JPH0472033A - Aluminum alloy piston for internal combustion engine - Google Patents

Abstract

PURPOSE:To promote the refining of primary crystal Si in an Al-Si alloy and to obtain the Al allay excellent in castability, mechanical strength and wear resistance by adding a specified trace amt. of P to an Al-Si allay as the Al alloy for a piston in an internal combustion engine. CONSTITUTION:An Al-Si allay constituted of, by weight, 15 to 18% Si, 2 to 4% Cu, 0.5 to 2% Mg, 0.5 to 2% Ni, 0.002 to 0.02% P, <0.8% Fe and <0.5% Mn, or in which <0.003% Ca and <0.001% Na are furthermore regulated and the balance Al is used as the material for a piston in an internal combustion engine. This is the Al-Si allay having a hyper-eutectic structure low in the coefficient of thermal expansion and excellent in heat resistance, in which the growth of primary crystal Si grains is suppressed by the addition of P and fine primary crystal Si having 20 to 60mum grain size is formed. Moreover, by regulating the amounts of Ca and Na suppressing the effect of P for refining the primary crystal Si into small ones, the refining of the primary crystal Si grains can be promoted.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an aluminum alloy that has excellent castability, workability, strength, etc., and is used as a piston of an internal combustion engine mounted particularly on a vehicle.

[Prior art] As an aluminum alloy for pistons, JISAC8A is used in consideration of 1ii1 heat resistance, wear resistance, low coefficient of thermal expansion, etc.
, AC8B, AC8C, etc.
Alloys and eutectic Aff-Si alloys have traditionally been used.

However, the hypoeutectic or eutectic Aβ-3i alloy is a material with a significantly large coefficient of thermal expansion. Therefore, when this type of alloy is used as a piston in an internal combustion engine, a large change in diameter occurs before and during engine operation. Therefore, a method has been adopted in which the expansion allowance is increased to accommodate the variation in diameter. However, as the expansion allowance increases,
The clearance with the cylinder bore on the low temperature side increases, increasing problems such as noise and vibration.

To solve this problem, J I 5-AC9A.

A hypereutectic AJ2-Si alloy with a small coefficient of thermal expansion, such as AC9B, is used in some parts. For example, JP-A-59-1
26750, Si 10-17% A℃-S
0.05 to 0.0 to improve heat cracking resistance to i alloy.
A product with 3% sb added is introduced.

[Problem to be solved by the invention] However, hypereutectic Al2- containing a relatively large amount of Si
In the Si alloy, 31 primary grains are significantly developed.

In addition, the melting temperature is high, eutectic or hypoeutectic Al2-3i is formed, and castability is poor. Moreover, due to the development and growth of 31 primary crystals during the casting process when the molten metal solidifies into the product shape,
The structure contains coarse crystal grains.

In particular, when casting a large-sized piston, the cooling rate during casting slows down, so the 31 primary crystals grow even larger, resulting in insufficient strength, and locally soft parts, which deteriorate wear resistance. do.

Due to these problems, hypereutectic A12-Si alloy is a material with a low thermal expansion coefficient and excellent heat resistance, but it is not used as a piston material for small engines installed in light cars and motorcycles. It remains in use.

The present invention was devised to solve such problems, and by suppressing the growth of 31 primary crystal grains and keeping the grain size as low as 20 to 60 tim, hypereutectic A.
The object of the present invention is to provide a piston material that takes advantage of the characteristics of the 12-Si alloy, such as low coefficient of thermal expansion, heat resistance, wear resistance, and high strength, while also having excellent castability and workability.

[Means for Solving the Problems] In order to achieve the object, the Al2-Si alloy of the present invention contains Si: 15+-18% by weight, Cu: 2-4% by weight, M
g: 0.5-2% by weight, Ni: 0.5-2% by weight, P:
0.002-0.02% by weight.

It has a hypereutectic composition containing Fe: 0.8% by weight or less and Mn: 0.5% by weight or less, and the average grain size of 31 primary crystals is 2.
It is characterized by being 0 to 60 μm.

Further, it is preferable to suppress the contents of Ca and Na contained as impurities in the Al2-Si alloy to 0.003% by weight or less and 0.001% by weight or less, respectively.

[Function] The coefficient of thermal expansion of the Aβ-Si alloy decreases in inverse proportion to the increase in Si content. However, when the Si content increases, the castability deteriorates due to the development of 31 primary crystals.

Therefore, in the present invention, the development of 31 primary crystals is suppressed by adding P. As a result, it becomes possible to utilize the excellent properties of the hypereutectic A° C.-3i alloy, such as a low coefficient of thermal expansion, heat resistance, and abrasion resistance 9 strength, while ensuring practical castability.

Furthermore, the refinement of primary Si grains by adding P is also effective in improving high-temperature fatigue strength, which is an important characteristic when used as a piston. This high temperature fatigue strength is also improved by increasing the Cu content.

Furthermore, the low permanent deformation required as a characteristic of the piston is
It has been found that stabilization can be achieved by adjusting the Cu content and Ni content.

The composition of the hypereutectic Al2-Si alloy of the present invention was designed based on such knowledge. The content and effects of each component will be explained below.

Si: An element effective in improving wear resistance, and particularly when 31 primary grains are crystallized in a hypereutectic region with a Si content of 12.7% or more, wear resistance is significantly improved. Also, Al
The coefficient of thermal expansion of the 2-Si alloy decreases inversely as the Si content increases, resulting in a material with a low coefficient of thermal expansion suitable for pistons. Such action of Si is caused when the Si content is 1
It becomes noticeable when it exceeds 5%. When the Si content is suppressed to 18% or less, a slab free of casting defects such as cavities can be produced by ordinary mold gravity casting, molten metal forging, and the like. However, if the Si content increases excessively, the AA-Si alloying point will be high and the castability will deteriorate. Therefore, the Si content was set in the range of 15 to 18%, preferably 16 to 17%.

Cu: # is a useful element for improving the mechanical strength and wear resistance of alloys in the warm and high temperature ranges. In order to obtain such an effect, it is necessary to contain 2% or more of Cu. However, when Cu is contained in an amount exceeding 4%, the amount of permanent deformation of the alloy increases, causing problems in piston design. Therefore, the Cu content was set in the range of 2 to 4%, preferably 2 to 3%.

Mg: Like Cu, Mg is a useful element for improving the mechanical strength and wear resistance of alloys at room temperature and high temperature ranges. In order to obtain such an effect, it is necessary to contain 0.5% or more of Mg. However, when Mg is contained in an amount exceeding 2%, a large amount of intermetallic compounds crystallize (
This results in a brittle material. From this point of view, the Mg content was determined to be in the range of 0.5 to 2%, preferably O15 to 1.0%.

Ni: It is necessary to contain 0.5% or more of Ni in order to improve high-temperature strength and wear resistance and to lower the coefficient of thermal expansion. However, when the N1 content exceeds 2%, the amount of intermetallic compounds crystallized increases, resulting in a brittle material. Therefore, the Ni content was set in the range of 0.5 to 2%, preferably 0.5 to 1.5%.

Fe: Similar to Ni, Fe is an element useful in improving high temperature strength and wear resistance as well as lowering the coefficient of thermal expansion. However, when the Fe content exceeds 0.8%, the amount of crystallized substances containing Fe increases, and the mechanical properties of the obtained alloy material deteriorate significantly.

Therefore, in the present invention, the Fe content is set to 0.8% or less.

P: An element that is indispensable for the refinement of primary Si grains, and 0.0
By containing 0.2% or more of P, coarsening of primary Si grains is suppressed. However, if P is contained in excess of more than 0.002%, defects such as poor hot water flow will occur, and castability will deteriorate6. Therefore, the P content is 0.002%.
-0.02%, preferably 0.005-0.01%.

Ca: In Al2-3i systems containing a large amount of Ca, it inhibits the action of P that refines primary Si grains, causing coarse primary Si grains to emerge, and improving castability, workability, etc. deteriorate. Therefore, the Ca content was suppressed to 00OO3% or less.

Na: Like Ca, it inhibits the action of P, so its content was suppressed to 0.001% or less.

In addition, in order to obtain the required wear resistance 1 mechanical strength and homogeneous casting structure, primary Si grains have an average grain size of 20 to 60
It was adjusted to a range of %m. When the particle size of the primary Si grains is less than 20 μm, sufficient wear resistance may not be obtained.

On the other hand, when the particle size exceeds 60 μm, the mechanical properties deteriorate and the distribution of primary Si particles becomes non-uniform, resulting in poor wear resistance. Therefore, the average grain size of primary Si grains was set to 20~
The thickness is 60 μm, preferably in the range of 20 to 40 μm.

"Example] Examples will be described below.

Aff-Si alloys having the components and composition shown in Table 1 were cast at a molten metal temperature of 760° C. into passenger car pistons and J1$ boat shapes, and test pieces were prepared. For this test piece, 510
Table 2 shows the properties of the Aρ-Si alloy after 6 heat treatments, in which the alloy was heated at 180° C. for 5 hours, cooled with water, heated to 180° C. for 6 hours, and cooled in air.

The mechanical properties were measured at 250''C after heating the test piece to 250°C for 100 hours, and the rotating bending fatigue strength Ow was measured in accordance with JIS Z2273. Using a Flictron friction and wear tester, the test piece was immersed in engine oil, and a load of 40 kg was applied.
Measurements were made under the conditions of a g-line velocity of 1 m/sec and a friction distance of 6 km.

Castability is expressed by the difficulty level of piston casting and the tendency for defects to occur. ○ indicates that the piston is easy to cast and has no defects, △ indicates that it is somewhat difficult to cast or there are occasional defects, and △ indicates that it is extremely difficult to cast or there are defects. Those with frequent defects were evaluated as ×.

Cutting performance is 0 for long tool life and good cutting surface.
Those with almost good tool life and cutting surface were evaluated as ○, those with some problems in both tool life and cutting surface were evaluated as △, and those with short tool life and rough cutting surface were evaluated as ×. In addition, the evaluation as a piston is 0 for the overall excellent one.
, those with satisfactory results were evaluated as △, and those with major problems in any of the characteristics were evaluated as ×.

As is clear from Table 2, the test piece 3 according to the present invention is equivalent to JI 5-AC8A, which is a typical conventional alloy, in terms of castability, mechanical properties, permanent deformation, etc. The abrasion resistance and coefficient of thermal expansion are comparable to AC9B. This shows that it is a useful material for pistons built into high-performance engines.

[Effects of the Invention] As explained above, in the aluminum alloy of the present invention, P suppresses the growth of primary Si grains formed during solidification of the hypereutectic Aε-5i alloy, and It is possible to obtain a cast slab that has the excellent castability of the crystalline Aff-3i alloy, has a homogeneous and fine casting structure, and has a small coefficient of thermal expansion that the hypereutectic C-3i alloy has. Therefore, when the obtained slab is used as a piston for an internal combustion engine, the clearance with the cylinder bore can be designed to be small. As a result, noise, vibration, etc. that occur when the internal combustion engine is operating are significantly reduced. Furthermore, the aluminum of the present invention has excellent mechanical strength, wear resistance, workability, etc., and can be used as a relatively large piston, making it suitable for vehicles equipped with large engines. Alloys can be useful materials.

Claims (2)

[Claims] (1) Si: 15-18% by weight, Cu: 2-4% by weight, Mg: 0.5-2% by weight, Ni: 0.5-2% by weight, P: 0.002-0.02% by weight , an aluminum for a piston having a hypereutectic composition containing 0.8% by weight or less of Fe and 0.5% by weight or less of Mn, and having an average particle size of primary Si grains of 20 to 60 μm. alloy. (2) Si: 15-18% by weight, Cu: 2-4% by weight, Mg: 0.5-2% by weight, Ni: 0.5-2% by weight, P: 0.002-0.02% by weight , having a hypereutectic composition containing Fe: 0.8% by weight or less and Mn: 0.5% by weight or less, and suppressing Ca and Na to 0.003% by weight or less and 0.001% by weight or less, respectively, An aluminum alloy for a piston, characterized in that the average grain size of primary Si grains is 20 to 60 μm.