WO2004002659A1

WO2004002659A1 - Compaction method for high density powder metal products

Info

Publication number: WO2004002659A1
Application number: PCT/SE2003/001095
Authority: WO
Inventors: Paul Skoglund; Mikhail Kejzelman
Original assignee: Höganäs Ab
Priority date: 2002-06-26
Filing date: 2003-06-25
Publication date: 2004-01-08
Also published as: SE0202010D0

Abstract

The present invention relates to the filed of powder metallurgy and a compacting method for powder metal products comprising the following steps: a) filling a metal powder, optionally comprising an internal lubricant, in a die cavity; b) compacting the powder to a compact with an intermediate density; c) moving the compact a predetermined distance towards the exit of the die cavity; d) performing a second compaction to achieve a compact with a target density; e) ejecting the compact.

Description

COMPACTION METHOD FOR HIGH DENSITY POWDER METAL PRODUCTS

Field of the invention

The present invention relates to the field of powder metallurgy. More particularly the invention concerns a compaction method for producing powder metal products.

Background of the invention

In industry the use of metal products manufactured by compacting and sintering metal-powder compositions is becoming increasingly widespread. A number of different products of varying shapes and thickness are being produced and the quality requirements are continuously raised. For many applications improved mechanical properties, such as tensile strength, impact strength, flexural strength, are needed. In general these properties are highly dependent on the density of the sintered product which in turn depends on the density of the green body, i.e. the density before sintering.

In order to obtain high green density one of the most important factors is the compacting pressure exerted on the metal powder. However, high compacting pressures lead to higher ejection forces and ejection energies. The risk is then that, when ejected from the die the green product will have an unacceptable surface finish. This is due to the fact that at high compaction pressures the radial forces in the compact become so high that the lubrication layer formed by the lubricant will collapse during ejection and cold welding will occur.

Another important factor limiting the achievable density is that a certain amount of lubricant has to be used in order to facilitate the compaction. However, the predominantly used lubricants have a lower volume to weight ratio compared with the iron based powder, hence in most cases the lubricant addition lowers the achievable density.

Briefly, in order to obtain compacts having high density, the amount of lubricant should be as low as possible but sufficiently high to avoid the risk of high ejection forces and ejection energies leading to tool wear and/or scratched compacts.

Objects of the invention An object of the invention is to provide a new and improved compaction method requiring less energy for ejecting the compact from the die.

A second object of the invention is to provide a compaction method resulting in compacts with high green densities.

A third object of the invention is to provide a compaction method wherein the amount of lubricant needed for the compaction is reduced.

A fourth object of the invention is to provide a compaction method enabling the production of high compacts .

Summary of the invention

These objects as well as other objects that will be apparent from the description below have now been obtained according to the present invention by providing a method for compacting a metal powder comprising the following steps: a) filling a metal powder, optionally comprising an internal lubricant, in a die cavity; b) compacting the powder to a compact with an intermediate density,. c) moving the compact a predetermined distance towards the exit of the die cavity; d) performing a second compaction to achieve a compact with a target density; e) ejecting the compact.

Detailed description of the invention In the method according to the present invention the compaction may be performed with any metal powder. Preferred metal powders are iron-based powders encompassing powders essentially made up of pure iron; iron powder that has been pre-alloyed with other substances improving the strength, the hardening properties, the electromagnetic properties or other desirable properties of the end products; and particles of iron mixed with particles of such alloying elements (diffusion annealed mixture or purely mechanical mixture) . Examples of alloying elements are copper, molybdenum, nickel, chromium, manganese, phosphorus, carbon in the form of graphite, and tungsten, which are used either separately or in combination, e.g. in the form of compounds (for example Fe₃P and FeMo) . Unexpectedly good results are obtained with iron powders having high compressibility. Generally, such powders have a low carbon content, preferably below 0.04% by weight. Such powders include e.g. Distaloy AE, Astaloy Mo, Astaloy CrM and ASC 100.29, all of which are commercially available from Hoganas AB, Sweden. The metal powder optionally includes a suitable lubricant. Alternatively the powder particles may be provided with a coating of a lubricant .

The lubricant may be selected from conventional lubricants such as metal soaps, waxes and thermoplastic materials, such as polyamides, polyimides, ployolefines, polyesters, polyalkoxides, polyalcohols . Specific examples of lubricants are zinc stearate, H-wax® and Kenolube® . The die cavity may have a shape of more or less complicated contour as known by the man skilled in the art. It has also been shown that cavities with a high filling height are especially suited for the compacting method according to the invention. Thus the compaction method enables the production of high compacts.

The compacting tool used in the method according to the invention may be provided with a stationary die and two punches moving symmetrically towards one another or a movable die and a stationary lower punch and an upper punch compressing the powder.

The compacting pressure used in step b) is suitably moderate, and the density received, i.e. the intermediate density, is low to moderate.

After the initial compaction the compact is moved a predetermined distance towards the exit of the die cavity in order to minimize the remaining ejection length. This is achieved by a relative movement of the die and lower punch in relation to each other. In one embodiment the die is lowered a predetermined distance in relation to the compact resting on the stationary lower punch. In another embodiment the lower punch carrying the compact is pushed a distance upwards towards the exit of the die cavity. This distance (X_SE) may be determined empirically by performing a series of compacting operations. In one embodiment of the invention this distance is 20-50% of the ejection length received during a conventional compaction process, i.e. a compaction operation including a single compaction step. In another embodiment X_SE is determined by calculating

where H_x is the height of the component after the first compaction and equals H_f*p_FD/pι, where H_f is the filling height of the powder, p_FD is the filling density of the powder and p_x is the density of the compact after the first compaction.

The pressure of the final compaction is higher than that of the previous compaction step leading to a compact with a final target density.

In one embodiment of the method according to the invention at least one additional compacting step and an accompanying step for moving the compact a predetermined distance towards the exit of the die cavity is inserted between step c and d.

In an additional embodiment of the method according to the invention at least one additional compacting step is inserted between step d and e.

Finally the compact is ejected from the die cavity. This ejection length may be 35-55% of the ejection length received during a conventional compaction process with only one compaction step. One embodiment of the method according to the invention concerns compaction processes using high compaction pressures to achieve high green densities.

A recently developed method of achieving high densities is "High velocity compaction" in the following abbreviated HVC. The method according to the invention is especially suited and favourable for this type of compaction which may be achieved with the aid of machines working with hydraulics according to the same principle as the percussion machine disclosed in the US 6202757 (WO 97/00751) which is hereby incorporated by reference. The compaction operation based on this principle is uni- axial and an impact ram is used for impacting the upper punch of the die including the metal powder in a cavity having a shape corresponding to the shape of the final compact.

According to a preferred embodiment of the invention the ram speed should be above 2 m/s, preferably above 5 m/s. The ram speed provides energy to the powder through the punch. The compaction obtained is further dependent on the amount of powder to be compacted, the weight of the impact body, the number of impacts, the impact length and the final geometry of the compact. The optimal conditions for the HVC compaction i.e. the amount of kinetic energy which should be transferred to the powder, may be decided by experiments performed by the man skilled in the art. The method according to the invention reduces the ejection energy due to two reasons. Firstly, at the lower density level achieved after the initial compaction step, during which the die is lowered, the residual radial pressure is moderate and the force and the energy required to move the die is therefore comparatively low. Secondly, the ejection length at high final density is shortened and this reduces the ejection energy considerably. The method according to the invention also facilitates the manufacture of high components, i.e. components which are difficult to eject due to a large surface to die wall ratio.

The invention will now be further described using a first embodiment indicated in the appended drawing and the following unlimiting examples.

Fig. 1 is a drawing illustrating the compaction method according to the invention.

The method according to the invention involves compacting metal powder to a compact according to the following steps indicated in Fig. 1: a) A metal powder -is filled in a die cavity 3 to a filling height H_f. b) The powder is densified by performing a first compaction with an upper punch 1 with a moderate pressure resulting in a compact 5 with an intermediate density. c) The die 2 is lowered a predetermined distance X_SE in relation to the compact 5 on the stationary lower punch 4. d) A second compaction is performed with a higher pressure than the first compaction resulting in a compact 6 with a final density. e) The compact is ejected by lowering the die 2 a distance X_SE in relation to the compact 6 to free the compact 6 from the die cavity. In the following examples HVC method as described above was used.

Example 1: Lower final ejection energy at equal final density

An iron based powder was compacted according to the conventional method and according to the method of the present invention. The final ejection length was measured and the final ejection energy determined. The outline and the results of the experiment are shown in Table I . The final ejection length at equal final density was shortened 32 mm or 56% and the ejection energy reduced 500 Nm or 22% with the method according to the invention compared with the conventional method.

TABLE I

Example 2 : Higher critical density level with respect to cold welding between the compact and the die

An iron based powder was compacted according to the conventional method and according to the method of the present invention. The critical density level of the compacts when cold welding occurred was determined. The outline and the results of the experiment are shown in Table II. The critical density level was increased from 7,10 to 7,32 g/cm³ with the method according to the present invention compared with the conventional method.

TABLE II

* The critical density level with respect to cold welding of the compact was determined by visual inspection.

Claims

1. A method for compacting a metal powder comprising the following steps: a) filling a metal powder, optionally comprising an internal lubricant, in a die cavity; b) compacting the powder to a compact with an intermediate density,. c) moving the compact a predetermined distance towards the exit of the die cavity; d) performing a second compaction to achieve a compact with a target density; e) ejecting the compact.

2. Method according to claim 1, wherein at least one additional compacting step and accompanying step for moving the compact a predetermined distance towards the exit of the die cavity is inserted between step c and d.

3. Method according to claim 1 or 2 , wherein the compact is moved a predetermined distance by a relative movement of the die and lower punch in relation to each other.

4. Method according to any one of the preceding claims, wherein the metal powder is an iron based powder.

5. Method according to any of the preceding claims, wherein at least one additional compacting step is inserted between step d and e.

6. Method for compacting a metal powder according to any of the preceding claims characterised in that the first and/or second compaction step(s) is (are) performed as HVC compaction with an impact ram speed above 2 m/s.