Metal treatment – Process of modifying or maintaining internal physical... – Heating or cooling of solid metal
Reexamination Certificate
1998-09-29
2001-10-23
Ip, Sikyin (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Heating or cooling of solid metal
C148S667000, C164S076100, C164S113000, C164S900000
Reexamination Certificate
active
06306231
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method of producing a material for plastic working made of a light metal alloy, particularly a magnesium alloy containing aluminum as an alloy component, and a method of producing a plastic-worked product by using the same.
PRIOR ART OF THE INVENTION
Light metal alloys containing aluminum or magnesium as a matrix, particularly magnesium alloys containing aluminum as an alloy component, have attracted special interest recently as materials, which are light-weight and capable of securing a predetermined mechanical strength by means of a plastic working such as forging. However, since these light metal alloys show good thermal shrinkage, the fluidity is lowered unless the casting temperature is raised in the gravity casting. Consequently, a perfect (fewer cavities) casting is not obtained. However, if the casting temperature is high, the cooling rate becomes smaller, resulting in coarse material structure, poor moldability and small working ratio. Therefore, the working process must be repeated again to obtain a molded article having a required shape. On the other hand, a fine structure can be obtained by die casting. However, since a molten metal is injected into a die under pressure in a spray state, a lot of fine cavities are contained in the casting to cause gas defects and, therefore, good forged materials can not be obtained.
To improve the forgeability of the light metal alloy containing aluminum and magnesium as the matrix, a cast material having a fine structure must be obtained by a method other than die casting. Therefore, as a result of intensive studies of the present invention, it has been found that, when the light metal alloy is injection-molded while adjusting the solid phase proportion or solid phase grain size using a semi-melt injection molding method, a material having good moldability can be obtained and a desirable molded article can be obtained by single-forging from the injection-molded material.
SUMMARY OF THE INVENTION
Therefore, a first object of the present invention is to provide a semi-melt injection molding method of producing a material having excellent plastic workability.
A second object of the present invention is to provide a method of injection-molding a material having excellent plastic workability and producing a forged article by means of single-step forging.
According to a first aspect of the present invention, there is provided a method of producing a material for plastic working made of a light metal alloy, which comprises preparing a light metal alloy into a molten state at a temperature just above a melting point or a semi-molten state wherein a solid phase and a liquid phase coexist and the solid phase proportion is not more than 20%; and subjecting the molten or semi-molten light metal alloy to injection molding.
According to a second aspect of the present invention, there is provided a method of producing a plastic worked light metal alloy product, which comprises preparing a light metal alloy into a molten state at a temperature just above a melting point or a semi-molten state wherein a solid phase and a liquid phase coexist and the solid phase proportion is not more than 20%; and subjecting the molten or semi-molten light metal alloy to an injection molding and further to a plastic working.
According to the present invention, it has been found that good moldability, wherein a limiting upsetting rate is not less than 70%, is obtained by adjusting the solid phase proportion to not more than 20% (see FIG.
1
). It has also been found that, in not only the semi-molten state but also completely molten state, when injection molding is performed at the temperature just above the melting point of the matrix, it is possible to obtain a material having excellent moldability compared with the case of die casting.
The reason why the solid phase proportion is adjusted to not more than 20% is as follows.
That is, the lower the solid phase proportion becomes, the smaller the solid phase average grain size in the semi-molten state becomes. Furthermore, the smaller the solid phase average grain size becomes, the more the moldability of the injection molding material is improved. It has been found that the solid phase average grain size is preferably not more than 300 &mgr;m and the limiting upsetting rate is rapidly decreased when it exceeds 300 &mgr;m (see FIG.
2
).
The reason why the above injection-molded article shows good moldability in case of injection molding at the solid phase proportion of not more than 20% is not clear, but is considered as follows. That is, the liquid phase portion is converted into a fine structure by injection molding in the semi-molten state and the moldability at the time of forging is good, whereas, the solid phase portion is liable to retain the form. Accordingly, when the proportion of the solid phase portion is too large or the grain size is too large, scatter in moldability occurs and the moldability is lowered as a whole.
Since the plastic workability, i.e. forgeability, of the material to be molded according to the present invention is improved, forging can be performed at the temperature of not more than 400° C. Consequently, the strength is improved. Since a net-shaped product can be produced by only single forging, in addition to injection molding, a plurality of forging dies and machining are not required, resulting in advantage such as excellent economical efficiency.
The method of the present invention is preferably applied to those containing magnesium as the matrix and 4 to 9% by weight of aluminum as the alloy component, as the light metal alloy. When the amount is smaller than 4% by weight, an enhancement of the mechanical strength is not expected. On the other hand, when the amount is more than 9% by weight, the moldability (limiting upsetting rate) is drastically lowered (see FIG.
3
).
The light metal alloy obtained in the present invention is preferably subjected to a T
6
heat treatment (composed of a solution treatment and an artificial age hardening treatment) as the condition of the heat treatment. As a result, the residual strain at the time of forging is removed and a change in shape with a lapse of time of the product does not occur and, furthermore, excellent ductility is further imparted.
According to the present invention, there can be provided an injection molding material having excellent moldability by means of continuous casting. Since the injection molding material is a billet having a rough shape, a final product can be obtained by means of single-step forging and the number of forging steps can be reduced. A perfect structure with fewer cavities is obtained and, therefore, the yield can be improved.
REFERENCES:
patent: 5693158 (1997-12-01), Yamamoto et al.
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Fujita Makoto
Ishida Kyoso
Sakamoto Kazuo
Yamamoto Yukio
Ip Sikyin
Mazda Motor Corporation
Nixon & Peabody LLP
Studebaker Donald R.
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