In the art of powder metallurgy, iron or steel powders are often admixed with one or more alloying elements, also in particulate form, followed by compaction and sintering. Because of their very fine size, these alloying powders are susceptible to the separatory phenomena known as dusting and segregation, but the incorporation of binding agents into the compositions reduces these problems, enhancing the homogeneity of the composition and therefore of the final sintered part. See, for example, U.S. Pat. No. 4,834,800 to Semel and U.S. Pat. No. 4,483,905 to Engstrom.
Metal powder compositions are also generally provided with a lubricant, such as a metal stearate, a paraffin, or a synthetic wax, in order to facilitate ejection of the compacted component from the die. The friction forces that must be overcome in order to remove a compacted part from the die, which generally increase with the pressure used to compact the part, are measured as the "stripping" and "sliding" pressures. The lubricants reduce these pressures.
Hundreds of thousands of tons of iron and steel powders worldwide are mixed each year and most of it, probably upwards of 95%, is done without the use of binders or, for that matter, even any consideration of the use of such. The addition of lubricants to these mixes is simple even to the point of being completely artless. Although lubricant type and content are important issues, method of addition is not. Accordingly, the lubricants are added directly along with the balance of the admix ingredients.
With the advent of bonding to prevent segregation and dusting and, particularly, with the use of solid binders as dispersed from solvent solutions, the method of lubricant addition and, more specifically, the timing of the addition relative to that of the binder additions has along with the issues of type and content also become an important issue.
In the very early development of the bonding technology, the aim was to achieve identically the same powder properties in a bonded mix as would be observed in the same composition mix but without bonding. The powder properties referred to include, particularly, the apparent density (ASTM B212-76), the flow rate (ASTM B213-77), the green density (ASTM B331-76) and the green strength (ASTM B312-76). Studies in connection with the development of the solid binders claimed in U.S. Pat. No. 4,834,800 showed that the best way to achieve parity with respect to these properties in a bonded mix versus an unbonded mix was to make the lubricant additions after the binder addition. More specifically, in this method, the iron-based powder and alloying powders are first mechanically blended, then a binding agent, (always) either dissolved or dispersed in a solvent, is thoroughly blended into the mixture and the solvent removed, usually by application of heat and vacuum, and finally at this point, the lubricants, (there could be more than one), in particulate form are added to the dry bonded powder mixture. The lubricant addition step may be carried out in the same vessel as employed to do the bonding treatment or, in a different vessel. In any case, the generally observed effects of this method of processing on the properties of the resultant mixes relative to unbonded mixes of the same composition were (1) to increase the apparent density slightly but not significantly; (2) to increase the flow rate by about 10%; (3) to decrease green strength by about 10%; and (4) to leave green density largely unaffected in the density range from about 6.2 g/cm.sup.3 to 6.9 g/cm.sup.3 which was the range of predominant industrial interest at the time.
Later studies of the type which led to this method showed that another method of adding the lubricant led to significant increases in the flow rates of bonded mixes. Improved flow rates are advantageous in that they increase efficiency of the compaction processing. According to this method, referred to as "flow-bonding," the lubricant is added to the dry admixture of iron-based and alloying powders prior to the addition of the binder agent. Specifically, the iron-based powder and alloying powders are blended together with the particulate lubricant. A solution of the binder agent in an appropriate organic solvent is then mixed into the powders in order to fully wet the powders. Finally, the solvent is removed, leaving a dry, flowable powder. This method generally increases the flow rate by as much as 25-75% as compared to the lubricated, non-bonded powder. However, this method typically increases the apparent density of the powder, usually by about 0.1 to about 0.25 g/cm.sup.3. Such a powder, although having the desired elemental composition and flow properties, may not be usable in retrofit applications involving fixed-fill compaction dies that have a limited latitude for accepting these higher apparent densities.
Therefore, a need exists in the powder metallurgical art for a method to prepare the metallurgical powder composition in which certain properties of the powder, especially the apparent density, can be altered while retaining desirable flow characteristics and not significantly altering other "green" (compacted) and sintered properties.