EP1294512A1

EP1294512A1 - Method and device for sintering aluminium based sintered parts

Info

Publication number: EP1294512A1
Application number: EP01956435A
Authority: EP
Inventors: Hartmut Weber
Original assignee: Eisenmann Anlagenbau GmbH and Co KG
Current assignee: Eisenmann Anlagenbau GmbH and Co KG
Priority date: 2000-06-28
Filing date: 2001-05-12
Publication date: 2003-03-26
Anticipated expiration: 2021-05-12
Also published as: DE10066005A1; EP1294512B1; US20030143098A1; ATE259267T1; ES2214435T3; US6821478B2; WO2002000377A1; DE10066005C2; AU2001278425A1

Abstract

The invention relates to a method for sintering aluminium-based sintered parts which are, initially, guided with the aid of a transport system T through a de-binding area (3) before being guided through followed by a sintering area (2) and finally being guided through a cooling area (4). Inert gas atmosphere prevails in the sintering area (2), provided with an oxygen content, corresponding to a thawing point of, maximum, 40° C. The sintered parts (23) are heated to the required sintering temperature of 560-620° C., by means of convection, whereby the inert gas atmosphere is accordingly heated, flowing around said sintered parts in a corresponding manner.

Description

Process for sintering aluminum-based sintered parts and device for sintering aluminum-based sintered parts

The invention relates to a method for sintering aluminum-based sintered parts, in which the following steps are carried out in separate atmospheres in spatially separated areas:

a) the sintered parts are debindered;

b-) the sintered parts are brought to the sintering temperature and held there for a certain time;

c) the sintered parts are cooled in a controlled manner;

such as

a device for sintering aluminum-based sintered parts with

a) a debinding area, in which the sintered parts are freed by heating binding aids;

b) a sintering area in which the sintered parts are subjected to a sintering process by heating to the sintering temperature and the corresponding one

Has heaters;

c) a cooling area in which the sintered parts can be cooled in a controlled manner after the sintering process, d) a transport system that continuously guides the sintered parts through the different areas;

e) Locks which keep the atmospheres of the different areas separate and which have to be crossed by the sintered parts when leaving a certain area.

The sintering of aluminum is becoming increasingly important in a wide variety of technical fields, in particular in the automotive industry, in view of the positive properties attached to this metal. In the last-mentioned area of application, weight saving, which is associated with the use of aluminum, plays a major role.

In general, pure aluminum powder is not processed; rather, powder mixtures or alloy powders, which contain silicon as an additive, are preferably used. All powders that contain aluminum as an important component are collectively called "aluminum-based" and are at risk of forming oxides during sintering. Aluminum sintered parts with a relatively high silicon content are particularly desirable. However, as the silicon content increases, the sintering process becomes more difficult. Another difficulty with the sintering of aluminum-based powders is that they require a higher binder content during the pressing process. While, for example, when sintering iron, such binding aids, which simultaneously serve as a lubricant for the pressing tool, make up a content of about 0.7 to 1.0 percent by weight, about 1.0 to 1.5 percent by weight of binding aids must be added for sintering aluminum , These binding aids must be complete again before the sintering process be removed. All in all, all requirements for accuracy, reproducibility and the homogeneity of the temperature distribution when sintering aluminum-based powder are much more critical than when sintering other powders, especially iron. For this reason, aluminum sintered parts have not yet been used wherever this would be desirable in and of itself.

A method and a device of the type mentioned are described in DE-PS 197 19 203. According to its title, this relates to a sintering process for molded parts pressed from metal powder, which would also linguistically include aluminum powder. In fact, however, this method and this device are only intended for sintering iron-based powders, since the rapid cooling of the sintered parts under a "martensite starting line" is only conceivable for such powders.

The object of the present invention is to provide a method of the type mentioned at the outset with which high-quality aluminum-based sintered parts can be produced.

This object is achieved in that in process step b) an inert gas is used as the atmosphere, the oxygen content of which corresponds to a dew point of at most -40 ° C., and in that the sintered parts are circulated to a sintering temperature of 560 - by circulating the correspondingly heated inert G Gaassess. 620 C can be heated

The invention . is based on a double insight: The fact that a maximum limit is set for the oxygen content of the inert atmosphere ensures that there are no undesired oxides in the sintering process can form, which adversely affect the sintering product. Characterized in that, unlike the subject of DE-PS 197 19 203 mentioned above, the sintered parts are not heated by radiant heat but by convection heat, for which purpose the highly pure inert mentioned

If gas is placed in a circulation flow, the sintered parts are heated with a homogeneity that would otherwise not be possible. It is only in the sum of these characteristics that the high quality of the sintered products is achieved.

Nitrogen is preferably used as the inert gas. This can be obtained commercially with the required purity and is much cheaper than noble gases, which in principle are also possible.

Another object of the present invention is to design a device of the type mentioned at the outset in such a way that it is suitable for producing high-quality aluminum-based sintered parts.

This object is achieved in that

f) the atmosphere in the sintering area is formed by an inert gas, the oxygen content of which

Corresponds to a dew point of at most -40 C;

g) the sintered area has at least one heating device for the sintered parts, which comprises indirectly heated heat exchange surfaces, a blower and an air guiding device such that a circulating flow of the inert gas flowing around the sintered parts can be set.

The meaning of these characteristics and the achievable with them Advantages coincide with what was said above for the method according to the invention.

The advantages of the exemplary embodiments specified in claims 4 and 5 have already been indicated above with reference to the method according to the invention.

The sintering area of a sintering device must have a length that corresponds to the time required for sintering at the selected transport speed. In general, it is advisable if a longer sintering area has a plurality of zones delimited by partitions, each of which has a heating device with heat exchange surfaces, blowers and air devices. As a result, defined flow conditions can be set anywhere, even with longer sintering areas.

In the heating zone of the sintered area in particular, the temperature of the inert gas differs from zones of the sintered area located one behind the other in the direction of movement.

The embodiment of the invention leads to particularly good sintering results due to the high homogeneity of the temperature profile, in which the flow around the sintered parts is different in zones of the sintered region lying one behind the other in the direction of movement. For example, the sintered parts can be washed from bottom to top, the other from top to bottom, once by a flow rotating clockwise, the other by a flow rotating counterclockwise.

It is also advantageous if a nozzle plate is provided over which the circulating inert gas is directed against the sintered parts. As a result, the flow in the area of the sintered parts and thus the heating experienced by the sintered parts can be further evened out.

As already mentioned above, the careful keeping of the atmosphere in the sintering area plays a very special role. As a result, special attention must be paid to introducing the sintered parts into the sintered area and discharging them from the sintered area. It is particularly preferred if the gates of the locks, which are adjacent to the inlet and / or the outlet of the sintering area, cannot be closed completely tightly and if the inert gas in the sintering area is under excess pressure. The deliberate "leak" in that of the two lock gates, which is adjacent to the sintered area, continuously maintains a flushing flow of the inert gas out of the sintered area into the respective lock, with which the

Interior of the lock is flushed and with the other hand, the penetration of foreign atmosphere from the lock into the sintered area is prevented.

An embodiment of the invention is explained below with reference to the drawing; show it

Figure 1 schematically shows a sintering furnace for sintering aluminum-based sintered parts;

Figure 2 shows a section on an enlarged scale through the sintering furnace of Figure 1 in the region of the sintering zone.

FIG. 1b shows a sintering furnace in vertical section, which is intended for sintering aluminum-based sintered parts. The entire sintering furnace is divided into different zones or areas, which are shown schematically in FIG. The sintered parts 23 (cf. FIG. 2) are conveyed through the sintering furnace from left to right in a continuous pass in the drawing with the aid of a transport system T. The sintering furnace contains, seen in the conveying direction, one after the other an inlet area 8, a debinding area 3, a sintering area 2, a cooling area 4 and an outlet area 9. Each of these areas 2, 3, 4, 8, 9 of the sintering oven is a separately drivable and controllable Assigned conveyor T2 to T9, which together form the above-mentioned conveyor system T.

To isolate the atmospheres in areas 3, 2, 4 and 9, locks 7 are arranged between these areas, each having two mechanical gates 6. These gates 6 are each arranged in a front shaft of the corresponding area 3, 2, 4, 9 and are preferably movable vertically, each lock 7 being assigned a conveyor which is also separately controllable and controllable (not shown in the drawing).

Details of the locks 7 can be found in FIG. 4 of the above-mentioned DE-PS 197 19 203.

The debinding area 3 preceding the sintering area 2 in the conveying direction is designed as a muffle furnace. That is, , above and below the path of movement to the sintered parts there is a partition 20, which is brought to temperature by means of electric heating elements 21 or the like, essentially heats the conveyed sintered parts by radiant heat and drives out the binding aids. While in the sintering furnace described in DE-PS 197 19 203.3 and intended for sintering iron powder parts, the sintering region also works with radiant heat, the sintering region 2 of the present sintering furnace differs from this in a manner which will now be described with reference to FIG. 2.

FIG. 2 shows a section perpendicular to the direction of movement of the sintered parts in the area of the sintering zone 2. The housing 22, which is provided with insulation, is at all points in which air could penetrate from the outside atmosphere or gases could escape from the inside atmosphere , well sealed. In the lower area of the housing 22, the transport system T2 is shown, the exact construction of which is deliberately left open. It is characterized by good gas permeability in the vertical direction; For example, roller or link belt systems are particularly suitable. With the help of the transport system T2, the sintered parts 23 are conveyed perpendicular to the drawing plane of FIG. 2, in the example shown on a carrier plate 24, which ideally should be permeable even in the vertical direction.

The area of the interior of the housing 22 which lies above the sintered parts 23 is divided into two chambers 26 and 27 by a partition 25 which runs parallel to the direction of movement of the sintered parts 23 and essentially vertically. In the chamber 26 on the left in FIG. 2 there are the heat exchanger surfaces 28 of an indirect heater 29, which can be operated, for example, electrically. At the upper end of the chamber 26 there are air baffles with a central opening 30, which represents the suction opening of a blower 31. The blower 31 is driven by a motor 32 mounted on the top of the housing 22.

The outlet side of the blower 31 is connected via an opening 33 to the chamber 27 on the right in FIG. 2 of the interior of the housing 22. This chamber 27 is closed at its lower end, just above the sintered parts 23, by a nozzle plate 34.

The entire sintered area 2 contains, in particular

FIG. 1b shows a plurality of identical sintered zones constructed in the manner described above, which are separated from one another by partition walls 35. The partitions 35 essentially only contain openings which just allow the passage of the sintered parts 23.

The cooling area 4 is designed essentially in the same manner as described in DE-PS 197 19 203.3. The manner in which the sintered parts are tempered and cooled in this area is of no interest in the present context. Also shown in the drawing is a type of "muffle furnace" with a similar design to that used in debinding zone 3.

The sintering furnace described works as follows:

The pressed sintered parts 23 are placed in the inlet area 8 on the conveyor system T8, introduced by the latter through a simple gate 6 into the debinding zone 3 and taken over by the conveyor system T3 there. With the help of the radiant heat emitted by the heated partition walls 20, the binding aids are expelled from the sintered parts 23 and essentially removed. Since all inner surfaces in the debinding zone 3 are hot, there is no risk of "sooting up" of binding agents. The sintered parts 23 enter individually or in small groups of sintered parts 23 lying next to and / or one above the other through the first gate of the lock 7, which lies between the debinding area 3 and the sintered area 2, into the space between the two gates of this lock 7. The second gate of this lock 7 leading to the sintering area 2 remains closed. Alternatively, it is possible to keep this second door open continuously with a small gap and to operate the interior of the sintering area 2 of the sintering furnace 1 under a certain excess pressure. Then the gas atmosphere prevailing there, which will be discussed further below, can constantly leak out of the sintered area 2 into the space between the two gates of the lock 7 and flush this space.

After the group of sintered parts 23 has entered the lock 7, the first gate leading to the debinding zone 3 is closed and the interspace of the lock 7 is rinsed and / or pumped out. As already mentioned above, the sintered parts 23 are conveyed by a separate transport system T7, the speed of which can differ from the speed in the other areas of the sintering furnace in order to keep the overall system short.

After a certain dwell time within the lock 7, the gate of the lock 7 adjacent to the sintering area 2 opens. The sintered parts 23 are now transferred to the conveyor system T2 and transferred from this to a picking zone which, for example, passes through the first three zones of the Sintered area 2 extends through. In the other zones of the sintering area 2, the actual sintering takes place at a temperature between 560 and 620 ° C instead.

The temperature of the gas present in the individual zones is monitored in each case by a temperature sensor 40 (see FIG. 4) which is arranged in the vicinity of the movement path of the sintered parts 23 and which controls the heater 29 via a control circuit.

As already noted above, all zones of the sintered area 2 are essentially those shown in FIG

Set up in this way and filled with high-purity nitrogen as an inert atmosphere. The oxygen content in this inert atmosphere must not exceed a dew point of -40 ° C. In each zone of the sintered area 2, a circulating current is generated with the aid of a blower 31

Maintain nitrogen atmosphere, which, coming from below in the area of the left chamber 26, past the heat exchanger surfaces 28 of the respective heater 29, through the chamber 26 to the blower 31, from there into the chamber 27 and through the nozzle plate 34 onto the sintered parts 23 , These hot nitrogen gases flow around the sintered parts 23, penetrate the carrier plate 24 and the transport system T22 and are fed back from there to the heater 29, which closes the circuit.

Low leakage losses in the inert atmosphere within the zones of the sintered area 2 are compensated for by a corresponding fresh gas supply. The temperature at the inlet of the heater 29 should differ as little as possible from the temperature at the outlet of the heater 29. This is equivalent to the statement that the circulating nitrogen gases in the interior of the housing 22 have essentially the same temperature everywhere. In order to further improve the uniformity of the heating of the sintered parts 23, it is possible to have the flow direction of the nitrogen gases alternate in the individual zones of the sintered area 2. In particular, it is conceivable for the flow in the area of the sintered parts 23 to alternate from top to bottom and from bottom to top.

At the end of the sintering area 2, the sintered parts 23 pass through the lock 7, which comprises two gates and is located between the sintering area 2 and the cooling area 4, the same operations taking place as described above for the lock 7 located between the debinding area 3 and the sintering area 2 has been. In the cooling area 4 there is then a controlled cooling of the finished sintered parts to a temperature at which the sintered parts 23 exit the cooling area 4 via a further lock 7 and are finally removed in the outlet area 9 from the conveyor system T9 there or transported to another location can.

Claims

claims

1. A method for sintering aluminum-based sintered parts, in which the following steps are carried out in separate atmospheres in spatially separated areas:

a) the sintered parts are debindered;

b) the sintered parts are brought to the sintering temperature and held there for a certain time;

c) the sintered parts are cooled in a controlled manner,

characterized in that

in process step b) an inert gas is used as the atmosphere, the oxygen content of which corresponds to a dew point of at most -40 ° C, and that the sintered parts (23) are heated by circulating the correspondingly heated inert G Gaasseess to a sintering temperature of 560 to 620 ° C.

2. The method according to claim 1, characterized in that nitrogen is used as the inert gas.

3. Device for sintering aluminum-based sintered parts with

a) a debinding area in which the sanitary parts are freed by heating binding aids; b) a sintering area in which the sintered parts are subjected to a sintering process by heating to the sintering temperature and which has heating devices corresponding thereto;

c) a cooling area in which the sintered parts can be cooled in a controlled manner after the sintering process;

d) a transport system that continuously guides the sintered parts through the various areas;

e) locks which keep the atmospheres of the different areas separate and which have to be crossed by the sintered parts when leaving a certain area,

characterized in that

f) the atmosphere in the sintered region (2) is formed by an inert gas, the oxygen content of which

Corresponds to a dew point of at most -40 C;

g) the sintered area (2) has at least one heating device for the sintered parts (23) which comprises indirectly heated heat exchange surfaces (28), a blower (31) and an air guiding device (25) in such a way that a circulation flow flowing around the sintered parts (23) of the inert gas can be adjusted.

4. The device according to claim 3, characterized in that the inert gas is nitrogen.

5. Apparatus according to claim 3 or 4, characterized in that the inert gas has a temperature of 560 to 620 ° C.

6. Device according to one of claims 2 to 5, characterized in that the sintered area (2) has a plurality of zones delimited by partitions (35), each having a heating device with heat exchange surfaces

(28), blower (31) and air guide device (25).

7. The device according to claim 6, characterized in that the temperature of the inert gas is different in zones of the sintered region (2) lying one behind the other in the direction of movement.

8. Apparatus according to claim 6 or 7, characterized in that the flow around the sintered parts (23) in zones lying one behind the other in the direction of movement of the sintered region (2) is different.

9. Device according to one of claims 3 to 8, characterized in that a nozzle plate (34) is provided, via which the circulating inert gas is directed against the sintered parts (2).

10. Device according to one of claims 3 to 9, characterized in that the gates of the locks (7), the inlet and / or the outlet of the sintering area

(2) are adjacent, cannot be completely sealed and the inert gases in the sintered area (2) are under excess pressure.