Threaded – headed fastener – or washer making: process and apparat – Process – Making externally threaded fastener – e.g. – screw or bolt
Reexamination Certificate
2000-03-27
2001-12-11
Tolan, Ed (Department: 3725)
Threaded, headed fastener, or washer making: process and apparat
Process
Making externally threaded fastener, e.g., screw or bolt
C470S009000, C470S010000, C470S011000
Reexamination Certificate
active
06328657
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a threaded fastener and a method of making it, wherein the fastener is rendered lighter in weight and more efficient to produce so that the number of processing steps is reduced to lower manufacture cost.
PRIOR ART
Recently, automobiles, household electric devices and apparatuses, office-automation (viz., ‘OA’) apparatuses and the like are all required to be as light as possible. In this connection, magnesium alloys are attracting attentions in the industries because they as a principal material forming those apparatuses are generally of a relatively low specific gravity and high specific strength. Thus, bolts, nuts and the like articles for assembling those apparatuses are also required to be lighter in weight.
However, magnesium alloys easy to mold and cut are chemically so active that dusts and chips produced during machining or cutting magnesium alloy articles are highly inflammable. Process control for and safety assurance of the alloys are not necessarily easy, thus rendering it problematic to mechanically treating with the magnesium alloys. In addition, the highly close-packed (viz., ‘hcp’) structure of magnesium microcrystals has made it considerably difficult to carry out processes for plastically deforming them at room temperatures.
JIS (viz., Japanese Industrial Standards) prescribes some ductile magnesium alloys of the types: ‘AZ31’, ‘AZ61’, ‘AZ80’ and ‘ZK60’, but applicants are not aware of any prior use of these alloys to make threaded fasteners for the reasons just mentioned above.
SUMMARY OF THE INVENTION
The present inventors have conducted researches and studies on pure magnesium of industrial grade and certain alloys thereof that had a superplastic fine-granulated metallographic (i.e., metallurgical) internal texture and showed superplasticity within a special range of temperatures. As a result, it has been found that threaded fasteners could be manufactured well by forging pure magnesium or its alloys at room temperatures, if their chemical composition as well as the size of their microcrystals and the processing temperature were restricted such that superplasticity would take place.
Objects of the present invention, that was made on the basis of such a surprising effect, are thus to provide a threaded fastener lighter in weight and easier to manufacture and also to provide a novel method of inexpensively making the fastener by a shortened process.
To achieve these objects, a threaded fastener provided herein will be produced by the moderate temperature-forging process, using a raw material selected from the group consisting of pure magnesium of industrial grade, magnesium alloys of a superplastic and fine-granulated texture, first composite materials whose matrix is pure magnesium, and second composite materials whose matrix is any of the magnesium alloys.
The temperature-forging is to be done at a raised moderate temperature falling within such a range of about 250-400° C. that superplasticity takes place, and using the raw material composed of microcrystals having an average diameter of 100 &mgr;m or less, whether material is pure magnesium of industrial grade or any of those fine-textured superplastic magnesium alloys or any of the composite materials whose matrix is pure magnesium or the alloy thereof.
Preferably, the magnesium alloys having such a superplastic fine-granulated metallographic texture may each be composed substantially of 1.0-12.0% by weight of aluminum (viz., Al), 0.3-2.5% by weight of zinc (viz., Zn), 0.2-0.3% by weight of manganese (viz., Mn), a balance of magnesium (viz., Mg) and unavoidable impurities contained therein. Alternatively, the magnesium alloys having such a superplastic fine-granulated metallographic texture may each be composed substantially of 2.0-8.0% by weight of Zn, 0.1-1.0% by weight of zirconium (viz., Zr), a balance of Mg and unavoidable impurities contained therein.
The composite materials whose matrixes are fine-granulated magnesium or its alloys of the superplastic texture are of improved mechanical strength, abrasion resistance and other properties. Ingredients for reinforcing the composite materials may be certain fibers or particles dispersed therein. Examples of such fibers and particles are: carbon fibers, glass fibers, whiskers, oxides, carbides, nitrides and the like.
The reasons for the above chemical composition of the magnesium alloys are as follows. Pure magnesium suffices if the threaded fastener need only be lightened in weight, whereas the magnesium alloys or composite materials whose matrix are any of the alloys are more preferable if the fastener must be of a higher strength in certain uses.
Each magnesium alloy consists of solid-solution elements, high-melting elements and a balance of Mg. Examples of the former elements are Zn and Al that are contained as rich as ensuring the solid-solution condition so as to miniaturize microscopic units (eutectic cells) in their finished fine metallurgical tissue. An excessive content of Zn or Al exceeding the upper limit discussed above will undesirably impair ductility and toughness of the alloys. Examples of the high-melting elements are Mn and Zr that will serve as pinning particles for stabilization of crystalline granules at elevated temperatures. In general, size of each pinning particle is 1 &mgr;m (one micrometer) or less. Richer contents of the high-melting elements will give a higher stabilization effect, but an excessive amount of them will make coarser the pinning particles and undesirably impair ductility and toughness of the alloys at room temperatures.
It is preferable that microcrystals have as small a diameter as possible in the raw material that may be pure magnesium, any of its superplastic and fine-granulated alloys, or any of composite materials whose matrix are pure magnesium or any of the magnesium alloys. If the diameter is 100 &mgr;m or less, then superplasticity will appear at temperatures raised to about 250-400° C. More preferably, the diameter may be 80&mgr;m in or less.
Next, the method of making a threaded fastener if of the folowing nature. Namely, the method is characterized in that the raw material selected from the group as below is heated to a temperature at which the material shows superplasticity when the fastener is formed by the moderate temperature-forging.
In detail, characteristic feature of the method is that the superplastic fine-granulated metallographic raw material forged at a moderate temperature is composed of microcrystals having a diameter of 100 &mgr;m or less, wherein the forging is carried out at a temperature falling within such a range of about 250-400° C. that superplasticity takes place.
The raw material used in this method has to be such as pure magnesium of industrial grade, a first kind of magnesium alloy, a second kind of magnesium alloy or any composite material whose matrix is any one of these magnesium and its alloys. The first kind of magnesium alloy is substantially composed of 1.0-12.0% of Al, 0.3-2.5% of Zn, 0.2-0.3% of Mn, a balance of Mg and unavoidable impurities contained therein (all ‘%’ values being ‘by weight’ hereinafter). The second kind of magnesium alloy is substantially composed of 2.0-8.0% by of Zn, 0.1-1.0% of Zr, a balance of Mg and unavoidable impurities contained therein. Any of these materials will be heated to 250-400° C. so as to make use of its superplasticity when moderate temperature-forging it into any desired shape of the fasteners.
Magnesium is of a relatively high thermal conductivity (being about twice that of iron). Therefore, the raw material (whether wire-shaped or rod-shaped) preliminarily heated to the above temperature will inevitably be cooled down to or below 100° C., by a forming punch and die at the first forging step for forming a bolt head. If a second forging step has to be done after the first step, the raw material must be heated again to and kept at 250-400° C. The following and final steps may be conducted using the forming die kept at 250° C. or higher so as to enable a continuous proce
Asaoka Takeyuki
Higashi Kenji
Kitagawa Masayoshi
Kohara Mitsuaki
Maekawa Keiichi
Antonelli Terry Stout & Kraus LLP
Kurimoto Ltd.
Tolan Ed
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