Patent Application
20060165549
The present invention relates to a method of fabricating ferrous-based powder compacts having high green densities.
The production of powder metallurgy (P/M) components with high static and dynamic properties for high performance is increasingly required by the P/M industry. In particular, it is well known that increasing the sintered density of parts results in a significant improvement in static and dynamic properties. The final sintered density and mechanical strength of P/M parts are not only dictated by the powder formulation, but also by the compaction process and compacting conditions used, the part characteristics and the sintering behavior. The densification and ejection performance of powder mixes remains however one of the main factors to address when targeting very high density.
In practice, lubricants are commonly admixed to the metallic powder formulations to reduce the friction between the powder particles themselves and with the die walls of the tooling. This is needed to improve the compressibility of metal powders, the uniformity of densification throughout the part, and also lowers the ejection force that is required to remove the compact from the die, thus minimizing die wear. For cold compaction, which is the process predominantly used, conventional lubricants are metallic stearates or amide-based waxes. The use of these conventional lubricants does not generally yield high green density at room or moderate temperature (lower than 100° C.).
A green-state compact is an intermediate step in the powder metallurgy manufacturing process, which is produced when a metal powder-lubricant mixture is compacted in a press. This compact is subsequently sintered in a furnace to produce the finished product. Different processes are becoming increasingly available to the P/M industry to improve the densification and ejection performance of metallic powder mixes. The warm pressing process, which consists in pressing a preheated powder mix in a heated die (most often between 100° C. and 180° C.), enables the fabrication of parts with high density and green strength by increasing the ductility of the ferrous powder particles. The gain in density achieved by warm compaction versus cold compaction generally ranges between 0.12 to 0.30 g/cm3. The density gain is usually larger for higher compacting pressure and less compressible powders. The warm pressing process requires the use of specific presses and toolings. In addition, to take advantage of the beneficial effect of an increase of the compacting temperature on densification, powder mixes must be properly designed, in particular the selection of the internal lubricant to provide adequate lubrication at die walls during both the compaction and ejection steps.
The die wall lubrication technique is also a promising avenue to promote green densities when high compacting pressures are used. This technique has been the object of several studies in recent years. The benefits of this technique consist in the possibility to significantly reduce the internal lubricant level in the powder mix, while maintaining good lubrication at die walls during the compaction and the ejection of parts. Even though the die wall lubrication has been extensively studied at the laboratory scale, it is not widely used on a production scale because of the difficulty in controlling the amount of lubricant and especially the thickness and uniformity of the film deposited on die walls, and also because of the risk that improper die wall lubrication occurs sporadically causing accelerated die wear and even tooling seizure.
Glyceryl esters of fatty acids are known as admixed lubricants for the cold compaction of iron-based powders. Molera et al. in “Possible alternative lubricants for processing iron powders”, Powder Metallurgy, 1988, vol.31, n°4, p.281, evaluated specific triglyceryl esters of fatty acids and showed that these substances lead to notably inferior green and sintered properties than conventional P/M lubricants and therefore dismissed its use. Meyer et al. in “Considerations on the practical effects of lubricants and binders commonly used in compacting metal powders”, Powder Metallurgy, 1969, vol.12, n°24, p.298 carried out a systematic compactibility study on an iron-based powder admixed with several lubricants in proportions of 0.25, 0.5, 0.75, 1% of the total weight. In particular, they refer in their article to the use of an “organic stearate” that corresponds to a specific glyceryl ester of fatty acid. Ramstedt et al. in “Powder composition” Patent Application Publication No: US 2003/0230166 A1, Dec. 18, 2003 filed a patent application claiming the use of glyceryl stearate as lubricant for iron-based powder. Their claim is based on the fact that according to their prior art, these substances have been used as binders in the powder metallurgy field but never admixed as lubricants. Hendrickson et al. in U.S. Pat. No. 6,602,315, Aug. 5, 2003 claim improved segregation-resistant and dust-resistant metallurgical compositions comprising a coating material also referred as solid binding agent containing preferably polyethylene but also solid hydrogenated vegetable oils defined as C14-24 alkyl moiety triglycerides and derivatives. Powder metallurgical compositions are prepared by mixing at low shear conditions the binding agent with the metal-based powder and alloying powder at temperature just below the melting point of the binding agent.
Glyceryl esters of fatty acids are known as admixed lubricants for the cold compaction of iron-based powders. However, such esters do not yield particular remarkable densities when compared to powder compositions comprising conventional lubricants, such as metallic stearates or amide-based waxes compacted under the same conditions.
The inventors have discovered that by compacting under specific conditions that iron-based powder compositions comprising specific glyceryl esters of fatty acids, powder compacts having unexpected high densities can be produced while keeping good shaping behavior. Accordingly, the present invention provides a method for the fabrication of high density iron-based powder compacts, comprising compacting at a temperature range of 50 to 100° C., iron-based powder compositions comprising a specific solid binder-lubricant having a melting point between 50 and 100° C. and containing glyceryl esters of fatty acids.
The solid binder-lubricant could consist of a single glyceryl ester with a melting point falling in the range specified, or alternatively could be a mixture of different glyceryl esters. In the latter case, the melting point of the resulting mixture should lie in the specified range, but it should be noted that the glyceryl esters forming the components of the mixture could have individual melting points lying outside the specified range provided the resulting mixture has a melting point falling within the range. Also, the binder lubricant may be mixed with an additional lubricant, such as a polyolefin wax, which also has a melting point outside the specified range. The important point is that the binder-lubricant consisting of the glyceryl esters of fatty acids, whether it be a mixture or single component, has a melting point between 50 and 100° C.
The invention also provides a metal powder composition comprising an iron-based metal powder and from about 0.01 to about 3 wt. % of a specific binder-lubricant based on the total weight of the composition, preferably from about 0.05 wt. % to about 1.5 wt. %. The specific binder-lubricant may be admixed to the metal powder in a solid state (comminuted, usually as a powder), in emulsion, in solution or in the melted state. The composition may further comprise other solid lubricants and/or binders and/or flowing agents to optimize either the compressibility and lubrication performance, or the flow and the segregation of the powder mixes. Alternatively, the specific binder-lubricant can be mixed with another lubricant in the melted state, granulated to yield a powder and then admixed with the metal powder composition. The specific binder-lubricant may also be sprayed on the die walls of the tooling as a powder or as a solution.
Examples of iron-based powders are pure iron powders, powders of iron pre-alloyed with other elements, and powders of iron to which such other elements have been diffusion-bonded. The composition may further contain powders of such alloying elements in the amount of up to 15 wt. % of said composition. Examples of alloying elements include, but are not limited to, elemental copper, nickel, molybdenum, manganese, phosphorous, metallurgical carbon (graphite) and ferro-alloys.
The glyceryl esters of fatty acids, also known as glycerides, can be natural or synthetic.
The metallurgical powder compositions of the invention can be compacted into parts in a die and subsequently sintered according to standard powder metallurgy techniques.
The invention will now be described in more detail, by way of example only, with reference to the accompanying drawings, in which:
Several powder compositions were prepared and tested for the fabrication of ferrous compacts for P/M applications. Exemplary metal powders suitable for the purpose of the present invention include iron-based powders used in the P/M industry, such as pure iron powders, pre-alloyed iron powders (including steel powders) and diffusion-bonded iron-based powders. Substantially any iron-based powder having a maximum particle size less than about 600 microns can be used in the composition of the invention. Typical iron-based powders are iron and steel powders including stainless steel and alloyed steel powders. ATOMET® steel powders manufactured by Quebec Metal Powders Limited of Tracy, Quebec, Canada are representative of such iron and steel powders. Typical ATOMET® powders contain in excess of 99.6 wt. % iron and pre-alloyed metals, less than 0.3 wt. % oxygen and less than 0.1 wt. % carbon, and have an apparent density of 2.50 g/cm3 or higher, and a flow rate of less than 30 seconds per 50 g.
Optionally, the iron-based powders can be admixed with alloying powders in the amount of preferably less than 15 wt. %. Examples of alloying powders include, but are not limited to, elemental copper, nickel, molybdenum, manganese, phosphorus, metallurgical carbon (i.e. graphite) and alloys of the above, with or without iron.
Powder compositions of the invention include a specific binder-lubricant in an amount from about 0.01 wt % to about 3 wt % based on the total weight of the composition, preferably from about 0.05 wt. % to about 1.5 wt. %. This specific binder-lubricant may be admixed to the metal powder in a solid state (comminuted, usually as a powder), in emulsion or in solution. It could also be admixed to the metal powder in a solid state) and subsequently melted to bind the secondary powders to the basic metal powder. The admixture may be carried out in a single operation or step, or in several steps. The composition may further comprise other solid lubricants or binders or flow agents to further improve either the compressibility and lubrication performance, the flow and/or the segregation of the powder mixes.
Typically, the specific binder-lubricant of the invention has a melting point between 50 and 100° C., and is a mixture of natural or synthetic glyceryl esters of fatty acids also known as glycerides. These glycerides are typically mono-esters, di-esters or tri-esters of glycerol (also known as monoglycerides, diglycerides or triglycerides), or a mixture of them. The C4-40 alkyl moiety of the glycerides can be saturated or unsaturated, linear or branched, and substituted or unsubstituted.
The binder lubricant of the invention may be used in combination with at least one additional lubricant, which is preferably selected from the group consisting of non-metallic fatty acid compounds, such as ethylene bis-stearamide, stearic acid, oleic acid, and/or polyolefinic waxes, such as polyethylene wax. In this case, the preferred embodiment is a binder-lubricant composition comprising 5-95 wt. % or preferably 30-95 wt. % of the binder lubricant of the invention and 95-5 wt. % or preferably 70-5 wt. % of additional lubricant.
In some embodiments, the binder-lubricant of the invention is soluble in standard solvents, which makes it possible to prepare the binder-treated powder composition using spray coating techniques, or by other known techniques.
The metallurgical powder compositions of the invention can be compacted under conventional powder metallurgy conditions. The compacting pressures are typically lower than 85 tsi and more specifically between 10 and 60 tsi. The metal powder compositions of the invention can be compacted into parts in a die and subsequently sintered according to standard powder metallurgy techniques. The compacting temperature suitable with the compositions of the invention is between 50 and 100° C.
The compaction and ejection characteristics of the ferrous powder compositions mixed using either dry, wet or melt-bonding procedures, were evaluated with a single action instrumented compacting device, known as the Powder Testing Center Model PTC 03DT, manufactured by KZK Powder Technologies Corporation, Cleveland, Ohio. This instrumented press allows continuous recording of the moving punch displacement, the forces applied to the moving punch and transmitted to the stationary punch and the IN-die density all along the compaction and ejection processes. The strippin g pressure, which corresponds to the force needed to start the ejection process divided by the friction area (contact surface between the compact and the die wall) and the ejection unit energy were estimated from the ejection curve in order to compare the lubricating performance of lubricants. The ejection unit energy is evaluated from the calculation of the area under the ejection curve (force vs. displacement) divided by the displacement of 2.54 mm and the friction area.
Tests were also conducted on the wet and melt-bonded mixes to evaluate the binding efficiency of the binder-lubricant of the invention. Dusting resistances were determined by fluidization with a stream of gas (in this case air). Air was directed at a constant flow rate of 6.0 liters/minute for ten minutes at the bottom of a 2.5 cm diameter tube in which the test material was placed. This causes finer secondary powders, such as graphite, to be entrained, as a result of a large surface-to-volume ratio, and low specific gravity (in the case of graphite), and to be deposited in the dust collector. The mixture remaining on the screen plate was then analyzed to determine the relative amount of alloying additive, which is a measure of the resistance to dusting when expressed as a percentage of the pre-test concentration.
The typical binder-lubricant of the invention that was used in the following examples is a mixture of mono, di and triglyceryl esters of fatty acids, and is referred as GEFA. The mono, di and tri-ester contents are respectively: 8 to 22 wt %, 40 to 60 wt % and 25 to 35 wt %. The fatty acids entering in the composition of these esters are palmitic acid (C16) and stearic acid (C18) in proportion of respectively 40 to 60 wt % and 40 to 60 wt %. This binder-lubricant is in a powder form having an average mean diameter of ˜30-40 μm, and has a melting point of 62° C.
Two ferrous powder compositions containing 96.65 wt. % ATOMET 1001 steel powder (Quebec Metal Powders Ltd.), 0.5 wt. % graphite powder (South Western 1651), 2 wt. % copper powder (MD 165) and 0.75 wt. % of lubricant were prepared by conventional dry-mixing in a V type mixer. The first powder composition referred as Control mix contained atomized ACRAWAX C powder from Lonza Inc. (EBS) as lubricant, while the second powder compositions contained the binder-lubricant of the invention described previously (GEFA).
As illustrated in
When comparing
Tests were conducted to evaluate the binding efficiency of the binder-lubricant of the invention using either a melt-bonding procedure or a wet procedure.
Melt-bonding procedure: The binder-treated Mix 1 was prepared by mixing at a temperature of 65° C. close to the melting point of the GEFA type binder-lubricant, 0.65 wt. % of GEFA with 96.5 wt. % ATOMET 1001 steel powder (Quebec Metal Powders Ltd.), 0.85 wt. % graphite powder (South Western 1651), 2 wt. % nickel (Nickel T123 PM, INCO Ltd). In this mix, the binder-lubricant of the invention was used both as binder and lubricant. The dusting resistance of the binder-treated Mix 1 powder composition was compared with the behavior of a dry mixture, referred as Control Mix 1 consisting in 95.5 wt % ATOMET 1001 steel powder (Quebec Metal Powders Ltd.), 0.85 wt. % graphite powder (South Western 1651), 2 wt. % nickel (Nickel T123 PM, INCO Ltd) and 0.75 wt. % of atomized ACRAWAX C powder from Lonza Inc. (EBS).
Wet procedure: The binder-treated Mix 2 was prepared by dissolving 0.15 wt. % of GEFA in a solvent and by mixing this solution with a mixture of 96.43 wt. % ATOMET 4201 steel powder (Quebec Metal Powders Ltd.) and 0.92 wt. % graphite powder (South Western 1651) and 2 wt. % copper powder (SCM 50ORL) and 0.15 wt % of molybdenum (Sylvania) and 0.5 wt. % of zinc stearate (Pompla). This mixture was then dried by evaporating the solvent. The dusting resistance of the binder-treated Mix 2 powder composition was compared with the behavior of a dry mixture, referred as Control Mix 2 consisting in 96.93 wt % ATOMET 4201 steel powder (Quebec Metal Powders Ltd.), 0.92 wt. % graphite powder (South Western 1651), 2 wt. % copper powder (SCM 500RL) and 0.15 wt % of molybdenum (Sylvania) and 0.5 wt. % of zinc stearate (Pompla).
As shown in Tables 1 and 2 below, the graphite, nickel, copper and molybdenum dusting resistances provided by the Binder-treated Mixes are significantly improved as compared to the Control Mixes.
This application claims the benefit under 35 USC 119(e) of prior U.S. provisional application no. 60/641,770 file Jan. 7, 2005, the contents of which are herein incorporated by reference.
| Number | Date | Country | |
|---|---|---|---|
| 60641770 | Jan 2005 | US |
