EP1299206A1

EP1299206A1 - Method and device for atomizing molten metals

Info

Publication number: EP1299206A1
Application number: EP01984153A
Authority: EP
Inventors: Alfred Edlinger
Original assignee: Tribovent Verfahrensentwicklung GmbH
Current assignee: Tribovent Verfahrensentwicklung GmbH
Priority date: 2000-07-07
Filing date: 2001-07-06
Publication date: 2003-04-09
Also published as: AU2002218757A1; ZA200201752B; AT410640B; JP2004502037A; US20020134198A1; IL148383A0; WO2002004154A1; ATA11692000A

Abstract

The invention relates to a method for atomizing molten metals. According to said method, the liquid molten metal bath is atomized from a tundish using a propellant gas via an outlet with gas into a refrigerated chamber or onto a surface to be coated to compact the reduced particles. The liquid molten metal is introduced via an annular gap into the outlet, in which hot gas with a temperature of between 250 DEG C and 1300 DEG C and a supercritical pressure of between 2 and 30 bar is ejected concentrically in relation to the outlet, by means of a Laval nozzle. The hot gas is brought into contact with the molten metal bath by a component that is directed radially outwards, or by a torsion whose speed is greater than the speed of sound. The device for carrying out said method has a molten metal tundish (1), an immersion pipe (4) that is immersed in the molten metal (2) and that forms an annular gap that surrounds the outlet for the molten metal (2) and a lance (7) for ejecting the propellant gas. The lance (7), whose height can be adjusted bears a Laval nozzle (9).

Description

Method and device for atomizing molten metal

The invention relates to a method for atomizing metal melts, in which the liquid metal bath is sprayed from a tundish via an outlet opening with gas into a cooling space or with compacting the comminuted particles onto a surface to be coated with propellant gas, and to a device for carrying it out process.

In order to achieve dense coatings of metals, it has already been proposed to expel such metals from a molten bath by means of propellant gases onto a surface or a target to be coated, the molten droplets solidifying and impinging upon impacting the surface to be coated or the target (substrate) a corresponding compression or compacting of the coating is achieved in this way. When atomizing molten metals with propellant gases, an inert jet of propellant gas at ambient temperature is generally used, the known processes consistently requiring a relatively high amount of propellant gas and generally also a relatively high propellant gas pressure. A number of nozzle geometries have been proposed for atomizing and compacting such atomized metal particles. The economic viability of such processes was, however, regularly largely determined by the required amount of propellant gas and the required propellant gas pressure.

The invention now aims to provide a method of the type mentioned at the beginning with which it is possible to atomize liquid metals efficiently and with significantly smaller devices while substantially reducing the amount of propellant gas required, at the same time achieving a much finer atomization and Possibility to offer the possibility of installing further components in the atomized metal melt. To achieve this object, the method according to the invention essentially consists in the liquid metal melt being introduced into the outlet opening via an annular gap. is brought into which hot gas with temperatures of 250 ° C to 1300 ° C and a supercritical pressure between 2 and 30 bar is expelled concentrically to the opening via a Laval nozzle, and that the hot gas with a radial, outwardly directed component or with a swirl is brought into contact with the melt pool at a speed exceeding the speed of sound. Because hot gases are used at temperatures of 250 ° C to 1300 ° C and a supercritical pressure between 2 and 30 bar, which differs from the known processes, the viscosity of the propellant gas is increased significantly compared to known processes, which means that shear forces are more effective and a finer division of the molten metal into particularly small particles with a diameter dso of less than 10 μm can be achieved. At the same time, it is possible to reduce propellant gas consumption compared to the use of propellant gases with the usual low temperatures from 1/3 to 1/5, which results in significant advantages in terms of economy in metal atomization. Another advantage is that the metal melt does not freeze in the melt outlet due to the lower temperature difference. Characterized in that the liquid melt is introduced into the outlet opening through an annular gap, the possibility is created by appropriately adjusting this annular gap to influence the inflow of the liquid melt, and thus the amount carried through in the unit of time, in a simple manner and in that the propellant gas is now introduced concentrically to the outlet opening, the possibility is created to use the component which determines the annular gap as a second concentric tube, as a suction tube for the suction of further substances. The fact that the hot gas is brought into contact with the molten pool with a radial, outwardly directed component or with a swirl at a speed exceeding the speed of sound, which is achieved in particular by the fact that the ejection takes place under supercritical pressure via a Lavalduse small amount of propellant gas to transmit high shear forces, the rapid deceleration of the propellant gas jet, which is more viscous due to the higher temperatures Impact on the molten metal results in a particularly efficient and rapid division. Characterized in that the hot gas is expelled inside the melt jacket and brought into contact with the radially outward component with the melt pool, gas is forced to pass through the melt jacket and thus the melt jacket is torn open. A very important advantage here is the formation of monograin powder, the formation of which is promoted by the radial tearing open of the hollow cylindrical melt jacket. When the enamel jacket is torn open radially, uniform ligament formation occurs in the radial direction and, subsequently, extremely uniform droplet formation. The monograin powder is ideal for use in powder metallurgy.

The flow conditions of the hot gas flowing out via the Lavalduse can also be set in such a way that an underexpanded propellant jet is created. This results in pressure surges in the area of the Mach 'see nodes, with expansion volumes lying between such Mach' see nodes. Vibration interference in the jet causes shear stresses to be introduced into the melt droplets, the frequency being correspondingly increased under increasingly supercritical conditions, as a result of which the distance of the Mach 'see nodes in the axial direction of the propellant gas jet is correspondingly reduced. The fact that an underexpanded jet is ejected leads to an immediate expansion after exiting the nozzle. The distance to a surface to be coated can be chosen to be extremely short in such a configuration, so that the size can be found with small-scale devices. The hot gas is advantageously expelled here via a guide body, so that the effective outlet cross section of the Lavalduse can be adapted to the respective requirements by suitable adjustment of the guide body. The use of a guide body also serves to give the outflowing hot gas a corresponding additional, radially outward flow component and / or a swirl. The process according to the invention is advantageously carried out in such a way that a lance with the Lavalduse for the hot gas is guided concentrically in a tube with the formation of an annular space and that reactive gases such as CO, H2, 02 or H2O-Da pf, and / or inert gases, such as N2 or Ar, and / or carbides, such as. B. WC, TiC or VC, are sucked in. The lower edge of the tube surrounding the lance with the Lavalduse defines the required annular gap for the access of the liquid metal melt, and an annular space for the suction of reactive gases and / or inert gases is simultaneously formed between the lance and the tube. Such an embodiment enables a preferred method of operation, in which metal powder or additives such as SiC, Al2O3 or Y2O3 and / or carbides are added to the gas stream which is sucked in, as a result of which, with a particularly simple, constructive design of the device, a high degree of adjustability of the atomization method to different needs is ensured.

The radiant heat of the molten metal ejected with the hot propellant gas, which is effectively atomized during the ejection, can be used to heat the hot gas, for which purpose the hot gas is preferably heated in a heat exchanger surrounding the ejected melt particles.

Because of the use of hot gas, particularly small particles are formed, as already mentioned at the beginning, and in addition to a downward flow in the cooling room, a vortex-like outward flow of very fine particles is formed. These fine particles are again sucked into the downward stream of the atomized melt and serve there, in part, for the rapid cooling of the atomized melt. In order to reduce the proportion of the very fine particles which are effective for cooling but partially prevent efficient particle size reduction, and in particular to ensure that such fine particles do not fall within the range of If the outlet opening or mouth of the tundish leads to growth, it is advantageously carried out in such a way that fine particles of the solidified melt rising in the cooling space are sucked off below the mouth of the melt stream and discharged via a lock. The possibility of sucking in additional solid materials, such as silicon carbide, Al2O3 or Y2O3, as fine powder via the annulus, also enables metal-matrix compound materials and ceramic-metal composite materials, and thus particularly wear-resistant coatings, to be achieved. Compared to complex, discrete spray nozzles, all of these tasks can be taken into account with a single Lavalduse with a downstream guide body, through which only the hot propellant gas is expelled, with a significantly lower fuel requirement, for which purpose only individual adjustment of the pipe is available Adjustment of the annular gap and an appropriate choice of the sucked gases is required. In addition, the desired jet geometry can be influenced in a simple manner by a corresponding axial adjustability of the hot gas nozzle or the guide body or by a corresponding exchange of the guide body and can be adapted to the selected substances. Overall, the method according to the invention enables efficient atomization of all possible metal melts, alloys and in particular ferro alloys such as FeV, FeCr, FeW, FeTi or FeMo also being atomized.

According to a preferred procedure, a pressure of 1.5 to 25 bar can be maintained in the tundish, preferably a pressure of 1.5 to 10 bar being maintained in the cold room. By observing these pressure values, a melt saturated with compressed gas can be achieved, argon, for example, being used as the compressed gas. The melt saturated with compressed gas leads to easier disintegration, so that overall a finer atomization is possible. The gas can be introduced using tundish floor nozzles or using an immersed lance. The device according to the invention for carrying out this method has a melt tundish and an immersion tube which plunges into the melt to form an annular gap surrounding the outlet opening for the melt, a lance also being provided for the ejection of propellant gas. The device according to the invention is essentially characterized in that the height-adjustable lance carries a Laval nozzle, preferably in or in the flow direction, a guide body is arranged in a height-adjustable manner after the widening mouth region of the Laval nozzle, the clear cross-section between the nozzle and the Guide body in the axial direction towards the outlet end is increasingly larger than the narrowest cross section of the Lavalduse. The guide body provided in or in the direction of flow next to the widening mouth area of the Lavalduse can be adjusted by its height adjustability to minimize the consumption of propellant gas. To achieve the desired supersonic speed, it is only necessary to ensure that the clear cross-section between the inner wall of the Lavalduse and the guide body in the axial direction to the outlet end is always larger than the narrowest cross section of the Lavalduse and is increasingly formed in the axial direction. However, the arrangement of a guide body is not absolutely necessary, and it has been shown that even without the guide body, efficient atomization can be achieved, with particularly good results being achieved if, as is a preferred development of the device according to the invention, the lance below the lower edge of the Dip tube opens into the outlet opening of the tundish. The lance is height adjustable for this purpose.

In order to achieve a corresponding annular space for the suction of additional components, the design is advantageously made such that the outside diameter of the lance is smaller than the inside diameter of the dip tube and the lance is sealingly guided through a cover of the dip tube and that a line for the supply of gases or / or reactive metal powder and / or additives opens into the space of the immersion tube surrounding the lance. An adjustable throttle valve can be provided in the line for the supply of gases and / or reactive metal powder so that, if necessary, the space between the lance and the immersion tube can be kept under a corresponding negative pressure, as a result of which pulsating flows can also be achieved. The valve can also remain completely closed.

The guide body is advantageously designed as a cone with guide surfaces arranged on the jacket. A pronounced radial component can be achieved with such a guide body if, as is in accordance with a preferred embodiment, the guide surfaces are curved in an S-shape and end in the direction of the circumference at the same angle to the tangent of the base circle of the conical body.

The invention is explained in more detail below on the basis of an exemplary embodiment schematically shown in the drawing of a device suitable for carrying out the method according to the invention.

In FIG. 1, 1 denotes a melt tundish shown in cross section, in which a metal bath 2 is held in a molten state. In order to keep this metal bath molten, inductive heating, as is indicated schematically by the windings 3, can be provided.

A tube 4 is immersed in the metal bath and defines an annular gap between the bottom of the tundish 1 and the lower edge of the tube. This tube 4 is adjustable in the height direction in the direction of the double arrow 5, so that the amount of metal bath flowing out of the tundish 1 in each time unit can be regulated in a simple manner. The tube 4 is closed with a cover 6, in which a lance 7 is sealingly guided in the direction of the double arrow 8 and is adjustable in the height direction. The lance 7 has a Laval nozzle 9 at its outlet end for hot gas. If hot gas is supplied under supercritical conditions, this design as a Laval nozzle in the narrowest cross-section of the Laval nozzle 9 results in exactly the speed of sound, the supersonic speed being achieved in the subsequent widening cross-section due to the rapid expansion. In this widening area, a guide body 10 is now arranged, which is adjustable via a corresponding linkage 11 in the direction of the double arrow 12, also in the axial direction. The beam shape can thus be influenced by appropriate adjustment of the guide body, it only being necessary to ensure that the respective effective cross section widens correspondingly in the axial direction following the narrowest point of the Lavalduse 9, so that the rapid expansion supersonic speed is achieved.

The propellant gas jet from the lance 7 now arrives in a subsequent cooling space 13, in which, for example, a target 14 can be arranged. The propellant gas jet collides with the supersonic speed and corresponding viscosity due to its high temperature with the outflowing metal bath, so that rapid and efficient comminution takes place, which can be applied to the target 14 as a coating. If such a target 14 is not installed, the correspondingly comminuted metal powder can be drawn off via a lock 15 at the lower end of the cooling chamber 13. The radiant heat of the solidifying metal droplets can be used in a heat exchanger 16 surrounding the cooling chamber, to which cold gas is supplied via a line 17 and from which hot gas is withdrawn via a line 18. If the temperature achieved in this way is sufficient for the desired purposes, this hot gas can be fed directly to the lance 7 via the line 18. A further heating can be carried out using conventional Recuperative heat exchanger, not shown, can be achieved.

In the interior of the cooling chamber 13, a ring line 19 can also be seen, through which fine particles can be suctioned off. These finest particles can be fed to a classifier 21 via line 20 and discharged as fine powder via a lock 22. The amount of very fine powder discharged thus no longer reaches the downward flow and thus has no influence on the solidification behavior of the droplets comminuted by the propellant gas jet.

The lance 7 is now guided, leaving an annular space 23 at a distance from the inner wall of the tube 4. Additional material can be drawn into this annular space via a line 24, with reactive gases such as CO, H2, N2, 02 or, in the event that partial oxidation of the metal particles is desired, also sucking in H2O vapor. The amount drawn in can be determined by an adjustable throttle valve 25. A series of powdery materials that can flow with a gas flow can also be sucked into this line as doping from a storage container 26. Metal powder, Sie, Al2O3 or also Y2O3 can primarily be sucked in as dispersible solids and can be introduced via line 24 into the annular space 23, from which they are sucked in via the hot gas flow and brought into rapid and intensive contact with the molten metal.

2 shows a modified embodiment of the propellant gas lance, in which the lance 7 opens below the lower edge of the dip tube 4 in the outlet opening of the tundish 1. The lance has a Laval nozzle 9, it being possible to dispense with the arrangement of a guide body. Experiments have shown that the deeper the propellant nozzle is pushed into the melt outlet, the better the atomization results. Inert gases, such as nitrogen, argon and helium, are primarily considered as propellant gases, but depending on the task, reactive gases such as CO, H2, optionally mixed with water vapor, can also be used if oxidative atomization is desired.

Al, Cu, Fe, Ni, Co, Ti, Mg or melts of rare earth metals or their alloys, in particular Co-based superalloys, can be used as the metal melts. The powders obtained are particularly suitable for use in sintering or powder metallurgy, for example for hot isostatic pressing, but also as feed material for MIM processes (metal injecting molding).

Claims

claims:

1. A process for atomizing metal melts, in which the liquid metal bath is sprayed from a tundish through an outlet opening with gas into a cooling room or with compacting the comminuted particles onto a surface to be coated with propellant gas, characterized in that the liquid metal melt is sprayed over an annular gap is introduced into the outlet opening, into which hot gas with temperatures of 250 ° C to 1300 ° C and a supercritical pressure between 2 and 30 bar is expelled concentrically to the opening via a Lavalduse, and that the hot gas with a radially outward component or with a swirl is brought into contact with the melt pool at a speed exceeding the speed of sound.

2. The method according to claim 1, characterized in that the hot gas is expelled via a guide body.

3. The method according to claim 1 or 2, characterized in that a lance with the Lavalduse for the hot gas is guided concentrically in a tube to form an annular space and that reactive gases such as e.g. CO, H2, 02 or H2O vapor, and / or inert gases, e.g. N2 or Ar, and / or carbides, such as. B. WC, TiC or VC, are sucked in.

4. The method according to claim 3, characterized in that reactive metal powder or additives such as e.g. SiC, Al2θ3, or Y2O3 can be abandoned.

5. The method according to any one of claims 1 to 4, characterized in that the hot gas is heated in a heat exchanger surrounding the ejected melt particles.

6. The method according to any one of claims 1 to 5, characterized in that ascending fine particles of the solidified melt in the cooling space are sucked off below the mouth of the melt flow and discharged via a lock.

7. The method according to any one of claims 1 to 6, characterized in that a pressure of 1.5 to 25 bar is maintained in the tundish.

8. The method according to any one of claims 1 to 7, characterized in that a pressure of 1.5 to 10 bar is maintained in the cold room.

9. Device for performing the method according to one of the

Claims 1 to 8 with a melt tundish (1) and an immersion tube immersed in the melt (2) to form an annular gap surrounding the outlet opening for the melt (2)

(4) and a lance (7) for the discharge of propellant gas, characterized in that the height-adjustable lance (7) carries a Laval nozzle (9).

10. The device according to claim 9, characterized in that a guide body (10) is arranged height-adjustable in or in the flow direction subsequent to the widening mouth region of the Lavalduse (9), the clear cross section between the nozzle (9) and the guide body (10 ) in the axial direction to the outlet end is increasingly larger than the narrowest cross section of the Lavalduse (9).

11. The device according to claim 9 or 10, characterized in that the lance (7) opens below the lower edge of the dip tube (4) in the outlet opening of the tundish (1).

12. Device according to one of claims 9 to 11, characterized in that the outer diameter of the lance (7) is smaller than the inside diameter of the dip tube (4) and the lance (7) sealingly by a cover (6) of the dip tube ( 4) and that a line (24) for the supply of gases and / or reactive metal powder and / or additives opens into the space of the immersion tube (4) surrounding the lance (7).

13. Device according to one of claims 10 to 12, characterized in that the guide body (10) is designed as a cone with guide surfaces arranged on the jacket.

14. The apparatus according to claim 13, characterized in that the guide surfaces are curved S-shaped and end directed in the circumferential direction with the same angle to the tangent of the base circle of the conical body.