Magnesium alloy cast and casting method thereof

Abstract

A magnesium alloy cast, featuring with few casting defects, having precipitation areas dispersed inside magnesium alloy grains is provided by melting magnesium alloy material in an oxygen-free atmosphere and cooling the molten magnesium alloy at a cooling rate in the 1-20K/sec range by the continuous casting for solidification.

Description

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a magnesium alloy cast for plastic forming and its casting method for use in manufacturing magnesium alloy components from magnesium alloys.

[0003] 2. Description of the Related Art

[0004] Recyclable metallic materials attract attention as a solution to recycling and environmental problems posed by mass-production home appliances, replacing conventional resin materials adopted in their exterior components. Compared with resin materials of which recycle rate is 20%, 90% of metallic materials are recycled.

[0005] Among metallic materials including alloys, magnesium alloys are relatively light and strong, and excellent in vibration-damping characteristics. Thus, they are commercially used in many applications for electronic mobile gears and auto parts. In addition, because the magnesium alloys melt at low temperatures, they consume less energy in recycling.

[0006] FIG. 8 is a diagram illustrating the process of manufacturing magnesium alloy components from magnesium alloys. The process of manufacturing magnesium alloy components from magnesium alloys can be roughly divided into casting such as die-casting and thixo-molding, and plastic forming such as pressing, bending and forging. Although casting has flexibility in forming, the product yield is low and manufacturing cost is high because of surface defects and bubbles in the cast product. For production of cabinets for home appliances of relatively simple shapes, a material piece is cast so as to have a shape close to the final shape of the housing and then the cast is directly formed into a desired product by plastic forming. Otherwise, the cast is made into a sheet by primary processing such as extrusion and rolling, the sheet is formed into a magnesium alloy formed product by secondary processing, and then the formed product is coated and dried to provide magnesium alloy components. Such combination of casting and plastic forming is considered to be advantageous in terms of shortening tact during processing, reducing surface defects, and saving hardware investment, compared with manufacturing techniques relying on casting alone.

[0007] FIG. 10 illustrates the metal microstructure of a typical product cast for plastic forming in the above-described manufacturing method. The cast 4 is a magnesium alloy, or AZ31, of which major elements are aluminum, zinc, and manganese as well as magnesium. Among magnesium alloys, the AZ-series magnesium alloys are suitable for applications in the cabinets of home appliances that often contact sweat or water since they are relatively strong and resistive to corrosion. Meanwhile, AM-series magnesium alloys, which include magnesium, aluminum and manganese as major elements, are suitable for applications in internal structural components since they have high ductility and shock-resistance.

[0008] On the other hand, since magnesium alloys have hexagonal crystalline structures of few sliding planes, their plastic formability is relatively low. In order to improve their plastic formability, there are some method proposed for reducing the grain size by controlling the cooling rate for magnesium alloys and adding agents for grain downsizing. It is known that grains may be made smaller to some extent by controlling the distortion and temperature during the primary processing such as extrusion and rolling.

[0009] However, if the cast has many molding defects and inclusions, it is not easy to remove such defects during the primary processing. The magnesium cast 4 shown in FIG. 10 is such an example where precipitation areas 6 exist in boundaries around &agr;-Mg phase 5 in the cast 4, and internal voids 7 formed during casting appear in grain boundaries.

[0010] In general, when a molten AZ-series magnesium alloy solidifies, such elements (metals and oxides) that are less soluble in solid magnesium gather in grain boundaries, as magnesium seed grains grow, to precipitate near grain boundaries in the form of intermetallic compounds. Meanwhile, gases that are less soluble in solid magnesium are likely to gather in magnesium grain boundaries during solidification and form voids 7 in the cast 4.

[0011] Such a cast 4 shows a poor formability because the points where casting defects and inclusions are gathered are likely to be origins of crack when the cast 4 is formed into a sheet and then a finished product by plastic forming. Particularly in magnesium alloys that have few sliding planes and poor ductility, the properties of the primary material determine much of formability. In order to improve the formability of magnesium alloys, it is important to reduce casting defects and inclusions in the primary cast from which sheets are formed and thereby to prevent defects segregation.

SUMMARY OF THE INVENTION

[0012] In view of the foregoing problems, an object of the invention is therefore to provide a magnesium alloy cast of reduced casting defects, inclusions and their segregation in texture for improving plastic formability in the primary processing such as extrusion and rolling into sheets or secondary processing such as forging of a processed sheet.

[0013] The magnesium alloy cast developed by the invention to meet the above goal is a magnesium alloy cast that is to be cast for plastic forming where a plurality of precipitation areas containing at least one of aluminum, zinc, and manganese exist inside magnesium alloy grains.

[0014] This magnesium cast is characterized in that the plurality of precipitation areas are separated from each other from zero to 400 &mgr;m exclusive, measured from an edge to another.

[0015] Metals other than magnesium, aluminum, zinc and manganese, as well as inclusions such as their oxides and carbides, are included in the alloy. Most of the inclusions do not easily dissolve in solid magnesium and exist either attached to or included in the precipitation areas. The state where a plurality of precipitation areas are dispersed inside grains is the state where magnesium alloy dendrites have disappeared and, in turn, grain boundaries have appeared, and a plurality of precipitation areas exist from each other separated by &agr;-Mg phase.

[0016] When the plurality of precipitation areas is dispersed inside grains, inclusions are also dispersed. Then the number of segregation of large inclusions that serve as origins of crack decreases, and the ductility of the cast is improved. When the cast has a high ductility, the sheet formed from the cast by primary processing such as extrusion and rolling also has a high ductility, and plastic formability is improved. In general, when a cast is subjected to primary processing such as extrusion and rolling, the grains near inclusions are more likely to become small than the other grains. As a result, in the present invention, since pluralities of precipitation areas are dispersed, the grains become fine and uniform over the whole range of the cast during primary processing such as extrusion and rolling. Then large grains that will degrade formability decrease, and plastic formability is improved.

[0017] For the magnesium alloys for practical use, their alloy compositions are standardized by ASTM. The above-described precipitation area is defined as an area where the content of any of the alloy elements other than magnesium is larger than that determined in ASTM specs.

[0018] In this invention, every precipitation area is to have an area from 25×10−12&pgr;m2 to 2500×10−12&pgr;m2. Now a method for calculating the area of a precipitation area will be explained. First, print a magnified cross-sectional photograph on paper and measure the whole weight (M) of the paper. Next, cut out the region of a precipitation area, and measure the weight (m) of the cut out paper piece. Defining the actual area of the cross-section of the real cast corresponding to the magnified cross-sectional photograph is S, the area, &sgr;, of each precipitation area is expressed by the equation of &sgr;=S·m/M. The diameter of the equivalent circle for a precipitation area is the diameter of a circle of which area is equal to the precipitation area identified in the cross-sectional photograph. The average diameter of precipitation areas is the average of the diameters of the equivalent circles. In a cast, precipitation areas extend three-dimensionally. The sizes of the three-dimensional regions were approximated by sectioning a cast and averaging a sufficient number of two-dimensional precipitation areas observed therein. The embodiment of the present invention determines that such a sufficient number is 20 or larger.

[0019] The above magnesium alloy cast (also called the cast) has a density that is 98-100% of the theoretical density calculated from the alloy composition. Namely, since there are few voids or bubbles in the metallic texture, the cast has a high ductility. When it is formed into a sheet by primary processing such as extrusion and rolling and when it is subjected to secondary processing such as forging, destruction due to casting defects (voids and pores) that will be origins of crack is prevented. Moreover, when the formed product is coated and dried, surface defects due to the burst of inner bubbles can be prevented.

[0020] The cast of the present invention contains aluminum as much as 1.8-9.1 wt. %. The present invention is emphasizing AZ-series magnesium alloys containing magnesium, aluminum, and zinc, in this order from most, and AM-series magnesium alloys containing magnesium, aluminum, and manganese, in this order from most. In the AZ-series magnesium alloys, the zinc content is smaller than the aluminum content, substantially less than 2 wt. %, preferably 1.5-0.5 wt. %. In the AM-series magnesium alloys, the manganese content is smaller than the aluminum content, substantially less than 1 wt. %, preferably 0.6-0.3 wt. %. As described above, the AZ-series magnesium alloys are strong and excellent in corrosion resistance, while the AM-series magnesium alloys show excellent ductility and shock-resistance, both being used in a wide range of applications for structural and cabinets materials.

[0021] Although a plurality of precipitation areas are separated from each other inside grains in this invention, when the aluminum content is less than 1.8 wt. %, the area occupied by precipitation areas becomes small in each grain. Then the effect of dispersing inclusions by dispersed precipitation areas decreases, and the plastic formability of the cast will degrade. In contrast, when the aluminum content is more than 9.1 wt. % in the cast, the precipitation areas grow in volume and link each other to form a network extending over a wide range. Then, relatively large-volume inclusions come to exist inside precipitation areas, and the plastic formability may degrade. Since in the invention the aluminum content in the magnesium alloy cast is 1.8-9.1 wt. %, inclusions are expected to disperse inside grains, and a cast of excellent plastic formability is provided.

[0022] The cylindrical bullet, which is a common cast product, is formed into a plate by extrusion or forging, and then rolled into a magnesium alloy plate that will be formed into cabinets by plastic forming. On the other hand, forming the cast product of the present invention into plate-like shape, it is easily to be formed into a magnesium alloy plate for use in cabinets only by several times of rolling for thickness adjustment to serve for plastic forming, without extrusion or forging. Then the manufacturing process is simplified, and the manufacturing cost is reduced.

[0023] Additionally, when the above plate-like cast product is 10 mm thick or less, an about 0.3-1.5 mm thick sheet which is suitable for use in cabinets of home appliances is provided only by several times of rolling, without extrusion or forging. Then the manufacturing process is further simplified and the cost is reduced.

[0024] It is found out that the above-described cast product has a preferable direction of crystal growth during solidification in its surface because its X-ray diffraction pattern indicates the existence of a strong orientation of the (0002) planes in surface grains. In general, since magnesium alloys cause an orientation of (0002) plane due to pressing, the X-ray diffraction pattern of the invention indicates that the cast product receives a pressure during casting.

[0025] In order to attain the above goal, the magnesium alloy casing method of the present invention includes a step of inputting magnesium alloy material in a crucible serving as a melting apparatus, melting the magnesium alloy material in the crucible under an oxygen-free atmosphere, supplying the molten alloy to a cooling mold, and cooling the molten magnesium alloy at a cooling rate in the 1-20K/sec range for solidification, and a step of drawing the solidified metallic ingot by drawing means keeping the ingot continuous.

[0026] By intermittently drawing the solidified metallic ingot by alternatively repeating drawing process and holding process during the solidification of the molten magnesium alloy in the cooling mold, gases and inclusions in the cast are reduced, because inertial forces and vibrations push gases and inclusions existing in the solid-liquid boundaries away to the liquid phase.

[0027] In this casting method, the molten magnesium alloy is cooled at a cooling rate in the 3-8K/sec ranges. The cooling rate of 3-8K/sec is suited for magnesium dendrites to grow by cooling and solidifying and then disappear by partial annealing due to thermal conductivity in the cast. As described above, when precipitation areas are left in the outer surfaces of dendrites that have been lost and dispersed inside grains, inclusions are also dispersed, and a cast of a high plastic formability is provided.

[0028] In this casting method, the combustion of magnesium alloy material is prevented during melting by isolating it from oxygen. For shutting out oxygen, the crucible is placed in an inert atmosphere, or a flux is added to the molten magnesium alloy to prevent the surface from contacting oxygen. As inert gases, argon, helium and air mixed with SF6 may be used. As a flux, a potassium chloride-based or a magnesium chloride-based flame-retardant flux may be used. The flux added also works to prevent metal elements from vaporizing out of the surface of the molten magnesium alloy.

[0029] In the above method, when the magnesium alloy material that will be thrown in the melting apparatus is granular solids and the atmosphere around the melting apparatus is kept inert during material feeding, casting is performed continuously with no shortage of material supply, by feeding the granular solids continuously or intermittently to the crucible, even if the crucible where a magnesium alloy is melted is not equipped with a temperature-holding furnace. Particularly, when an oxygen-shielding protective atmosphere is formed on the surface of magnesium alloy in the crucible, time and effort are needed to rebuild another inert atmosphere, when the inert atmosphere is released for material supply. According to this method, however, such time and effort may be saved because material is added without releasing the built atmosphere.

[0030] In the above method, the cross-section of the inner wall of the cooling mold is rectangular and at least one part of the inner wall is tapered so that the short side of the rectangle on the melting apparatus side is different from that on the drawing means side, thus the force applied to the magnesium alloy may be increased or decreased during drawing. Then it becomes possible to improve material characteristics and prevent the ingot from being discontinuous by reducing friction between the ingot and the mold.

[0031] In the above method, if the alloy holding part of the melting apparatus (crucible) and/or the material of the cooling mold are/is of graphite, it becomes possible to heat/cool material smoothly due to their excellent thermal conductivities. Materials other than graphite, which react little with magnesium alloys, may also be used in the crucible, but such materials that include copper, nickel and iron that will degrade the corrosion-resistance of the cast if they move in the cast should be avoided. It has been reported that graphite mixed in a magnesium alloy makes grain size of the alloy smaller.

[0032] In the above method, it is preferable to make a structure for casting that shuts out oxygen from the surface of the molten magnesium alloy in the melting apparatus and the vicinity of the melting apparatus. For example, a chamber may be built outside the melting apparatus for evacuation or inert gas replacement to control the atmosphere around the melting apparatus. Because the molten magnesium alloy emits a lot of vapor, a chamber should be equipped to prevent metallic vapors from being discharged to the air.

[0033] In the above method, with the melting apparatus having a replaceable lid, the diffusion of metallic vapors to outside the melting apparatus may be reduced, and the generation of powder metal that is produced by diffused metallic vapors when the vapors adhere to the chamber inner walls and solidify thereon may be reduced. Then the decreasing in material yield due to the generation and accumulation of powder metal and the fear of spontaneous firing can be eliminated. As described previously, vaporization from the surface of the molten magnesium alloy may be controlled by mixing a flux in the material as means for shutting out oxygen from the magnesium alloy material held in the crucible. In such a case, however, there is a possibility that the flux become mixed in the cast product. Accordingly, it is better to add a lid on the melting apparatus.

[0034] With a lateral type cooling mold connected to the side of the melting apparatus, having capability to draw the magnesium alloy in the horizontal direction, mixing of impurities floating on the surface of molten magnesium alloy and those accumulated in the bottom of the crucible into the cast product is prevented. Accordingly, a magnesium alloy cast having high ductility is manufactured.

[0035] These and other objects and features of the present invention will be more fully apparent from the following detailed description taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0036] FIG. 1 is a sectional view schematically illustrating the metallic texture of an AZ-series magnesium alloy cast according to an embodiment of the invention.

[0037] FIG. 2 is a scanning electron microscopic (SEM) photograph showing a cross section of the AZ-series magnesium alloy cast of the embodiment.

[0038] FIGS. 3A-3C are optical microscopic photographs of three AZ-series magnesium alloy plates of different thickness of the embodiment; FIG. 3A is a sectional view of the 10 mm-thick plate; FIG. 3B is a sectional view of the 5 mm-thick plate; and FIG. 3C is a sectional view of the 3 mm-thick plate.

[0039] FIG. 4 is an X-ray diffraction pattern of the surface of the magnesium alloy cast of the embodiment.

[0040] FIG. 5 is a longitudinal sectional view schematically illustrating a casting apparatus of the embodiment.

[0041] FIG. 6 is a longitudinal sectional view schematically illustrating the major part of a molding apparatus that is tapered to widen toward pinch rolls.

[0042] FIG. 7 is a longitudinal sectional view schematically illustrating the major part of the molding apparatus that is tapered to shrink toward the pinch rolls.

[0043] FIG. 8 is a diagram illustrating the typical process for manufacturing magnesium alloy components from magnesium alloy material.

[0044] FIG. 9 is a schematic diagram illustrating a magnesium alloy sheet blank for use in estimating the performance of square-cylinder drawing.

[0045] FIG. 10 is a sectional view schematically illustrating the metallic texture of an AZ-series magnesium alloy cast manufactured by a conventional casting method with a conventional casting apparatus.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0046] Now the embodiments of the invention will be described below with reference to FIG. 1 to FIG. 8. The following embodiments, however, are only examples of the invention and do not restrict any technical scope of the invention.

[0047] FIG. 1 is a schematic cross-sectional view of a metallic texture of magnesium alloy cast 1 according to an embodiment of the invention. This magnesium alloy cast 1 is an AZ31 magnesium alloy containing about 3% aluminum and about 1% zinc and has been formed into a plate by a casting method that will be described later. In a magnesium alloy grain, &agr;-Mg phase 2 exists in a way of separating a plurality of precipitation areas 3 (containing at least one of components excluding magnesium, i.e., aluminum, zinc, and manganese) from each other. This is considered to be a state where the elements of precipitation areas 3 that are separated by magnesium dendrite frames are kept dispersed in grains after the dendrites have disappeared and, in turn, grain boundaries have appeared.

[0048] The following table 1 shows the specifications of AZ31 magnesium alloys determined by the American Society for Testing and Materials (ASTM), and composition of magnesium alloy cast 1 according to the embodiment. The unit is weight percent. The composition was measured by the ICP emission spectrochemical analysis in the embodiment. 1 TABLE 1 Al Zn Mn Fe Si Cu Ni Ca ASTM specs 2.5˜3.5 0.5˜1.5 ≧0.20 ≦0.03 ≦0.10 ≦0.1 ≦0.005 <0.04 Cast of 3.0 0.99 0.22 0.0047 0.022 0.0017 0.0012 0.0001 the present embodiment Remarks: The remainder is magnesium.

[0049] FIG. 2 is a scanning electron microscopic (SEM) photograph showing a cross section of a metallic texture of the magnesium alloy cast 1 of the embodiment. Denoted 31-34 in the photograph are the measurement points of which compositions were measured with a SEM-EDS (scanning electron microscopy-energy disperse spectrometry for characteristic X-ray detection) analyzer; and Table 2 shows the composition (by weight percent) at each measurement point. 2 TABLE 2 Measurement Measurement Measurement Measurement Point 34 Point 31 Point 32 Point 33 Wt. Element Wt. % Element Wt. % Element Wt. % Element % Mg 100.00 C 17.76 Mg 55.96 Mg 92.78 O 13.39 Al 21.16 Al 4.63 Mg 35.91 Zn 22.88 Zn 2.59 Al 15.40 Mn 17.12 Zn 0.41

[0050] As shown in Table 1, the total composition of the cast product of the embodiment meets the ASTM specifications. Although such points as measurement points 32-34 in FIG. 2 are “precipitation areas” of the embodiment containing at least one of aluminum, zinc and manganese other than magnesium, the points have at least one of the elements such as aluminum and zinc other than magnesium, of which content is higher than the ASTM spec as shown in Table 2. The measurement point 32 has high contents of carbon and oxygen that are elements not referred to in the ASTM specs. This indicates that the precipitation area including this point may contain carbides and oxides.

[0051] Next, the distribution of the above precipitation areas in grains will be described with reference to FIGS. 3A-3C. FIG. 3A is an optical microscopic cross-sectional photograph showing part of a 10 mm thick plate-like magnesium alloy cast la of the above embodiment. Grain boundaries 8 are identified and the average grain size is about 200 &mgr;m; and a plurality of precipitation areas 3 of which sizes lie between a few &mgr;m and about 30 &mgr;m, expressed by the diameter of the equivalent circle, exist separated from each other inside grains. Most of the areas of the precipitation areas 3 in this texture lie between 25×10−12 &pgr;m2 and 2500×10−12&pgr;m2. In the cross-sectional photograph of FIG. 3A, about 30-40 precipitation areas 3 are identified inside each grain. The individual precipitation areas 3 are separated at least about 2-30 &mgr;m from each other, and two precipitation areas 3 are seen that are separated about 200 &mgr;m at most in one magnesium alloy grain.

[0052] FIG. 3B is an optical microscopic cross-sectional photograph showing part of a 5 mm thick plate-shaped magnesium alloy cast 1b. Grain boundaries 8 are identified, and the average grain size is about 80 &mgr;m. In this photograph, about 3-20 precipitation areas 3 of which areas lie between 25×10−12&pgr;m2 and 2500×10−12&pgr;m2 exist inside each grain. Here, the individual precipitation areas 3 are separated at least about 1-40 &mgr;m from each other, and two precipitation areas 3 are seen that are separated about 180 &mgr;m at most in a magnesium alloy grain.

[0053] FIG. 3C is an optical microscopic cross-sectional photograph showing part of a 3 mm thick plate-shaped magnesium alloy cast 1c. Grain boundaries 8 are identified, and the average grain size is about 250 &mgr;m. In this photograph, about 60-160 precipitation areas 3 of which areas lie between 25×1031 12&pgr;m2 and 2500×10−12&pgr;m2 exist inside each grain. Here, the individual precipitation areas 3 are separated at least about 1-30 &mgr;m from each other, and two precipitation areas 3 are seen that are separated about 400 &mgr;m at most in a magnesium alloy grain.

[0054] Unlike in the conventional material 4 shown in FIG. 10, in the magnesium alloy cast 1 of the embodiment, precipitation areas 6 do not segregate in the grain boundaries of magnesium alloy grains 5, and voids 7 that are created during casting do not precipitate in grain boundaries. Instead, about 3-160 precipitation areas 3 of relatively small sizes are distributed in each grain. As a result, inclusions are also distributed, and therefore the magnesium alloy cast 1 becomes more ductile and less likely to fracture during extrusion or rolling when formed into a sheet. Even after formed into a sheet, the sheet is rich in ductility and shows excellent formability during secondary forming such as pressing and forging. In case that the precipitation areas 3 included in one grain are two or less, the effect of inclusion distribution due to distributed precipitation areas is not sufficient, and the cast and the sheet provided by a primary forming of the cast have poor plastic formability. Meanwhile, in case that 160 or more precipitation areas are included, the distances between individual precipitation areas become small and precipitation areas are combined. As a result, relatively large inclusions exist segregated, and the plastic formability degrades.

[0055] Table 3 shows the ratio of the density against the theoretical density for each test piece arbitrarily cut out from the magnesium alloy cast 1 of the embodiment. The volume of each test piece was calculated from its dimensions after formed by machining. 3 TABLE 3 Sample Number 1 2 3 Volume (cm3) 1.241 4.526 8.355 Weight (g) 2.177 7.977 14.78 Density (g/cm3) 1.754 1.763 1.769 Ratio to the theoretical density (1.78 g/cm3) 98.6% 99.0% 99.4%

[0056] The part other than the test pieces had densities that were 98-100% of the theoretical density, 1.78 g/cm3, calculated from the composition of the magnesium alloy cast 1 of the embodiment. On the other hand, the material 4 of FIG. 10 having the same composition as the magnesium alloy cast 1 had a number of voids 7 therein, and thus many parts of this material 4 had densities that were smaller than 98% (no more than 1.744 g/cm3) of the theoretical density, 1.78g/cm3. The magnesium alloy cast 1 of the embodiment has a dense structure with few internal voids, and as a result, it is hard to fracture during the downstream processing of extrusion and rolling as well as subsequent secondary processing. In addition, it is possible to prevent surface defects due to burst of inner bubbles during the coating-drying process (FIG. 8).

[0057] In this embodiment, the magnesium alloy cast 1 was formed by casting (described later) into 3 mm-, 5 mm- and 10 mm-thick plates. There was no distinct difference in the characteristics and ratio of the textures between the three plates due to thickness difference.

[0058] Although the embodiment used the AZ31 magnesium alloy, other AZ-series magnesium alloys may be used. In order to obtain the above characteristic microstructure, the AZ-series magnesium alloy should contain aluminum of 1.8-9.1 wt. % of the alloy.

[0059] An X-ray diffraction analysis of the surface of the cast 1 of the embodiment shows a clear orientation of the (0002) plane, as shown in FIG. 4. This is probably because a force was applied vertically to the ingot from its surface during solidification.

[0060] FIG. 5 shows the structure of a casting apparatus 21 for manufacturing the magnesium alloy cast 1 of the embodiment.

[0061] This casting apparatus 21 comprises a crucible 11 that holds magnesium alloy material 13 and serves as a melting apparatus for melting magnesium alloy material 13 using a heater (heating means) 12, a cooling mold 15 that is attached to the crucible 11 and cools the molten magnesium alloy 13a into a piece of a desired shape with a cooling pipe (cooling means) 20, and a dummy bar 16 and a pinch roll 17 that work as drawing means for continuously drawing an ingot of solidified magnesium alloy 13b from the cooling mold 15.

[0062] In this casting apparatus 21, the magnesium alloy material 13 placed in the crucible 11 is heated with the heater 12 to be molten magnesium alloy 13a. Because molten magnesium alloys easily burn when exposed to oxygen, they should be shielded from oxygen during melting. In the present embodiment, for shutting oxygen out, the crucible 11 is placed in a chamber 14, and the chamber has a mechanism (not shown) for evacuation and introducing inert gas.

[0063] Molten magnesium alloy 13a is held in the crucible 11 and flows in the cooling mold 15 directly connected to the crucible 11. The pinch rolls 17 moves the dummy bar 16 to push in and pull out of the cooling mold 15 from the lower side of the chamber 14. During casting, the solidified magnesium alloy 13b that has been drawn out, instead of the dummy bar 16, prevents the leak of molten magnesium alloy 13a.

[0064] The cooling mold 15 takes heat from the molten magnesium alloy 13a and dissipates heat to its periphery. The present embodiment has cooling means such as a cooling pipe 20 provided around the cooling mold 15 to remove the discharged heat. The water flow rate and temperature are controllable in this structure. In addition to the above water-cooling means, an air-cooling technique may be employed in such cooling means.

[0065] The solidified magnesium alloy 13b or magnesium alloy cast 1, which solidifies in the cooling mold, has the same sectional shape as that formed by the inner walls of the cooling mold 15. In the embodiment, the shape of the inner walls of the cooling mold 15 is determined to provide a magnesium alloy cast 1 of which sectional shape is a rectangle of 50 mm-width×3 mm-, 5 mm- or 10 mm-thickness. Although the length of the cooling zone of the cooling mold 15 of this embodiment is 170 mm, the length of the cooling zone is dependable upon the shape of the magnesium alloy cast molded with the cooling mold 15 and flow rate of cooling water.

[0066] During the casting of magnesium alloy cast 1 with the above configured casting apparatus, when the solidified magnesium alloy 13b (ingot) is likely to be discontinuous, it is possible to prevent ingot fracture by forming a taper 15a in the inner walls of the cooling mold 15 so that it widens toward the pinch rolls 17 against the crucible 11 to reduce friction with the cooling mold 15, as shown in FIG. 6. In contrast, with a taper 15b formed in the connection between the cooling mold 15 and the crucible 11, as shown in FIG. 7, so that it shrinks toward the pinch rolls 17 against the crucible 11, an appropriate force perpendicular to the drawing direction is applied to the solidifying region of the molten magnesium alloy 13a, and thus inclusions and gases included in the ingot may be reduced.

[0067] The crucible 11 that holds and melts magnesium alloy material 13 has an inner depth of 220 mm, coupled with the cooling mold 15 about 10 mm above its bottom, and has a structure where the magnesium alloy 13b does not easily include impurities 19 gathered in the bottom of the crucible 11 when the molten magnesium alloy 13a is drawn into the cooling mold 15.

[0068] The material shooter (material supplying means) 18 shown in FIG. 5 is installed above the chamber 14 so that magnesium alloy material 13 may be continuously added in the crucible 11 without opening/closing the chamber 14. If the magnesium alloy material 13 initially placed into in the crucible 11 is small enough to fit there, no problems arise. However, when the alloy material is added through the material shooter 18, the alloy material should be particles or chips with diameters of about 1-10 mm to prevent destruction of the crucible 11 or spill-out of molten magnesium alloy 13a from the crucible 11.

[0069] Although the casting apparatus 21 of this embodiment is a lateral type that has the cooling mold 15 attached to the side of the crucible 11, a vertical type may be used that has a cooling mold 15 in the bottom of the crucible 11.

[0070] The heater 12 serving as heating means is a high-frequency heater capable of providing an output power of around 10 kW in the present embodiment. However, the heater is not limited to a high-frequency heater but may be any heating means capable of heating materials at least 780° C.

[0071] In the present embodiment, the crucible 11 and cooling mold 15 are made of graphite, which is a material having an excellent thermal conductivity and hard to react with the molten magnesium alloy 13a. Materials, containing copper, nickel or iron, that significantly lower the corrosion resistance of the cast when included in the magnesium alloy are not suitable materials for the crucible 11 or cooling mold 15.

[0072] Molten magnesium alloy 13a constantly discharges metal vapors, and the metal vapors solidify and turn into powder when they contact the inner walls of the chamber 14. This leads to lower material yields and causes a danger of spontaneous firing. Thus, the crucible 11 should be equipped with a replaceable lid (not shown). It is also preferable to open the lid only when adding material in the crucible, in order to prevent metallic vapors from flowing out of the crucible 11.

[0073] Dummy bar 16 should be made of a material that is strong and holds high temperatures (about 650-800° C.) at which magnesium alloy material 13 is melted. In the present embodiment, stainless steel is employed.

[0074] Now the continuous casting of magnesium alloy materials using the casting apparatus 21 above will be described specifically.

[0075] First, magnesium alloy material 13 is placed in the crucible 11, and the chamber 14 is closed to be airtight.

[0076] Next, the chamber 14 is evacuated and filled with inert gas, preferably argon. Then the pressure in the chamber 14 may be, for example, 0.1-0.2 Torr (13.3-26.6 Pa) in the case of evacuation, while 14-18 Torr (1870-2400 Pa) in the case of introducing inert gas. However, the pressure is not limited to such values. Furthermore, with a gas outlet formed in the chamber 14 and internal air discharged through the outlet during inert gas introduction, the evacuation process may be eliminated.

[0077] Next, run water in the cooling pipe 20 that is cooling means installed around the cooling mold 15 to cool the cooling mold 15 and dummy bar 16. The water flow rate in the pipe is 0.5-2.01/min, and water temperature is controlled at 20-35° C.; however, these conditions are not essential.

[0078] Next, heat the crucible 11 with the heater 12 to melt magnesium alloy material 13. AZ31 magnesium alloy melts if heated up to its melting point 630° C. or higher. In the present embodiment, temperature was kept between 750-780° C., considering the fluidity of molten metal and temperature gradient in the metal, based upon the experimental result that the ingot (solidified magnesium alloy 13b) was likely to become discontinuous if the temperature of molten metal was low. This is assumed because when the temperature of molten metal is low, the metal is cooled unevenly, and solid-liquid boundaries appear in many spots, preventing continuous solidification during casting.

[0079] Meanwhile, when the temperature of molten metal is controlled at the aforesaid temperatures, fluidity of the molten metal rises and its surface tension decreases. Then gases included in the molten metal are likely to be discharged from the metal. As a result, casting defects such as voids and pores remaining in the cast product will decrease. However, raising the temperature of molten metal too much causes problems such as waste of energy and increased amount of powder metal adhering to the crucible 11 and chamber 14 due to an increased vapor pressure of molten metal.

[0080] Next, rotate the pinch rolls 17 and draw the dummy bar 16. In the embodiment, the drawing speed is controlled at 45-125 mm/min. The present embodiment performed this drawing intermittently. For example, when the drawing speed was set at 10 mm/min, the ingot was drawn as long as 5 mm at a rate of 10 mm/sec and held there for 2.5 seconds to make the overall drawing speed of 100 mm/min. Furthermore, it is possible to draw magnesium alloy 13b, repeating drawing and pushing by rotating the pinch rolls 17 in the forward and reverse directions in combination.

[0081] Compared with the case where magnesium alloy 13b is continuously drawn at a constant speed with no holding, drawing the alloy intermittently in combination with drawing and holding, an inertial force and vibrations are applied to the solid-liquid boundaries where the solidification of molten magnesium alloy 13a progresses, and therefore inclusions and voids are pushed away to the liquid phase. As a result, such a cast product will be provided that has an excellent plastic formability with few inclusions and voids.

[0082] When the dummy bar 16 is sufficiently pulled and solidified magnesium alloy 13b has reached the pinch rolls 17, the magnesium alloy 13b then plays a role of the dummy bar 16.

[0083] The temperature of magnesium alloy 13b immediately after being drawn from the cooling mold 15 is about 100° C. or lower. In the casting apparatus 21, a cooling mold 15 with a 170 mm long cooling zone was used. Then, the magnesium alloy 13b is pulled at 100 mm/min. The magnesium alloy of which temperature was 780° C. in the crucible is cooled down to 100° C. while moving the 170 mm long cooling zone in 1.7 min (102 seconds). Eventually, the cooling rate becomes about 6.7K/sec. Similarly, at 45 mm/min of speed for drawing the magnesium alloy 13b, the cooling rate becomes 3.0K/sec; while at 125 mm/min of speed for drawing the magnesium alloy 13b, the cooling rate becomes 8.3K/sec.

[0084] As magnesium alloy material 13a decreases in the crucible 11 as casting progresses, the pressure applied to the solid-liquid boundaries where molten alloy solidifies decreases, and the ingot (magnesium alloy 13b) is likely to become discontinuous. In the present embodiment, alloy material is added from the material shooter 18 when the alloy material in the crucible 11 has decreased to about half the initial input amount. Then it is unnecessary to leak the atmosphere inside the chamber 14. In some instances, alloy material may be added during casting. Alloy material may be added intermittently in batches after a prescribed amount of material has been consumed, or continuously as material gradually decreases during casting.

[0085] Next, a simplified casting experiment was conducted to confirm the cooling rate of molten metal and resulting characteristics of the cast product. Although the basic structure of the apparatus used in this Experiment-1 is the same as that employed in the aforementioned embodiment, the chamber 14 and the shooter 18 were removed to simplify the apparatus structure. Although alloy material could not be added, instead, a lid was attached to the crucible 11 to downsize the apparatus.

[0086] In this experiment, the same AZ31 alloy was used as that used in the above-described embodiment, and the cooling rate was set at 0.5K/sec, 1.0K/sec, 3.0K/sec, 8.0K/sec, 16K/sec, 20K/sec and 24K/sec for casting experiment. The followings are the results of the experiment.

[0087] When the cooling rate was 0.5K/sec, the grain boundaries were thick and clear, and any precipitation area of which area lay from 25×10−12&pgr;m2 to 2500×10−12&pgr;m2 inside grains was not identified. Such a cast product would have a poor plastic formability because of precipitation areas segregated in grain boundaries.

[0088] When the cooling rate was 1.0K/sec, the grain boundaries were clear, but about 2-10 precipitation areas of which areas were from 25×10−12&pgr;m2 to 2500×10−12&pgr;m2 per grain were identified. When the cooling rate became larger from 3.0 K/sec, 8.0 K/sec to 16K/sec, the number of precipitation areas from 25×10−12&pgr;m2 to 2500×10−12&pgr;m2 grew, and the areas as many as about 5-160 per grain were identified.

[0089] On the other hand, when the cooling rate was 20K/sec, traces of dendrites did not disappear and grain boundaries were difficult to be recognized, yet still recognizable, in the microstructure.

[0090] At the cooling rate of 24K/sec, dendrites remained and grains were not identified. Such a cast product is considered to present a state where dendrites of poor formability exist in the whole area of the cast product, degrading its plastic formability.

[0091] Table 4 summarizes the characteristics of the cross-sectional texture of the AZ31 alloy cast at individual cooling rates. 4 TABLE 4 Cooling rate 0.5 1.0 3.0 8.0 16 20 24 (K/sec) Presence/Absence ∘ ∘ ∘ ∘ ∘ &Dgr; x of grain (Hard to (Not boundaries identify) identified) Dendrites None Almost Almost Almost Remained Remained Remained Disappeared Disappeared Disappeared a little a little Number of None 2˜10 3˜130 5˜120 10˜160 20˜160 precipitation areas from 25 × 10−12&pgr;m2 to 2500 × 10−12&pgr;m2 per grain

[0092] The above results indicate that the cooling rate should be in the 1-20K/sec range for manufacturing magnesium alloy cast products featured by a cross-sectional texture where precipitation areas are dispersed inside grains. Cooling rates of about 3-8K/sec are most preferable for realizing a microstructure where dendrites of poor formability have disappeared and precipitation areas incorporating inclusions are well dispersed.

[0093] Next, Experiment-2 was conducted, which is similar to the casting experiment of the Experiment-1, using pure magnesium, AZ21, AZ61, AZ91, AM60 and a material containing about 11% aluminum. Here, the cooling rates were set at 3K/sec and 8K/sec for casting to make sure each material featuring such a cross-sectional texture where precipitation areas were dispersed inside grains.

[0094] Table 5 shows the measured compositions of pure magnesium, AZ21, AZ61, AZ91, AM60 and the material containing about 11% aluminum which were cast in the experiment. The compositions are expressed by weight percent in the table. 5 TABLE 5 Al Zn Mn Fe Si Cu Ni Ca Pure 0.002 0.003 0.001 0.002 0.005 0.0010 0.0002 0.0012 Mg Material AZ21 1.8 1.11 0.10 0.0043 0.018 0.0011 0.0010 0.0001 Alloy AZ61 5.9 0.95 0.25 0.0041 0.020 0.0015 0.0013 0.0001 Alloy AZ91 9.1 0.85 0.30 0.0037 0.028 0.0011 0.0008 0.0002 Alloy AM60 6.1 0.11 0.42 0.0033 0.022 0.0022 0.0007 0.0001 Alloy Al11% 11.2 1.15 0.34 0.0035 0.041 0.0025 0.0013 0.0001 Material Remarks: The remainder is Mg.

[0095] Casting of the above alloys was conducted at the cooling rates of 3K/sec and 8K/sec, and observed the cross-section of each cast product. As a result, the cast products of the same alloy cast at the cooling rates of 3K/sec and 8K/sec showed almost the same texture.

[0096] Although relatively clear grain boundaries were identified in the cast of pure magnesium, no precipitation area was observed inside grains.

[0097] Meanwhile, in AZ21, AZ61, AZ91 and AM60, precipitation areas from 25×10−12&pgr;m2 to 2500×10−12&pgr;m2 were recognized, like the AZ31 in the above-described embodiment, which were separated from each other inside grains.

[0098] In the cast of the high-aluminum content (11%) alloy, the precipitation areas extended over a wide range, presenting a network texture; and clear grain boundaries were not recognized.

[0099] Table 6 summarizes the characteristics of cross-sectional texture of the cast product of each alloy which was cooled at 3K/sec or 8K/sec. 6 TABLE 6 Pure Mg Al11% Material Material AZ21 AZ61 AZ91 AM60 Material Microstructure No A plurality of Precipitation precipitation precipitation areas areas formed area separate from each a network identified other no more than texture. inside 400 &mgr;m from an end to grains. another inside each grain (about 3-160 precipitation areas per grain).

[0100] The above results indicate that, when manufacturing AZ- or AM-series magnesium alloy casts by the method of the present invention, the aluminum content should be 1.8-9.1 wt % in order to provide a magnesium alloy cast featured by precipitation areas distributed inside grains. Even when third elements other than magnesium or aluminum are contained in the alloy, when the aluminum content is 1.8-9.1 wt. % ant and contents of the third elements are smaller than the aluminum content, the alloy is expected to have the characteristic that precipitation areas are distributed inside grains.

[0101] With respect to the cooling rate, it is expected that, within a range of 1-20K/sec as confirmed in AZ31 alloy, a magnesium alloy cast where a plurality of precipitation areas exist separated from each other inside grains will be provided from AZ- or AM-series magnesium alloys containing aluminum in the 1.8-9.1 wt % range.

[0102] When the cooling rate is larger than 20K/sec, dendrites remain in the texture and magnesium grain boundaries do not appear. Meanwhile, When the cooling rate is smaller than 1K/sec, dendrites completely disappear, and the cast presents a microstructure where precipitation areas 3 have dissolved, moved or segregated in &agr;-Mg phase 2 of the magnesium alloy.

[0103] According to the present invention, without solution heat treatment, cast products where grain boundaries are identified and a plurality of precipitation areas exist separated from each other inside grains are manufactured.

[0104] The followings are experimental examples demonstrating the merits of magnesium alloy cast 1 of the present invention with respect to plastic formability.

[0105] The AZ31 magnesium alloy cast 1 of the present embodiment was formed into 3.0 mm-, 5.0 mm- and 10.0 mm-thick plates, and then they were rolled into 0.5 mm-thick sheets (test sheet-1, -2 and -3). So as not to cause cracks in the cast, cold rolling was carried out several times at pressing ratios of about 10-30%, and the plate was annealed for 10 minutes at 450° C. between individual rolling times. At the final cold rolling, the plate was formed into a 0.5 mm-thick sheet, and then annealed at about 200° C. for one hour to release stress. The actual rolling times were 6, 7 and 12 for the test sheet-1, -2 and -3, respectively.

[0106] Also, a commercially available 0.5 mm-thick magnesium alloy sheet (test sheet-4) made by extrusion and rolling that were different from the embodiment was prepared. The average grain sizes were about 7-12 &mgr;m in those four test sheets.

[0107] Those test sheets were cut into octagonal blank sheets (about 40 mm×40 mm, corners were removed 6 mm) shown in FIG. 9, and each pressed into an angular cylinder with a punch of which shoulder was R2.0 mm, outer size 25 mm×25 mm and R2.0 mm at corners. With one-side clearance being kept 0.1 mm, the pressing speed was varied by controlling the die descending speed to conduct experiments. Specifically, the pressing speed was varied as 100 mm/sec, 70 mm/sec, 50 mm/sec, 20 mm/sec, 10 mm/sec, 5 mm/sec, 2 mm/sec, 1 mm/sec, 0.75 mm/sec, 0.5 mm/sec and 0.1 mm/sec.

[0108] High pressing speeds caused cracks in the four corners of the formed product. If the sheet is pressed at low speeds, the formed product has no cracks. Determining the maximum pressing speed that did not cause a crack as the pressing speed limit, the maximum pressing speed of each test sheet was measured. The die temperatures at 250° C., 200° C. and 150° C. were set, and a spray of molybdenum disulfide was used as lubricant. In the pressing test, more than one blank were prepared, and pressing was conducted on at least three times per each experimental condition to visually check for cracks. Then, the pressing speed limit was decided based on the average state.

[0109] Table 7 shows the pressing speed limits of the four test sheets. The unit of the pressing speed limit is mm/sec. 7 TABLE 7 Sheets fabricated from the cast of the Commercially Die present embodiment available sheet temperature Test sheet-1 Test sheet-2 Test sheet-3 Test sheet-4 250° C. 100 100 100 10 200° C. 20 10 10 1 150° C. 1 1 1 0.1

[0110] The experimental result indicates that the test sheet-1, -2 and -3 fabricated by rolling the magnesium alloy cast 1 of the present embodiment are not easily to have cracks even at a pressing speed which is about ten times as large as the pressing speed limit of the commercially available sheet (test sheet-4), namely, have excellent plastic formability. This experimental result is an example demonstrating the advantage of the magnesium alloy cast 1 of the present embodiment with respect to plastic formability. Based upon this result, the same advantage not only in pressing but also in other plastic forming such as forging and bending are expected.

[0111] As described so far, since the present invention eliminates casting defects such as voids and pores in the metallic texture as well as segregation of inclusions, it provides cast products of high ductility. In the cast product of the invention where a plurality of precipitation areas are separated from each other and dispersed inside grains, the grain size becomes small and uniform over the whole range of the product formed by primary processing such as extrusion and rolling. Such a cast product having fewer large grains that degrade formability presents excellent plastic formability. When such a cast product is formed into a sheet by primary processing such as extrusion and rolling or is subjected to secondary processing such as forging, destruction due to casting defects that tend to be origins of crack is prevented. Thus, it becomes also possible to simplify the forming process by rolling the cast product into a sheet, and thereby to reduce manufacturing cost. Moreover, when the formed product is coated and dried, surface defects due to the burst of inner bubbles is prevented.

[0112] Although the present invention has been fully described in connection with the preferred embodiment thereof, it is to be noted that various changes and modifications apparent to those skilled in the art are to be understood as included within the scope of the present invention as defined by the appended claims unless they depart therefrom.

Claims

1. A magnesium alloy cast having a plurality of precipitation areas existing inside magnesium alloy grains, the precipitation area containing at least one of aluminum, zinc, and manganese.

2. The magnesium alloy cast according to claim 1, wherein the plurality of precipitation areas is separated from each other from zero to 400 &mgr;m measured from an edge to another.

3. The magnesium alloy cast according to claim 1, wherein the area of each precipitation area is from 25×10−12&pgr;m2 to 2500×10−12&pgr;m2.

4. The magnesium alloy cast according to claim 1, wherein an amount of aluminum contained is 1.8-9.1 wt. %.

5. The magnesium alloy cast according to claim 1, wherein the cast is formed in a plate-like shape.

6. A magnesium alloy casting method comprising:

a step of putting magnesium alloy material in a melting apparatus, melting the magnesium alloy material in the melting apparatus under an oxygen-free atmosphere, supplying the molten alloy to a cooling mold, and cooling the molten magnesium alloy at a cooling rate in the 1-20K/sec range for solidification; and

a step of drawing the solidified metallic ingot by drawing means for keeping the ingot continuous.

7. The magnesium alloy casting method according to claim 6, wherein the solidified metallic ingot is intermittently drawn by alternatively repeating drawing process and holding process.

8. The magnesium alloy casting method according to claim 6, wherein the molten magnesium alloy is cooled at a cooling rate in the 3-8K/sec range.

9. The magnesium alloy casting method according to claim 6, wherein the magnesium alloy material added in the melting apparatus is a granular solid, and the molten magnesium alloy is molded into the cooling mold with the atmosphere around the melting apparatus being kept inert.

10. The magnesium alloy casting method according to claim 6, wherein a cross-section of an inner wall of the cooling mold is rectangular and at least one part of the inner wall is tapered so that a short side of the rectangle on the melting apparatus side is different from that on the drawing means side.

11. The magnesium alloy casting method according to claim 6, wherein an alloy holding part of the melting apparatus and/or the cooling mold are/is made of graphite.

12. The magnesium alloy casting method according to claim 6, wherein the cooling mold is connected to a side of the melting apparatus and the magnesium alloy is drawn in a horizontal direction.