GB2117417A

GB2117417A - Producing high-purity ceramics- free metallic powders

Info

Publication number: GB2117417A
Application number: GB08307970A
Authority: GB
Inventors: Franz-Georg Knell; Otto Stenzel; Rolf Ruthardt; Ernst Weingartner
Original assignee: Leybold Heraeus GmbH
Current assignee: Balzers und Leybold Deutschland Holding AG
Priority date: 1982-03-31
Filing date: 1983-03-23
Publication date: 1983-10-12
Also published as: GB2117417B; SE8301165L; SE8301165D0; GB8307970D0; DE3211861A1; JPS58177403A; FR2524356A1

Abstract

Ceramic free metal powder is produced by forming a metal melt in container 16 from consumable arc electrode 15 and controlling the heat balance in the container so that a solidified metal skull is formed on the inside wall of the container the molten metal overflows from the container at 18 and is atomised by a gas stream at 19. The overflow may be heated by plasma flame 27.

Description

SPECIFICATION Method and apparatus for producing highpurity ceramics-free metallic powders The invention concerns a method and apparatus for producing high-purity ceramics-free metallic powders by atomization of a melt by means of a stream of gas and subsequent solidification of the resulting droplets.

Methods of the above kind are often referred to as "metal-spraying methods". The prior art practice involves passing the melt from an induction furnace having a ceramic lining to the atomizing means by way of a pouring funnel which is usually also made of ceramic material.

The ceramic lining of the induction furnace as well as the ceramic pouring funnel can result in the formed powder becoming contaminated with ceramic particles, so that the quality of the metallic powder is adversely affected in a decisive manner.

For the purpose of producing ceramics-free metallic powders, use has previously been made of the centrifugal method for comminution of the metallic melt. In this process the starting material takes the form of ingots, which are freed from ceramic constituents by being remelted in a vacuum arc furnace, an electron-beam melting furnace or an electro-slag remelting installation.

The ingots concerned are brought to the molten state again, and the dripping or trickling molten material is directed onto what is called a centrifuging dish, on which it was broken down into very fine droplets by the action of centrifugal forces (DE-OS 25 28 999).

Although metallic powders of high purity can be produced by methods of this kind, there arises in practice the disadvantage of the low specific throughput of the apparatus, combined with coarser particles as compared with those occurring in gas-spraying.

It is also known to use an ingot as the consumable electrode and to establish between the ingot and a rotating watercooled drum an arc which melts away the ingot. This method also results in a wide range of particle sizes, and the specific throughput of the apparatus is extremely low.

Combination of the electron-beam melting method with the metal-spraying process is unsuccessful because, in practice, it is not possible, during metal spraying, to establish a sufficiently high vacuum at reasonable cost, because of the need for continuously supplying atomizing gas. A vacuum that is as complete as possible is, however, necessary for spreading the electron beams.

Nor has a combination of the metal-spraying method with a plasma-melting process been found to be applicable, since, because of the unfavourable efficiency and energy balance, it has been necessary to install equipment providing uneconomically high plasma-melting performance.

The object of the present invention is, therefore, to provide a method of the initially described kind whereby, despite the involvement of an atomizing process and the necessary large quantities of molten material supplied per unit of time, it becomes possible to produce powders which are free from ceramic particles.

According to the invention, there is provided a method of producing high-purity ceramics-free metallic powders by atomization of a melt, which comprises producing and maintaining the melt in a melt container by means of an arc electrode, controlling the heat balance of the melt container to form a solidified layer of metal in the container, allowing the melt to flow freely down over an overflow on the melt container, atomizing the flowing melt below the overflow by means of a stream of gas, and subsequently solidifying the resulting droplets to form a powder.

Compared with electron-beam melting, melting by means of an arc electrode can be carried out under atmospheric pressure so that the considerable quantities of atomizing gas do not interfere with the melting process. The establishment of a solidified coating of metal on the inner wall of the melt container effectively prevents interaction between the melt and the melt container. The solidified layer of metal is often referred to as the "skull". Although a melting process of this kind is known in principle, it has previously been used only for the production of precision castings in a vacuum.

The thickness of the solidified coating of metal depends upon the heat balance of the melt container, i.e. upon the quantity of heat supplied by the arc process, on the one hand, and upon the quantity of heat removed by a cooling medium, on the other. Thus, by controlling the quantities of heat supplied and removed, it becomes possible to influence the thickness of the solidified layer of metal.

In this method it is important that the molten metal, before coming into contact with the material of the melt container, should flow down over an overflow, i.e. by way of the highest point of the molten pool, and should be atomized below the overflow. The quantity of heat supplied, which also determines the fusion rate, can be varied within wide limits so that the melting capacity necessary for a high throughput can be fully established by the method of the invention. Metal spraying also constitutes a process whereby high atomization rates can be achieved. A wide range as regards the variation in particle size becomes possible. The particle-size range can be narrowed or widened depending upon the parameters of the method; the top limit of the particle-size range can also be varied.An additional advantage is constituted by the favourable cooling properties, associated with metal spraying, which result in extremely rapid solidification of the melt.

Expressed in simple terms, the invention consists of a combination of the "skuli-melting method" and the metal-spraying method.

Particular advantage accrues if the overflow is heated by a plasma burner to regulate its output in such manner that the flow of molten metal is kept substantially constant. This step influences two characteristic factors of the melt, namely its viscosity and the surface tension. By applying heat to a lesser or greater extent, it is possible to hold back the melt or to accelerate its flow. This of course assumes that melt flows in sufficient quantity, a process which again can be influenced by regulating the electric arc heating. Apart from this, however, the quantity of melt within the melt container constitutes a certain buffer volume, since a meniscus, as it were, is formed above the highest point of the overflow, and the magnitude of the meniscus is dependent upon surface tension.If the overflow is then heated to a greater extent, this balance is intentionally influenced and, in fact, in the sense of causing the melt to flow away in a greater quantity per unit of time.

Furthermore, the overflow is kept free of melt that may be starting to solidify.

The invention also concerns apparatus for performing tne method of the invention, such apparatus comprising a melt and atomization chamber and, within said chamber, a melt container designed as a liquid-cooled melt crucible having an overflow, an arc electrode heating means arranged above the melt container for forming and maintaining a melt in the container, an atomization diffuser arranged below the overflow for atomizing the freely falling melt from the overflow, and a cooling portion for solidifying the resulting droplets; and means for collecting the powder.

Arc electrodes that can be used are permanent electrodes (graphite electrodes, water-cooled metal electrodes), as well as-and as referred what are called consumable electrodes, which are made of the same metal as the powder to be produced.

Advantageously the melt and atomization chamber has, in its lower part, a lateral extension which lies in the same direction as the overflow and encompasses the cooling portion, and a powder-receiving container is arranged at the end of the extension.

What are known as enclosed electric-art furnaces, in which the melting process is carried out in an inert gas atmosphere, are generally constructions the main axis of which extends in the vertical direction. A further feature of the invention therefore mainly consists in providing such a furnace, extending substantially vertically, with an extension which is disposed in the lateral, i.e. horizontal, direction. This horizontal part also includes the cooling zone for the metallic powder which solidifies relatively rapidly on account of the high gas pressure.

In this cooling zone, the metallic particles spread out in a substantially horizontal direction, but in the manner of a trajectory parabola having a very pronounced lateral component. This lateral component is important as regards the functionability of the apparatus of the invention, since this constructional feature means that the vertical dimension of the entire apparatus does not require to be increased. In this system it has to be taken into account that, in the known metal-spraying methods wherein gas emerging at high velocity from a circular-slot diffuser blows the metallic particles mainly in a downward direction, it is necessary to provide which is called a "well", which considerably increases the vertical dimension of such apparatus.

An embodiment of apparatus for performing the method of the invention will now be described by reference to the accompanying schematic drawing.

The apparatus comprises a melt and atomization chamber 1 which encloses a furnace head 2, the upper and median part of which is substantially cylindrical. The chamber consists of an upper part 3 and a lower part 4, which parts are interconnected by a releasable flanged joint 5 for charging purposes. As illustrated, the upper part and lower part of the chamber are of doublewalled construction and are provided with pipes 6, 7 for supplying a cooling medium and pipes 8, 9 for discharging it. The upper part 3 of the chamber is secured to a pivoting column, not illustrated, and can therefore be swung out to the side.

An electrode rod 11 extends into the upper part of the chamber through a slide packing ring 10 and is connected to a current supply unit by a lead 12. Secured to the lower end of the electrode rod 11 is a clamping device 13 in which is releasably fitted an electrode stub 14 in a shape-locking manner. The electrode stub 14 is connected by welding to an electrode 15 which is of the consumable kind.

Located below the electrode 1 5 is a melt container 16 which is connected by a lead 17 to the other pole of the currant supply means. The melt container is designed as a double-walled liquid-cooled crucible which, at one side, is provided with an overflow 18 in the form of what is known as a pouring lip. During the melting process an arc burns between the electrode 1 5 and the molten metal within the melt container 16, and this arc keeps the molten metal in the liquid state and continuously causes fusion of the material of the electrode 1 5. As already described above, a layer of solidified material, also known as the "skull", is formed between the molten metal and the inner wall of this melt container. This layer (not shown in the drawing) prevents contact between the molten metal and the melt container.

The metal flowing downwardly from the overflow then moves to a position in front of an atomization diffuser 19 which is connected by a pipe 20 to a source of pressurised gas, not illustrated. If a stream of gas were not provided, the metal flowing over the overflow 18 would move along immediately in front of the front face of the atomization diffuser 19. However, suitable pressurisation of the atomization diffuser 1 9 causes the stream of metal to be broken up into extremely fine droplets, which are flung away to the right (as seen in the drawing) in a substantially horizontal direction.

For this purpose, a lateral extension 21 is provided on the lower part 4 of the chamber and in the lower zone thereof; the double-wall of the extension is connected to the cooling system, and a cooling zone 22 is present in the extension. It will be seen that the extension 21 is disposed more or less in the same direction as the overflow 1 8 and in that of the flow of the gas emerging from the atomization diffuser 19. At the far end of the extension 21 its lower delimiting wall is formed as a funnel 23 at the lower end of which a powder-collecting container 25 is attached by way of a lock 24.

Arranged at the point where the lower part 4 of the chamber joins the lateral extension 21 is a plasma burner 26 which is fitted in the chamber wall and is directed towards the overflow 18 so that the latter can be heated by the plasma flame 27.

Claims

1. A method of producing high-purity ceramics-free metallic powders by atomization of a melt, which comprises producing and maintaining the melt in a melt container by means of an arc electrode, controlling the heat balance of the melt container to form a solidified layer of metal in the container, allowing the melt to flow freely down over an overflow on the melt container, atomizing the flowing melt below the overflow by means of a stream of gas, and subsequently solidifying the resulting droplets to form a powder.

2. A method according to Claim 1, wherein the overflow is heated by a plasma burner to regulate its output in such manner that the flow of melt is kept substantially constant.

3. Apparatus for performing the method according to Claim 1, comprising a melt and atomization chamber and, within said chamber, a melt container designed as a liquid-cooled melt crucible having an overflow, an arc electrode heating means arranged above the melt container for forming and maintaining a melt in the container, an atomization diffuser arranged below the overflow, for atomizing the freely falling melt from the overflow, and a cooling portion for solidifying the resulting droplets; and means for collecting the powder.

4. Apparatus according to Claim 3, wherein the lower portion of the melt and atomization chamber has a lateral extension in the same direction as the overflow and which encompasses the cooling portion, and the collecting means comprises a powder-receiving container arranged at the end of the extension.

5. Apparatus according to Claim 3, wherein the melt and atomization chamber includes a plasma burner which is directed towards the overflow.

6. A method of producing high-purity ceramics-free metallic powders by atomization of a melt according to Claim 1, and substantially as hereinbefore described.

7. Apparatus for producing high-purity ceramics-free metallic powders by atomization of a melt, constructed, arranged and adapted to operate substantially as hereinbefore described with reference to the accompanying drawing.