JP4285188B2 - Heat-resistant magnesium alloy for casting, casting made of magnesium alloy and method for producing the same - Google Patents

Description

  The present invention relates to a heat-resistant magnesium alloy having excellent castability such as die casting.

  In recent years, the need for lighter materials has increased, and magnesium alloys that are lighter than aluminum alloys have attracted attention. Magnesium alloys are the lightest among practical metals and are being used as materials for automobiles in addition to aircraft materials. For example, magnesium alloys have already been used for automobile wheels and engine head cover materials. Furthermore, due to the recent demand for strong weight reduction, the applicable range of this magnesium alloy is further expanding. For example, it is considered to apply a magnesium alloy to a structural part such as an engine block that becomes high temperature and a functional part such as a piston.

By the way, a magnesium alloy product is usually manufactured by die casting or the like in many cases. For this reason, in order to spread the magnesium alloy, its castability is also important. In other words, it is necessary to suppress and prevent the occurrence of casting cracks and to improve yield and cost.
Under such circumstances, various magnesium alloys aimed at improving heat resistance and castability have been developed so far. For example, the following Patent Documents 1 to 3 propose magnesium alloys that achieve both heat resistance and castability.

Japanese Patent Laid-Open No. 7-331375 JP 2001-59125 A Japanese National Patent Publication No. 10-513225

  Patent Document 1 discloses an Mg—Al—Zn—RE—Mn—Ca alloy. However, according to the study of the present inventor, Al improves the castability of the magnesium alloy, but rather lowers the heat resistance such as creep resistance. Therefore, an Mg alloy containing a large amount of Al is not preferable as a heat-resistant magnesium alloy for casting.

  The Mg—Zn—Ca—R.E.—Zr alloy disclosed in Patent Document 2 exhibits excellent heat resistance and castability. However, when a large amount of Zn is contained, although a large casting crack does not occur, a fine casting crack can occur on the surface of the casting depending on the shape of the casting.

  Patent Document 3 discloses an Mg-Zn-RE alloy having improved heat resistance, castability and corrosion resistance, and also describes that it is preferable to add Ca for improving castability. . However, examples in which Ca is actually added are not disclosed. According to the inventor's research, as will be described later, Ca is an element that improves heat resistance, but does not improve castability in the presence of Zn.

  The present invention has been made in view of such circumstances, and based on an Mg- (Zn) -Ca-RE alloy, the heat-resistant magnesium for casting that can improve heat resistance and castability more than ever. Provide alloy. Another object of the present invention is to provide a magnesium alloy casting that has excellent heat resistance and suppresses casting cracks and the like, and a method for producing the same.

  As a result of intensive studies and various systematic experiments to solve this problem, the present inventors have found that the coexistence of Ca and Zn has a great influence on the cast cracking property. Thus, the present inventors have completed the present invention by newly finding an alloy composition capable of achieving both compatibility and cast cracking at a high level.

(Heat-resistant magnesium alloy for casting)
That is, the heat-resistant magnesium alloy for casting of the present invention is 0.2 to 3% by mass of calcium (Ca) and 1 to 4% by mass of rare earth element (RE) when the whole is 100% by mass. And less than 1% by mass of zinc (Zn) and 0.3 to 1% by mass of zirconium (Zr) , the balance consisting of magnesium (Mg) and inevitable impurities, and the inclusion of Ca in the Zn The quantity ratio (Ca / Zn) is 0.2 to 4, and is characterized by excellent castability and heat resistance.

(Magnesium alloy castings)
Moreover, this invention can be grasped | ascertained not only as the said heat-resistant magnesium alloy for casting but as a magnesium alloy casting casted using it.
That is, according to the present invention, when the total is 100% by mass, 0.2 to 3% by mass of Ca, 1 to 4% by mass of RE, less than 1% by mass of Zn, and 0.3 % -1% by mass of Zr , the balance being Mg and inevitable impurities, the content ratio of Ca to Zn (Ca / Zn) is 0.2-4, excellent heat resistance and casting It may be a magnesium alloy casting characterized by almost no cracks.

(Manufacturing method of magnesium alloy casting)
Furthermore, this invention can be grasped | ascertained also as a manufacturing method of the said magnesium alloy casting.
That is, in the method for producing a magnesium alloy casting according to the present invention, when the whole is 100% by mass, 0.2 to 3% by mass of Ca, 1 to 4% by mass of RE, and 1% by mass. Less than Zn and 0.3-1 mass% Zr , the balance consists of Mg and inevitable impurities, and the content ratio of Ca to Zn (Ca / Zn) is 0.2-4 an injection step of the molten magnesium alloy is injected into a mold Ru Oh, and a solidifying step of solidifying the molten metal injected into the template, that the magnesium alloy casting casting cracks hardly with excellent heat resistance obtained It is good also as the manufacturing method of the casting made from the magnesium alloy characterized.

(Effects of the invention)
When cast using the heat-resistant magnesium alloy for casting of the present invention, a casting made of a magnesium alloy having extremely low heat resistance such as high-temperature strength and creep resistance can be obtained. The reason for this is not necessarily clear, but it can be considered as follows.

  The reason why the heat-resistant magnesium alloy for casting of the present invention (hereinafter simply referred to as “magnesium alloy”) is excellent in heat resistance is that it contains appropriate amounts of RE and Ca. For example, according to the research of the present inventors, the contribution of each alloy element to the heat resistance of the magnesium alloy is in the order of RE> Ca> Zr> Zn for the elements related to the present invention. Therefore, Ca is a very effective element for improving the heat resistance of the magnesium alloy together with RE.

  However, conventionally, Ca has been said to be an element that lowers the castability of magnesium alloys. For this reason, for example, as described in Patent Document 2, it has been recommended to suppress the Ca content (for example, 0.3% by mass or less) from the viewpoint of improving castability.

  However, further research by the present inventors has revealed that when Ca coexists with a relatively large amount of Zn, its castability is greatly reduced. This is considered to be because when Ca coexists with Zn, the solidus temperature decreases and the solidification temperature of the molten alloy increases. This is because castability can be evaluated mainly by casting cracks, but the casting cracks are caused by strain accompanying solidification shrinkage during casting. When the solidification temperature range is expanded, the shrinkage stress accompanying solidification shrinkage concentrates on the weak liquid phase portion, and casting cracks are likely to occur.

  Therefore, in the heat-resistant magnesium alloy for casting according to the present invention, heat resistance is ensured by leaving an appropriate amount of Ca, and by suppressing Zn that expands the solidification temperature range, the solidus temperature during casting is increased. It is considered that castability is secured.

  In the present specification, the composition range of each element is shown in the form of “x to y mass%”, but this also includes the lower limit (x mass%) and the upper limit (y mass%) unless otherwise specified. . Further, the “castability” in the present specification can be evaluated by, for example, the presence or absence of occurrence of casting cracks when the molten alloy is cooled and solidified. Here, unless otherwise specified, the casting referred to in this specification is not intended for a specific casting method, but is intended for all casting methods such as sand casting, die casting, gravity casting, and high pressure casting. However, the heat-resistant magnesium alloy for casting of the present invention is particularly effective for a casting method having a high cooling rate such as die casting. This is because the amount of Zn is small, the solidification temperature range of the molten metal is narrow, casting cracks and the like hardly occur, and the cooling rate is fast, and the structure of the magnesium alloy casting becomes fine. “Heat resistance” can be evaluated by, for example, the mechanical properties (creep characteristics, high temperature strength, axial force retention, etc.) of a magnesium alloy in a high temperature atmosphere.

  The magnesium alloy for casting referred to in the present invention is mainly used as a casting raw material. For example, it has a primary form such as an ingot. The magnesium alloy casting may be a raw material, an intermediate material, an intermediate product, an end product, or the like. Further, it may be in an as-cast state, heat-treated, or finished. Regardless of the form, for example, in addition to a rod shape, a plate shape, an annular shape, or the like, the shape may be a final product shape or a shape close thereto.

  The present invention will be described in more detail with reference to embodiments of the invention. In addition, the content described in this specification including the following embodiments includes not only the heat-resistant magnesium alloy for casting of the present invention but also a magnesium alloy casting (hereinafter, both are simply referred to as “magnesium alloy”) and the same. Applicable to the manufacturing method as appropriate. Which embodiment is the best depends on the target, required performance, and the like.

(1) Alloy composition The magnesium alloy of the present invention contains 0.2 to 3% by mass of Ca, 1 to 4% by mass of RE, and less than 1% by mass of Zn.

  Ca is solid-solved in the Mg base to strengthen α-Mg and form a compound with Mg to form a structure as a fine precipitate or crystallized substance, thereby improving the heat resistance and proof stress of the magnesium alloy. It is an element. In particular, the heat resistance is improved by the fine precipitates, and the proof stress is improved by the Mg—Ca compound crystallized at the grain boundary. When Ca is less than 0.2% by mass, such an effect is weak. When Ca exceeds 3% by mass, a large amount of Ca compound (grain boundary compound) is crystallized at the grain boundary, Reduces elongation and toughness. Moreover, excess of Ca also causes casting cracks. More preferably, the lower limit of Ca is 0.2% by mass, further 0.5% by mass, and the upper limit of Ca is 3% by mass, 1.5% by mass, and further 1% by mass.

  RE is an element that improves the heat resistance of the magnesium alloy by solid solution in the Mg matrix and crystallization (precipitation) at the grain boundary. If the RE is less than 1% by mass, such an effect is thin, and if the RE exceeds 4% by mass, the toughness of the magnesium alloy decreases. More preferably, the lower limit of RE is 1% by mass and 1.5% by mass, and the upper limit of RE is 3% by mass and 2.5% by mass.

  Re.E. includes scandium (Sc), yttrium (Y), lanthanoids, and actinoids. Typical examples of R.E. in the present invention are lanthanum, cerium, praseodymium, neodymium, and the like. is there. These RE may be one type or two or more types. In particular, it is preferable to use misch metal (Mm) which is a mixture of lanthanum, cerium, praseodymium, neodymium and the like because it is relatively easy to obtain and inexpensive.

  Zn is an element that strengthens the α-Mg phase, which is the parent phase, by solid solution strengthening and improves the room temperature strength of the magnesium alloy. Zn is also effective in improving the heat resistance of the magnesium alloy. However, as described above, when Zn coexists with Ca, the castability of the magnesium alloy is greatly reduced. Therefore, in the present invention, the amount of Zn is relatively reduced to generally increase the solidus temperature and narrow the solidification temperature range. The solidus temperature and the liquidus temperature also vary depending on the alloy composition. However, as a result of investigation by the present inventors, the magnesium alloy of the present invention has a solidus temperature of approximately 490 ° C. or higher, and the liquid phase at that time. The line temperature is 640-650 ° C. Therefore, the temperature difference between the liquidus temperature and solidus temperature indicating the solidification temperature range is 160 ° C. or less. In this invention, although the upper limit of Zn was 1 mass%, it is more preferable in it being 0.8 mass% further 0.5 mass%. The lower limit of Zn may be 0% by mass. However, in order to improve room temperature strength and heat resistance, it is more preferable that the lower limit is 0.1% by mass and further 0.3% by mass.

Here, when the Zn content increases in the presence of Ca and Zn, the castability of the heat-resistant magnesium alloy for casting decreases because the solidus temperature of the alloy decreases due to the increase of the Zn content, and the solidification temperature range is It is thought that this is because the fragile residual liquid phase becomes the starting point of the casting crack by expanding.
From such a viewpoint, the content ratio of Ca to Zn (Ca / Zn) is preferably 0.2 to 4. When Ca / Zn is less than 0.2, the heat resistance is inferior, and when Ca / Zn exceeds 4, castability and toughness are lowered. The Ca / Zn is more preferably 0.2 to 2.5.

  The magnesium alloy of the present invention preferably further contains 0.1 to 1% by mass of strontium (Sr). Further, it is preferable that one or more of zirconium (Zr) and manganese (Mn) are contained in a total amount of 0.3 to 1% by mass.

  Sr is an element that improves castability. If Sr is less than 0.1% by mass, the effect is small, and if it exceeds 1% by mass, a large amount of Sr-based compound is crystallized and the alloy becomes brittle. Sr is more preferably 0.3 to 0.8% by mass.

  Zr and Mn serve to refine the crystal grains of the alloy. Zr is an element that also contributes to improvement in heat resistance. If the total is less than 0.3% by mass, the effect is small, and if it exceeds 1% by mass, the toughness is lowered, which is not preferable. Zr and Mn are more preferably 0.3 to 0.8% by mass in total.

(2) Manufacturing method of magnesium alloy casting The casting method of the magnesium alloy casting of the present invention is not particularly limited. For example, as described above, a molten magnesium alloy having the above composition is used as a mold. And a solidification step of solidifying the molten metal injected into the mold. As a result, a magnesium alloy casting having excellent heat resistance and almost no casting cracks (surface cracks) can be obtained. Here, when mass-producing magnesium alloy castings, die casting is mainly used. If die casting is performed by the production method of the present invention, casting cracks and the like are few even in a thin-walled, complex-shaped casting.

  By the way, in die casting, a molten alloy is supplied from a plunger or the like to a cavity of a set mold at a high speed and rapidly solidified while being pressurized. Usually, it is performed almost automatically and continuously with a short tact by a die casting machine or the like.

  The die casting conditions are substantially defined by, for example, casting pressure, injection temperature, injection speed (or molten metal injection time), cooling speed, and the like. The casting pressure is, for example, 20 to 60 MPa. The injection speed (plunger speed) is, for example, 1 to 4 m / sec, or 1 to 7 m / sec. Speaking of this as the injection time of the molten metal into the product cavity, it is, for example, 300 msec or less, further 100 msec or less. Strictly speaking, the cooling rate differs depending on the casting site, and thus cannot be specified uniformly. However, the cooling rate is, for example, 20 ° C./sec or more at the latest. In the case of such die casting, the injection step is a step of injecting the molten metal into the mold at a high speed, and the solidification step is a step of rapidly cooling and solidifying the injected molten metal.

(3) Use of magnesium alloy casting The magnesium alloy casting of the present invention may be subjected to heat treatment such as solution treatment or aging treatment after casting, but has excellent heat resistance in an as-cast state without heat treatment. Demonstrate. For this reason, a highly heat-resistant magnesium alloy casting can be obtained with a good yield and at a low cost. In particular, in the case of a die cast product, the cycle time required for casting is short, and post-casting processing is almost unnecessary, so that mass production and cost reduction of a magnesium alloy casting can be further achieved.

  There are various uses of the magnesium alloy casting of the present invention, for example, the following. In the field of automobiles and motorcycles, body structural members, chassis members, wheels, space frames, steering wheels (core bars), seat frames, suspension members, transmission cases, pulleys, oil pans, shift levers, instrument panels, doors Impact panels, intake surge tanks, pedal brackets, front shroud panels, etc. In particular, since the magnesium alloy casting of the present invention has excellent heat resistance, when used in a product used in a high temperature environment, for example, an engine, a transmission, or a related product disposed in an engine room of an automobile. More preferred.

  In order to specifically evaluate the castability and heat resistance of the magnesium alloy of the present invention, test pieces (magnesium alloy castings) produced by die casting with various changes in Zn content and Ca content were produced. Hereinafter, the manufacturing method, test method, and test result of those test pieces will be described.

(1) Relationship between casting crack and Zn content (Manufacture of test pieces)
First, magnesium chloride-based flux was applied to the inner surface of a crucible made of high chromium alloy steel (SUS430) preheated in an electric furnace, and pure magnesium metal was put into the crucible and melted. Ca, Zn and misch metal (Mm) were added to the molten metal maintained at 700 ° C. Further, the molten metal was heated to 780 ° C., and then an Mg—Zr alloy was added. After sufficiently stirring them to dissolve completely, the molten metal was kept at 780 ° C.

  At this time, the alloy composition of the test piece was Ca: 0.2 mass%, Misch metal: 2.0 mass%, Zr: 0.5 mass%, and Zn: x mass% (x varies depending on the test piece), and the balance. Each raw material was prepared so that becomes Mg. The Zn amount was 0% by mass, 0.25% by mass, 0.5% by mass, 0.75% by mass, 1% by mass or 2% by mass, respectively. Mm used was 52.2% by mass of cerium (Ce), 25.47% by mass of lanthanum (La), 16.1% by mass of praseodymium (Pr), 5.4% by mass of neodymium (Nd), samarium (Sm ) The composition ratio was 0.1% by mass.

In order to prevent combustion during the melting operation, a mixed gas of carbon dioxide gas and SF ₆ gas was sprayed on the surface of the molten metal at a flow rate of 0.2 L / min, and flux was appropriately sprayed on the surface of the molten metal. The molten alloy thus obtained was poured into a mold having a test piece-shaped cavity shown in FIG. 1 (injection step), and then rapidly solidified (solidification step) to obtain a die-cast casting.

  The die casting performed at this time was performed using a vertical die casting machine. That is, various molten metals prepared to the above composition were injected by pressure into the mold cavity with a plunger (φ40 mm) (injection step), and then solidified at a cooling rate of about 100 ° C./second (solidification step). The casting conditions at this time were casting pressure: 64 MPa, injection (plunger) speed: 0.6 m / s, injection (melting) temperature: liquidus temperature + 20 ° C. The mold temperature was set to 50 to 100 ° C.

  The test piece shown in FIG. 1 includes a thin plate portion having a width 40 × length 70 × 3 mm thickness connected to the gate, a width 40 × length 65 × thickness plate 11 mm continuous from the thin plate portion, and the center of the thin plate portion. The ribs protrude from the surface and are bridged from the thick plate portion to the gate. The ribs are 2x length 70x height 8mm. And the back surfaces of the thin plate portion and the thick plate portion are flush with each other.

(Measurement and evaluation)
It was examined whether or not casting cracks occurred on the rib back surface (flat surface on the rib back side) of each test piece and the corner R portion formed at the base portion of the rib and the thin plate portion. That is, the depth of the casting crack was measured by microstructure observation on the rib back surface and the corner R portion at a position of 50 mm in the height direction from the gate. The result is shown in FIG. In FIG. 2, the sum (Δ) is an arithmetic sum of the crack depth (♦) on the rib back surface and the crack depth (■) at the corner R portion. FIG. 2 also shows the crack depth when AZ91, which is a typical heat-resistant magnesium alloy for casting, is shown for reference. As is clear from FIG. 2, in the case of Mg-x% Zn-0.2% Ca-2% Mm-0.5% Zr (unit: mass%), Zn is less than 1 mass% (in particular, Zn: 0 .5 mass%), it was confirmed that the crack depth was very small.

(2) Relationship between casting crack and Ca content Based on the above results, Mg-0.5% Zn-x% Ca-2% Mm-0.5% Zr (unit: mass%) as in the above case ) Was manufactured, and the relationship between casting crack and Ca content was investigated. The result is shown in FIG. The Ca amount (x) of each prepared test piece was 0.2%, 1%, 2%, and 3%.

  However, when Zn was 0.5%, even if the amount of Ca changed, fine casting cracks were as small as or less than that of the AZ91 alloy. However, when the amount of Ca exceeded 3% by mass, the casting was broken by restraint of the mold. The broken portion was the boundary portion between the thick plate portion and the thin plate portion in FIG. Incidentally, casting cracks occur in the middle of the solidification of the molten metal (liquidus temperature to solidus temperature), but the casting breaks after solidification (at a temperature below the solidus temperature). is there.

  From this result, it was found that when the Zn content was less than 1% by mass, excellent castability was obtained regardless of the Ca content (however, within 3% by mass). In other words, in order to ensure the high heat resistance of the magnesium alloy, even if Ca and Mm (R.E.) are contained in appropriate amounts (or relatively large amounts), the amount of Zn is suppressed by suppressing the amount of Zn. It can be seen that the property is also secured.

  For reference, the creep resistance of the test piece of this example (Mg-0.2% Zn-0.8% Ca-2% Mm-0.5% Zr) and the test pieces having other compositions The creep resistance is shown in comparison with FIG. In this creep test, a tensile stress of 100 MPa was applied to the test piece in a high temperature environment of 150 ° C. The vertical axis of FIG. 4 represents the strain ε generated at that time, and the horizontal axis represents time (sec).

  As described above, when the heat-resistant magnesium alloy for casting of the present invention is used, not only heat resistance but also casting cracks and the like can be extremely reduced even in a thin and complex-shaped casting. That is, the robustness in the case of performing die casting or the like is improved, the conditions under which casting can be performed can be expanded, and a magnesium alloy casting having sufficient heat resistance can be manufactured even with existing equipment.

It is a perspective view showing the shape of a test piece. It is a figure which shows the relationship between Zn amount and the depth of a casting crack. It is a figure which shows the relationship between Ca amount and the depth of a casting crack. It is a graph which shows the creep characteristic (150 degreeC x 100 Mpa) of each test piece.

Claims (6)

When the total is 100% by mass,
0.2-3 mass% calcium (Ca),
1-4% by weight of rare earth elements (RE),
Less than 1% by weight of zinc (Zn);
0.3 to 1% by mass of zirconium (Zr) ,
The balance consists of magnesium (Mg) and inevitable impurities,
The content ratio of Ca to Zn (Ca / Zn) is 0.2 to 4,
A heat-resistant magnesium alloy for casting characterized by excellent castability and heat resistance.   The heat-resistant magnesium alloy for casting according to claim 1, which is a heat-resistant magnesium alloy for die-casting used in die-casting. The heat-resistant magnesium alloy for casting according to claim 1 or 2 , wherein a temperature difference between the liquidus temperature and the solidus temperature is 160 ° C or less. When the total is 100% by mass,
0.2-3 mass% Ca,
1-4% by weight of RE,
Less than 1% by weight of Zn,
0.3 to 1% by mass of Zr ,
The balance consists of Mg and inevitable impurities,
The content ratio of Ca to Zn (Ca / Zn) is 0.2 to 4,
Magnesium alloy casting characterized by excellent heat resistance and almost no casting cracks.   The magnesium alloy casting according to claim 4, which is die-cast. 0.2 to 3% by mass of Ca, 1 to 4% by mass of R.E., less than 1% by mass of Zn, and 0.3 to 1% by mass of Zr when the whole is taken as 100% by mass. wherein the door, the balance Ri Do from the Mg and inevitable impurities, implanting step the content ratio relative to the Zn of the Ca (Ca / Zn) injects a melt of 0.2-4 der Ru magnesium alloy into a mold When,
A solidification step of solidifying the molten metal injected into the mold,
A method for producing a magnesium alloy casting, characterized in that a magnesium alloy casting having excellent heat resistance and almost no casting cracks is obtained.

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