Specialized metallurgical processes – compositions for use therei – Compositions – Consolidated metal powder compositions
Reexamination Certificate
2000-11-30
2002-08-13
Mai, Ngoclan (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Compositions
Consolidated metal powder compositions
C075S247000, C419S021000, C419S026000, C419S031000, C419S048000
Reexamination Certificate
active
06432157
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a method of producing an Ag—ZnO electric contact material.
BACKGROUND ART
Conventionally, Ag—ZnO electric contact materials have been known to have considerably low contact resistance, but also to have unsatisfactory welding resistance and wear resistance. Therefore, enhancement of welding resistance and wear resistance of Ag—ZnO electric contact materials is technically important for employment of such materials in make-and-break contacts, such as relays and switches, which are required to possess particularly excellent welding resistance and wear resistance.
A basic approach for enhancing welding resistance and wear resistance of Ag—ZnO electric contact materials resides in uniformly dispersing ZnO micrograms in Ag. In order to attain uniform dispersion of ZnO micrograms, a variety of techniques have been proposed in the fields of powder metallurgy and internal oxidation, in relation to methods of producing Ag—ZnO electric contact materials.
In powder metallurgy, powdered Ag and ZnO are mixed, and the mixture is shaped and sintered. Thus, reduction in the particle size of the powders to be mixed and sufficient mixing of ZnO micrograms result in a certain degree of dispersion. However, in powder metallurgy, the dispersion state of ZnO depends on the particle size of Ag powder and ZnO powder, and, therefore, the target uniformity in the dispersion state of ZnO grains of smaller size is considered to be limited. In addition, since Ag and ZnO have poor sinterability, voids are possibly formed in the produced sintered material, thereby lowering welding resistance and wear resistance in some cases. Thus, make-and-break contacts having highly satisfactory characteristics have never been produced from Ag—ZnO material. Furthermore, powder metallurgy is not economically preferred for producing Ag—ZnO material, in view of generally high production costs.
In internal oxidation, a predetermined amount of an Ag—Zn alloy is sequentially cast, rolled, blanked, and cut, to thereby produce an alloy product of specific shape. The product is heated in an oxidizing atmosphere, to thereby selectively oxidize Zn in the Ag—Zn alloy, causing dispersion of ZnO in Ag. As disclosed in Japanese Patent Publication (kokoku) No. 57-13613, dispersion of ZnO micrograms is attained through internal oxidation concomitant with addition of a third metallic element which causes dispersion of ZnO micrograms.
When ZnO micrograms are dispersed through internal oxidation concomitant with addition of a third metallic element, the ZnO micrograms dispersed in Ag tend to become acicular, and in many cases the acicular oxide is deposited in a streak-like manner. Dispersion becomes more distinct with increasing Zn content. Because this differs from the case of spherical ZnO micrograms dispersed through powder metallurgy, the acicular oxide deposited in a streak-like manner insufficiently enhances welding resistance and wear resistance. In addition, since the third metallic element added to disperse micrograms may affect the characteristics of Ag—ZnO electric contact material, depending on the amount of addition, the amount of the third element to uniformly disperse ZnO micrograms is considered to be limited when conventionally-employed internal oxidation is carried out.
On the basis of the features describe above, Ag—ZnO electric contact materials produced through powder metallurgy have often been employed. However, problems; e.g., controlling of powder particles and sinterability, still remain in the production of Ag—ZnO electric contact material, even when powder metallurgy as described above is employed. In addition, at present, reducing production costs thereof is demanded.
The present invention has been accomplished in view of the foregoing, and an object of the present invention is to provide a method of producing an Ag—ZnO electric contact material, which method can more uniformly disperse, in Ag, ZnO grains of smaller grain size; maintain low contact resistance of the contact material; enhance welding resistance and wear resistance; and produce the material at reasonable cost.
DISCLOSURE OF THE INVENTION
In order to solve the aforementioned problems, the present inventors have improved a method of producing an Ag—ZnO electric contact material including internal oxidation, and have achieved production of an Ag—ZnO electric contact material in which ZnO micrograms are uniformly dispersed at a level which had never before been attained. Accordingly, the invention provides a method of producing an Ag—ZnO electrical contact material which comprises casting a predetermined amount of Ag and Zn and subjecting the resultant Ag—ZnO alloy to internal oxidation to disperse ZnO in Ag, the method being characterized in that an Ag—Zn alloy comprising 5-10 wt. % (as reduced to weight of metal) Zn, the balance being Ag, is formed into chips thereof; the chips are subjected to internal oxidation; the internally oxidized chips are compacted to thereby form billets; the billets are pressed and sintered; and subsequently, the sintered billets are extruded. The present inventors have found that this method can effect highly uniform dispersion, in Ag, of ZnO micrograms.
When the cast Ag—Zn alloy is formed into chips for carrying out internal oxidation, the chips are compacted into billets, and the billets are pressed and sintered. The deposited ZnO assumes a streak-like dispersion state. However, when the billets are further extruded, the streak-like dispersion state of ZnO is converted to a uniform dispersion state of ZnO micrograms. The present inventors assume that the phenomenon occurs due to good wettability of ZnO to Ag.
When billets are formed into material such as wire rods through extrusion, a large shear stress is imposed on the billets in the longitudinal direction during deformation. The deformation during extrusion induces shear of ZnO dispersed in the billets in a streak-like manner, thereby yielding dispersion of ZnO micrograms in Ag. The present inventors have confirmed that a uniform dispersion state of oxide micrograms as yielded in the Ag—ZnO electric contact material of the present invention cannot be attained in an Ag—SnO
2
electric contact material; i.e., a material containing an oxide of poor wettability to Ag. SnO
2
that is an oxide having poor wettability to Ag cannot be formed into micrograms even though a large amount of shear stress is applied to a billet in the longitudinal direction during extrusion. In contrast, ZnO that is an oxide having good wettability to Ag is subjected to shear stress concomitant with deformation of Ag when a large amount of shear stress is applied to the billet in the longitudinal direction during extrusion. Thus, ZnO deposited in a streak-like manner in the billet is further fractured to form micrograms thereof, thereby yielding a very uniform dispersion state of ZnO micrograms to an extent which has never before been attained.
In order to produce the Ag—ZnO electric contact material of the present invention, particularly, the following process conditions must be satisfied. One condition concerns pressing and sintering to which the billets produced by compacting internally oxidized chips are subjected. The pressing and sintering must be carried out until residual voids and defects in the billets disappear. For example, pressing and sintering of the billets must be performed repeatedly, to thereby sufficiently remove voids and defects in the billets.
The other process condition concerns extrusion which is carried out as a final process. Extrusion must be carried out to a relatively large extrusion ratio. Preferably, the extrusion ratio of the surface area of a billet to that of a produced rod is controlled to 51:1 or higher. The reason for such a high extrusion ratio is that ZnO contained in Ag can be considerably uniformly dispersed in the form of ZnO micrograms by employment of the ratio, thereby enhancing production yield. Typical extruders have an extrusion capacity; i.e., an achievable extrusion ratio, of approximately 350:1. I
Kusamori Hiroyuki
Matsuzawa Osamu
Nakamura Tetsuya
Sakaguchi Osamu
Takahashi Masahiro
Arent Fox Kintner & Plotkin & Kahn, PLLC
Mai Ngoclan
Tanaka Kikinzoku Kogyo K.K.
LandOfFree
Method for preparing Ag-ZnO electric contact material and... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method for preparing Ag-ZnO electric contact material and..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method for preparing Ag-ZnO electric contact material and... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2909263