JP2005187895A - Heat resistant magnesium alloy casting - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a heat resistant magnesium alloy casting which has excellent mechanical properties and creep properties, and has improved castability and creep strength as well. <P>SOLUTION: The heat resistant magnesium alloy casting has a composition comprising, by mass, 1.5 to 6% Al, 0.3 to 3% Ca, 0.1 to 1% Mn and 0.01 to 1% Sr, further comprising one or more kinds of metals selected from 0.1 to 3% La, 0.1 to 3% Ce and 0.1 to 3% Nd, and, if required, further comprising 0.1 to 1% Si and 0.2 to 1% Zn, and the balance Mg with inevitable impurities. Thus, a casting with a desired shape having satisfactory castability can be obtained without causing casting cracks by diecast molding or the like. The casting has excellent high temperature creep strength, can be used, e.g., for components concerned with an automobile engine, and has excellent corrosion resistance as well. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a heat-resistant magnesium alloy cast suitable for use in automobile parts that are required to be reduced in weight, particularly transmission parts or engine parts that are required to have heat resistance.

In recent years, magnesium alloys have been attracting attention in order to reduce weight in transportation equipment such as automobiles. Such magnesium alloys, particularly magnesium alloys for casting, include Mg-Al alloys (ASTM standard AM60B, AM50A, AM20A, etc.) containing 2 to 6 wt% of A1, or 8 to 10 wt% of A1 and 1 to 1 wt%. An Mg—Al—Zn alloy (ASTM standard AZ91D or the like) containing 3 wt% Zn is known. These magnesium alloys have good castability and can be die-cast.
In addition, AE42 alloy added with rare earth elements, alloys added with Ca to Mg-Al alloys (for example, Patent Documents 1 and 2), and alloys added with Ca to Mg-Al-Mn alloys have been developed. (For example, Patent Document 3). In particular, Patent Document 3 states that when the Ca / Al ratio is set to 0.7 or more, the Mg—Ca intermetallic compound is crystallized to obtain a high creep strength.
JP 7-11374 A JP-A-9-291332 JP-A-6-25790

However, the above-mentioned AM and AZ standard alloys have low creep strength at a high temperature of 125 to 175 ° C., for example, 150 ° C., and when used for parts around the engine, they tend to cause dripping during use. There is a risk that the bolts tightening the bolt may be loosened. For example, AZ91D, which is a typical alloy for die casting, has good castability, strength, and corrosion resistance, but is inferior in creep strength.
In addition, the AE42 standard alloy to which rare earth elements are added does not improve to such an extent that castability and creep strength are sufficient.
Moreover, although the Mg alloy to which Ca is added has improved creep strength, it cannot be said to be sufficient as compared with the ADC12 aluminum alloy, and further, there is a problem that poor hot water and casting cracks are likely to occur due to the addition of Ca. Yes.

  The present invention has been made against the background of the above circumstances, and provides a heat-resistant magnesium alloy casting that is excellent in mechanical properties and creep properties and has improved castability and creep strength suitable for automobiles and the like. Objective.

  That is, the heat-resistant magnesium alloy casting of the present invention is a magnesium alloy excellent in heat resistance and castability, and is 1.5 to 6% Al, 0.3 to 3% Ca, and 0 to Mn by mass%. 0.1 to 1%, Sr 0.01 to 1%, and La 0.1 to 3%, Ce 0.1 to 3%, Nd 0.1 to 3% of one or more, The balance is made of Mg and inevitable impurities.

  In addition, the heat-resistant magnesium alloy casting of this invention can contain 0.1 to 1% of Si further as a component. Furthermore, 0.2 to 1% of Zn can be contained. Further, Be1 to 100 ppm can be contained.

  The composition of the magnesium alloy used for the heat-resistant magnesium alloy casting according to the present invention will be described below. Unless otherwise specified in the present specification, when a range is specified by connecting two numerical values with “˜”, the following is shown. For example, 1 to 3% indicates a range of 1% or more and 3% or less unless otherwise specified. Furthermore, unless otherwise specified,% in the present specification indicates mass%.

Al (1.5 to 6% by mass): Up to 6% of Al is dissolved in magnesium, the strength is increased by solid solution hardening, and is bonded to Ca to form Al-Ca on the grain boundary. The creep characteristics of magnesium alloy are improved by forming a network of crystallized crystals. Furthermore, there exists an effect | action which improves castability. However, if the content is less than 1.5%, these effects are small, and the strength is particularly low, which is not practical. On the other hand, when the Al content exceeds 6%, the creep characteristics deteriorate rapidly. Therefore, the Al content is set in the range of 1.5 to 6%.
Further, when the Al content is 1.5 to 4.0%, the tensile strength and proof stress are relatively small. Therefore, when high tensile strength and proof strength are required, the Al content is 4.0. More preferably, it is in the range of more than 6% and 6% or less.

  Ca (0.3 to 3% by mass): Ca has an effect of improving creep resistance with an increase in the content thereof, but if the content is less than 0.3% by mass, the improvement effect is small. On the other hand, when it exceeds 3%, the improvement effect is saturated. Further, Ca has an effect of increasing the flame retardancy of the molten metal, but less than 0.3% is not sufficient. Ca originally tends to lower the castability, but the deterioration of castability can be greatly improved by the inclusion of Sr described later. However, when the Ca content exceeds 3%, the castability is significantly lowered even when Sr is contained. Therefore, the Ca content is in the range of 0.3 to 3% by mass. Within that range, the lower limit is preferably 0.5% and the upper limit is 2.5%, and the lower limit is preferably 1.0% and the upper limit is more preferably 2.0%. Furthermore, an upper limit of 1.5% is more preferable.

  Sr (0.01 to 1% by mass): Sr has the effect of increasing the creep resistance as the content thereof increases and at the same time making it difficult to cause casting cracks. In addition, there is an effect of suppressing the generation of micro shrinkage porosity generated in the casting, enhancing the soundness of the casting, and improving the tensile strength and elongation. However, if it is less than 0.01%, those improvement effects are small, while if it exceeds 1%, those improvement effects are saturated. Therefore, the Sr content is set in the range of 0.01 to 1% by mass%. More preferred is a range of 0.21 to 0.5%.

One or more of La 0.1 to 3%, Ce 0.1 to 3%, Nd 0.1 to 3%, or a compound of aluminum and La, Ce or Nd (hereinafter referred to as La, etc.) is produced to improve the creep characteristics. To do. However, when the content of these elements is less than 0.1%, sufficient effects cannot be obtained. When the content exceeds 3%, cracking and seizure are likely to occur during casting. Further, when La or the like is added, a marked improvement in corrosion resistance is recognized, and 0.2% or more is preferable for obtaining this effect. In order to improve the above creep characteristics and corrosion resistance, the lower limit is preferably 0.5%, and the upper limit is preferably 1.5% for the same reason as described above. Conventionally, it has been intended to improve the creep characteristics by adding rare earth elements, but since only a relatively inexpensive misch metal has been used, a sufficient effect has not been obtained. Although the reason is not sufficiently clear, the inventors of the present application have found that a remarkable effect different from the conventional misch metal can be obtained by containing only La and the like, and have led to the present invention. It is.

Mn (0.1 to 1% by mass): Mn improves corrosion resistance, and further dissolves in the ground of magnesium to improve creep strength and proof strength, particularly high temperature proof strength. However, if the content is less than 0.1%, these effects are small. On the other hand, if the content exceeds 1%, a large amount of Mn single layer is crystallized, so that the magnesium alloy becomes brittle and the strength decreases. Therefore, the content of Mn is set in the range of 0.1 to 1%. Preferably it is 0.4 to 0.7% of range.

Si: 0.1 to 1%
Zn: 0.2 to 1%
When Si and Zn are contained in the above proportions, the following characteristics are further exhibited. That is, the magnesium alloy of the present invention may contain 0.1 to 1% (preferably 0.2 to 0.6%) of Si in addition to the above components. In addition to the above components, Zn may be contained in an amount of 0.2 to 1% (preferably 0.4 to 0.8%). That is, the magnesium alloy of the present invention that further contains Si in the above-described proportion tends to improve castability and hardly cause casting cracks. In addition, the magnesium alloy of the present invention that further contains Zn in the above-described proportion has a feature that the strength is improved by solid solution hardening.

Be (1-100 ppm)
Be has the effect of preventing oxidation during casting, improving castability, and improving strength. If the amount is less than 1 ppm, the effect is small. On the other hand, if the amount exceeds 100 ppm, the oxide film formed by adding excessive Be occurs, and the strength and elongation are lowered. For the same reason, the lower limit is preferably 5 ppm and the upper limit is preferably 50 ppm.

  As described above, according to the heat-resistant magnesium alloy casting of the present invention, by mass, Al is 1.5 to 6%, Ca is 0.3 to 3%, Mn is 0.1 to 1%, and Sr. Since it contains 0.01 to 1%, further contains one or more of La 0.1 to 3%, Ce 0.1 to 3%, Nd 0.1 to 3%, and the balance is composed of Mg and inevitable impurities Castability is good, and good casting is possible by many casting methods including die casting. The cast product has excellent creep strength at high temperatures, can be used for parts around the engine, and has excellent corrosion resistance.

The magnesium alloy of the present invention described above can be manufactured by a general Mg alloy melting technique. For example, it can be obtained by melting using an iron crucible or using an antioxidant gas such as a mixed gas of SF ₆ / CO ₂ / Air.

The heat-resistant magnesium alloy casting having the above alloy components can be produced by a general magnesium alloy casting technique. For example, it can be melted using an iron crucible. In casting, various casting methods such as a die casting method and a gravity casting method can be used. In the present invention, the die casting method and the gravity casting method are not particularly limited to the die casting method and the gravity casting method. In general, it is effective to manufacture by.
In the heat-resistant magnesium alloy casting according to the present invention, the cooling rate in the casting process is preferably in the range of 0.1 ° C./second or more and 100 ° C./second or less. This is because if the cooling rate is less than 0.1 ° C./second, the amount of crystallized matter that precipitates at the grain boundaries of Mg crystal grains increases, and the amount of Al solid solution decreases, resulting in a decrease in strength. If it exceeds 100 ° C./second, the area ratio of the crystallized product that crystallizes at the grain boundaries of the Mg crystal grains decreases, and the creep strength decreases.

  Next, specific applications of the magnesium alloy for die casting having the above-described composition include parts related to automobile engines such as cylinder blocks, cylinder heads, cylinder head covers, oil pans, oil pump bodies, oil pump covers, intake manifolds, etc. The structural members around the engine or the cases can be applied to case members around the engine such as a transmission case, a transfer case, a chain case steering case, a joint cover, and an oil pump cover.

Hereinafter, the effect of the present invention will be clarified using examples. However, the present invention is not limited to the following examples. A magnesium alloy having the composition shown in Table 1 below is melted in an electric furnace in a mixed gas atmosphere of SF ₆ / CO ₂ / Air using an iron crucible, and is cooled by a cold chamber die casting machine as shown in FIGS. 1 (a) and 1 (b). The castings having the shapes shown in FIG. The cooling rate during casting in this example was 70 ° C./second. The cast product 1 shown in FIGS. 1 (a) and 1 (b) is a plate material having a width of 70 mm and a height of 150 mm as a whole, and a 1/3 portion 4 in the height direction from the lower part of the plate material is 3 mm in thickness. The next 1/3 portion 3 has a thickness of 2 mm, and the remaining 1/3 portion 2 has a thickness of 1 mm. A portion 4 having a thickness of 3 mm is arranged on the side of the biscuit portion 5 on the side where the molten metal is poured into the mold, and an overflow portion 6 is formed so that the cast metal overflows from the portion 2 having a thickness of 1 mm. Is formed.

  The die castability of the magnesium alloy casting according to the present invention was evaluated based on casting cracks (hot cracking) during casting and seizure to the mold. Cast cracking occurs due to stress concentration during solidification shrinkage in the vicinity where the thickness of the cast product shown in FIG. 1 changes from 1 mm to 2 mm. For each alloy having the composition shown in Table 1 below, 100 shots were produced, the first 30 shots were discarded, and the average cast crack length per piece was determined for the remaining 70 shots, and the cast crack was evaluated. . The magnesium alloy casting of each sample was visually evaluated for seizure to the mold.

  Moreover, the plate-shaped test piece was cut out from the 3 mm-thick part of the casting 1 shown in FIG. 1, and the tensile test and the creep test were implemented. The tensile test was performed using a 10-ton autograph tester at a tensile speed of 5 mm / min at room temperature. The creep test was performed at 150 ° C. with a load of 50 MPa and a test time of 100 hours, and the minimum creep rate was determined from the creep curve to evaluate the creep characteristics. The results of evaluation carried out under the above conditions are shown in Table 2 below. Moreover, about a part of Example and the comparative example, the corrosion rate at the time of spraying salt water for 240 hours with respect to the test piece was shown as a parameter | index of corrosion resistance.

As shown in Table 2, it can be seen that the heat-resistant magnesium alloy castings of Examples 1 to 8 satisfying the requirements of the present invention have excellent tensile strength and proof stress, a minimum creep rate is small, and a casting crack length is short. . Further, it can be seen that the heat-resistant magnesium alloy castings of these Examples are castings having excellent characteristics with no occurrence of seizure during casting.
With respect to the heat-resistant magnesium alloy casting of the above example, the casting of Comparative Example 1 in which the Al content was 1% lower than the range of the present invention had a longer casting crack length. Further, the samples of Comparative Examples 1 and 4 were seized during casting.

Since the sample of Comparative Example 3 contains 0.1% of Ca lower than the range of the present invention, there is a problem that the minimum creep rate is large.
In a cast product with an Al content of 5%, Comparative Example 3 has a high minimum creep rate due to a low Ca content, and Comparative Example 4 has a too large Ca content and a significantly large casting crack length. It was.
The sample of Comparative Example 2 had a high Al content and a high minimum creep rate. Comparative Example 6 was a sample containing a large amount of Mn, but had a problem that the tensile strength was low.

  Moreover, since Comparative Example 5 does not contain La or the like, it can be seen that the creep strength is inferior. In Comparative Example 7, the other components were almost the same as those in Example 3. Instead of La, etc., misch metal (52.8% Ce, 27.4% La, 15% Nd, 4.7% Pr, 0 .1% Sm) is contained in approximately the same amount. As a result, it can be seen that Comparative Example 7 is clearly inferior in creep strength as compared with Example 3.

(A) is a side view of the casting obtained in the Example of this invention, (b) is a top view of a casting.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Cast product 2 Thickness 1mm part 3 Thickness 2mm part 4 Thickness 3mm part 5 Biscuit part 6 Overflow part

Claims (4)

Magnesium alloy excellent in heat resistance and castability, and in mass%, Al is 1.5 to 6%, Ca is 0.3 to 3%, Mn is 0.1 to 1%, and Sr is 0.01 to Containing 1%, further containing one or more of La 0.1 to 3%, Ce 0.1 to 3%, Nd 0.1 to 3%, the balance being composed of Mg and inevitable impurities Heat-resistant magnesium alloy castings. The heat-resistant magnesium alloy casting according to claim 1, further comprising 0.1 to 1% of Si as a component. The heat-resistant magnesium alloy casting according to claim 1 or 2, wherein 0.2 to 1% of Zn is contained. The heat-resistant magnesium alloy casting according to any one of claims 1 to 3, further comprising Be1 to 100 ppm as a component.

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