Specialized metallurgical processes – compositions for use therei – Processes – Producing or treating free metal
Reexamination Certificate
2001-11-13
2003-11-25
King, Roy (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Processes
Producing or treating free metal
C148S420000, C420S407000
Reexamination Certificate
active
06652621
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a method of manufacturing a magnesium alloy member, which is a thixotropic material in which a solid material coexists with a liquid material.
BACKGROUND ART
A magnesium alloy member, which is excellent in light weight, high intensity, accuracy and fire retardancy and is a large-scaled thin member, can be enumerated as one of members which constitute the principal portion of a motor vehicle, an aircraft or the like. As technologies for shaping the member, an injection molding method for a thixotropic material, which is disclosed in Japanese Patent KOKOKU No. 33541/89 and Japanese Patent KOKOKU No. 15620/90, is known.
According to this injection molding method, athixotropic material such as a magnesium alloy having a dendrite structure is heated to a temperature in the range of from the liquidus temperature or more to the solidus temperature thereof or less in a molding machine so as to make a solid-liquid coexistent state; and a dendrite is sheared with a screw in the molding machine while the solid-liquid coexistent state is kept, so that the dendrite can be inhibited from growing until the dendrite is injected into a mold.
According to a method of casting a thixotropic material such as a magnesium alloy through an injection molding method, the granulation and growth of a dendrite are inhibited until the dendrite is injected into a mold. However, a thixotropic material such as a magnesium alloy is very high in thermal conductivity, and therefore, after the material is injected into a mold, it is quenched in the mold. This causes a rapid coagulation, which has been the main cause of the following problems.
That is, in the above injection molding method, the dendrite of the thixotropic material in a solid-liquid coexistent state at a temperature in the range of from the liquidus temperature or more to the solidus temperature or less in the mold is sheared and granulated so as to inhibit the growth. However, the thixotropic material exists in a solid-liquid coexistent state before it is injected into the mold, and thus there is a small difference between the temperature of the thixotropic material and the coagulation temperature thereof, which is commonly in the range of from 130° C. to 160° C. Therefore, the thixotropic material as injected into the mold begins to coagulate in a moment of time, whereby the flow pass of the thixotropic material in the mold rapidly becomes narrower. Hence, it is difficult to fill a mold for a thin shaped article, in particular, for a large-scaled complicated thin shaped product such as a motor vehicle with the thixotropic material to the end, and thus it is difficult to improve a large-scaled thin injection molded product in quality. In addition, since the flow pass of the thixotropic material in the mold rapidly becomes narrower, a liquid phase in the thixotropic material, which is easy to flow, escapes to the end of the mold, and/or can contribute to a molding sink, which makes the improvement of a large-scaled thin injection molded article in quality still more difficult.
Against the above-mentioned problems, countermeasures for keeping the temperature of a thixotropic material to the end of a mold have been taken. However, none of them has provided a solution for the above-mentioned problems.
For example, there exists a countermeasure, which comprises increasing the injection speed of a thixotropic material into a mold. That is, this countermeasure is intended to increase the injection speed of the thixotropic material into the mold for a large-scaled thin shaped product to five times or more as compared with the one in a resin injection molding method, or to 35 m/sec or more in some cases, so that the mold can be filled with the thixotropic material to the end in a minute range of temperature decrease. However, when the injection speed of the thixotropic material into the mold has been increased as mentioned above, a mold cavity and/or vortical traces on the surface of an injection molded product are often observed due to turbulence in the flow of the thixotropic material.
As another example, there exists a countermeasure, which comprises applying metal plating or coating of a heat insulating material to the surface of a mold. That is, metal plating or coating of a heat insulating material is applied to the surface over which a thixotropic material in the mold flows so that the heat insulating material can inhibit the temperature of the thixotropic material from decreasing when the thixotropic material is injected thereinto. In this case, the heat insulating material is largely different from a base material of the mold in coefficient of thermal expansion, and therefore, when a material which is heated to a high temperature of 500° C. or more, with which the interior of the mold is filled, is repeatedly cooled in the mold, the plated metal or the coating of the heat insulating material is peeled in earliest stages, and thus the length of life is apt to be shortened. Furthermore, since the injection speed of the thixotropic material is rapid, the surface of the mold is intensely abraded by a solid portion of the thixotropic material, and there by the plated metal or the coating of the heat insulating material is worn away in earliest stages, whereby the life of the mold is further shortened.
Besides, it has been carried out to improve the flowability of a thixotropic material in a mold. For example, a material such as silica or potassium is added to a magnesium alloy so that a solid-phase particle of the magnesium alloy in a semi-molten state becomes minute and spherical so as to improve its flowability. However, with respect to this type of magnesium alloy, the improvement effect of flowability thereof is observed when the magnesium alloy is molded, while the material characteristics of the magnesium alloy member after molding molten, such as strength, cannot be improved.
Accordingly, the material characteristics of the magnesium alloy member after molding are generally inferior to those of an aluminum alloy member, and it has been said that it is difficult to improve the material characteristics thereof. For example, a magnesium-based magnesium alloy is largely weak in tensile strength and fatigue strength as compared with an aluminum-based aluminum alloy. As to tensile strength, the magnesium alloy has its strength of 230 Mpa, while the aluminum alloy has its strength of 315 Mpa. As to fatigue strength, the magnesium alloy has its strength of 70 Mpa, while the aluminum alloy has its strength of 130 Mpa.
Thus, as a countermeasure for increasing the strength of a magnesium alloy, carbon fibers have been used as a reinforcing material for magnesium alloy die-casting. That is, the carbon fibers and the magnesium alloy have been kneaded at a temperature of the solidus temperature or more (about 700° C. or more) so that the magnesium alloy member can be reinforced with the carbon fibers. However, in this case, according to experimental results by the present inventors, as shown in
FIG. 6
(which is a graph illustrating the relationship between “the content of C
3
Al
4
in a carbon fiber” and “the temperature of a molten Al liquid”), when the carbon fiber and the magnesium alloy are kneaded at a temperature of 700° C. or more, an aluminum component in the magnesium alloy reacts with the carbon fiber, whereby the carbon fiber becomes remarkably fragile, and thus it is difficult to improve the strength of a magnesium alloy member with the carbon fiber.
Furthermore, as a means for inhibiting the reaction of an aluminum component in a magnesium alloy with a carbon fiber whereby the carbon fiber is fragile when the magnesium alloy and the carbon fiber are kneaded at a temperature of 700° C. or more, the surface of the carbon fiber is previously treated with metal plating or the like. However, it is difficult to treat the surface of a carbon fiber as described above from the viewpoint of a manufacturing process and capital investment, whereby the manufacturing cost of a magnesium alloy member becomes c
Matsuda Yutaka
Oishibashi Hiroji
King Roy
Pitney Hardin Kipp & Szuch LLP
Wessman Andrew
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