casting method can be calculated (estimated) using the mold temperature and this linear approximation equation, and the cooling rate of the molten metal in the mold casting method (mold) shown in Table 1 is a value calculated by this linear approximation equation. The estimated value of the cooling rate of the molten metal calculated by this linear approximation equation was almost the same as the cooling rate calculated from the temperature of the molten metal measured by the direct measurement method described in embodiment 1.

According to the graph of FIG. 7 and the linear approximation equation, when the mold temperature is 326° C., the cooling rate of the molten metal is estimated to be 1.87° C./sec., but in Table 1, this is rounded to one decimal place and shown as 1.9° C./sec.
Similarly, when the mold temperature is 332° C., the cooling rate of the molten metal is estimated to be 1.80° C./sec, but in Table 1 it is shown as 1.8° C./sec, rounded off to one decimal place.
Similarly, when the mold temperature is 336° C., the cooling rate of the molten metal is estimated to be 1.76° C./sec, but in Table 1 it is shown as 1.8° C./sec, rounded off to one decimal place.
Similarly, when the mold temperature is 393° C., the cooling rate of the molten metal is estimated to be 1.13° C./sec, but in Table 1 it is shown as 1.1° C./sec, rounded off to one decimal place.
As described above, when the mold temperature was 200° C., the cooling rate of the molten metal was 3.25° C./sec according to the actual measured values, the graph in FIG. 7, and the linear approximation equation. However, as described above, in Table 1, this value was rounded off to one decimal place to give a value of 3.3° C./sec.

On the other hand, based on the contents explained in embodiment 2 with reference to FIG. 1, the cooling rate of the molten metal in the DC casting method was measured and calculated. The cooling rate of the molten metal in the DC casting method is shown in Table 1.

(Observation of Microstructure)
The microstructure of the prepared T61 treated test material was observed as follows.
From the test material, a test piece of about 10 mm square was cut out, mechanically polished, buffed, and then the metal structure of the cross section perpendicular to the metal flow (L direction) of the test material was observed and photographed in 2 to 3 fields of view at 100 times magnification using an optical microscope (Nikon Corporation ECLIPSE MA200). The observed and photographed structural photographs were analyzed using analysis software (Mitani Shoji Co., Ltd. WinROOF2018). The crystal grain size (μm) was measured, and the average circle equivalent diameter (μm), area ratio (%) in the field of view, and number density (pieces/mm ² ) of the intermetallic compounds were calculated.
In addition, the distances between all the intermetallic compounds having an average equivalent circle diameter of 0.4 μm or more that could be observed were measured in the ST direction, and the variation in the distances was evaluated in terms of standard deviation. The standard deviation was calculated using the formula (1) described in the embodiment.
The results are shown in Table 2.

(tensile test, creep test)
A tensile test piece (test piece TP) having a total length of about 85 mm, a parallel portion length of 25 mm, and a parallel portion diameter of 6.35 mm in accordance with ASTM E8 and ASTM B357 was prepared from the position shown in the hatched portion in FIG. 5 and FIG. 6, and a tensile test was performed.
Also, a creep test piece (test piece TP) with a total length of about 90 mm, a parallel portion length of 40 mm, and a parallel portion diameter of 6 mm was prepared from the same position, and a creep test was performed according to JIS Z 2271:2010 at 180°C and a load stress of 220 MPa until the test piece broke. As a result of the creep test, a specimen with a break time of 220 hours or more was rated as pass (◯), and a specimen without such a break time was rated as fail (×). The creep test temperature and load stress, as well as the results of the creep test, i.e., creep characteristics (break time (hr)), are shown in Table 2.

(Larson-Miller Parameter (LMP))
When creep properties are applied to design criteria, the stress that causes a creep rate of 0.01% per 1000 hours, or the 100,000-hour fracture stress, is often used. However, it takes a long time to obtain these values. Therefore, the Larson-Miller parameter (LMP) is often used as a method for estimating long-term creep properties from short-term test results ("Basic Industrial Technology of Aluminum Materials," published by the Japan Aluminum Association, p. 307). LMP is defined as, for example, LMP = T (20 + Log (tr)), T: temperature (absolute temperature, K), tr: fracture time (hr). As this formula shows, low-temperature, long-term creep properties can be estimated from high-temperature, short-term creep test results. The LMP calculated from this formula is shown in Table 2 together with the creep test results.

Note that the underlines in Tables 1 and 2 indicate that the requirements or pass criteria are not met. The "-" in Table 1 indicates that the component is below the detection limit, and indicates that the mold temperature was not measured because a water-cooled mold is used in the DC casting method.

As shown in Tables 1 and 2, the aluminum alloy forgings according to Nos. 1 to 5 (all of which are examples) had excellent creep properties. In addition, the aluminum alloy forgings according to Nos. 1 to 5 all had tensile properties that satisfied the JIS standard.
The aluminum alloy forgings according to Nos. 6 to 9 satisfied the JIS standard in terms of tensile properties, but were inferior in creep properties (all of which are comparative examples).

The aluminum alloy forgings No. 1 to 9 that gave the above results are analyzed. Figures 8 to 11 are graphs of several items based on the obtained results. Figure 8 is a graph showing the relationship between the total content of Mn, Cr, and Zr and the total content of Fe and Ni for the examples and comparative examples. Figure 9 is a graph showing the relationship between the average circle equivalent diameter of the intermetallic compounds and the variation in the distance between the intermetallic compounds in the ST direction for the examples and comparative examples. Figure 10 is a graph showing the relationship between the variation in the distance between the intermetallic compounds in the ST direction and the rupture time in the creep test for the examples and comparative examples. Figure 11 is a graph showing the relationship between the average circle equivalent diameter of the intermetallic compounds and the rupture time in the creep test for the examples and comparative examples. In all of Figures 8 to 11, the examples are plotted with "◆" and the comparative examples are plotted with "□".

As shown in Figure 8, the examples have low total contents of Mn, Cr, and Zr, and low total contents of Fe and Ni. Also, as shown in Figure 9, the examples have low average circle-equivalent diameters of intermetallic compounds and low variations in the distance between intermetallic compounds in the ST direction. As shown in Tables 1, 2, Figures 10, and 11, in order for the rupture time of the creep test to pass, both the variation in the distance between intermetallic compounds in the ST direction and the average circle-equivalent diameter of the intermetallic compounds must be low.

Optical micrographs of the aluminum alloy forgings according to Examples No. 1 to No. 5 and Comparative Examples No. 6 to No. 9 were analyzed using analysis software.
As a result, in the aluminum alloy forgings according to Nos. 1 to 5, the intermetallic compounds were uniformly dispersed.
On the other hand, in the aluminum alloy forgings according to Nos. 6 to 9, the intermetallic compounds were either closely connected or distant from each other, and were unevenly dispersed.

From the above, it was confirmed that the aluminum alloy forgings according to Examples No. 1 to 5 have a total Fe and Ni content of 2.2 mass% or less, a total Mn, Cr, and Zr content of 0.20 mass% or less, an average circle equivalent diameter of the intermetallic compounds of 4.5 μm or less, and a variation in the distance between the intermetallic compounds in the ST direction of 2.3 or less. It was confirmed that by adopting such a configuration, the aluminum alloy forgings according to Examples No. 1 to 5 can be made to have excellent creep properties.

On the other hand, some of the aluminum alloy forgings Nos. 6 to 9 did not meet the requirements for the total content of Fe and Ni, the total content of Mn, Cr, and Zr, the average circle equivalent diameter of the intermetallic compounds, and the variation in the distance between the intermetallic compounds in the ST direction. Therefore, the aluminum alloy forgings Nos. 6 to 9 had poor creep properties.

Specifically, for the comparative examples, the aluminum alloy forgings according to Nos. 6 and 9 had a total content of Mn, Cr, and Zr exceeding 0.20 mass%, so the variation in the distance between intermetallic compounds in the ST direction exceeded 2.3. In other words, the intermetallic compounds in the aluminum alloy forgings according to Nos. 6 and 9 were not uniformly dispersed, but were unevenly distributed. As a result, the creep properties of the aluminum alloy forgings according to Nos. 6 and 9 were poor.

In addition, the aluminum alloy forgings according to Nos. 7 and 8 both had a total content of Fe and Ni exceeding 2.2 mass%.
Here, in the aluminum alloy forging material according to No. 7, the cooling rate of the molten metal was less than 1.2° C./sec, so that the average equivalent circle diameter of the intermetallic compounds exceeded 4.5 μm, and therefore the creep properties of the aluminum alloy forging material according to No. 7 were poor.
On the other hand, in the aluminum alloy forging according to No. 8, the cooling rate of the molten metal was 1.2°C/sec or more, so that the average equivalent circle diameter of the intermetallic compounds was 4.5 μm or less, but the variation in the distance between the intermetallic compounds in the ST direction exceeded 2.3. In the aluminum alloy forging according to No. 8, the intermetallic compounds were not uniformly dispersed, but were unevenly distributed. Therefore, the aluminum alloy forging according to No. 8 had poor creep properties.
From the results of the aluminum alloy forgings Nos. 7 and 8, it was found that the cooling rate of the molten metal has a large effect on the size of the intermetallic compounds. From these results, it was inferred that it is essential to appropriately control the cooling rate of the molten metal.

The aluminum alloy forging material and its manufacturing method according to the present invention have been described in detail above through embodiments and examples, but the gist of the present invention is not limited to these and should be interpreted broadly based on the scope of the claims.

Claims (2)

Si: 0.10 to 0.25 mass%;
Fe: 0.9 to 1.3 mass%,
Cu: 1.9 to 2.7 mass%,
Mg: 1.3 to 1.8% by mass,
Zn: 0.10% by mass or less,
Ni: 0.9 to 1.2 mass%,
An aluminum alloy forging material comprising Ti: 0.01 to 0.1 mass%, the balance being Al and unavoidable impurities,
An aluminum alloy forging having a total content of Fe and Ni of 2.2 mass% or less, a total content of Mn, Cr and Zr of 0.20 mass% or less, an average circle equivalent diameter of intermetallic compounds of 4.5 μm or less, and a variation in distance between intermetallic compounds in the ST direction of 2.3 or less. A method for producing an aluminum alloy forging according to claim 1,
13. A method for producing an aluminum alloy forging, comprising the steps of: cooling a molten metal having the alloy composition according to claim 1 at a cooling rate of 1.2° C./sec or more to cast an ingot; and subjecting the ingot to a homogenization treatment, hot forging, solution treatment, quenching, and aging treatment to produce an aluminum alloy forging.

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