ATOMISATION OF METALS

Abstract: A device for gas atomising a liquid stream, such as a stream of molten metal or metal alloy, has an atomising de­ vice including, for example, an annular opening for receiving the stream. The atomising device is arranged for applying atomising gas to the stream so as to form a spray of atomised particles. At least a part of the atomising gas, and preferably all, is applied by means movable relative to the stream whereby movement is imparted to the spray. This movement leads to improved uniformity or control of deposition.

Description: This invention relates to a device for gas atomising a liquid stream, such as a stream of molten metal or metal alloy. The atomising and spray depositing of a stream of liquid metal has been known for many years, for example from British Patent Specification No: 1262471, and our own British Patent Specification Nos : 1379261 and 1472939. However, it has always been a problem to achieve precise control of the mass deposition in the metal on the deposition surface. One proposal to improve the control of the mass distribution of the deposited layer of gas atomised of metal is set out in British Patent Specification No: 1455862 where it is proposed to oscillate the spray of atomised particles by the use of a primary set of gas jets for atomisation and two sets of secondary jets which are rapidly switched on and off to impart an oscillatory motion to the spray of atomised metal. However, it was found that the arrangement did not give ideal control of the mass distribution of the metal deposited. Therefore, an alternative proposal for imparting a direction to a spray was suggested as diclosed in European Patent Publication No: 0127303A. That arrangement involves the switching on and off of individual gas jets which accomplish the function of both atomizing and oscillating the spray. However, both these methods are very difficult to control, and in particular lack flexibility in operation. In the first proposal, the use of secondary jets can result in excess cooling of the deposited metal meaning that subsequently arriving particles do not coalesce properly with the already deposited metal. In the second method the shape and properties (eg. temperature) of the spray can change as individual jets are switched on and off which makes it extremely difficult to ensure uniform deposition and solidification conditions. In DE-A-2043882 there is disclosed a movable atomizing device which is used to distribute liquid metal within a cooled casting mould. An object of the present invention is to provide an improved device for gas atomizing a liquid stream, such as a stream of molten metal or metal alloy and for imparting controlled and precise movements to the atomized liquid stream. According to the present invention, there is provided apparatus for gas atomizing a liquid stream such as a stream of molten metal or metal alloy, comprising: an atomising device for applying atomizing gas to the stream for atomizing the stream into a spray of droplets, said atomizing device being angularly movable about a fixed axis; and means for moving the atomizing device about said axis and relative to the stream whereby the application of said movement may impart movement to the spray characterized by the provision of control means controlling movement of the atomizing device whereby the atomizing device may undergo a predetermined cycle of movements for imparting oscillatory movement to the spray of deoplets, the control means controlling the angle of oscillation, rate of oscillation and speed of oscillation during each cycle of oscillation. The improved method of the present invention does not involve the switching on and off of gas jets to oscillate the spray. Instead, despite the proximity to the nozzle from which molten metal issues, we have devised a system whereby the spray is moved by moving the atomizing jets themselves or the whole atomizing device. This has the following particular advantages over previous method: (a) on average the atomizing conditions can be kept relatively constant because gas jets are not being switched on and off, ie. the atomizing conditions may be the same or otherwise controlled regardless of the degree of movement of the spray; (b) the movement imparted is preferably an oscillation and the angle of oscillation can be changed very easily merely by increasing the angle of tilt of the whole or part of the atomiser during each cycle; (c) the rate of oscillation can be easily varied; and (d) the speed of oscillaton at any instant during each cycle of oscillation can be easily varied. Consequently, the apparatus and method of the present invention provides a very high degree of control over the atomising device and the movement of the spray which previously has not been attainable. This enables the oscillation conditions to be varied to suit the shape of deposit being produced or to control the deposition conditions and/or the profile of the spray on the surface of the collector. In one form of the method of the invention the liquid stream is molten metal or metal alloy, the spray is directed at a substrate moving continuously through the spray and the spray is moved transverse to the direction of movement to achieve uniformity of thickness of deposition across the width of the substrate whereby strip, coated strip, plate or coated plate products may be formed. The invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a perspective diagrammatic view of a preferred apparatus; Figure 2 illustrates diagrammatically the mode of movement of the atomising device and hence the movement imparted to a spray; Figure 3 is a plan and side elevation of a preferred atomiser; Figure 4 is sectional side elevation of the atomiser; Figure 5 is a diagrammatic perspective view of the invention as applied to the manufacture of strip; Figure 6 is a diagrammatic perspective view of the invention as applied to the coating of strip; and Figure 7 is a diagrammatic view of an alternative atomising device where only part of the device is movable. In Figure 1 of the drawings a liquid stream 1, such as molten metal or metal alloy, is teemed through an atomising device 2. The device 2 is generally annular in shape and is supported by diametrically projecting supports 3. The supports 3 also serve to supply atomising gas to the atomising device in order to atomise the stream 1 into a spray 4. In order to impart movement to the spray 4 the projecting supports 3 are mounted in bearings (not shown in Figure 1) so that the whole atomising device 2 is able to tilt about the axis defined by the projecting supports 3. The control of the tilting of the atomising device 2 comprises an eccentric cam 5 and a cam follower 6 connected to one of the supports 3 as will be explained. By altering the speed of rotation of the cam 5 the rate of oscillation of the atomising device 2 can be varied. In addition, by changing the surface profile of the cam 5, the speed of oscillation at any instant during the cycle of oscillation can be varied. The oscillation typically can be up to 30° from the stream axis although the movement may not necessarily be centered on the stream axis, this will depend upon the shape of the deposit being formed. From Figure 2 it can be seen that the atomising device 2 comprises a plenum chamber 7 and a plurality of gas atomising means consisting of nozzles 8. In the preferred embodiment the whole atomising device 2 is tiltable as indicated by Figure 2 so that, as it is tilted the gas issuing from the nozzles 8 imparts lateral movement to the spray. Figures 3 and 4 illustrate a preferred embodiment of the invention in more detail. In those Figures an atomising device 10 is positioned within an atomiser housing 11 and below the nozzle opening 12 of tundish 13. The atomising device 10 includes a plenum chamber 14 and has atomising gas jet openings 15. The atomising device 10 is substantially annular in shape having a central opening 16 through which a stream 17 from the tundish 13 is arranged to pass. The atomising device is supported within the housing 11 by diametrically opposed supports 18, 19 which project outwardly from the atomising device 10 and is positioned sufficiently away from the bottom of the tundish 13 and has a central opening 16 dimensioned so that the atomising device may be made to undergo a tilting motion. So that this tilting motion may be achieved the supports 18, 19 are mounted within respective bearings 20, 21 in the atomiser housing 11. One of the supports 18, also serves as a conduit 22 to supply atomising gas to the plenum chamber 14. The movement of the atomising device 10 is effected by mechanical means consisting of a drum cam 23 rotated by drive means (not shown) and, a cam follower 24 pivoted at 25 and held against the cam profile by means of a pneumatic cylinder 26. The cam follower 24 has a connecting arm 27 pivoted to it at 28 and the arm 27 extends to a further pivotal connection 29 on a plate 30. The plate 30 is freely movable and is fixed to the support 19, as clearly shown in Figure 4, at a position offset from the pivotal connection 29. Accordingly, it will be understood that movement of the drum cam 23 is translated into movement of the atomising device 10 via the cam follower 24, connecting arc 27 and plate 30. The cam profile may be designed to define a predetermined degree of movement and the speed of rotation of the drum cam, which may be readily controlled in a known manner by an electric motor, the speed of movement of the atomising device. Movement of the atomising device, suitably a to and fro oscillatory movement, imparts a corresponding movement to the spray since the atomising device 10 carries with it the atomising gas jet openings 15. The atomising device of the present invention is particularly useful for producing strip or plate 31 as illustrated in Figure 5. Also, the apparatus may be used for producing spray coated strip or plate products 32 as shown in Figure 6. In producing these products the spray is moved to and fro at right angles to the direction of movement of a collector 33 moving continuously through the spray as indicated by the arrows in the Figures. This ensure that the deposit 34 is formed uniformly across the width of the collector, or substrate, preferably in the thickness range 0.5mm - 50mm. Preferably the substrate or collector will pass a plurality of atomising devices aligned along the axis of the movement of the substrate. In respect of coated strip or plate 31 the substrate to be coated may suitably be unwound from a decoiler 35 diagrammatically illustrated in Figure 6. Although the present invention is particularly suitable for forming strip, plate and coated strip and plate it will be understood, that the atomiser can be used beneficially for producing many other products including ingots, bars, tubes, rings, rolls, conical shapes forging and extrusion blanks, spray coated products, laminates, composites, and products for thixotropic deformation etc. The substrate or collector may be an flat substrate, an endless belt or a rotatable mandrel. The formation of strip will now be described by way of example: EXAMPLE OF STRIP PRODUCTION: WIDTH = 300mm DEPOSITED MATERIAL 0.15% CARBON STEEL POURING TEMP. 1580 degrees centigrade METAL POURING NOZZLE 9.0mm bore SPRAY HEIGHT 630mm (ie Distance from the underside of the atomiser to collector) OSCILLATING SPEED 10 cycles/sec OSCILLATING ANGLE 13° about a vertical axis ATOMISING GAS Nitrogen COLLECTOR 0.5mm thick X 300mm wide X 1000mm length mild steel plate - grit blasted. COLLECTOR MOVEMENT 40mm/sec LIQUID METAL FLOW RATE INTO ATOMISER 58kg/min GAS/METAL RATIO 0.3 Kg/Kg DEPOSIT THICKNESS 8mm STRIP PRODUCTION: WIDTH = 155mm DEPOSITED METAL 0.15% CARBON STEEL POURING TEMP. °570° Centigrade METAL POURING NOZZLE 9.0mm bore SPRAY HEIGHT 630mm OSCILLATING ANGLE +/-7 degrees about a vertical axis OSCILLATING SPEED 10 cycles/sec ATOMISING GAS Nitrogen COLLECTOR 0.5mm x 155mm wide x 1000mm length mild steel plate COLLECTOR MOVEMENT 60mm/sec LIQUID METAL FLOW RATE INTO ATOMISER 60kg/min GAS/METAL RATIO 0.35 Kg/Kg DEPOSIT THICKNESS 10mm In the present invention the spray cone generated by the atomising device is always maintained and the gas jets which, in prior inventions, were used to impart an oscilation to the spray, are used merely for atomisation. Not all the jets need necessarily be moved. For example in Figure 7 an atomising device 40 is substantially square shaped in plan and comprises pairs of opposed atomising jets 41, 42. Atomising jets 41 are movable so as to move a spray, formed by passing a liquid stream through the centre of the device 40, in a to and fro direction indcated by arrow 43. However, opposed jets 42 are fixed to provide side curtains of gas which keep the oscillating spray within confined lateral limits. As an alternative the atomising gas means may simply be a single gas opening such as an annulus. Whilst the invention has been particularly described with reference to the atomisation of liquid metal streams, the invention may be applicable to the atomisation of other liquid streams such as liquid ceramics or liquid stream or spray into which solid metallic or non-metallic particles or fibres are injected or incorporated. Also, whilst the present invention has been described with reference to mechanical control means, preferred methods for controlling the movement of the atomiser may be electro-mechanical means such as a programme controlled stepper motor, or hydraulic means such as a programme controlled electro-hydraulic servo mechanism using a linear actuator to control oscillation movement. The above devices can also be used for producing gas atomised metal powders whereby the movement of the spray can impart improved cooling to the atomised particles.

Claims: Apparatus for gas atomizing a liquid stream (1;17) such as a stream of molten metal or metal alloy, comprising: an atomizing device (2;10) for applying atomizing gas to the stream for atomizing the stream (1;17) into a spray of droplets, said atomizing device (2;10) being angularly movable about a fixed axis (3); and means (3,6;18,19,24) for moving the atomizing device (2;10) about said axis (3) and relative to the stream whereby the application of said movement may impart movement to the spray characterized by the provision of control means (5;23) controlling movement of the atomizing device whereby the atomizing device may undergo a predetermined cycle of movements for imparting oscillatory movement to the spray of droplets, the control means controlling the angle of oscillation, rate of oscillation and speed of oscillation during each cycle of oscillation. Apparatus according to claim 1, wherein the control means (5;23) comprises a movable cam the surface profile of which defines said predetermined cycle of movements, and a cam follower (6;24) connected to the atomizing device (2;10) by mechanical connection (3;18,19). Apparatus according to claim 1, wherein the control means is selected from mechanical means comprising a co-operating cam and cam follower, electro-mechanical means comprising a programme controlled stepper motor, or hydraulic means comprising a programme controlled electro-hydraulic servo mechanism. Apparatus according to claim 1, wherein the atomizing device (2;10) is annular and includes a plurality of atomizing jets (8;15) for applying the atomizing gas. Apparatus according to claim 4, wherein the atomizing device (2;10) is supported at diametrically opposed positions and includes a plenum chamber (7;14) supplied with atomizing gas through at least one of said opposed supports (3;18,19).

Inventors: COOMBS, JEFFREY S.
Applicants: OSPREY METALS LIMITED
Application year: 1986
Main classification: C23C4
IPC codes: B22D23, B22F9, C23C4, B22F3, B22D27