EP2138595A1

EP2138595A1 - Magnesium alloy for casting and magnesium alloy cast

Info

Publication number: EP2138595A1
Application number: EP08740692A
Authority: EP
Inventors: Yuki Okamoto; Kyoichi Kinoshita; Motoharu Tanizawa; Kazuhiko Yoshida
Original assignee: Toyota Industries Corp
Current assignee: Toyota Industries Corp
Priority date: 2007-04-20
Filing date: 2008-04-14
Publication date: 2009-12-30
Also published as: US20100209285A1; JP2008266733A; WO2008133218A1

Abstract

Amagnesiumalloy for casting according to the present invention is characterized in that, when the entirety is taken as 100% by mass, it includes copper (Cu) in an amount of from 1% by mass or more to 5% by mass or less, calcium (Ca) in an amount of from 0.1% by mass or more to 5% by mass or less, tin (Sn) in an amount of from 0.1 or more to 3 or less by mass ratio with respect to the Ca (Sn/Ca); and the balance comprising magnesium (Mg) and inevitable impurities.

By means of including Cu, Ca and Sn, crystallized substances of Mg-Ca-Sn compounds crystallize in crystalline grain boundaries between Mg crystalline grains as network shapes (three-dimensionally mesh shapes), along with Mg-Cu compounds. By means of the three-dimensionally mesh constructions, grain-boundary sliding, which becomes active especially when becoming high temperature, is suppressed, and thereby high-temperature strength and creep resistance at high temperature improve. Moreover, since Sn forms compounds with Ca preferentially, it influence less on the heat conductivity, compared with the other additive elements.

Description

TECHNICAL FIELD

The present invention is one which relates to a magnesium alloy for casing, magnesium alloy which is suitable for service under high temperature.

BACKGROUND ART

Magnesium alloy, which is much more light weight than aluminum alloy is, is about to come to be used widely for aircraft material, vehicle material, and the like, from the viewpoint of weight saving. However, in magnesium alloy, since the strength and heat resistance are not sufficient depending on applications, further improvement of the characteristics has been sought.
For example, as a general magnesium alloy, AZ91D (ASTM code) is present, for instance. Since the heat conductivity of AZ91D is 73 W/mK approximately, when it is used in member whose service environment is high temperature, or in member that generates heat in service, the radiation of heat cannot be carried out satisfactorily, and thereby thermal deformation might occur in the member. In particular, when a magnesium alloy whose heat conductivity is low is used as a magnesium alloy that is used in a cylinder head or cylinder block of internal combustion engine, the cylinder head undergoes thermal deformation, or heat dwells within the cylinder block so that the cylinder bores deform, and thereby adverse affects, such as increased friction and declined air tightness, occur. Consequently, a magnesium alloy has been sought, magnesium alloy in which the radiation of heat is carried out satisfactorily by possessing a high heat conductivity, and magnesium alloy which is thereby suitable for usage under high temperature.
For example, the heat conductivity of a magnesium alloy that has an alloy composition of Mg-3%Cu-1%Ca (the units being "% by mass") is higher than the heat conductivity of AZ91D, because Cu whose heat conductivity is high is included. However, there might be a case where the creep resistance at high temperature is not sufficient, depending on service conditions.
In Japanese Unexamined Patent Publication (KOKAI) Gazette No. 6-25, 791 , a magnesium alloy is disclosed, magnesium alloy which includes calcium (Ca) in an amount of 0.8-5% by mass, copper (Cu) in an amount of 0-10% by mass, and zinc (Zn) in an amount of 3-8% by mass. Although the magnesium alloy set forth in Japanese Unexamined Patent Publication (KOKAI) Gazette No. 6-25,791 exhibits high strength at room temperature and high temperature, nothing is set forth on the heat conductivity, and accordingly it is unclear whether the addition of zinc influences the heat conductivity of magnesium alloy or not.

DISCLOSURE OF THE INVENTION

In view of the aforementioned problematic issues, it is an object for the present invention to provide a magnesium alloy for casting, magnesium alloy which is suitable for usage under high temperature. Moreover, it is an object to provide a cast product, which comprises that magnesium alloy for casting.
As a result of wholehearted studies, the present inventors found out that it is possible to improve the creep resistance of magnesium alloy at high temperature by adding tin along with copper and calcium as another alloying element of the magnesium alloy, without ever affecting the heat conductivity adversely, and then arrived at completing the present invention based on this.
Specifically, a magnesium alloy for casting according to the present invention is characterized in that, when the entirety is taken as 100% by mass, it includes:

copper (Cu) in an amount of from 1% by mass or more to 5% by mass or less;
calcium (Ca) in an amount of from 0.1% by mass or more to 5% by mass or less;
tin (Sn) in an amount of from 0.1 or more to 3 or less by mass ratio with respect to the Ca (Sn/Ca); and
the balance comprising magnesium (Mg) and inevitable impurities.

Since the magnesium alloy for casting according to the present invention includes Cu, Ca and Sn, crystallized substances of Mg-Ca-Sn compounds crystallize in crystalline grain boundaries between Mg crystalline grains as network shapes (three-dimensionally mesh shapes), along with Mg-Cu compounds. By means of the three-dimensionally mesh constructions, grain-boundary sliding, which becomes active especially when becoming high temperature, is suppressed, and thereby high-temperature strength and creep resistance at high temperature improve. Moreover, Mg-Ca compounds are brittle comparatively; however, since strength is high in the Mg-Ca-Sn compounds in which Sn substitutes for a part of Ca of the Mg-Ca compounds, the strength of the three-dimensionally mesh constructions and eventually the strength of the magnesium alloy improve.
Moreover, it will be detailed later that, since Sn forms compounds with Ca preferentially, it influence less on the heat conductivity, compared with the other additive elements such as aluminum.
Note that, in the present description, the expressions such as "X-Y compounds" are compounds, in which "X" and "Y" make the major components, like those which are represented with such a compositional formula as "X₂Y," for instance.
Moreover, a magnesium-alloy cast product according to the present invention is a cast product that comprises the magnesium alloy for casting according to the present invention. The magnesium-alloy cast product according to the present invention is characterized in that it is obtainable by way of the following:

a molten-metal pouring step of pouring an alloy molten metal into a casting mold, the alloy molten metal including: copper (Cu) in an amount of from 1% by mass or more to 5% by mass or less; calcium (Ca) in an amount of from 0.1% by mass or more to 5% by mass or less; tin (Sn) in an amount of from 0.1 or more to 3 or less by mass ratio with respect to the Ca (Sn/Ca) ; and the balance comprising magnesium (Mg) and inevitable impurities; when the entirety is taken as 100% by mass; and
a solidifying step of solidifying the alloy molten metal after the molten-metal pouring step by cooling it.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a graph for illustrating the heat conductivities of magnesium alloys whose alloy compositions differ.
Fig. 2 is a graph for illustrating the stress lowering magnitudes from the beginning of testing to 40 hours later in a stress relaxation test on magnesium alloys whose alloy compositions differ.
Fig. 3 is a graph in which compression stresses being applied to test specimens are plotted with respect to the testing time of the stress relaxation test for every 10 minutes.
Fig. 4A and Fig. 4B are photographs for substituting for drawings that show the metallic structure of Mg-3%-by-mass Cu-1%-by-mass Ca alloy (#01).
Fig. 5A and Fig. 5B are photographs for substituting for drawings that show the metallic structure of Mg-3%-by-mass Cu-1%-by-mass Ca-0.1%-by-mass Sn alloy (#02).
Fig. 6A and Fig. 6B are photographs for substituting for drawings that show the metallic structure of Mg-3%-by-mass Cu-1%-by-mass Ca-1%-by-mass Sn alloy (#03).
Fig. 7A and Fig. 7B are photographs for substituting for drawings that show the metallic structure of Mg-3%-by-mass Cu-1%-by-mass Ca-2%-by-mass Sn alloy (#04).
Fig. 8A and Fig. 8B are photographs for substituting for drawings that show the metallic structure of Mg-3%-by-mass Cu-1%-by-mass Ca-4%-by-mass Sn alloy (#05).

Best Mode for Carrying Out the Invention

Hereinafter, the best mode for carrying out the magnesium alloy for casting according to the present invention will be explained.
The magnesium alloy for casting according to the present invention is characterized in that it includes copper (Cu), calcium (Ca), tin (Sn), and the balance comprising magnesium (Mg) and inevitable impurities.
In the magnesium alloy for casting according to the present invention, the crystallized substances of Mg-Ca-Sn compounds crystallize as network shapes (three-dimensionally mesh shapes) in crystalline grain boundaries between Mg crystalline grains, along with Mg-Cu compounds, by setting the contents of Cu, Ca and Sn to appropriate amounts. Since they are network shapes with less discontinuous parts, the effect of suppressing grain-boundary slip is high.
When the entire magnesium alloy for casting is taken as 100% by mass, the content of Cu is from 1% by mass or more to 5% by mass or less. When the content of Cu is 1% by mass or more, the Mg-Cu compounds crystallize sufficiently in the crystalline grain boundaries. When the content of Cu is less than 1% by mass, the strength is low because the crystallization of the Mg-Cu compounds to the crystalline grain boundaries is insufficient. A preferable content of Cu can be 2% by mass or more. Meanwhile, the greater the content of Cu is, the more excessive the amount of the Mg-Cu compounds that crystallize in the crystalline grain boundaries becomes, thereby turning into brittle structures so that the strength lowers. A preferable content of Cu can be 4% by mass or less.
The magnesium alloy for casting according to the present invention includes Ca and Sn along with Cu. Ca and Sn exist in the crystalline grain boundaries along with Cu, and thereby contribute to the formation of three-dimensionally mesh constructions. To be concrete, the Mg-Ca-Sn compounds crystallize in the crystalline grain boundaries along with the Mg-Cu compounds, and then favorable three-dimensionally mesh constructions with less discontinuous parts are formed.
When the entire magnesium alloy for casting is taken as 100% by mass, the content of Ca is from 0.1% by mass or more to 5% by mass or less. When the content of Ca is 0.1% by mass or more, the Mg-Ca-Sn compounds crystallize sufficiently in the crystalline grain boundaries. Moreover, since the ignition temperature of magnesium alloy rises when adding Ca to the magnesium alloy, the combustion that might occur when making the magnesium alloy into a molten metal is prevented. A preferable content of Ca can be 0.5% by mass or more. Meanwhile, when the content proportion of Ca surpasses 5% by mass, problems might arise in post-processing because the generation amount of the grain-boundary crystallized substances has become too much so that the mechanical strengths, such as the tensile strength and the elongation, lower. A preferable content of Ca can be 3% by mass or less, further preferably 2% by mass or less.
The content of Sn is from 0.1 or more to 3 or less by mass ratio with respect to calcium (Ca) (Sn/Ca). When the content of Sn is 0.1% by mass or more, the Mg-Ca-Sn compounds crystallize in the crystalline grain boundaries sufficiently. Meanwhile, when the content proportion of Sn becomes great, the Mg-Ca-Sn compounds separate from the three-dimensionally mesh constructions so that they come to crystallize even within the crystalline grains, and thereby favorable three-dimensional mesh constructions become less likely to be formed. As a result, the creep resistance tends to lower. Moreover, in the case of adding Sn excessively, not only the Mg-Ca-Sn compounds but also Mg-Sn compounds arise. Since the Mg-Sn compounds are compounds with low melting points, they degrade the creep resistance. Consequently, the content of Sn is set to 3 or less by Sn/Ca. When Sn/Ca is 3 or less, the formation of the low-melting-point compounds is suppressed. Further, when Sn/Ca is 2 or less, favorable three-dimensionally mesh constructions with less discontinuous parts are formed so that the high-temperature strength and the creep resistance at high temperature improve. Specifically, a preferable content of Sn can be 2 or less, further preferably 1.5 or less, by Sn/Ca.
Note that the Sn makes compounds with Ca more preferentially than with Mg and Cu. Consequently, it does not have a bad influence on Cu and the Mg-Cu compounds whose heat conductivities are high; and, as a result of this, it is believed that the heat conductivity of magnesium alloy becomes less likely to lower. Considering this fact, namely, in view of the stoichiometric ratio of Sn/Ca in the Mg-Ca-Sn compounds, it is preferable to set the mass ratio of Sn with respect to Ca (Sn/Ca) to 3 or less, further preferably from 0.1 or more to 2 or less; there is hardly any Sn that forms compounds with the Mg-Cu compounds, and accordingly the crystallization of above-described low-melting-point compounds is also suppressed.
Beginning with the fields of space, military and aviation, the magnesium alloy for casting according to the present invention being explained as above can be used in various fields, such as automobiles and electric instruments. Moreover, as a member comprising the magnesium alloy for casting according to the present invention, the following can be given, taking advantage of its characteristics at high temperature: products being utilized in high-temperature environments, for example, component parts constituting compressor, pumps and various cases that become high temperatures in service; moreover, engine component parts being used under high temperature and high load, especially, cylinder heads, cylinder blocks and oil pans of internal combustion engine, impellers for turbocharger of internal combustion engine, transmission cases being used for automobile and the like, and so forth.
Moreover, the magnesium-alloy cast product according to the present invention is a cast product that comprises the magnesium alloy for casting according to the present invention being detailed as above. Specifically, the magnesium-alloy cast product according to the present is a cast product that is obtainable by way of a molten-metal pouring step, and a solidifying step; the molten-metal pouring step is a step of pouring an alloy molten metal into a casting mold, the alloy molten metal including: copper (Cu) in an amount of from 1% by mass or more to 5% by mass or less; calcium (Ca) in an amount of from 0.1% by mass or more to 5% by mass or less; Sn in an amount of from 0.1 or more to 3 or less by mass ratio with respect to the Ca (Sn/Ca) ; and the balance comprising magnesium (Mg) and inevitable impurities; when the entirety is taken as 100% by mass; and the solidifying step is a step of solidifying the alloy molten metal after the molten-metal pouring step by cooling it.
The magnesium-alloy cast product according to the present invention is not limited to those made by ordinary gravity casting and pressure casting, but can even be those made by die-cast casting. Moreover, even the casting mold being utilized for the casting does not matter if it is sand molds, metallic molds, and the like. Since even the solidification rate (cooling rate) in the solidifying step is not limited in particular, it is allowable to properly select such an extent of solidification rate, which permits to form the three-dimensionally mesh constructions, depending on the size of ingots. Note that, when it is solidified at a usual solidification rate, the network-shaped metallic structure is obtainable.
Moreover, it is desirable that themagnesiumalloy for casting and magnesium-alloy cast product according to the present invention can be an as-cast material. It is even allowable to improve the characteristics of the cast product by means of heat treating it after casting.
So far, the embodimentmodes of themagnesiumalloy for casting and magnesium-alloy cast product according to the present invention have been explained, however, the present invention is not one which is limited to the aforementioned embodiment modes. It can be conducted in various modes to which modifications, improvements, and the like, which one of ordinary skill in the art can carry out, are performed, within a range not departing from the scope of the present invention.
Hereinafter, while giving specific examples, the present invention will be explained in detail.
Test specimens whose contents of alloying elements in magnesium alloys were varied were made in a quantity of plural pieces, and then the evaluation of their characteristics and the observation of their metallic structures were carried out.
(Making of Test Specimens #01-#05)
A chloride-system flux was coated onto the inner surface of a crucible being made of iron that had been preheated within an electric furnace, and then a weighed pure magnesium base metal, pure Cu, and pure Ag, if needed, were charged into it and were then melted. Further, weighed Ca was added into this molten metal that was held at 750 °C (i.e., a molten-metal preparing step).
After fully stirring this molten metal to melt the raw materials completely, it was held calmly at the same temperature for a while. The thus obtained various alloy molten metals were poured into a metallic mold with a predetermined configuration (i.e., a molten-metal pouring step), and were then solidified in air atmosphere (i.e., a solidifying step), thereby casting test specimens (i.e., magnesium-alloy cast products) being labeled #01-#05. Note that the obtained test specimens had a size of 30 mm × 30 mm × 200 mm. The chemical compositions of the respective test specimens were specified in Table 1.
(Measurement of Heat Conductivity)
In addition to regarding the test specimens being labeled #01-#05 that were made with the aforementioned procedures, regarding similar test specimens that were made from the commercially available AZ91D (the composition is set forth in Table 1), the heat conductivities were found by means of laser flash method. The test results are specified in Table 1 and Fig. 1.
(Stress Relaxation Test)
Onto test specimens being made from Test Specimens #01-#05 and AZ91D that are specified in Table 1, a stress relaxation test was carried out, thereby examining the creep resistances of the magnesium alloys. In the stress relaxation test, processes of how stresses decreased with time were measured, stresses which arose when a load was applied to the test specimens in the course of testing time until they exhibited a predetermined deformation magnitude. To be concrete, in 200 °C air atmosphere, a compression stress of 100 MPa was loaded to the test specimens, and then the compression stress was lowered in agreement with the elapse of time so as to keep the displacements of the test specimens at that time constant. The stress lowering magnitudes from the beginning of testing to 40 hours later are specified in Table 1 and Fig. 2. Moreover, a graph is illustrated in Fig. 3, graph which was prepared by plotting the compression stresses, which were applied to the test specimens, for every 10 minutes.
(Observation of Metallic Structure)
The surfaces of Test Specimens #01-#05 given in Table 1 were observed. The surface observation was carried out by observing cross sections, which were cut out of the respective test specimens, with a metallographic microscope. Although the metallic structures in the surfaces being labeled #01-#05 are shown in Fig. 4A-Fig. 8A and in Fig. 4B-8B, respectively, Fig. 4A-Fig. 8A are for observing identical cross sections with (a) lowmagnification; and Fig. 4B-Fig. 8B are for observing identical cross sections with (b) high magnification, in the respective drawings.
In Test Specimen #01, the three-dimensionally mesh constructions were confirmed, three-dimensionally mesh constructions which comprised intermetallic compounds that crystallized in crystalline grain boundaries, as can be appreciated from Fig. 4A. Moreover, it was ascertained by means of EPMA (electron probe microanalyzer) and XRD (X-ray diffraction) that, in Fig. 4B, those looking bright at the crystalline grain boundaries are CuMg₂ and those looking dark thereat are Mg₂Ca . Moreover, in Test Specimens #02 and #03, the three-dimensionally mesh constructions were confirmed, three-dimensionally mesh constructions whose meshes were finer and continuities were higher than those of #01, as can be appreciated from Fig. 5A and Fig. 6A. Moreover, it was ascertained by means of EPMA and XRD that, in Fig. 5B and Fig. 6B, those looking bright at the crystalline grain boundaries are CuMg₂ and those looking dark thereat are Mg₂Ca and MgCaSn. Meanwhile, in Test Specimens #04 and #05, the three-dimensionally mesh constructions were not formed completely because of the fact that the crystallized substances crystallized even inside the crystalline grains, as canbe appreciated from the other respective diagrams, namely, Fig. 7A as well as Fig. 7B and Fig. 8A as well as Fig. 8B. Here, in Fig. 7B and Fig. 8B, the crystallized substances that crystallized at the grain boundaries and within the grains are designated with the arrows for one of their locations at least. In Fig. 7B and Fig. 8B, "Ga" is the crystallized substances within a grain, and "Gb" is the crystallized substances at a grain boundary. It was ascertained by means of EPMA and XRD that those crystallizing within the grains were MgCaSn. Further, in Test Specimen #05, Mg₂Sn being a low-melting-point compound was detected by means of XRD.

(TABLE 1)

No.	Alloy Composition (% by mass)	Heat Conductivity (W/mK)	Stress Lowering Magnitude (MPa)
#01	Mg-3%Cu-1%Ca	155	65
#02	Mg-3%Cu-1%Ca-0.1%Sn	163	54
#03	Mg-3%Cu-1%Ca-1%Sn	154	52
#04	Mg-3%Cu-1%Ca-2%Sn	148	59
#05	Mg-3%Cu-1%Ca-4%Sn	122	67
AZ91D	Mg-9%Al-1%Zn	73	60

Any one of the test specimens being labeled #01-#05 were better than AZ91D in terms of the heat conductivity. Although the heat conductivity of the test specimens being labeled #01 that did not include any Sn was 155 W/mK, lowering of the heat conductivity that resulted from the addition of Sn was not seen in test specimens being labeled #02-#04. Meanwhile, in #05 in which the content of Sn was excessive, although the heat conductivity was better than that of AZ91D, the difference was great between the heat conductivity and that of #01.
Moreover, in the test specimens being labeled #02-#04, the stress lowering magnitudes were less than that of Test Specimen #01 from the beginning of testing to 40 hours later in the stress relaxation test at 200 °C. Specifically, the creep resistance at high temperature was improved by adding Sn to an Mg-Cu-Ca alloy (i.e., #01). This is presumed that #02-#04 had the three-dimensionally mesh constructions with high continuities more, compared with #01 in which discontinuous parts are present more in the three-dimensionally mesh constructions. On the contrary, in #04 including Sn in an amount of 2% by mass, although the creep resistance after 40 hours was better than that of #01 that did not include any Sn, it was inferior to those of #02 and #03. It is believed that the metallic structure of #04 was more incomplete than those of #02 and #03. In #05 including Sn in an amount of 4% by mass, it is believed that the creep resistance degraded because the three-dimensionally mesh constructions were incomplete and included Mg₂Sn being a low-melting-point compound.
Further, in test specimens being labeled #02 and #03, although the stress lowering magnitudes were greater than that of #01 from the beginning of testing to about 3 hours later, the changing magnitudes of the stress from 3 up to 40 hours were small and stabilized. Moreover, in test specimens being labeled #04, although the stress lowering magnitude was greater than those of #01 and AZ91D from the beginning of testing to about 3 hours later, the changing magnitude of the stress from 3 up to 40 hours was small and stabilized. (Fig. 3).
Note that, in the aforementioned respective test specimens, the content of Cu was kept constant at 3% by mass and the content of Ca was kept constant at 1% by mass. Even in any one of the test specimens, they exhibit the heat conductivity and creep resistance to the same extent as those of the aforementioned respective test specimens when they fall in a range of from 2.7% by mass or more to 3.3% by mass or less for the case of being the content of Cu, and in a range of from 0.7% by mass or more to 1.3% by mass or less for the case of being the content of Ca.
Specifically, a magnesium alloy that includes Cu, Ca and Sn in the appropriate amounts does not show any lowering of the heat conductivity resulting from the addition of Sn, and is good in terms of the creep resistance at high temperature.

Claims

A magnesium alloy for casting, the magnesium alloy being characterized in that, when the entirety is taken as 100% by mass, it includes:
copper (Cu) in an amount of from 1% by mass or more to 5% by mass or less;

calcium (Ca) in an amount of from 0.1% by mass or more to 5% by mass or less;

tin (Sn) in an amount of from 0.1 or more to 3 or less by mass ratio with respect to the Ca (Sn/Ca); and

the balance comprising magnesium (Mg) and inevitable impurities.
The magnesium alloy for casting as set forth in claim 1, wherein the content of said copper (Cu) is from 2% by mass or more to 4% by mass or less.
The magnesium alloy for casting as set forth in claim 1, wherein the content of said calcium (Ca) is from 0.5% by mass or more to 3% by mass or less.
The magnesium alloy for casting as set forth in claim 1, wherein the content of said tin (Sn) is from 0.1 or more to 2 or less by mass ratio with respect to said Ca (Sn/Ca).
The magnesium alloy for casting as set forth in claim 1 having a structure in which Mg-Cu compounds and Mg-Ca-Sn compounds are crystallized as network shapes in crystalline grain boundaries between Mg crystalline grains.
A magnesium-alloy cast product being characterized in that it is obtainable by way of the following:
a molten-metal pouring step of pouring an alloy molten metal into a casting mold, the alloy molten metal including: copper (Cu) in an amount of from 1% by mass or more to 5% by mass or less; calcium (Ca) in an amount of from 0.1% by mass or more to 5% by mass or less; tin (Sn) in an amount of from 0.1 or more to 3 or less by mass ratio with respect to the Ca (Sn/Ca) ; and the balance comprising magnesium (Mg) and inevitable impurities; when the entirety is taken as 100% by mass; and

a solidifying step of solidifying the alloy molten metal after the molten-metal pouring step by cooling it.
The magnesium-alloy cast product as set forth in claim 6 having a structure in which Mg-Cu compounds and Mg-Ca-Sn compounds are crystallized as network shapes in crystalline grain boundaries between Mg crystalline grains.