IL23213A - Roll compacting of metal powders - Google Patents

Description

« ions mpas « ROLL COHPACTIHG OP METAL POWDERS n This invention relates to methods by which metal and metal coated particles are roll compacted into metallic strips and sheets and, more particularly, to improvements in such methods which permit the roll compaction of fine metallic pow-ders of normally poor flow characteristics .

Methods are known in the powder metallurgy art by means of which particles of metal and metal coated metallic and non-metallic compounds can be compacted to form sheets and strips Generally, the powder is first compacted into a "green" strip or sheet by feeding it, usually at a predetermined, uniform rate, vertically into the roll gap of a roll compacting unit comprised of a pair of oppositely positioned, horizontally disposed pressure rolls . The roll diameter and roll gap are selected to provide a self-supporting, partially densified "green" strip of the desired thickness. The "green" strip is generally sintered and hot rolled and otherwise hot and/or cold worked to produce a finished, or semi-finished metallic product of substantially 100 density.

Problems are encountered when these methods are ap-plied to metal powders having poor flow characteristics as such powders either will not flow into the roll gap at all or they do not flow uniformly and "green" strip of very poor quality is produced.

The flow characteristics of metal powders are gene-rally measured by reference to the standard flow test for metal powders which is set out in ASTM Standards 196l, part 3, page 1047 (ASTM designation B213-48) . Some metal powders do not flow at all under the conditions of this test; such powders are said to have a flow rate of infinity. Other powders may have measur-able flow rates according to the standard test but nonetheless do not have flow characteristics which enable them to be uniformly fed by conventional means to the roll gap.

Although the particle size is not the only characteristic which affects the flow characteristics of metal powders, it is generally found that metal and metal coated powders having a particle size up to 10 microns, Fisher sub-sieve size, have very high or infinite flow rates according to the standard test and do not flow readily into the roll gap of conventional powder rolling units which rely on a gravity feed from a vertically arranged hopper. For the purpose of the present description such powders will be referred to as "fine" powders and all particle sizes given in microns will be Fisher sub-sieve sizes .

Metal powders for roll compacting into strips are not always available in the particle size range, size distribution and shape generally found to result in flow characteristics most suitable for roll compacting. In some cases, the process by which the powder is obtained may inherently produce relatively fine metal particles having poor flow characteristics. Also, it is sometimes desirable to specifically employ very fine particles for roll compacting in order to produce wrought strip possessing certain desirable qualities. For example, in the production of dispersion strengthened metal strip from composite metal-metal oxide powders such as nickel coated thoria, it is generally desirable to utilize fine powder in order to obtain uniform, close spaced distribution of the dispersed phase and, also, to ensure a fine, preferably less than 10 microns, grain size in the matrix metal .

Previous efforts to overcome roll compacting mill feeding problems resulting from poor flow characteristics in fine metal powders have included expedients such as slowing down the roll speed; roughening the surfaces of the rolls; force feeding powder to the roll gap; surrounding the vicinity of the roll gap with an atmosphere of a gas of a lower viscosity than air, such as hydrogen; maintaining a partial vacuum in the vicinity of the roll gap; agglomerating the powder such as by sintering or other means; vibrating means on the hopper or like powder feed mechanisms; and various combinations of the foregoing.

None of these methods has been found to provide an entirely adequate solution to the problem of compacting fine metal τ lie powders having poor flow characteristics. We have found that generally they are ineffective, unduly expensive, or impractical for commercial utilization.

We have found that the problems encountered in forming powders with poor flow characteristics formed of finely divided metal and metal coated particles of a size below 10 microns into self-sustaining green strip by gravity feeding such particles into the roll gap between a pair of oppositely positioned rolls of a roll compacting unit can be overcome by mixing said particles, prior to compaction, with a volatilizable, non-residue forming liquid in amount within the range of 1-6$ by weight, said amount being sufficient to change the flow characteristics of the powder from those dependent on the free flow of the powder particles to those dependent on the mutual adhesion of the particles, and roll compacting the liquid containing particles .

The invention will be described in detail hereinafter with reference mainly to water as the liquid additive; however, it is to be understood that other liquids, such as naphtha, benzene, kerosene, glycol, and the like volatilizable liquids may be equally suitable under proper circumstances, such as, for example, where oxidation of the powder by water is a problem.

The precise amount of liquid to be added for any given powder depends primarily on the particle size of the powder, although particle shape and apparent density of the powder also have some affect. In general, the amount of liquid addition pro vided in accordance with this invention is in the range of from 1 to 6 percent by weight. For most fine powders, an amount with in this range will cause mutual adhesive forces to be set up between the particles comprising the powder. The amount of liquid addition required for optimum results increases with de-creasing particle size. We have found, for example, that fine powders having a particle size in the range of 1 micron to 5 microns generally require a water addition of 2.5 to 4.0 percent by weight, and powders in the to 10 micron size range will require 1.0 to 2.5 weight percent water addition.

The liquid additions provided in accordance with this invention do not function as a lubricant to increase the free flowing characteristics of the powder or to provide lubrication at the compression zone in the roll gap; rather, the effect is to change the flowability of the powder from dependenc on the free flowing characteristics of the powder particles under gravi ty to dependency on mutual adhesion of the particles. According to the present invention, the criterion for satisfactory roll compaction is thus no longer the ability of the powder to flow evenly and uniformly into the roll gap under the influence of gravity but is the ability of the powder particles in the roll gap to pull other particles down into the roll gap through adhesive forces set up between the particles. This phenomenon is marked by very definite limits for any given powder. An excess 'liquid addition weakens the adhesive forces and too little will not set up the necessary adhesive forces, and in both cases the result Is that particles are not drawn into the roll gap at all or, at best, insufficient amounts enter the roll gap to produce strong, dense green strip. Thus, for satisfactory roll compaction, it is essential that the liquid addition be carefully con-trolled within the upper and lower limits applicable for any given powder. It is simple for one ordinarily skilled in the art to determine, by a few trial runs conducted in accordance with the teachings set out herein, the precise optimum liquid require* ments for a given powder.

The liquid is mixed with the powder by any conventional mixing procedure, such as in double cone blenders, which ensures uniform distribution of the liquid through the powder. The moistened powder is then fed into the gravity feed hopper of the compacting mill and roll compacted in the conventional manner.

The chemical properties and the precise physical properties of the liquid additive are not critical factors. In general all that is essential is that the liquid flows readily at the normal mixing and feeding temperatures and that it be volatilizable and non-residue forming at the temperatures at which the green strip is sintered. For economic reasons water is the preferred liquid additive, but where oxidation of the metal powder is a problem an equivalent amount of another liquid such as naphtha will be equally effective.

A factor which must be taken into account in carrying out the method of this invention is that during compacting, some liquid is squeezed out of the moistened powder and tends to build up in the roll gap and on the surfaces of the rolls . If this build-up becomes excessive, the adhesive forces between particles becomes too weak, and the powder ceases to be drawn into the roll gap and the quality of the "green" strip rapidly deteriorates.

This problem is usually avoided by providing means, such as an absorbent wiper pad or rubber or plastic scrapers, to continuously remove excess moisture from the rolls. In addition, where tho rolling operation is continuous, the amount of liquid initially added may be decreased somewhat from that required to roll compact short sections of the same powder to ensure that when equilibrium is reached, the liquid content at the roll gap is at the optimum level for uniform feeding.

The invention is further illustrated by the following Examples: Example 1 In this Example, a nickel-thoria composite powder, prepared for producing dispersion strengthened wrought nickel, was utilized as the starting material. The powder contained 98 3$. nickel and 1.7 weight percent thoria; the Fisher sub-sieve size was 3.7 micrOns and the apparent density was 1.66 grams per cubic centimetre. The powder initially had a flow rate of infinity under the conditions of the standard ASTM flow test, i.e., it did not flow at all. Samples of this powder were mixed with various amounts of water and compacted in a laboratory compacting mill.

The mill was gravity fed from a feed hopper positioned above the rolls and having a feed throat extending vertically into the roll gap. The diameter of the rolls was 6 inches and the roll gap in every case was set at 0.005 inch. The results of these tests are shown in Table 1.

TABLE 1 Green Strip Properties Weight/ Water Thickarea Content ness Density gm/in Wt. % in. % (centre) Strip quality 0 .014 only edges compacted 0 .5 0.19 — tail and edges compacted 1 .018 - same as 0.5 but improved 1 .5 .022 73 1.94 complete strip but very brittle 2 .022 74. 5 2.23 good continuous flexible strip _ ll _ 2 . 5 - 77 2 .79 n _ .025 78 . 5 2 .73 II 3 - 5 - 77 - 5 2. 1 4 .026 76.5 2 .77 small holes in strip near the tail end .026 75 2.72 longer and more numerous holes in strip near tail end of sample 6 .024 75 2.52 low density area and holes near edge of strip 7 broken pieces of strip These results illustrate the effect of water additions on the compacting behaviour of the nickel-thoria composite pow-der under gravity feed conditions . There are definite limits (2.0 to 3.5 weight percent in this specific example) which define the optimum water content for production of sound green strip. Above and below this region, poor quality strip is produced. The results also indicate the effect of water addition on strip thick-ness, density and weight per unit area, all of which increase with increasing water content to maximum values then decrease with further increases in water content.

It can be noted further that with this particular powder with water additions of $ and higher the defects in the strip tend to occur near the tail end which indicates that there is a build-up of excess moisture in the roll gap after a period of time, thus .causing the quality of the strip to deteriorate.

Example 2 In this Example, the starting material consisted of samples of pure nickel powder of various sizes . The compacting and mixing procedures were the same as those of Example 1. The results are set out in Table 2.

TABLE 2 Green Strip Properties Powder Water Thick- Size Content ness Density Wt ./area Strip quality 0 powder would not flow and did not compact 2 -7 1 .015 51 - 5 1 - 17 continuous strip, very weak 2 .015 58.0 1 .27 good strip 0 .011 - continuous strip. very weak 4.0 1 .015 64. 5 1 .42 weak strip 3 .016 68. 5 1 .60 good .015 64. 0 1 .40 holes in tail 0 .011 59 - 5 0.96 weak but continuous strip 2 .015 71 .8 I.57 good - 2 4 .016 70.6 1.64 good 6 .016 70.2 1.64 good (hole in tail) 8 poor, pieces of strip only 0 .013 64.3 1 .22 continuous strip, weak 2 .016 74.0 I.73 good 7 - 3 4 .018 74.0 I. 4 good, weak towards tail 6 .016 70.7 I.65 good, holes in extreme tail 8 - - - holes across strip.

Example 3 Samples of fine iron powder having an apparent density of I.71 gms/cc, Fisher sub-sieve size of 5 · 7 microns and a flow rate of infinity according to the ASTM Standard test were mixed with various amounts of water and compacted in the roll compact- ing unit employed in Examples 1 and 2. The roll gap was set 0.025 inch.

The results are shown in Table 3 - TABLE 3 Water Addition Strip Thickness Strip Quality 1 .0 Ο.32 edges compacted only 2.0 Ο.32 very weak centre 3.0 Ο.38 good green strip 4.0 Ο.43 .0 Ο.43 All of the above green strips were badly oxidized. 5-0 weight percent naphtha was substituted for water in another scrapie Good quality green strip 0.40 inch thick and free from any oxidation was produced.

Although the present invention has been described in conjunction with the preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and appended claims. l VING NOW particularly described and ascertained the nat ure of my/our said invention and in what manner th§ iftmt is to be performed, I/We declare that what l/We elalm i§ i=

Claims (4)

1. A method for forming finely divided metal and metal coated particles of a size below 10 microns into self-sustaining green strip by gravity feeding said particles into the roll gap defined between a pair of oppositely positioned rolls of a roll compacting unit which comprises mixing1 said particles prior to compaction with a volatilizable, non-residue forming liquid in amount within the range of 1 to 6 weight percent, said amount being sufficient to cause said particles to adhere together such that their flow characteristics become dependent on mutual adhesion and they are drawn into the roll gap of said roll compacting unitj and roll compacting the liquid containing particles to produce green strip.

2. The method according to claim 1 in which the liquid is one of the group consisting of water, naphtha, benzene, kerosene and glycol.

3. The method according to claim 1 in which the particles are within the size range of 1 micron to 5 microns and the liquid is water added in amount within the range of 2.5 to 4.0 percent by weight .

4. The method according to claim 1 in which the particles are in the size range of 5 to 10 microns and the liquid is water added in an amount within the range of 1.0 to 2.5 percent by weight .