JPH07224340A - Hypereutectic al-si alloy excellent in machinability and its production - Google Patents

Abstract

PURPOSE:To improve the machinability of a hypereutectic Al-Si alloy used as internal combustion engine parts or the like. CONSTITUTION:This hypereutectic Al-Si alloy has a compsn. contg. 13 to 21% Si, 0.5 to 5% Cu, 0.3 to 2.0% Mg, 0.2 to l.5% Pb or Bi, 6 to 120ppm Ca and 40 to 130ppm P, and in which the weight ratio of P/Ca is regulated to the range of 0.6 to 6. Both Pb and Bi may be incorporated therein by the total content of 0.2 to l.5%. It is subjected to DC casting or die casting at a temp. of (the liquidus temp. +70 deg.C) or above. Preferably, the diameter or thickness of the cast ingot is regulated to <=150mm as for the DC casting material, and the diameter or thickness of the cast ingot is regulated to <=30mm as for the die casting material. The obtd. cast ingot is formed into the stock for a piston or the like having a cast structure in which the average grain size of primary crystal Si is regulated to <=20mum. By the addition of the Pb and Bi, its machinability is improved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hypereutectic Al-Si alloy having improved machinability and a manufacturing method.

[0002]

2. Description of the Related Art A hypereutectic Al-Si alloy containing 12.6% by weight or more of Si has a small thermal expansion coefficient and excellent heat resistance. In addition, since high-hardness primary silicon crystallizes when the molten metal solidifies, it is used as a component for internal combustion engines such as pistons, crankcases, brake drums, and cylinder liners that require wear resistance. The hypereutectic Al-Si alloy exhibits excellent wear resistance due to the crystallization of hard primary Si, but tends to have a cast structure in which primary Si is largely grown. When cutting an ingot in which large primary crystal Si is crystallized, the hard primary crystal Si causes the cutting tool to wear in a short period of time, shortening the tool life. as a result,
Cutting cost increases. In addition, the mechanical properties are not sufficient, and the finished dimensions due to cutting cannot be controlled with high precision.

Primary crystal Si is refined by rapid solidification. For example, a powder method is adopted, or Japanese Patent Laid-Open No. 52-12
As disclosed in Japanese Patent No. 9607, the molten aluminum alloy is rapidly solidified by the molten metal rolling method to refine the cast structure. In addition, molten aluminum alloy is Al-Cu.
The primary crystal Si can also be made fine by P treatment with -P, Cu-P, Ni-P, or the like. By adding P, the primary crystal Si is refined and the workability and mechanical properties are improved. The added P forms an intermetallic compound AlP,
It is considered that this intermetallic compound AlP acts on the refinement of primary crystal Si. However, in a method of passing through an ingot such as a die casting or a DC casting excluding a special one such as a rapid solidification method, the primary crystal Si cannot be sufficiently refined only by adding P in many cases.

It has been reported that when casting a hypereutectic Al-Si alloy melt containing Ca, Ca inhibits the action of P for refining primary crystal Si. The present inventors, in the process of investigating and studying the mechanism by which Ca suppresses the refinement of the primary crystal Si, when the appropriate amounts of P and Ca are contained, the primary crystal Si is compared with the case of only P. It was found that the device was made finer, and the application was filed as Japanese Patent Application No. 4-244259. That is, P / Ca weight ratio: 0.6-
Under the conditions of 6, P: 40 to 130 ppm and Ca: 6-1
In the hypereutectic Al-Si alloy melt containing 20 ppm, the primary crystal Si refining effect of P becomes remarkable, and a product having a fine cast structure can be obtained. It was found that the machinability is improved with the refinement of the primary crystal Si, and Japanese Patent Application No. 5-
No. 71804 and Japanese Patent Application No. 5-100626.

[0005]

The hypereutectic Al-Si alloy has improved workability due to the refinement of the primary crystal Si, but the wear of the tool during cutting is still not negligible.
Especially when a high-level cutting process is performed, the cutting tool is easily worn by hard primary crystal Si. The present invention has been devised to solve such a problem, and Pb and / or Bi
Hypereutectic Al-Si in which primary crystal Si is refined by addition of
The purpose is to improve the machinability of the alloy.

[0006]

Hypereutectic Al-S of the present invention
In order to achieve the purpose, the i alloy has Si: 13-21.
% By weight, Cu: 0.5-5% by weight, Mg: 0.3-2.
0% by weight, Pb or Bi: 0.2 to 1.5% by weight, C
It contains a: 6 to 120 ppm and P: 40 to 130 ppm, and is characterized in that P / Ca is in the range of 0.6 to 6 in weight ratio. The hypereutectic Al-Si alloy may also contain both Pb and Bi in a total amount of 0.2 to 1.5% by weight. A hypereutectic Al-Si alloy having this composition is
DC casting or die casting is performed at a temperature of (liquidus line + 70 ° C.) or higher. DC casting material has ingot diameter or thickness of 150 m
It is preferably m or less. The die casting material preferably has a diameter or thickness of the ingot of 30 mm or less. The obtained ingot becomes a material such as a piston having a casting structure in which the average grain size of primary crystal Si (hereinafter, simply referred to as grain size) is 20 μm or less.

[0007]

When P is added to the hypereutectic Al-Si alloy, A
An IP compound is formed. The AlP compound acts as a nucleus of primary crystal Si, and the cast structure is refined. At this time,
When Ca coexists, a large number of Ca-P compounds are formed in addition to AlP, and when the Ca-P compound is in a preferable state, it acts as an effective nucleus of primary crystal Si. It is speculated that the Ca-P compound acts as a more effective crystal nucleus when the primary crystal Si crystallizes, and further refines the primary crystal Si, as compared with AlP. In order for the Ca-P compound to act as the nucleus of primary crystal Si, it is necessary to adjust the weight ratio P / Ca in the molten aluminum alloy immediately before casting to a range of 0.6 to 6. The weight ratio P / Ca immediately before casting is 0.6 to
As long as it is maintained at 6, even if any of the following methods is adopted or these methods are combined,
A finer cast structure can be obtained as compared with the conventional P treatment.

A method of preliminarily blending a predetermined amount of Ca and P with a melting raw material in consideration of consumption of Ca and P in the process from the start of melting to casting. A method of adding Ca after melting a hypereutectic Al-Si alloy in a step of melting and casting a hypereutectic Al-Si alloy containing a predetermined amount of P to obtain an ingot. A method in which a hypereutectic Al-Si-based alloy containing no Ca and P is melted, and then predetermined amounts of Ca and P are added simultaneously or in tandem. A method of analyzing the composition of a hypereutectic Al-Si alloy containing Ca and P according to any one of the above items immediately before casting, and adding Ca and P when the Ca and P are insufficient. The composition of a hypereutectic Al-Si alloy containing Ca and P in any of the above-mentioned items is analyzed immediately before casting. If the Ca content is excessive, either raise the melt temperature or increase the holding time. The method of reducing Ca content by doing.

Up to now, the reason why Ca has been considered to be detrimental to the refining effect of P treatment and harmful to the refining of primary crystal Si is that P / Ca ratio and Ca and P relative to Si content.
It is considered that this was due to insufficient studies on the content (different from the amount added), the time from melting to casting, the casting temperature, the cooling rate in the primary Si crystallization temperature range, and the like. That is, either condition is not appropriate, and Ca-
The reason is that the P compound did not work as an effective nucleus. Therefore, the present inventors conducted a detailed study on these conditions, as introduced in Japanese Patent Application No. 4-244259. The following conditions are Cu and M
The same holds true for a hypereutectic Al-Si alloy containing g.

Addition of Ca Ca is contained in the hypereutectic Al-Si alloy by either preliminarily containing it in the molten raw material or by adding it to the melted hypereutectic Al-Si alloy. be able to. In any case, since Ca is highly worn in the melting and holding processes, it is necessary to grasp the content not by the amount added but by the content. It should be noted that Ca is added in the form of lumps, rods, wires, powders, granules, melts, etc. as a Ca-containing Al-Ca-based mother alloy, compound, mixture, and the like. In order to control the Ca content with high accuracy, a predetermined amount of Ca should be added to the hypereutectic Al-Si alloy after melting.
Is preferably added. That is, if Ca is blended before melting, C is added in the steps such as melting, holding at high temperature, and degassing.
a is worn, and it becomes difficult to accurately control the Ca content in the ingot. In particular, when handling a large amount of metal such as continuous casting, the target Ca content cannot be obtained, and the probability of failure increases. Further, since the yield of Ca transferred to the ingot is low, it is necessary to add a larger amount of Ca in consideration of the amount of wear.

When Ca is added to the hypereutectic Al-Si alloy after melting, the Ca content in the ingot can be controlled relatively accurately, and the primary crystal Si can be refined as desired. For example, when Ca is mixed as a cold material into a melting raw material and cast immediately after melting, the yield of Ca is 45
There was a large variation in the range of ~ 85%. On the other hand, when Ca is added to the hypereutectic Al-Si alloy after melting and immediately cast, the yield of Ca is improved to 76 to 94%, and the Ca content of the ingot does not vary greatly. It was The Ca content also changes depending on other manufacturing conditions.
In particular, the Ca content is greatly reduced by the degassing treatment. The decrease in the Ca content at this time shows different tendencies depending on the type of gas used for degassing, the degassing time, and the like. Therefore, it is preferable to previously obtain the change rate of the Ca content corresponding to the degassing condition and control the Ca content based on this change rate.

When the weight ratio P / Ca in the hypereutectic Al-Si alloy just before casting is in the range of 0.6 to 6.0,
The refinement effect of the Ca-P compound is effectively exhibited. However, the Ca content gradually decreases when the hypereutectic Al-Si alloy is held in the molten state, and the P / Ca content increases accordingly.
Will increase. In addition, the decrease rate of Ca content is
The larger the temperature of the molten Si-based alloy, the larger it becomes. Therefore, it is preferable to adjust the Ca content to a predetermined range prior to casting and then perform casting without a long holding time.
When the Ca content decreases and the weight ratio P / Ca exceeds 6.0, the Ca-P compound has an insufficient refining effect. Also, if the weight ratio P / Ca is less than 0.6, the miniaturization effect cannot be obtained. Ca content further increases and P / Ca
The lower the value, the more the primary crystal Si becomes coarser than in the case where no Ca is added. When the weight ratio P / Ca becomes less than 0.6,
The Ca concentration in the Ca-P compound also rises, which is the primary crystal Si.
It is considered that an unfavorable state where it does not work as a crystal nucleus of As a result, the miniaturization effect of P treatment is impaired, as reported previously. Further, when the Ca content exceeds 120 ppm, if the weight ratio P / Ca is less than 0.6, the primary crystal Si is refined, but the fluidity of the molten metal is remarkably reduced, and casting defects such as a molten metal boundary occur. It tends to occur. From this point, the upper limit of the Ca content is set to 120 ppm. On the other hand, the lower limit of the Ca content is set to 6 ppm in order to secure the action of Ca.

Addition of P, P is less reactive than Ca. Therefore, even if P is added to the dissolution raw material in advance, or if P is added after the dissolution, the effect of P addition does not substantially change.
Therefore, the addition timing of P may be any of the following. Further, a hypereutectic Al containing a predetermined amount of P in advance
After melting the -Si alloy or the melting raw material, the remaining P can be added before or after the addition of Ca. P is added to the P-containing mother alloy, compound, mixture, etc. in the form of a lump, rod, wire, powder, granule, melt or the like.

Preparation of hypereutectic Al-Si alloy containing P or melting raw material → melting → Ca addition → casting Preparation of hypereutectic Al-Si alloy containing P-free or melting raw material → melting → Ca and P Simultaneous addition →
Casting Preparation of hypereutectic Al-Si alloy containing no P or molten raw material → Addition of molten P → Addition of Ca →
Casting Preparation of hypereutectic Al-Si-based alloy or P-free P-containing alloy → Melting → Ca addition → P addition
→ casting

The P content is the primary crystal S due to the Ca-P compound.
In order to promote the miniaturization of i, it is necessary to maintain it in the range of 40 to 130 ppm. Unlike the Ca content, the P content is not significantly affected by the holding time even when the hypereutectic Al-Si alloy is held in the molten state, and the decrease amount is small. If the P content is less than 40 ppm, the effect of refining the primary crystal Si is insufficient. However, if the P content exceeds 130 ppm, although there is an effect of refining the primary crystal Si, the fluidity of the molten alloy is reduced, and casting defects such as a molten metal boundary are likely to occur. Further, when the concentration of P is high, the melting yield is extremely reduced.
It is very difficult to contain P of 30 ppm or more.

P / Ca ratio The P / Ca ratio is a factor that has a great influence on the miniaturization effect. It is presumed that by maintaining P / Ca in the range of 0.6 to 6 by weight, a Ca-P compound effective for refining primary crystal Si is produced. That is, the generated Ca
The -P compound is uniformly dispersed in the alloy as fine nuclei, and primary crystals of Si crystallize from these nuclei. As a result, a fine cast structure can be obtained. If the P / Ca weight ratio is less than 0.6, Ca-P having a high Ca concentration that does not act as a crystal nucleus of primary Si is formed, and Ca in the Ca-P compound is retained by the molten metal for a long time. It is considered that when the amount decreases, it becomes a preferable state and exhibits an action as a crystal nucleus.
On the other hand, if the P / Ca weight ratio exceeds 6, Ca will run short,
The number of Ca-P compounds formed is insufficient.

The phenomenon that primary crystal Si is refined by the Si contents Ca and P is a hypereutectic Al—Si having a Si content in the range of 13 to 21% by weight.
Found in alloys. As the Si content increases, it is necessary to contain a larger amount of Ca and P, and it is also necessary to strictly control the casting conditions. Moreover, the miniaturization effect decreases depending on the Si content. Therefore, the upper limit of the Si content is set to 21% by weight. In addition, in order to obtain the characteristics of the hypereutectic Al-Si alloy,
The lower limit of Si content was set to 13% by weight.

Cu content Cu forms an intermetallic compound CuAl ₂ and forms a hypereutectic Al
-It is an important alloying element that improves the strength and altitude of the Si-based alloy. In order to obtain this effect, Cu content of 0.5% by weight or more is required. However, even if a large amount of Cu exceeding 5% by weight is contained, the action of Cu is saturated and the strength cannot be improved in proportion to the increase in the amount. Therefore, in the present invention, the Cu content is set in the range of 0.5 to 5% by weight. Mg content Mg is an alloying element that contributes to the improvement of strength in the coexistence with Si, and the effect of Mg appears remarkably when the content is 0.3% by weight or more. However, when a large amount of Mg is contained in excess of 2.0% by weight, the elongation decreases as the Mg content increases. Further, a large amount of Mg content makes it difficult to control the molten metal because of the progress of oxidation, and causes a casting defect. Therefore, in the present invention, M
The g content was set in the range of 0.3 to 2.0% by weight.

Pb and Bi Pb and Bi are effective in improving the machinability, either alone or in combination. If the single addition amount or the combined addition amount is less than 0.2% by weight, the effect of adding Pb and / or Bi is not sufficient. However, a large amount of P exceeding 1.5% by weight
When b and / or Bi is contained, the machinability is improved in proportion to the increase in the amount, but the gravity segregation of Pb and Bi becomes remarkable. As a result, the properties of the obtained alloy casting become non-uniform. In order to effectively exert the refining effect of the melting temperatures Ca and P, Si
Of the hypereutectic Al-Si alloy so that
It is preferable to dissolve in a temperature range of 60 to 850 ° C.
The molten metal temperature is set high in proportion to the Si content. However, melting at an excessively high temperature not only causes energy loss for melting, but also tends to change conditions in the process up to casting. Therefore, the upper limit of the melting temperature is
Set to 850 ° C.

The refining action by the molten metal holding time Ca appears immediately after Ca addition in the hypereutectic Al-Si alloy containing Ca whose weight ratio P / Ca exceeds 0.6. This refinement action disappears when the molten alloy is held for a long time. The time when the action of Ca disappears is C
a Depending on the content and holding temperature, it is approximately 60-600
Minutes. In this respect, it is preferable to adjust the Ca content to a setting range in which the weight ratio P / Ca is 0.6 to 6.0 and then enter the casting process without the holding process for a long time. On the other hand, in the hypereutectic Al-Si alloy containing excess Ca such that the weight ratio P / Ca is less than 0.6, the refining effect of Ca does not appear immediately after the addition of Ca, and the molten alloy is Appears after holding for a period of time. There is a so-called incubation period. The incubation period becomes longer as the Ca content immediately after addition increases. For example, 61 ppm P and 18
A hypereutectic Al-Si alloy containing 0 ppm of Ca
When kept at 60 ° C., after about 100 minutes, the refining effect of Ca appears.

The latent period observed when a large amount of Ca is contained is derived from the fact that Ca is reduced while holding the molten alloy, and as a result, the weight ratio P / Ca is increased to 0.6 or more. it is conceivable that. That is, when the weight ratio P / Ca becomes 0.6 or more, the refining effect of Ca is exhibited for the first time. If the molten alloy is held for a long time, Ca
When the weight ratio P / Ca exceeds 0.6 as the content decreases, the refining effect disappears. This means that as the amount of Ca decreases, Ca- that acts as an effective nucleus for crystallization of primary Si
It suggests that the number of P compounds is insufficient. When the Ca content is high, the molten metal holding time becomes long until the weight ratio P / Ca becomes 0.6 or more.
It becomes difficult to control the a content. However, when a large amount of alloy is produced using a large melting furnace, preparation and casting take a long time. In such a case, the latent period and the period during which the Ca decreases and the weight ratio P / Ca exceeds 6.0 after the latent period is effective can be used for a long miniaturization. That is, when the time until casting is long, Ca is excessively added and the weight ratio P / Ca is adjusted to fall within the range of 0.6 to 6.0 at the time of casting.

[0022] In terms of refining the primary Si by casting temperatures higher cooling rate, it is preferable to set the casting temperature as high as possible. However, as the molten alloy temperature becomes higher, the wear of Ca becomes more severe, and it becomes difficult to control the Ca content during casting.
Therefore, it is preferable that the casting temperature is as low as possible within a range in which the refining effect can be obtained by the high cooling rate.
Specifically, it depends on the composition and content of the hypereutectic Al-Si alloy such as Si content, but it is (liquidus line + 70 ° C) or more, preferably (liquid phase) in the Al-Si binary system phase diagram. Phase line + 70 ~
The casting temperature is set in the temperature range of 170) ° C. For example, a hypereutectic Al-Si alloy containing 15% by weight of Si has a casting temperature of 680 ° C or higher, and a hypereutectic Al-Si alloy containing 17% by weight of Si has a casting temperature of 710 ° C or more. For a hypereutectic Al-Si alloy containing 20% by weight of Si, the casting temperature is set to 760 ° C or higher.

Size of ingot In order to refine primary crystal Si to a grain size of 20 μm or less, the diameter or thickness of the ingot is restricted to 150 mm or less in DC casting.
In die casting, it is necessary to regulate the diameter or thickness of the ingot to 30 mm or less and to secure the cooling rate of the central portion of the ingot. As a result, the refinement of the primary crystal Si proceeds. Particle size primary crystal Si primary crystal Si is, P content, by adjusting the Ca content and P / Ca ratio, being finely divided in the following average particle size 20 [mu] m. If the grain size of the primary crystal Si exceeds 20 μm, the machinability deteriorates. Further, the cast structure in which large primary Si crystallizes reduces the toughness and mechanical properties. Therefore, in the present invention, the grain size of primary crystal Si is limited to 20 μm or less in consideration of machinability, mechanical properties, and the like.

[0024]

EXAMPLE A hypereutectic Al-Si alloy was dissolved in a 50 kg crucible, and P and Ca were 100 ppm and 50 p, respectively.
pm was added to the target. The hypereutectic Al-Si alloy used was Si: 17 wt%, Cu: 4.5 wt% and Mg:
It is an A390 alloy containing 0.5% by weight, and the other components are shown in Table 1.

[0025]

[Table 1]

Each molten alloy was hot-top casted under the conditions of a casting temperature of 780 ° C., a casting speed of 150 mm / min, a cooling water amount of 110 liters / min and a diameter of 84 mm.
And DC casting into a 1500 mm long ingot. As a P source, Al-19% Cu-1.4% P mother alloy was added in the furnace,
An Al-5% Ca mother alloy as a Ca source was continuously put into the gutter during casting. As the Pb source and the Bi source, metal Pb and metal Bi were added in the furnace. The obtained ingot was chamfered to a diameter of 79 mm to remove the skin, and then subjected to a cutting test. A diamond sintered body is used as the cutting tool, and the cutting edge tilt angle is 0
Degree, vertical scooping angle 5 degrees, crossing angle 15 degrees, cutting speed 600
m / min, feed rate 0.1 mm / rev, cutting depth 0.
A cutting test was performed under a condition of 5 mm and a cutting distance of 36000 m without using a lubricant. Then, the machinability was evaluated by measuring the wear amount of the tool flank. Cutting time 6
The test results after the lapse of 0 minutes (cutting distance 36000 m) are shown in Table 2 together with the grain size of primary crystal Si. When the wear amount of the cutting tool was 0.17 mm or more, it was determined that the cutting property was poor.
With respect to Alloy Nos. 1, 5, 10, and 14, the wear amount of the cutting tool was regularly measured, and its change with time was investigated. FIG. 1 shows the change in wear amount at this time.

[0027]

[Table 2]

In the comparative examples of alloy numbers 6, 11, and 15, the amount of wear on the flanks is small. This is a large amount of Pb
It shows that the machinability is improved by the inclusion of Bi and Bi. However, when the microstructure of the ingot was observed, Pb and Bi were segregated at the center of the ingot into particles of 10 μm or more, and large particles of several tens of μm. Also,
In the comparative examples of Alloy Nos. 1 containing no Pb and Bi and alloys Nos. 2, 7 and 12 having a small content of Pb or Bi, the wear amount of the flank was large and the life of the cutting tool was short. On the other hand, alloy numbers 3 to 5 and 8 to according to the present invention
In Examples 10, 13 and 14, Pb and Bi were finely dispersed in a size of 1 to 5 μm at the crystal grain boundaries and the cell boundaries of dendrites, and good machinability was exhibited. Moreover, the length of the cutting tip was about 10 mm, and the tip cutting property was excellent.

[0029]

As described above, the hypereutectic Al-Si alloy of the present invention is a primary S alloy due to the quantitative control of P and Ca.
The machinability is improved by adding Pb and / or Bi to a system in which i is finely dispersed. Pb and / or Bi added in a predetermined amount finely disperse at the grain boundaries and cell boundaries of dendrites without becoming large segregated particles. Therefore, a hypereutectic Al-Si alloy casting excellent in both machinability and workability is obtained, and exhibits excellent characteristics as various internal combustion engine parts and the like.

[Brief description of drawings]

FIG. 1 is a graph showing the machinability of various hypereutectic Al—Si alloys as the amount of wear of the tool flank over time.

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Eikichi Sagisaka 1-34-1 Kambara, Kambara-cho, Anbara-gun, Shizuoka Nipparu Giken Co., Ltd. (72) Inventor Yamaji Kitaoka 3-13 Mita, Minato-ku, Tokyo No. 12 Nippon Light Metal Co., Ltd.

Claims (6)

[Claims] 1. Si: 13 to 21% by weight, Cu: 0.5
~ 5 wt%, Mg: 0.3-2.0 wt%, Pb or B
i: 0.2 to 1.5% by weight, Ca: 6 to 120 ppm and P: 40 to 130 ppm, and P / Ca is in the range of 0.6 to 6 by weight ratio. Excellent hypereutectic Al-Si alloy. 2. A hypereutectic Al—Si alloy excellent in machinability according to claim 1, which contains both Pb and Bi, and the total amount thereof is 0.2 to 1.5% by weight. 3. The hypereutectic Al—Si alloy according to claim 1, wherein the grain size of the primary crystal Si is 20 μm or less. 4. The alloy according to claim 1 or 2 (liquidus line +
70 ° C.) or more, and a method for producing a cast material, the method comprising casting. 5. The hypereutectic Al—Si according to claim 1 or 2.
A DC casting material made of an alloy and having an ingot diameter or thickness of 150 mm or less. 6. The hypereutectic Al—Si according to claim 1 or 2.
A mold casting material made of an alloy and having an ingot diameter or thickness of 30 mm or less.