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Title:
IRON-BASED POWDER COMPOSITION COMPRISING A COMBINATION OF BINDER-LUBRICANTS AND PREPARATION OF THE POWDER COMPOSITION
Document Type and Number:
WIPO Patent Application WO/2005/061157
Kind Code:
A1
Abstract:
The invention concerns an improved segregation-resistant and dust-resistant metallurgical composition for making compacted parts, comprising at least about 80 percent by weight of an iron or iron-based powder; at least one alloying powder; and (c) about 0.05 to about 2 percent by weight of a binding/lubricating combination of polyethylene wax and ethylene bis-stearamide, the polyethylene wax having a weight average molecular weight below about 1000 and a melting point below that of ethylene bisstearamide, and being present in amount between 10 and 90% by weight of the binding/lubricating combination.

Inventors:
LARSSON MATS (SE)
AHLIN AASA (SE)
RAMSTEDT MARIA (SE)
VIDARSSON HILMAR (SE)
Application Number:
PCT/SE2004/001905
Publication Date:
July 07, 2005
Filing Date:
December 17, 2004
Export Citation:
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Assignee:
HOEGANAES AB (SE)
LARSSON MATS (SE)
AHLIN AASA (SE)
RAMSTEDT MARIA (SE)
VIDARSSON HILMAR (SE)
International Classes:
B22F1/00; C22C1/05; C22C33/02; B22F; (IPC1-7): B22F1/00; C22C33/02
Domestic Patent References:
WO1999020689A11999-04-29
Foreign References:
US20010038802A12001-11-08
US6605251B12003-08-12
US6533836B22003-03-18
US5840469A1998-11-24
US6602315B22003-08-05
EP1179607A22002-02-13
Attorney, Agent or Firm:
AWAPATENT AB (Malmö, SE)
Download PDF:
Claims:
CLAIMS
1. An improved segregationresistant and dustresistant metallurgical composition for making compacted parts, comprising: (a) at least about 80 percent by weight of an iron or ironbased powder; (b) at least one alloying powder; and (c) about 0.05 to about 2 percent by weight of a binding/lubricating combination of polyethylene wax and ethylene bisstearamide, the polyethylene wax having a weight average molecular weight below about 1000 and a melting point below that of ethylene bisstearamide, and being present in amount between 10 and 90% by weight of the binding/lubricating combination.
2. Composition according to claim 1 wherein the particles of the iron or ironbased powder are coated with a layer of polyethylene wax for binding particles of the alloying element (s) and particles of the ethylene bisstearamide.
3. Composition according to claim 1 or 2 wherein the polyethylene wax has a weight average molecular weight between 400 and 800.
4. Composition according to any one of the claims 13, wherein the binding/lubricating combination is made up by 2070% by weight of the polyethylene wax and 8030% by weight of the ethylene bisstearamide.
5. Composition according to any one of the claims 14 wherein the binding/lubricating combination is present in an amount of 0.51. 5 % by weight of the total composi tion.
6. Composition according to any one of the claims 15 further including a fatty acid in an amount of 0.005 0.15, preferably 0.0100. 08 and most preferably 0.015 0. 07% by weight of the composition.
7. Composition according to claim 6, wherein the fatty acid is stearic acid.
8. Composition according to any one of the claims 17 further including a flow agent in an amount of 0.011 percent by weight, preferably 0.0250. 5 percent by weight of the total composition.
9. Composition according to any one of the claims 18 wherein the flow agent is silicon dioxide.
10. Method of preparing an improved segregationresistant and dustresistant metallurgical composition containing alloying powder bound to ironbased powder comprising the steps of mixing and heating an iron or ironbased powder, an al loying element powder, etylene bis stearamide and a pul verulent polyethylene wax and optionally a fatty acid to a temperature above the melting point of the polyethylene wax and below the melting point of the ethylene bisstera mide cooling the obtained mixture to a temperature below the melting point of the polyethylene wax for a period of time sufficient to solidify the polyethylene wax and bind the particles of the alloying element to the ironcon taining particles in order to form aggregate particles, and optionally mixing a pulverulent flow agent having a particle size below 200 nanometers, preferably below 40 nanometers, with the obtained mixture in an amount between 0.005 to about 2% by weight of the composition. 11. Method according to claim 10 wherein the mixture is heated to a temperature between 70 and 150°C for a period between 1 and 60 minutes.
Description:
IRON-BASED POWDER COMPOSITION COMPRISING A COMBINATION OF BINDER-LUBRICANTS AND PREPARATION OF THE. POWDER COMPOSITION.

FIELD OF THE INVENTION The present invention relates to a new metal powder com- position for the powder metallurgical industry. Particu- larly the invention relates to an iron-based powder com- position which contains a binding composition which also provides lubrication during the compaction process used to form a part.

BACKGROUND OF THE INVENTION In industry the use of metal products manufactured by compacting and sintering iron-based powder compositions is becoming increasingly widespread. The quality require- ments of these metal products are continuously raised, and as a consequence new powder compositions having im- proved properties are developed. One of the most impor- tant properties of the final, sintered products is the density and dimensional tolerances, which above all have to be consistent. Problems with size variations in the final product often originates from inhomogenities in the powder mixture to be compacted. These problems are espe- cially pronounced with powder mixtures including pulver- ulent components, which differ in size, density and shape, a reason why segregation occurs during the trans- port, storage and handling of the powder composition.

This segregation implies that the composition will be non-uniformly composed, which in turn means that parts made of the powder composition are differently composed and consequently have different properties. A further problem is that fine particles, particularly those of lower density such as graphite, cause dusting in the handling of the powder mixture.

The small particle size of additives also create problems with the flow properties of the powder, i. e. the capacity of the powder to behave as a free-flowing powder. An im-

paired flow manifests itself in increased time for fill- ing dies with powder, which means lower productivity and an increased risk of variations in density in the com- pacted component, which may lead to unacceptable deforma- tions after sintering.

Attempts have been made at solving the problems described above by adding different binding agents and lubricants to the powder composition. The purpose of the binder is to bind firmly and effectively the small size particles of additives, such as alloying components, to the surface of the base metal particles and, consequently, reduce the problems of segregation and dusting. The purpose of the lubricant is to reduce the internal and external friction during compaction of the powder composition and also re- duce the ejection force, i. e. the force required to eject the finally compacted product from the die.

Various organic binding agents are disclosed in for exam- ple the US patent 4 483 905 (Engstrom) which teaches the use of a binding agent that is broadly described as being of"a sticky or fat character". The US patent 4 676 831 (Engstrom) discloses the use of certain tall oils as binding agents. Furthermore the US patent 4 834 800 (Semel) discloses the use of certain film-forming poly- meric resins that are insoluble or substantially insolu- ble in water as binding agents.

Other types of binding agents set forth in the patent literature are polyalkylene oxides having molecular weights of at least about 7000, which are disclosed in the US patent 5 298 055 (Semel). Combinations of dibasic organic acid and one or more additional components such as solid polyethers, liquid polyethers, and acrylic res- ins as binding agents are disclosed in the US patent 5 290 336. Binding agents that can be used with high tem-

perature compaction lubricants are disclosed in the US patent 5 368 630 (Luk).

Furthermore, the US patent US 5 480 469 (Storstrom) pro- vides a brief review of the use of binding agents in the powder metallurgy industry. The patent notes that it is important to have not only a powder composition that has the alloying powder adhered to the iron-based powder by way of the binding agent, but to also have a lubricant present to achieve adequate compressibility of the powder composition within the die and to decrease the forces re- quired to remove the part from the die.

Specifically, the US patent 5 480 469 teaches a method for binding additives in an iron-based powder metallurgi- cal mixture to the iron or iron-based powder particles by the use of a diamide wax binder. In order to achieve an effective binding between the iron or iron-based parti- cles and the additive particles the powder metallurgical mixture including the binder is mixed and heated to about 90-160° C during mixing and melting of the binder, and subsequently the mixture is cooled during mixing, until the binder has solidified. By this method the flow and apparent density is substantially improved and the prob- lem with dusting can be reduced or eliminated.

A property of a powder mix which is not specifically dis- cussed in the US patent 5 480 469 is the lubricating property. This property is of particular importance when components having high density and/or a complex shape are required. In connection with the production of such com- ponents it is essential that the lubricating properties of the used powder metallurgical mixture are good which in turn means that the energy needed in order to eject to component from the die, i. e. the ejection energy, should be low which is a pre-requisite for a satisfactory sur-

face finish of the ejected component, i. e. a surface fin- ish without any scratches or other defects.

We have now developed a new iron or iron based composi- tion which is distinguished by low segregation and low dusting, good flow and high apparent density and which is also distinguished by good lubricating properties i. e. properties which are all important for powders to be com- pacted and sintered to high quality products.

SUMMARY OF THE INVENTION In brief the iron or iron-based composition according to the present invention includes at least about 80 percent by weight of an iron or iron-based powder; at least one alloying powder in an amount up to 20 percent by weight; and about 0.05 to about 2 percent by weight of a combina- tion of polyethylene wax and ethylene bisstearamide. The polyethylene wax should have a weight average molecular weight below about 1000 and a melting point below that of ethylene bisstearamide. Furthermore, the amount of the polyethylene wax should vary between 10 and 90% by weight of the total weight of the binding/lubricating combina- tion of polyethylene wax and ethylene bisstearamide. In the powder composition used for compaction the polyethy- lene wax is present as a layer or coating on the iron or iron-based particles and binds the alloying element par- ticles and the ethylene bisstearamide particles to the iron or iron-based particles. It is preferred that the composition also includes a fatty acid and a flow agent.

The invention also concerns a method of preparing the powder composition to be compacted.

DETAILED DESCRIPTION OF THE INVENTION As used in the description and the appended claims, the expression"iron or iron-based powder"encompasses pow- ders prepared by atomisation, preferably water atomisa- tion. Alternatively, the powder may be based on sponge

iron. The powders may be essentially pure iron powders preferably such powders, which have high compressibility.

Generally, such powders have a low carbon content, such as below 0. 04% by weight. Other examples of powder are iron powders that have been pre-alloyed or partially al- loyed with other substances improving the strength, the hardening properties, the electromagnetic properties or other desirable properties of the end products. Examples of powders are e. g. Distaloy AE, Astaloy Mo and ASC 100.29, all of which are commercially available from Hoganas AB, Sweden.

The particle size of the iron or iron-based particles normally have a maximum weight average particle size up to about 500 microns; more preferably the particles will have a weight average particle size in the range of about 25-150 microns, and most preferably 40-100 microns.

Examples of alloying elements are copper, molybdenum, chromium, nickel, manganese, phosphorus, carbon in the form of graphite, and tungsten, which are used either separately or in combination. These additives are gener- ally powders having a smaller particle size than the base iron powder and most additives have a particle size smaller than about 20 um.

The'molecular weight of polyethylene wax has an impact on the powder properties and it has been found that a combi- nation of good flow, high apparent density and low ejec- tion energy may be obtained with a low molecular weight polyethylene which in connection with the present inven- tion means a linear polyethylene having a weight average molecular weight below 1000, particularly below 800 and above 300 particularly above 400. In addition to the mo- lecular weight of the polyethyelene wax the ratio between the ethylene bis stearamide and the polyethylene wax in- fluences these properties. Ethylene bis stearamide is

available as e. g. Acrawax (D or Licowax@. Polyethylene wax is available from Allied Signal and Baker Petrolite.

According to the present invention and as is illustrated by the examples the relative amounts of polyethylene wax and ethylene bisstearamide are important. In the bind- ing/lubricating combination of polyethylene wax and eth- ylene bisstearamide it has thus been found that 10-90% by weight should be polyethylene wax. According to the pre- sently most preferred embodiment the amount of polyethy- lene wax should be present in 20-70% by weight of the binding/lubricating combination. If more than 90% by weight of polyethylene wax is used, the lubrication will be in most cases insufficient and if more than 90% by weight of ethylene bisstearamide is used, the binding will be insufficient. The total amount of bind- ing/lubricating combination in the composition is pref- erably between 0.5 and 1% by weight.

The improved segregation-resistant and dust-resistant metallurgical composition according to the invention can be defined as a composition containing at least about 80 percent by weight of iron-based powder; at least one al- loying powder; and about 0.05 to about 2 percent by weight of a partially melted and subsequently solidified binding/lubricating combination adhering the alloying powder particles to the iron or iron-based powder parti- cles.

Low molecular polyethylene waxes have been mentioned in connection with iron-based metal powders for the PM-in- dustry in e. g. the US patent 6 605 251 (Vidarsson) wherein it is disclosed that polyethylene waxes can be used as lubricants in warm or cold compaction of iron or iron based powders. When used in warm compaction the mix- ture including the polyethylene wax is heated to a tem- perature below the melting point of the polyethylene wax

before compaction. The US patent 6 602 315 (Hendrickson) and the related US patent 6 280 683 (Hendrickson) dis- close the use of low molecular polyethylene wax in bonded mixtures. The bonding effect is achieved by the wax at an elevated temperature which is below the melting point of the wax. The illustrating examples which concern iron or iron-based powders indicate that none of the samples ex- hibited flow. Furthermore the US patents 6 533 836 (Uenosono) and 6 464 751 (Uenosono) disclose a free lubri- cant of low molecular polyethylene wax and etylenbis- stearamid in combination with a binder which comprises at least one member selected from the group consisting of stearic acid, oleamide, stearamide, a melted mixture of stearamide and ethylenbis (stearamide) and ethylen- bis (stearamide). The binder may also comprise zinc stearate and at least one member selected form the group consisting of oleic acid, spindle oil and turbine oil.

According to the present invention it is also preferred that the starting mix in addition to the iron or iron- based powder, the alloying powder and the polyethylene wax and the ethylene bisstearamide also includes a fatty acid, preferably a fatty acid having 10-22 C atoms. Exam- ples of such acids are oleic acid, stearic acid and palmitic acid. The amount of the fatty acid is normally 0.005-0. 15, preferably 0.010-0. 08 and most preferably 0.015-0. 07% calculated on the total weight of the powder composition. Fatty acid contents below 0.005 make it dif- ficult to achieve an even distribution of the fatty acid.

If the content is higher than 0.15 there is a consider- able risk that the flow will deteriorate.

It is furthermore preferred that a flow agent of the type disclosed in the US patent 5 782 954 (Luk) is included in the composition after the bonding has been completed.

Preferably this flow agent is silicon oxide, most pref- erably silicon dioxide having an average particle size of

below about 40, preferably from about 1-35 nanometers and it is used in an amount from about 0.005 to about 2, preferably 0.01-1 percent by weight, most preferably from 0.025 to 0.5 percent by weight of the total composition.

Other metals that can be used as flow agents in either its metal or metaloxide forms include aluminium, copper, iron, nickel, titanium, gold, silver, platinum, palla- dium, bismuth, cobalt, manganese, lead, tin, vanadium, yttrium, niobium, tungsten and zirconium with a particle size of less than 200 nm.

The process for preparing the new powder composition in- cludes the steps of - mixing and heating a mix of an iron or iron-based pow- der, an alloying element powder, ethylene-bisstearamide and a pulverulent polyethyelene wax and optionally a fatty acid to a temperature above the melting point of the polyethylene wax, and below the melting point of EBS - cooling the obtained mixture to a temperature below the melting point of the polyethyelene wax for a period of time sufficient to solidify the polyethyelene wax and bind the particles of the alloying element to the iron- containing particles in order to form aggregate parti- cles, and, optionally, - mixing a pulverulent flow agent having a particle size below 200 nanometers, preferably below 40 nanometers, with the obtained mixture in an amount between 0.005 to about 2% by weight of the composition. The heating is suitably performed at a temperature between 70 and 150°C for a period between 1 and 60 minutes.

The invention is further illustrated by the following non limiting examples, wherein the following ingredients and methods were used: Iron powder-AHC 100.29 from Hoganas AB (Sweden) Graphite uf4 from Kropfmuhl

Polyethylene wax 400,500, 655,750 and 1000 from Baker Petrolite (USA).

Ethylene bisstearamide (EBS) available as Licowax) from Clariant (Germany) The stearic acid is available from Faci (Italy) Aerosil is available from Degussa AG (Germany).

The flow was measured according to according to ISO 4490.

The apparent density was measured according to ISO 3923.

The Ejection Energy was evaluated in an instrumented 125 tons hydraulic uniaxial laboratory press. Force and dis- placement are registered during ejection of the compact.

Ejection energy is calculated by integrating the force with respect to the displacement of the ejected part.

Ejection energy is expressed as energy per envelope sur- face area.

Dusting was measured by subjecting 5 grams of the sample to a flow of air of 1,7 liter/minutes, particles less then 10 microns transported by the air stream were counted by a measuring instrument Dust Track Aerosol Monitor model 8520. Dusting is expressed as mg/m3.

The part bonded graphite and lubricant was measured by an instrument Roller Air Analyzer or Roller particle size Analyzer from Aminco. The instrument is an air classi- fier, which separates material by diameter and density.

50 grams of sample was used. The fraction of bonded graphite is calculated by comparing the content of graph- ite before and after the air classification. Bonding in this case is expressed as % bonded graphite.

EXAMPLE 1 Mixtures including iron powder, 0. 5% by weight of graph- ite and 0. 8% by weight of a binding/lubricating combina- tion of polyethylene wax with different weight average molecular weight and ethylene bisstearamide, according to table 1, and 0, 05% of stearic acid were thoroughly heated and mixed at temperature above the melting point of the polyethylene wax but below the melting point of the eth- ylene bisstearamide. The mixtures were then allowed to cool in order to obtain a bonded powder mixture wherein the graphite particles were bonded to the iron particles.

During cooling 0. 06% of an inorganic particulate flow agent was added. Powder properties such as flow, apparent density and dusting were measured. In order to measure the lubricating properties rings with outer diameter of 55 mm, inner diameter of 45 mm and a height of 10 mm were compacted at three different compaction pressures and the energy needed in order to eject the body from the mould after compaction, i. e. ejection energy, were measured.

Table 1 Mixture binding/lubricating combination 1 75% EBS/25% Polywax 400 2 75% EBS/25% Polywax 500 3 75% EBS/25% Polywax 655 4 75% EBS/25% Polywax 750 5 75% EBS/25% Polywax 1000 EXAMPLE 2 Mixtures including iron powder, 0. 5% by weight of graph- ite and 0.8% by weight of a binding/lubricating combina- tion of polyethylene wax and ethylene bisstearamide in different proportions, and 0, 05% of stearic acid, accor-

ding to table 2, were thoroughly heated and mixed at tem- perature above the melting point of the polyethylene wax but below the melting point of the ethylene bissteara- mide. The mixtures were then allowed to cool in order to obtain a bonded powder mixture wherein the graphite par- ticles were bonded to the iron particles. During cooling 0. 06% of an inorganic particulate flow agent was added.

Powder properties such as flow, apparent density and dusting were measured. In order to measure the lubrica- ting properties rings with outer diameter of 55 mm, inner diameter of 45 mm and a height of 10 mm were compacted at three different compaction pressures and the energy needed in order to eject the body from the mould after compaction, i. e. ejection energy, were measured.

Table 2 Mixture binding/lubricating combination 6 90% EBS/10% Polywax 655 7 75% EBS/25% Polywax 655 8 60% EBS/40% Polywax 655 9 40% EBS/60% Polywax 655 10 100% Polywax 655 EXAMPLE 3-COMPARATIVE EXAMPLE Two mixtures including iron powder, 0. 5% by weight of graphite and 0. 8% by weight of ethylene bisstearamide but with no polyethylene wax were prepared. Mixture no 11 in- cluding 0.05% by weight of stearic acid was thoroughly heated and mixed at temperature above the melting point of the ethylene bisstearamide. The mixture was then al- lowed to cool in order to obtain a bonded powder mixture wherein the graphite particles were bonded to the iron particles. During cooling 0. 06% of an inorganic particu-

late flow agent was added. Mixture no 12 were thoroughly mixed without heating. Powder properties such as flow, apparent density and dusting were measured. In order to measure the lubricating properties rings with outer dia- meter of 55 mm, inner diameter of 45 mm and a height of 10 mm were compacted at three different compaction pres- sures and the energy needed in order to eject the body from the mould after compaction, i. e. ejection energy, were measured.

As can be seen from table 4 best combination of AD, flow, bonding and lubrication properties for the powder metal- lurgical composition containing a binding/lubricating combination including the polyethylene wax and ethylene bis stearamide is achieved when the mean molecular weight of the polyethylene wax is between 500 and 750, the con- tent of polyethylene wax is between 10-90% and the con- tent of ethylene bis stearamide is between 90 and 10% in the binding/lubricating combination.

As can be seen from the following table 4 best combina- tion of AD, flow, bonding and lubrication properties for the powder metallurgical composition containing the bind- ing/lubricating combination including polyethylene wax and ethylene bis stearamide is achieved when the mean mo- lecular weight of the polyethylene wax is between 500 and 750, the content of polyethylene wax is between 20-80% and the content of ethylene bis stearamide is between 80 and 20% of the binding/lubricating combination.

Table 4 Mixture AD Flow Dust no Bonding g/cm3 sek mg/m3 % 1 3.03 27.5 26 97.8 2 3.09 26.5 23 97.0 3 3.13 24.3 46 100.0 4 3.13 24.8 67 98.6 5 3.17 24.3 36 100.0 6 3.07 24.7 112 97.3 7 3.13 24.3 46 100.0 8 3.16 24.1 29 99.2 9 3.23 22.9 22 100.0 10 2.92 25.8 31 100 11 3.28 24.4 39 99.8 12 2. 98 33. 5 288 54. 9 Table 4 Mixture GD GD GD no 400 MPa 600 MPa 800 MPa g/cm3 g/cm3 g/cm3 1 6.75 7.10 7.23 2 6.74 7.09 7.22 3 6.70 7.06 7.20 4 6.70 7. 05 7.19 5 6.69 7.04 7.19 6 6.69 7.04 7.19 7 6.70 7.06 7.20 8 6.69 7.06 7.20 9 6.67 7.04 7.18 10 6.69 7.03 7.16 11 6.63 7.00 7.17 12 6.66 7.04 7. 18

Table 4 Ejection Ejection Ejection Mixture Energy Energy Energy no 400 MPa 600 MPa 800 MPa 3/cm2 J/cm2 J/cm2 1 20.0 28.9 31.4 2 19.8 29.2 31.5 3 20.1 25.9 32.4 4 20.1 30.1 32.5 5 20.1 30.5 34.0 6 20.1 30.6 33.2 7 20.1 25.9 32.4 8 19.4 29.3 33.3 9 18.9 27.3 31.5 10 23.6 31.0 34.9 11 20.1 31.6 38.7 12 19. 3 29. 0 33. 5