Metal powder compression tool

Details Metal powder compression tool Metal powder compression tool

2000-04-13

2003-05-06

Jenkins, Daniel J. (Department: 1742)

Plastic article or earthenware shaping or treating: apparatus

Means for molding powdered metal

Reexamination Certificate

active

06558144

ABSTRACT:

The present invention relates generally to the manufacture of articles by sintering techniques and more specifically to a powder compression tool for forming a work piece herein termed a compact, which is then placed in a sintering furnace.
BACKGROUND OF THE INVENTION
In general terms, sintering consists of compressing metal powder, generally a steel powder, to obtain a compact of definitive shape. This compact whose shape is maintained only by cohesion of the powder, is then passed through a furnace at a sintering temperature below the melting temperature, but sufficient for the powder particles to join.
After sintering, the product will typically exhibit a final density which approaches, but does not equal the density of the metal in question. In the case of steel powder, it is possible to achieve final densities on the order of 7.4-7.5 g/cc, using the conventional pressing and sintering techniques described below, whereas the density of steel itself is on the order of 7.8-7.9. For ease of reference, this will be referred to as the maximum density.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a modified pressing process and apparatus capable of operation to yield sintered products having a final sintered density which more nearly approaches the maximum density of the material, and in the case of steel, a final sintered density of over 7.5. According to the present invention, this is achieved by a single press, single sinter process in which a metal powder mix containing from about 0.3 to 0.5 weight % of a solid lubricant is pressed in a single step in a die having a working clearance of at least 45 &mgr;m under a pressure of at least 800 MPa to form a compact for subsequent sintering.
For better understanding of the basic technology, conventional powder compression processes will now be described with the aid of
FIGS. 1A
,
1
B and
1
C.
FIGS. 1A
to
1
C illustrate the operation of a powder compression tool. The tool includes a die
10
with a cavity
12
arranged through it. This cavity
12
defines the shape or profile of the desired compact, including features such as a smooth surface, an indentation, or other characteristic. Die
10
co-operates with an upper punch
14
and a lower punch
15
which penetrate through both ends of the cavity
12
.
In
FIG. 1A
, the cavity
12
is filled with metal powder flush with the upper surface of die
10
. Lower punch
15
is at a specific position determined by the volume of powder required to obtain the desired height and density of the final produce Once cavity
12
is filled with powder, upper punch
14
is lowered.
In
FIG. 1B
, upper punch
14
reaches an end position determined by the pressure applied to both punches. A compact
17
of desired shape is then obtained in cavity
12
, formed of powder particles sufficiently cohered together to allow it to be handled and carried to a sintering furnace (not shown).
In
FIG. 1C
, upper punch
14
is withdrawn while lower punch
15
is raised to eject compact
17
from the cavity
12
. The compact is then carried to the sintering furnace. To eject Impact
17
, instead of raising the punch
15
, die
10
could be lowered. It will be appreciated that various options are possible.
As illustrated in
FIGS. 1A and 1B
, the volume of the powder decreases considerably on application of pressure. For conventional pressures, on the order of 700 MPa. the volume decreases by a factor 2.3 to 2.5. This decrease in volume is accompanied by rubbing of the powder against the wails of the cavity
12
over the length of travel of the punch. It is thus essential to lubricate the walls of the cavity
12
to minimise friction.
Lubrication of the walls of the cavity
12
being impractical in production, it is preferred to include the lubricant in the metal powder. For the powder to be able to properly flow to fill up the cavity, the lubricant also comes as a powder.
Of course, lubrication also facilitates ejection of the compact
17
, without damage.
The proportion of lubricant commonly used in the metal powder is from 0.6 to 0.8% in weight. However, the lubricant is about eight times less dense that the metal powder, and occupies an incompressible volume which cannot be replaced by metal during the compression. As a result, especially upon elimination of the lubricant while sintering. the obtained compacts are porous and have a mechanical strength which is substantially lower than that of pure metal.
Thus in practical terms, conventional pressing and sintering processes can yield products with a final density (in the case of steel) of up to 7.5. More typically, using a pressure of 700 MPa and 0.8% lubricant, the final density is only around 7.15. In theory, higher pressures would tend to increase the final density, but in practice, pressures exceeding about 800 MPa have been observed to result in rapid tool damage, even though the tool itself is, in isolation, capable of withstanding more than 2000 MPa.
It is appropriate to mention that final sintered density is much more significant than the so-called “theoretical maximum density” of the compact, including lubricant, before sintering. Reducing the lubricant quantity may make it possible to achieve a higher percentage of the maximum theoretical density of a particular metal powder/lubricant mixture, but even values such as 96% of maximum theoretical density correspond only to a final sintered density of 7.15 in the case of steel powder containing 0.8% lubricant.
A final density, in the case of steel powder, of around 7.15 thus is typical of that obtained through a conventional single press/single sinter process, in which a single powder compression step is performed, at about 700 MPa, followed by sintering to obtain the final product.
To obtain sintered compacts with higher densities, a double press/double sinter process can be used, in which, after compression under the above-mentioned conditions, the compact undergoes a pre-sintering treatment to vaporise the lubricant, so as to empty the pores that it occupies. The compact is then submitted, before a final sintering, to a second compression during which the material, not yet generally integral, tends, through plastic deformation, to occupy the empty pores. With such a process, however, final densities above 7.5 cannot be achieved. Further, such a two-stage process is more expensive to implement than a single press/single sinter process.
There is also a warm forming process in which the die and powder are heated to about 100-150° C. to liquefy the lubricant which then escapes by draining from the pores. The maximum densities obtained are on the order of 7.4 (in the case of steel) and the process is also expensive to implement.
A further object of the present invention is to provide a compression tool which can more successfully withstand operation at higher than normal pressures.
Yet another object of the present invention is to provide such a tool which enable compacts of particularly high final density to be obtained through a single pressing process.
In conventional compression tools, the clearance between punches and dies has always been made as small as possible. This is to avoid or at least minimise extension of powder through the clearance, as well as the formation of moulding flash, generally referred to as “beards”. The clearance commonly found in typical tools ranges from 10 to 20 &mgr;m.
FIG. 2
illustrates on an enlarged scale the clearance in the tool and the deformation which take place during a compression operation. The nominal diameters of moving punch
14
and of cavity
12
of the die are indicated in dotted lines. During compression, punch
14
tends to undergo barrel deformation. At a certain pressure level, the punch comes into contact with the die along As entire circumference while still moving. The resulting friction increases as punch
14
comes closer to its final position and the deformation also increases.
If the friction were uniform over the punch circumference, the tool would be able to better withstand high pressure. However, in practi

Affiliated with

Also associated with

Rating

Metal powder compression tool does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Metal powder compression tool, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Metal powder compression tool will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-3019011