CA1151384A

CA1151384A - Liquid phase compacting

Info

Publication number: CA1151384A
Application number: CA000336663A
Authority: CA
Inventors: Sang-Kee Suh; Sydney M. Kaufman
Original assignee: Ford Motor Company of Canada Ltd
Current assignee: Ford Motor Company of Canada Ltd
Priority date: 1978-11-24
Filing date: 1979-09-28
Publication date: 1983-08-09
Also published as: JPS5573801A; EP0011981B1; DE2966491D1; JPS5653601B2; EP0011981A1

Abstract

ABSTRACT OF THE DISCLOSURE
A method of making high density powdered metal compacts and also a method for making high density powder metal sintered products is disclosed. With respect to the compacts, iron powder of a coarse or fine configuration is mixed or coated with a low melting metal additive selected from the group consisting of tin, copper-tin, copper-lead, or lead in a percentage by weight of 2 5 of the admixture. The admixture is subjected simultaneously to both heat (at a temperature level such that the low-melting alloy is in a liquid phase) and pressure which is in the range of 10-30 tsi. Upon relief of heat and pressure, the agglomerated or compacted product will possess an increased density in excess of 80% and an improved hardness and strength level.
With respect to a sintered product, the powder metal is selected to have a line particle size (average range of about 4-5 microns) and the powder is dry impact coated by ball milling with a low-melting additive such as tin, copper-tin, copper-lead or lead; the coated particles are warm briquetted and subjected to a sintering operation whereby the resulting product has a minimum density of 97% or more.

Description

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LIQUID PHASE C,OMPACTING
The present invention relates to liquid phase compacting.
Powdered iron parts have been used in some industrial 5 application, but have not generally found full acceptance because the extra method steps and the added cost required to obtain a reasonable strength level and density in the powdered part have been excessive, particularly when compared to similar parts obtained by melt formation.
The conventional commercial mode of processing powdered metal parts typically comprises (a) blending and milling together selected powder elements in the presence of a lubricant, (b) the mechanically blended charge is then compacted, (c) the compact is heated under 15 a reducing atmosphere for a period of 30 min. at 810C
to volatilize the lubricant, and (d) the compact is sintered at an appropriate temperature. This has resulted at best in a green density prior to sintering of about 79-86% and a sintered density of 80-89% of theoretical.
20 A higher density level would require additional hot forging to increase the strength level and reduce porosity.
What is needed is a method by which high strength, high density parts can be obtained without secondary consolidation (hot forging) and in many cases without 25 sintering. Development to this end by the prior art has included cold briquetting or cold compacting with a low melting matrix element effective to close pores;
this processing is then followed by conventional sintering.
U.S. Patents representing this state of the art include 30 1,922,548 and 1,793,757. Another method is to raise the temperature of the blended powders to a liquid-solid condition and then sinter such powders in such multiphase condition while under pressure. This is exemplified in U.S. Patent 3,393,630. The principal difficulty that 35 is experienced with cold briquetting is that it promotes a negative effect on density even when low melting filler agents are employed. The principal difficulty of sintering under pressure while in the liquid-solid condition without 38~

any pre-compaction is that the equipment is severely stressed and subjected to considerable wear resulting in increased cost of processing.
In accordance with the present invention, there is provided a method of making high density powdered metal compacts, which comprises~ uniformly adding 1 to 5% by weight of a low melting metal to an iron-based powder supply having a uniform particle size to form a mixture; and warm briquetting the mixture at a temperature effective to melt the low melting metal without melting the iron-based powder while employing a compacting pressure no greater than about 30 Tsi, whereby the low melting metal acts as a lubricant during the warm briquetting and facilitates compressibility.
The invention produces powdered metal parts having higher strength and higher densities without the necessity for secondary consolidation, such as sintering and/or hot forging.
The unprecedented strength and density level of the warm briquetted compact makes it possible to produce certain powder metal parts without sintering, resulting in significant energy savings. Candidate parts would be heat sinks (such as diodes) and steering column collars.
A preferred method for carrying out the method Of this invention is as follows:
(1) An iron powder is prepared preferably by water atomization which may have either a coarse or a fine particle configuration. If a coarse configuration is employed, the average particle size should be in the range of 80 to 100 microns and if a fine particle size configuration is used, the average particle size should be about 4 to 5 microns. When the powder is produced by water atomization, the chemical content of the powder will typically consist of 99.8% Fe, 0.05%C, 0.04% Mn, and .03% Si. Since the water atomization technique will require subsequent grinding and screening, the ultimate product from this production cycle will result in a screen analysis such as 20% coarse, 60~ fine, and 20% ultrafine.
The particle size of the powder to be used herein should ~.

~1384 preferably be uniform, but may be a blend of coarse and fine particle configurations provided the ratio is within the range of 1/3 or less. If other powder making techniques are employed, such as the carbonyl process, the particle 5 size range and chemistry will be affected.

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(2~ The selected a~d sized iron powder is dry impact coated with a low-melting metal selected from the group consisting of tin~ copper~tin, lead, and copper-lead.
One preferred method for carrying out this dry impact coating is by use of a ball milling apparatus employing large impact or milling balls consisting solely of the low-melting metal~ such as tin~ The powder is placed preferably in a ball milling machine ~for purposes of-the trials herein, an interior~chamber of 8" high x 8" in diamater 10 was used). Tin laden milling elements~ preferably in the form of solid tin balls of about ~5" diameter, may be used~ The powder charge was a~out 10 cubic inches and the milling time about 48 hours. Milling time and the milling rate depend upsn mill volume, mill diameter, size 15 of the tin balls, and tAe speed of rotation. The ball milling elements should have a diameter at least 50 times the largest dimension of any of the particle shapes of the powder~
The function of this step is to transfer, by impact, 20 a portion of the tin ingredient, carried by the ball milling elements, to ~orm a tin shell about su~stantially each particle of the powder. The coating is generated by -abrasion or scratching of the powder particle against the surface of the ball milling elements. This obviously is 25 accomplished by rotating the housing of the ball mill machine to impart a predetermined abrading force from the balls~ The ball milling operation will cold work or generate defect sites in substantially aLl of the powder particles above.l24 microns; since the majority of the 30 particles selected for the process will be below said size range r they will generally be free of cold wor~ or defect sites~ The ball milling operation should be carried sufficiently long so that su~stantially each particle will be fully coated~ this requires statistically 35 a minimum period of time so that tin coating will preferabl~ be continuous. ~hen this step is completed, the particles will be in a condition where they will all 3~34 substantially have a continuous tin envelope (coating or shell). Although the shell should preferabiy be an imprevious continuous envelope about each particle, it is not critical that it be absolutely impervious.
(3) The dry impact coated iron particles are then subjected to a heating treatment while an agglomerating pressure is applied. Heat is applied to raise the tempera-ture of the mass of particles to a level slightly above the melting temperature of the metal coating, which should be in the range of 450-650F, 419F being necessary to melt pure tin, 446F being necessary to melt an alloy of 99.6% tin and .4% copper, a melting temperature of 594F is necessary to melt an alloy of 95~ lead and 5% tin, and a melting tem-perature of 561F is necessary to melt an alloy of 63% tin and 37% lead and a melting temperature of 618F is necessary to melt pure lead. T~e warm briquetting temperature could be raised to as much as 1350F (728C~ if necessitated by the requirement to melt the metal coating r or to improve densifi-cation by plastic deformation 450-1358F encompass warm briquetting herein. The pressure applied should be no then 30 tsi (and preferably in the range of lO-30 tsi) so that wear on the tooling used for agglomeration is reduced to a minimum. Agglomeration is reduced to a minimum.
Agglomeration or compaction may be carried out by a conventional press to obtain the maximum desired densities herein. Compact densities herein are considerably improved to 80~ or more of theoretical density. The presence of the solid tin or low melting envelope about the particles improves compressibility acting as a lubricant, and the liquified metal phase acts as a pore filler during the compaction operation. With prior uncoated powders~ a density of about 82~ of theoretical or 6.4 gm~ per cubic centimeter is typically obtained using a compressive force of about 30 tsi; with a dry impact coated powder herein, densities of about 7 0 gm~ per cubic centimeter can be obtained at the same force level.

~ 1 5~384 (4) The application of heat and pressure is removed allowing the low-melting metal to solidify and form an auxiliary bond between the particles in addition to the normal compressive and mechanical interlocking 5 bond therebetween. Test results of this kind of a warm briquetted product shows that it can be subjected to a hardness test evidencing a Rockwell value of about RB 45. Moreover, a special bend test for such a product will show that it has a strength level of at least 2000 10 psi, which is 400% greater than that of a product produced without the use of the low-melting metal lubricant.
This method may be varied in either or both of two respects. First, the iron powder selected is limited to a fine particle size, averaging 4 to 5 microns.
15 This can preferably be obtained by extracting the iron powder as a by-product of processing of scrap or machining chips from industrial metal work. To this end, scrap metal in the form of machine turnings are segregated or selected. Machine turnings are segments of ribbons 20 of low carbon or alloy steel; the turnings should be selected to have a surface to volume ratio of at least 60:1. The machine turnings may be shavings cut from alloy bar, and the bar may have a chemistry which includes alloying ingredients such as manganese, silicon, chromium, 25 nickel and molybdenum. The turnings will have a size characterized by a width of 0.1 to 1.0", thickness of 0.005 to 0.03", and a length of 1 to 100". Machine turnings are usually not suitable for melting in an electric furnace because they prevent efficient melt down due to such 30 surface to volume ratio. The turnings should be selected to be generally compatible in chemistry when in the final product; this is achieved optimally when the turnings are selected from a common machining operation where the same metal stock was utilized in forming all the 35 turnings.
The selected scrap pieces are put into a suitable charging passage leading to a ball milling machine.

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(or equi~alent impacting device~ Within the passage means, an ingredient for freezing the metal pieces is introduced, such as l~quid nitrogen, it is sprayed directly onto the metal pieces~ Mere contact of the liquid nitrogen with the scrap pieces will freeze them instantly~ The liquid nitrogen should be applied uniformly throughout its path to the point of impaction~ The ball milling elements are motivated preferably by rotation of the housing to contact and impact the frozen pieces of scrap metal causing them to fracture and be comminuted~ Such impaction is carried out to apply a sufficient fracturing force for a sufficient period of time and rate to reduce said scrap pieces to a powder form. The resulting powder will be layered or flake in configuration and typically have both coarse and fine powder proportions~ A typical screen analysis for a cryogenic powder would be as follows (for a 100 gm. sample):
Mesh Wt. in Gm. After 72 hrs. Wt. in Gm.
. . . _ . . _ 60.0 31.5 100 19.5 11.0 14~ 5.5 7~5 200 6.5 18.0 325 4.5 22.5 ~325 4.0 9.5 Secondly, the method is varied in another important aspect: the compact or briquetted product is subjected to sintering. This treatment can be carried out in a conventional sintering furnace with heating to a temperature preferably about 2000F~ The temperature to which the briquetted or compact is heated should be at least to the plastic region for the metal constituting the powder. A controlled or protective atmosphere may be maintained in the furnace~ preferably consisting of inert or reducing gases~
~ith sintering~ a final density of about 97% has been achieved without the necessity for secondary consolidation such as forging~ This is an extremely high ~513~34 ~ 8~
density for a process which is essentially two step and devoid of secondary consolidation~ Since the compact does not contain a volatile lubricating agent, such as Acrowax, the delubrication step is eliminated from the process or from the zone in a sintering furnace. The resulting mechanical properties for such a sintered product would be as follows: tensile strength 115,000 psi, ~ elongation 10%, and hardness RB 78.
Test results to support the above methods are depicted in Ta~le I. In one test example, an iron powder specimen identified as Atomet 28 was employed which has a chemistry of 99.8% Fe 0.05%C and an average particle size of 70-80 microns. The second powder specimen consisted of carbonyl powder, having a chemistry of 99.9% Fe 0.1%C
and a particle size range of 4~5 microns. Each of the powders were subjected to dry impact coating according to the step (2~ described in the preferred method. Tin was employed in amount of 2.5% weight percentage of the powder mass. This required ball milling to be carried out for a period of 48 hours to achieve a coating thickness of about 0.1 micron. Each powder specimen was heated to a temperature level of 473F, Density measurements were oktained and compared with the same powder specimen but uncoated and prepared according to a conventional technique using a 1% Zinc Stearate admixed lubricant.

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f~5~384 Although percentages of tin above 5% can be employed and ~ill operably work within the system as described, it is suggested that more than 5~ tin is not desirable because of economic reasons, tin ~eing considerably more expensive than iron. Also certain ph~sical characteristics are affected by the presence of large amounts of tin in the resulting product. It is important to point out that the use of tin by itself without the warm temperature to effect the liquid phase will create a negative effect on density.
The variation of density as a function of the percentage of tin employed is shown. It increases for those compacts which are produced with warm compaction at a temperature of about 473F (245C~, the tin being in a liquid form during the compaction. Use of 2.5 or 5%
tin in the solid state reduces the density that can be obtained from that achieved when tin is not present. It is therefore important to emphasize that the use of a low-melting temperature metal additive is only effective when compaction takes place with heat such that the additive is in liquid phase~

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method of making high density powdered metal compacts, which comprises:
uniformly adding 1 to 5% by weight of a low melting metal to an iron-based powder supply having a uniform particle size to form a mixture; and warm briquetting said mixture at a temperature effective to melt said low melting metal without melting said iron-based powder while employing a compacting pressure no greater than about 30 Tsi, whereby said low melting metal acts as a lubricant during said warm briquetting and facilitates compressibility.

2. The method of claim 1 wherein said low melting metal is selected from the group consisting of tin, copper/tin alloy, copper/lead alloy and lead.

3. A method of making high density powdered metal compacts, which comprises:
uniformly adding 1 to 5% by weight of tin to an iron-based powder supply having a uniform particle size to form a mixture, and warm briquetting said mixture at a temperature of about 450°F effective to melt said tin without melting said iron-based powder while employing a compacting pressure of no greater than about 30 Tsi.

4. The method of claim 3 wherein the tin is added to the powder by ball milling in which tin balls transfer tin to the iron-based powder by constant and continuous abrasion when carried out for a period of time sufficient to add the 1 to 5% by weight of tin to the iron-based powder and substantially each particle of iron-based powder is coated with tin.