Magnesium alloy and magnesium alloy die casting

Abstract

A magnesium alloy and a die casting produced from the alloy include an AZ91-based magnesium alloy including 6.0 to 11.0 percent by weight of aluminum, 0.1 to 2.5 percent by weight of zinc, 0.1 to 0.5 percent by weight of manganese, and strontium and at least one of calcium and antimony added in an amount sufficient to act as a grain refining agent.

Description

The present invention relates to a lightweight magnesium alloy having a high specific rigidity. In particular, the present invention relates to a die casting magnesium alloy and a magnesium die casting including the same.

BACKGROUND OF THE INVENTION

Among widely used alloys, magnesium alloys (hereafter referred to as ¢Mg alloys”) are lightest high specific rigidity raw materials having a high electromagnetic shielding property and high thermal conductivity. AZ91 Mg alloy is the most widely used Mg casting alloy for forming housing materials of portable electronic equipment, such as notebook personal computers and cellular phones, portable electric tools, and the like.

This AZ91 alloy has excellent strength, corrosion resistance, moldability, and the like, and is widely used as a balanced casting alloy for die casting. However, this AZ91 alloy is believed to be unsuitable for applications to automobiles, motorcycles, and the like required to have high mechanical properties with respect to elongation, bending, and heat resistance. Consequently, in general, an AM60 alloy and an AM50 alloy containing a reduced amount of aluminum and having improved elongation are used for these applications.

On the other hand, in recent years, demands for weight reduction of automobile components have been intensified. In addition, covers disposed in the vicinity of car mounted engines also tend to be made of Mg alloys. Therefore, development of new alloys provided with heat resistance has been actively pursued, as described for example in U.S. Pat. No. 6,322,644.

One of the prior art heat resistant alloys is produced by improving the AM 50 alloy or the AM 60 alloy. The base thereof is Al: 2% to 9% and Sr: 0.5% to 7%, preferably is Al: 4% to 6%, and is Al: 4.5% to 5.5% and Zn: 0.35% or less (AM alloy level). The comparison between the alloy based on the above-described known technology and the AZ91 alloy is described, for example, in Pekguleryuz and Baril, Development of Creep Resistant Mg—Al—Sr Alloys, Magnesium Technology 2001, J. Hryn, Ed., pages 119-125. FIG. 17 is a diagram showing the content of comparisons described in this document.

However, the above-described alloys produced by improving the AM 50 alloy or the AM 60 alloy include the following problems. In FIG. 17, the lower row indicates an example of the above mentioned prior art alloys hereafter referred to as “known alloys”). The tensile strength of this alloy at room temperature (ambient temperature) is about 15% lower than that of the AZ91 alloy. Although the tensile strength at 175° C. is improved by about 7%, the elongation values at room temperature and at 175° C. are reduced. The creep characteristic value and the like is actually improved. However, when the material is used practically, since the material is exposed to from room temperature to a high temperature of about 175° C., the properties at room temperature cannot be neglected. In the above-described known technology, such a point is not taken into consideration and, therefore, reduction of the strength at room temperature cannot be prevented.

SUMMARY

Embodiments of the invention provide a magnesium alloy and a die casting produced from the alloy comprising an AZ91-based magnesium alloy comprising 6.0 to 11.0 percent by weight of aluminum, 0.1 to 2.5 percent by weight of zinc, 0.1 to 0.5 percent by weight of manganese, and strontium and at least one of calcium and antimony added in an amount sufficient to act as a grain refining agent.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are diagrams showing the melting point characteristic of an alloy in which Ca and Sr, and Sb, Ca and Sr, respectively, are added to an AZ91 alloy.

FIG. 1C is a diagram showing the melting point characteristic of alloys of the third preferred embodiment of the invention.

FIG. 2 is a diagram showing the change characteristic of the crystal grain size in the case where Ca is added to the AZ91 alloy.

FIG. 3A is a diagram showing the change characteristic of the crystal grain size in the case where Ca and Sr are added to the AZ91 alloy.

FIG. 3B is a diagram showing the effect of an addition of a grain refining agent to an AZ91 base alloy.

FIG. 3C is a diagram showing the effect of addition of Sr to an AZ91SbCa alloy.

FIG. 4 is a diagram showing an example of melting of an AZ91CaSr alloy.

FIGS. 5A and 5B are diagrams showing the range of proportion of addition of grain refining agents to the AZ91 alloy in first and second embodiments of the present invention, respectively.

FIGS. 6A and 6B are diagrams showing the behavior of change in crystal grain size ratio of the AZ91 alloy versus the AZ91CaSr alloy and the AZ91SbCaSr alloy, respectively.

FIGS. 7A and 7B are diagrams showing compositions of alloy ingots according to the first and third embodiments of the invention.

FIGS. 8A and 8B are diagrams showing the behavior of filling factor of the alloys.

FIGS. 9A and 9B are diagrams showing the behavior of room temperature tensile strength of the alloys.

FIGS. 10A and 10B are diagrams showing the behavior of room temperature elongation characteristic of the alloys.

FIGS. 11A, 11B and 11C are diagrams showing the behavior of the relationship between the high-temperature strain rate and the flow stress of the alloys.

FIGS. 12A, 12B and 12C are diagrams showing the behavior of the relationship between the high-temperature strain rate and the creep elongation value of the alloys.

FIGS. 13A, 13B and 13C are diagrams showing the relationship between the high-temperature creep rate and the stress of various magnesium alloys.

FIG. 14 is a diagram showing test conditions of a creep test.

FIGS. 15A, 15B and 15C are diagrams showing compositions of raw ingots used in a corrosion resistance test.

FIGS. 16A, 16B and 16C are diagrams showing the results of the corrosion resistance tests.

FIG. 17 is a diagram showing the content of comparisons of alloy characteristics described in known technical documents.

DETAILED DESCRIPTION OF THE EMBODIMENTS OF THE INVENTION

The embodiments of the invention provide a die casting magnesium alloy capable of improving the high-temperature creep characteristics without causing reduction of the room temperature strength and a magnesium die casting including the same. The magnesium alloy comprises an AZ91 alloy preferably containing a grain refining agent, which preferably comprises at least two different grain refining elements. Preferably, the alloy contains strontium (Sr) and at least one of calcium (Ca) and antimony (Sb) as the grain refining agent.

In a first embodiment of the invention, the alloy contains Sr and Ca as the grain refining agent. In a second preferred embodiment of the invention, the alloy contains Sr and Sb, or Sr, Sb and Ca as the grain refining agent. In a third embodiment of the invention, the base alloy contains at least one element selected from silicon (Si), misch metal containing a simple substance of rare earth (RE), zirconium (Zr), scandium (Sc), yttrium (Y), tin (Sn), and barium (Ba), and a grain refining agent selected from Sr and at least one of Sb and Ca.

The AZ91 die casting magnesium alloy preferably comprises a base alloy of magnesium containing Al, Zn and Mn alloying elements, and grain refining agents selected from Sr and at least one of Sb and Ca. The base alloy preferably contains at least 70 percent Mg by weight, such as at least 80 percent Mg by weight, for example about 85 to about 93 percent by weight Mg. The base alloy further comprises 6.0 to 11.0 percent by weight of aluminum, such as 7.0 to 11.0 percent aluminum, preferably 8.5 to 10 percent aluminum, 0.1 to 2.5 percent by weight of zinc, preferably 0.3 to 2.1 percent zinc, and 0.1 to 0.5 percent by weight of manganese, preferably 0.18 to 0.36 percent manganese. The alloy may also contain other unavoidable impurities in a trace amount.

Since the base alloy is AZ91, reduction of the strength characteristics at room temperature can be prevented in contrast to the AM60-based alloys. By adding Sr and at least one of Ca and Sb as grain refining agents to the AZ91-based alloy, the alloy structure can be improved and the crystal grain size can be made fine, and excellent high-temperature creep characteristics equivalent to or better than the characteristics of an AS21 alloy known as a heat-resistant magnesium alloy can be attained. As a result, an alloy having improved high-temperature creep characteristics can be realized without causing reduction of the room temperature strength.

In the first preferred embodiment, the grain refining agents preferably comprise 1.1 to 5 weight percent of the AZ91 alloy. Preferably, the grain refining agents comprise 1.0 to 3.5 percent by weight of calcium and 0.1 to 1.5 percent by weight of strontium which are added to the AZ91-based alloy. Specifically, when calcium comprises 1.0 to 3.5 percent by weight and strontium 0.1 to 1.5 percent by weight of the alloy, the average crystal grain size can be reliably controlled at 20 μm or less in a casting, such as 13 to 19 microns, for example. Since Ca and Sr are added to the AZ91 alloy, reduction of the strength characteristics at room temperature is prevented. Furthermore, the alloy structure can be improved by the addition of Ca and Sr, the crystal grain size can be made fine, and the high-temperature creep characteristics can be improved.

In a second embodiment of the present invention, the above described AZ91 base alloy contains grain refining agents which comprise Sr and at least one of Sb and Ca. Preferably, the grain refining agents comprise all three of Sr, Sb and Ca. Specifically, the alloy preferably contains 0.1 to 2.5 percent by weight Sr, such as 0.1 to 1.5 percent Sr, preferably 0.5 to 2.1 percent Sr. The alloy also preferably contains 0 to 1.5 percent by weight Sb, such as 0.1 to 1.5 percent Sb, preferably 0.3 to 1.2 percent Sb. The alloy also preferably contains 0.05 to 3.5 percent by weight Ca, such as 0.2 to 3.5 percent Ca, preferably 0.5 to 2.1 percent Ca.

Thus, by adding the grain refining agents, the crystal grain size can be reliably controlled at 20 μm or less, and excellent high-temperature creep characteristics equivalent to or better than the characteristics of an AS21 alloy known as a heat-resistant magnesium alloy can be reliably attained. As a result, an alloy having improved high-temperature creep characteristics can be reliably realized without causing reduction of the room temperature strength.

In a third embodiment of the present invention, the AZ91 die casting magnesium alloy comprises a base alloy of magnesium containing Al, Zn and Mn alloying elements, at least one alloy element which enters gaps of grain boundary products Mg₁₇Al₁₂ (β phase) and/or Al₂Ca crystals generated by addition of Ca and divide these phases, and grain refining agents selected from Sr and at least one of Sb and Ca.

The base alloy preferably contains at least 70 percent Mg by weight, such as at least 80 percent Mg by weight, for example about 85 to about 93 percent by weight Mg. The base alloy further comprises 6.0 to 11.0 percent by weight of aluminum, such as 7.0 to 11.0 percent aluminum, preferably 8.5 to 10 percent aluminum, 0.1 to 2.5 percent by weight of zinc, preferably 0.3 to 2.1 percent zinc, and 0.1 to 0.5 percent by weight of manganese, preferably 0.18 to 0.36 percent manganese.

The base alloy also contains at least one element which enters and divides the beta phase and Al₂Ca crystals, selected from at least any one of silicon (Si): 0.1 to 1.5 percent by weight, misch metal containing a simple substance of rare earth (RE): 0.1 to 1.2 percent by weight, zirconium (Zr): 0.2 to 0.8 percent by weight, scandium (Sc): 0.2 to 3.0 percent by weight, yttrium (Y): 0.2 to 3.0 percent by weight, tin (Sn): 0.2 to 3.0 percent by weight, and barium (Ba): 0.2 to 3.0 percent by weight. Any combination of these elements may be provided in the alloy.

The alloy also contains the grain refining agents which comprise Sr and at least one of Sb and Ca. Preferably, the grain refining agents comprise all three of Sr, Sb and Ca. Specifically, the alloy preferably contains 0.1 to 2.5 percent by weight Sr, such as 0.1 to 1.5 percent Sr, preferably 0.5 to 2.1 percent Sr. The alloy also preferably contains 0 to 1.5 percent by weight Sb, such as 0.1 to 1.5 percent Sb, preferably 0.3 to 1.2 percent Sb. The alloy also preferably contains 0.05 to 3.5 percent by weight Ca, such as 0.2 to 3.5 percent Ca, preferably 0.5 to 2.1 percent Ca. The alloy may also contain unavoidable impurities.

In the third embodiment, the AZ91 alloy includes elements which improve the high-temperature creep characteristics of the alloy while maintaining the high moldability and the high ambient temperature strength. The addition of Sb, Ca, and Sr grain refining agents improves the alloy structure and the crystal grain size can be made fine.

Thus, as described above, one or more of silicon, misch metal containing a simple substance of rare earth, zirconium, scandium, yttrium, tin, and barium are added to the AZ91 alloy. These alloying elements enter the gaps of grain boundary products Mg₁₇Al₁₂ (β phase) and Al₂Ca crystals generated by addition of Ca, which are assumed to be weak points in the characteristics of the AZ91 alloy, and divide those phases, so that the alloy strength can be increased. However, if excessively high amounts of these alloying elements are added, harmful effects, such as deterioration of moldability, occur. Therefore, preferably, silicon is added up to 1.5 percent by weight, misch metal containing a simple substance of rare earth is added up to 1.2 percent by weight, zirconium is added up to 0.8 percent by weight, and scandium, yttrium, tin, and barium are added up to 3.0 percent by weight. As a result, high moldability is retained and the thermal creep resistance can be further improved. Thus, AZ91 alloy contains at least any one of Si, RE, Zr, Sc, Y, Sn, and Ba, which are added to the AZ91 alloy to the extent that the moldability is not significantly changed. Consequently, reduction of the room temperature strength is prevented, and the moldability is maintained. Since these elements deposit (i.e., are located) in the gaps of β phases, which deposit at grain boundaries and become a cause of weakness, and divide those phases, the high-temperature creep characteristics can be improved reliably.

Furthermore, a magnesium die casting (i.e., a die cast part) is made by die-casting of the die casting AZ91-based magnesium alloy of the first, second or third embodiments. The die casting can be produced with good moldability by the use of the above described AZ91 alloys having improved high-temperature creep characteristics without causing reduction of the room temperature strength.

The inventors of the present invention conducted various experiments on die casting magnesium alloys and die castings to achieve an alloy having improved high-temperature creep characteristics while maintaining the excellent characteristics of the AZ91 alloy and without causing reduction of the room temperature strength in contrast to known alloys.

(1) Reduction of Crystal Grain Size

When the AZ91 alloy is subjected to die-casting, the crystal grain size becomes about 40 μm, and crystal grains are significantly made fine compared with the crystal grain size of about 200 to 300 μm of the alloy subjected to usual gravity casting. Therefore, grain refining agent used previously in the gravity casting were believed in the prior art to be unnecessary for the die-casting. The inventors of the present invention intentionally added the grain refining agent to the die casting alloy, and attempted die-casting of an ingot of this alloy.

Among various grain refining agents described in the prior art, some grain refining agents, such as hexachloroethane, have a high refining effect but release a chlorine gas when being added. Other grain refining agents, such as metal Na, are attended with significant danger in its handling.

Thus, the inventors selected Sr and at least one of Sb and Ca as grain refining agents. The effects of these agents are not impaired by remelting for die-casting.

Initially, melting points of the AZ91 alloy and an alloy according to the first embodiment in which 1% of Ca and 0.5% of Sr were added to the AZ91 alloy were measured by a furnace cooling method. As a result, it was made clear that the alloy including Ca and Sr had a melting point slightly lower than the melting point of the AZ91 alloy, as shown in FIG. 1A. Likewise, as shown in FIG. 1B, an alloy according to the second embodiment which contains 0.5% Sr, 0.5% Sb and 0.5% Ca had slightly lower melting point and freezing point temperatures than the alloy of the first embodiment. Furthermore, with respect to the alloy of the third embodiment, as shown in FIG. 1C, when the elements which enter the beta phase gaps are added to the alloy, the alloy melting point temperature is either maintained or reduced compared to the prior art AZ91 alloy. In FIG. 1C, a mixed rare earth metal (“MM”) containing 51% Ce by weight is used as the rare earth source. When each of the melts was remelted and poured into a respective casting mold, it was ascertained that every melt fluidity (corresponding to die castability in the case of application to a die casting) was good without causing any problem.

Next, 3 kg of AZ91 alloy was melted in an iron crucible coated with alumina, and was kept at 680° C. Thereafter, predetermined amounts of Ca and Sr were added, and the resulting melt was quickly cast by 100 g with a ladle in a pipe mold (thickness 3 mm, inner diameter 32 mm, depth 53 mm) heated to 100° C., so that a sample was prepared. This sample was transversely cut at the middle portion, and was subjected to a solution treatment at 410° C. for 2 hours in order to make grain boundaries clear. Polishing was performed to attain a mirror-finished surface, etching was performed with a 6% picric acid alcohol solution, and microscopic examination was performed. The crystal grain size was determined by a microtome method of a crystal grain size number measuring method in compliance with JIS on iron and steel.

FIG. 2 shows the measurement results of the crystal grain size in the case where Ca was added to the AZ91 alloy. Likewise, FIG. 3A shows the results in the case where Sr was added to a melt in which 1% of Ca was added to the AZ91 alloy of the first embodiment. FIG. 3B shows the effect of addition of refining agents to the alloy of the second embodiment. FIG. 3C shows the result when Sr is added to the alloy of the second embodiment containing 0.5% Sb and 0.5% Ca. FIG. 4 shows an example of melting of an AZ91CaSr alloy. FIG. 5A collectively shows the results of measurement of the crystal grain size on a sample basis where the amounts of addition of Ca and Sr were changed in the alloy of the first embodiment. FIG. 5B shows the same results for the alloy of the second and third embodiments.

As is clear from FIG. 2, FIG. 3A, FIG. 4, and FIG. 5A, in the case of no addition of the grain refining agents, the grain size was 40 μm, and even when Ca or Sr was added alone, it was unable to make the grain size 20 μm or less. However, it was made clear that when combined addition was performed within the range surrounded by a dotted line in FIG. 5A, the grain size was allowed to become 20 μm or less. In this range, Ca is 1.0 percent by weight or more and 3.5 percent by weight or less, and Sr is 0.1 percent by weight or more and 1.5 percent by weight or less in the alloy of the first embodiment. It should be noted that the grain refining elements may be added in amounts outside the above ranges to still achieve a decreased average grain size, such as a grain size of about 20 to about 30 microns. Furthermore, only one grain refining element may be added to still achieve a decreased average grain size, such as a grain size of about 20 to about 30 microns.

FIG. 3B shows the results of addition of Sb and Ca to the AZ91 alloy. The result indicated by Δ is derived from addition of Sb up to 1% followed by addition of Ca up to 2.5%. It is clear that the effect of Sb alone is slightly smaller, but an effect substantially equivalent to the effect of Ca is provided by the addition of Sb and Ca in combination. Thus, an average grain size between 20 and 30 microns may still be obtained without adding Sr.

FIG. 3C shows an example in which Sr was added to the melt after the combined Sb and Ca addition. When 0.6% or more of Sr was added, the crystal grain size was reduced to 20 μm or less. Likewise, an alloy in which 0.5% to 1.0% each of Si, RE, and Zr are added was subjected to the test, and similar results were exhibited with respect to the crystal grain size. Although respective crystals specific to Si, RE, and Zr appeared dispersedly (i.e., were dispersed in the alloy), the entire crystal grain size was not changed.

FIG. 5B shows an example of the examination results of the relationship between Sb, Ca, and Sr specified to be at various levels and the crystal grain size. In this case, the AZ alloys comprise the alloys of the second and third embodiments and refer to AZ91+0.5% Si alloy, AZ91+0.5% RE alloy, and AZ91+0.5% Zr alloy, in addition to the AZ91 alloy. As is clear from FIG. 5B, the grain size can be made 20 μm or less when combined addition is performed within the range surrounded by a dotted line, i.e. Sb: 0.5%, Ca: 0.5% to 3.0%, and Sr: 0.1 to 2.5%. As shown in FIG. 5B, the grain size ranges from 13 to 19 microns in the area bounded by the dashed lines. It should be noted that the grain refining elements may be added in amounts outside the above ranges to still achieve a decreased average grain size, such as a grain size of about 20 to about 30 microns. Furthermore, only one grain refining element may be added to still achieve a decreased average grain size, such as a grain size of about 20 to about 30 microns. It should also be noted that the Sb content in the alloy should not be considered to be limited to 0.5%, and the Sb content in the alloy is held constant at 0.5% in FIG. 5B for illustration of the preferred variation of Ca and Sr content only.

The present inventors conducted another experiment, and found out that with respect to the above-described Sb and Ca, even when only one of them was added, the refining effect similar to that provided by the alloys within the above-described range surrounded by the dotted line was attained as long as at least any one of Sb: 0.1% to 1.5% and Ca: 0.05% to 3.5% is added. Therefore, this grain refining effect can be attained by adding Sr: 0.1% to 2.5% and at least any one of Sb: 0.1% to 1.5% and Ca: 0.05% to 3.5% in combination.

Furthermore, the present inventors also conducted another experiment, and found out that it was not necessary to add the elements of the third embodiment which enter the beta phase gaps (i.e., Si, RE, Zr, and the like) to the AZ91 alloy to reduce the grain size as described above, and the refining effect as in the above description was able to be attained when Sr: 0.1% to 2.5% and at least any one of Sb: 0.1% to 1.5% and Ca: 0.05% to 3.5% were added.

When an alloy in which Ca and Sr are added to the AZ91 alloy is subjected to die-casting and is compared with the AZ91 alloy, the crystal grain size of the die casting was substantially equal to (i.e., about 1 to 1.03 times) the crystal grain size of the alloy cast into a pipe mold. On the other hand, as is clear from FIGS. 6A and 6B, the crystal grain size ratio of the AZ91CaSr alloy of the first embodiment and the AZ91SbCaSr alloy of the second embodiment to the AZ91 alloy was about 0.33 to about 0.34, even under different casting and molding conditions and, therefore, the size was made fine. Since it has been previously believed in the art that the grain refining agent is useless in die castings, this is believed to be a new finding.

When the crystal grain size is made fine as described above, the network of grain boundaries becomes fine-grained and, thereby, the strength of the material is increased. In addition, the thickness of the β phase deposited at grain boundaries is reduced and, thereby, coarse intermetallic compounds, which cause corrosion and which tend to be generated at grain boundaries, become hard to be generated, so that the corrosion resistance can be improved. The strength and the corrosion resistance of the AZ91 alloy of the third embodiment, in which Si, RE, Zr, Sc, Y, Sn, and Ba are added to the AZ91 alloy, are improved. These elements enter between intermetallic compounds at grain boundaries, and prevent the β phases and the like from becoming coarse. As a result, the characteristics are improved. It is believed that the operation mechanisms in the two cases are extremely similar to each other.

(2) Room Temperature Strength and Elongation Characteristics and Melt Fluidity

In consideration of the results in the above-described grain refining section, the present inventors determined the room temperature strength characteristics and the room temperature elongation characteristics of the above-described alloys of the first embodiment in which Ca and Sr were added to the AZ91 alloy (hereafter referred to as “Ca and Sr containing alloy”) and the AZ91 alloy of the third embodiment in which at least any one of Si, RE, Zr, Sc, Y, Sn, and Ba is added to the AZ91 alloy, and in which Sb, Ca, and Sr are also added to the AZ91 alloy (hereafter referred to as “Sb, Ca, and Sr containing alloy”).

Ingots shown in FIG. 7A (first embodiment) and FIG. 7B (third embodiment) were used. By using a test mold for a B5-size plate of 1.5 mm in thickness, 80 plates of each alloy were molded at a molding temperature (melting furnace temperature) of 650° C. and a mold temperature of 200° C. with a cold chamber die casting machine. Each of 5 molded plates was divided into 3 equal parts in the transverse direction and 2 equal parts in the longitudinal direction, so that 6 pieces were prepared. These pieces were subjected to a density measurement by a water displacement method, the theoretical density was determined by summation based on component values analyzed separately and a density table of atoms described in Kagaku Binran, and the filling factor in the mold was calculated. Furthermore, test pieces for ambient temperature tensile strength test were cut from 5 molded plates, and the tensile strength and the elongation value at room temperature were measured with an Instron type tensile tester.

FIGS. 8A and 8B show the filling factor of the above-described die-cast test plate for the Sr and Ca containing alloy and the Sb, Ca and Sr containing alloy. It is clear that the filling rate is increased when Sb and/or Ca and Sr are added. Specifically, the alloys according to the first and third embodiments had a filling rate of greater than 98% (i.e., about 98.6%).

FIGS. 9A and 9B show the room temperature tensile strength of the die casting (first and third bars) of the above described alloys. The measurement results of the die-cast test piece subjected to a solution treatment at 410° C. for 2 hours are also shown (second and fourth bars). As is clear from the drawing, in the case of as-cast, the Ca and Sr containing alloy of the first embodiment exhibits a value about 7% higher than the value of the AZ91 alloy. Specifically, the Sr and Ca containing alloy of the first embodiment exhibited a tensile strength of about 245 MPa and the Sb, Ca and Sr containing alloy exhibited a tensile strength of about 250 MPa. Thus, the alloys exhibited a tensile strength of above 230 MPa at room temperature. The strength of the AZ91 alloy was reduced by the solution treatment. As a result of observation of the test piece, bubbles were observed in some locations of the AZ91 alloy. It is believed that these were responsible for deterioration of the properties and reduction of the filling factor. In contrast, even when the Ca and Sr containing alloy and the Sb, Ca and Sr containing alloy were subjected to the solution treatment, reduction of the strength did not occur, nor were any bubbles observed. The tensile strength of these alloys was slightly higher than 250 MPa after the solution treatment.

FIGS. 10A and 10B show the elongation value of the alloys of the first and second embodiments, respectively. The elongation of the Ca and Sr containing alloy and the Sb, Ca and Sr containing alloy was substantially equivalent to the AZ91 alloy. When Sb and/or Ca and Sr were added to the AZ91 alloy, no bubble was involved in the die casting, the filling factor was increased, and the tensile strength was increased. Consequently, it is clear that the die castability is improved. The alloy of the first and third embodiments exhibited an elongation of about 3.5%.

The inventors of the present invention determined an influence exerted on the room temperature tensile strength by the base alloy components of the AZ91 alloy, to which Sb and/or Ca and Sr were added. They found that if the content of Al was less than 6 percent by weight, the above-described effect of improving the room temperature tensile strength was not observed. Therefore, it was assumed to be appropriate that the content of Al in the alloy should be at least 6 percent by weight, preferably 7 percent by weight or more in order to improve the room temperature tensile strength.

With respect to the effect of the addition of Si, RE, and Zr, in the third embodiment, the melt fluidity was visually inspected as described above, and thereby, the above-described upper limits were determined. It was ascertained that when the contents exceeded these upper limits, the viscosity was increased, and the melt fluidity was adversely affected. With respect to the lower limit values, the room temperature tensile strength of the alloy including them was checked, and the amounts at which the strength was improved were specified to be lower limit values.

Consequently, the present inventors determined that it was appropriate to specify the AZ91-based base alloy to be an alloy in which a predetermined amount of at least any one of Si, RE, Zr, Sc, Y, Sn, and Ba was added to the AZ91 alloy of Al: 6% to 11.0%, Zn: 0.1% to 2.5%, and Mn: 0.1% to 0.5%. These AZ91-based base alloys are collectively called AZ91-based alloys, and each graph shows an average of the entire thereof.

(3) High Temperature Creep Characteristics

Next, in consideration of the results in the above-described sections, the inventors of the present invention determined the high-temperature creep characteristics of the Ca and/or Sb and Sr containing alloy.

Test pieces were cut from 5 die castings, and creep data were determined at 175° C. with a constant-speed high-temperature creep tester. For the purpose of comparison, a common AZ91 alloy or other AZ-based alloy was subjected to a similar measurement.

FIGS. 11A, 11B, 11C, 12A, 12B, 12C, 13A, 13B and 13C show the results of the constant-speed high-temperature creep test at 175° C. for the alloys of the embodiments of the present invention (light bars) and for the alloys of the comparative examples (dark bars)

FIGS. 11A, 11B and 11C show the relationship between the strain rate and the flow stress. It is clear that with respect to the Ca and Sr containing alloy of the first embodiment in which Ca and Sr are added to the AZ91 alloy, the flow stress is improved by 10% to 20% at each strain rate and the creep resistance is increased compared with those of the AZ91 alloy. Likewise, with respect to the Sb, Ca and Sr alloy of the third embodiment, the flow stress is improved by 15% to 30%. Specifically, the flow stress of the alloy of the first embodiment was about 130 MPa, about 170 MPa and about 210 MPa for strain rates of 10⁻⁵, 10⁻⁴ and 10⁻³ s⁻¹, respectively. For the alloys of the third embodiment shown in FIGS. 11B and 11C, these values were about 150 MPa, about 180 to 190 MPa and about 220 to 240 MPa for strain rates of 10⁻⁵, 10⁻⁴ and 10⁻³ s⁻¹, respectively. Thus, the flow stress for the alloys of the embodiments of the invention ranges from about 130 MPa to about 150 MPa, from about 170 MPa to about 190 MPa and from about 210 MPa to about 240 MPa for strain rates of 10⁻⁵, 10⁻⁴ and 10³ s⁻¹, respectively.

FIGS. 12A, 12B and 12C show data of the creep elongation value at 175 degrees Celsius. The AZ61 alloy exhibits an elongation value of 25% or less depending on the strain rate, whereas the alloys of the first and third embodiment exhibit an elongation value of 29% or more at every strain rate. Specifically, the elongation for the alloy of the first and third embodiments ranged from about 29% to about 41%.

FIGS. 13A, 13B and 13C are diagrams in which the above-described results are additionally provided on a graph based on the data base of the Japan Magnesium Association for the purpose of comparison with other alloys. The measuring method of the above-described Association is a constant-stress method. According to the principle, both methods evaluate the same property. Data of other alloys at 175° C. based on documents are also shown. In FIGS. 13A-C, the lines labeled “Mercer” correspond to data from the following article: W. E. Mercer, II “Magnesium Die Cast Alloys for Elevated Temperature Applications”, SAE Paper No. 900788, SAE Warrendale, Pa., U.S.A., 1990. The lines labeled “Nagaoka University of Technology” correspond to data from the following article: Yasuhiro Gokan, Shigeharu Kamado, Suguru Takeda, et al. “Mg—Zn—Al—Ca—RE kei Goukin daikasutozaino mikurososhiki oyobi kouonkyodotokusei (Microstructure and high temperature strength characteristics of Mg—Zn—Al—Ca—RE-based alloy die casting)” the Japan Institute of Light Metals Dai 103 kai shukitaikai kouengaiyoshu (No. 103 Annual Meeting in Fall Lecture Summary) P-16, P. 375. As can be seen from FIGS. 13A-C, the lines corresponding to an alloys according to the first and third embodiments of the invention are located in the upper right hand corner of Figures. These lines indicate that the alloys according to the embodiments of the invention have a creep rate ranging from about 5×10⁻⁵ S⁻¹ to about 10⁻³ s⁻¹ for a load stress of about 160 MPa to about 240 MPa, respectively, at 175 degrees Celsius.

FIG. 14 shows measurement conditions of the creep test of the inventors of the present invention and the creep test of the Japan Magnesium Association, shown in FIGS. 13A-C.

In FIGS. 13A-C, a curve of the ZACE05411 alloy of the Nagaoka University of Technology rises crossing the AS21 alloy curve. Other data are the data on basic alloys of Mercer, and levels of the creep resistance of heat-resistant magnesium alloys, AS41, AS21, and AS42, can be understood.

The curves corresponding to the alloys according to the embodiments of the present invention are an extension of the creep characteristic curve of the AS21 alloy and, therefore, the creep resistance at 175° C. is assumed to be equivalent to that of the AS21. Therefore, it is clear that an alloy having high-temperature creep characteristics equivalent to or even better than that of the AS21 having excellent high-temperature creep characteristics can be attained by adding Sb and/or Ca and Sr to the AZ91 based alloy.

The inventors of the present invention separately determined an influence exerted on the room temperature tensile strength by the component of the AZ91 based alloy, to which Sb and/or Ca and Sr were added, instead of the amounts of the addition of Sb, Ca and Sr, as in the above description. When the content of Al exceeded 11 percent by weight, deterioration of the elongation value exceeded 1% and, thereby, it was assumed to be appropriate that the content of Al was specified to be 11 percent by weight or less in order to improve the high-temperature creep characteristics.

(4) Corrosion Resistance

The AZ91 alloy is an alloy having excellent corrosion resistance among the magnesium alloys. With respect to the alloy of the first and second embodiments, new elements, Sb and/or Ca and Sr, are added as grain refining agents. With respect to the alloy of the third embodiment, Si, RE, Zr, Sc, Y, Sn, and/or Ba are added to the AZ91 alloy so as to produce the AZ91-based base alloy, and new elements, Sb and/or Ca and Sr, are added as grain refining agents.

The present inventors determined that these alloys have sufficient corrosion resistance by conducting a salt spray test on these alloys and on a conventional AZ91 alloy as a comparative example.

The outline of the salt spray test conducted is described below. Raw ingots having compositions shown in FIG. 15A (alloys of the first embodiment) and in FIGS. 15B and 15C (alloys of the third embodiment) were used. With respect to die-casting, alloys A and B to be tested were die-cast at each molding temperature (melting furnace temperature) of 620° C., 650° C., and 680° C., so that plates were prepared. With respect to the shape of a salt spray test sample, the molded plate had a thickness of 0.7 mm, and was cut into 95 mm×130 mm. With respect to pretreatment conditions, no conversion treatment was conducted, and the surface was wiped with acetone.

With respect to the test method, Salt Spray CASS Test Instrument (produced by Suga Test Instruments Co., Ltd.) was used, the temperature in a test vessel was 35° C., and a spray pressure was 0.098 MPa (1 kgf/cm²). Under the above-described conditions, after spraying was performed continuously for 2 hours, the sample was washed with running water, and was left standing for 16 hours. The degree of occurrence of corrosion was visually evaluated on a 1-to-5 scale, “almost no corrosion was observed: −5″, “slight corrosion was observed: +4″, “corrosion was observed: ++3″, “corrosion was observed all over the surface: +++2″, and “significant corrosion was observed all over the surface: 1″.

FIGS. 16A, 16B and 16C show the results thereof. As shown in FIG. 16A, the AZ91+1.0% Ca+0.5% Sr alloy according to the first embodiment was evaluated as the above-described “corrosion was observed: ++3″, and the common AZ91 alloy was also evaluated as “corrosion was observed: ++3″. Therefore, it was made clear that there was not a significant difference in the corrosion resistance between the above-described two alloys, and the Ca and Sr containing alloy of the first embodiment was able to ensure the corrosion resistance substantially equivalent to that of the common AZ alloy. Likewise, FIGS. 16B and 16C show the results from the ingots of FIGS. 15B and 15C, respectively. The results in FIGS. 16B and 16C are similar to those of FIG. 16A.

As described above, according to the embodiments of the invention, a die casting magnesium alloy provided with a room temperature tensile strength equivalent to the AZ91 alloy and provided with excellent high-temperature creep characteristics while ensuring excellent die castability and corrosion resistance can be attained. In particular, the alloy according to the embodiments of the present invention is useful for magnesium die castings which covers from a room temperature region to a high-temperature region in applications to automotive parts, such as transmission covers and oil pans in which a weight reduction effect can be exerted, car air conditioner piston portion housings, airbag covers, engine covers, or the like.

The alloy of the embodiments of the present invention may be die cast into a die casting (i.e., die cast part) using any suitable die casting method. For example, the alloy of the embodiments of the present invention may be provided in a melted state, such as in a liquid state, into an injection chamber of a die casting machine. The melted alloy is then injected into a mold cavity from the injection chamber by a plunger. The alloy then solidifies into a casting in the mold cavity and is then subsequently removed from the mold cavity after solidification.

Japanese Priority Applications Numbers 2004-252764, 2004-175334 and 2004-004285 are incorporated herein by reference in their entirety. The foregoing description of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention. The description was chosen in order to explain the principles of the invention and its practical application. It is intended that the scope of the invention be defined by the claims appended hereto, and their equivalents.

Claims

1. An AZ91-based magnesium alloy, comprising: 6.0 to 11.0 percent by weight of aluminum, 0.1 to 2.5 percent by weight of zinc, 0.1 to 0.5 percent by weight of manganese, and strontium and at least one of calcium and antimony added in an amount sufficient to act as a grain refining agent.

2. The alloy of claim 1, wherein the grain refining agent comprises Sr and Ca.

3. The alloy of claim 2, wherein the alloy comprises 1 to 3.5 percent by weight of Ca and 0.1 to 1.5 percent by weight of Sr.

4. The alloy of claim 1, wherein the grain refining agent comprises Sr and Sb.

5. The alloy of claim 1, wherein the grain refining agent comprises Sr, Ca and Sb.

6. The alloy of claim 5, wherein the alloy comprises 0.05 to 3.5 percent by weight of Ca, 0.1 to 1.5 percent by weight of Sb and 0.1 to 2.5 percent by weight of Sr.

7. The alloy of claim 6, wherein the alloy comprises 0.5 to 3.5 percent by weight of Ca, 0.5 to 1.5 percent by weight of Sb and 0.1 to 2.5 percent by weight of Sr.

8. The alloy of claim 6, further comprising at least one element which enters gaps in a beta phase of the alloy and which divides the beta phase.

9. The alloy of claim 1, further comprising at least one element which enters gaps in a beta phase of the alloy and which divides the beta phase.

10. The alloy of claim 9, wherein the at least one element comprises at least one of silicon (Si): 0.1 to 1.5 percent by weight, rare earth misch metal (RE): 0.1 to 1.2 percent by weight, zirconium (Zr): 0.2 to 0.8 percent by weight, scandium (Sc): 0.2 to 3.0 percent by weight, yttrium (Y): 0.2 to 3.0 percent by weight, tin (Sn): 0.2 to 3.0 percent by weight, and barium (Ba): 0.2 to 3.0 percent by weight.

11. The alloy of claim 1, wherein the alloy comprises at least 70 percent by weight of magnesium.

12. The alloy of claim 1, comprising 7 to 10 percent by weight of aluminum, 0.3 to 2.1 percent by weight of zinc, 0.18 to 0.36 percent by weight of manganese, 0.1 to 1.5 by weight of Sr, and at least one of 0.5 to 2.1 percent by weight of Ca or a combination of Ca and Sb.

13. A magnesium die casting comprising the alloy of claim 1.

14. The casting of claim 13, wherein the casting comprises an average grain size of 20 microns or less.

15. The casting of claim 14, wherein the casting comprises:

an average grain size of 13 to 19 microns;

a room temperature tensile strength of greater than 230 MPa;

a high temperature elongation of at least 29% at 175° C.;

a high temperature flow stress of at least 130 MPa at a strain rate of 10−5 s−1; and

a high temperature flow stress of at least 210 MPa at a strain rate of 10−5 s−1, where the high temperature flow stress is measured at 175 ° C.

16. The casting of claim 13, wherein the casting comprises an automotive part.

17. An AZ91-based magnesium alloy containing a grain refining agent.

18. The alloy of claim 17, wherein the grain refining agent comprises at least one grain refining element.

19. The alloy of claim 18, wherein the grain refining agent comprises Sr and at least one of Sb and Ca.

20. A method of making a die casting, comprising:

providing a melted magnesium alloy into an injection chamber of a die casting machine, wherein the alloy comprises an AZ91-based magnesium alloy which comprises 6.0 to 11.0 percent by weight of aluminum, 0.1 to 2.5 percent by weight of zinc, 0.1 to 0.5 percent by weight of manganese, and strontium and at least one of calcium and antimony added in an amount sufficient to act as a grain refining agent; and

injecting the alloy into a mold cavity to form the magnesium die casting.

21. The method of claim 20, wherein the casting comprises an automotive part having a grain size of 20 microns or less.