SEMI-SOLIDIFIED SLURRY PRODUCTION METHOD, MOLDED BODY PRODUCTION METHOD, AND MOLDED BODY

TECHNICAL FIELD

The present invention relates to a semi-solidified slurry production method, a molded body production method, and a molded body.

BACKGROUND ART

In a production method for a semi-solidified slurry of metal consisting of a solid phase and a liquid phase, Patent Document 1 discloses a technique for producing a semi-solidified slurry by revolving and rotating (spinning) a plurality of rods in a molten metal.

PRIOR ART DOCUMENT
Patent Document

- Patent Document 1: Japanese Patent Application Laid-Open (kokai) No. 2018-15771

SUMMARY OF THE INVENTION
Problem to be Solved by the Invention

In the conventional art, the molten metal is stirred by revolving and rotating (spinning) the rods, but there is a problem that the solid phase and the liquid phase are not mixed sufficiently, so that the solid phase ratio of the obtained semi-solidified slurry varies at each part.

The present invention has been made to solve the above problem, and an object of the present invention is to provide a semi-solidified slurry production method capable of reducing the variation of a solid phase ratio at each part of a semi-solidified slurry, a molded body production method capable of reducing the variation of the size of crystal grains, and a molded body.

Means for Solving the Problem

To attain the above object, a production method for a semi-solidified slurry of the present invention includes: a preparation step of placing a molten metal into a bottomed container; and a stirring step of stirring the molten metal by performing reciprocating movement of a rod placed in the molten metal in a length direction of the rod until a solid phase ratio at any portion of the molten metal in the container reaches 80% or more.

A production method for a molded body of the present invention includes a molding step of pressurizing the semi-solidified slurry to deform the semi-solidified slurry to mold the semi-solidified slurry after the semi-solidified slurry is obtained by the production method for the semi-solidified slurry.

In a molded body of the present invention, when a plurality of eutectic area ratios each of which is a ratio of an area of eutectic crystals appearing in a field of view on a predetermined cross-section to an area of the field of view are measured for each field of view, a variation coefficient obtained by dividing a standard deviation of the eutectic area ratios by an average value of the eutectic area ratios is 0.15 or less.

Advantageous Effects of the Invention

In the production method for the semi-solidified slurry according to a first aspect, since the molten metal is stirred by performing reciprocating movement of the rod placed in the molten metal in the length direction of the rod in the stirring step, even if the solid phase ratio is high, the solid phase and the liquid phase are easily stirred homogeneously when the molten metal solidifies. Furthermore, since the molten metal is stirred until the solid phase ratio of the molten metal reaches 80% or more, the variation of the distribution of the liquid phase around the solid phase is reduced while the liquid phase portion that solidifies and grows into a solid phase is reduced. Therefore, the variation of the solid phase ratio at each part of the obtained semi-solidified slurry can be reduced, and the variation of the size of crystal grains can also be reduced.

In the production method for the semi-solidified slurry according to a second aspect, in the production method for the semi-solidified slurry according to the first aspect, the position, orthogonal to the length direction of the rod, of the reciprocating movement of the rod is different from that of the immediately previous reciprocating movement, so that the solid phase and the liquid phase are easily stirred homogeneously at a plurality of positions orthogonal to the length direction of the rod in the molten metal when the molten metal solidifies. Therefore, the variation of the solid phase ratio at each part of the obtained semi-solidified slurry can be further reduced.

In the production method for the semi-solidified slurry according to a third aspect, in the production method for the semi-solidified slurry according to the first or second aspect, on a cross-section orthogonal to the length direction of the rod, a plurality of the rods exist so as to be spaced apart from each other, and the two adjacent rods are located such that a center of one of the rods is positioned in a circle centered on a center of the other of the rods and having a radius equal to a length which is seven times a thickness of the other of the rods. Thus, the influence of the reciprocating movement of the rods is given to the molten metal between the rods, and the solid phase and the liquid phase are easily stirred homogeneously when the molten metal solidifies. Therefore, the variation of the solid phase ratio at each part of the obtained semi-solidified slurry can be further reduced.

The production method for the semi-solidified slurry according to a fourth aspect includes an electromagnetic stirring step performed prior to or simultaneously with the stirring step in the production method for the semi-solidified slurry according to any one of the first to third aspects. Therefore, the size of crystal grains of the obtained semi-solidified slurry can be further reduced by the electromagnetic stirring step.

In the production method for the molded body according to a fifth aspect, a semi-solidified slurry having small variation of the solid phase ratio at each part and having small variation of the size of crystal grains is obtained by the production method for the semi-solidified slurry according to any one of the first to fourth aspects. The semi-solidified slurry is pressurized and deformed to be molded, so that a molded body having stable proof stress at any part can be obtained.

In the molded body according to the sixth aspect, since the variation coefficient of the eutectic area ratio of the molded body is 0.15 or less, the variation of the proof stress, which correlates with the eutectic area ratio, at each part can be reduced. Therefore, a molded body having stable proof stress at any part can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(a) is a cross-sectional view of a container when rods are outside a molten metal, and FIG. 1(b) is a cross-sectional view of the container when the rods are in the molten metal.

FIG. 2(a) is a cross-sectional view of the container when the rods are outside the molten metal at a horizontal position different from that in FIG. 1(a), and FIG. 2(b) is a cross-sectional view of the container when the rods are in the molten metal at a horizontal position different from that in FIG. 1(b).

FIG. 3(a) is a cross-sectional view of the container along a line IIIa-IIIa in FIG. 1(b), and FIG. 3(b) is a cross-sectional view of the container along a line IIIb-IIIb in FIG. 2(b).

FIG. 4(a) is a perspective view of a semi-solidified slurry, and FIG. 4(b) is a cross-sectional view of the semi-solidified slurry shown in an arrow IVb in FIG. 4(a).

FIG. 5 shows schematic diagrams representing the movement of a solid phase and a liquid phase in the molten metal in a stirring step.

FIG. 6(a) is a perspective view of a molded body, and FIG. 6(b) is a projection view of the molded body drawn on a plane perpendicular to a line A in FIG. 6(a).

FIG. 7 shows the relationship between a stirring method and a solid phase ratio.

FIG. 8 shows the relationship between the stirring method and a eutectic area ratio.

MODES FOR CARRYING OUT THE INVENTION

Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings. A production method for a semi-solidified slurry 40 and a production method for a molded body 100 according to one embodiment will be described with reference to FIG. 1(a) to FIG. 4. FIG. 1(a) is a cross-sectional view of a container 20 when rods 30 are outside a molten metal 10. FIG. 1(b) is a cross-sectional view of the container 20 when the rods 30 are in the molten metal 10. In FIG. 1(a) and FIG. 1(b), the up-down direction of the drawing sheet, the right-left direction of the drawing sheet, and the direction perpendicular to the drawing sheet are referred to as up-down direction, right-left direction, and front-back direction of the container 20, respectively (the same applies to FIG. 2(a), FIG. 2(b), and FIG. 5(a) to FIG. 5(d)).

As shown in FIG. 1(a), the molten metal 10 is placed in the bottomed container 20. The molten metal 10 is obtained by melting a metal, and is obtained, for example, by melting an aluminum alloy, a magnesium alloy, a copper alloy, or an iron alloy. The molten metal 10 may be obtained by mixing a powder or a fiber in a molten metal. Examples of the materials of the powder and the fiber include ceramics such as SiC and Al₂O₃, and carbon. In a preparation step, the molten metal 10 is placed into the container 20 by pouring the molten metal 10 from a spout of a furnace (not shown) set at a high temperature at which a molten state can be maintained, or placing a vessel into the furnace, taking the molten metal 10 into the vessel, and pouring the molten metal 10 from the vessel into the container 20.

The metal is preferably a heat-treatable alloy whose mechanical properties (especially proof stress) are improved by performing a solution treatment and an aging treatment thereon. Examples of the metal include A6000 series heat-treatable alloys such as A6051 and A6061, A2000 series heat-treatable alloys such as A2011, A2017, and A2618, and A7000 series heat-treatable alloys, all of which are aluminum alloys. In addition, the metal may be, for example, a metal based on series such as Mg—Zn(—Zr) (ZK series), Mg—Zn—Cu (ZC series), Mg—Zn—RE (ZE and EZ series; here, RE means a rare earth element), Mg—Zn—Mn(—Al) (ZM series), Mg—Al—Zn(Mn) (AZ and AM series), Mg—Y—RE(˜Zr) (WE series), Mg—Ag—RE(˜Zr) (QE and EQ series), and Mg—Sn(—Zn, Al, Si) base alloys, all of which are magnesium alloys. In addition, copper alloys and iron alloys are employed as the metal. In the present embodiment, the metal is a heat-treatable alloy of aluminum.

As the container 20, a container made of a metallic or non-metallic material can be used as long as the container has high-temperature strength with respect to the temperature of the molten metal 10 and does not react with the molten metal 10. The container 20 is open at an upper side thereof, has a bottom 21 at a lower side thereof, and has a rectangular cross-section orthogonal to the up-down direction. In the right-left direction and the front-back direction (hereinafter referred to as “horizontal direction”) of the container 20, flat walls 22 are arranged in the right, left, front, and back directions, respectively, and are connected to each other. The lower end of each wall 22 is connected to each end in the horizontal direction of the bottom 21.

A plurality of rods 30 are spaced apart from each other and connected to one surface of a base 32. Each rod 30 has a pointed conical end portion 31 at an end on the side opposite to the side where the rod 30 is connected to the base 32. As the shape of a cross-section of the rod 30, a circular shape, an elliptical shape, a quadrangular shape, a triangular shape, a polygonal shape, and a star shape are employed. The shape of the cross-section of the rod 30 is preferably a circular shape having only a few portions protruding in an outer shape.

The plurality of rods 30 are arranged parallel to each other. The length of each rod 30 is set longer than the distance from a liquid surface 11 of the molten metal 10 in the container 20 to the bottom 21 of the container 20. In the preparation step, the rods 30 are outside the liquid surface 11 of the molten metal 10.

Each rod 30 is made of a metallic or non-metallic material that can withstand the temperature of the molten metal 10. At least the surface of the rod 30 is coated with DLC (diamond-like carbon) or the like. Owing to the coating, the rod 30 suppresses wear due to friction with the molten metal 10 and adhesion of the molten metal 10.

As shown in FIG. 1(b), in a stirring step, immediately after the molten metal 10 is placed in the container 20, or after the container 20 is kept in (exposed to) the air atmosphere, a vacuum, or an inert gas such as argon gas or nitrogen gas for a predetermined time after the molten metal 10 is placed in the container 20, the rods 30 are inserted from the liquid surface 11 of the molten metal 10 toward the bottom 21 of the container 20 (downward). Then, the rods 30 are reciprocated in the length direction of the rods 30 to stir the molten metal 10. The predetermined time for keeping the container 20 in the air atmosphere, the vacuum, or the inert gas is set as appropriate according to the amount of the molten metal 10, the type of the metal of the molten metal 10, the shape and the size of the container 20, the atmosphere temperature of the air atmosphere, the vacuum, or the inert gas, etc.

FIG. 2(a) is a cross-sectional view of the container 20 when the rods 30 are outside the molten metal 10 at a horizontal position different from that in FIG. 1(a), and FIG. 2(b) is a cross-sectional view of the container 20 when the rods 30 are in the molten metal 10 at a horizontal position different from that in FIG. 1(b).

The horizontal position of the rods 30 in FIG. 2(a) is a position to which the horizontal position of the rod 30 in FIG. 1(a) is moved in the right direction by a distance a. As shown in FIG. 1(b), once the rods 30 are inserted toward the bottom 21 of the container 20, the rods 30 rise from the bottom 21 of the container 20 toward the liquid surface 11 of the molten metal 10 and return to the position of the rods 30 in FIG. 1(a). Then, as shown in FIG. 2(a), the rods 30 move in the right direction by the distance a.

Next, as shown in FIG. 2(b), from the horizontal position to which the rods 30 have moved in the right direction by the distance a, the rods 30 are inserted from the liquid surface 11 of the molten metal 10 toward the bottom 21 of the container 20. Then, the rods 30 further rise from the bottom 21 of the container 20 toward the liquid surface 11 of the molten metal 10 and return to the position of the rods 30 in FIG. 2(a).

FIG. 3(a) is a cross-sectional view of the container 20 along a line IIIa-IIIa in FIG. 1(b), and FIG. 3(b) is a cross-sectional view of the container 20 along a line IIIb-IIIb in FIG. 2(b). In FIG. 3(a) and FIG. 3(b), the up-down direction of the drawing sheet, the right-left direction of the drawing sheet, and the direction perpendicular to the drawing sheet are the front-back direction, the right-left direction, and the up-down direction of the container 20, respectively. In FIG. 3(b), portions shown by dotted lines are four representative cross-sections of cross-sections of the plurality of rods 30 in FIG. 3(a), and indicate the positions before moving in the horizontal direction by the distance a.

As shown in FIG. 3(a), in the cross-section orthogonal to the length direction of the rods 30, the plurality of rods 30 exist so as to be spaced apart from each other. Two adjacent rods 30 are located such that the center of one rod 30 is positioned in a circle centered on the center of the other rod 30 and having a radius equal to a length L2 which is seven times a thickness L1 of the other rod 30. The thicknesses of the adjacent rods 30 may be different from each other or may be the same.

The distance a is a variable value (a value including the same magnitude) whose magnitude changes for each reciprocating movement of the rods 30. The direction of movement by the distance a also changes in the right-left direction and the front-back direction or any of both directions for each reciprocating movement of the rods 30. Therefore, in the stirring step, the rods 30 can be reciprocated at a horizontal position different from that of the immediately previous reciprocating movement, so that a solid phase 50 (described later) and a liquid phase 60 (described later) are easily stirred homogeneously at a plurality of horizontal positions in the molten metal 10 when the molten metal 10 solidifies.

The reciprocating movement of the rods 30 shown in FIG. 1(a) to FIG. 3(b) is repeated until a solid phase ratio at any portion of the molten metal 10 reaches 80% or more. The solid phase ratio at any portion of the molten metal 10 reaching 80% or more means that the solid phase ratio becomes 80% or more at a portion at which the solid phase ratio of the molten metal 10 is the lowest.

FIG. 4(a) is a perspective view of the semi-solidified slurry 40, and FIG. 4(b) is a cross-sectional view of the semi-solidified slurry 40 shown in an arrow IVb in FIG. 4(a). The semi-solidified slurry 40 in FIG. 4(a) is a semi-solidified slurry 40 taken out of the container 20 by inverting the up-down direction after the molten metal 10 is stirred in the stirring step. In the semi-solidified slurry 40, the solid phase 50 and the liquid phase 60 coexist.

As shown in FIG. 4(a) and FIG. 4(b), the solid phase ratio of the molten metal 10 after the stirring step, that is, the semi-solidified slurry 40, can be determined by taking the semi-solidified slurry 40 out of the container 20 immediately after the stirring step, placing the semi-solidified slurry 40 into water to rapidly cool the semi-solidified slurry 40, and observing cross-sections at 15 locations with a metallurgical microscope (200× magnification). The cross-sections at 15 locations of the semi-solidified slurry 40 are cross-sections at a total of 15 locations including five locations of wall side portions 44, 44, intermediate portions 45, 45, and a center portion 46 from a portion in contact with the wall 22 of the container 20 toward the inner side at three locations of an upper portion 41, a central portion 42, and a lower portion 43 from the upper side toward the lower side of the semi-solidified slurry 40. The rapidly cooled and solidified portion is the liquid phase 60 portion of the semi-solidified slurry 40. The solid phase ratio (%) can be obtained for each of the cross-sections at 15 locations by dividing the area of the solid phase 50 appearing in a field of view on the cross-sections by the area of the field of view and multiplying the resulting quotient by 100. The field of view is a rectangular region of 450 μm in length and 600 μm in width.

As for the solid phase ratio of the molten metal 10 in the stirring step, a semi-solidified slurry 40 having a desired solid phase ratio can be obtained, for example, by creating in advance a graph of the correlation between the kinematic viscosity and the solid phase ratio of the molten metal 10, providing a viscometer to the rod 30, and measuring the kinematic viscosity of the molten metal 10 when stirring the molten metal 10. In addition, a semi-solidified slurry 40 having a desired solid phase ratio can be obtained by creating in advance a graph of the correlation between a stirring time and a solid phase ratio, and stirring the molten metal 10 until the time when the desired solid phase ratio is obtained from the graph.

The end portion 31 is placed such that the pointed end thereof is directed from the liquid surface 11 of the molten metal 10 toward the bottom 21 of the container 20. Since the end portion 31 is pointed at the end thereof, it is easy to insert the rod 30 into the molten metal 10 even when the molten metal 10 has a high solid phase ratio of 80% or more.

In the stirring step, since two adjacent rods 30 are located such that the center of one rod 30 is positioned in a circle centered on the center of the other rod 30 and having a radius equal to the length L2 which is seven times the thickness L1 of the other rod 30, and stirring is performed until the solid phase ratio becomes a high solid phase ratio of 80% or more, even if the distance between the centers of the adjacent rods 30 is long, the molten metal 10 therebetween can be sufficiently stirred.

Next, the movement of the solid phase 50 and the liquid phase 60 in the molten metal 10 in the stirring step will be described with reference to FIG. 5(a) to FIG. 5(d). FIG. 5(a) is a schematic diagram when the molten metal 10 is placed into the container 20 in the preparation step. FIG. 5(b) is a schematic diagram representing the state of the molten metal 10 when the rod 30 is inserted into the molten metal 10 in the stirring step. FIG. 5(c) is a schematic diagram representing the state of the molten metal 10 after a predetermined time from FIG. 5(b). FIG. 5(d) is a schematic diagram representing the state of the molten metal 10 after a further predetermined time from FIG. 5(c).

In FIG. 5(a) to FIG. 5(d), the solid phase 50 represented by squares and the liquid phase 60 represented by triangles schematically represent the distribution of the solid phase 50 and the liquid phase 60 in the molten metal 10. In FIG. 5(a) to FIG. 5(d), a portion at the liquid surface 11 of the molten metal 10 at which the amount of the solid phase 50 is large is omitted. Also, in FIG. 5(b) to FIG. 5(d), the rods 30 are omitted except for one rod 30 to simplify the drawings.

As shown in FIG. 5(a), the molten metal 10 placed into the container 20 from the furnace is cooled from portions closer to the walls 22 and the bottom 21 of the container 20 rather than from a central portion in the horizontal direction of the container 20, and a first layer 70 in which the amount of the solid phase 50 is large is formed from the portions closer to the walls 22 and the bottom 21 of the container 20. On the inner side of the first layer 70, a second layer 80 containing the amount of the liquid phase 60 larger than that of the first layer 70 is formed. On the inner side with respect to the second layer 80, a third layer 90 including the central portion in the horizontal direction of the container 20 and containing the amount of the liquid phase 60 larger than that of the second layer 80 is formed. The first layer 70 to the third layer 90 all include the vicinity of the liquid surface 11 of the molten metal 10 (except for the portion at the liquid surface 11 of the molten metal 10 at which the amount of the solid phase 50 is large). The boundary between the first layer 70 and the second layer 80 and the boundary between the second layer 80 and the third layer 90 are each not a clear boundary, but boundary lines are provided for the sake of description.

As shown in FIG. 5(b), the rod 30 is inserted from the liquid surface 11 toward the bottom 21 while pushing away the solid phase 50 and the liquid phase 60 in the molten metal 10 to the outside of the rod 30. The pushed-away solid phase 50 and liquid phase 60 exist around the rod 30.

Next, as shown in FIG. 5(c), the rod 30 is raised from the bottom 21 of the molten metal 10 toward the liquid surface 11, the liquid phase 60 which has higher fluidity and easily moves than the solid phase 50 flows to the portion at which the rod 30 has existed in the molten metal 10, earlier than the solid phase 50. Since the liquid phase 60 flows to the portion at which the rod 30 has existed and which is close to the bottom 21, earlier than the solid phase 50, the molten metal 10 in which the amount of the liquid phase 60 is large moves to the portion close to the bottom 21.

Then, as shown in FIG. 5(d), after the molten metal 10 in which the amount of the liquid phase 60 is large moves to the portion at which the rod 30 has existed, the molten metal 10 in which the amount of the solid phase 50 is large flows thereto. The solid phase 50 in the first layer 70 in which a large amount of the solid phase 50 exists can be pushed away, and the molten metal 10 in which the amount of the liquid phase 60 is large can flow to that portion, so that the solid phase 50 and the liquid phase 60 are easily mixed.

The molten metal 10 in which the amount of the liquid phase 60 is large and which has moved to the first layer 70 is located in the first layer 70, which is closer to the walls 22 and the bottom 21 than the positions of the second layer 80 and the third layer 90 are, and thus is easily cooled to become a solid phase 50. Since the molten metal 10 in which the amount of the liquid phase 60 that exits in the second layer 80 and the third layer 90, in which the liquid phase 60 is less likely to become a solid phase 50, is large moves to the first layer 70 in which the liquid phase 60 easily becomes a solid phase 50, no portion at which the amount of the liquid phase 60 is large remains, and the liquid phase 60 can be homogeneously made into a solid phase 50. Therefore, a semi-solidified slurry 40 having small variation of the solid phase ratio at each part can be obtained.

In the stirring step, the molten metal 10 is stirred by reciprocating the rods 30 until the solid phase ratio of the molten metal 10 reaches 80% or more, thereby reducing the variation of the distribution of the liquid phase 60 around the solid phase 50 while reducing the amount of the liquid phase 60 existing around the solid phase 50. Since the liquid phase 60 having small distribution variation causes the solid phase 50 to further grow with the nearby solid phase 50 as a nucleus, the size variation of the particles (crystal grains) of the solid phase 50 can be reduced.

In the embodiment, the speed of the reciprocating movement of the rods 30 depends on the magnitude of the reciprocating movement, and is, for example, 200 mm/sec to 300 mm/sec, and preferably, two or more reciprocations are performed per second. Since two or more reciprocations are performed per second, the negative pressure generated in the portion at which each rod 30 has existed when the rod 30 rises from the bottom 21 of the molten metal 10 toward the liquid surface 11 is increased as compared to the case where the speed is lower than that of this reciprocating movement, thereby promoting the flow of the solid phase 50 and the liquid phase 60. Therefore, the time for stirring can be shortened.

Since the horizontal position at which the rods 30 are reciprocated is different from the horizontal position at which the immediately previous reciprocating movement of the rods 30 is performed, the effect of the reciprocating movement can be obtained at a plurality of different horizontal positions in the molten metal 10. At any horizontal position in the molten metal 10, the solid phase 50 and the liquid phase 60 are easily mixed, so that the distribution of the solid phase 50 and the liquid phase 60 in the semi-solidified slurry 40 is easily made homogeneous. Therefore, the variation of the solid phase ratio at each part of the semi-solidified slurry 40 can be reduced.

The surface temperature of each rod 30 before coming into contact with the molten metal 10 is lower than the temperature of the molten metal 10. Therefore, the molten metal 10 that comes into contact with the surface of the rod 30 tends to solidify quickly. Since there is almost no difference between the surface temperatures of the plurality of rods 30, the molten metal 10 close to the center in the horizontal direction of the container 20 and the molten metal 10 close to the walls 22 of the container 20 are cooled regardless of the distance from the molten metal 10 to the walls 22. Therefore, the variation of the solid phase ratio at each part can be reduced.

In the stirring step, when the rods 30 are raised from the bottom 21 of the container 20 toward the liquid surface 11 of the molten metal 10 during the reciprocating movement of the rods 30, at least a portion of each rod 30 is exposed to the air atmosphere or the like, so that the rod 30 is cooled. When the portions of the rods 30 exposed to the air atmosphere and the cooled portions of the rods 30 come into contact with the molten metal 10, the formation of a solid phase 50 is promoted. Therefore, the time to solidify the molten metal 10 can be shortened.

In the stirring step, when the rods 30 are raised from the bottom 21 of the container 20 toward the liquid surface 11 of the molten metal 10 during the reciprocating movement of the rods 30, each rod 30 has only a few portions protruding in the outer shape of the rod 30, so that the molten metal 10 is less likely to adhere to the rod 30. Therefore, a semi-solidified slurry 40 having a stable volume can be repeatedly obtained. In the stirring step, the rods 30 may be vibrated while being reciprocated. In this case, when the rods 30 are raised from the bottom 21 of the container 20 toward the liquid surface 11 of the molten metal 10, the vibration can make the molten metal 10 further less likely to adhere to the rods 30. The direction of vibration is particularly preferably the length direction of the rods 30. In addition, even if the molten metal 10 adheres to the rod 30, the adhered molten metal 10 can be easily blown off with air or the like. An electromagnetic stirring step of electromagnetically stirring the molten metal 10 may be performed after the preparation step and prior to or simultaneously with the stirring step. When electromagnetic stirring is performed prior to the stirring step, the stirring step is preferably performed without any time interval after the electromagnetic stirring step.

When electromagnetic stirring is performed, the size of the particles (crystal grains) of the solid phase 50 of the obtained semi-solidified slurry 40 becomes smaller. If the size of the particles of the solid phase 50 becomes smaller, the area where the solid phase 50 is in contact with the liquid phase 60 can be increased, and more eutectic crystals can be formed between the solid phases 50.

Then, the obtained semi-solidified slurry 40 is set from the container 20 into a molding mold. The molding mold is closed, and the semi-solidified slurry 40 is pressurized to deform the semi-solidified slurry 40 to form the molded body 100 (described later). The molding mold is opened, and the molded body 100 is taken out therefrom. Then, the molded body 100 is subjected to a solution treatment and an artificial aging treatment (collectively referred to as “T6 treatment”).

The liquid phase 60 portion of the semi-solidified slurry 40 includes a portion that exists around primary crystals (solid phase 50) and causes the primary crystals to grow, and a portion that becomes eutectic crystals containing the component of the primary crystals (solid phase 50) and another element, during molding. The portion that becomes eutectic crystals contains the element of the primary crystals and an element different from that of the primary crystals. In the eutectic crystals, the element other than the primary crystals exists to a certain extent around the primary crystals. Thus, after the molded body 100 is formed, when the solution treatment and the aging treatment are performed on the obtained molded body 100, a deposition phase which is a strengthening mechanism is deposited (formed) around the primary crystals. When the deposition phase is formed, the movement of the primary crystals due to sliding between particles is inhibited by the deposition phase, so that the mechanical properties (especially proof stress) of the molded body 100 are improved.

Since the obtained molded body 100 is formed using the semi-solidified slurry 40 having a high solid phase ratio and having small variation of the solid phase ratio at each part, the variation of the distribution of the eutectic crystals formed between the solid phases 50 (primary crystals) is also small. Therefore, the molded body 100 subjected to the T6 treatment can be obtained as a molded body 100 having small variation of the distribution of the deposition phase and having small variation of the distribution of mechanical properties (especially proof stress).

The molded body 100 is, for example, a press-molded product used for a housing. As press-molding, drawing using a molding mold, forging, etc., are employed. In the molded body 100, a fiber flow (grain flow), which is a flow in a crystalline structure of metal, continuously exists.

Next, methods for obtaining the eutectic area ratio of the molded body 100 and a variation coefficient of the eutectic area ratio will be described with reference to FIG. 6(a) and FIG. 6(b). FIG. 6(a) is a perspective view of the molded body 100 which is pressurized in the molding mold, taken out therefrom, and subjected to the T6 treatment. In FIG. 6(a), a line A is a line along a direction in which the molded body 100 is pressurized in the molding mold. The direction in which the molded body 100 is pressurized may be a direction from an upper surface 101 toward a lower surface 102, a direction from the lower surface 102 toward the upper surface 101, or the directions from both the upper surface 101 and the lower surface 102. The upper surface 101 and the lower surface 102 may be uneven, or the shape of the upper surface 101 and the shape of the lower surface 102 may be different from each other.

FIG. 6(b) is a projection view of the molded body 100 drawn on a plane perpendicular to the line A in FIG. 6(a). As shown in FIG. 6(b), first, a longest line segment 112 among line segments each connecting two points 110 and 111 on a contour line 103 in the projection view of the molded body 100 is drawn. Next, the molded body 100 is cut by six cutting planes 113 that perpendicularly intersect the line segment 112 and divide the line segment 112 into six equal parts. From the cross-section of the sample obtained from each of these six equal parts, the eutectic area ratio at each part can be obtained. In the present embodiment, the eutectic area ratio is obtained from the cross-section of the sample at each of the six equal parts into which the molded body 100 is divided by the cutting planes 113. However, the eutectic area ratio may be obtained from the cross-section of the sample at each of six or more equal parts into which the molded body 100 is divided by six or more cutting planes 113.

The eutectic area ratio is the ratio of the area of the eutectic crystals appearing in the field of view on the cross-section, to the area of the field of view. When a plurality of eutectic area ratios are measured for each field of view, the molded body 100 has a variation coefficient of 0.15 or less, which is obtained by dividing the standard deviation of the eutectic area ratios by the average value of the eutectic area ratios. If a longitudinal direction exists in the orthogonal direction of the molded body 100, the cross-section of the molded body 100 is a cross-section obtained from six or more equal parts into which the molded body 100 is divided in the longitudinal direction. The field of view on the cross-section is a rectangular region of 450 μm in length and 600 μm in width.

Since the variation coefficient of the eutectic area ratio of the molded body 100 is 0.15 or less, the molded body 100 obtained can have small variation of the eutectic area ratio with respect to the magnitude of the average value of the eutectic area ratios, and thus can have reduced variation of the proof stress, which correlates with the eutectic area ratio, at each part. Therefore, the molded body 100 obtained can have stable proof stress at any part.

EXAMPLES

The present invention will be described in more detail with reference to examples, but the present invention is not limited to the examples.

Samples 1 to 3 of semi-solidified slurries were produced by stirring the molten metal 10 by different stirring methods. As the metal, A6061, which is a heat-treatable alloy of aluminum, was used. As the container 20 into which the molten metal 10 was placed, a bottomed stainless steel container having a size of 60 mm wide, 60 mm long, 50 mm high, and 0.8 mm thick and having a rectangular cross-sectional shape was used. For Sample 2 and Sample 3, the speeds at which the rods 30 were moved were 200 mm/sec to 300 mm/sec and were substantially the same. All other conditions were the same.

Sample 1 is a semi-solidified slurry obtained by performing only electromagnetic stirring for 10 seconds without any time interval after the preparation step. Sample 2 is a semi-solidified slurry obtained by performing electromagnetic stirring for 10 seconds without any time interval after the preparation step, and then further stirring by revolving the rods 30 for 30 seconds without any time interval. Sample 3 is a semi-solidified slurry obtained by performing electromagnetic stirring for 10 seconds without any time interval after the preparation step, and then further stirring by reciprocating the rods 30 for 30 seconds without any time interval.

The obtained Samples 1 to 3 were placed into water to be rapidly cooled, and the cross-section of each sample was observed with a metallurgical microscope (200× magnification) at a total of 15 locations including five locations of wall side portions, intermediate portions, and a center portion from a portion in contact with the wall 22 of the container 20 toward the inner side at three locations of an upper portion, a central portion, and a lower portion of each sample, to determine a solid phase ratio. The rapidly cooled and solidified portion is the liquid phase 60 portion of each sample. The solid phase ratio (%) was obtained for each cross-section by dividing the area of the solid phase 50 appearing in a field of view on the cross-section by the area of the field of view and multiplying the resulting quotient by 100. The field of view is a rectangular region of 450 μm in length and 600 μm in width at each of the 15 locations. FIG. 7 shows the relationship between each sample for which the stirring method is different and the solid phase ratio.

As shown in FIG. 7, the solid phase ratio of Sample 1 was in the range of 55% to 100%, and the standard deviation thereof was 13.51%. The solid phase ratio of Sample 2 was in the range of 36% to 100%, and the standard deviation thereof was 19.40%. The solid phase ratio of Sample 3 was in the range of 85.2% to 100%, and the standard deviation thereof was 4.10%. Sample 3 also had a solid phase ratio of 80% or more even at a portion at which the solid phase ratio was the lowest.

Next, Sample 1 and Sample 3 were molded by applying a pressure of about 70 MPa to 140 MPa in a molding mold by a press machine including the molding mold, to obtain molded bodies, respectively. A solution treatment and an artificial aging treatment were performed on each of the obtained molded bodies, and six tensile test pieces were made from each molded body and measured for 0.2% proof stress. The six tensile test pieces were made from six equal parts into which the molded body was divided in a longitudinal direction in an orthogonal direction orthogonal to the direction in which each sample was pressurized. The 0.2% proof stress was measured in accordance with JIS Z2241: 2011. Each tensile test piece was made as a No. 14B test piece in accordance with JIS Z2241: 2011. The 0.2% proof stress of Sample 1 was in the range of 78 MPa to 271 MPa, and the standard deviation thereof was 81.58 MPa. The 0.2% proof stress of Sample 3 was in the range of 220 MPa to 255 MPa, and the standard deviation thereof was 8.54 MPa.

Then, each of the fracture surfaces of the six tensile test pieces in the tensile test was polished and observed with a metallurgical microscope (200× magnification) to measure a eutectic area ratio. Specifically, a rectangular region of 450 μm in length and 600 μm in width was arbitrarily set on each of the fracture surfaces of the six tensile test pieces, and the eutectic area ratio in this region was measured. FIG. 8 shows the relationship between each sample for which the stirring method is different and the eutectic area ratio.

As shown in FIG. 8, the eutectic area ratio of Sample 1 was in the range of 0.4% to 1.5%, the standard deviation thereof was 0.41%, and the average value thereof was 1.09%. The variation coefficient of the eutectic area ratio of Sample 1 was 0.38. The eutectic area ratio of Sample 3 was in the range of 1.1% to 1.6%, the standard deviation thereof was 0.20%, and the average value thereof was 1.41%. The variation coefficient of the eutectic area ratio of Sample 3 was 0.15.

It is inferred that in Sample 3, as compared to Sample 1 and Sample 2, the solid phase 50 and the liquid phase 60 in the molten metal 10 were able to be homogenously stirred by reciprocating the rods 30 in the up-down direction, so that the variation of the solid phase ratio at each part became smaller.

Sample 3 has small variation of the eutectic area ratio at each part and therefore has small variation of the distribution of eutectic crystals between primary crystals. Since the variation of the distribution of eutectic crystals between primary crystals is small, it is inferred that the variation of the distribution of a deposition phase obtained by the solution treatment and the artificial aging treatment became smaller, and the variation of the 0.2% proof stress at each part became smaller.

Although the present invention has been described above based on the embodiment, the present invention is not limited to the above embodiment at all. It can be easily understood that various modifications may be made without departing from the gist of the present invention.

In the embodiment, the case where the molten metal 10 to be placed into the container 20 is a molten metal 10 of a heat-treatable alloy A6061 of aluminum has been described, but the molten metal 10 is not necessarily limited thereto. As the metal, any other heat-treatable alloy or non-heat-treatable alloy can be employed as long as the semi-solidified slurry 40 can be produced. In this case, even if the metal is a non-heat-treatable alloy, the molded body 100 formed by pressurizing the semi-solidified slurry 40 obtained by the stirring step has no deposition phase formed as a strengthening mechanism between primary crystals, but the variation of the size of crystal grains is small and the variation of the distribution of primary crystals and eutectic crystals is small, so that the molded body 100 obtained can have stable proof stress at any part.

In the embodiment, the container 20 has been described as one having a rectangular cross-section orthogonal to the up-down direction, but is not necessarily limited thereto. As a matter of course, a container whose cross-section orthogonal to the up-down direction has a circular, elliptical, or polygonal shape may be employed as the container 20.

In the embodiment, the rods 30 have been described as a plurality of rods 30 arranged parallel to each other, but are not necessarily limited thereto. The rods 30 may be rods that are not parallel to each other. As a matter of course, the arrangement of each rod 30 may be set as appropriate according to the shape of the container 20. In addition, as a matter of course, the arrangement of each rod 30 may be set regardless of the shape of the container 20. For example, the respective rods 30 may be arranged so as to spread out toward the bottom 21 and the walls 22 of the container 20.

In the embodiment, the case where there are a plurality of rods 30 has been described, but the present invention is not necessarily limited thereto. The number of rods 30 may be one.

In the embodiment, the case where the rods 30 move upward in the reciprocating movement until the end portions 31 come out of the liquid surface 11 of the molten metal 10 in the stirring step, has been described, but the present invention is not necessarily limited thereto. As a matter of course, the rods 30 may be repeatedly reciprocated in the up-down direction with the end portions 31 positioned in the molten metal 10.

In the embodiment, the production method for the semi-solidified slurry 40 in which the electromagnetic stirring step is performed prior to or simultaneously with the stirring process has been described, but is not necessarily limited thereto. As a matter of course, a stirring method other than electromagnetic stirring may be combined with the stirring step for stirring. Examples of the stirring method other than electromagnetic stirring include stirring by ultrasonic vibration, stirring by injecting gas into the molten metal 10, stirring by high frequency induction, stirring by rotating, revolving, or vibrating the rod 30, and stirring by a combination of these methods.

In the embodiment, the case where the semi-solidified slurry 40 is set from the container 20 into the molding mold and pressurized to be molded in the molding step, has been described, but the present invention is not necessarily limited thereto. The molten metal 10 may be placed directly into the mold in the preparation step, and the semi-solidified slurry 40 may be obtained in the mold by a stirring step and then pressurized to be molded as it is. In this case, the container 20 is a mold into which the molten metal 10 is placed.

DESCRIPTION OF REFERENCE NUMERALS

- 10 molten metal
- 20 container
- 21 bottom
- 30 rod
- 40 semi-solidified slurry
- 50 solid phase
- 60 liquid phase
- 100 molded body

SEMI-SOLIDIFIED SLURRY PRODUCTION METHOD, MOLDED BODY PRODUCTION METHOD, AND MOLDED BODY

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