JP2005029847A - Aluminum alloy for casting having excellent high temperature strength - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an aluminum alloy for casting which has excellent strength particularly at high temperatures. <P>SOLUTION: The aluminum alloy for casting having excellent high temperature strength has a composition comprising, by mass, 4.0 to 7.0% Si and 21 to 31.5% Cu, and the balance Al with inevitable impurities, and in which the Si content and the Cu content satisfy the following expressions (1) to (3): (Si mass%)≤-0.19×(Cu mass%)+11 (1); (Si mass%)≥-0.5×(Cu mass%)+17.5 (2); and (Si mass%)≥6×(Cu mass%)-184 (3). <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an aluminum alloy having excellent high temperature strength. In particular, the present invention relates to an aluminum alloy suitable for a piston, a brake disk rotor and the like of an internal combustion engine used at a high temperature and a manufacturing method thereof.

  Conventionally, when an aluminum alloy material used at a high temperature is produced by casting, an alloy obtained by adding Cu or Ni to a eutectic Al-Si alloy having excellent castability has been used. For example, JIS standard AC8A (12% Si-1% Mg-1% Cu-1% Ni-Al) alloy, or AC8A alloy further added with Cu (12% Si-1% Mg-4% Cu-1) % Ni—Al) alloys have been used. For example, Patent Document 1 by the applicant of the present invention proposes an alloy in which 6% Cu is added to increase the high temperature strength.

It is considered that the main cause of the decrease in the high temperature strength of the Al alloy is that the α-Al phase is softened. Conventional heat-resistant alloys improve the strength of the matrix by adding Cu or Mg, and Al-Cu-based crystals or Al-Ni-based crystals with relatively high high-temperature strength by adding Cu or Ni. The product was crystallized and the high temperature strength was increased by finely dividing the α-Al phase.
However, even in such an alloy with increased high-temperature strength, the mechanical strength sharply decreases when the use temperature exceeds 200 ° C., and is less than 1/2 of the normal temperature strength at 250 ° C., and 1/4 at 350 ° C. It may be reduced to the following.
JP-A-8-104937

In recent years, further high-temperature combustion of engines has progressed from the viewpoint of energy saving, and aluminum alloys have been required to have improved strength in a higher-temperature environment than before.
Accordingly, an object of the present invention is to provide an aluminum alloy for casting that is superior in high-temperature strength.

  As a result of intensive studies by the present inventors, by adopting the following specific alloy composition, the crystallized product is crystallized extremely finely and uniformly, and the α-Al phase is finely divided, and high high-temperature strength is obtained. In addition, the solid-liquid coexistence region is narrow, so that the crystallized product was crystallized almost at the same time and dispersed finely and uniformly, and the present invention was made.

Therefore, the aluminum alloy of the present invention contains Si: 2.0 to 7.0 mass%, Cu: 21.0 to 31.5 mass%, the balance is made of Al and inevitable impurities, and the Si amount and the Cu amount Satisfies the following relational expressions (1) to (3).
(Si mass%) ≦ −0.19 × (Cu mass%) + 11 (1)
(Si mass%) ≧ −0.5 × (Cu mass%) + 17.5 (2)
(Si mass%) ≧ 6 × (Cu mass%) − 184 (3)
The alloy of the present invention further includes Ti: 0.001 to 0.20%, B: 0.0005 to 0.02%, Zr: 0.0005 to 0.002%, V: 0.0005 to 0.02% Any one or more of them may be included.
An aluminum alloy cast material of this alloy composition shows a high high-temperature strength value as it is, but when cast and maintained at 150 to 240 ° C. for 2 to 8 hours, an aluminum alloy having higher high-temperature strength is obtained. A material can be obtained.

  The aluminum alloy according to the present invention contains a predetermined amount of Si and Cu, and by making the amount of Si and Cu a specific ratio, the strength in a high temperature environment exceeding 300 ° C. is particularly improved, The occurrence of microporosity (casting defects) can be reduced.

The 1st characteristic of the aluminum alloy which concerns on this invention is that 2.0-7.0 mass% Si and 21-31.5 mass% Cu are included.
For example, when 27% Cu is added to an Al alloy containing 5% Si and cast, as shown in the micrograph of FIG. 1, the crystallized product crystallizes extremely finely and the α-Al phase is finely divided. Has a structured structure. On the other hand, 12% Si-1% Mg-1% Cu-1% Ni-Al (AC8A) alloy, which is a conventional heat-resistant aluminum alloy, and 12% Si-1% Mg-4% Cu in which the amount of Cu is further increased. A micrograph of the microstructure of a 1% Ni—Al (hereinafter referred to as “AC8A + Cu”) alloy is shown in FIGS. 2 and 3, respectively. As is clear from comparison of these micrographs, the Al alloy of the present invention containing 5% Si and 27% Cu crystallizes much finer than the conventional Al alloy, and α-Al The phases are finely divided. As a result, it was found that extremely high temperature strength can be obtained.

Further, in order to crystallize the crystallized product finely and uniformly, it is preferable to narrow the solid-liquid coexistence region and to crystallize a large amount of crystallized product at the time of casting. This is because if the solid-liquid coexistence region is wide, the first crystallized product becomes coarse, and elements are used for the coarsening and other crystallized products cannot be formed.
The three kinds of Al alloys, namely 5% Si-27% Cu-Al alloy (invention), 12% Si-1% Mg-1% Cu-1% Ni-Al (AC8A) alloy, and 12% Si -1% Mg-4.3% Cu-1% Ni-Al (AC8A + Cu) alloy calculated by thermal equilibrium calculation, showing the amount of each solid phase crystallized from the liquid phase as a distribution with respect to temperature, Shown in FIGS.

In the case of the alloy of the present invention shown in FIG. 4, when the molten metal is cooled, the α-Al phase is first crystallized at 529 ° C., and a solid-liquid coexistence state in which the solid phase (α-Al phase) and the liquid phase coexist is obtained. . When the temperature reaches about 528 ° C., Al ₂ Cu crystallizes, and when cooled to about 524 ° C., only the solid phase is obtained.
In the case of the AC8A alloy shown in FIG. 5, when the molten metal is cooled, the α-Al phase first crystallizes at about 573 ° C., and a solid-liquid coexistence state in which the solid phase (α-Al phase) and the liquid phase coexist is obtained. . Next, when the temperature drops to about 568 ° C., Si crystallizes, and the temperature further decreases. At about 540 ° C., Al ₃ Ni and Mg ₂ Si crystallize, and when the temperature reaches about 535 ° C., only the solid phase It becomes a state.
In the case of the AC8A + Cu alloy shown in FIG. 6, when the molten metal is cooled, Si crystallizes at about 572 ° C., and a solid-liquid coexistence state in which the solid phase (Si) and the liquid phase coexist is obtained. Next, when the temperature is lowered to about 562 ° C., an α-Al phase is crystallized, and when the temperature is further lowered, β-AlFeSi, Al ₉ FeNi, Al ₅ Cu ₂ Mg ₈ Si ₆ , and Al ₃ Ni are crystallized. When the temperature further reaches about 519 ° C., only the solid phase is obtained.

  That is, as shown in these figures, in the AC8A alloy, which is a conventional high-temperature high-strength alloy, and the AC8A + Cu alloy in which Cu is further increased, their solid-liquid coexistence regions are 535-573 ° C. and 519-572 ° C., respectively. In contrast, in the alloy of the present invention, the solid-liquid coexistence region is very narrow at 524 to 529 ° C. That is, the Al alloy of the present invention containing 5% Si and 27% Cu has a very narrow solid-liquid coexistence region, so that the crystallized product crystallizes almost at the same time, and as a result, it is considered that finely and uniformly dispersed. .

The second feature of the aluminum alloy according to the present invention is that the Si content and the Cu content satisfy a predetermined relational expression.
FIG. 7 shows the amounts of Si and Cu constituting the aluminum alloy of the present invention. In FIG. 7, the horizontal axis (x axis) represents the mass% of Cu, and the vertical axis (y axis) represents the mass% of Si. The straight line described in the figure is represented by the following formula.
y = −0.19 × x + 11 (1 ′)
y = −0.5 × x + 17.5 (2 ′)
y = 6 × x−184 (3 ′)

A region surrounded by three straight lines represented by the above formulas (1 ′) to (3 ′), that is, when expressed by xy coordinates, approximately point A (21.0, 7.0), point B (31.5, 5.0) ) And a region surrounded by a triangle having the C point (31.0, 2.0) as a vertex represents a mass% range of Si and Cu constituting the aluminum alloy of the present invention.
That is, when Cu is 21 to 31% by mass, the range of Si mass% is −0.5Cu + 17.5 ≦ Si ≦ −0.19Cu + 11, and when Cu is 31 to 31.5% by mass, 6Cu−184 ≦ Si ≦ −0.19Cu + 11.

A region where the amount of Si is excessive (Si region in FIG. 7), a region where the amount of Cu is excessive (Al ₂ Cu region in FIG. 7), and Si and Cu are insufficient. It was found that microporosity occurs in any region (α-Al region in FIG. 7).

  The aluminum alloy of the present invention can obtain sufficient high-temperature strength even by a casting method with a slow cooling rate such as gravity casting, but when cast by a casting method with a fast cooling rate such as a die casting method, the crystallized substance is more Since it is finely and uniformly dispersed, a cast material with further excellent high-temperature strength can be obtained.

Hereinafter, the action of each component element constituting the aluminum alloy of the present invention will be outlined. In addition,% in this specification shall mean the mass%.
・ Si: 2.0-7.0%
In addition to mechanical strength, Si exhibits an effect of improving castability, wear resistance, low thermal expansion, and vibration resistance. Furthermore, in the case of the alloy of the present invention, it is finely and uniformly dispersed as a Si-based crystallized product, and the α-Al phase is divided to improve the high temperature strength. This effect becomes significant at 2.0% or more. On the other hand, if it exceeds 7.0%, primary crystal Si crystallizes and grows from the liquid phase, forming coarse particles, and particularly tends to reduce fatigue strength.

Cu: 21.0-31.5%
Cu strengthens the matrix by solid solution / precipitation, and finely and uniformly disperses it as an Al-Cu-based crystallized product, thereby improving the high-temperature strength. When the content is less than 21.0%, the solid-liquid coexistence region becomes wide and the crystallized product cannot be dispersed sufficiently finely and uniformly, and it is difficult to obtain sufficient high-temperature strength. Conversely, if it exceeds 31.5%, primary Al ₂ Cu crystallizes and grows from the liquid phase, forming coarse particles, and particularly tends to reduce fatigue strength.

・ Si% ≦ −0.19 × Cu% + 11
When Si% exceeds −0.19 Cu% + 11, primary crystal Si crystallizes and grows to form coarse particles, and the strength, particularly elongation, is remarkably lowered, and the machinability tends to deteriorate. Further, in casting, there may be a shrinkage accompanying segregation, which may deteriorate internal quality.
・ Si% ≧ −0.5 × Cu% + 17.5
If Si% is less than -0.5 Cu% + 17.5, the α-Al phase grows and lowers the high-temperature strength. During casting, the liquid phase remains between the grown α-Al phases and shrinkage occurs. Or trap gas trapping and tend to degrade internal quality.
・ Si% ≧ 6 × Cu% −184
When Si% is less than ₆ Cu% -184, coarse Al ₂ Cu crystallizes and decreases strength, particularly elongation. Also, castability tends to deteriorate.

Ti: 0.001 to 0.20%, B: 0.0005 to 0.02%, Zr: 0.0005 to 0.02%, V: 0.0005 to 0.02%
These elements have the effect of refining crystal grains and improving castability. However, if the amount added is too large, a coarse compound is formed and the elongation is lowered.

-Inevitable impurities Fe is an element that is inevitably mixed from raw materials such as scrap, but Fe has an effect of improving mechanical strength and an effect of preventing die seizure during die casting. If it exceeds 1.0%, a coarse compound is formed and the elongation is lowered. Therefore, it is preferably 1.0% or less, and more preferably 0.3% or less.
Mg is an element that strengthens the matrix by solid solution / precipitation, but when it is added to the alloy of the present invention, a low melting point Al—Si—Cu—Mg phase is generated, causing casting defects (microporosity). It becomes. Therefore, the content needs to be suppressed to about 0.3%.
Elements other than Fe and Mg, for example, elements normally contained in aluminum alloys such as Mn, P, Ca, Sr, Sb, Na, Zn, Pb, Bi, Sn, etc. are also up to about 0.5% in total. Preferably, up to about 0.3% is allowed.

Aging treatment: 150 to 240 ° C. × 2 to 8 hours When an aging treatment is performed, Cu—Al-based precipitates are deposited, and mechanical strength is improved. Moreover, permanent growth is suppressed and dimensional stability is improved. This effect becomes remarkable at 150 ° C. or more × 2 hours or more. Conversely, if the aging treatment is performed at over 240 ° C. or over 8 hours, it becomes over-aged and the mechanical strength decreases. Preferably, it is 200-240 degreeC x 3-5 hours.

-Quenching: Cooling between 450-150 ° C at an average cooling rate of 100 ° C / sec or more. When excluding after casting, precipitates in which Si and Cu dissolved in the matrix do not contribute to strength improvement To be deposited. When such precipitation occurs, the amount of precipitates that contribute to strength improvement during aging treatment is reduced. Therefore, in order to prevent precipitation of precipitates that do not contribute to strength, it is necessary to cool at an average rate of 100 ° C./second or higher between 450 and 150 ° C.

An aluminum alloy having the composition shown in Table 1 was cast into a boat shape (dimensions 200 × 30 × 40 mm) at a casting temperature of 760 ° C., and when the temperature of the cast material reached 450 ° C., the cast material was The mold was taken out and immediately quenched with water (cooling temperature 300 ° C./second). After cooling, aging treatment was performed by heating and holding at 220 ° C. for 4 hours. After the aging treatment, the cast material was cut into the shape of a JIS standard CT73 type tensile test piece, preheated at 300 ° C. and 400 ° C. for 100 hours, and then subjected to a tensile test at each temperature. The results are shown in Table 2.
Moreover, each cast material was cut | disconnected, the cross section was color-checked, and the porosity number from a casting bottom surface to 20 mm was measured. The results are shown in Table 3.
As a comparative example, a similar experiment was performed on a JIS standard AC8A alloy and the results are also shown in Tables 2 and 3.

From Table 2, the alloy of the present invention (alloy numbers 1 to 11) has a markedly improved high-temperature strength compared to the alloy number 17 (corresponding to the AC8A + Cu alloy in FIG. 3) in which the amount of Cu in the AC8A alloy or AC8A is increased. I understand.
Further, in the alloys of Comparative Examples 12 to 16, the content of one or both of Si and Cu is within the range of the present invention (Si: 4.0 to 7.0 mass%, Cu: 21 to 31.5). Some are included in (mass%), but both are examples in which the Si amount and the Cu amount do not satisfy the relational expressions (1) to (3) of the present invention. As a result, the high temperature strength is comparable to that of the alloy of the present invention, but in particular, as shown in Table 3, it has been found that the porosity is large and the castability is poor.

The hardness (HRF) at 150 ° C., 250 ° C. and 350 ° C. was measured for the alloy 1 of the present invention shown in Table 1, and the conventional alloy 17 (AC8A + Cu) and AC8A. The results are shown in Table 4 below.

  As is apparent from Table 4, the alloy of the present invention (Alloy No. 1) is harder than the conventional heat resistant alloys AC8A + Cu (Alloy No. 17) and AC8A, and in particular, the hardness of the conventional alloy decreases as the temperature increases. On the other hand, the alloy of the present invention has almost no change in hardness, and as a result, the difference in hardness from the conventional alloy increases as the temperature increases. That is, it can be seen that the alloy of the present invention can sufficiently withstand use under severe conditions at high temperatures.

It is a figure which shows the thermodynamic state figure of Al alloy of this invention. It is a figure which shows the thermodynamic state figure of the conventional heat resistant Al (AC8A) alloy. It is a figure which shows the thermodynamic state figure of the conventional heat resistant Al (AC8A + Cu) alloy. It is a microscope picture which shows the structure of the alloy of this invention. It is a microscope picture which shows the structure of the conventional heat resistant Al (AC8A) alloy. It is a microscope picture which shows the structure | tissue structure of the conventional heat resistant Al (AC8A + Cu) alloy. It is a graph which shows the relationship of the mass% of Si and Cu which comprise Al alloy which concerns on this invention.

Claims (4)

Si: 2.0-7.0% by mass, Cu: 21.0-31.5% by mass, the balance is made of Al and inevitable impurities, and the amount of Si and the amount of Cu are expressed by the following relational expression (1) To (3):
(Si mass%) ≦ −0.19 × (Cu mass%) + 11 (1)
(Si mass%) ≧ −0.5 × (Cu mass%) + 17.5 (2)
(Si mass%) ≧ 6 × (Cu mass%) − 184 (3)
An aluminum alloy for casting with excellent high-temperature strength that meets the requirements. Furthermore, any of Ti: 0.001 to 0.20%, B: 0.0005 to 0.02%, Zr: 0.0005 to 0.02%, V: 0.0005 to 0.02% The aluminum alloy for casting excellent in high temperature strength according to claim 1, comprising at least one kind. A method for producing an aluminum alloy cast material excellent in high-temperature strength, wherein the aluminum alloy having the composition according to claim 1 or 2 is cast and then subjected to aging treatment at 150 to 240 ° C for 2 to 8 hours. . After casting the aluminum alloy having the composition according to claim 1 or 2, the alloy is cooled at an average cooling rate of 100 ° C./second or more between 450 to 150 ° C., and then held at 150 to 240 ° C. for 2 to 8 hours. The method for producing an aluminum alloy cast material excellent in high-temperature strength according to claim 3, wherein the aging treatment is performed.

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