WO2002022295A1

WO2002022295A1 - Method for producing a moulded body from foamed metal

Info

Publication number: WO2002022295A1
Application number: PCT/DE2001/003477
Authority: WO
Inventors: Robert F. Singer; Carolin KÖRNER
Original assignee: Neue Materialien Fürth GmbH
Priority date: 2000-09-13
Filing date: 2001-09-11
Publication date: 2002-03-21
Also published as: DE10045494A1; EP1318884B1; DE50109977D1; JP2004508202A; EP1318884A1; ATE327851T1; DE10045494C2

Abstract

The invention relates to a method for producing a moulded body from foamed metal comprising the following steps: a) preparing a first powder consisting of the metal and a second powder consisting of a foaming agent b) feeding the first and the second powder to an extruder device, the first and second powder having a non-compact form c) transporting a powder mixture of the first and second powders in the extruder device towards a mould, whereby said mixture is partially melted and the partially melted powder mixture is then subjected to a pressure that is greater than the gas pressure caused by the foaming agent d) injecting the at least partially melted powder mixture into the mould, and e) relaxing the pressure to a value that is less than the gas pressure, so that the mould can be filled completely with the generated foamed metal.

Description

Process for producing a shaped body made of metal sheath

The invention relates to a method for producing a molded body from metal foam.

EP 0 804 982 A2 discloses a process for producing molded parts from metal foam, in which a compact mixture of gas-releasing propellant and metal powder is used as the starting material. Such a compacted mixture is usually produced by extrusion. The compacted mixture can be in the form of rods, tubes or as granules. To produce a metal foam, the compacted mixture is heated in a heatable chamber until the metal melts and the blowing agent decomposes. The released gas foams the metal. The metal foam formed is then pressed from the chamber into a mold with a piston.

DE 197 34 394 AI discloses a method in which a compact semi-finished product is also used to manufacture a molded part from a metal foam. According to an alternative, non-foamable metal can also be used as the starting material and gas or a blowing agent can be added separately to the melt to form a metal show.

From DE 42 06 303 Cl it is known to use a compacted starting material for the production of molded articles from metal foam, which is produced by a continuous extrusion process and then comminuted.

DE 197 44 300 AI discloses another method for producing a molded body formed from a metal foam. This turns a gas-releasing blowing agent and a semi-finished product made of a metal powder is heated in a recipient. The metal foam that forms arrives in a downstream casting mold.

The aforementioned methods disadvantageously require the costly and time-consuming production of a compact starting material or semi-finished product. This usually requires a special device, e.g. an extrusion press.

DE 1 164 102 discloses a further method for producing molded articles from metal foam. A molten metal is fed to a mixer and mixed there with a gas-generating substance. The method requires the provision of a separate melting device to produce the molten metal.

A method for producing molded bodies from a metal foam is known from US Pat. No. 5,865,237. A heatable chamber is provided which is connected to a casting mold. A compacted mixture which is accommodated in the heatable chamber and is produced by powder metallurgy and contains the metal and a blowing agent is heated until a metal foam is formed. The metal foam is then pressed into the mold.

The structure of the shaped body depends on the filling of the chamber, on the quality of the mixture of the powder with the blowing agent and on the heating of the powder in the chamber. These parameters cannot always be set optimally and reproducibly. In order to lose as little blowing agent as possible and work economically, it is cheap to heat up quickly. On the other hand, rapid heating leads to an uneven temperature distribution and an uneven cell or pore structure of the molded body. The components produced using the known method have irregularities in their structure. 5

The object of the invention is to eliminate the disadvantages of the prior art. In particular, a method is to be specified by means of which molded articles made of metal foam of constant L0 quality can be produced in the simplest and cheapest possible way.

This object is solved by the features of claim 1. Appropriate configurations result from the features of claims 2 to 19. L5

According to the invention, a method for producing a molded body from metal foam is provided, comprising the following steps:

20 a) providing a first powder formed from the metal and a second powder formed from a blowing agent,

b) feeding the first and second powder to an extrusion device, the first and second powder being in non-compacted form,

c) conveying a powder mixture formed from the first and the second powder in the extrusion device in

30 direction of a mold, wherein it is at least partially melted and a pressure is applied to the at least partially melted powder mixture, which is greater is as a gas pressure caused by the propellant,

d) injecting the at least partially melted powder mixture into the casting mold and

e) Relaxing the pressure to a value less than the gas pressure, so that the mold is completely filled with a metal foam that forms.

In the method according to the invention, non-compacted powder is fed directly to an extrusion device, mixed therein and at least partially melted. The costly and time-consuming step of producing a compacted starting powder is avoided. The provision of a special device for producing the compacted mixture is omitted.

With the proposed method, thorough mixing of the at least partially melted powder mixture is achieved. A pressure can also be applied in the extrusion device without great additional effort, which counteracts an undesired premature decomposition of the blowing agent in the extrusion device. The heating of the powder mixture can be precisely regulated along the conveying path. By releasing the at least partially melted powder mixture into the casting mold only after it has left the extrusion device, the formation of the metal foam is shifted into the casting mold. The formation of an uneven cell or pore structure is avoided. The moldings produced can be produced with consistent quality. The first powder expediently has an average grain diameter in the range from 50 to 250 μm, preferably from 100 μm. The second powder can have an average grain diameter in the range from 5 to 20 μm, preferably from 10 μm. According to a further design feature, it is provided that the first and the second powder are mixed before being fed to the extrusion device. The first and / or second powder can be fed to the extrusion device under an inert gas atmosphere. This prevents undesired oxidation of the powder. The quality of the components and their reproducibility are increased.

The first and the second powder are expediently mixed in the extrusion device under the action of shear forces. The mixing and / or conveying of the powder mixture can be carried out by rotating a screw of the extrusion device. In this case, shear forces are also applied to the at least partially melted powder mixture. Undesired growth of dendritic crystals is avoided. An advantageous one

The speed of rotation of the screw is about 100 revolutions per minute.

The powder or the powder mixture is advantageously continuously heated along a conveying path extending from a feed opening in the direction of an antechamber. Along the conveying path, the powder mixture is at least partially converted into a foamable melt in a single process step. Both the powder mixture and the at least partially melted are thereby

Powder mixture constantly mixed. Furthermore, the powder mixture is conveyed in the direction of an at least partially melted powder mixture on receiving antechambers. The process Ren can be carried out inexpensively using a single extrusion device. The separate production of a compacted starting material by extrusion, the subsequent subsequent crushing of the extruded semi-finished product and the transfer of the compacted starting material into the extrusion device are eliminated.

In the extrusion device, the powder mixture is heated to a temperature above the solidus temperature. The melt is advantageously heated in the extrusion device to a temperature of at most 50 ° C., preferably 20 ° C., above the liquidus temperature. It has proven to be particularly advantageous to heat the powder mixture to a temperature in the range between the solidus and the liquidus temperature. In this case, the powder mixture is only partially melted. The at least partially melted powder mixture can have a solid phase content of 20 to 50%, preferably 30 to 40%. The at least partially melted powder mixture is advantageously accumulated in a semi-solid thixotropic state in the antechamber. In the partially melted state, the viscosity is significantly increased compared to the completely melted state. This advantageously ensures that a closed melting front is formed when it is injected into the casting mold. This means that undesired atomization of the material injected into the casting mold and premature escape of the gas formed by the blowing agent are avoided. Because of the pressure release in the mold, the injected material is completely melted. The release of the gas formed from the propellant is carried out when a powder mixture that is only partially melted is injected. but delayed. In this case, moldings with a homogeneous foam structure are formed, ie the formation of giant bubbles is prevented in particular.

In order to keep the resulting gas pressure as low as possible, it is advisable to use a pre-oxidized blowing agent as the blowing agent.

The powder mixture can be fed into the extrusion device by means of an external heating device, e.g. by means of external heating tapes or an induction device. Such a heating device enables a precise adjustment of the heating rate of the powder mixture in the extrusion device. In this way, premature decomposition of the blowing agent and thus expansion of the melt can be avoided. Furthermore, the temperature can be adjusted so that undesired growth of dendritic metals is avoided. The build-up of a suitable pressure can be controlled via known mechanical and process engineering measures. A pressure of more than 10 bar, preferably more than 30 bar, is expediently applied in the extrusion device from a temperature of more than 300 ° C.

The at least partially melted powder mixture can be injected by an axial movement of the screw in the direction of the casting mold. The extrusion device is expediently provided with a mechanical valve for optionally opening and closing an antechamber downstream of the screw. Suitable devices are known for example from US 5,040,589 or EP 0 409 966 B1, the disclosure content of which is hereby incorporated. According to another design feature, the mold is preheated. This prevents the melt from solidifying too quickly in the contact area with the casting mold. To carry out the proposed method, magnesium or a magnesium alloy is expediently used as metal 5.

According to a further design feature, it is provided that a mold cavity enclosed by the casting mold after the

L0 spray is enlarged. In this case, a plunge edge tool is expediently used as the casting mold. With such a tool, at least one wall of the casting mold can be moved in the manner of a stamp and the size of the mold cavity can thus be changed.

L5

Magnesium, aluminum, a magnesium or aluminum alloy can be used as the metal. A metal hydride, preferably TiH ₂ or MgH ₂ , can be used as the blowing agent. The proportion of the blowing agent in the total weight of the powder is

20 usually carries 0.5% by weight.

An exemplary embodiment of the invention is explained in more detail below with reference to the drawing. Show here:

1 shows a schematic cross-sectional view of a device suitable for carrying out the method according to the invention,

Fig. 2 is a sectional view of a first molded body and

30 Fig. 3 is a sectional view of a second molded body. An injection molding machine, generally designated by the reference number 1 in FIG. 1, has a feed hopper 2 which is suitable for receiving granules or powder. The granulate or powder is conveyed to a feed opening 3 of an extrusion cylinder 4 via a conveying device (not described in more detail).

In order to avoid oxidation of the supplied material, the conveying device and the feed funnel 2 can be flushed with L0 inert gas. The inert gas can be e.g. Argon or nitrogen can be used.

A screw 5 accommodated in the extrusion cylinder 4 is rotatable and axially movable. The screw 5 has a spindle-L5-shaped circumferential wing 6. The free end of the screw 5 is designated by the reference number 7. The extrusion cylinder 4 has a nozzle 8 at its outlet end. The nozzle 8 opens into a gate of a two-part casting mold 9. It can be connected by means of a valve (not shown here)

20 locks will be. The two mold halves of the casting mold 9 form a mold cavity 10.

The opposite end of the screw 5 is connected to a high-speed injection apparatus 11 known per se. This has an accumulator 12 and a cylinder 13 which is accommodated in a fixed bearing 14, 16. A shot or injection ram 15 is arranged downstream of the cylinder 13 and extends into a back pressure bearing of a clutch 17. This can be done in a manner known per se

30 a connection to a drive shaft 18 can be made, so that the injection plunger 15 can only move back and forth when required, but not rotate. The drive shaft 18 extends in a conventional manner through a rotary drive 19. This allows a horizontal reciprocating movement of the drive shaft 18 depending on the movement of the injection plunger 15. The drive shaft 18 is coupled to the screw 5 via a drive coupling 20 in a known manner in order to transmit the rotary movement to the screw 5. In the same way, an axial movement can be transmitted to the screw 5.

Example 1: A powder is added via the feed hopper 2. The powder consists of a Mg alloy, eg type AZ 91. This powder has an average grain diameter of 100 μm. It is mixed with a MgH ₂ powder with an average grain diameter of 10 μm, which serves as a blowing agent. The blowing agent has a proportion of 0.5% by weight of the powder.

The premixed powder is conveyed through a conveyor under an inert gas atmosphere, e.g. Argon gas, conveyed to the feed opening 3 of the extrusion cylinder 4. Due to the action of the screw 5, the powder mixture is moved further in the direction of the nozzle 8. At the same time, the powder mixture is increasingly heated and pressurized. The powder mixture is heated e.g. by external heaters, e.g. Heating tapes.

The screw 5 is rotated at about 100 revolutions per minute. The powder mixture is heated with an increasingly closer distance to the nozzle 8 to a temperature above the solidus temperature of 465 ° C. The pressure in the prechamber designated by reference number 21 in the extrusion cylinder 4 is more than 30 bar. It can be up to 500 or 1000 bar. Shortly before the injection, the temperature is about 20 ° C higher than the liquidus temperature of the alloy, which is 596 ° C. The powder mixture is therefore in the completely melted state. The melt is mixed homogeneously. The pressure acting on the melt in the prechamber 21 is greater than the gas pressure generated by the blowing agent at the aforementioned temperature. The melt does not foam up.

As soon as there is enough melt in the prechamber 21, the nozzle 8 can be opened. At the same time, the free end 7 on the screw 5 is shot in the direction of the nozzle 8 via the high-speed injection device 11. The melt enters the mold cavity 10. The pressure there relaxes. The gas pressure is greater than the ambient pressure. The melt foams suddenly and completely fills the mold cavity 10. The mold halves of the casting mold 9 can advantageously be preheated.

Example 2: The first powder used is an alloy made from 99% aluminum and 1% magnesium, which contains a small amount of silicon. The first powder has an average grain size of approximately 100 μm. TiH ₂ with an average grain size of approximately 10 μm is used as the second powder. The first and the second powder are conveyed to the feed opening 3 of the extrusion cylinder 4 under an inert gas atmosphere, for example argon gas. The powder mixture is kneaded intensively along the conveying path extending from the feed opening 3 to the pre-chamber 21 and by external heating devices to a temperature of approximately 20 ° C. below the

Liquidus temperature, in this case about 630 ° C, heated. An argon pressure of 100 bar is applied. This melts the powder mixture partially. The melt has a solid phase content of about 35%.

As soon as the pre-chamber 21 is filled with the partially melted powder mixture, the nozzle 8 is opened. The partially melted powder mixture is injected into the mold in the thixotropic state. The pressure in the casting mold is released to 26 bar.

FIG. 2 shows a cut-away microscopic view of a molded body produced by the aforementioned method. The molded body has a bubble-free edge zone. Because the material is injected in the semi-solid thixotropic state, uncontrolled foaming of the melt in the casting mold is avoided. The pore structure is homogeneous.

Example 3:

The procedure is as in Example 2. In the casting mold, however, the pressure is released to 11 bar.

FIG. 3 shows an incident light microscope image of a cross section through a shaped body produced in this way. The length of the sample is again 20 mm. It can be seen that the edge zone is again essentially free of bubbles. The pores formed in the interior of the molded body are homogeneously distributed. However, their average pore size is larger than that of the shaped body shown in FIG. 2. This is attributed to the fact that the melt has been expanded here against a smaller counter pressure. LIST OF REFERENCE NUMBERS

1 injection molding machine

2 feed hoppers

3 feed opening

4 extrusion cylinders

5 snail

6 wings

7 free end

8 nozzle

9 mold

10 mold cavity

11 High speed injection equipment

12 accumulator

13 cylinders

14 bearings

15 injection plungers

16 bearings

17 clutch

18 drive shaft

19 rotary drive

20 drive coupling

21 antechamber

Claims

claims

1. A method for producing a molded body from metal foam with the following steps:

a) providing a first powder formed from the metal and a second powder formed from a blowing agent,

b) feeding the first and second powder to an extrusion device (4, 5, 6, 7), the first and second powder being in non-compacted form,

c) conveying a powder mixture formed from the first and the second powder in the extrusion device (4, 5, 6, 7) in the direction of a casting mold (9), wherein it is at least partially melted and pressure is applied to the at least partially melted powder mixture which is greater than a gas pressure caused by the propellant,

d) injecting the at least partially melted powder mixture into the casting mold (9), and

e) Relaxing the pressure to a value less than the gas pressure, so that the casting mold (9) is completely filled with a metal foam that forms.

2. The method according to claim 1, wherein the first powder has an average grain diameter in the range from 50 to 250 μm, preferably from 100 μm.

3. The method of claim 1 or 2, wherein the second powder has an average grain diameter in the range of 5 to 50 microns, preferably from 10 microns.

4. The method according to claim 1, wherein the first and the second powder are mixed before being fed to the extrusion device (4, 5, 6, 7).

5. The method according to any one of the preceding claims, wherein LO the first and / or second powder is fed to the extrusion device (4,5,6,7) under an inert gas atmosphere.

6. The method according to any one of the preceding claims, wherein the first and the second powder in the extrusion device

L5 (4,5,6,7) is mixed under the action of shear forces.

7. The method according to any one of the preceding claims, wherein the mixing and / or conveying of the powder mixture by the rotary movement of a screw (5) of the extrusion device

20 (4,5,6,7) is carried out.

8. The method according to any one of the preceding claims, wherein the powder mixture along a from a feed opening (3) in the direction of a prechamber (21) extending conveyor

25 way is continuously heated.

9. The method according to any one of the preceding claims, wherein the powder mixture in the extrusion device (4,5,6,7) to a temperature of at most 50 ° C, preferably 20 ° C, above

30 heated to half the liquidus temperature.

10. The method according to any one of the preceding claims, wherein the at least partially melted powder mixture has a solid phase content of 20 to 50%, preferably from 30 to 40%.

11. The method according to any one of the preceding claims, wherein the at least partially melted powder mixture in a semi-solid thixotropic state is accumulated in the prechamber (21).

L O

12. The method according to any one of the preceding claims, wherein pre-oxidized blowing agent is used as blowing agent.

13. The method according to any one of the preceding claims, wherein L5 the powder mixture in the extrusion device (4,5,6,7) is heated by means of an external heating device.

14. The method according to any one of the preceding claims, wherein in the extrusion device (4,5,6,7) from a temperature of

20 more than 300 ° C, a pressure of more than 10 bar, preferably more than 30 bar, is applied.

15. The method according to any one of the preceding claims, wherein the injection of the at least partially melted powder

25 mixed is caused by an axial movement of the screw (5) in the direction of the casting mold (9).

16. The method according to any one of the preceding claims, wherein the casting mold (5) is preheated.

30

17. The method according to any one of the preceding claims, wherein a mold cavity enclosed by the casting mold (9) is enlarged after the injection.

18. The method according to any one of the preceding claims, wherein magnesium, aluminum, a magnesium or aluminum alloy is used as the metal.

19. The method according to any one of the preceding claims, wherein a metal hydride, preferably TiH ₂ or MgH ₂ , is used as blowing agent.