Powder metallurgy processes – Forming articles by uniting randomly associated metal particles – Consolidation of powders
Reexamination Certificate
1999-12-21
2001-02-20
Mai, Ngoclan (Department: 1742)
Powder metallurgy processes
Forming articles by uniting randomly associated metal particles
Consolidation of powders
C419S036000
Reexamination Certificate
active
06190605
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to lubrication of molds used for the compaction of metal powders, as is done in preparing metal powder compacts for sintering.
BACKGROUND OF THE INVENTION
Powder metallurgy is a well established process for the manufacture of a wide range of products for various applications. In its simplest form, the process involves pouring fine powders into a precision metal mold which has moveable elements (FIG. 1) and then applying pressure to the powder to form a “compact”. The compact is then ejected from the mold by a relative upwards motion of the bottom tool element (punch). Holes can be formed in the compact by use of “core-pins”. The compact is then subjected to a thermal process called “sintering” which involves heating the compact in a temperature controlled furnace under a protective atmosphere to effect powder particle bonding and alloying which results in a strong metal product that can be used for structural and mechanical purposes.
It is also well known that the physical and mechanical properties of the “sintered product” are highly dependent upon its density. Since both static and dynamic strength are highly valued properties of materials, there has been extensive work in both academic and industrial arenas to increase the density at low cost. There are several costly ways of achieving this high density goal: double processing which involves restriking the sintered product and then resintering it, hot forging the sintered product, and recently “warm pressing” of powder mixes involving special expensive lubricants and binder powder additives plus a system for precision heating of the powder mixture prior to compaction in warm tooling.
The present invention, however, is an improvement on another approach which involves lubrication of the vertical surfaces of the mold elements (tools). This invention allows elimination of powder lubricants normally added to the mix to facilitate ejection of the compact from the mold to occur without scoring or galling of the tools from cold welding of metal powder particles to metal tool elements. Elimination of the pressing lubricants which are light soap-like powders such as an organic stearate, clears the way for extra metal powder densification at high compacting pressures.
Mold wall lubrication is not new. In fact, it has been practiced commercially at Zenith Sintered Products, Germantown, Wis. U.S.A. since before 1985 under the trade name Z95 Plus. This, however, involved a liquid lubricant spray onto the tool surfaces. A drawback to the process is that the resulting compact surface is wet, and this collects and holds loose powder which bonds to the compact in the sintering stage. The result can be unacceptable quality products. The washing of compacts has been used to overcome this problem, but the washing process has its own problems. The liquid carrier medium also presents problems since it must be volatile yet meet stringent safety regulations.
The search for a dry powder sprayed on mold coating was therefore a direction of research. Recently a process involving charging the lubricant powder particles electrostatically and spraying them onto the mold which is electrically grounded has been developed and the results published widely. A major limitation with this process is with respect to the depth of mold that can be effectively coated to permit ejection of a compact under high pressing pressures. A variety of lubricant powders were sprayed onto mold wall surfaces using a “Tribostatic Sprayer” which was attached to a production compacting press using production tooling to make a right cylinder of approximate dimensions 1.5 inches outer diameter, 1.0 inches inner diameter. It was found by experimentation that at high pressing pressures (above 50 tons per square inch) the maximum density achievable of an iron-carbon-copper powder mix was limited to 7.25 grams per cubic centimeter and the vertical length (height) was limited to about 0.5 inches. The limiting mechanism governing the height of the compact was the removal of powder lubricant from the top half of the mold surface during the powder compaction stage. Since the powder height is about halved during compaction, the top half of the mold wall, past which the compact must be ejected, is dry and unlubricated prior to ejection. This leads to scoring and galling of the mold surface on ejection. It was confirmed that wet spraying of the mold surface did not suffer from this effect, since a wet residue is left on the upper half of the mold wall during compaction, that provides lubrication during compact ejection.
SUMMARY OF THE INVENTION
An object of the invention, therefore, is to take advantage of the dry powder spray system yet provide a residual “wet” type wall lubrication, and yet avoid a wet compact when it is ejected to avoid powder adherence.
In practicing the invention the mold is precisely heated prior to and during compaction to a narrow temperature band, and a dry powder lubricant is selected that has an ideal softening and melting characteristic to match that temperature range. As a result, on contact with the warm mold wall the lubricant powder particle softens and sticks to the surface. When the metallic powder is compacted, it “wipes” the soft lubricant powder down the mold wall surface, thereby smearing a residual film for effective subsequent ejection. Careful selection and control of lubricant type, condition, and mold temperature range is essential for optimum performance.
Using this process has resulted in the ability to compact rings on the annular tooling described earlier to above 7.35 grams per cubic centimeter density, with a height of at least 1.0 inches which is at the limitation of the tooling. On ejection, the compacts were non-adherent to loose powder.
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PCT Search Report dated Jul. 24, 1998 in International Appln. No. PCT/US98/07090.
Patent Abstracts of Japan, Publ. No. 01172502, publ. date Jul. 7, 1989, “Sintering Cold Forging Method,” inventor Y. Kazuyuki, applicant Mazda Motor Corp.
Patent Abstracts of Japan, Publ. No. 04173903, publ. date Jun. 22, 1992, “Method for Forging Powder of Aluminum Alloy,” inventor Akechi Kiyoaki, applicant Sumitomo Electric Ind. Ltd.
Cadle Terry M.
Mandel Joel H.
Roskopf Paul R.
Mai Ngoclan
Quarles & Brady LLP
Zenith Sintered Products, Inc.
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