Pack plating process

Abstract: A crucible used in a powder pack plating process has an aperture below the upper surface of the powder pack to allow a density driven flow of the plating gas generated by the powder pack to move through the powder pack.

Description: This invention relates to a pack plating process, particularly a pack aluminising process. A pack plating process is a process where the surfaces of objects are plated with metal by heating them with a metalising powder pack. A conventional pack aluminising process is shown diagrammatically in figure 1. An object to be aluminised, for example a gas turbine blade 1, is placed in a powder pack 2 formed by a shallow open topped tray 3 containing a quantity of aluminising powder 4. This is carried out by putting the blade horizontally on top of a layer of aluminising powder 4 in the tray 3 and then adding further aluminising powder 4 to cover the blade 1. The blade 1 is laid horizontally in order to minimise the total mass of the powder pack 2 and the thickness of the powder 4 around the blade 1 in order to minimise the thermal response time of the powder pack 2. The aluminising powder 3 is a mixture of metallic aluminium, a volatile halide and a refractory bulking agent such as aluminium oxide. The powder pack 2 is then placed inside a retort 5 which is sealed apart from an inlet port 6 and an outlet port 7 at the bottom and top of the retort 5 respectively. Argon gas is pumped into the retort 5 through the lower inlet port 6. Argon is denser than air and so displaces the air within the retort upwards and out of the upper outlet port 7. When all the air has been flushed out of the retort 5 a flow of argon is maintained and the retort 5 is heated. This heating causes the metallic aluminium and the volatile halide to react to produce aluminium halide gas within the aluminising powder 3, where this gas contacts the blade 1 it decomposes, depositing a layer of aluminium on the surface of the blade 1. The aluminium halide gas is denser that argon or air and so it displaces the argon and any air trapped in the powder from the tray 3. It is essential to purge the air from the retort 5 because the aluminium halide gas is a powerful reducing agent and would decompose on contact with the oxygen in the air. There is a problem with such a system. If the object to be aluminised has narrow holes in it, such as cooling air channels in a gas turbine blade, the aluminium halide gas tends not to penetrate very far down them and as a result the inner surfaces of such holes can prove to be unplateable or plateable only by keeping the powder pack in a heated retort for an unacceptable length of time. A particular prior art pack plating process is described in British Patent 933,200, which discloses a process for the diffusion of a first metal into an article made of a second metal. The article is placed in a powder mixture within an enclosure which is then sealed. The powder mixture contains a proportion of the first metal. The sealed enclosure is heated at a temperature above the melting point of the first metal so as to bring about diffusion of plating gas through the powder and enclosure. The enclosure is provided with a non-return valve to permit gas and vapour to be forced out of the enclosure during the heating process but to prevent their ingress into the sealed enclosure. Technological background prior art which we are aware of comprises US patent 1,893,782 and 4,347,267 and British Patent 770,109. US Patent 1,893,782 describes a process for coating cast iron with eg aluminium by enclosing an iron casting in a chamber together with aluminium powder and a halogen compound such as bleaching powder (CaOCl) or ferric chloride and heating so as to cause the halogen compound to react with the aluminium to form gaseous aluminium chloride. The aluminium chloride diffuses through the chamber and contacts the cast iron where is dissociates to coat the casting with aluminium. US Patent 4,347,267 effects diffusion coating of the internal surface of a workpiece accessible only through a very narrow passageway by coating that surface with a uniform layer of particles of the metal to be diffused into it, and then holding the thus-prepared workpiece at diffusion-coating temperature in a diffusion coating atmosphere. British Patent 770,109 produces a metal chromium coating on a metal article by treatment with a gaseous chromium halide. The article is placed in a sealed container containing a non-oxidising gas, a refractory material impregnated with a chromium halide, and metallic chromium. On heating, gaseous chromium halide is produced and dissociates on contact with the metal article to produce a chromium coating. According to the present invention there is provided a a pack plating process of the kind in which an object (18) to be plated is buried in a pack of plating powder (1O) held within a crucible (11) which is heated during the step of plating to generate a plating gas, characterised in that, throughout the plating step the plating gas exits the crucible, at a point below the object to be plated, as a continuous density driven flow through the plating pack. Where the object (8) to be plated has a channel (9) passing through it the object (8) is arranged within the pack (10) so that a portion of the density driven gas flow passes through the channel (9) during the plating step. Pack plating systems embodying the invention will now be described by way of reference only, with reference to the accompanying diagrammatic figures, in which: Figure 2 shows a cross section through plating apparatus employing a first crucible and a first process according to the present invention, Figure 3 shows a cross section through plating apparatus using the crucible of Figure 1 and a second process according to the present invention, Figure 4 shows a perspective view of a second crucible according to the present invention, and Figure 5 shows a cross section through the crucible of Figure 4 used in a first process according to the present invention, similar parts having the same reference numerals throughout. Referring to figure 2 a gas turbine blade 8 having an internal cooling passage 9 running lengthways through it The blade 8 is placed vertically on top of a layer of conventional aluminising powder 10 in a crucible 11. More aluminising powder 10 is then added to cover the blade 8. The crucible 11 has a hole 13 in its base and when all of the aluminising powder 10 has been added a lid 14 is fitted over the top of the crucible 11. The crucible 11 is then placed in a retort 15 which is sealed apart from an inlet port 16 and an outlet port 17 at the top and bottom of the retort 15 respectively. The retort 15 is then flushed with argon pumped into the retort 15 through the lower inlet port 16. This displaces the air within the retort 15 out through the upper outlet port 17. After this initial flushing a flow of argon is maintained through the retort 15. The retort 15 is then heated so that the aluminising powder 10 reacts to generate an aluminium halide plating gas. The aluminium halide gas produced is denser than both air and argon and as result it flows downward through the aluminising powder and out through the hole 13 in the base of the crucible 11. The lid 14 increases this flow by reducing the amount of aluminium halide gas escaping from the upper surface of the aluminising powder 10. The aluminium halide gas escaping through the hole 13 is entrained in the argon flow through the retort 15 and is carried with this argon flow out of the upper outlet port 17. The aluminium halide gas flowing through the aluminising powder 10 decomposes on contact with the surface of the turbine blade 10 and deposits a layer of aluminium on it. Additionally, as the aluminium halide gas flows downwards through the aluminium powder 10 some of it flows into and along the internal cooling passage 9 within the turbine blade 8. The aluminium halide gas flowing along the internal cooling passage 9 decomposes on contact with the walls of the internal cooling passage 9 and deposits a layer of aluminium on them. If the grain size of the aluminising powder 10 or the grain sizes of any of its constituents are equal to or smaller than the width of the internal cooling passage 9 a problem can arise due to the aluminising powder 10 entering the cooling passage 9. Any grains of the aluminising powder 10 inside the cooling passage 9 may stick together or to the walls of the cooling passage 9 when the retort 15 is heated and form an obstruction in the cooling passage 9. In order to prevent this the arrangement shown in figure 3 is used. In this a gas turbine blade 8 is placed on top of a first block 18 of a porous refractory material on the bottom of the crucible 11. A second block 19 of a porous refractory material is then placed on top of the blade 8. After this the process is carried out in the same way as in the previous example, aluminising powder is poured into the crucible 11 to cover the blade 8 and the second block 19, and a lid 14 is placed on the crucible 11. The crucible 11 is then placed in a retort 15 which is flushed with argon and then heated. The first and second blocks 18 and 19 are porous and so allow the aluminium halide gas to flow downwards through the aluminising powder 10 and through the cooling passage 9. This allows the aluminising process to operate as before, but the blocks 18 and 19 prevent the aluminising powder 10 getting inside the cooling passage 9 because the aluminising powder 10 cannot pass through them. In order to make efficient use of retort space and simplify handling it is useful to simultaneously aluminise a number of blades 8 in a single crucible 20, as shown in figures 4 and 5. The crucible 20 is circular in the shape of an annular trough having a circular central aperture 21. There are a plurality of holes 13 evenly spaced around the bottom of the crucible 20, and an annular lid 22 fits over the top of the crucible 20. The crucible 20 is shaped as an annulus to minimise its mass and thermal response time and so speed the aluminising process. In use, a plurality of turbine blades 8 are placed on top of a layer of aluminising powder 10 on the bottom of the crucible 20. More aluminising powder 10 is then poured into the crucible 20 to cover the blades 8 and the annular lid 22 is placed on top of the crucible 20. The crucible 20 is then put into a retort 15 as before and the retort 15 is flushed with argon and then heated. The aluminium halide gas produced flows down through the aluminising powder 10 and cooling passages 9 as before, the only difference being that it leaves the crucible 20 through a plurality of holes 13 instead of only one. It has been found that by using these techniques, as well as allowing plating of the insides of relatively narrow holes, the rate of plating of the outside of objects placed in the aluminising powder 10 can be increased, for a given temperature over time profile. It is believed that it is because in the prior art aluminising process air trapped between the grains of the aluminising powder will not be displaced by argon unless the powder pack is allowed to stand in the argon atmosphere within the retort for a very long time. Generally waiting for such a long time will make an industrial plating process unacceptably slow and as a result when the prior art powder pack is heated the aluminium halide gas produced immediately comes into contact with this trapped air and reacts with the oxygen in the air, destroying the aluminium halide gas and so reducing the amount of aluminium halide gas which comes into contact with the object to be plated. Using a crucible with a hole in its bottom, the aluminium halide gas produced, being denser than either air or argon, will flow downwards and drive any trapped air or argon out of the hole. As a result any trapped air is rapidly removed so no oxygen remains within the powder pack to react with and reduce the concentration of the aluminium halide gas. It is not essential to use a lid on top of the crucible. However, if a lid is not used the aluminium halide gas produced toward the top of the aluminising powder tends to diffuse upwards and into the argon above the aluminising powder and is then entrained by the argon gas flow through the retort and carried away. As a result it has been found that in order to produce the same aluminium halide gas concentration around the object a greater depth of aluminising powder must be used above it. This increases the bulk and thermal mass of the powder pack, both of which increases are undesirable, and so it is preferred to use a lid. In practice a number of crucibles 11 or 20 may be simultaneously used in a single retort. This invention can be applied to any pack plating process, such as boronising or siliconising as well as aluminising, by use of appropriate plating powder mixtures. Other gasses than argon could be used for purging, providing that they did not react undesirably with the plating powder or plating gasses evolved. Although the processes above are described using separate retorts and crucibles, it would of course be possible to use a crucible which is integral with a retort, or to place a crucible in a controlled atmosphere furnace.

Claims: A pack plating process of the kind in which an object (8) to be plated is buried in a pack of plating powder (10) held within a crucible (11) which is heated during the step of plating to generate a plating gas, characterised in that throughout the plating step the plating gas exits the crucible, at a point below the object to be plated, as a continuous density driven flow through the plating pack. A packing plating process according to claim 1 wherein the object (8) to be plated has a channel (9) passing through it and the object (8) is arranged within the pack (1) so that a portion of plating gas passes through the channel (9) during the plating step. A pack plating process according to claim 2 wherein at least one end of the channel (9) is covered with a porous refractory member (8) so as to prevent plating (11) from entering the channel (9). A pack plating process according to any preceding claim wherein the top of the crucible (11) is maintained closed throughout the plating step. A pack plating process according to any preceding claim wherein the crucible (11) is heated in a retort (15) which is continuously flushed with an inert gas. A pack plating process according to any preceding claim wherein the process is an aluminising process and the plating gas is an aluminium halide gas.

Inventors: GILLETT IAN KENNETH, JOHNSON ROBERT WINSTON, MAGRATH PAUL SEAN JOHN, WEAVER COLIN RODNEY, GILLETT, IAN KENNETH, JOHNSON, ROBERT WINSTON, MAGRATH, PAUL SEAN JOHN, WEAVER, COLIN RODNEY
Applicants: ROLLS ROYCE PLC, ROLLS-ROYCE PLC
Application year: 1991
Main classification: C23C10
IPC codes: C23C24, C23C10, C23C8