Powder metallurgy processes – Powder metallurgy processes with heating or sintering – Making composite or hollow article
Patent
1982-02-05
1984-09-11
Padgett, Benjamin R.
Powder metallurgy processes
Powder metallurgy processes with heating or sintering
Making composite or hollow article
419 27, 419 30, 419 38, 419 40, 4285395, 428547, 428553, B22F 700
Patent
active
044709536
DESCRIPTION:
BRIEF SUMMARY
This invention relates to a process of manufacturing sintered metallic compacts from sinterable powder.
It is generally known to mould powder compacts from metal powder or metal powder mixtures and to increase the compactness of the powder mass to the desired density by vibration or pressing, for example isostatic pressing, and to sinter the compacts at a suitable sintering temperature. Infiltration of another metal having a lower melting temperature than that of the powder mixture can be effected in an infiltration step after sintering or during the sintering proper by means of an infiltrant metal supplied from outside, or by incorporating with the metal powder mixture a weighed amount of powder of infiltrant metal. In both cases the intention is to wet the metal powder grains and to fill the voids between them to provide a dense structure. The voids between the metal grains in the compact are interconnected by a network of fine passages, and the infiltration operation proper takes place by the intermediary of capillary forces.
In the mass production of sintered articles it is desirable to use metal powder mixtures which are as cheap as possible for the product quality one wishes to obtain. To attain as uniform an infiltration as possible of the infiltrant metal in the sintered metal powder compact it has been considered necessary to mould the metal powder compact from a mixture of powder particles which are of varying sizes and are well mixed with each other. However, even though the powder particles have been thoroughly mixed from the very beginning, the moulding and compaction of the metal powder mass may involve a tendency towards separation into coarse and fine grains with accumulations of finer grains among coarser gains. This tendency towards separation will become more pronounced when the powder mass is vibrated for compaction. The price of metal powder is dependent not only on the metal price itself but also on the screening accuracy. A metal powder of very narrow grain size distribution is more expensive than a metal powder of larger grain size distribution. It is generally considered that a very dense structure of a sintered product requires the product to be produced from very fine grains and pressing of the powder mass.
The use of powder mixtures of varying grain size is considered desirable in order that the interstices between coarse grains be filled with fine grains. Powder mixtures of varying grain size are considered advantageous in that a good compaction can be attained even at a moderate compaction pressure and that the screening accuracy can be reduced. However, the tendency towards separation and formation of finer and coarser local regions constitutes a problem.
A sintered compact prepared from a coarse-grained powder mixture may certainly display a relatively strong structure, but it is difficult to manufacture coarse-grained structures of good surface quality even if the structure is well filled with infiltration material. Moreover, a sintered compact displaying a structure of irregular distribution of fine and coarse grains, is of poorer quality than a sintered compact of uniform structure. This applies especially to sintered compacts which shall be subjected to machining or which shall for example have a uniform surface hardness, and a tendency towards separation into fine grains and coarse grains can therefore be devastating to product quality.
The invention has for its object to provide a process which permits manufacturing sintered compacts of very dense structure in the surfaces and in a layer, of desired thickness closest to the surfaces with the use of a sinterable powder which totally, i.e. for the entire sintered product, is relatively cheap and in many cases cheaper than a corresponding total amount of powder used for conventional manufacture of sintered compacts under equivalent requirements for quality. For the attainment of the last-mentioned object the present invention is based on the following theory.
If the structure of a sintered compact displays large or small
REFERENCES:
patent: 2695231 (1954-11-01), Causley
patent: 2843501 (1958-07-01), Ellis et al.
patent: 2979401 (1961-04-01), Szymaszek
patent: 3697261 (1972-10-01), Jump et al.
patent: 3804575 (1974-04-01), Chaudhok
patent: 4314399 (1982-02-01), Severinsson
Brookes Anne
Padgett Benjamin R.
Uddeholms Aktiebolag
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