PRODUCTION OF SPRAY DEPOSITS

Abstract: A method and apparatus for the production of spray deposited ingot, disc or bar is provided in which a spray of gas atomised molten metal or metal alloy particles generated with an atomising device is directed onto a collector. The collector is rotated about an axis of rotation and a controlled amount of heat is extracted from the atomised particles in flight and on deposition. In order to maintain a substantially constant distance between the atomising device and the surface of the deposited metal or metal alloy there is relative movement between the atomising device and the collector. The spray is directed so that the main axis of the spray and the axis of rotation of the collector are inclined at an angle to one another; and the spray is oscillated so that the main axis of the spray oscillates relative to the axis of rotation of the collector. By the method of the invention the shape of deposit is not dependent on the shape or physical containment of the collector ie, for example, a mould as used in a casting process is not required.

Description: This invention relates to method and apparatus for the production of spray deposited ingots, discs, billet or bar. At present a disc or ingot can be formed by directing a spray of gas atomized molten metal or metal alloy at a collector which is tilted at an angle to the spray axis in order to provide a more favourable angle of impingement of the atomised particles onto the already deposited metal. The collector is rotated and simultaneously oscillated and may be moved away from the spray to maintain a constant spray distance. For example, U.S. 4066117 discloses such an apparatus but in that arrangement it is essential that the collector is a mould which includes side walls. As clearly indicated in Figure 11 of that Patent, when the depth of the mould is exceeded, the dimensional control of the deposit is completely lost. Another problem, even if the deposit were to continue to be built up in the uncontrolled manner, would be that, as the collector is tilted at an angle, the centre of gravity of the deposit the collector so making the deposit unstable and reciprocation of the collector more difficult, if not impossible. DE-C-810223 discloses apparatus for forming generally thin deposits by means of a spray gun. The shape of the deposits are determined by an aperture in a screen through which the sprayed material passes prior to deposition. US-A-4064295 discloses apparatus for distributing liquid metal particles in a uniform layer across a substrate by deflecting the particles with a secondary gas stream. According to the present invention, a method for the production of spray deposited bar or ingot comprising the steps of generating a spray of gas atomized molten metal or metal alloy particles with an atomising device; directing the spray of particles onto a collector; rotating the collector about an axis of rotation; extracting heat from the atomized particles in flight and on deposition by means of the atomizing gas, directing the spray so that the main axis of the spray and the axis of rotation of the collector are inclined at an angle to one another, oscillating the spray so that the main axis of the spray oscillates relative to the axis of rotation of the collector; controlling the extraction of heat from the atomized particles and from the deposit forming on the collector such that the surface of the already deposited metal consists of a relatively thin layer of semi-solid/semi-liquid metal or metal alloy; moving the collector along its axis in order to maintain a substantially constant distance between the atomizing device and the surface of the deposited metal; and, determining the shape of the deposit without physical containment by controlling the oscillating movement of the spray, the rate of deposition and the rate of withdrawal of the collector along its axis. With this method it is possible to position a collector horizontally or vertically or any other direction. The invention also includes an apparatus for the production of spray deposited bar comprising; an atomizing device including an opening through which a stream of molten metal or metal alloy may be passed in use; means for passing an atomizing gas to the atomizing device; a plurality of atomizing jets in the atomizing device through which the atomizing gas may issue to generate a spray of particles from the stream of molten metal or metal alloy; a collector rotatable about an axis of rotation and positioned relative to the atomizing device to receive a spray of molten metal or metal alloy generated by the atomizing device; means for oscillating the spray across the surface of the collector or a deposit building up thereon, the atomizing device and the collector being so positioned relative to one another that the spray generated by the atomizing device and the collector are inclined at an angle to one another; and means far effecting relative movement between the atomizing device and the collector in order to maintain a substantially constant distance between the atomizing device and the surface of the metal or metal alloy being deposited upon the collector. The atomizing device is preferably a device including means movable relative to the stream of liquid metal from which the spray is formed whereby movement is imparted to the spray. The invention will now be described by way of example with reference to the accompanying diagrammatic drawings in which: Figure 1 illustrates one embodiment of the invention applied to the formation of bar; Figure 2 illustrates a second embodiment of the invention applied to the formation of bar; Figure 3 is an end view in the direction of arrow C in Figure 2; Figure 4 shows a further embodiment of the invention as applied to a disc or ingot; Figure 5 shows another embodiment of the invention as applied to a disc or ingot; and, Figure 6 illustrates a diagrammatic view of apparatus for moving the spray. In Figure 1 a collector (1) is rotatable about an axis of rotation (2) and is movable along said axis as indicated by the arrow A. An atomizing device (3) is positioned so as to be inclined to the axis of rotation (2) so that the spray of metal or metal alloy droplets created by the atomizing device (3) arrives at the surface of the collector at an angle to the axis of rotation. The atomising device 3 is arranged to tilt about an axis passing through the atomiser so that the main axis of the spray oscillates across the surface of the collector and the deposit building up thereon as indicated by the arrow B. As the deposit increases in size the collector 1 is withdrawn so that the distance between the surface of the deposit and the atomising device remains substantially constant. In order to key the deposit to the collector 1, the collector 1 is suitably formed with a central projection 4 (or depression) about which the initial layers of the deposit form. Moreover, as the deposit 5 grows in size, the deposit may be stablised by side stabilising devices 6 which include bearing rollers 7 to allow continued withdrawal as the deposit increases in size. After the initial support provided by the collector 1 the diameter or cross-sectional shape of the deposit, within limits, is substantially determined and controlled by the movements of the spray, the rate of withdrawal of the collector and the rate of deposition. Providing the metal or metal alloy being deposited is in the correct "state" at and on deposition i.e. the correct and controlled amount of heat has been extracted including the superheat and a large proportion or all of the latent heat, a mould to apply predetermined dimensions to the deposit as it builds up is not required. The heat extraction from the atomised particles before and after deposition occurs in three main stages: (i) in-flightcooling mainly by convective heat transfer to the atomising gas. Cooling will typically be in the range 10 -3 - 10 -6 °C/sec depending mainly on the size of the atomised particles. (Typically atomised particle sizes are in the size range 1-500 microns); (ii) on deposition, cooling both by convection to the atomising gas as it flows over the surface of the spray deposit and also by conduction to the already deposited metal; and (iii) after deposition cooling by conduction to the already deposited metal. It is essential to carefully control the heat extraction in each of the three above stages. It is also important to ensure that the surface of the already deposited metal consists of a relatively thin layer of semi-solid/semi-liquid metal into which newly arriving atomised particles are deposited. This is achieved by extracting heat from the atomised particles by supplying gas to the atomising device under carefully controlled conditions of flow, pressure, temperature and gas to metal ratio and by controlling the further extraction heat after deposition. If desired the rate of the conduction of heat on and after deposition may be increased by applying cold injected particles as disclosed in our European Patent Application published under No: 0198613. In addition a metal matrix composite bar, ingot or disc an be produced by incorporating metallic or non-metallic particles or fibres into the atomised spray. In Figure 2 a similar arrangement to Figure 1 is shown except that the collector 1 is positioned vertically as opposed to horizontally. This arrangement is preferable for continuous production methods but additionally requires stablising supports 8, similar to the stabilising devices 6 which hold the formed bar as it is withdrawn in the direction of arrow A in order to maintain the spray distance between the atomising device 3 and the surface of the deposit substantially constant. As seen from Figure 3 and the arrows included on the stabilising supports 8, the stabilising supports 8 are movable axially so that end bearing rollers 9 can accommodate surface irregularities without preventing continued withdrawal of the deposit. In Figure 4 a disc or ingot deposit 10 is formed on a collector 11 which is rotated under the spray 12 about an axis 13 transverse to the mean axis 14 of the spray. As with the embodiment of Figure 1, the spray 12 is oscillated as indicated by the arrow so as to scan the surface of the deposit as it is rotated about axis 13. As the deposit 10 builds up on the collector 11, the collector is retracted in an axial direction in order to maintain a substantially constant spray distance. The arrangement of Figure 5 is similar to that of Figure 4 except that the collector and the axis of rotation are inclined to a spray 15 generated so as to have a generally vertical mean axis 14. The shape of the deposit is again determined solely by the inter-relationship between the movement of the spray 15, the rate of deposition and the withdrawal of the collector and, after inital deposition, is not dependent in any way on the shape or physical containment of the collector (ie for example a mould as used in a casting process is not required). The oscillation of the spray in the embodiment is preferably achieved by oscillation of the atomising device itself. For example the atomising device may be as diagrammatically illustrated in Figure 6 and mounted at an inclined angle. In Figure 6 a metal steam 21 is teemed through an atomising device 22. The device 22 is generally annular in shape and is supported by diametrically projecting supports 23. The supports 23 also serve to supply atomising gas to the atomising device in order to atomise the stream 21 into a spray 24. In order to impart movement to the spray 24 the projecting supports 23 are mounted in bearings (not shown) so that the whole atomising device 22 is able to tilt about the axis defined by the projecting supports 23. The control of the tilting of the atomising device 22 comprises an eccentric cam 25 and a cam follower 26 connected to one of the supports 23. By altering the speed of rotation of the cam 25 the rate of oscillation of the atomising device 22 can be varied. In addition, by changing the surface profile of the cam 25, the speed of oscillation at any instant during the cycle of cam 25, can be varied. The oscillation can be of the order of 5 to 30 cycles per second for obtaining a particular desired shape to a deposit. Full details of the preferred apparatus may be obtained from our co-pending application filed herewith to which reference is directed. The oscillations of the spray are suitably a to and fro motion so that, as the collector rotates, a deposition pattern is created on the already deposited metal. If the speed of oscillation relative to the speed of collector rotation is kept low, the patterns can be made discernable by arranging for the oscillations per revolution to be in phase with the rotation of the collector. If the number of oscillatons is, say exactly four per revolution, a deposit with four axes of symmetry can be formed, for example square bar. Alternatively, the cross-section of the deposit may be effected by varying the speed of rotation and the spread of the oscillation of the spray such that the whole surface of the deposit is substantially covered at some time during the cycle by the main axis of the spray. The size of the deposit is determined as a function of the rate of withdrawal and the metal deposition rate. Although, the atomising conditions can be maintained substantially constant some variations may occur in practice. Accordingly, in order to maintain a constant size of bar, the diameter or cross-sectional area may be monitored and the speed of withdrawal varied to compensate for, for example, changes in metal flow rate. Whilst the invention has been particularly described with reference to moving the collector, it will be understood that it may be desirable alternatively to raise the atomising device instead in order to maintain a substantially constant spray distance. Moreover, for simplicity, the chamber in which spray deposition takes place has been omitted from all the Figures except Figure 1. In that Figure a chamber 30 is shown for providing a desired atmosphere such as, an inert atmosphere, atomising gas is exhausted at 31, and any overspray powder is collected at 32.

Claims: A method for the production of spray deposited bar or ingot comprising the steps of generating a spray of gas atomized molten metal or metal alloy particles with an atomizing device; directing the spray of particles onto a collector; rotating the collector about an axis of rotation; extracting heat from the atomized particles in flight and on deposition by means of the atomizing gas, directing the spray so that the main axis of the spray and the axis of rotation of the collector are inclined at an angle to one another, oscillating the spray so that the main axis of the spray oscillates relative to the axis of rotation of the collector; controlling the extraction of heat from the atomized particles and from the deposit forming on the collector such that the surface of the already deposited metal consists of a relatively thin layer of semi-solid/semi-liquid metal or metal alloy; moving the collector along its axis in order to maintain a substantially constant distance between the atomizing device and the surface of the deposited metal; and, determining the shape of the deposit without physical containment by controlling the oscillating movement of the spray, the rate of deposition and the rate of withdrawal of the collector along its axis. A method according to Claim 1 wherein the relative movement between the atomizing device and the collector comprises withdrawing the collector in the direction of said axis of rotation. A method according to Claim 1 wherein the extraction of a controlled amount of heat comprises in-flight cooling substantially by convection to the atomizing gas, on deposition cooling by convection to the atomizing gas as it flows over the surface of the deposit, and on deposition cooling by conduction, the extraction of heat, the relative movement effected between the atomizing device and the collector and the oscillating of the spray being controlled such that the surface of the already deposited metal consists of the layer of semi-solid/semi-liquid metal into which the newly arriving atomized particles are deposited. A method according to any of Claims 1 to 3 wherein the speed of oscillation of the spray is an integer multiple of the speed of rotation of the collector. A method according to Claim 4 wherein the integer multiple is four to form a deposit with four axes of symmetry, and the spread of the spray is controlled such that the deposit formed is a bar substantially square in cross-section. A method according to Claim 2 comprising varying the rotational speed of the collector within each revolution. Apparatus for the production of spray deposited bar comprising; an atomizing device (3; 22) including an opening through which a stream (21) of molten metal or metal alloy may be passed in use; means (23) for passing an atomizing gas to the atomizing device (3; 22); a plurality of atomizing jets in the atomizing device (3; 22) through which the atomizing gas may issue to generate a spray of particles (24) from the stream (21) of molten metal or metal alloy; a collector (1) rotatable about an axis of rotation (A) and positioned relative to the atomizing device (3; 22) to receive a spray of molten metal or metal alloy generated by the atomizing device (3; 22); means (23; 25; 26) for oscillating the spray across the surface of the collector (1) or a deposit building up thereon, the atomizing device (3; 22) and the collector (1) being so positioned relative to one another that the spray generated by the atomizing device and the collector are inclined at an angle to one another; and means for effecting relative movement between the atomizing device and the collector in order to maintain a substantially constant distance between the atomizing device and the surface of the metal or metal alloy being deposited upon the collector. Apparatus according to Claim 7 wherein the means for oscillating the spray comprises means for moving the atomizing device relative to the stream of molten metal from which the spray is formed to impart movement to the spray. Apparatus according to Claim 7 including means (6, 7; 8, 9) for stabilizing the deposit as it grows in size following deposition on the collector. Apparatus according to Claim 7, 8, or 9 wherein the collector is substantially horizontal. Apparatus according to Claim 7, 8, or 9 wherein the collector is substantially vertical. Apparatus according to any of Claims 7 to 11 wherein the means for effecting relative movement comprises means for withdrawing the collector relative to the atomizing head. Apparatus according to any of Claims 7 to 12 wherein the collector (1) includes a substantially central projection (4) on which the deposit is keyed.

Inventors: COOMBS, JEFFREY S., DAVIS, PAUL B., LEATHAM, ALAN G.
Applicants: OSPREY METALS LIMITED
Application year: 1986
Main classification: C23C4
IPC codes: C23C4, B22F9, B22F3, B22D23