JP4539572B2 - Magnesium alloys and castings for casting - Google Patents

Description

The present invention relates to a magnesium alloy for casting and a casting excellent in heat resistance and castability.

  In industries such as the automobile industry, magnesium alloys have been applied for the purpose of weight reduction. It is expected that the scope of application will be further expanded in the future. In particular, it is considered to be applied to engine peripheral parts that are effective in reducing the weight. However, engine peripheral parts and the like are required to have high heat resistance, and development of a magnesium alloy having excellent heat resistance is required. In contrast to conventional magnesium-aluminum alloys, magnesium-aluminum-silicon alloys, magnesium-aluminum-RE alloys and the like have been developed with the aim of increasing heat resistance, but they are not necessarily in terms of corrosion resistance, castability, and cost. It was not enough. In contrast, magnesium-aluminum-calcium alloys having excellent heat resistance, corrosion resistance, and castability have been developed. For example, Patent Document 1 presents a surface having high strength and excellent castability of a magnesium-aluminum-calcium alloy. Patent Document 2 shows an increase in strength of a magnesium-aluminum-calcium alloy by adding strontium. Patent Document 3 and Patent Document 4 show an increase in strength due to an increase in aluminum and calcium relative to the conventional invention.

In other words, Patent Document 1 discloses a magnesium-aluminum-calcium alloy containing 1.0 to 5.0% aluminum and 0.3 to 3.0% calcium. Patent Document 2 discloses a magnesium alloy for die casting containing 2 to 6% aluminum, 0.3 to 2% calcium, and 0.01 to 1% strontium. Patent Document 3 contains 4.7 to 7.3% aluminum, 1.8 to 3.2% calcium, 0.0 to 0.8% zinc, and 0.3 to 2.2% tin. A magnesium alloy is disclosed. Patent Document 4 contains aluminum exceeding 6% to 10%, calcium 1.8 to 5%, strontium 0.05 to 1.0%, manganese 0.1 to 0.6%, calcium / Magnesium having a mass ratio of aluminum set to 0.3 to 0.5 is disclosed.
JP-A-8-269609 2001-316752 gazette 2004-238676 No. 2005-113260

  The industry demands that magnesium alloys be used in harsh use at higher temperatures and higher load stress conditions. The heat resistance of magnesium-aluminum-calcium alloys that have been proposed so far is not necessarily sufficient in such a severe environment. Therefore, further improvement in heat resistance is required.

The present invention has been made in view of the above-described circumstances, and provides a magnesium alloy for casting that is excellent in heat resistance and castability, in which the formation of β phase (Mg ₁₇ Al ₁₂ ) is suppressed and heat resistance can be further improved. Is an issue.

The present inventor has intensively developed a magnesium-aluminum-calcium alloy. When the structure of a magnesium-aluminum-calcium alloy is observed, this alloy generally has an Mg phase (including Mg—Al solid solution and Mg—Ca solid solution), β phase (Mg ₁₇ Al), depending on the alloy composition. ₁₂ ), Al ₂ Ca (Mg) phase, and Mg ₂ Ca (Al) phase are often included as a main structure.

In a relatively low temperature region, that is, in a temperature region of 120 ° C. or lower, if a β phase, an Al ₂ Ca (Mg) phase, or an Mg ₂ Ca (Al) phase exists at the grain boundary of the Mg phase, the grain boundary slip is suppressed. Therefore, the creep resistance is easily improved.

However, according to the research results of the present inventors, it has been found that in the high temperature range (temperature range exceeding 120 ° C.), the β phase tends to hinder the improvement of creep resistance. It also found that Al 2 _Ca (Mg) phase of the phases described above are effective in improving the creep resistance.

As a result of studying the present inventors based on such an idea, the present inventors have found that the magnesium alloy contains aluminum, calcium, and manganese, and is 6 to 12% aluminum and 0.1 to 1. In a magnesium-based alloy containing 5%, if the mass ratio of calcium / aluminum is set in the range of 0.55 to 1.0, the basic structure is two phases of Mg phase and Al ₂ Ca (Mg) phase ( (Some may have a slight Mg ₂ Ca (Al) phase), a structure in which the formation of β phase can be suppressed, and magnesium having excellent heat resistance (for example, creep resistance) and castability It was found that an alloy was obtained, confirmed by tests, and a magnesium alloy according to the present invention was completed.

That is, the magnesium alloy for casting according to the present invention is a magnesium alloy that contains aluminum, calcium, and manganese and does not contain tin, and is 6% to 12% aluminum and 0.1 to 1.% manganese in mass%. 5% are contained, the calcium / aluminum mass ratio is 0.55 to 1.0, the balance Ri is Do magnesium and inevitable impurities, beta-phase _(Mg 17 _{Al 12)} 0.5% by area ratio or less magnesium, characterized in der Rukoto alloy (excluding semi-molten molding magnesium is an alloy).

According to the present invention, the generation of β-phase (Mg 17 _{Al 12)} is suppressed, it is possible to provide a casting magnesium alloy and casting excellent in heat resistance and castability.

The magnesium alloy for casting according to the present invention is a magnesium alloy containing aluminum, calcium and manganese, and contains 6 to 12% aluminum and 0.1 to 1.5% manganese in mass%. The mass ratio of calcium / aluminum is 0.55 to 1.0, and the balance consists of magnesium and inevitable impurities. Therefore, the magnesium alloy according to the present invention is a magnesium-aluminum-calcium-manganese alloy. Hereinafter, the reasons for limiting the composition will be described. In the present specification, unless otherwise specified,% regarding the content is mass%.

(Aluminum 6-12%)
Aluminum contributes to the improvement of castability, in particular, the flowability of molten metal, and also contributes to the strengthening of the alloy and improves the mechanical properties. However, when it is excessive, the ductility and strength tend to decrease. If the amount is too small, the absolute amount of the Al ₂ Ca (Mg) phase is insufficient, and it is difficult to obtain sufficient heat resistance. Further, the liquidus temperature of the alloy becomes high, the solid-liquid coexistence range is widened, and casting cracks are likely to occur. When aluminum exceeds 12%, a coarse Al ₂ Ca (Mg) phase as a primary crystal is easily crystallized, and the castability is remarkably lowered. Considering these circumstances, aluminum is made 6 to 12%.

  In this case, it can be 6% or more and over 6%. Therefore, aluminum can be 6 to 10%, 6.1 to 9%, and 6.2 to 8.5%. Examples of the lower limit of aluminum include 6.05%, 6.1%, 6.2%, 6.4%, and 6.6%. The upper limit that can be combined with this lower limit is 11.5%. Examples are 10.5% and 9.5%, but are not limited thereto. In the present specification, the phrase “below” means including the numerical content. The phrase “exceeding” and “less than” means not including the numerical content.

(Mass ratio of calcium / aluminum is 0.55 to 1.0)
The mass ratio of calcium / aluminum affects the formation of β phase (Mg ₁₇ Al ₁₂ ). Here, the β phase (Mg ₁₇ Al ₁₂ ) has a low melting point and is easily generated at the crystal grain boundary. When the amount of β-phase generated is large, grain boundary slipping is likely to occur in a high temperature region, and satisfactory heat resistance is difficult to obtain. If the above-described mass ratio is less than 0.55, the β phase is likely to appear and the heat resistance is lowered. On the other hand, when the mass ratio of calcium / aluminum exceeds 1.0, the Mg ₂ Ca (Al) phase is relatively increased, and the castability is greatly lowered. If the calcium / aluminum mass ratio is 0.55 to 1.0, the formation of the β phase in the tissue can be suppressed, so that the β phase is hardly generated or not generated. Therefore, in the field of view of the microscope, the β phase is preferably suppressed as much as possible by the area ratio, and is preferably 0.5% or less, more preferably 0.2% or less, and 0.1% or less. Or it is preferable that it does not exist substantially as (beta) phase, and it is especially preferable to set it as 0%.

  Considering the above circumstances, the mass ratio of calcium / aluminum is preferably 0.58 to 0.90, or 0.60 to 0.88. Examples of the lower limit value of the mass ratio of calcium / aluminum include 0.58, 0.60, 0.62, 0.65, and the like. The upper limit values that can be combined as the lower limit values are 0.98, 0.95, 0.90, 0.88, etc. are exemplified. However, it is not limited to these.

  Here, when the mass ratio of calcium / aluminum is 0.55 to 1.0, the minimum value of calcium is 3.3% (6% × 0.55 = 3.3%), and the maximum value of calcium Is 12% (12% × 1.0 = 12%). Therefore, although calcium is 3.3 to 12%, 4 to 11%, 5 to 10%, and 6 to 9% are exemplified, but not limited thereto.

  FIG. 1 shows the relationship between the amount of aluminum and the amount of calcium in the magnesium alloy described above. As shown in FIG. 1, the characteristic line K1 shows Ca / Al mass ratio = 1.00 (atomic ratio: 1 / 1.49). The characteristic line K2 shows Ca / Al mass ratio = 0.550 (atomic ratio: 1 / 2.7). The characteristic line K3 shows Ca / Al mass ratio = 0.500 (atomic number ratio: 1 / 2.98). The characteristic line K4 shows the mass ratio of Ca / Al = 0.300 (atomic ratio: 1 / 4.95).

  In FIG. 1, ♦ indicates an alloy whose axial force retention at 175 ° C. is 90 or more when ADC12 is 100. A circle indicates an alloy whose axial force retention at 175 ° C. is 80 or more when ADC12 is 100. In FIG. 1, a region KA indicates a range according to the product of the present invention. A region KB indicates a range according to Patent Document 4. In the region KA, the generation of β phase is suppressed. Considering the ratio of Ca / Al, a β phase is easily generated in the region KB.

(Manganese 0.1-1.5%)
Manganese contributes to the improvement of corrosion resistance, but if it is too small, the corrosion resistance is lowered, and if it is excessive, it cannot be completely dissolved in the molten metal, and a sufficient effect on corrosion resistance and heat resistance cannot be obtained. Considering these circumstances, manganese is made 0.1 to 1.5%. Considering the above circumstances, for example, it can be 0.12 to 1.3%, 0.2 to 1.0%, and 0.3 to 0.8%. Examples of the lower limit value of manganese include 0.15%, 0.20%, and 0.30%. The upper limit value of manganese that can be combined with this lower limit value is 1.3%, 1.2%, 1. Examples are 0% and 0.8%. However, it is not limited to these.

According to the magnesium alloy according to the present invention, preferably, strontium 1.5% or less, 2.5% rare earth element or less, may contain at least one of 1% or less under the silicon. Strontium, rare earth elements, if the silicon down is _added, although the Al 2 Ca (Mg) phase is different phases, phases show similar effects as _Al 2 Ca (Mg) phase with respect to heat resistance formed And can contribute to further improvement in heat resistance. In addition, the addition of strontium and rare earth elements improves the corrosion resistance of the casting. Incidentally, silicon emissions is also effective in improving the castability. However, according to the magnesium alloy according to the present invention, strontium, rare earth elements, if silicon down is no particular need, may without containing.

(Strontium 1.5% or less)
Strontium is advantageous for improving heat resistance. However, if the amount of strontium is more than the above amount, the amount of Mg—Al—Sr compound or Al ₄ Sr produced increases, and the ductility may be lowered. Considering this situation, when strontium is contained, the content is made 1.5% or less. In this case, it can be made 1.3% or less and 1.1% or less. Examples of the lower limit of strontium include 0.1%, 0.2%, and 0.3%. Examples of the upper limit of rare earth elements that can be combined with this lower limit include 1.4% and 1.3%. it can. However, it is not limited to these.

(Rare earth elements 2.5% or less)
Rare earth elements dissolve in the primary α-magnesium matrix and contribute to improving heat resistance by solid solution strengthening. In addition, rare earth elements form a compound phase at the crystal grain boundary of the primary α-magnesium matrix, thereby suppressing grain boundary sliding and contributing to improved heat resistance. However, if the rare earth element is excessive, the ductility and strength, the hot water flowability, and the corrosion resistance tend to decrease. Considering these circumstances, when a rare earth element is contained, the rare earth element is made 2.5% or less. In this case, it can be 2.3% or less and 2.0% or less. Examples of the lower limit of the rare earth element include 0.1%, 0.2%, 0.4%, and 0.6%. The upper limit of the rare earth element that can be combined with this lower limit is 2.4%, 2% .3% can be exemplified. However, it is not limited to these.

  Since it is expensive to separate the rare earth element as a simple substance, misch metal can be used as the rare earth element. Misch metal is generally a rare earth alloy mainly composed of at least one of cerium, lanthanum, praseodymium and neodymium. A cerium-based misch metal, a neodymium-based misch metal, or a lanthanum-based misch metal may be used. In some cases, the rare earth element may be a simple substance such as cerium, lanthanum, praseodymium, neodymium, or another rare earth element. In addition, when there is a relatively large amount of calcium, castability may be slightly lowered, so the amount of rare earth elements can be suppressed to 0 to 2%. However, it is not limited to this.

(Silicon 1% or less)
Silicon is effective in improving heat resistance and castability, but if it is excessive, the amount of Mg ₂ Si compound crystallized increases, and ductility and strength are likely to decrease. Therefore, when silicon is contained, the silicon is made 1% or less. In particular, it is preferably 0.8% or less and 0.6% or less . However Shi is not limited thereto.

(For the scan's)
Tin contributes to improvement in heat resistance by, for example, solid solution in the primary crystal α-magnesium matrix. Tin also crystallizes at the grain boundary and dendrite cell gap at the end of solidification, thereby contributing to improvement of castability. However, since tin has a large specific gravity of about 7.3, an excess of tin is disadvantageous as a magnesium alloy for reducing weight. In view of the above circumstances, tin is not contained as an active element.

(casting)
The magnesium alloy according to the present invention has good castability and is suitable for die casting, die gravity casting, sand casting and the like. The die casting may be performed by a cold chamber method or a hot chamber method. The magnesium alloy according to the present invention can be applied to components that require both weight reduction and heat resistance. For example, a cylinder head cover, a cylinder block, a piston, a transmission case, and the like of a vehicle are exemplified, but not limited thereto.

  Examples of the present invention will be specifically described below.

In the series of Example 1, manganese was fixed at 0.3% so as to have the composition shown in Table 1, and the raw materials were blended so as to change the aluminum amount, the calcium amount, and the Ca / Al ratio. Similarly, as a series of Example 2, raw materials were blended so that the amount of aluminum, the amount of calcium and the amount of manganese were changed and the ratio of Ca / Al was changed so as to have the composition shown in Table 2. The series of Example 2 may include a strontium amount, a mesh metal amount, and a silicon amount .

The raw material was melted in a gas melting furnace without flux. Then, after maintaining the molten metal at a molten metal temperature of 690 ° C., the molten metal was loaded into a molding cavity of a die casting die of a 7.8MN die casting machine, and a cast product (die casting product) was cast as a test piece. The compositions shown in Tables 1 and 2 are target values. In Table 2, a magnesium alloy containing tin was used as a reference example.

  According to this example, misch metal was used as the rare earth element. The basic composition of misch metal is 50% cerium, 27% lanthanum, 11% neodymium, 5% praseodymium, and other rare earth elements as the balance. ing. When the misch metal used in this example is 100%, the total of cerium, lanthanum, neodymium, and praseodymium, which are the main components, accounts for 93% of 100%.

  According to the present example, analytical values of cerium, lanthanum, neodymium, and praseodymium are obtained from the magnesium alloy, the total amount (%) thereof is obtained, and the total amount (%) × (100/93) is calculated as Misch metal (Mm ) Amount (%). The amount of the misch metal is Mm, and this is shown in the column of Table 1. Therefore, the content of misch metal (Mm) shown in Table 1 corresponds to the content of misch metal containing other rare earth elements in addition to containing cerium, lanthanum, neodymium, and praseodymium.

  And as characteristic evaluation of the series of Example 1, (beta) phase rate (area ratio), axial force retention (ratio with respect to ADC12), and castability (casting crack) were measured. As a characteristic evaluation of the series of Example 2, axial force retention (ratio to ADC12) was measured. Tables 1 and 2 show the measurement results together with the composition.

  As the β phase ratio, a sample cut from a casting was polished and etched using a 10% by mass acetic acid aqueous solution as an observation sample. The observation sample was observed using a scanning electron microscope (SEM) to classify the compounds. Analysis by EDAX was also performed to confirm the presence or absence of β phase. In this case, an SEM photograph was taken, the area ratio of the β phase was determined using image analysis software (ImagePro, etc.), and this was defined as the β phase ratio. The area ratio was an average of 5 fields of view.

  In the axial force test, a U-shaped die cast 150 having arms 151 and 152 made of magnesium alloy was formed as shown in FIG. As casting conditions, the injection speed (plunger moving speed) is set to 0.3 to 0.35 meters / second, the injection pressure is set to 28 MPa, the injection molten metal temperature is set to the liquidus temperature + 30 ° C., and the pressurizing time is set to 5 seconds. The mold temperature was room temperature to 40 ° C. The ring-shaped test piece 100 was cut out from one arm portion 151 of the die cast casting 150. In the test piece 100, it was set as the fastening part (outer diameter 20mm, internal diameter (bolt through-hole) 9mm, thickness about 10mm). Then, as schematically shown in FIG. 3, a bolt 200 having a male screw is inserted through the insertion hole of the test piece 100 through a washer 105 (outer diameter 18 mm, thickness 3 mm, A6061-T6), and the mating member Fastened to 300 screw holes 301. The bolt 200 used was made of steel of MS × 25 and strength category 10.9. The mating member 300 was a JIS standard ADC12 aluminum die casting alloy member. The bolt 200 was fastened with an initial axial force of 8 kN. The axial force was measured using a strain gauge 400 attached to the bolt 200. Then, the test piece which consists of the test piece 100 fastened with the volt | bolt 200 and the other party material 300 was inserted into the atmospheric furnace, and it hold | maintained high temperature on the conditions of 175 degreeC and 300 hours, and cooled to room temperature after that. Thereafter, the axial force of the bolt 200 was measured again. Then, the axial force retention ratio with respect to the initial axial force described above was obtained. In this case, the axial force retention was determined as an average of a plurality of values. Here, the axial force retention rate of 76% means that the axial force has decreased to an axial force of an initial axial force of 8 kN × 0.76 due to the high temperature retention under the above-described conditions. In addition, although the axial force of the bolt 200 was measured also by the ultrasonic axial force measuring method, the same result as the case where the strain gauge 400 was used was obtained. As the axial force retention rate, the ratio (ADC12 ratio) of the axial force retention rate of each alloy when the axial force retention rate of the ADC12 alloy, which is a general-purpose aluminum alloy, is defined as 100 was obtained. The case where the axial force retention rate exceeded 80 was evaluated as ◯, and the case where the axial force retention rate did not exceed was evaluated as ×.

  Also, a die cast casting 300 having the shape shown in FIG. 4 was made as a prototype, and the presence or absence of cracks occurring in the casting 300 was examined with the naked eye. As casting conditions, the injection speed (plunger moving speed) was 1 meter / second, the injection pressure was 64 MPa, the mold temperature was 200 ° C., and the molten metal temperature was the liquidus temperature + 30 ° C.

  As a comparative example shown in Table 1, either the aluminum amount or the Ca / Al ratio is out of the present invention. In many comparative examples shown in Table 1, since either the amount of aluminum or Ca / Al is not appropriate, as shown in Table 1, many have a high β phase ratio and many have a low axial force retention. The overall evaluation was x. Although Comparative Example 1-7 and Comparative Example 1-8 had high axial force retention, casting cracks occurred and the overall evaluation was x.

  On the other hand, according to Example 1 series, the amount of aluminum, the amount of calcium, and the ratio of Ca / Al are appropriate, and as shown in Table 1, in the β phase ratio, the axial force retention, and the prevention of casting cracks Overall excellent. The reason for this is presumed that the occurrence of β-phase can be suppressed, so that grain boundary sliding can be effectively prevented in a high temperature region.

  For the Example 2 series, the axial force retention was determined in the same manner as the Example 1 series. The case where the axial force retention rate exceeded 80 was evaluated as ◯, and the case where the axial force retention rate did not exceed was evaluated as ×. According to Example 2 series, the amount of aluminum and the ratio of Ca / Al are appropriate, and as shown in Table 2, the β phase ratio and the axial force retention are comprehensively excellent. The prevention of casting cracks was also good. As shown in Table 2, in Comparative Example 2-1, Comparative Example 2-2, and Comparative Example 2-3, since the ratio of Ca / Al is not appropriate, the axial force retention is low, and the overall evaluation is x. It was.

Furthermore, a creep resistance test in a high temperature region was conducted. The test conditions were a measurement temperature of 180 ° C., an initial stress of 104 MPa, a test piece shape of a round bar (parallel portion: 6 mm in diameter), and a measurement time of 300 hours. The result of the minimum strain rate is shown in FIG. As shown in FIG. 5, in Comparative Example 3 (Ca / Al = 0.43, Mg-7% Al-3% Ca-0.3% Mn), the strain rate was considerably high. In contrast, For Example, strain rate, Example 3-1 (Ca / Al = 0.71, Mg-7% Al-5% Ca-0.3% Mn), Example 3-2 (Ca / Al = 0.71, Mg-7% Al-5% Ca-0.3% Mn-0.5% Sr), Example 3-3 (Ca / Al = 0.75, Mg-12% Al-9% Ca-0.3Mn-0.5% Sr) in this order. Comparing Sr of Example 3-1 and Example 3-2, it can be seen that the addition of Sr is effective in reducing the strain rate in the creep resistance test.

(Metal structure)
6 to 13 show photographs (SEM) of metal structures. 10 and 13 show the product of the present invention. In the photograph, the part indicated by the black triangle mark indicates the β phase. The metal structure was observed after etching with a 10% by mass acetic acid aqueous solution. As can be understood from the photograph, in the alloys other than the product of the present invention, the β phase is generated at the grain boundary. In some cases, it is generated in the crystal grains. On the other hand, with the composition of the product of the present invention, the formation of β phase was suppressed and was substantially 0%. Thus, since the production | generation of (beta) phase is suppressed, it is guessed that a grain boundary slip can be prevented effectively and the heat resistance (creep resistance etc.) of a magnesium alloy can be improved. The β phase was identified by an apparatus (SEM-EDX) having a scanning electron microscope section and an energy dispersive X-ray analysis section.

  In addition, the present invention is not limited only to the embodiments described above and shown in the drawings. One or more of thulium, erbium, eurobium, ytterbium, etc. may be used, and the like can be implemented with appropriate modifications as necessary. The content of each alloy element described in Table 1 can be described as an upper limit or a lower limit that defines the composition of each claim.

  The present invention can be used for parts of vehicles and industrial machines that are expected to be reduced in weight. In a vehicle, it can be used for engine-related parts such as an oil pan, a transmission case, a cylinder block, a cylinder head, and a piston, or parts that require both lightness and heat resistance.

It is a graph which shows the relationship between the amount of aluminum, the amount of calcium, and the mass ratio of Ca / Al in a magnesium alloy. It is a conceptual diagram which shows typically the cutout form of the ring-shaped test piece which measures the axial force of a volt | bolt. It is a conceptual structure figure which shows typically the state which measures the axial force of the bolt which fastened. It is a perspective view of the test piece which evaluates a casting crack. It is a graph which shows the result of the minimum distortion speed. It is a photograph figure of a metal structure. It is a photograph figure of a metal structure. It is a photograph figure of a metal structure. It is a photograph figure of a metal structure. It is a photograph figure of a metal structure. It is a photograph figure of a metal structure. It is a photograph figure of a metal structure. It is a photograph figure of a metal structure.

  In the figure, 100 is a test piece, 200 is a bolt, and 300 is a mating member.

Claims (7)

A magnesium alloy containing aluminum, calcium and manganese and not containing tin, and containing 6 to 12% aluminum and 0.1 to 1.5% manganese in mass%, and the mass of calcium / aluminum ratio is 0.55 to 1.0, Ri Do the balance is magnesium and inevitable impurities,
β phase (excluding semi-molten molding magnesium is an alloy) _(Mg 17 _{Al 12)} is casting magnesium alloy according to claim der Rukoto 0.5% or less in area ratio. In claim 1, further containing, by mass%, strontium 1.5% or less, 2.5% rare earth elements less, casting, characterized by containing at least one of 1% or less of silicon Magnesium alloy. 3. The magnesium alloy for casting according to claim 1, wherein a mass ratio of calcium / aluminum is 0.60 to 0.88. The magnesium alloy for casting according to any one of claims 1 to 3, wherein manganese is 0.2 to 1.0% or less by mass. The magnesium alloy for casting according to any one of claims 1 to 4, wherein the calcium is 4% or more by mass. The magnesium alloy for casting according to any one of claims 1 to 4, wherein the calcium is 5% or more by mass. Casting characterized by Tei Rukoto formed either magnesium alloy according to one of claims 1 to 6.

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