JP2008266733A - Magnesium alloy for casting, and magnesium alloy casting - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a magnesium alloy for casting suitable for use at high temperature. <P>SOLUTION: The magnesium alloy for casting includes, by mass, 1 to 5% copper (Cu), 0.1 to 5% calcium (Ca) and tin (Sn) of 0.1 to 3 by a mass ratio (Sn/Ca) to the Ca, and the balance magnesium (Mg) with inevitable impurities. By the incorporation of Cu, Ca and Sn, the crystallized products of an Mg-Cu based compound and an Mg-Ca-Sn based compound are crystallized out on the crystal grain boundaries of Mg crystal grains, so as to be a network shape (three-dimensional network shape). By the three-dimensional network structure, boundary sliding particularly activated when temperature is made high is suppressed, so as to improve its high temperature strength and creep strength. Further, since Sn preferentially forms a compound with Ca, its influence on thermal conductivity is smaller than the case of the other additional elements. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a magnesium alloy for casting suitable for use at high temperatures.

  Magnesium alloys that are lighter than aluminum alloys are being widely used as aircraft materials and vehicle materials from the viewpoint of weight reduction. However, magnesium alloys are not sufficient in strength, heat resistance, and the like depending on applications, and thus further improvement in characteristics is required.

  For example, as a general magnesium alloy, there is AZ91D (ASTM symbol). Since the thermal conductivity of AZ91D is about 73 W / mK, when used in a member that is used at a high temperature or generates heat during use, heat dissipation is not performed well, and the member may be thermally deformed. . In particular, if a magnesium alloy with low thermal conductivity is used as the magnesium alloy used in the cylinder head or cylinder block of an internal combustion engine, the cylinder head is thermally deformed, the heat is accumulated in the cylinder block, and the cylinder bore is deformed. Adverse effects such as increase in the airtightness and decrease in airtightness occur. For this reason, there is a demand for a magnesium alloy that has a high thermal conductivity so that heat can be radiated well and is suitable for use at high temperatures.

  For example, the heat conductivity of a magnesium alloy having an alloy composition (unit: “mass%”) of Mg—3% Cu—1% Ca is higher than that of AZ91D due to the inclusion of Cu with high heat conductivity. Is also expensive. However, the creep resistance at a high temperature may not be sufficient depending on the use conditions.

Patent Document 1 discloses a magnesium alloy containing 0.8 to 5% by mass of calcium (Ca), 0 to 10% by mass of copper (Cu), and 3 to 8% by mass of zinc (Zn). Has been. The magnesium alloy of Patent Document 1 shows high strength at room temperature and high temperature, but there is no description about the thermal conductivity, and it is unclear whether or not the addition of zinc affects the thermal conductivity of the magnesium alloy.
JP-A-6-25791

  An object of this invention is to provide the magnesium alloy for casting suitable for use under high temperature in view of the said problem. Moreover, it aims at providing the casting which consists of the magnesium alloy for casting.

  As a result of diligent research, the inventors have improved the creep resistance at high temperatures without adversely affecting the thermal conductivity of the magnesium alloy by adding tin together with copper and calcium as the alloying element of the magnesium alloy. The present invention has been completed based on this finding.

  That is, the casting magnesium alloy of the present invention has a copper (Cu) content of 1% by mass to 5% by mass and calcium (Ca) of 0.1% by mass to 5% by mass when the total is 100% by mass. ) And tin (Sn) of 0.1 to 3 in terms of mass ratio (Sn / Ca) with respect to Ca, and the balance is made of magnesium (Mg) and inevitable impurities.

The magnesium alloy casting of the present invention is a casting made of the magnesium alloy for casting of the present invention. The magnesium alloy casting of the present invention is
When the total is 100% by mass, 1% by mass to 5% by mass of copper (Cu), 0.1% by mass to 5% by mass of calcium (Ca), and a mass ratio to the Ca (Sn / A pouring step of pouring a molten alloy containing 0.1 to 3 tin (Sn) and a balance of magnesium (Mg) and inevitable impurities into a mold;
A solidification step of cooling and solidifying the molten alloy after the pouring step;
It is obtained through the process.

  The magnesium alloy for casting of the present invention contains Cu, Ca, and Sn, so that the crystallized product of the Mg—Ca—Sn compound together with the Mg—Cu compound is network-like (third order) at the crystal grain boundary of the Mg crystal grains. Crystallized in the original mesh shape. The three-dimensional network structure suppresses intergranular sliding that becomes particularly active at high temperatures, and improves high-temperature strength and creep resistance at high temperatures. In addition, although Mg—Ca compounds are relatively brittle, Mg—Ca—Sn compounds in which part of Ca of Mg—Ca compounds is replaced with Sn have high strength, so that the strength of the three-dimensional network structure, and hence magnesium The strength of the alloy is improved.

  As will be described in detail later, since Sn forms a compound preferentially with Ca, it has less influence on thermal conductivity than other additive elements such as aluminum.

In the present specification, the description of “XY compound” and the like is, for example, a compound containing X and Y as main components as indicated by X ₂ Y in the composition formula.

  Below, the best form for implementing the magnesium alloy for casting of this invention is demonstrated.

  The magnesium alloy for casting according to the present invention includes copper (Cu), calcium (Ca), and tin (Sn), and the balance is composed of magnesium (Mg) and inevitable impurities.

  In the magnesium alloy for casting according to the present invention, when the content of Cu, Ca and Sn is set to an appropriate amount, the crystallized product of the Mg—Ca—Sn compound together with the Mg—Cu compound is converted into a crystal of Mg crystal grains. Crystallizes in the form of a network (three-dimensional network) at the grain boundary. Since it is a network with few discontinuities, the effect of suppressing grain boundary sliding is high.

  The content of Cu is 1% by mass or more and 5% by mass or less when the entire magnesium alloy for casting is 100% by mass. When the Cu content is 1% by mass or more, the Mg—Cu compound is sufficiently crystallized at the crystal grain boundary. When the Cu content is less than 1% by mass, the Mg—Cu compound is insufficiently crystallized at the crystal grain boundary, and therefore the strength is low. The preferable Cu content is 2% by mass or more. On the other hand, as the amount of Cu increases, the amount of the Mg—Cu-based compound crystallized at the crystal grain boundary becomes excessive and a brittle structure is formed, so that the strength decreases. The preferable Cu content is 4% by mass or less.

  The magnesium alloy for casting according to the present invention contains Ca and Sn together with Cu. Ca and Sn are present in the grain boundary together with Cu and contribute to the formation of a three-dimensional network structure. Specifically, the Mg—Ca—Sn compound is crystallized at the grain boundary together with the Mg—Cu compound, and a good three-dimensional network structure with few discontinuities is formed.

  The content of Ca is 0.1% by mass or more and 5% by mass or less when the entire magnesium alloy for casting is 100% by mass. When the content of Ca is 0.1% by mass or more, the Mg—Ca—Sn compound is sufficiently crystallized at the crystal grain boundary. Further, when Ca is added to the magnesium alloy, the ignition temperature of the magnesium alloy rises, so that combustion that may occur when the magnesium alloy is made into a molten metal is prevented. The preferable Ca content is 0.5% by mass or more. On the other hand, when the content ratio of Ca exceeds 5% by mass, the amount of grain boundary crystallized product is excessively increased, and mechanical properties such as tensile strength and elongation are lowered, which may cause problems in post-processing. . The preferable Ca content is 3% by mass or less, and further 2% by mass or less.

  The content of Sn is 0.1 to 3 in terms of mass ratio (Sn / Ca) to calcium (Ca). When the Sn content is 0.1% by mass or more, the Mg—Ca—Sn compound is sufficiently crystallized at the crystal grain boundary. On the other hand, when the content ratio of Sn increases, the Mg—Ca—Sn-based compound separates from the three-dimensional network structure and crystallizes in the crystal grains, and it becomes difficult to form a good three-dimensional network structure. As a result, creep resistance tends to decrease. Moreover, when Sn is added excessively, not only a Mg-Ca-Sn type compound but a Mg-Sn type compound is produced. Since the Mg—Sn compound is a low melting point compound, it deteriorates creep resistance. Therefore, the Sn content is 3 or less in terms of Sn / Ca. If Sn / Ca is 3 or less, formation of a low melting-point compound will be suppressed. Furthermore, if Sn / Ca is 2 or less, a good three-dimensional network structure with few discontinuities is formed, and high temperature strength and creep resistance at high temperature are improved. That is, the preferable Sn content is 2 or less, further 1.5 or less in terms of Sn / Ca.

  Sn makes a compound preferentially with Ca over Mg and Cu. Therefore, it is considered that Cu or Mg—Cu-based compounds having high thermal conductivity are not adversely affected, and as a result, the thermal conductivity of the magnesium alloy is hardly lowered. Therefore, considering the Sn / Ca stoichiometric ratio of the Mg—Ca—Sn compound, the mass ratio of Sn to Ca (Sn / Ca) is 3 or less, more preferably 0.1 or more and 2 or less. Preferably, there is almost no Sn forming a compound with the Mg—Cu-based compound, and crystallization of the low melting point compound is suppressed.

  The magnesium alloy for casting of the present invention described above can be used in various fields such as space and aviation, automobiles, electrical equipment and the like. In addition, as a member made of the magnesium alloy for casting according to the present invention, a product used under a high temperature environment, for example, a compressor, a pump, various cases, etc., which become high temperature during use, taking advantage of its high temperature characteristics. Component parts, engine parts used under high temperature and high load, especially cylinder heads of internal combustion engines, cylinder blocks and oil pans, impellers for turbochargers of internal combustion engines, transmission cases used in automobiles, etc. It is done.

  The magnesium alloy casting of the present invention is a casting made of the magnesium alloy for casting of the present invention described in detail above. That is, the magnesium alloy casting according to the present invention is a casting obtained through a pouring step and a solidification step, and the pouring step is performed in an amount of not less than 1% by mass and not more than 5% by mass when the total is 100% by mass. (Cu), 0.1 mass% or more and 5 mass% or less calcium (Ca), and 0.1 to 3 or less tin (Sn) by mass ratio (Sn / Ca) to Ca, and the balance The step of pouring molten alloy consisting of magnesium (Mg) and inevitable impurities into the mold, and the solidification step are steps of cooling and solidifying the molten alloy after the pouring step.

  The magnesium alloy casting of the present invention is not limited to ordinary gravity casting or pressure casting, but may be die casting. The mold used for casting may be a sand mold, a mold, or the like. There is no particular limitation on the solidification rate (cooling rate) in the solidification step, and a solidification rate at which a three-dimensional network structure is formed may be appropriately selected according to the size of the ingot. If solidified at a general solidification rate, a network-like metal structure can be obtained.

  Moreover, it is desirable that the magnesium alloy for casting and the magnesium alloy casting of the present invention be an as-cast material. Furthermore, you may improve the characteristic of a casting by heat-processing after casting.

  As mentioned above, although embodiment of the magnesium alloy for casting of this invention and magnesium alloy casting was described, this invention is not limited to the said embodiment. The present invention can be implemented in various forms without departing from the gist of the present invention, with modifications and improvements that can be made by those skilled in the art.

  The present invention will be specifically described below with reference to examples.

  A plurality of test pieces having different alloy element contents in the magnesium alloy were manufactured, their characteristics were evaluated, and the metal structure was observed.

[Production of test pieces # 01 to # 05]
Chloride-based flux was applied to the inner surface of an iron crucible preheated in an electric furnace, and weighed pure magnesium ingot, pure Cu, and pure Sn as needed, were dissolved. Furthermore, weighed Ca was added to the molten metal maintained at 750 ° C. (molten preparation step).

  The molten metal was sufficiently stirred to completely dissolve the raw material, and then kept calm at the same temperature for a while. The various alloy melts thus obtained were poured into a mold having a predetermined shape (pouring process) and solidified in an air atmosphere (solidification process) to cast # 01- # 05 test pieces (magnesium alloy castings). . In addition, the obtained test piece was 30 mm x 30 mm x 200 mm. Table 1 shows the chemical composition of each test piece.

[Measurement of thermal conductivity]
In addition to the # 01 to # 05 test pieces prepared by the above procedure, the thermal conductivity of the same test piece prepared from commercially available AZ91D (composition is described in Table 1) was determined by the laser flash method. The test results are shown in Table 1 and FIG.

[Stress relaxation test]
The test pieces prepared from the test pieces # 01 to # 05 and AZ91D shown in Table 1 were subjected to a stress relaxation test to examine the creep resistance of the magnesium alloy. The stress relaxation test measures a process in which stress when a load is applied to a test piece during a test time up to a predetermined deformation amount decreases with time. Specifically, in an air atmosphere at 200 ° C., a compressive stress of 100 MPa is applied to the test piece, and the compressive stress is adjusted with the passage of time so that the displacement of the test piece is kept constant. It was lowered. The amount of stress reduction 40 hours after the start of the test is shown in Table 1 and FIG. Moreover, the graph produced by plotting the compressive stress added to a test piece every 10 minutes is shown in FIG.

[Observation of metal structure]
The surfaces of test pieces # 01 to # 05 shown in Table 1 were observed. Surface observation was performed by observing a cross section cut out from each test piece with a metallographic microscope. The metal structures on the surfaces of # 01 to # 05 are shown in FIGS. 4 to 8, respectively. In each figure, (a) is a low magnification, and (b) is a high magnification, the same cross section was observed.

In test piece # 01, as can be seen from FIG. 4A, a three-dimensional network structure in which an intermetallic compound was crystallized at the grain boundary was confirmed. Further, in FIG. 4B, it was confirmed by EPMA (Electron Probe Microanalyzer) and XRD (X-ray diffraction) that CuMg ₂ appears bright at the grain boundary and Mg ₂ Ca appears dark. . Further, in test pieces # 02 and # 03, as can be seen from FIGS. 5 (a) and 6 (a), a three-dimensional network structure having finer mesh and higher continuity than # 01 was confirmed. Further, in FIGS. 5B and 6B, it was confirmed by EPMA and XRD that CuMg ₂ appeared bright at the grain boundaries and Mg ₂ Ca and MgCaSn appeared dark. On the other hand, in test pieces # 04 and # 05, as can be seen from each of FIGS. 7 and 8, the three-dimensional network structure was not completely formed because the crystallized product also crystallized inside the crystal grains. It was. Here, in FIG. 7 (b) and FIG. 8 (b), at least one crystallized substance crystallized in the grain boundary and in the grain is indicated by an arrow. 7 (b) and 8 (b), Ga is an intragranular crystallized product, and Gb is a grain boundary crystallized product. It was confirmed by EPMA and XRD that MgCaSn crystallized in the grains. Further, in test piece # 05, Mg ₂ Sn, which is a low melting point compound, was detected by XRD.

  All of the test pieces of # 01 to # 05 were superior in thermal conductivity to AZ91D. The thermal conductivity of the # 01 test piece not containing Sn was 155 W / mK, but no decrease in thermal conductivity due to the addition of Sn was observed in the # 02 to # 04 test pieces. On the other hand, in # 05 where the Sn content is excessive, although the thermal conductivity is superior to AZ91D, the difference in thermal conductivity from # 01 was large.

In addition, the test pieces of # 02 to # 04 made of a magnesium alloy containing Sn had less stress reduction after 40 hours from the start of the test in the stress relaxation test at 200 ° C. than the test piece # 01. That is, the addition of Sn to the Mg—Cu—Ca alloy (# 01) improved the creep resistance at high temperatures. This is presumed to be because # 02 to # 04 have a highly continuous three-dimensional network structure as compared to # 01 where there are many discontinuous parts in the three-dimensional network structure. On the other hand, # 04 containing 2% by mass of Sn was inferior to # 02 and # 03, although it had better creep resistance after 40 hours than # 01 containing no Sn. It is thought that the # 04 metal structure was due to the incomplete three-dimensional network structure than # 02 and # 03. # 05 containing 4% by mass of Sn is considered to have deteriorated creep resistance because the three-dimensional network structure is incomplete and Mg ₂ Sn which is a low melting point compound is contained.

  Further, the test pieces of # 02 and # 03 had a larger amount of stress reduction after about 3 hours after the start of the test than # 01, but the amount of change in stress from 3 to 40 hours was small and stable. In addition, the # 04 test piece had a greater amount of stress reduction after about 3 hours after the start of the test than # 01 and AZ91D, but the amount of change in stress from 3 to 40 hours was small and stable. (Figure 3)

  In addition, each said test piece made constant 3 mass% Cu and 1 mass% Ca. In any test piece, in the range of 2.7 mass% to 3.3 mass% for Cu and 0.7 mass% to 1.3 mass% for Ca, Shows similar thermal conductivity and creep resistance.

  That is, a magnesium alloy containing Cu, Ca, and Sn with appropriate contents does not show a decrease in thermal conductivity due to the addition of Sn, and is excellent in creep resistance at high temperatures.

It is a graph which shows the heat conductivity of the magnesium alloy from which an alloy composition differs. It is a graph which shows the stress fall amount 40 hours after a test start in the stress relaxation test of the magnesium alloy from which an alloy composition differs. It is the graph which plotted the compressive stress added to a test piece every 10 minutes with respect to the test time of a stress relaxation test. It is a drawing substitute photograph which shows the metal structure of Mg-3 mass% Cu-1 mass% Ca alloy (# 01). It is a drawing substitute photograph which shows the metal structure of Mg-3 mass% Cu-1 mass% Ca-0.1 mass% Sn (# 02) alloy. It is a drawing substitute photograph which shows the metal structure of Mg-3 mass% Cu-1 mass% Ca-1 mass% Sn (# 03) alloy. It is a drawing substitute photograph which shows the metal structure of Mg-3 mass% Cu-1 mass% Ca-2 mass% Sn (# 04) alloy. It is a drawing substitute photograph which shows the metal structure of Mg-3 mass% Cu-1 mass% Ca-4 mass% Sn (# 05) alloy.

Claims (5)

  When the total is 100% by mass, 1% by mass to 5% by mass of copper (Cu), 0.1% by mass to 5% by mass of calcium (Ca), and a mass ratio to the Ca (Sn / A magnesium alloy for casting which contains 0.1 to 3 tin (Sn) in Ca), the balance being magnesium (Mg) and inevitable impurities.   The magnesium alloy for casting according to claim 1, wherein the copper (Cu) is 2% by mass or more and 4% by mass or less.   The magnesium alloy for casting according to claim 1, wherein the calcium (Ca) is 0.5 mass% or more and 3 mass% or less.   The magnesium alloy for casting according to claim 1, wherein the tin (Sn) has a mass ratio (Sn / Ca) to the calcium (Ca) of 0.1 or more and 2 or less. When the total is 100% by mass, 1% by mass to 5% by mass of copper (Cu), 0.1% by mass to 5% by mass of calcium (Ca), and a mass ratio to the Ca (Sn / A pouring step of pouring a molten alloy containing 0.1 to 3 tin (Sn) and a balance of magnesium (Mg) and inevitable impurities into a mold;
A solidification step of cooling and solidifying the molten alloy after the pouring step;
Magnesium alloy casting characterized by being obtained through

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