Die casting magnesium alloy

Abstract: The present invention provides a die casting magnesium alloy having excellent heat resistance and castability, comprising 2 to 6% by weight of Al, 0.3 to 2% by weight of Ca, 0.01 to 1% by weight of Sr, 0.1 to 1% by weight of Mn, the balance magnesium and unavoidable impurities. A further improvement to the creep strength may be achieved if preferrably 0.1 to 3% by weight of Rare Earth Elements is added to the above composition.

Description: Field of the Invention The present invention relates to a die casting magnesium alloy having excellent heat resistance and castability. Background of the Invention For the purpose of weight saving, magnesium alloys have recently become of major interest in modes of transport, including automobiles. As these magnesium alloys, particularly casting magnesium alloys, for example, Mg-Al alloys containing 2 to 6% by weight of Al (e.g. AM60B, AM50A, or AM20A defined in ASTM [American Society for Testing and Materials] standard) or Mg-Al-Zn alloys containing 8 to 10% by weight of Al and 1 to 3% by weight of Zn (e.g. AZ91D defined in ASTM standard) have been known. These magnesium alloys have good castability and can be applied to die casting. However, in case such a magnesium alloy is used for parts for the proximity of an engine, the magnesium alloy is liable to cause yielding during use because of low creep strength at high temperature ranging from 125 to 175°C, e.g. 150°C, thus loosening bolts by which parts are clamped. For example, typical die casting alloy AZ91D has poor creep strength, although it has good castability, tensile strength and corrosion resistance. AE42 is known as a heat-resistant die casting alloy containing rare earth metals, but this alloy does not have good castability and also has poor creep strength. Therefore, there have recently been suggested alloys wherein Ca is added to a Mg-Al alloy (Japanese Patent Application, First Publication No. Hei 7-11374 and Japanese Patent Application, First Publication No. Hei 9-291332). However, these Mg-Al-Ca alloys have poor creep strength as compared with an aluminum alloy ADC12 (Al-1.5-3.5Cu-9.6-12.0Si;corresponding to AA A384.0), although the creep strength is improved. Furthermore, these Mg-Al-Ca alloys have a problem that misrun and casting cracks are caused by deterioration of the die-castability. Although these alloys contain rare earth elements as essential components, the cost increases when rare earth elements are added in a large amount. A thixocasting technique has recently been started to be applied to casting of magnesium alloys, unlike the die casting technique described above. This technique is considered to be effective to inhibit the occurrence of casting crack of the Mg-Al-Ca alloys because it is a method of performing injection molding in a semi-solid state. However, this technique has never been completed and is not applied to automobile parts at present. Therefore, the die casting technique is still used exclusively as a method of casting Mg alloys. EP-A-0 799 901 discloses a heat resistant magnesium alloy member having good creep resistance and containing no more than 0.15 wt.% SR. As disclosed in Japanese Patent Application, First Publication No. Hei 4-231435 (U.S. Patent No. 5,147,603), the application relating to a magnesium alloy having a load at tensile rupture of at least 290 MPa and an elongation at tensile rupture of at least 5%, essentially consisting of to 11% by weight of Al, 0 to 1% by weight of Mn, 0.1 to 6% by weight of Sr, the balance Mg, and less than 0.6% by weight of Si, less than 0.2% by weight of Cu, less than 0.1% by weight of Fe and less than 0.01% by weight of Ni as principal impurities has already been filed. The magnesium alloys of this patent application are alloys having high mechanical strength and excellent corrosion resistance produced by a rapid solidification method, and is produced in the form of band, powder or tip from a molten alloy by a roller quenching, spraying or atomization method. The patent described above discloses a technique of obtaining a product having a desired shape by consolidating the resulting band, powder or tip to form a billet, and subjecting the billet to conventional extrusion or hydrostatic extrusion. The alloy of the above patent application is an alloy produced by the rapid solidification process and has very high load at tensile rupture of 290 MPa or more, but this alloy is an alloy obtained only as a solid in the form of band, powder or tip by the rapid solidification process. In order to be formed into a desired shape of the product, alloy powders or alloy granules in the form of bands, powder or tips obtained by the rapid solidification process must be compacted by a heat consolidation molding method such as conventional extrusion or hydrostatic extrusion. Furthermore, finally obtainable shapes are limited. An object to be attained by the present invention is to provide a die casting magnesium alloy which has excellent heat resistance and castability and also has excellent creep properties. Another object to be attained by the present invention is to provide a die casting magnesium alloy which has the excellent properties described above and can be formed into a free shape by casting and can also be provided at low cost. Still another object to be attained by the present invention is to provide a die casting magnesium alloy which is suited to the production of parts having a complicated shape around the engine or thin-wall parts and has excellent heat resistance and castability, and also has excellent creep properties. Summary of the Invention As a result of an intensive study of the influence of additional elements on the castability and the creep strength of Mg-Al-Ca alloys containing Ca, the present inventors have found that the die-castability deteriorated by the addition of Ca can be remarkably improved and the creep strength can be further improved by adding Sr, thus completing the present invention. The present invention has been attained based on such knowledge, and the objects described above can be attained by die casting magnesium alloys having excellent heat resistance and castability, comprising the compositional as given by the claims. 2 to 6% by weight (hereinafter "to" indicates a numerical limitation range including an upper limit and a lower limit unless otherwise specified, and "2 to 6% by weight" represents the range of not less than 2% by weight and not more than 6% by weight) of Al, 0.3 to 2% by weight of Ca, 0.01 to 1% by weight of Sr, 0.1 to 1% by weight of Mn, the balance magnesium and unavoidable impurities. The Al content was limited to "2 to 6% by weight" based on the results of the test described below. When the Al content is not more than 6% by weight, a great portion of Al is incorporated into the matrix of Mg in the solid state. The tensile strength of the alloy is enhanced by solid-solution hardening. Also, the creep properties of the alloy are improved by the network-like structure of an Al-Ca compound crystallized out at grain boundary as a result of bonding with Ca. Al also improves the castability of the alloy. However, when the Al content exceeds 6% by weight, the creep properties rapidly deteriorate. On the contrary, when the Al content is less than 2% by weight, the above effects (effect of improving the tensile strength of the alloy by solid-solution hardening, effect of improving the creep properties) are poor. Particularly, when the Al content is less than 2% by weight, the resulting alloy is liable to have low strength and poor practicability. In light of the background described above, the Al content was set within a range from 2 to 6% by weight. The Al content is preferably within a range from 4.0 exclusive to 6% by weight, within the above range. And the creep properties is improved with the increase of the Ca content. When the Ca content is less than 0.3% by weight, the improvement effect is small. However, when the Ca content exceeds 2% by weight, the casting crack is liable to occur. In light of the background described above, the Ca content was set within a range from 0.3 to 2% by weight. The Ca content is preferably within a range from 0.5 to 1.5% by weight, within the above range. Further the creep properties improved with the increase of the Sr content and it becomes hard to cause casting crack. This effect is small when the Sr content is less than 0.01% by weight. On the other hand, when the Sr content exceeds 1% by weight, the effect reaches the saturated state. In the present invention, the Sr content was set within a range of more than 0.2 to 1% by weight. Under the circumstances described above, the maximum Sr content is preferably 0.5% by weight, and more preferably 0.4% by weight. In case Mn is added to this kind of an alloy, the corrosion resistance is improved and the creep strength is also improved. Furthermore, the proof stress, particularly high temperature proof stress is improved. This effect is small when the Mn content is less than 0.1% by weight. However, when the Mn content exceeds 1% by weight, a large mount of a primary elemental Mn particle is crystallized. Therefore, the resulting alloy becomes brittle, thereby lowering the tensile strength. For the reasons described above, the Mn content was set within a range from 0.1 to 1% by weight. The Mn content is more preferably within a range from 0.2 to 0.7% by weight. The essential elements in the Mg alloy of the present invention as given in the claims include Al, Ca, Sr and Mn, in addition to Mg. The other elements are either optional or contained as unavoidable impurities. However, when Si, Zn, and rare earth elements are contained in the proportion described below, the following advantages are also obtained. Sometimes, the die casting magnesium alloy of the present invention further contains 0.1 to 1% by weight (preferably 0.2 to 0.6% by weight) of Si, in addition to the components described above. Sometimes, the die casting magnesium alloy further contains 0.2 to 1% by weight (preferably 0.4 to 0.8% by weight) of Zn, in addition to the components described above. Sometimes, the die casting magnesium alloy further contains 0.1 to 3% by weight (preferably 0.5 to 2.0% by weight, more preferably 0.8 to 1.5% by weight) of rare earth elements, in addition to the components described above. Regarding the die casting magnesium alloy further containing Si in the proportion described above, it is made possible to obtain the advantage that the castability is further improved, thereby making it difficult to cause casting crack. Regarding the die casting magnesium alloy further containing Zn in the proportion described above, it is made possible to obtain the advantage that the tensile strength is improved by solid-solution hardening. Regarding the die casting magnesium alloy further containing rare earth elements in the proportion described above, it is made possible to obtain the advantage that the creep strength are further improved. Concretely, the alloy containing rare earth elements contains 2 to 6% by weight of Al, 0.3 to 2% by weight of Ca, more than 0.2 to 1% by weight of Sr, 0.1 to 1% by weight of Mn, 0.1 to 3% by weight of rare earth elements (one or more kinds of La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu), the balance Mg and unavoidable impurities. When the content of rare earth elements exceeds 3% by weight, casting crack increases and die-sticking becomes severe, thereby deteriorating the castability. Also, coarsening of the Al-RE compound in the constitution occurs, thereby deteriorating the mechanical properties. Furthermore, since rare earth elements are expensive elements, the smaller the amount, the better, in view of the cost. The die casting magnesium alloy of the present invention such as Mg-Al-Ca-Mn-Sr alloy is produced by a general technique of melting the Mg alloy. For example, the alloy can be obtained by melting in an iron crucible using a protective gas such as SF 6 /CO 2 /Air. The die casting magnesium alloy of the present invention has excellent mechanical properties such as tensile strength, proof stress, elongation, and the like and has excellent castability free from die-sticking during the casting, and also has excellent creep properties and corrosion resistance which are markedly excellent features for die casting magnesium alloys. According to the magnesium alloy of the present invention, it is made possible to obtain an excellent casting made of magnesium alloy, which is free from cracking and defects, even in case when thin-wall cast parts are produced. The die casting magnesium alloy of the present invention is markedly preferred as an alloy to produce by die casting parts for the proximity of an engine, and can provide an excellent die casting product. Brief Description of the Drawings FIG. 1 is a graph showing the relationship between the Ca content and the minimum creep rate. FIG. 2 is a graph showing the relationship between the Ca content and the average casting crack length. FIG. 3 is a graph showing the relationship between the Sr content and the minimum creep rate. FIG. 4 is a graph showing the relationship between the Sr content and the average casting crack length. FIG. 5 is schematic view showing a casting obtained in the embodiment, in which FIG. 5(a) is a side view of the casting and FIG. 5(b) is a plan view of the casting. Description of Preferred Embodiments The die casting magnesium alloy with the present invention can be applied to automobile parts around the engine, for example, structural members around an engine, such as cylinder blocks, cylinder heads, cylinder head covers, oil pans, oil pump bodies, oil pump covers, and intake manifolds; and cases, for example, case members around an engine, such as transmission cases, transfer cases, chain case stealing cases, joint covers, and oil pump covers. The Al content was limited to "2 to 6% by weight" based on the results of the test described below. When the Al content is not more than 6% by weight, a great portion of Al is incorporated into the matrix of Mg in the solid state. The tesile strength of the alloy is enhanced by solid-solution hardening. Also, the creep properties of the alloy are improved by the network-like structure of an Al-Ca compound crystallized out at grain boundary as a result of bonding with Ca. Al also improves the castability of the alloy. However, when the Al content exceeds 6% by weight, the creep properties rapidly deteriorate. On the contrary, when the Al content is less than 2% by weight, the above effects (effect of improving the tensile strength of the alloy by solid-solution hardening, effect of improving the creep properties) are poor. Particularly, when the Al content is less than 2% by weight, the resulting alloy is liable to have low strength and poor practicability. In light of the background described above, the Al content was set within a range from 2 to 6% by weight. The Al content is preferably within a range from 4.0 exclusive to 6% by weight, within the above range. The reason why the Ca content was limited within a range from 0.3 to 2% by weight in the embodiments is as follows. FIG. 1 is a graph showing an influence of the Ca content exerted on the minimum creep rate of the Mg alloy in case the Al content is 5% by weight, and FIG. 2 is a graph showing an influence of the Ca content exerted on the average casting crack length of the Mg alloy in case the Al content is 5% by weight. As is apparent from FIG. 1, the minimum creep rate decreases with the increase of the Ca content. When the Ca content is less than 0.3% by weight, the improvement effect is small. However, when the Ca content exceeds 2% by weight, the improvement effect is saturated and casting crack is liable to occur as shown in FIG. 2. In light of the background described above, the Ca content was set within a range from 0.3 to 2% by weight. The Ca content is preferably within a range from 0.5 to 1.5% by weight, within the above range. The reason why the Sr content was limited within a range from more than 0.2 to 1% by weight in the embodiments is as follows. FIG. 3 is a graph showing an influence of the Sr content exerted on the minimum creep rate of the Mg alloy in case the Al content is 5% by weight and the Ca content is 1.5% by weight, and FIG. 4 is a graph showing an influence of the Sr content exerted on the average casting crack length of the Mg alloy in case the Al content is 5% by weight and the Ca content is 1.5% by weight. As is apparent from FIG. 3 and FIG. 4, the minimum creep rate tends to decrease with the increase of the Sr content and it becomes hard to cause casting crack. This effect is small when the Sr content is less than 0.01% by weight. On the other hand, when the Sr content exceeds 1% by weight, the effect reaches the saturated state. As is apparent from the decrease of the creep rate shown in FIG. 3, low creep rate is maintained within a range from 0.1 to 0.5% by weight and a slight increase is observed within a higher content. Referring to FIG. 4, when the Sr content slightly increases within a range of not more than 0.1% by weight, the casting crack length rapidly decreases and a rapid decrease continues up to about 0.05% by weight. On the other hand, when the Sr content exceeds 0.05% by weight, the average casting crack length is certainly under 10 mm. When the Sr content exceeds 0.1% by weight, the casting crack length decreases to a sufficiently small value, although the decrease proportion of the casting crack length slightly reduces. When the Sr content exceeds 0.2% by weight, the casting crack length decreases to a degree which does not matter in practical use. In light of the background described above, the Sr content was set within a range from more than 0.2 to 1% by weight in the present invention. under the circumstances described above, the Sr content is preferably within a range to 0.4% by weight, within the above range. In case Mn is added to the compound to this kind of an alloy, the corrosion resistance is improved and the creep properties is also improved. Furthermore, the proof stress, particularly high temperature proof stress, is improved. This effect is small when the Mn content is less than 0.1% by weight. However, when the Mn content exceeds 1% by weight, a large amount of a primary elemental Mn particle is crystallized. Therefore, the resulting alloy becomes brittle, thereby lowering the tensile strength. For the reasons described above, the Mn content was set within a range from 0.1 to 1% by weight. The Mn content is more preferably within a range from 0.2 to 0.7% by weight. The essential elements in the Mg alloy of the present invention include Al, Ca, Sr, and Mn, in addition to Mg. The other elements are basically contained as unavoidable impurities. However, when Si, Zn, and rare earth elements are contained in the proportions described below, the following advantages are obtained. Sometimes, the die casting magnesium alloy of the present invention further contains 0.1 to 1% by weight (preferably 0.2 to 0.6% by weight) of Si, in addition to the components described above. Sometimes, the die casting magnesium alloy further contains 0.2 to 1% by weight (preferably 0.4 to 0.8% by weight) of Zn, in addition to the components described above. Sometimes, the die casting magnesium alloy further contains 0.1 to 3% by weight (preferably 0.5 to 2.0% by weight, more preferably 0.8 to 1.5% by weight) of rare earth elements, in addition to the components described above. Regarding the die casting magnesium alloy further containing Si in the proportion described above, it is made possible to obtain the advantage that the castability is further improved, thereby making it difficult to cause casting crack. Regarding the die casting magnesium alloy further containing Zn in the proportion described above, it is made possible to obtain the advantage that the tensile strength is improved by solid-solution hardening. Regarding the die casting magnesium alloy further containing rare earth elements in the proportion described above, it is made possible to obtain the advantage that the creep properties are further improved. Concretely, the alloys containing rare earth elements contain 2 to 6% by weight of Al, 0.3 to 2% by weight of Ca, more than 0.2 to 1% by weight of Sr, 0.1 to 1.0% by weight of Mn, 0.1 to 3% by weight of rare earth elements (one or more kinds of La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu), the balance being Mg and unavoidable impurities. When the content of rare earth elements exceeds 3% by weight, casting crack increases and die-sticking to the die becomes severe, thereby deteriorating the castability. Also, coarsening of the Al-RE compound in the constitution occurs, thereby deteriorating the mechanical properties. Furthermore, since rare earth elements are expensive elements, the smaller the amount, the better, in view of the cost. The die casting magnesium alloy of the present invention such as Mg-Al-Ca-Mn-Sr alloy is produced by a general technique of melting the Mg alloy. For example, the alloy can be obtained by melting in an iron crucible, using a protective gas such as SF 6 /CO 2 /Air. The present invention will be described by way of more specific embodiments, but the present invention is not limited by the following embodiments. Mg alloys with the composition show in Table 1 and Table 2 belowweremelted in an iron crucible using an electric furnace under an atmosphere of a mixed gas of SF 6 /CO 2 /Air to form a molten alloy, followed by casting using a cold chamber die casting machine to obtain a casting 1 having the shape show in FIG. 5(a) and FIG. 5(b). The casting 1 shown in FIG. 5(a) and FIG. 5(b) is a plate material generally having a width of 70 mm and a height of 150 mm, and a one-third portion of this plate material is a first portion 1 having a thickness of 3 mm, another one-third portion thereof is a second portion 3 having a thickness of 2 mm, and still another one-third portion thereof is a third portion 4 having a thickness of 1 mm. The first portion having a thickness of 3 mm is arranged at the side of a biscuit portion 5, which is the side where a molten metal is poured into a die, followed by continuous formation of the second portion 3 having a thickness of 2 mm and the third portion 4 having a thickness of 1 mm and further formation of an overflow portion 6 where the poured metal overflows at the tip end of the third portion 4. Rare earth elements were added to the molten metal in the form of a misch metal (52.8% Ce, 27.4% La, 15% Nd, 4.7% Pr and 0.1% Sm). During the casting, the die-castability was evaluated by the presence or absence of the occurrence of casting crack (hot cracking) and die-sticking. Casting crack is caused by stress concentration during the solidification shrinkage in the vicinity of the portion where the thickness of the casting 1 shown in FIG. 5(a) and FIG. 5(b) changes from 1 mm to 2 mm. With respect to samples of the respective alloys, casting of 100 shots was performed and the first 30 shots were scrapped. With respect to the remainder 70 shots, the average casting crack length per one shot was determined and casting crackability was evaluated by this casting crack length. Die-sticking was visually observed. Furthermore, plate-shaped test samples were cut from the portion having a thickness of 3 mm out of the casting, and then the tensile test and the creep test were performed. The tensile test was performed at room temperature under the conditions of a cross head speed of 5 mm/minute using a 10-tons Instron-type testing machine. The creep test was performed at a temperature of 150°C under a load of 50 MPa for 100 hours, and then the minimum creep rate was determined from a creep curve and creep properties were evaluated by the minimum creep rate. The smaller the minimum creep rate, the better the creep properties. In case salt water is sprayed over the sample for 240 hours, the measured corrosion weight loss is shown as an index of the corrosion resistance. These results are shown in Table 3 and Table 4 below. As is apparent from the results shown in Table 1 to Table 4, the alloy with the composition within the range of the present invention makes it possible to produce a die casting alloy which has excellent tensile strength and proof stress and exhibits small minimum creep rate and short casting crack length, and which has excellent corrosion resistance (small corrosion weight loss) and does not cause die-sticking during the casting. The sample of Comparative Embodiment 1 is a sample containing Al in the amount of 1.0% by weight smaller than 2% by weight as the lower limit of the range of the present invention, and it exhibited large minimum creep rate and large casting crack length and caused die-sticking and decrease in tensile strength, and also exhibited large corrosion weight loss. The sample of Comparative Embodiment 2 is a sample containing Al incorporated therein in the amount of 7.0% by weight with greater than 6% by weight as the upper limit of the range of the present invention, and the minimum creep rate increased. The sample of Comparative Embodiment 3 is a sample containing Ca in the amount of 0.1% by weight with less than 0.3% by weight as the lower limit of the range of the present invention, and the minimum creep rate increased, while the sample of Comparative Embodiment 4 is a sample containing Ca in the amount of 2.5% by weight with greater than 2% by weight as the upper limit of the range of the present invention, and the casting crack length drastically increased and die-sticking also occurred. The sample of Comparative Embodiment 5 is a Sr-free sample, and it exhibited large minimum creep rate and large casting crack length, while the sample of Comparative Embodiment 6 is a sample containing Mn in the amount of 1.5% by weight with greater than 1.0% by weight within the range of the present invention, and the proof stress decreased and the minimum creep rate increased. The samples of Comparative Embodiments 7, 8, 9, and 10 are samples wherein the amount of rare earth elements exceeds 3% by weight and any of Mn, Si and Zn is added or the addition of any one of them is omitted, and they exhibited excellent creep properties, but the casting crack length slight increased and die-sticking also occurred. The sample of Comparative Embodiment 12 is a sample containing Sr in an amount less than the lower limit of the range of the present invention, and the minimum creep rate was slightly large and the casting crack length increased. Comparative Embodiments 13 show the measurement results of the sample containing Ca in the amount less than the lower limit in the state where Si is contained, while Comparative Embodiments 14 show the measurement results of the sample containing Sr in the amount smaller than the lower limit in the state where Zn is contained. The samples of Comparative Embodiments 13 exhibited slight large minimum creep rate, the sample of Comparative Embodiment 14 exhibited slight large minimum creep rate and large casting crack length. As is apparent from the above description, the alloys (comparative embodiments) with the composition departing from that of the present invention are inferior in any of tensile strength, proof stress, elongation, creep properties, casting crack length, die-sticking, and corrosion resistance to the alloys with the composition of the embodiments.

Claims: A die casting magnesium alloy having excellent heat resistance and castability consisting of: 2 to 6% by weight of Al, 0.3 to 2% by weight of Ca, more than 0.2 to 1% by weight of Sr, 0.1 to 1% by weight of Mn, optionally 0.1 to 1% by weight of Si, optionally 0.2 to 1% by weight of Zn, optionally 0.1 to 3% by weight of rare earth elements, and the balance being magnesium and unavoidable impurities. A die casting magnesium alloy according to Claim 1, comprising 0.1 to 1% by weight of Si and 0.2 to 1% by weight of Zn. A die casting magnesium alloy according to Claim 1, comprising 0.1 to 1% by weight of Si and 0.1 to 3% by weight of rare earth elements. A die casting magnesium alloy according to Claim 1, comprising 0.2 to 1% by weight of Zn and 0.1 to 3% by weight of rare earth elements. A die casting magnesium alloy according to Claim 1, comprising 0.1 to 1% by weight of Si, 0.2 to 1% by weight of Zn and 0.1 to 3% by weight of rare earth elements. A structural member for use around an engine, the member being formed from a die casting magnesium alloy having a composition as defined in any preceding Claim.

Inventors: NAKAURA YUSUKE, OHORI KOICHI, SAKAGAMI TAKESHI, NAKAURA, YUSUKE, OHORI, KOICHI, SAKAGAMI, TAKESHI, Nakaura, Yusuke, Mitsubishi Aluminum Co., Ltd., Ohori, Koichi, Mitsubishi Aluminum Co., Ltd., Sakagami, Takeshi, Mitsubishi Aluminum Co., Ltd.
Applicants: MITSUBISHI ALUMINIUM, MITSUBISHI ALUMINUM CO.,LTD.
Application year: 2001
Main classification: C22C23
IPC codes: C22C23, B22D21