EP0011981B1

EP0011981B1 - Method of manufacturing powder compacts

Info

Publication number: EP0011981B1
Application number: EP79302649A
Authority: EP
Inventors: Sydney Morton Kaufman; Sang-Kee Suh
Original assignee: Ford Werke GmbH; Ford France SA; Ford Motor Co Ltd
Current assignee: Ford Werke GmbH; Ford France SA; Ford Motor Co Ltd
Priority date: 1978-11-24
Filing date: 1979-11-21
Publication date: 1983-12-14
Also published as: CA1151384A; JPS5653601B2; JPS5573801A; EP0011981A1; DE2966491D1

Description

This invention relates to a method of making powder compacts.
Powdered metal compacts have been used in some industrial applications, 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 metal compact have been excessive, particularly when compared to similar parts obtained by melt formation.
The conventional commercial mode of processing powdered metal to form compacts typically comprises (a) blending and milling together selected powder elements in the presence of a lubricant, (b) compacting the .nechanically blended charge (c) heating the compact under a reducing atmosphere for a period of 30 min. at 810°C to volatilize the lubricant, and (d) sintering the compact 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. A higher density level would require additional hot forging to increase the strength level and reduce porosity.
In order to increase the strength and density of the parts without secondary consolidation (hot forging), it has been proposed to effect briquetting or compacting with a low melting matrix element effective to close pores; this processing is then followed by conventional sintering (U.S. Patent No. 1,793,757 discloses such a proposal). Another method is to raise the temperature of the blended powders to a liquid-solid condition and then sinter such powders in such multi-phase condition while under pressure. The principal difficulty that 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 any pre- compaction is that the equipment is severely stressed and subjected to considerable wear resulting in increased cost of processing.
It is also known from U.S. Patent Specification No. 4,050,933 to form a metallic object by compressing particles which are previously coated with a metal having a melting point lower than that of the metal particules themselves. The coating metal fills the gaps between the particles so that the object is impervious to fluids.
The coating in the latter Patent Specification amounts to between 12% and 30% by weight of the metallic object. The latter proposal also suggests a pressure of between 540 and 772 MPa (35 and 50 tsi). In U.S. Patent Specification No. 1,986,197, there is described a process in which the metal powder compressed is electro-plated with a low melting point metal. The latter Patent also provides for high pressures.
The present invention seeks to provide a method of producing a metal powder compact which is less expensive to implement and causes less wear on the tools involved in the manufacture.
According to the present invention, there is provided a method of producing a metal powder compact which comprises compacting iron powder in the presence of a low melting point metal at a temperature sufficient to melt the low melting point metal, characterised in that the iron powder is compacted in the presence of from 1% to 5% by weight of the low melting point metal, that the iron powder is compacted at a pressure no greater than 463 MPa (30 tsi) and that the iron powder particles are coated with the low melting point metal by dry impact coating.
Preferably, the dry impact coating is effected by ball milling and milling elements composed of or carrying the low melting point metal.
In the preferred embodiment of the invention 2-4% of a low melting metal ingredient (preferably selected from tin, a copper-tin alloy, a copper-lead alloy and lead) is added to a coarse or fine iron powder and the mixture is subsequently warm-briquetted (preferably at a temperature of 230 to 345°C (450-650°F)) to form a liquid phase of said addition agent while consolidating the powder. The particle size of the base iron powder is preferably relatively fine (4-5 microns) and about 2.5% of a liquid phase promoting ingredient, particularly tin, is preferably added thereto, the blended mixture being warm briquetted at a temperature of 230 to 345°C (450 to 650°F). The compact may then be sintered. Desirably a cryogenically- produced fine powder is used as the source of the powdered metal.
The present invention includes two preferred embodiments:- (1) the addition of 1-5% of a low-melting metal, such as tin, to an iron powder which results, after warm briquetting, in a compact having 25% less porosity and increased strength compared with a conventional cold briquetted compact; and (II) the addition of 1-5% of a low melting metal, such as tin or lead, to a fine particle size iron powder, which results, after warm briquetting and sintering, in a product having only 3% porosity compared with 20% porosity when tin or lead is not used. 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. With respect to the 97% dense sintered powder product, it can be produced with existing equipment (no forging) which represents a major potential for a new class of powder metal business.
A preferred method for carrying out the first aspect 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 60 to 120 ,am and preferably 80-100 µm and if a fine particle size configuration is used, the average particle size should be about 4-5 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 preferably be uniform but may be a blend of coarse and fine particle configurations provided the ratio is within the range of t or less. If other powder making techniques are employed, such as the carbonyl process, the particle size range and chemistry will be affected.
(2) The selected and 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 in which the coating is of tin, involves the use of a ball milling apparatus employing large impact or milling balls carrying the tin. The powder is placed preferably in a ball milling machine (for purposes of the trials herein, an interior chamber of 20 cm high by 20 cm in diameter (8" high x 8" in diameter) was used). Tin laden milling elements, preferably in the form of solid tin balls of about 1.25 cm (.5") diameter, may be used. The powder charge was about 164 cm³ (10 cubic inches) and the milling time about 48 hours. Milling time and the milling rate depend upon mill volume, mill diameter, size of the tin balls, and the 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, a portion of the tin ingredient, carried by the ball milling elements, to form a tin shell about substantially 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 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 124 microns; since the majority of the particles selected for the process will be below said size range, they will generally be free of cold work or defect sites. The ball milling operation should be carried sufficiently long so that substantially each particle will be fully coated; this requires statistically a minimum period of time so that tin coating will preferably be continuous. When this step is completed, the particles will be in a condition where they will all substantially have a continuous tin envelope (coating or shell). Although the shell should preferably be an impervious 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 temperature of the mass of particles to a level slightly above the melting temperature of the metal coating, which should be in the range of 230-345°C (450-650°F), 215°C (419°F) being necessary to melt pure tin, 230°C (446°F) being necessary to melt an alloy of 99.6% tin and .4% copper, a melting temperature of 312.2°C (594°F) is necessary to melt an alloy of 95% lead and 5% tin, and a melting temperature of 294°C (561 °F) is necessary to melt an alloy of 63% tin and 37% lead and a melting temperature of 325°C (618°F) is necessary to melt pure lead. The warm briquetting temperature could be raised to as much as 732°C (1350°F) if necessitated by the requirement to melt the metal coating, or to improve densification by plastic deformation. The pressure applied should be no more than 463 MPa (30 tsi), and preferably in the range of 154-463 MPa (10-30 tsi), so that wear on the tooling used for 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 g. per cubic centimeter is typically obtained using a compressive force of about 463 MPa (30 tsi); with a dry impact coated powder herein; densities of about 7.0 g. per cubic centimeter can be obtained at the same force level.
(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 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 R_s45. Moreover, a special bond test for such a product will show that it has a strength level of at least 13780 KPa (2000 psi), which is 400% greater than that of.a a product produced without the use of the low-melting metal lubricant.
With respect to the second aspect of this invention, the first method is varied in either or both of two respects. First, the iron powder selected is limited to a fine particle size, averaging 4-5 microns. This can preferably be obtained by extracting the iron powder as a byproduct 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 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, nickel and molybdenum. The turnings will have a size characterized by a width of .254-2.54 cm (.1-1.0"), thickness of .0127-.0762 cm (.005-.03"), and a length of 2.54-254 cm (1-100"). Machine turnings are usually not suitable for melting in an electric furnace because they prevent efficient melt down due to such 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 turnings.
The selected scrap pieces are put into a suitable charging passage leading to a ball milling machine (or equivalent impacting device). Within the passage means, an ingredient for freezing the metal pieces is introduced, such as liquid 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):
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 1093°C (2000⁰F). 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.
With sintering, a final density of about 97% has been achieved without the necessity for secondary consolidation such as forging. This is an extremely high 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 7.9 x 10⁵ KPa (115,000 psi), % elongation 10%, and hardness R₈78.
Test results to support the above methods are depicted in Table 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 an 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 245°C (473°F). Density measurements were obtained and compared with the same powder specimen but uncoated and prepared according to a conventional technique using a 1% Zinc Stearate admixed lubricant.
Although percentage of tin above 5% can be employed and will operably work within the system as described, it is suggested that more than 5% tin is not desirable because of economic reasons, tin being considerably more expensive than iron. Also certain physical 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 473°F (245°C), 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 therefor 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

1. A method of producing a metal powder compact which comprises compacting iron powder in the presence of a low melting point metal at a temperature sufficient to melt the low melting point metal, characterised in that the iron powder is compacted in the presence of from 1% to 5% by weight of the low melting point metal, that the iron powder is compacted at a pressure no greater than 463 MPa (30 tsi), _ and that the iron powder particles are coated with the low melting point metai by dry impact coating.

2. A method as claimed in Claim 1, wherein the dry impact coating is effected by ball milling with milling elements composed of or carrying the low melting point metal.

3. A method as claimed in Claim 1 or 2, wherein the iron powder is coated with 2% to 4% by weight of the low melting point metal.

4. A method as claimed in any preceding Claim, in which the low melting point metal is chosen from tin, a copper-tin alloy, a copper-lead alloy and lead.

5. A method as claimed in Claim 4, wherein the low melting point metal is tin.

6. A method as claimed in any preceding Claim, wherein the iron powder is compacted at a temperature of from 230°C to 345°C.

7. A method as claimed in any preceding Claim, wherein the iron powder has a particle size of from 60 to 120 ,um.

8. A method as claimed in any preceding Claim, wherein the metal powder compact is sintered.