Powder metallurgy processes – Powder metallurgy processes with heating or sintering – Consolidation of powder prior to sintering
Reexamination Certificate
2001-10-10
2003-03-25
Jenkins, Daniel J. (Department: 1742)
Powder metallurgy processes
Powder metallurgy processes with heating or sintering
Consolidation of powder prior to sintering
Reexamination Certificate
active
06537489
ABSTRACT:
This invention relates to the general field of powder metallurgy and it is particularly concerned with high density products and a compacting operation for achieving such products.
As is well known the usual powder metallurgy practice employs raw materials in the form of powders suitably prepared, compacting such material by means of one of the various processes and then treating the product so obtained adopting suitable conditions of temperature, pressure and composition of the environmental atmosphere in order to obtain a product of desired characteristics. It is also well known to those skilled in the art that the final characteristics obtained remarkably depend on the density increase obtained by means of the forming operation.
A method of compacting metal or non-metal powders in order to increase the density and get compacts having a density of 92 to 98% of that of the solid body is the explosive compaction method. According to this method the powder is normally encapsulated in a can around which an explosive is placed. Some experiments have also been made in which a body was launched by explosion of the explosive to impact on the powder, whereby the speed of the body varied about 200 m/sec. The main advantage of this technique is that rods of high density can be produced, which, according to need, may have large dimensions without large capital expenditure. The method of compacting powder by using explosives is, however, not easy. It is not industrially used for mass production as it is not controllable. Furthermore, it can be dangerous for the operator.
A method and apparatus for doing away with the disadvantages of the explosive compaction method is disclosed in the U.S. Pat. No. 4,255,374. According to this patent the speed of the impact member relative to the powder to be compacted should be at least 300 m/s. However, no actual examples reporting results are given and, to the best of our knowledge, this method has not been accepted or used within the PM industry.
Industrially, there are currently different ways of achieving high densities, e.g. the composition of the metal powder may be changed, the compacting pressure may be increased, the compacting operation may be repeated or the compacting may be performed with a preheated powder or pre-compact in a heated die.
The U.S. Pat. No. 6,202,757 discloses a percussion machine. This machine is primarily intended for cutting metal rods but it is also stated that this machine might be used for the compaction of metal powders. It is suggested that the machine is adapted for compacting spherical, gas atomised powders. It is also stated that the compaction should be performed in several steps and that each step or stroke has a specific function. Thus it is taught that stroke
1
should be an extremely light stroke, stroke
2
should be a high-energy stroke and stroke
3
should be a stroke with medium high energy. There are however no examples demonstrating the effects of compaction of metal powders with the use of this machine. Nor does this patent state anything about the effects of such a compaction except that densities of 94-99% of the density of the corresponding homogenous material might be obtained if the compaction is performed in a spherical mould.
The WO publication 99/36214 discloses compaction of spherical metal powders with a compaction method which seems to be of the type described in the above U.S. Pat. No. 6,202,757. According to this known method a thermoreversible hydrocolloid, such as gelatine, is necessary as a binder in order to form the agglomerates of the powder particles before the compaction is performed. This WO publication also teaches that no significant effect has been seen regarding improvement in density or mechanical properties with powders having irregular particles. The highest green density which is obtained when compacting the agglomerates of these powders having spherical particles is reported to be 95.2% of the theoretical density. Only by subjecting the green bodies to a subsequent sintering step products having near full density are obtained.
In contrast to the teaching in the WO publication 99/36214 it has now been found that very high green densities can be obtained by subjecting iron powders or iron based powders having irregularly shaped particles to compaction with the aid of a percussion machine of the type disclosed in the U.S. Pat. No. 6,202,757. This type of compaction referred to as “high velocity compaction” or HVC in the following text is further described below.
According to the present invention and contrary to the teaching in the WO publication it has also been found that the powder particles should be subjected to the HVC compaction without preceding agglomeration of the powder particles with a thermoreversibe hydrocolloid such as gelatine. The use of a hydrocolloid in the manner disclosed in the WO publication would destroy the iron or iron-based powder by corrosion.
Water-atomised powders is a type of powder which have irregularly shaped particles and which preferably can be used according to the present invention. Another type of iron powder or iron based powder having irregularly shaped particles which can be used according to the present invention is sponge iron powder. Examples of such powders are pure iron powders, such as the water-atomised powder ASC 100.29. The pure iron powder could optionally be mixed with alloying elements such as phosphorus, copper, nickel, molybdenum, sulphur, chromium, manganese, vanadium, tungsten, cobalt etc. to a total amount of alloying elements of at most 20% by weight of the metal powder. Other iron-based powders which may be compacted according to this method are partially alloyed (diffusion alloyed) powders including the elements copper, nickel and molybdenum. The amount of copper may be up to 25 wt %, the amount of nickel may be up to 10 wt % and the amount of molybdenum up to 3 wt %. Another type of powders are fully pre-alloyed water-atomised powders including in addition to iron e.g. molybdenum, nickel, manganese and/or chromium. Also iron-based powders including a pre-alloyed base powder having elements such as copper, nickel and/or molybdenum diffusion annealed thereto may be used. So far, however, our results do not indicate that non-aggregated, spherical, gas-atomised powders generally can be HVC compacted to densities near theoretical density.
Before the compaction the powder may also be mixed with different particular additives selected from the group consisting of graphite, ferrophosphorus, hard phase materials, machinability enhancing agents, flow enhancing agents, lubricants. The graphite may be added in amounts up to 1% by weight of the powder in order to increase the mechanical properties. The powder fill can be performed as in conventional compaction.
The compaction may be performed in a lubricated die. Alternatively the iron or iron-based powder is mixed with a lubricant before compaction (=internal lubrication or bulk lubrication). It is also possible to use a combination of die wall lubrication and internal or bulk lubrication. Another alternative is to provide the powder particles with a lubricant coating or film before the compaction is performed. In this case the die walls may or may not be lubricated. Such a film may also be formed in situ during the compaction operation. The amount of lubricant required is at most 1% by weight, preferably less than 0.6% by weight and most preferably less than 0.3% by weight. The lubricant can be selected among conventionally used lubricants such as metal soaps, waxes and thermoplastic materials, such as polyamides, polyimides, polyolefins, polyesters, polyalkoxides, polyalcohols. Specific examples of lubricants are zinc stearate, H-wax® and Kenolube®. These lubricants may also be used for die wall lubrication.
In order to obtain the products having the desired high density according to the present invention the compacting method is important. Normally used compaction equipment does not work quite satisfactorily, as the strain on the equipment will be too great. It
Allroth Sven
Johansson Björn
Skoglund Paul
Burns Doane Swecker & Mathis L.L.P.
Hëganäs AB
Jenkins Daniel J.
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