Specialized metallurgical processes – compositions for use therei – Processes – Producing solid particulate free metal directly from liquid...
Reexamination Certificate
1998-04-13
2001-07-03
Wyszomierski, George (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Processes
Producing solid particulate free metal directly from liquid...
C075S338000, C075S339000
Reexamination Certificate
active
06254661
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method and apparatus for production of metal powder by spraying. In particular the invention is intended to provide fine powder of spherical or granular shape suitable for metal injection shaping of sintered products.
2. Description of Related Art
Metal powder is ordinarily produced by mechanical grinding, electrolysis, chemical reduction or spraying. Among these processes, spraying is widely adopted because of capability of mass production and applicability to a variety of metals. Spraying, also called atomizing, is a method to pulverize molten metal by spraying with injection of gas or liquid into a down flow of molten metal flowing from a small hole in the bottom of a vessel like a tundish or a crucible. In this process inert gas is usually used as gas and water is usually used as liquid; the former process is called gas atomizing method and the latter process is called water atomizing method.
The gas atomizing method usually provides metal powder of spherical shape with high tapping density and low oxygen content. Therefore, this method has advantage of effectively pulverizing metals of high affinity to oxygen such as Ti and Al, or alloys containing these metals. However, this method has the disadvantage of difficulty in obtaining finer particles than the water atomizing method, especially ultra-fine particles below 10 &mgr;m, because of smaller energy of the inert gas as atomizing medium. Also, the high price of the inert gas tends to result in high costs of the powder.
On the other hand, water atomizing usually produces powder of irregular shape and low tapping density. Further, reaction between the metal and water vapor generated from the water jet leads to oxidation of the metal and increase of oxygen content in the powder. However as mentioned above, the water atomizing method enables easy production of finer powder because of its high energy of water relative to gas as atomizing medium, and has the advantage of low price of the produced powder due to use of water.
Metal powder is used for a variety of applications such as metal injection molding process (MIM), composite materials, catalysts, paints and others. The market for these applications has a strong demand for production of fine metal powder with low cost in large quantities. In particular, the market for the MIM process has a increasing demand for a low-cost supply of fine powder of spherical or granular shapes with low oxygen content, whereas the MIM process is recently drawing attention for production of metal parts of three-dimensional complex shapes. This demand includes application of water atomizing for low-cost production having a powder of metals of strong affinity to oxygen such as aforementioned Ti or Al, and also alloys of these metals.
The MIM process produces metal products through injection molding of raw material (pellets) provided with enough fluidity by mixing of binders such as wax or thermoplastic resin, followed by removal of the binders and sintering. The reason why powder of spherical or granular shape is necessary for MIM process is to give sufficient fluidity to pellets. The fluidity of pellets is considered to become higher with an increase in tapping density of metal powder, and the powder shape of high sphericity is effective to increase the tapping density (tapping density is defined in the JIS Z 2500 as “mass of powder per a unit volume in a vessel after vibration”).
Moreover in the MIM process the binders should be removed easily. For good fluidity and stable shape the binders usually contain as much as 50 to 35% in volume in accordance with the amount of 50 to 65% of metal powder. As they must be removed completely in the removal process, the quantity of the binders is required to be as small as possible. Also in this instance powder of spherical or granular shape, namely high tapping density is advantageous, since the necessary amount of binders is effectively reduced and the time for binder removal is saved.
Further, fine powder is necessary for the MIM process. Generally speaking, fine powder increases the points of contact among particles and can be sintered with a higher density at a lower sintering temperature. The density of metal parts produced by MIM process is evaluated in terms of relative density. The relative density after sintering becomes higher with a decrease in the size of particles, so in general for MIM applications it is said that the average size of powder should be about 10 &mgr;m (relative density is defined in JIS Z 2500 as “ratio of density of a porous article in reference to density of an article of the same constituents free of pores”).
Moreover, for the MIM process the oxygen content in metal powder is required to be low. High oxygen content leads to retention of oxygen as nonmetallic inclusion in the MIM processed metal parts and to their poor mechanical properties.
In summary, powder for the MIM process is necessary to be small in size, spherical or granular in shape, high in tapping density, and low in oxygen content. For powder of irregular shape sufficient fluidity for injection molding can be obtained by increasing the quantity of binders, however, the cost for removal of binders becomes higher and the products do not have sufficient uniformity of metallic powder leading to poor performance. In the early stage of development of MIM process, powder manufactured by carbonyl method was mainly used because of their stable supply, however, powder of carbonyl method was limited to pure metals such as iron and nickel. Recently, as MIM products are attempted to be extended to wider applications with development of the MIM technique, powder of a variety of alloys prepared by atomizing has gotten attention as the material for the MIM process. However as stated above, although the gas atomized powder products are suitable for the MIM process because of their spherical shape, high tapping density and low oxygen content, there are the drawbacks of high production cost and difficulty in obtaining fine particles.
On the other hand, although the water atomizing has the advantage of easiness in obtaining fine particles and low production cost, it has a problem in application to the MIM process due to irregular shape of the particles and low tapping density of the powder products. Use of such water atomized powder of irregular particle shape in the MIM process has the problem that injection into intricate portion is difficult. Therefore the use is limited by applicable size of metallic articles and inferiority of dimensional accuracy in the products because of the non-uniformity at the injection.
From the above-mentioned reason, a technique for low-cost mass production of metal powder for the MIM process by water atomizing is necessary, however, no satisfactory procedure is currently available. As an example of a prior art process for production of metal powder by spraying method, there is Japanese published patent No. Sho.52-19540 “Spraying and pulverizing apparatus for molten metal”. It is described in the patent publication that “the present invention secures production of metal powder with suitable properties for powder metallurgy by controlling the spray form through selection of appropriate number of spray nozzles, aperture diameter of the nozzles, and surface characteristics of front edge of guide for liquid flow facing to the nozzle apertures”. Therefore, the prior invention covers the same category of technique as this invention, however, it is intended for a “pulverizing apparatus to be used in mass production of powder of irregular shape suitable to powder metallurgy”, as described. In this prior invention no disclosure is made of the technical aspect concerned with production of metal powder of spherical or granular shape which is the aim of this invention.
SUMMARY OF THE INVENTION
In consideration of the above mentioned current status of the art, the present invention is intended to produce fine particles by spraying at a low costs. In particular, the intention
Abo Hideo
Azuma Hiroyuki
Kato Yoshiyuki
Nakabayashi Koei
Sasaki Masami
Pacific Metals & Co. Ltd.
Wenderoth , Lind & Ponack, L.L.P.
Wyszomierski George
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